1. PROGRESS of FAST NEUTRON FLUX MEASUREMENT in CJPL
School of Physical Science and Technology, Sichuan University
CDEX Collaboration
PROGRESS of FAST NEUTRON FLUX MEASUREMENT in CJPL 
On behalf of Sichuan University CDEX Group
Haoyang Xing
2018/11/15
Symposium of Sino-German GDT

2. Outline Background Neutron in CJPL Neutron Detector Design
Fabrication of Detector
Energy Calibration
Detection Efficiency Calibration (Discrimination between γ and n)
Next Work
Summery
2018/11/15
Symposium of Sino-German GDT

3. Background Neutron in CJPL
2018/11/15
Symposium of Sino-German GDT

4. Source and Characteristic
μ induced neutrons Spontaneous fission of U238
(α,n) reactions from U, Th series
Low neutron flux <10-7 n/cm2/s.
Energy range from thermal to some tens of MeV.
Uranium, Thorium, Muon
2018/11/15
Symposium of Sino-German GDT

5. Requirement of Detector
Therefore, to measure the neutron flux and energy spectra, we need:
– A high sensitivity neutron detector.
– Effective way to eliminate the gamma
background since most of the neutron
detectors are also sensitive to gammas.
2018/11/15
Symposium of Sino-German GDT

6. Neutron Detector Design
2018/11/15
Symposium of Sino-German GDT

7. Neutron Spectrometer Characteristics
We can have several types of Neutron Detector
2018/11/15
Symposium of Sino-German GDT

8. Liquid Scintillator Choice
Because of the low neutron flux, the liquid scintillator loaded with Gd is adopted to confirm a neutron event.
Schematic diagram：
PSD could be applied
Fast and slow signal coincidence measurement p Gd γ n
It show us
2018/11/15
Symposium of Sino-German GDT

9. Configuration of Detector
LS : EJ335
PMT: R5912
2018/11/15
Symposium of Sino-German GDT

10. Geometrical Simulation
200,000 Neutron
Energy :10MeV
Location&Direction: Outside of detector and point to detector
To help design the detector, we did also
Red, proton recoiling;
Blue, captured by Gd
Distance distribution
2018/11/15
Symposium of Sino-German GDT

11. Fast-Slow event time simulation
For setting Time Window of Fast-Slow Identification.
2018/11/15
Symposium of Sino-German GDT

12. Structure and Shape of LS Detector
The considered factors of Large Volume Detector :
Fast-Slow coincidence
Light collecting efficiency
Effect from optic absorption length
Two PMT
2018/11/15
Symposium of Sino-German GDT

13. Fabrication of Detector
2018/11/15
Symposium of Sino-German GDT

14. Assembling and Shielding
Quartz glass container + EJ335 + Copper shield + Two PMTs
Lead shield
Quartz
2018/11/15
Symposium of Sino-German GDT

15. DAQ system Diagram of DAQ system PMT1 PMT2 FIFO V1721 A2818 trigger
Voltage： left（PMT1）： -850V
right（PMT2）： -880V
Threshold of discriminator：
channel1（PMT1） 10 (15mv)
channel2（PMT2） 12 (15mv)
Outgoing pulse width of discriminator： 140 ns
Diagram of DAQ system
PMT1
PMT2
FIFO
V1721
A2818
trigger
logical And
Discriminator
2018/11/15
Symposium of Sino-German GDT

16. Energy Calibration with Gamma Source for Detector
2018/11/15
Symposium of Sino-German GDT

17. Background Measurement
1721 setting：
PreTrigger：
TimeWindow： （240us）
Total events： ,000,000
Time： s
Average Rate： /s
40K Potassium, 60Co Cobalt, 137 Cesium, 232TI Thallium,
Background Energy Spectrum
Background Experiment
2018/11/15
Symposium of Sino-German GDT

18. Energy zero-point
Using a signal generator to get a 1KHZ square signal for the random trigger signal of FADC and collect the output from the detector.
From this energy spectrum,we know that the energy zero-point is at the zero channel.
Energy Spectrum of Random Trigger
2018/11/15
Symposium of Sino-German GDT

19. Gamma Experiment The position of the γ source
The γ source was placed near to the centre of one side of the detector through a hole which was made in the roof of the lead shield.
2018/11/15
Symposium of Sino-German GDT

20. Energy spectrum measurement for γ source
60Co
137Cs
Total events： ,000,000
Time： s
Average rate： /s
Total events： ,000,000
Time： s
Average rate： /s
Co experiment
60Co spectrum (red)， background (blue)
137Cs spectrum (red)，background (blue)
We adopt two Gamma source.
2018/11/15
Symposium of Sino-German GDT
60Co spectrum after subtracting background
137Cs spectrum after subtracting background

21. Energy calibration of the detector
Through the simulation by Geant4, we get the energy deposition spectrum of the γ source in the detector. We perform a broadening for this energy spectrum by using After adjusting the parameters, which is in order to make the simulation and the experiment more coherently. We can calibrate the energy of the detector through the comparison between the simulation and the experiment.
仿真谱怎么得到的？按G4中放射性GPS软件包，写脚本指定原子序数质量数等。1.17Mev Compt 0.96;1.33MeV，1.11;后两叠加；中间部分不完全沉积的叠加；Cs KeV；Cs137光电峰前面的是什么？
60Co energy deposition spectrum by simulation
137Cs energy deposition spectrum by simulation
2018/11/15
Symposium of Sino-German GDT

22. When α= and β= 0.35 (unit 10KeV) ， the simulation and the experiment get the most coincidence.
60Co simulation(red) and experiment(blue) spectrum
137Cs simulation(red) and experiment(blue) spectrum
Correspondence：
Peak position
Energy(MeV)
高低能不一致的原因?林：反散射? 张：隧道效应？Cut的位置为什么这样？
Energy calibration curve
2018/11/15
Symposium of Sino-German GDT

23. Detection Efficiency Calibration
Discrimination Between Gamma and Neutron
(Neutron Identification)
Haven't finished, only show the work of Discrimination Between Gamma and Neutron,
2018/11/15
Symposium of Sino-German GDT

24. Neutron Experiment
We use the Am-Be neutron source to test the detector
The distance between neutron source and lead shield is1m
Total events： 200,000
Average Rate： 416 /s
道数按面积算,所以6M以上有小峰, Americium, Beryllium
Energy spectrum of all events (red), background (blue)
2018/11/15
Symposium of Sino-German GDT

25. n- γ discrimination One point stand for 2ns PSD:
Charge comparison method is adopted to perform the n-gamma discrimination. “Qtotal” stands for the total charge of the pulse, and the integrating range is from 40ns before the peak position to 80ns after the peak position. “Qpart” stands for the charge of the pulse rising edge, and the integrating range from 30ns after the peak position to 80ns after the peak position. We define the discriminative factor：Dis = Qpart/Qtotal
2018/11/15
Symposium of Sino-German GDT

26. n- γ discriminative diagram
Left gamma, right neutron.
Over1.6MeV, it is hard to discriminate neutron and gamma.
2018/11/15
Symposium of Sino-German GDT

27. More precise standard to identify n
Analysis data from
Background Experiment
Co60 Experiment
2018/11/15
Symposium of Sino-German GDT

28. Neutron experiment + Background
The red part is background
From this diagram, we can clearly know the distribution of the background in the n-γ discriminative diagram. This can help us to establish the standard of identifying the neutron( or recoil proton) signal.
2018/11/15
Symposium of Sino-German GDT

29. Neutron experiment + Background + Co
Am-Be
From this diagram, we can get a clear boundary of the γ signals.
γ signal should have the same characteristics whether it comes from Gd capture or the 60Co.
2018/11/15
Symposium of Sino-German GDT

30. B A c Region A, gamma? but can be used in fast-slow coincidence.
The fitted boundary equation: exp( *x) *x+4.188*x2
Am-Be
Background
60Co A B
beryllium c
(<0.145)
Region A, gamma? but can be used in fast-slow coincidence.
2018/11/15
Symposium of Sino-German GDT

31. Region B, events can be identified to be Neutron with very small error.
Total events
Valid events
Region B
Mono-pulse events above the red curve
Multi-pulse events above the red curve
Neutron Exp
200,000
186839
37409(0.2002)
21477(0.574)
15932(0.426)
Background Exp
20,000
13331
62(0.0047)
39(0.629)
33(0.371)
Co60 Exp
50,000
40758
116(0.0029)
84(0.724)
32(0.276)
2018/11/15
Symposium of Sino-German GDT

32. fast-slow coincidence
Region C, can be used to retrieve more neutron by F-S coincidence.
Fast signal
(or recoil proton signal)
Slow signal
(or γ signal)
Threshold ?
Region B was also inspected by F-S coincidence.
2018/11/15
Symposium of Sino-German GDT

33. fast-slow coincidence Gamma energy Threshold (MeV)
(Region B)
Gamma energy Threshold (MeV)
in Region A
Neutron Exp
(multi-pulse events 15932)
Background Exp
(multi-pulse events 33)
Co60 Exp
(multi-pulse events 32)
>2.6
4778(0.299)
12(0.364)
4(0.125)
>2.4
5348(0.335)
14(0.424)
>2.2
5964(0.374)
15(0.455)
5(0.156)
>2.0
6625(0.415)
16(0.485)
>1.8
7216(0.452)
17(0.515)
6(0.188)
>1.6
7835(0.491)
8(0.250)
>1.4
8446(0.530)
9(0.281)
>1.2
9147(0.574)
18(0.545)
13(0.406)
>1.0
9899(0.621)
19(0.576)
15(0.469)
>0.8
10669(0.669)
20(0.606)
>0.6
11293(0.708)
21(0.636)
>0.4
11796(0.740)
22(0.667)
16(0.500)
>0.2
12247(0.768)
For Bg and Co, although the ratio is relatively high, while the absolute quantity is small, so don’t worry.
2018/11/15
Symposium of Sino-German GDT

34. Region C, events probably includes amount of Neutron
Total events
Valid events
Events inside the red line
Mono-pulse event inside the red curve
Multi-pulse events inside the red curve
Neutron Exp
200,000
186839
43856(0.2347)
27772(0.633)
16084(0.367)
Background Exp
20,000
13331
747(0.0560)
712(0.953)
35(0.047)
Co60 Exp
50,000
40758
7925(0.1944)
6329(0.798)
1596(0.202)
2018/11/15
Symposium of Sino-German GDT

35. fast-slow coincidence Gamma energy Threshold (MeV)
in Region A
Neutron Exp
(16084)
Background Exp
(35)
Co60 Exp
(1596)
>2.6
3785(0.235)
8(0.229)
2(0.001)
>2.4
4252(0.264)
3(0.002)
>2.2
4707(0.293)
7(0.004)
>2.0
5227(0.325)
20(0.013)
>1.8
5774(0.359)
9(0.257)
30(0.019)
>1.6
6253(0.389)
11(0.314)
38(0.024)
>1.4
6729(0.418)
52(0.033)
>1.2
7290(0.453)
13(0.371)
125(0.078)
>1.0
7909(0.492)
278(0.174)
>0.8
8522(0.530)
16(0.457)
407(0.255)
>0.6
9064(0.564)
19(0.543)
471(0.295)
>0.4
9510(0.591)
21(0.600)
517(0.324)
>0.2
9999(0.622)
24(0.686)
563(0.353)
Using this way, can be get from Neuron Exp event (22%).
2018/11/15
Symposium of Sino-German GDT

36. Neutron Identification Standard
PSD over 1.6MeV
Fast - slow coincidence, Gamma threshold over 1.2 MeV
2018/11/15
Symposium of Sino-German GDT

37. Next work Calibration of detection efficiency for the detector.
2018/11/15
Symposium of Sino-German GDT

38. Steps to Calibrati detection efficiency of n
A known γ HpGe detector
Radioactivity measurement of Gamma source
γ/n is known for AmBe Source
Neutron radioactivity
Calibrating neutron detector
2018/11/15
Symposium of Sino-German GDT

39. Summery Design Detector, finished Fabrication, finished
Energy calibration, almost finished
Efficiency calibration, in work
Measure the neutron flux, in recent future
2018/11/15
Symposium of Sino-German GDT

40. Thank you! Americium, Beryllium, Cesium, 2018/11/15
Symposium of Sino-German GDT