GLAST LAT Project
March 24, 2003
GLAST Large Area Telescope:
Gamma-ray Large
Area Space
Telescope
Tracker Subsystem
WBS 4.1.4
1C: Performance Predictions
vs
Requirements
Bill Atwood
Santa Cruz Institute for Particle Physics
University of California at Santa Cruz
atwood@scipp.ucsc.edu
1C
Tracker Peer Review, WBS 4.1.4
1
GLAST LAT Project
March 24, 2003
Level III Performance Specification
• Level III Performance for the Tracker Subsystem
– LAT-SS-0017-1 LAT TKR Subsystem Specification - Level III Specification
• Applicable LAT documents to this review:
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–
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1C
LAT-TD-00178
LAT-SS-00177
LAT-TD-01128
LAT-TD-00201
LAT-TD-00086
LAT-TD-00128
LAT-TD-01078
LAT-TD-01060
LAT-TD-01090
Tracker Reliability Analysis
LAT TKR Tracker Dimensions and Masses
Mini TKR Trigger Timing Study
End-of-Mission Noise in GLAST Silicon Detectors
Results from Testing of the LAT SSD Prototypes
Heavy Ion Irradiation of LAT Silicon Strip Detectors
SSD QA: Irradiations up to October 2002
Trigger Noise Rate Measurements on a Tracker Ladder
Test results on the Tracker ASIC prototypes
Tracker Peer Review, WBS 4.1.4
2
GLAST LAT Project
March 24, 2003
Tracker Subsystem Requirements Flow Down
Level I
Project
Specifica
tions
FIGURE 2-1 SPECIFICATION TREE
Program Plan
Requirements
Lev Le
el II vel
Sys II(
tem a)
Spe Sy
cifi ste
cati m
ons
Le
vel
II(
b)
Ele
me
nt
Mission Assurance
Requirements
GLAST00110
Spacecraft
Performance Spec.
Mission System
Specification
GLAST00074
SGL Comm
Interface Spec.
Science Support
Center Spec.
LAT - ACD Spec.
LAT-SS-00016
GBM Instrument
Performance Spec.
Inter-Center
Interface Spec.
Mission Operations
Center Spec.
LAT - TKR Spec.
LAT-SS-00017
T&DF Spec.
LAT-SS-00019
1C
SC-SI Interface
Specification
GLAST00038
LV Interface
Specification
LAT Instrument
Performance Spec.
LAT-SS-00010
Level III
Subsyste
m
Specifica
tions
Science Req’ts
Document
GLAST00010
Operations Concept
Document
GLAST00089
LAT IOC
Performance Spec.
LAT-SS-00015
LAT - CAL Spec.
LAT-SS-00018
Aux. Subsystem Spec.
LAT-SS-000xx
GBM IOC
Specification
LAT Interface Spec.
LAT-SS-000xx
Tracker Peer Review, WBS 4.1.4
LAT SAS Spec.
LAT-SS-00020
LAT IOC Spec.
LAT-SS-00021
3
GLAST LAT Project
March 24, 2003
Verifications
5.2 Gamma Ray Conversion Efficiency
Requirement: The TKR shall convert at least 65% of the gamma rays with
energy >10 GeV impinging upon the device at normal incidence.
Science impact: effective area
Design impact: amount of W converter
Verification Method: Analysis
Prob(conv) = 1-e-7/9x0 where x0 is the radiation lengths in the Tracker
x0 = 12  0.03 + 4  0.18 + 18  0.015 = 1.35
Hence Prob.(conv) = 65.01%
MC cross check: #Triggers = 34000 per 100K over 6 m2.
Geo Area of Tracker = 2.205 m2
No. that hit Geo. Area of Tracker = 36750
Percent triggering: 23223/36750 = 63.2%
Differences: Wall material, tray closeouts, MCM boards, MC run at 0o-5o off
normal
1C
Tracker Peer Review, WBS 4.1.4
4
GLAST LAT Project
March 24, 2003
Verifications
5.3 Converter Configuration
Requirement: At least 25% of the gamma-ray conversions shall occur in high-Z
converter foils no greater than 3.5% radiation lengths thick, with the remainder
occurring in other material and converter material no more than 25% radiation
lengths thick.
Method: Analysis
Total Thin Converters = 120.03 = .36 rl.
Total everything else = 40.18 + 180.015 = 0.99 rl.
Hence: 0.36/1.35 = 26.7%
The implicit assumption in this approach is that the beam intensity does not
vary with depth. So Alternatively:
Prob. of Conversion in Thin = 1-e-7/9x0 where x0 = 120.03 = 0.36. Hence
Prob. (Thin) = 24.4%
Thus percentage of gamma’s converting in the thin of all gamma conversions is
Prob. –Thin relative to all Conversions = .244/.6501 = 37.56%
1C
Tracker Peer Review, WBS 4.1.4
5
GLAST LAT Project
March 24, 2003
Verifications
5.3.1 Converter/Sensor Spacing
Requirement: The converter material shall not lie more than 3 mm
above the corresponding sensors.
Method: Analysis
Bond Thickness:
.1 mm
Bias Plane Thickness: .1 mm
Bias Plane-SSD Bond: .15mm
SSD’s:
.4 mm
Gap Between Planes: 2.13 mm.
Hence from converter exit side to top side of furthest SSD = 2.88 mm
1C
Tracker Peer Review, WBS 4.1.4
6
GLAST LAT Project
March 24, 2003
Verifications
5.4.1 Within a module active volume
Requirement: At normal incidence and within the active area of the
thin-converter region, no more than 35% of the gamma-ray
conversions shall occur in material other than the high-Z converter
foils.
Method: Analysis
Converters:
.03 rl.
Other materials (SSD + Tray): .0145rl.
Total:
.0445 rl.
Percentage in Converter = .03/.0445 = 67.4% .
Hence 32.6% of conversion do not occur in converters
1C
Tracker Peer Review, WBS 4.1.4
7
GLAST LAT Project
March 24, 2003
Verifications
5.4.2 Material Between Modules
Requirement: A particle traversing the boundary between TKR modules and
at normal incidence to the module wall shall not encounter more than 5%
radiation lengths of material, on average.
Method: Analysis
The items crossed are Tower Wall (1.5mm): .0098 r.l.
Tray Closeout (5 mm):
.0137 r.l.
MCM (1.1 mm):
.0093 r.l.)
Closeouts cover 91% of total wall area
MCM cover 71% of every other wall
Traversal between towers doubles Wall & Closeout numbers
Average material = 2 x Wall + 2 x .9xCloseout + .7xMCM = .0511
(Requirement was .05)
1C
Tracker Peer Review, WBS 4.1.4
8
GLAST LAT Project
March 24, 2003
Verifications
5.5 Geometric Area
Requirement: The TKR shall have an active area at
normal incidence of at least 19,000 cm2.
Method: Analysis
The active area of an SSD:
ASSD = (89.5-2*.974)mm x (89.5-2*.964)mm =
87.55mm x 87.57mm
ASSD = 7666.9 mm2 = 7.667 x 10-3 m2
No. of SSD’s in a layer : 16 per tower x 16 towers = 256
AACTIVE = 256 * ASSD = 1.963 m2 = 19,630 cm2
1C
Tracker Peer Review, WBS 4.1.4
9
GLAST LAT Project
March 24, 2003
Verifications
5.6 Aspect Ratio
Requirement: The ratio of height to width of the TKR active volume
shall not exceed 0.45 (for large field-of-view).
Method: Analysis
Module Height: 624.7 mm
Module Width: 1486.5 mm (4 Module width)
Aspect Ratio: 624.7 mm / 1486.5 mm = .4202
1C
Tracker Peer Review, WBS 4.1.4
10
GLAST LAT Project
March 24, 2003
Verifications
5.7 Charged Particle Detection
Requirement: The TKR shall measure immediately following each converter foil,
in both x and y views and within the active area, the position of passage of a
minimum ionizing particle at normal incidence with an efficiency of greater than
98%.
Method: Test & Analysis
SSD Eff. measured to be > 99% Eff.
At normal incidence the average signal to the SSD comparitor is > .5*32000 eequiv. To be compared with a noise of < 2000 e- equiv. For a threshold of 8000
e- equiv ( 4+s above the noise) or 1/4 MIP, would require a fluctuation of 20s
(16000-8000 / sqrt(16000) = 8000/400) to not be counted.
101
100
Hit Efficiency
The plot at the right shows the Hit Efficiency for the
BTEM using both cosmic rays as well as the well as
the electron beam at SLAC. The Electron beam
measurements are the more reliable as the direction,
energy and nature of the incoming is controlled.
99
98
Cosmic Rays
Electron Beam
97
Layer 6x
96
95
1
2
3
4
5
Detector Ladder
1C
Tracker Peer Review, WBS 4.1.4
11
GLAST LAT Project
March 24, 2003
Verifications
5.8 Spatial Measurement Resolution
Requirement: The TKR shall measure the direction of a charged particle of
straight trajectory (negligible multiple scattering), in both x and y views and
using just two consecutive measurement planes, to a precision of no worse
than 0.2.
Method: Analysis
SSD Pitch = .228 mm with a > 32.1 distance between measuring planes.
Hence the projected angle error is:
  2 
.228 1

 2.9 mrad = .17o
12 32.1
This accuracy persists even as the trajectory departs for normal incidence
Due to the finite thickness of the SSDs.
Note however that the space angle error is ~ 1.5 times larger or .25o
1C
Tracker Peer Review, WBS 4.1.4
12
GLAST LAT Project
March 24, 2003
Verifications
5.9 Dead Area
Requirement: The fraction of non-active area presented by
the top of the TKR shall not exceed 12%.
Method: Analysis
From 5.5: AACTIVE =
AGEOM =
=
=
1.963 m2
(Tower Pitch x 4 – 1 Tower Gap)2
(372*4 – 1.5)
2.210 m2
Percentage Dead Area:
ADEAD = 1 – AACTIVE/AGEOM = 11.2%
1C
Tracker Peer Review, WBS 4.1.4
13
GLAST LAT Project
March 24, 2003
Verifications
5.10 Ionization Measurement
Requirement: For an event with a single track detected, the TKR shall
distinguish, on the basis of charge deposition, a single minimum-ionizing
particle from two minimum-ionizing particles to a level of at least 1-sigma.
Method: Analysis and Test
The right-hand plot shows the Monte Carlo expectation for 1 GeV photon conversions. The first
peak is caused by single charged particles crossing a strip while the second peak results when both
the e+ and e- from the conversion cross a single strip. The expectation for a single particle is a TOT
of 6.5 msec with and rms of 2.3 msec. The GLAST detectors and front end electronics have been
adapted for use in low energy proton beams for use in medical studies and an extensive set of
measurements and test were done to measure both the linearity, resolution, and absolute calibration
of the TOT system. Shown in the figure on the left are TOT distributions for various energy protons
which can then be used to estimate the ionization depositions. Excellent agreement with the
predicted values was found and a single MIP was measure to have a mean TOT of 7.0 msec with an
rms of 1.7 msec. This places the 1 vs 2 MIP distributions more then > 4s apart.
1C
Tracker Peer Review, WBS 4.1.4
14
GLAST LAT Project
March 24, 2003
Verifications
5.11 Self Trigger
Requirement: The TKR shall provide prompt signals to be used by
the T&DF system to form a trigger for readout of the TKR and other
sub-detectors.
Method: Demonstration
The layered OR’s form each plane is made into a 6 fold coincidence
with nearest 5 Planes (X or Y. Note that planes come in X-Y pairs) –
or more simply put the 3-In-A-Row trigger demands 3 XY pairs of
adjacent hits. This trigger was tested and used in the 1999 Beam
test and the subsequent Balloon flight successfully.
1C
Tracker Peer Review, WBS 4.1.4
15
GLAST LAT Project
March 24, 2003
Verifications
5.11.1 Trigger Efficiency
Requirement: The TKR trigger shall be on average at least 90% efficient for the
set of gamma-ray conversions from which the conversion products traverse the
active areas of at least 3 consecutive measurement planes.
Method: Analysis and Test
Tests validate the assumption that the individual SSDs are > 99% Eff. Thus the
major component of Trigger inefficiency comes from the various dead areas.
WHAT % OF EVENTS HAVE JUST 3 LAYERS? From MC at 100 MeV
percentage of found tracks with 6 hits = 2182/17075 = 12.8%. This is an over
estimate for “just 3 layer events” as a portion of these are events with more hits,
but only 6 are picked up and used in the reconstruction.
OF THOSE WHAT PERCENT TRIGGER? Chance of getting all 3 layers: .893
= 70% thus we miss 30% of these 3-layer events. Overall we miss < .30 * .128
= 3.84% of all events due to dead areas in the tracker.
1C
Tracker Peer Review, WBS 4.1.4
16
GLAST LAT Project
March 24, 2003
Verifications
5.11.2 Trigger Noise
Requirement: In the case that no charged particles or gamma rays are incident
upon the TKR, the TKR trigger shall not exceed a rate of 500Hz.
Method: Test and Analysis
Until a full tower is available, estimating the trigger noise from single SSD
noise rates is the best that is available. A component of the false trigger rate is
driven by the stochastic noise rate (see 5.12 below). This is a calculable
probability whose solution is:
The measure occupancy rate for the
LAT SSD is below 10-5 which should
yield a trigger rate well below 500 Hz
(estimate ~ few Hz for the full LAT
instrument.)
1C
Tracker Peer Review, WBS 4.1.4
17
GLAST LAT Project
March 24, 2003
Verifications
5.11.3 Trigger Saturation Recovery Time
Requirement: The trigger signals from a TKR readout module shall not hold true
for longer than 250 ms in the case of passage of a high-momentum fully-ionized
iron nucleus.
Method: Analysis and Test
The output from the comparitor was observed directly (via test pad, probe, and
scope) with varying amounts of charged injected into the front end amplifier. The
signal reached a maximum length of150 msec for 550 fC of charge at which
point it saturated and did not increase out to the maximum tested (4000 fC). The
passage of a fully ionized Fe nucleus would deposit 3448 fC.
1C
Tracker Peer Review, WBS 4.1.4
18
GLAST LAT Project
March 24, 2003
Verifications
5.12 Data Noise Occupancy
Requirement: The noise occupancy in the TKR data stream shall not
exceed one in 10,000 channels per trigger.
Method: Test
The layer-OR rate was measured as a
function of the threshold as shown at the right.
The envisione trigger thresshold is ~ 1.3 fC (or
¼ MIP). At that point in the plot the per
channel rate is < 1 Hz per. For a gate of 1
msec this will yield a chance coincidence of ~
10-6 well below the 10-4. This measurement
has been done several times on different sets
of chips; all have given similar results
1C
Tracker Peer Review, WBS 4.1.4
19
GLAST LAT Project
March 24, 2003
Verifications
5.13 Dead Time
Requirement: The dead time imposed by the TKR readout shall not exceed 10% at
a cosmic-ray trigger rate of 10 kHz.
Method: Test and Analysis
From the initiation of a Layer-OR trigger signal, it takes < 2 msec for a trigger to be returned
to the tracker front end to latch in the high (hit) channels. For each channel there is a four
deep buffer allowing for successive triggers to occur without causing additional dead time
associated with latching data into the front end chips. In the worse case where a cosmic ray
causes 2 front end chips to fire in a layer with only one functional read-controller chip, a total
of 200 clock cycles @ 20 MHz (10 msec) is required to transfer the data to the read-controller
chip. Lastly the transfer of data down the tower from the read-controllers (9 read-controllers
per transfer line), again for this worse case 531 clock cycles (26.6 msec) are required. This
is the ultimate limit – namely when the sustained trigger rate approaches 37.6 kHz, the read
controller chip buffers are full and there is 100% dead time. Because of the highly buffered
nature of the system, appreciable dead time will mount very slowly prior to this point (e.g. at
10 kHz the dead time due to latching will be ~2%). We also note however that the tracker
does not set the trigger dead time. The calorimeter holds off new triggers for ~ 20 msec,
required to digitize the data and this system has no buffering (i.e. ~ 20% dead time at 10
kHz)
1C
Tracker Peer Review, WBS 4.1.4
20
GLAST LAT Project
March 24, 2003
Verifications
5.14 Tracker Mass
Requirement: The mass of the TKR shall not exceed 530 kg.
Method: Test
Since no complete towers have yet been constructed it presently is
not possible to measure the total mass. However the masses of all of
the pieces is now well known and measured. The results of a
complete mass audit for the tracker now stands at 517.27 kg
(including the redesigned bottom tray and flexture mounts)
1C
Tracker Peer Review, WBS 4.1.4
21
GLAST LAT Project
March 24, 2003
Verifications
5.15 Tracker Power
Requirement: The power consumption of the TKR shall not
exceed 184 W of conditioned power.
Method: Test
The power required to run completed MCM have been measured
to be 250 mW using the G chip and 239 mW using the F chip.
These are for running with a 20 MHz clock. Hence the total
tracker power is anticipated to be 144 W (138 W) for the G (F)
chip version.
1C
Tracker Peer Review, WBS 4.1.4
22
GLAST LAT Project
March 24, 2003
Verifications
5.16 Control Signals
Requirement: The TKR shall receive control signals from the T&DF system to
reset and to configure its readout, to perform in-flight calibration tasks, to
trigger acquisition of events, and to control the flow of data to the T&DF.
Method: Demonstration
The various proto-type units have been operated in this manner.
1C
Tracker Peer Review, WBS 4.1.4
23
GLAST LAT Project
March 24, 2003
Verifications
5.16.1 Configuration Read-Back
Requirement: The TKR shall provide a facility for read-back
of all electronic configuration settings.
Method: Demonstration
The various proto-type units have been shown to have this
functionality
1C
Tracker Peer Review, WBS 4.1.4
24
GLAST LAT Project
March 24, 2003
Verifications
5.17 Data Flow
Requirement: The TKR shall deliver its data in a zero-suppressed
format to the T&DF system, by way of at least two independent paths.
Method: Demonstration
The various proto-type units have been operated in this manner.
1C
Tracker Peer Review, WBS 4.1.4
25
GLAST LAT Project
March 24, 2003
Verifications
5.18 Internal Calibration System
Requirement: The TKR readout electronics shall include a system
for calibration and test by means of injection of signals and trigger
from the T&DF system.
Method: Demonstration
The various proto-type units have been shown to have this
functionality. The system is comprised of a DAC onboard each of
the front end chips (64 channels) which is programmable. This DAC
controls the amount of charge injected into the pre-amps during test.
This has been one of the ways used to measure the noise and gain of
individual channels.
1C
Tracker Peer Review, WBS 4.1.4
26
GLAST LAT Project
March 24, 2003
Verifications
5.19 Temperature Monitoring
Requirement: The TKR shall provide temperature sensors within its
volume for monitoring the TKR temperature and temperature gradient.
Method: Demonstration
On each tower there will be a total of 16 thermistors (2 per controllerreadout cable – 8 cables per tower). These sensors are spread out along the
tower’s length and various sides to provide for the monitoring of the
temperatures at there locations.
1C
Tracker Peer Review, WBS 4.1.4
27
GLAST LAT Project
March 24, 2003
Verifications
5.20 Thermal Control
Requirement: The TKR shall be cooled by passive flow of
heat through its interface to the Grid.
Method: Test and Analysis
As no completed towers have yet been made a full test
has yet to be done. However a complete thermal model
of the tower has been made and shows that with the
chosen side wall material and connection to the LAT Grid,
a thermal gradient of less then 10 oC from the Grid to the
top of a tower is expected.
1C
Tracker Peer Review, WBS 4.1.4
28
GLAST LAT Project
March 24, 2003
Verifications
5.21 Environmental
Requirement: The TKR shall meet the structural and thermal
environment requirements defined in its interface control
document.
Method: Test
As no completed towers have yet been made a full test has
yet to be done. However, to date, we expect the design to
meet or exceed all of the ICD requirements
1C
Tracker Peer Review, WBS 4.1.4
29
GLAST LAT Project
March 24, 2003
Verifications
5.22 Reliability
Requirement: The reliability of the tracker shall be at least 96% in five years.
Reliability is the probability that the tracker will not experience a reduction in
operability below 90% due to failure of its components. Operability is the
percentage of tracker channels that are operational.
Method: Analysis
This has been analyized by assessing what possible failure modes exist (16 were
indentified). For each its impact on the mission (ranked 1 – 5 from most to least sever) and
its probability of occurrence (again ranked 1- 5 where 1 is > 10% , 3 < 1%, and 4 and 5 are
considered very unlikely). Of the 16, 2 were judged to have a severity level of 2 and one
had a level of 3. The rest were either at level 4 or 5 with essentially little impact on the
mission. The 2 at severity level 2 involve the loss of a non-redundant TEM part or the lost
of a redundant TEM part. This would cause the loss of a tower (1/16 of the detector).
The probability of this to occur for the identified parts was deemed to be to small to allow for
a credible estimate. In the case of a redundant part failure of course the impact is nil. The
one item flagged at severity level 3 was an MCM failure either through corruption of the data
transmission electronics or through the bias voltage distribution. This could cause the loss
of one side of a tray ( 1 out of 36 per tower). After burn-in of these electronic modules the
probability of failure in less then 5 years was deemed to small to allow for a reliable
estimate.
1C
Tracker Peer Review, WBS 4.1.4
30