Calibration of the focal plane detectors for a compact MPR neutron
spectrometer with proton beam from HI-13 tandem accelerator
Guoguang Zhangab*,Xia Lib , Ji Baob , Jianfu Zhanga, Long Houb ,Zuying Zhoub,
a
Northwest Institute of Nuclear Technology, Xi’an, Shaanxi, 710024, P.R.C
b
China Institute of Atomic Energy, Beijing, 102413, P.R.C
ABSTRACT
A new compact magnetic proton recoil (MPR) type spectrometer has been developed for diagnosing fusion
neutrons. The designs of beam optics and focal plane detectors (FPD) of the MPR spectrometer are presented. The
calibration experiment of the FPD, which is consisted of silicon micro strips, has been performed on the HI-13
tandem accelerator at China Institute of Atomic Energy (CIAE). The uncertainty of the measurement data is
discussed.
Keywords: focal plane detector, energy calibration, magnetic proton recoil spectrometer, beam optics
1. INTRODUCTION
The magnetic proton recoil (MPR) spectrometer has been utilized in the measurement of the thermonuclear
neutrons from a fusion reactor
[1, 2]
. The instrument is based on the principle that neutrons can be converted to
protons through elastic scattering in a proton-rich conversion foil and the fact that protons’ traveling in a magnetic
field can be separated in space due to their momentum difference. When protons enter the magnetic spectrometer,
momentum selection takes place and their energies can be deduced by their positions on a focal plane detector array
at the exit of the spectrometer. The protons are only recoiled from the elastic scattering process and there is
practically no other extraneous production of protons. Furthermore, only forward recoiled protons are used so the
energy downshift is small. This helps obtaining good energy resolution and low background interference. This
spatial distribution can be related back to neutron energy at the foil.
The MPR spectrometer was installed at the Joint European Torus (JET) in 1996 and has provided diagnostics
of mainly for high neutron yield rates from both deuterium-deuterium and deuterium-tritium plasmas since February
1997. The MPR weights about 90 tons including the surrounding concrete shielding that is needed for reducing
background interference in the hostile environment of the Torus hall. A novel spectrometer for the measurement of
neutrons has been designed for OMEGA and the NIF [3, 4].
In this work, for solving the problems of fusion neutron energy measurement in a very limited space, a
compact MPR type spectrometer has been developed at Northwest Institute of Nuclear Technology (NINT). The
whole MPR weights only about 1.5 tons. The size of the MPR spectrometer is about 1.30.90.5 m3. A silicon PIN
detector array, which consists of 40 micro strips, is used as the focal plane detectors (FPD). Each strip size is
2200.5 mm3. The dimension of the proton collimator’s slit is 2mm20mm. The electric conductivity of silicon
material, manufactured by Wake Company in Germany, is about 6000 cm. The dark current of each strip is below
1 nA at 100V bias. A quadrupole-dipole (QD) system has been developed to focus and adjust the proton beam. The
QD system has a target-to-image demagnification in both transverse directions and therefore produces a spectrum in
a small area on the dispersive plane, which can be recorded by the FPD. The beam optics of the spectrometer was
calculated using TRANSPORT [5] and Turtle codes. The bending angle of the dipole is 60 and 14MeV protons can
be cast in the central trajectory when the magnetic field is 18 kG .
2. EXPERIMENTAL METHOD
In order to test the focal plane of the system, a calibration experiment of the FPD using proton beam was
carried out at the HI-13 tendon accelerator at China Institute of Atomic Energy (CIAE). The diagram of the
spectrometer is shown in Fig.1. The whole system consists of four parts. The first part is the Au scattering chamber,
the second part is the vacuum chamber for proton transport, the third part is the magnetic system and the last part is
the focal plane detector. An Au foil is set in the center of the scattering chamber and it can deliver appropriate proton
beam without losing much of its energy before entering the proton collimator. The Au target is 10mm in diameter
and with a mass thickness of 4.7mg/cm2. The energy of incoming proton beam is 14MeV. Then the scattered protons
go through a 2.2 cm long collimator to the magnetic analysis system and are focused on the focal plane detector. As
shown in Fig.1, a corrugated tube and the back-to-front equipment are installed to adjust the distance from the
chamber to the focal plane.
Au target
proton dump
beam line
proton
collimator
vacuum chamber
quadrupole magnetic
dipole magnetic
proton dump
corrugated tube
focal plane detector
Fig.1. The schematic diagram of experimental system
The MPR data are collected by the data acquisition (DAQ) system during an experiment run. In addition to
collecting data from the detector, the DAQ is also used to monitor the status of detector array, the DAQ electronics
and the dipole magnets. The schematic view of the data acquisition is shown in Fig. 2.
CF8000 OR
M32 Charge
Sensitive
Preamplifiers
Detectors Bias
Preamplifier Power
Supply
VME DAQ Systam
Focal Plane Detector
32 Strips Si-PIN
Detectors
CAEN N568B
16 Channel
Amplifier
CAEN V785N
CAEN N568B
16 Channel
Amplifier
DAQ trig
CF8000 OR
Fig.2. The schematic view of MPRs data acquisition system
When a proton hits on the silicon strip of the detector array, the output charge signal, related to the energy
deposited in the detector, are fed into M32 charge sensitivity preamplifiers. The output signal from preamplifier is
then split into two signals and fed into two CAEN 568B linear amplifiers, which constitutes two separate branches
of the DAQ system, one analogue and one digital branch. The digital branch, which possesses high throughput,
records the counts of signals (time resolved) by using standard CAEN V785N VME system, which is triggered by
the signal
from the PS CF8000, fed by CAEN 568B fast output signal. In the analogue branch, pulse height
histograms are recorded by using analogue-to-digital conversion (ADC) VME modules. The pulse height histograms
are used for off-line background corrections of the data. However, only silicon signals above a given absolute
voltage (threshold) are recorded so that low-energy events are rejected automatically in the DAQ.
When the pulse height histogram at the central channel appears more distinct than that at other channels by
adjusting the magnetic current, the current value of the power supply for the magnetic is fixed. After the position of
14MeV proton on the focal plane was obtained, the energy of proton beam was changed step by step from 12MeV to
16MeV by changing the high voltage of the HI-13 tandem accelerator. Then the position spectrum and the pulse
height histograms are measured for all silicon strip detectors. An Am-241 alpha standard source and ORTEC 448
pulse generator were used for energy calibration. The yield of proton is monitored by using an ORTEC 439 digital
current integrator. The spectrum of protons at each detector position can be deduced from the parameters of VME
data acquisition system.
3. RESULTS AND DISCUSSION
Fig.3 shows the position distribution of 14MeV protons on focal plane detector. It is clear that only three
channels can record the proton beam from the magnetic analysis vacuum chamber. The peak is at the 15th channel.
The pulsed height distribution of 15th channel is shown in Fig.4.
Fig.3. The position distribution on focal plane detector
Fig.4. The pulse height distribution of 15th channel
After obtaining the position of 14MeV proton on the focal plane, the energy of proton is changed from
12MeV to 16MeV by changing the high voltage of HI-13 tandem accelerator. The position distributions of different
protons on focal plane detector are shown in Fig.5. So the relationship between the energy of protons and the peak
of position can be obtained, which is shown in Fig.6.
Fig. 5. The position distributions of different protons on focal plane detector
Fig. 6. The relationship of energy and the position of silicon strip detector
The results show that the position resolution of the system can be up to 50keV/mm or 0.1MeV per channel.
The whole range of the focal plane detector can cover about 4MeV, while 40 silicon strips are used together. The
uncertainty of the measurement data at 14MeV is around 8%. The energy resolution of 14MeV proton is about 1.4%.
It is believed that this compact MPRs can be used as a neutron diagnostic system for the pulsed reactors, neutron
generators or other facilities in the future.
ACKNOWLEDGEMENTS
We would like to thank all technical staff in the HI-13 tandem accelerator laboratory for operating the
accelerator. We also thank Prof. Ping Yuan and Hanliang Sheng from the Modern Physics Institute of Chinese
Academy of Sciences for their technical support in the mechanical design of MPRs. The authors are also grateful to
the staff of the neutron team at CIAE.
REFERENCES
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2 J. Källne, L. Ballabio, and G. Gorini et al. Rev. Sci. Instrum.Vol.70,No.1(1999) 1181-1184.
3 Tomas J.Murphy et al. Review of Scientific Instruments Vol.72 No.1 2001
4 F.H.Seguin et al.Physica of Plasmas.Vol.9 No.6 2002
5 http://fermitools.fnal.gov/abstracts/transport
*Corresponding author: Tel: 029-84765156, Fax: 029-83366333, Email: kogunchiang@gmail.com.