Rev. ECO
01
32-148
Description
Initial Release
Author Approved
Date
JCKasper
06/20/2006
Functional Instrument Description
and
Performance Verification Plan
Dwg. No. 32-05002
Revision 01
June 20, 2006
32-05002
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Revision 01
Table of Contents
PREFACE ....................................................................................................................... 5
1
INTRODUCTION .............................................................................................. 6
1.1
The Cosmic Ray Telescope for the Effects of Radiation ......................................................................6
1.2
Scope of this document ............................................................................................................................6
1.3
Outline of this document .........................................................................................................................6
1.4
1.4.1
1.4.2
Related documents ...................................................................................................................................6
GSFC Configuration Controlled Documents .............................................................................................6
CRaTER Configuration Controlled Documents ........................................................................................6
2
CRATER OVERVIEW ...................................................................................... 7
2.1
2.1.1
2.1.2
LRO Level 1 Measurement Objectives Relevant to CRaTER .............................................................7
RLEP-LRO-M10 .......................................................................................................................................7
RLEP-LRO-M20 .......................................................................................................................................7
2.2
CRaTER Level 2 and Level 3 Requirements .........................................................................................8
2.3
Overall Design ..........................................................................................................................................9
3
TELESCOPE DESIGN ................................................................................... 11
3.1
Overview ................................................................................................................................................. 11
3.2
3.2.1
3.2.2
3.2.3
3.2.4
Detectors ................................................................................................................................................. 12
Detector Description ................................................................................................................................ 13
Thin and Thick Detectors ........................................................................................................................ 14
Detector Diameter and Maximum Event Rates ....................................................................................... 16
Leakage Current, Detector Noise, and Operating Temperature ............................................................... 18
3.3
3.3.1
3.3.2
Tissue Equivalent Plastic....................................................................................................................... 19
Description and Composition .................................................................................................................. 19
Use of TEP in CRaTER ........................................................................................................................... 19
3.4
3.4.1
3.4.2
Telescope Stack ...................................................................................................................................... 20
Fields of Regard ....................................................................................................................................... 20
Optimizing Fields of View and Geometric Factors ................................................................................. 21
3.5
Telescope Board ..................................................................................................................................... 23
3.6
Electronics Box....................................................................................................................................... 23
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4
ELECTRICAL DESIGN .................................................................................. 23
4.1
Overview ................................................................................................................................................. 23
4.2
Telescope Board ..................................................................................................................................... 24
4.3
Analog Processing Board ...................................................................................................................... 24
4.4
4.4.1
4.4.2
Digital Processing Board ....................................................................................................................... 26
Test Pulse Generator ................................................................................................................................ 27
Low Level Discriminator ......................................................................................................................... 28
4.5
4.5.1
4.5.2
Power ...................................................................................................................................................... 29
DC-DC conversion .................................................................................................................................. 29
Bias Supplies ........................................................................................................................................... 29
5
MEASUREMENT PROCESS ......................................................................... 30
5.1
5.1.1
5.1.2
5.1.3
5.1.4
5.1.5
5.1.6
Science Measurements ........................................................................................................................... 30
Overview ................................................................................................................................................. 30
Description of Pulse Height Analysis ...................................................................................................... 30
Internal Calibration Capability ................................................................................................................ 31
Responding to Solar Energetic Particle Events ........................................................................................ 31
Primary Science Data Products ................................................................................................................ 31
Secondary Science Data Products ............................................................................................................ 32
5.2
5.2.1
5.2.2
Housekeeping ......................................................................................................................................... 33
Overview ................................................................................................................................................. 33
Variables Monitored ................................................................................................................................ 33
5.3
Commands .............................................................................................................................................. 34
6
INSTRUMENT REQUIREMENTS VERIFICATION PLAN ............................. 37
6.1
Description ............................................................................................................................................. 37
6.2
Level 2 Requirements Verification Matrix .......................................................................................... 38
6.3
6.3.1
6.3.2
6.3.3
6.3.4
6.3.5
6.3.6
6.3.7
6.3.8
Level 2 Requirements Verification Plan .............................................................................................. 39
CRaTER-L2-01 Measure the Linear Energy Transfer Spectrum ............................................................. 39
CRaTER-L2-02 Measure Change in LET Spectrum through TEP .......................................................... 39
CRaTER-L2-03 Minimum Pathlength through total TEP ....................................................................... 39
CRaTER-L2-04 Two asymmetric TEP components................................................................................ 39
CRaTER-L2-05 Minimum LET measurement ........................................................................................ 40
CRaTER-L2-06 Maximum LET measurement ........................................................................................ 40
CRaTER-L2-07 Energy deposition resolution ......................................................................................... 40
CRaTER-L2-08 Geometrical factor ......................................................................................................... 40
6.4
Level 3 Requirements Verification Matrix .......................................................................................... 41
6.5
6.5.1
6.5.2
6.5.3
Level 3 Requirements Verification Plan .............................................................................................. 42
CRaTER-L3-01 Thin and thick detector pairs ......................................................................................... 42
CRaTER-L3-02 Minimum energy ........................................................................................................... 42
CRaTER-L3-03 Nominal instrument shielding ....................................................................................... 42
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6.5.4
6.5.5
6.5.6
6.5.7
6.5.8
6.5.9
6.5.10
6.5.11
7
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CRaTER-L3-04 Nadir and zenith field of view shielding ....................................................................... 42
CRaTER-L3-05 Telescope stack ............................................................................................................. 42
CRaTER-L3-06 Full telescope pathlength constraint .............................................................................. 42
CRaTER-L3-07 Zenith field of view ....................................................................................................... 42
CRaTER-L3-08 Nadir field of view ........................................................................................................ 43
CRaTER-L3-09 Calibration system ......................................................................................................... 43
CRaTER-L3-10 Event selection .............................................................................................................. 43
CRaTER-L3-11 Maximum event rate ..................................................................................................... 43
GLOSSARY ................................................................................................... 44
Page 4 of 44
Revision 01
Preface
Revision 01 of this document is being released for CDR.
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1 Introduction
1.1
The Cosmic Ray Telescope for the Effects of Radiation
The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument is designed
to characterize the lunar radiation environment on the Lunar Reconnaissance Orbiter (LRO)
spacecraft. CRaTER will investigate the effects of solar and galactic cosmic rays on tissueequivalent plastics as a constraint on models of biological response to radiation in the lunar
environment.
1.2
Scope of this document
This Functional Instrument Description (FID) document provides an overview of the
CRaTER instrument design as it took shape before the Critical Design Review in June 2006.
The design described in this FID was developed to meet the requirements levied by the Level 2
through Level 3 requirements in the Instrument Requirements Document (IRD). The FID has
two purposes, namely to describe the design that has been developed to meet the requirements
levied in the IRD, and to present the procedures we have developed to verify that CRaTER meets
the requirements.
1.3
Outline of this document
Section 2 presents an overview of the functioning of the instrument. Section 3 discusses
the mechanical design. Section 4 covers the electrical design.
1.4
Related documents
1.4.1









GSFC Configuration Controlled Documents
ESMD-RLEP-0010 (Revision A effective November 30 2005)
LRO Mission Requirements Document (MRD) – 431-RQMT-00004
LRO Technical Resource Allocation Requirements – 431-RQMT-000112
LRO Electrical ICD – 431-ICD-00008
CRaTER Electrical ICD – 431-ICD-000094
CRaTER Data ICD – 431-ICD-000104
Mechanical Environments and Verification Requirements – 431-RQMT-00012
CRaTER Mechanical ICD – 431-ICD-000085
CRaTER Thermal ICD – 431-ICD-000118
1.4.2







CRaTER Configuration Controlled Documents
32-01203
Contamination Control Plan
32-01204
Performance Assurance Implementation Plan
32-01205
Instrument Requirements Document
32-01206
Performance and Environmental Verification Plan
32-01207
Calibration Plan
32-02003.02 Mechanical Interface Document
32-02052
Analog to Digital Subsystem Electrical Interface Control Document
32-05002
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





32-03001.01
32-03010
32-04003.01
32-04011.01
32-05001
32-05203
Electrical Grounding Diagram
Digital Subsystem Functional Description Document
Reliability Assessment Document Drawing
Analog Electronics Worst Case Analysis Drawing
Detector Specification Document
Electronics Subsystem Mechanical Interface Control Document
2 CRaTER Overview
NASA has established investigation measurement requirements for LRO based on RLEP
Requirements and the LRO AO and refined further from the mission instrument selections and
Project trade studies. In this section, the LRO Level 1 measurement requirements and rationales
relevant to CRaTER are reproduced from Section 3.1.1 of ESMD-RLEP-0010, along with the
associated product listed in Section 6.2 the instrument will produce in response to the LRO
measurement requirements.
2.1
LRO Level 1 Measurement Objectives Relevant to CRaTER
2.1.1 RLEP-LRO-M10
2.1.1.1 Requirement
The LRO shall characterize the deep space radiation environment at energies in excess of
10 MeV in lunar orbit, including neutron albedo.
2.1.1.2 Rationale
LRO should characterize the global lunar radiation environment in order to assess the
biological impacts on people exploring the moon and to develop mitigation strategies.
2.1.1.3 Data Product
Provide Linear Energy Transfer (LET) spectra of cosmic rays (particularly above 10
MeV), most critically important to the engineering and modeling communities to assure safe,
long-term, human presence in space.
2.1.2 RLEP-LRO-M20
2.1.2.1 Requirement
The LRO shall measure the deposition of deep space radiation on human equivalent
tissue while in the lunar orbit environment.
2.1.2.2 Rationale
The radiation environment needs to be characterized in order to assess its biological
impacts and potential mitigation approaches, including shielding capabilities of materials and
validation of other deep space radiation mitigation strategies.
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2.1.2.3 Data Product
Provide LET spectra behind different amounts and types of areal density, including tissue
equivalent plastic.
2.2
CRaTER Level 2 and Level 3 Requirements
Tabulated copies of the CRaTER Level 2 and Level 3 requirements listed in the IRD are
repeated in this section for reference.
Item
CRaTER-L2-01
Sec
4.1
CRaTER-L2-02
4.2
CRaTER-L2-03
4.3
Item
CRaTER-L3-01
Ref
6.1
CRaTER-L3-02
CRaTER-L3-03
6.2
6.3
CRaTER-L3-04
6.4
CRaTER-L3-05
6.5
CRaTER-L3-06
6.6
Requirement
Measure the Linear Energy
Transfer (LET) spectrum
Measure change in LET
spectrum through Tissue
Equivalent Plastic (TEP)
Minimum
pathlength
through total TEP
Two
asymmetric
TEP
components
Minimum LET measurement
Quantity
LET
Parent
RLEP-LRO-M10
TEP
RLEP-LRO-M20
RLEP-LRO-M10,
RLEP-LRO-M20
CRaTER-L2-04
4.4
1/3 and 2/3 RLEP-LRO-M20
total length
CRaTER-L2-05
4.5
0.2 keV per RLEP-LRO-M10,
micron
RLEP-LRO-M20
CRaTER-L2-06
4.6
Maximum
LET 7 MeV per RLEP-LRO-M10,
measurement
micron
RLEP-LRO-M20
CRaTER-L2-07
4.7
Energy deposition resolution < 0.5% max RLEP-LRO-M10,
energy
RLEP-LRO-M20
CRaTER-L2-08
4.8
Minimum full telescope 0.1 cm2 sr
RLEP-LRO-M10
geometrical factor
Table 2.2.1: CRaTER Level 2 instrument requirements and LRO parent Level 1 requirements.
32-05002
> 60 mm
Requirement
Quantity
Parent
Thin and thick detector 140 and 1000 CRaTER-L2-01,
pairs
microns
CRaTER-L2-05,
CRaTER-L2-06,
CRaTER-L2-07
Minimum energy
< 250 keV
CRaTER-L2-01
Nominal
instrument >
1524 CRaTER-L2-01
shielding
micron Al
Nadir and zenith field of <=
762 CRaTER-L2-01
view shielding
micron Al
Telescope stack
Shield,
CRaTER-L2-01,
D1D2, A1, CRaTER-L2-02,
D3D4, A2, CRaTER-L2-04
D5D6, shield
Pathlength constraint
< 10% for CRaTER-L2-01,
D1D6
CRaTER-L2-02,
CRaTER-L2-03
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CRaTER-L3-07
6.7
Zenith field of view
<=
34 CRaTER-L2-01,
degrees
CRaTER-L2-02
D2D5
CRaTER-L3-08
6.8
Nadir field of view
<=
70 CRaTER-L2-01
degrees
D4D5
CRaTER-L3-09
6.9
Calibration system
Variable rate CRaTER-L2-07
and
amplitude
CRaTER-L3-10
6.10 Event selection
64-bit mask
CRaTER-L2-01
CRaTER-L3-11
6.11 Maximum
event >=
1000 CRaTER-L2-01
transmission rate
events/sec
CRaTER-L3-12
6.12 Telemetry interface
32-02001
CRaTER-L3-13
6.13 Power interface
32-02002
CRaTER-L3-14
6.14 Thermal interface
32-02004
CRaTER-L3-15
6.15 Mechanical interface
32-02003
Table 2.2.2: CRaTER Level 3 instrument requirements and parent Level 2 requirements.
2.3
Overall Design
The two drawings in Figure 2.3.1 illustrate the overall mechanical design of CRaTER. The
drawing on the left is of the entire assembled instrument. CRaTER consists of a rectangular
electronics box with a tilted top cover (visible on the left) and a telescope assembly (visible on
the right side of the first drawing and rendered in cross section in the drawing on the right).
Figure 3.2.1: Drawings of the CRaTER instrument. The image on the left is from CRaTER
Mechanical Interface Document 32-02003.02 and shows the entire assembled instrument. The
four electrical connectors all visible on the left side of the instrument, consists of the two
redundant 1553 communications interfaces, temperature monitors to the spacecraft, and power
from the spacecraft. The telescope assembly and the electronics box are assembled separately.
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The figure on the right is a cross-section of the telescope assembly, showing the stack of
detectors and TEP, the connections between the detectors and their preamplifiers, and the single
connector to the electronics box.
The electronics box houses a digital processing board (DPB) and an analog processing
board (APB). The DPB interfaces with the spacecraft through the four connecters seen on the
side of the instrument in the drawing on the left in Figure 3.2.1. From the left, there are
redundant 1553 command and telemetry interfaces, feedthroughs for thermometers used for
survival heaters and monitoring the instrument health by the spacecraft, and a connector for the
spacecraft supplied 28V DC power.
The telescope assembly holds the telescope stack and the telescope electronics board
(TEB). The TEB connects the telescope to the electronics box, delivers bias voltages to the
detectors, and sends detector signals and calibration pulses through preamplifiers back to the
APB. The preamplifiers are very sensitive, therefore it is desirable to limit electrical interference
near the detectors. Therefore the telescope assembly is electrically isolated from the electronics
box, and grounded on the same path as the signals from the preamplifiers to the APB. The
telescope stack, visible in the cross section in the drawing on the right in Figure 3.2.1, consists of
aluminum shields on the nadir and zenith sides to block low energy particles, followed by pairs
of thin and thick silicon detectors surrounding sections of A-150 Tissue Equivalent Plastic
(TEP).
Figure 3.2.2 is a functional block diagram of the entire instrument. It is based on the
CRaTER reliability assessment and shows the critical components necessary for the functioning
of the instrument. As described above, the telescope assembly houses the telescope stack and the
telescope board. The electronics box assembly houses the APB and the DPB. The APB is
described in detail in Section. It acts to shape the pulses from each of the detector preamplifiers,
to further amplify the signals, and to generate calibration pulses for testing the response of each
signal path. The DPB identifies and processes particle events and generates scientific
measurements, controls power distribution within the instrument, records housekeeping data, and
receives commands and sends telemetry to the spacecraft.
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Figure 3.2.3: Block diagram of CRaTER showing the critical components of the instrument
(Similar to reliability assessment drawing 32-04003.01). The telescope assembly consists of the
telescope stack of detectors and TEP, and a telescope board with preamplifiers. The electronics
box assembly consists of an analog processing board (APB) on which the pulses from the
detectors are shaped and the digital processing board (DPB) which monitors the detectors for
particle events, conducts the pulse height analysis, and tracks housekeeping data. The DPB also
interfaces with the spacecraft through 1553 for telemetry, and receives a 1 Hz timing signal and
28V for power.
3 Telescope Design
3.1
Overview
In this section, we outline the physical construction of CRaTER, with a focus on the
sensing portion of the instrument, or the telescope assembly. As described in the instrument
overview in Section 2.3, CRaTER consists of two physical parts, the telescope assembly and the
electronics box. The telescope is mechanically mounted to the electronics box, but the two
structures are electrically isolated from one another. The entire telescope assembly is instead
grounded to the digital signal ground. This is done to reduce noise on the spacecraft chassis
ground reaching the detectors and the preamplifiers.
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As specified in CRaTER L2 requirement CRaTER-L3-05 the telescope stack consists of
components three pairs of thin and thick detectors surrounding two pieces of TEP. From the
zenith side of the stack the components are the zenith shield (S1), the first pair of thin (D1) and
thick (D2) detectors, the first TEP absorber (A1), the second pair of thin (D3) and thick (D4)
detectors, the second TEP absorber (A2), the third pair of thin (D5) and thick (D6) detectors, and
the final nadir shield (S2).
Figure 3.1: Cross section of the telescope assembly, showing the pairs of thin and thick detectors
and the TEP in the telescope stack, and the associated telescope electronics board. A pigtail with
signal and ground runs from each of the detectors to one of six preamplifiers on the telescope
electronics board.
These components may be seen in Figure 3.1.1 below. Pairs of thin and thick silicon
detectors are used to measure the LET spectrum. Section 3.2 reviews the detectors selected for
CRaTER and discusses the need for the pairs of thin and thick detectors to cover the full range of
LET possible in silicon. The three pairs are needed to cover the range in LET expected by SEP
and GCR ions in silicon and after evolving through the TEP. Section 3.3 describes the A-150
Tissue Equivalent Plastic. Section 3.4 describes the optimization of the location of components
in the telescope stack.
3.2
Detectors
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3.2.1 Detector Description
The rate (dE/dx) at which ions will loose energy (E) in the detectors is given by the
stopping power of the material,
dE

 Selectronic  S nuclear ,
(1)
dx
which is a combination of electronic and nuclear interactions. For the energy range we are
interested in Selectronic Snuclear , and the dominant process for energy loss by ions in the detectors
is through electromagnetic interactions. Note that nuclear interactions such as fragmentation will
occur, especially for the higher energy galactic cosmic rays we will be measuring with CRaTER.
Nonetheless, the signal produced by a shower of fragments will still be produced by
electromagnetic processes. Within the detector the most of the ionizing energy is used to free
electron-hole pairs in the detector. The signal collected from the detector is the current pulse
produced by these electron-hole pairs. A small fraction, several percent, of the energy loss will
also go into generating phonons in the detectors. Since we do not measure the energy deposited
into phonons, it is important to calibrate the relationship between energy deposited into the
detectors and the final signal.
Figure 3.2 depicts a simplified detector cross-section illustrating the key components of the
CRaTER detectors. Silicon semiconductor detectors are useful for characterizing the energy loss
of ionizing radiation as it traversed the detector. The silicon is doped to make it a
semiconductor, with electron-hole pairs in valance levels with energy levels much smaller than
the first ionizing potential of an atom, making the detector much more sensitive than a
proportional counter. The silicon is composed of both an N-type and a P-type substrate, making
it a large diode that can be biased at a relatively high voltage with little leakage current. When
an energetic charged particle passes through the silicon, it liberates electron-hole pairs, which
rapidly drift to either end of the detector in the electric field established by the bias voltage. The
resulting signal from the drifting electron-hole pairs is linearly proportional to the total energy
deposited. The purpose of the instrument is to amplify these signals when they occur in multiple
detectors, and to stably and accurately determine the height of each pulse.
Two types of detectors, thin and thick circular disks, have been procured from Micron
Semiconductor Ltd for the engineering and flight models of CRaTER. The thin detectors are
nominally 140μm thick and the thick detectors are 1000μm thick. Both detectors are ion
implanted totally depleted structures formed from an N-type substrate. The Phosphorousimplanted N-type substrate is the ohmic side of the detector and the Boron-implanted P-side is
the junction. These implants require lower energy and result in low implant depths of ~0.3μm.
Both detectors are circular, have thin junction and ohmic windows, and have fast timing
capability (i.e., although fast timing is not critical for CRaTER, it is desired to have the
metallization is made in such a fashion as to reduce surface resistivity). There is a guard ring (
indicated by the green region labeled Guard) around the active junction to improve edge
uniformity and a neighboring field plate (FP) ring to aid discharge of oxide stray charge. Each
thin and thick detector is mounted to its own small passive PCB and connected to the telescope
electronics board by a wire pigtail. Details of the expected detector performance and the detector
validation procedures are described in the Detector Specification document 32-05001.
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Figure 3.2: Schematic cross section of the silicon detectors ordered from Micron Semiconductor
Ltd for the CRaTER instrument (Figure 1 of Detector Specification 32-05001).
3.2.2 Thin and Thick Detectors
Thin and thick detector pairs are used to span the entire range of LET and to provide a
system that is less sensitive to low energy protons in the event of intense Solar Energetic
Particles (SEPs) from solar flares or coronal mass ejections.
The need for pairs of thin and thick detectors is reflected in CRaTER-L3-01, namely that
he telescope stack shall contain adjacent pairs of thin and thick Silicon detectors. The thickness
of the thin detectors will be approximately 140 microns and the thick detectors will be
approximately 1000 microns. The need for pairs of detectors arises from the large range in LET
that ions can produce in silicon through ionizing radiation and from the different amplitudes of
energy loss expected from light and heavy ions.
To understand the range of energy deposits expected in the detectors, consider the BetheBloch equation for the energy loss of a particle passing through matter and generating ionization.
The relativistically invariant version is,
2

Zq 2   12 me  2 v  
 dE 
2
2
2


 4 N A re me c 
ln 
(2)
 ,


A 2  
Eion
 dx  Bethe Bloch





where NA is Avogadro’s number, re is the classical radius of the electron, me is the electron mass,
ρ is the mass density of the target, A and Z are the atomic mass and number of the beam, and Eion
is the ionization energy of the stopping medium. Corrections exist at both high energies, where
polarization and shielding effects become more difficult to treat, and at low energies where
assumptions about the adiabaticity of the collisions become invalid. Additionally, an ion will
experience multiple Coulomb scatterings, leading to a trajectory that deviated from a straight
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line. This leads to a statistical distribution in the pathlength a given species of particles even
with identical initial energies will traverse in passing through or stopping in the detector. This
results in the phenomenon of straggling, which will manifest itself as a finite width in the range
of energy deposited by a type of particle, no matter how fine the energy resolution of the detector
or how low the noise level. This “noise” in the signal due to straggling places a lower limit on
the energy resolution needed to distinguish events.
The two key implications of Equation (2) are that:
 The energy deposited in the detectors increases with the square of the charge of the
ionizing particle, so iron will deposit much more energy than hydrogen
 The peak of the energy deposition occurs as the particle is just stopping in the detector
and the minimum ionizing radiation rate for particles passing through the detector is
reached asymptotically at ultrarelativistic energies.
The minimum LET is set by ultra-relativistic protons. The maximum LET is set by the
energy that an iron nucleus will deposit when it just stops in the thick detector.
Figure 3.3: The upper energy range of the thin detectors is determined by the maximum
energy expected from an iron nucleus. This is a plot of the projected range and LET of
iron in silicon showing a peak energy of approximately 700 MeV (J. B. Blake).
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150 MeV Protons
150 MeV Protons
Nominal Threshold
100
10
0.1
1
Detector 1
Detector 3
Detector 6
20,000 incident protons
Thick detectors (1000 um)
1000
Events
Events
1000
1
0.01
10000
20,000 incident protons
Thin detectors (140 um)
Nominal Threshold
10000
100
10
10
1
0.01
100
0.1
1 GeV Protons
100
20,000 incident protons
Thick detectors (1000 um)
Detector 2
Detector 4
Detector 5
Detector 1
Detector 3
Detector 6
1000
Events
Events
10
1 GeV Protons
10000
20,000 incident protons
Thin detectors (140 um)
1000
100
10
1
0.01
1
Deposited Energy (MeV)
Deposited Energy (MeV)
10000
Detector 2
Detector 4
Detector 5
100
10
0.1
1
10
100
Deposited Energy (MeV)
1
0.01
0.1
1
10
100
Deposited Energy (MeV)
Figure: Simulations of the energy deposited in MeV by protons at 150 MeV and 1 GeV for
the thin (left) and thick (right) detectors for 150 MeV (top) and 1 GeV (bottom) initial
energies (GEANT simulation output provided by M. Looper). The dashed orange lines
indicate the minimum energy threshold settings for the thin and thick detectors. As can be
seen, the thin detectors, which are optimized for high-LET measurements, are insensitive
to protons in a broad range of incident energies, while the thick detectors respond to almost
all protons.
3.2.3 Detector Diameter and Maximum Event Rates
We also conducted a trade study to select the diameter of the CRaTER detectors. We
examined 25mm and 35mm diameter detectors. The differences in diameter did not lead to a
large difference in the structural stability of the detectors. Our study therefore focused on the
benefit of a larger diameter detector in the form of larger collecting areas and geometrical
factors, versus the risk of unacceptably high single particle fluxes during major solar energetic
particle (SEP) events. Figure 3.2.3.1 is a plot of selected proton and heavy ion differential fluxes
for a six day interval in January 2005. This period was associated with an enhancement of the
interplanetary radiation environment several orders of magnitude above typical background
values. As the most intense radiation interval in the last solar cycle, we used this period to make
estimates of the worst case event rates. As can be seen in the plot, while the heavy ions were
also enhanced during this event, the flux of hydrogen was many orders of magnitude larger. The
hydrogen therefore was the focus of this study.
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Figure 3.2.3.1: Particle spectra recorded by instruments on the ACE spacecraft for the
period from January 14-20, 2005.
The first thick detector, D2, is designed to see protons and is therefore most susceptible
to enhanced fluxes during SEP events. The other thick detectors lie behind shielding in the form
of the TEP and telescope mass, and are therefore expected to have lower rates. The concern is
not that the detector could be damaged by the high flux, but more that it could experience pileup, in which multiple protons pass through the detector at the same time and corrupt the signal.
If the D2 signal became corrupted and drove the acquisition system it could lead to a decrease in
the utility of the measurements returned to the ground. The predicted singles count rate for D2 is
shown in Figure 3.2.3.2
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Figure 3.2.3.3: Calculated singles counting rate for D2 during the January 2005 SEP event using
several measurements of proton fluxes from the ACE spacecraft and 25 mm and 35 mm diameter
detectors. The lower horizontal line indicates the 1200 event/second rate that the primary
science stream transmits events to the ground. The upper horizontal line indicates the threshold
above which multiple protons would enter the detector within the 2 μs shaping time of the pulse
forming network.
3.2.4 Leakage Current, Detector Noise, and Operating Temperature
Figure: Estimates of the thick detector noise operating at 20C as a function of the detector
leakage current.
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3.3
Tissue Equivalent Plastic
3.3.1 Description and Composition
The muscle equivalent tissue equivalent plastic (TEP) A-150 is the most popular material
used today in the construction of instruments that measure the evolution of ionizing radiation in
the presence of materials similar to human tissue. A150 is an Exradin proprietary plastic blend
designed to be tissue equivalent. It is also known as Shonka tissue equivalent plastic after its
original creator, Francis Shonka1. TEP is composed of a mixture of polyethylene, polyamide,
carbon black, and calcium fluoride (See Tables 3.3.1 and 3.3.2).
TEP has the appearance of a black crayon but is very stiff and is easily machined. The
TEP is slightly hydroscopic, and will absorb up to 3% water by weight in a 50% humidity
environment. Our tests showed that other than the water little material outgassed from TEP
samples after they were first baked out in a vacuum.
Material
% by Weight
Polyethylene, (CH2)n
45.14
Polyamide, nylon, Zytel 69 35.22
Carbon black, C
16.06
Calcium fluoride, CaF2
3.58
Table 3.3.1: Material composition of A-150 TEP by weight.
Element % by Weight
H
10.2
C
76.8
O
5.9
N
3.6
Ca
1.8
F
1.7
Al
0.015
K
<0.01
Mg
0.068
Fe
0.039
Table 3.3.2: Elemental composition of A-150 TEP
by weight. The portion of the table shaded in gray
indicates trace elements.
3.3.2 Use of TEP in CRaTER
1
F.R. Shonka, J.E. Rose, and G. Failla, 2nd United National International Conference on the Peaceful Uses of the
Atom; Health and Safety; Dosimetry and Standard, 21, 753, 184, 1958.
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Two pieces of TEP are used in the telescope stack. The pieces are right cylinders and are
held firmly in place at each end.
3.4
Telescope Stack
3.4.1 Fields of Regard
Motivated by the desire for simplicity and by the fact that the capability it not needed for
the CRaTER Level 1 data products, the telescope does not determine the trajectory of individual
particles (through position sensing strips, for example). Additionally, no mechanism is
employed to identify particles that pass through the side of the instrument (using a scintillator
shroud surrounding the entire telescope stack coupled to a photo-multiplier tube, for example).
These techniques can be used to constrain the pathlength traversed in the detectors and to reject
events with multiple particles or scattering out of the instrument. Since we are interested in
studying the spectrum of all particles within the telescope, including those that scatter at large
angles, and because the technologies described above introduce a great deal of complexity,
telemetry, and mass, we instead define the fields of view of the instrument through detector
coincidences.
The CRaTER Level 2 and Level 3 requirements place constraints on the field of view of
the instrument only indirectly, through restrictions on the uncertainty in the pathlength traversed
through the TEP and the detectors, or through lower limits on geometrical factors, for example.
While large fields of view are not a requirement, they are desirable in that they increase the
geometric factor and therefore the statistical accuracy of the LET spectral measurements. In
addition, since it is possible that SEPs and GCRs reflecting off the lunar surface – especially at
small grazing angles of incidence – could provide significant diagnostics of the composition of
the moon, it is desirable that the nadir side of the telescope have as large a field of view as
possible.
The optimization of the CRaTER fields of view is discussed in the following Section.
Here we note the fields of regard that the spacecraft is able to support for the instrument. Due to
the lack of trajectory determination, it is essential that the fields of view of CRaTER not be
obstructed by components of the spacecraft, as this instrument will not be able to distinguish
between the primary spectrum of GCRs and SEPs and the secondary spectrum produced by
interactions with spacecraft components. The fields of regard maintained by the spacecraft for
CRaTER are shown in Figure 3.4.1.
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Figure 3.4.1: Clear fields of regard of the nadir (top in this image) and zenith (bottom)
sides of the instrument (Figure 2 of Telescope to Electronics Subsystem Mechanical ICD
32-05203). The spacecraft shall not obstruct an 85o full width nadir field of regard and a
35o zenith field of regard.
3.4.2 Optimizing Fields of View and Geometric Factors
The final spacing between the components of the telescope stack was determined by
conducting a trade study that identified the configuration that optimized the parameters captured
in the Level 2 and Level 3 design requirements. The requirements from the IRD that are effected
by the telescope stack are listed in Table 3.4.1.
Item
Requirement
Quantity
CRaTER-L2-03 Minimum pathlength through total TEP
> 60 mm
CRaTER-L2-04 Two asymmetric TEP components
1/3 and 2/3 total length
CRaTER-L2-08 Minimum full telescope geometrical factor 0.1 cm2 sr
CRaTER-L3-06 Pathlength constraint
< 10% for D1D6
CRaTER-L3-07 Zenith field of view
<= 34 degrees D2D5
CRaTER-L3-08 Nadir field of view
<= 70 degrees D4D5
Table 3.4.1: CRaTER Level 2 and Level 3 instrument requirements used in determining the
optimum telescope configuration.
We found that the telescope stack optimization could be expressed by varying two free
parameters, which we called L1 and L2. L1 is the length between the top of detector D2 and the
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top of detector D4, and L2 is the length between the top of detector D2 and the top of detector
D6. We considered other heights less important. For example, the spacing between the thin and
thick detectors in each pair is set by the natural value of the PCB the detectors are mounted on.
We found that placing the zenith and nadir shields further away from the closest detector pair
only increased the necessary diameter of the shield needed to provide a uniform thickness over
the field of view. Therefore the shields are simply best put close to the adjacent detectors, as
limited solely by sensible design practices. Since the TEP sections have a sufficiently larger
diameter than the detectors, it did not make a difference where the TEP was mounted relative to
the detector pairs. Thus, L1 and L2 summarize the two unique parameters.
Figure presents the results of this trade study, plotting L2 VS L1. Each line on the figure
indicates a constraint on the accessible space. The black dashed vertical and horizontal lines are
the minimum values of L1 and L2 respectively due to the finite sizes of the components of the
telescope. The lengths cannot be smaller than these values. The solid red line indicates the
range of L1 and L2 where the nadir field of view defined by an event seen by both D4 and D5
(the worst case for a particle impinging on CRaTER from the lunar surface) is less that 75 o, or
the nadir field of regard. For all the colored lines the region ruled out is indicated by the downstrokes on the line. The light green line indicates the region where the field of view of a zenith
particle incident from deep space passes through both D2 and D5 with a field of view less than
the zenith field of regard of 35o. Finally, the Blue line indicates the range where the geometric
factor for a particle passing through the whole telescope is greater than 0.7 cm2 sr. The blue line
demonstrates that we found that the
Figure 3.4.1: Optimization of the separations between the pairs of thin and thick detectors in the
telescope stack. The constraints derived from the Level 2 and Level 3 requirements described in
the Instrument Requirements Document are that the geometrical factor for events passing
through all detectors be greater than 0.7 cm2 sr, that the zenith field of view be less than 35o, and
that the nadir field of view be less then 75o. The dashed dark lines indicate the minimum heights
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possible due to the physical size of the components of the telescope, including the TEP and the
detectors.
3.5
Telescope Board
The telescope board is mounted within the telescope assembly and connects the telescope
electronics to the electronics box through a single large connector. The components of the board
are discussed in Section 4.2.
3.6
Electronics Box
The electronics box houses the Analog Processing Board (APB) and the Digital
Processing Board (DPB). The electronics box also acts as the electrical, mechanical, and thermal
interface between CRaTER and the spacecraft.
4 Electrical Design
4.1
Overview
In this Section we cover the flow of electrical signals from the detectors through the
telescope board to the analog processing board and finally the digital processing board.
A key electrical aspect of the overall design of CRaTER is that the telescope housing is
connected to the analog ground inside the telescope assembly, and nowhere else. The telescope
housing is electrically isolated from the electronics box in order to prevent noise coupling from
the spacecraft into the detector front-end. The electronics box housing is connected to the digital
ground inside the electronics box.
Figure 4.1.1: Grounding diagram for the instrument, illustrating the connections between the
electronics box and the telescope (CRaTER Electrical Grounding Diagram 32-03001.01).
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4.2
Telescope Board
The purpose of the telescope board is to bias each of the detectors, and to provide for an
initial pre-amplification of the signals close to the detectors to reduce noise as the small initial
signal is transported. Figure 4.2.1 is a block diagram of the components of the telescope board
for a single detector.
Figure 4.2.1: Schematic illustrating the components of the telescope board for a single detector.
The N-contact surface of the detector is grounded, and the guard ring and the active surface of
the P-contact are separately biased, with current from the guard ring shunted to ground. Current
from the detector is AC coupled to a jFET and A250 amplifier and the amplified signal is sent to
the electronics box. A feedback network dissipates the collected signal from the detector and
accepts external test pulses from the electronics box for calibration purposes.
4.3
Analog Processing Board
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Figure 4.3.1: Block flow diagram illustrating the pulse shaping process (adapted from figure 2 of
the Analog to Digital Subsystem Electrical Interface Control Document 32-02052). The preamp
processes the signals from either the detector or the test pulser. The signal is then amplified and
given an approximately 1 us pulse profile. A baseline restorer is used to help maintain the
baseline voltage with larger events. The baseline restorer can also be used to generate a lowlevel trigger - this feature is not used in the current design in flight but is used on the ground
while calibrating the system.
The APB provides a linear transfer function of output signal amplitude to detector energy
deposit for three thin detectors and three thick detectors. A functional block diagram of a single
amplifier string is shown in Figure 4.3.1. The signal from the preamplifier is passed through a
system to remove constant offsets and give the signal a symmetric Gaussian profile. The thin
and thick detector signals are Gaussian shaped pulses with a peaking time of 1 usec +/- 20%.
This pulse shaping time was set by a consideration of the noise level of the thick detectors at 20C
as a function of the shaping time. This process is illustrated in Figure 4.3.1, which plots the
expected noise levels of the thin and thick detectors at two temperatures as a function of the time
constant.
Our calculations suggest that the thin detector noise voltage will be less than 1.5 mVrms.
The thin detector transfer function shall be nominally 3 mV/MeV into 1 Meg-ohm. The actual
transfer function will be determined during electrical calibration of the APB with the DPB. The
thick detector transfer function shall be nominally 30 mV/MeV into 1 Meg-ohm.
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Figure 4.3.1: Selecting the optimum peaking time for the detectors to minimize expected noise
level (Figure 1 of Analog Electronics Worst Case Analysis 32-04011.01).
4.4
Digital Processing Board
Figure 4.4.1: Schematic illustration of the interface between the analog processing system
designed by Aerospace and the digital processing and power supply designed by MIT (adapted
from figure 1 of the Analog to Digital Subsystem Electrical Interface Control Document 3202052).
The purpose of the Digital Processing Board (DPB) is to identify events that are valid for
pulse height analysis, digitize the pulse-heights of the APB output signals, provide control of the
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APB test pulser, supply power and detector bias, and provide the command and telemetry
interface to the spacecraft. Figure 4.4.1 details the connections between the APB and the DPB.
Figure 4.4.2 is a block diagram of the functions of the FPGA and the DPB.
Figure 4.4.2: Block diagram of the digital system adapted from Figure 1 of CRaTER Digital
Subsystem Functional Description Document 32-03010. The FPGA generates test pulses for
calibration and broadcasts the LLD for the think and thick detectors, performs the PHA, enables
bias voltages, and monitors housekeeping data such as temperatures, voltages, currents, and total
dosage within the telescope. The FPGA communicates to the spacecraft through the 1553
interface and receives the 1 Hz clock from the spacecraft.
4.4.1 Test Pulse Generator
The test pulser function is used during ground test phases and on-orbit to monitor the
transfer function stability with time. The test pulser injects a known charge into the front of each
preamplifier at a known rate. The DPB will supply one programmable voltage level and two
clocking signals; one for a high range input to test amplifier saturation and one for a low range
input to test the thresholds. The APB will convert these signals into a charge injection into the
detector preamplifiers. The resolution for level settings shall be 8-bits.
Two timing pulses are forwarded to the Analog Board. One frequency control (command I/F:
ECAL Rate) causes both pulses to operate at either the high (period = 512μs) or low (period =
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524 ms) rate. Duty cycle is approximately 50%, but there may be a slight discontinuity at the
one-second tick. Each pulse can be independently enabled (command I/F: ECAL High/Low
On/Off). When disabled, the pulse output is held at logic zero. Additionally an “Electrical Cal
Amplitude” voltage is generated by converting a Pulse Width Modulated (PWM) FPGA output
to a DC voltage. The amplitude, set via the 8-bit command I/F register, ranges from 0 to
approximately 5V-DC. (Default is 0x00 => 0V.)
PWM Operation: The PWM subsystem produces a bit stream that operates at 1MHz. The
number of “high” bits is determined by the “DAC” register value. At the zero setting, the PWM
output of the FPGA is always low. A setting of one results in a PWM output that is low for
255μs, and high for 1μs. At the 0x80 (halfway) setting, the PWM output produces a square wave
(128μs high +128μs low). At the 0xFF setting (full-scale), the PWM output is high for 255μs,
and low for 1μs.
The High Range Clock is a clock whose frequency corresponds to the rate of charge
injection into the preamplifiers and whose amplitude is controlled by the pulser level, which is
scaled on the APB to test the APB amplifiers mid range and overload performance. The source
of this signal is the DPB and the destination is the APB. This signal will be used for both thin
and thick detector strings. The High Range Clock is a CMOS digital signal with 0-volts
corresponding to logic 0 and 5-volts corresponding to logic 1. A logic 0 triggers the test pulser
circuit on the APB to inject charge into the front of both the thin and thick detector preamps. The
amount of charge injected is determined by the pulser level and by a fixed resistor on the APB.
The Low Range Clock is a clock whose frequency corresponds to the rate of charge
injection into the preamplifiers and whose amplitude is controlled by the pulser level and scaled
on the APB to test the APB amplifiers threshold and mid range performance. The source of this
signal is the DPB and the destination is the APB. This signal will be used for both thin and thick
detector strings. The Low Range Clock is a CMOS digital signal with 0-volts corresponding to
logic 0 and 5-volts corresponding to logic 1. A logic 0 triggers the test pulser circuit on the APB
to inject charge into the front of both the thin and thick detector preamps. The amount of charge
injected is determined by the pulser level and by a fixed resistor on the APB.
4.4.2 Low Level Discriminator
Two Low Level Discriminator (LLD) threshold voltages are generated by converting Pulse
Width Modulated FPGA outputs to a DC voltages. The amplitude, set via the 8-bit command I/F
registers, ranges from -0.047 to 0.141 V-DC. The Event Amplitude Discriminator - Thin
Detector setting drives the detector 1, 3 and 5 comparators; the Event Amplitude Discriminator –
Thick Detector drives the detector 2, 4 and 6 comparators. (Default setting for both LLD
thresholds: 0x80 => 0.047V.) (See paragraph above, in Section 2.4, for a description of the
PWM operation.)
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4.5
Power
4.5.1 DC-DC conversion
The DPB monitors and enables a low voltage DC-DC converter that changes the 28V
provided by the spacecraft into +/- 5V for use with the digital electronics components and to feed
to the high voltage bias supplies.
4.5.2 Bias Supplies
The specifications for the bias supplies are derived from several system parameters: range
of detector operating voltage, size of detector bias resistor, and maximum practical loading of the
bias source. Because the detector specification allows for a wide range of depletion voltages, the
bias supplies must be capable of much larger voltages than what might be necessary once the
detectors are delivered. In light of this, a voltage divider on the Telescope board will allow the
bias voltage to be reduced as necessary to operate the detector in the safe range. Nominally, the
bias will be set to 30 volts higher than the full depletion voltage. The total equivalent resistance,
including the divider and the series bias resistor, is chosen to keep the bias point stable within 15
volts, so that the detectors will always be biased 15 volts higher than depletion over a large range
of leakage currents. These factors contribute to the maximum load capability required of the bias
supplies.
4.5.2.1 Thin detector bias
The thin detector bias is supplied by the power system through the DPB. The thin
detector bias is a stable positive DC voltage of +75V +/- 2.5%. This voltage will be tuned on the
APB/Detector Board using a resistive divider to meet the bias requirements of the thin detector.
The detector leakage current in addition to the resistive divider current creates the total load on
the bias supply. The total current drawn by the thin detector bias supply (i.e., sum of all three
thin detectors) shall not exceed 50 uA.
4.5.2.2 Thick detector bias
The thick detector bias is also supplied to the APB/Detector Board by the power system
through the DPB. The thick detector bias is a stable positive DC voltage of +225V +/- 2.5%.
This voltage will be tuned on the APB/Detector Board using a resistive divider to meet the bias
requirements of the thick detector. The detector leakage current in addition to the resistive
divider current creates the total load on the bias supply. The total current drawn by the thick
detector bias supply (i.e., sum of all three thin detectors) shall not exceed 150 uA.
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5 Measurement Process
5.1
Science Measurements
5.1.1 Overview
The primary science measurements consist of the amplitudes of pulses detected from each
of the detectors. The six signals from the pulse-forming and amplification networks on the APB
are sent to the digital board, where the signals from the thin and thick detectors are separately
compared to a low level discriminator (LLD) reference voltage. If any of the six signals crosses
the corresponding LLD threshold, then a pulse height analysis (PHA) of all six signals is
triggered.
After the PHA is complete, the list of triggered detectors is compared with a mask that
encodes the sets of detector coincidences that correspond to events of interest for subsequent
analysis on the ground. If an event falls within the mask set, then it is added to a buffer
connected to the FPGA, otherwise it is discarded.
5.1.2 Description of Pulse Height Analysis
At some time T<0μs a current pulse produced by either migrating electron-hole pairs in
the detector or a test pulse from the digital board passes through the preamplifier in the telescope
board and arrives at the analog board. At T=0μs the signal from the preamplifier crosses the low
level discriminator (LLD) and triggers the PHA. At T=2μs the peak detect is set to hold and the
ADC is commanded to power up. At T=4μs the ADC is powered up and does a sample and hold
in the following two clock cycles. The ADC then performs a serial conversion over the next 12
clock cycles, clocking the results out to the digital board. About 1/3 of the way through the ADC
the peak detector is reset. The ADC and the measurement process complete at T=12μs, defining
the fixed measurement deadtime.
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Figure: Schematic illustration of the pulse height analysis (PHA) process on the analog and
digital boards in the electronics box.
5.1.3 Internal Calibration Capability
5.1.4 Responding to Solar Energetic Particle Events
Section 3.2.3 examined the maximum fluxes expected in the D2 detector due to the
largest SEP event of the previous solar cycle. During these large events, the effects of the
enhanced heavy ion spectrum may be more important for meeting the Level 1 measurement
goals of LRO that CRaTER is responsive to than just measuring the large proton spectrum.
Several features have been incorporated into the CRaTER signal processing design to allow for a
reduction in the sensitivity of the instrument to these SEP protons. First, the thickness and LET
ranges of the thin and thick detectors have been optimized such that the thin detectors that
measure high LET events do not see many of the protons (See the discussion in Section 3.2.2).
The FPGA is designed to deactivate one or more of the detector signals for triggering a PHA by
exceeding the LLD value. In the event that D2 rates rose above a specified value, the FPGA
could either deactivate the D2 signal or raise the threshold for low LET events on the thin
detectors.
5.1.5 Primary Science Data Products
The event data for a single event in the primary science packet consists of a 9 byte block
containing the 6 12-bit numbers corresponding to the Pulse Height Analysis of the six detectors.
Primary science telemetry packets queries would optimally occur every 40 ms, resulting in a
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maximum primary science throughput of 1200 events, or 86400 data bits per second. There are
no hardware restrictions on when or how often such 1553 bus queries occur, however. The
current implementation of the FPGA contains two buffers. While primary science data are being
written to one buffer during a given one second interval, the FPGA reads out the data stored in
the previous second from the other buffer. Event writes and primary science telemetry reads
share a common memory (32Kx8 SRAM). This EV-SRAM is split into two buffers (determined
by the most significant address bit), with one buffer always allocated to event writes, the other to
telemetry reads. The buffer select is toggled every second, so that events written during the Nth
second are read-out during the (N+1)th second
5.1.6 Secondary Science Data Products
Secondary science telemetry packets are retrieved from the 1553 Remote Terminal in the
interval between 100 and 900 milliseconds following a 1 Hz Reference Pulse. Secondary
science packets are read during every 1 second data interval in which primary science data is
being collected.
A summary of the variables in the Secondary Science Data Packet is provided in Table
5.1.6.1. These Secondary Science values set the time resolution that the state of the instrument
(calibration commands, bias supply status, and disabling of detector processing) can be tracked
on the ground.
A critical second component of this data stream is the telemetry stall, event reject, and good event counters,
which are used to determine the deadtime of the instrument and convert the primary science data into absolute fluxes
during periods of intense SEP radiation. Event counters, detailed below, are all 16-bit counters that “freeze” at the
maximum count until reset. Every second (upon TK1S) the current-count value is transferred to a
holding register (which is inserted into the Secondary Science Telemetry) and the counter is
cleared.
The event reject counter is a 16-bit counter that is clocked every time an event is rejected by
the Discriminator Accept Mask test.
The Total Event Counter is a 16-bit counter is clocked every time an event is processed. This
includes rejected events, events written into memory, as well as accepted events that aren’t
written (due to buffer overflow).
The Good Event Counter is a 16-bit counter which is clocked every time an qualifying event is
processed. This counter is clocked for all events, even if they are not written into SRAM (due to
buffer overflow) or read-out of SRAM the following second (telemetry overflow).
The Singles Counters: For each detector: a 16-bit counter which is clocked by the “Singles
Count” signals received from the AE subsystem. Since these inputs are asynchronous, they are
sampled and filtered. Singles “incidents” with pulse widths < approximately 200ns may not be
recognized.
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Relative Bit
Position
0
1
2
3
4
5
6
7
8
9
10
11-15
16-31
32-47
48-63
64-79
Data Description
Electrical Cal On = 1
Electrical Cal Rate High = 1
Thin Detector Bias Voltage On = 1
Thick Detector Bias Voltage On = 1
Detector D1 Processing Enabled = 1
Detector D2 Processing Enabled = 1
Detector D3 Processing Enabled = 1
Detector D4 Processing Enabled = 1
Detector D5 Processing Enabled = 1
Detector D6 Processing Enabled = 1
Fixed value = 0
RT SubAddr of Last Command
Contents of Last Command
Telemetry Stall Counter
Event Reject Counter
Good Event Counter
Table 5.1.6.1: Description of the data contained in the Secondary Science Data Packet,
taken from Section 4.1.7.2 of the Data ICD.
5.2
Housekeeping
5.2.1 Overview
Housekeeping packets are read every 16 seconds while 28VDC power is supplied to the
instrument at in the same time window as the secondary science data products. The following
Section discusses the variables that are monitored in the housekeeping data stream.
5.2.2 Variables Monitored
5.2.2.1 Bias Voltages
Input 28VDC bus power will be monitored. We expect variations as a function of bus
voltage, but – at a given voltage – the power numbers should be stable. Prior to orbiter
integration the data will be collected manually. After integration the power numbers will be
extracted from spacecraft housekeeping if available.
5.2.2.2 Temperature
The APB provides temperatures recorded in the telescope and at the APB itself. The
source of telescope signal is the APB, however the temperature transducer will actually be
located in the telescope. The circuit implementation shall be an AD590, with its source terminal
connected to the –5 volt supply and the other terminal connected to +5 volts through a 10 kohm
resistor. Additional temperature measurements are made of the bulkhead, the digital board, and
the bias power supply.
5.2.2.3 Total Dose Monitor
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The APB board will contain a device for measuring the total radiation dose experienced
by the electronics on the APB board. The device will provide three analog outputs corresponding
linearly to total dose with three respective coefficients.
(3)
Total Dose  D1  R1  D2  R2  D3  R3 .
The actual values of the coefficients will be obtained during device testing. The first output will
have the highest sensitivity, with D1 ~ 0.8 V/mRad. The second output provides a medium
resolution with D1 ~ 3.12 V/Rad and the final output provides the lowest resolution, with
D1 ~ 12.2 mV/Rad. The maximum total dose the system can record, set by the 0-4V range of the
outputs and these approximate calibrations, is several thousand Rads.
5.2.2.4 Current Monitor
The detector leakage currents will be monitored separately for the thin and thick
detectors. For the thin detectors the monitor will cover the current range from 0-0.24 μA. For
the thick detectors the monitor will cover the range from 0-0.12 μA.
5.3
Commands
The CRaTER command structure is described in the Spacecraft to CRaTER Data Interface
Control Document 32-02001. The structure of the commands is summarized in Table 5.3.1.
Key commands include the time of the next sync pulse. The DPB accepts and increments a 1 Hz
timing pulse from the spacecraft. The value of the time of the next sync pulse send in the
command packet is used by the FPGA to determine the time the data were recorded. The value
is valid on the next received 1Hz Reference Pulse (Section Error! Reference source not
found.). This command is sent during each (1 second) data interval, at least 100 milliseconds
before the Reference Pulse to which its value applies, and at least 100 milliseconds following the
previous Reference Pulse. The Thin and Thick Detector Bias on/off controls the application of
(fixed) bias voltage to the thin silicon detectors. Electrical Cal on/off controls a fixed energy
calibration signal injected into the event chains, and the Electrical Cal Rate selects between a low
(1 Hz) rate and a high (2KHz) rate cal signal.
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Sub-Address
No. of Description
Default
Data Bits
1
64
Time of next sync pulse
(none)
Bits 0-31 = seconds
Bits 32-63 = sub-seconds
2
16
Command Echo
(none)
3
16
Global Discrete Commands
Bit 0 = Thin Detector Bias Off
Off
Bit 1 = Thin Detector Bias On
Bit 2 = Thick Detector Bias Off
Off
Bit 3 = Thick Detector Bias On
Bit 4 = Electrical Cal Off
Off
Bit 5 = Electrical Cal On
Bit 6 = Electrical Cal Low Rate
Low
Bit 7 = Electrical Cal High Rate
Bit 11 = Data Test Mode
No action
Bit 14 = Clear all Commands
Bit 15 = System Reset
(none)
4
16
Video Processing Commands
Bit 0 = Detector D1 Processing Off
Bit 1 = Detector D1 Processing On
On
Bit 2 = Detector D2 Processing Off
Bit 3 = Detector D2 Processing On
On
Bit 4 = Detector D3 Processing Off
Bit 5 = Detector D3 Processing On
On
Bit 6 = Detector D4 Processing Off
Bit 7 = Detector D4 Processing On
On
Bit 8 = Detector D5 Processing Off
Bit 9 = Detector D5 Processing On
On
Bit 10 = Detector D6 Processing Off
Bit 11 = Detector D6 Processing On
On
5
64
Discriminator Accept mask
All “1”s
6
16
Event Amplitude Discriminator,
255,0
Thin Detectors {1,3,6}
7
16
Event Amplitude Discriminator,
255,0
Thick Detectors {2,4,5}
Table 5.3.1: Description of commands that are sent to CRaTER.
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6
Instrument Requirements Verification Plan
6.1
Description
This section outlines the steps in the verification plan for demonstrating that the CRaTER
instrument and subsystems meet the Level 2 and Level 3 requirements described in this
Instrument Requirements Document. Aspects of the full verification plan for CRaTER are
addressed in the CRaTER Performance and Environmental Verification Plan (32-01206), the
CRaTER Calibration Plan (32-01207), the CRaTER Performance Assurance Implementation
Plan (32-01204), and the CRaTER Detector Specification (32-05001). The purpose of this
section is to demonstrate that the requirements levied on CRaTER in this document may be
verified and to provide a snapshot of that verification plan, but the aforementioned documents
will take precedence and describe the final verification plans.
We categorize the verification methods into three categories, “inspection”, “test”, and
“analysis”, which are described below:
Inspection: This is used to determine system characteristics by examination of and comparison
with engineering drawings or flow diagrams and computer program listings during product
development to verify conformance with specified requirements. Inspection is generally nondestructive and consists of visual examinations or simple measurements without the use of
precision measurement equipment.
Test: Test is used to verify conformance of functional characteristics with operational and
technical requirements. The test process will generate data, and precision measurement
equipment or procedures normally record these data. Analysis or review is subsequently
performed on the data derived from the testing. Analysis as described here is an integral part of
this method and should not be confused with the "analysis" described in the third verification
category.
Analysis: Analysis or review of simulation data is a study method resulting in data used to verify
conformance of characteristics with specified requirements. Worst case data may be derived
from design solutions where quantitative performance cannot be demonstrated cost-effectively.
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6.2
Level 2 Requirements Verification Matrix
Item
CRaTER-L201
CRaTER-L202
Sec
8.3.1
8.3.4
Requirement
Measure the Linear Energy
Transfer (LET) spectrum
Measure change in LET spectrum
through Tissue Equivalent Plastic
(TEP)
Minimum pathlength through total
TEP
Two asymmetric TEP components
CRaTER-L203
CRaTER-L204
CRaTER-L205
CRaTER-L206
CRaTER-L207
CRaTER-L208
8.3.3
8.3.5
Minimum LET measurement
8.3.6
Maximum LET measurement
8.3.7
Energy deposition resolution
8.3.8
Minimum
factor
8.3.2
D1D6
Quantity
LET
Verification
A
TEP
> 60 mm
A
I
1/3 and 2/3 (27 and I
54 mm nominal)
< 0.25 keV per
micron
> 7 MeV per
micron
< 0.5% max energy
geometrical > 0.1 cm2 sr
T
T
T
I
Table 8.1: Verification matrix for Level 2 requirements, listing the relevant part of Section 8.3 ,
the requirement, the section in which the verification plan is outlined, and indicating the planned
use of inspection, test, demonstration, and analysis for each requirement.
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6.3
Level 2 Requirements Verification Plan
6.3.1 CRaTER-L2-01 Measure the Linear Energy Transfer Spectrum
6.3.1.1 Current Value
6.3.1.2 Verification by Analysis
One dimensional numerical simulations will be used to predict the energy deposition in
the silicon detectors as a function of input and evolving LET spectra through the instrument.
These simulations will demonstrate that the energy deposition in the silicon detectors is
sufficient to measure the local LET spectrum and provide predictions for comparison with the
beam and radiation tests.
6.3.2 CRaTER-L2-02 Measure Change in LET Spectrum through TEP
6.3.2.1 Current Value
This has been demonstrated.
6.3.2.2 Verification by Analysis
We have used numerical simulations to model the expected evolution the LET spectrum
of ions through the TEP sections. These simulations have agreed very well (within our needed
measurement accuracy) with measurements at the proton beam at Mass General Hospital.
6.3.3 CRaTER-L2-03 Minimum Pathlength through total TEP
6.3.3.1 Current Value
The current value of the minimum pathlength through the TEP in the engineering model
is 61mm.
6.3.3.2 Verification by Inspection
Mechanical diagrams will be reviewed to verify that the total length of TEP traversed by
particles passing through the telescope is at least 60 mm of TEP. The length of the TEP
components will be measured during fabrication. We also found that the length of the TEP could
be double checked by examining beam data.
6.3.4 CRaTER-L2-04 Two asymmetric TEP components
6.3.4.1 Current Value
The short section of TEP is 27 mm long and the long piece of TEP is 54 mm long.
6.3.4.2 Verification by Inspection
Mechanical diagrams will be reviewed to verify that the lengths of the two components of
TEP are 27 mm and 54 mm respectively. The flight sections of TEP will be measured at low
resolution to verify the length.
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6.3.5 CRaTER-L2-05 Minimum LET measurement
6.3.5.1 Test
The minimum LET threshold of the thick detectors will be measured in an accelerator
facility.
6.3.6 CRaTER-L2-06 Maximum LET measurement
6.3.6.1 Test
The maximum LET we can measure in the thin detectors is greater than what we would
expect from a stopping iron nucleus and therefore we are unlikely to be able to produce the
maximum signal with a real beam. The maximum LET threshold of the thin detectors will be
extrapolated based on the performance of the analog and digital electronics and beam testing at
lower LET values. We have demonstrated this procedure using the MGH measurements and the
TEP Test Assembly.
6.3.7 CRaTER-L2-07 Energy deposition resolution
6.3.7.1 Test
The detector provider will produce specifications of the energy resolution of each of the
detectors, as determined with a pulser test and with an alpha source. The energy deposition
resolution will be determined through analysis of pulsar data and through the use of lineemission from gamma-ray sources. We have performed this in the lab with old Micron detectors
to demonstrate that it can be done successfully.
6.3.8 CRaTER-L2-08 Geometrical factor
6.3.8.1 Inspection
The geometrical factor will be determined through review of the telescope mechanical
drawings. The geometrical factor is a function of the separation between the detectors and the
radius of the detectors.
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6.4
Level 3 Requirements Verification Matrix
Item
Ref
Requirement
Quantity
Verification
CRaTER-L3- 8.5.1 Thin and thick detector 140 and 1000 microns
I
01
pairs
CRaTER-L3- 8.5.2 Minimum energy
< 250 keV
T
02
CRaTER-L3- 8.5.3 Nominal
instrument 0.060” Al
I
02
shielding
CRaTER-L3- 8.5.4 Nadir and zenith field of 0.030” Al
I
03
view shielding
CRaTER-L3- 8.5.5 Telescope stack
Shield, D1D2, A1, D3D4, I
04
A2, D5D6, shield
CRaTER-L3- 8.5.6 Pathlength constraint
10% for D1D6
I
05
CRaTER-L3- 8.5.7 Zenith field of view
35 degrees D1D4
I
06
CRaTER-L3- 8.5.8 Nadir field of view
75 degrees D3D6
I
07
CRaTER-L3- 8.5.9 Calibration system
Variable rate and gain
T
08
CRaTER-L3- 8.5.10 Event selection
64-bit mask
T
09
CRaTER-L3- 8.5.11 Maximum
event 1,200 events/sec
T
10
transmission rate
Table 8.2: Verification matrix for Level 3 requirements, listing the relevant part of Section 8.5,
the requirement, the section in which the verification plan is outlined, and indicating the planned
use of inspection, test, demonstration, and analysis for each requirement.
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6.5
Level 3 Requirements Verification Plan
6.5.1 CRaTER-L3-01 Thin and thick detector pairs
6.5.1.1 Inspection
The detector provider will report the sizes of the thin and thick detectors pairs.
6.5.2 CRaTER-L3-02 Minimum energy
6.5.2.1 Test
The CRaTER silicon detectors are delivered from the provider, Micron Semiconductor
Ltd, in boards with one thin and one thick detector. Before integration into the telescope stack,
these boards will be taken to a beam facility and the minimum energy will be measured.
Additionally, we demonstrated at BNL that an alpha source may be used to quickly place an
upper limit on the thickness of any dead layers on the detectors.
6.5.3 CRaTER-L3-03 Nominal instrument shielding
6.5.3.1 Inspection
Mechanical drawings of the instrument will be reviewed to visually gauge the range of
shielding of the detectors.
6.5.4 CRaTER-L3-04 Nadir and zenith field of view shielding
6.5.4.1 Inspection
The thickness of the nadir and zenith aluminum plates will be measured with a
micrometer at a minimum of five locations.
6.5.5 CRaTER-L3-05 Telescope stack
6.5.5.1 Inspection
The detector boards will be designed so they can only be mounted in the correct
orientation (thin detector in zenith or deep space direction). The assembly will be inspected to
verify the stack configuration.
6.5.6 CRaTER-L3-06 Full telescope pathlength constraint
6.5.6.1 Inspection
The minimum and maximum pathlength through pairs of detectors is determined through
review of the mechanical drawings.
6.5.7 CRaTER-L3-07 Zenith field of view
6.5.7.1 Inspection
The zenith field of view will be determined by reviewing mechanical drawings of the
telescope.
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6.5.8 CRaTER-L3-08 Nadir field of view
6.5.8.1 Inspection
The nadir field of view will be determined by reviewing mechanical drawings of the
telescope.
6.5.9 CRaTER-L3-09 Calibration system
6.5.9.1 Test
The pulse heights due to pulses from the calibration system will be compared with
predictions derived from an analysis of the analog electronics.
6.5.10 CRaTER-L3-10 Event selection
6.5.10.1 Test
An automated program will be used to activate the calibration system on all combinations
of detectors (64) and to step through all possible detector coincidences (63) and record the events
that are sent to the ground support equipment. The resulting data will be analyzed to verify that
the coincidence system functions correctly.
6.5.11 CRaTER-L3-11 Maximum event rate
6.5.11.1 8.5.10.2 Test
The calibration system will be commanded into a mode such that the synthesized event
rate exceeds the maximum rate the digital system is capable of passing through the 1553
interface and it will be verified that the first 1200 events are correctly transmitted.
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7 Glossary
APB
CRaTER
DPB
GCR
LRO
PHA
SEP
TEB
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Analog Processing Board
Cosmic Ray Telescope for the Effects of Radiation
Digital Processing Board
Galactic Cosmic Ray
Lunar Reconnaissance Orbiter
Pulse Height Analysis
Solar Energetic Particle
Telescope Electronics Board
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