Tokamak fusion neutron spectrometer based on PXI bus∗

JIANG Xiao-Fei(蒋小菲),CAO Jing(曹靖),JIANG Chun-Yu(蒋春雨), CAO Hong-Rui(曹宏睿),,SONG Xian-Ying(宋先瑛),and YIN Ze-Jie(阴泽杰)

1State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China,Hefei 230026,China

2Department of Modern Physics,University of Science and Technology of China,Hefei 230026,China

3Southwestern Institute of Physics,Chengdu 610041,China

Tokamak fusion neutron spectrometer based on PXI bus∗

JIANG Xiao-Fei(蒋小菲),1,2CAO Jing(曹靖),1,2JIANG Chun-Yu(蒋春雨),1,2CAO Hong-Rui(曹宏睿),1,2,†SONG Xian-Ying(宋先瑛),3and YIN Ze-Jie(阴泽杰)1,2

1State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China,Hefei 230026,China

2Department of Modern Physics,University of Science and Technology of China,Hefei 230026,China

3Southwestern Institute of Physics,Chengdu 610041,China

In order to realize on-line real-time measurement of dynamic and time-sharing neutron spectrum of HL-2A, a tokamak fusion neutron spectrometer based on PXI bus was developed.It consists of electronics system and eight thermal neutron detectors,namely SP93He proportional counter,embedded in eight polyethylene spheres in different diameters.Response function of the eight polyethylene spheres was the key to calculate the neutron spectrum accurately.In this paper,response function of the eight polyethylene spheres is simulated by adopting Geant4 code,and neutron counts from an241Am-Be neutron source are measured by the eight detectors.The calculated spectrum of the Am-Be neutron is accurate in 0–2MeV region,and is similar to the theoretical spectrum.The tokamak fusion neutron spectrometer was used in HL-2A device to monitor the dynamic neutron spectrum of HL-2A on-line and real-time.

HL-2A,Response function,Calibration,Dynamic time-sharing energy spectrum,241Am-Be

I.INTRODUCTION

HL-2A[1,2]is a tokamak fusion experiment device of China,which laid the foundation of advanced tokamak physics experiment and the research of ITER and fusion reactor.HL-2A presented many challenges for fusion neutron spectrum measurements.One of the major difficulties is on-line and real-time measurement of the dynamic and time-sharing fusion neutron spectrum.In this regards,we developed a tokamak fusion neutron spectrometer based on PXI bus.It consists of an electronics system and eight thermal neutron detectors,namely SP93He proportional counters embedded in eight polyethylene(PE)spheres of different diameters.The neutron spectrometer uploaded eight timesharing neutron counts to the system controller of PXI chassis through PXI bus.The detectors and electronics will be finally activated by neutrons in the fusion reactor.

As a common radioactive neutron source used in laboratory,an241Am-Be neutron source is featured by its small size, long term use,simple protection,and stable neutron emission.Activity of the neutron source at any time can be calculated accurately according to the half-lives of radionuclide decay.Spectrum measurement of the Am-Be neutron source can verify the tokamak fusion neutron spectrometer,without activating the detectors and electronics by fusion neutrons. Response functions of the PE spheres are the key to calculating neutron spectrum accurately.In this paper,response functions of the eight PE spheres are simulated by the Geant4code,and neutron spectra of an Am-Be neutron source are measured.

II.PRINCIPLE OF MEASUREMENT

The eight PE sphere spectrometers are of 4,5,6,7,8,9, 10,and 12 inches in outer diameter,respectively.An SP93He proportional counter of 33-mm outer diameter is located in each of the sphere center.This kind of counters is usually used for thermal neutron measurement because of larger reaction cross sections[3]and lower γ sensitivity.

Assuming the following parameters:m,number of PE spheres in different diameters;Ri(E),the response function of the ithsphere[4,5];Φ(E),the fluence of neutrons in differentenergies,andN1,···,Nm,countsofthePEspheres, then the ithsphere will have:

Neutron spectrum measurements are finally converted into calculating the linear equation of

where,R(E)is the coefficient matrix about response function,N is the data of eight measurements,and Φ(E)is the neutron spectrum.Nican be experimentally obtained,while the response function Ri(E)can be obtained by Geant4 simulation.So,simulation of response function is the key to calculating the spectrum of Am-Be neutron source.

Fig.1.(Color online)System block diagram of the tokamak fusion neutron spectrometer based on PXI bus.

Fig.2.(Color online)Schematics of the Geant4 simulation.

A block diagram of the fusion neutron spectrometer based on PXI bus is shown in Fig.1. It consists of eight detector/pre-amplifier/main-amplifier,and system controller of PXI chassis.The charge-sensitive pre-amplifiers use NIM chassis and the main electronics use PXI chassis.The detector signals amplified by the pre-amplifier are amplified, shaped and filtered by the main electronics.The DACs output the upper and lower thresholds to the comparator through the FPGA.The comparators compare the main amplifier output with the upper and lower thresholds,and the FPGA processes the comparison results.The FPGA uploads eight timesharing neutron counts to the system controller of PXI chassis through PXI bus.The system controller works out and displays real-time neutron spectrum.

By measuring counts of the eight PF spheres simulating the response function,and using Eq.(2),spectrum of the Am-Be neutron source can be calculated.

III.GEANT4 SIMULATION

The Geant4 simulation of response function is shown schematically in Fig.2[6–8],with eight PE spheres of different diameters(yellow colored)and the neutron source in the center.The red dots in the spheres are eight embedded SP93He proportional counters.The SP93He counter is 40cm from the neutron source.The neutrons are emitted in 4π solid angle in the simulation.

Fig.3.(Color online)Response function of the eight PE spheres of Φ 4 inches to Φ 12 inches.

Fig.4.(Color online)Experimental measurement environment.

The fast neutrons are slowed down to thermal neutrons by elastic collisions with hydrogen atoms or carbon atoms.For the PE spheres of small diameters of PS,low energy neutrons after moderating shall have great chance to reach the detector in the sphere center,while high energy neutrons after moderating are likely to escape.For the PE spheres of large diameters,a large number of low energy neutrons are absorbed in the moderation,while thermal neutrons being slowed down from high energy neutrons shall reach the detector.Neutrons in different energies are moderated in different degrees in the PE spheres of different diameters.The response functions simulated by Geant4 are shown in Fig.3.The response functions are peaked in the low energy region for smaller diameter PS(e.g.4 inches),but the peak position moves toward the high energy region as the sphere diameter increases.The influence of neutrons escaped from the small PE spheres and scattered from the floor,should be considered.

Fig.5.Background counts of Tokamak fusion neutron spectrometer, at the low threshold of 100–160mV.

IV.EXPERIMENTAL MEASUREMENT

The experiment was designed to calibrate[9]the response function simulated by Geant4 and verify the tokamak fusion neutron spectrometer.An Am-Be neutron source[10,11] was used.Placement of the eight PE spheres(Fig.4)was consistent with simulation.The fusion neutron spectrometer identifies the neutron signals through amplitude.As described in Sec.II,the signal amplitude between the lower and upper thresholds is neutron signal,and the count Niadds one. There is always noise in an electronics system.To avoid the noise influence,the lower threshold must be higher than most of the noise.

The background counts without a neutron source can be obtained by setting a lower threshold. Fig.5 shows background counts at lower thresholds of 100mV,120mV, 140mV and 160mV.It can be seen that the background counts are very small at lower threshold of≥ 140mV.So, the lower threshold for the neutron source is 140mV.

With the lower threshold of 140mV and the upper threshold of 500mV,the counts of 9-inch PE sphere detecting the Am-Be neutron source is shown in Fig.6.

Fig.6.241Am-Be neutron source counts of the 9-inch PE sphere,at 140mV low threshold.

Using the241Am-Be neutron source,the time-sharing counts of the PE spheres in diameters of 4,5,6,7,8,9,10 and 12 inches were measured as Ni=534,1077,1161,1717, 1768,2367,1959 and 662,respectively.Combined with the simulated response function,the Am-Be neutron source spectrum was calculated with Eq.(2),as shown in Fig.7.The black squares are the theoretical results in Ref.[12],and the red dots are the calculated spectrum with the response function and measured data.

It can be seen that the calculated spectrum of Am-Be neutron is accurate in the 0–2MeV region,but in high energy region,relative errors of the calculated spectrum are large.This is because of neutron scattering from the floor.The calculated spectrum of Am-Be neutron is similar to the theoretical spectrum.The experimental results show that the response function can be simulated by Geant4 for the tokamak fusion neutron spectrometer to monitor the dynamic and time-sharing neutron energy spectrum on-line and real-time.

Fig.7.(Color online)Spectrum of the Am-Be neutron source.

V.CONCLUSION

The fusion neutron spectrometer,with eight time-sharing neutron counters connected to the system controller of PXI chassis through PXI bus,was developed to monitor spectrum of the tokamak fusion neutrons.The response function of the eight PE spheres was simulated by adopting software Geant4. The experiment using Am-Be neutron source to verify the simulated response function was performed.The calculated spectrum of Am-Be neutron is accurate at 0–2MeV region. The calculated spectrum of Am-Be neutron is similar to the theoretical spectrum.The experiment verifies the response function simulated by Geant4 and the tokamak fusion neutron spectrometer.The tokamak fusion neutron spectrometer was used in HL-2A device to monitor the dynamic neutron spectrum of HL-2A on-line and real-time.

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(Received November 13,2013;accepted in revised form March 3,2014;published online July 17,2014)

10.13538/j.1001-8042/nst.25.040401

∗Supported by Ministry of science and technology major projects(No. 2013GB104003),National Natural Science Foundation of China(No. 11375195),Ministry of Education and Youth Innovation Fund(No. WK2030040037)

†Corresponding author,caohr@ustc.edu.cn