1. GEM 中子束监测器的研制进展 周健荣, 孙志嘉 zhoujr@ihep.ac.cn 中国科学院 “ 核探测技术与核电子学 ” 重点实验室 中国科学院高能物理研究所, CSNS 探测器组 2010 年 8 月 17 日 zhoujr@ihep.ac.cn

2. Page 散裂中子源进展汇报 September 17, 2015 2 Outline Background & History Physical Design Performance Test Prospects

3. Page 散裂中子源进展汇报 September 17, 2015 3 CSNS Development of Neutron Source 年份 Reactor Spallation

4. Page 散裂中子源进展汇报 September 17, 2015 China Spallation Neutron Source (CSNS) 4 CSNS, with the power of 100kW and the pulse repetition frequency of 25Hz, is scheduled to begin operation in September, 2016.

5. Page 散裂中子源进展汇报 September 17, 2015 Neutron Beam Monitor Intensity fluctuations of the incident beam due to the accelerator or reactor power changes Need beam monitor to correct the experimental data of each neutron scattering instrument Requirements to meet the need of new generation neutron facilities –Timing resolution(~1μs) Wavelength resolution –Spatial resolution(~1mm) More accurate corrections –High transmission(>95%) & low efficiency(~1‰) least perturbation –Gamma to neutron separation(<10 -6 ) γ insensitivity –High counting rate(~10 6 Hz) High intensity (>10 8 n/ cm 2 ﹒ s) 5 Neutron GuideNeutron Beam Monitor Sample Neutron Detector Neutron Beam

6. Page 散裂中子源进展汇报 September 17, 2015 History YearWhereStyleSpatial Resolution Counting ratePerformance 1942 Stanford UniversityBF 3 ionization chamber NO Slow 1970s 1980s GEBF 3 Ionization chamber Stable in practice EG&G OrtecSi surface -barrier with 6 LiRadiation damage IPNS, Argorme 3 He gas proportional counter unknown composition plated on the anode ISISGS20 glassSensitive to gamma, in practice 1990s 2000s Ordela, Canberra, Mirrotron 3 He gas proportional counter No10kHzwell in practice ISIS 3 He MSGDσ=0.1mm~100kHzWell in development CERNMicomegas with 235 U 、 10 B <50μm~1MHzExcellent in development SNSMicomegas with 10 B~1MHzExcellent in development J-PARCGEM with 10 B~1mm1MHzExcellent in development 6

7. Page 散裂中子源进展汇报 September 17, 2015 Why GEM? Fast signal: ~100ns High counting rate: 10MHz Flexible readout: pad, strip, CCD Convenient to use neutron convertor Spatial resolution: ~1mm γ insensitivity: <10 -6 Radiation endurance Long life Cost less Safety compared with Micomegas 7

8. Page 散裂中子源进展汇报 September 17, 2015 The same work in KEK firstly 8 RequirementMeasurement Neutron sensitivity~0.1% 0.042  0.002% Data transfer rate~1 MHz~1.1 MHz Position resolution~1 mm (FWHM)~1.3 mm (FWHM) Wavelength separation capability RequiredOK Active area~50 mm  ~50 mm100 mm  100 mm H. Ohshita et al, KEK, 2009

9. Page 散裂中子源进展汇报 September 17, 2015 Physical Design 9

10. Page 散裂中子源进展汇报 September 17, 2015 Object for Detection Beam monitor for Spallation or Reactor Neutron Source Size of Beam: ~4cm*4cm Flux at Guide Exit CSNS: ~10 8 n/cm 2.s(MS, HIPD), ~10 7 n/cm 2.s(SANS) Reactor: ~ 10 7 n/cm 2.s Repeated Pulse ~ms, CSNS 40ms Wavelength Cutoff with Choppers, Velocity Selector Custom Extent: SANS 0.4~8 angstrom HIPD 0.3~4.8 angstrom MS 1.9~6.2 angstrom 10

11. Page 散裂中子源进展汇报 September 17, 2015 11 neutron 7 Li or α Drift Transfer Induction n+ 10 B  α+ 7 Li+2.79MeV 7% E α =1.78MeV E Li =1.0MeV n+ 10 B  α+ 7 Li * + 2.31MeV 93% τ=73fs α+ 7 Li+γ+2.31MeV E α =1.47MeV E Li =0.84MeV GEM1 GEM2 Readout pad: 4mm*4mm, 1mm gap Gas ： Ar/CO 2 = 70:30 Cathode plane coated with 10 B -e-e Structure 10 B: easy to get, cost less, chemical stability Gain: ~10 3 Charge input: 0.1~1 pC Pulse: ~100ns wide With the thin conversion layer, the time and the location of the emitted α or 7 Li can be just treated as those of incident neutron. Active area: 50mm*50mm

12. Page 散裂中子源进展汇报 September 17, 2015 Neutron Convertor-- 10 B 12 Due to the multiple Coulomb scattering: 1) Range of 1.47 MeV α : 3.6μm Range of 1 MeV 7 Li : 2μm (SRIM) 2)Maximum efficiency (~5%) for thermal neutrons: ~2.5 μm Low efficiency5‰: ~0.1 μm coated on Al using evaporation or sputtering techniques. 3)Energy of charged particles is extended down. Thin foil can improve capacity of γ discrimination 4) Cone emission α or 7 Li 10 B 99% Al cathode plate 7 Li or α 0.1μm 10 B1μm 10 B η=5.2‰ η=3.7%Geant4 Simulation for Thermal Neutron Neutron

13. Page 散裂中子源进展汇报 September 17, 2015 GEM detector 13 CERN Standard GEM: 50 µm Kapton 5 µm Copper 70 µm holes at 140 µm pitch 1)Operation in flow mode with Ar/CO 2 (70/30) mixtures at atmospheric pressure to avoid ageing effects. 2)Range of 1.47 MeV α in the mixture gas is 6.7mm and that of 1.78MeV α is 9mm calculated by SRIM. 3) If the drift gap is chosen corresponding to the range of the ions so as to energy deposited completely in the drift gas volume, the neutron can be detected with maximal pulse-height related to good γ discrimination. 4) Track of ions and emission angle determine spatial resolution.

14. Page 散裂中子源进展汇报 September 17, 2015 Pad readout In order to fulfill counting rate 1MHz/cm 2 for fast readout, an array of pads is used and the special electronics is being designed in IHEP, each channel of which is independent completely. Electronics & DAQ The best solution is ASIC Readout electronics 14 Pad Preamplifier +Shaper Counting individually Discriminator Timing counting Ethernet &USB DAC All channel FPGA Pad Preamplifier +Shaper Counting individually Discriminator Timing counting DAQ PC DAC All channel

15. Page 散裂中子源进展汇报 September 17, 2015 Readout PCB 15

16. Page 散裂中子源进展汇报 September 17, 2015 Shielding & Background Shielding – Al shell  electromagnetism –Boron-rich plastic and Cd  absorbing neutrons Background – Due to low-Z materials used, counting gas with low density and thin  low probability –Hardly secondary charged particles –Al without neutron activation –High energy of charged conversion products: α and 7 Li –Insensitive to γ background 16

17. Page 散裂中子源进展汇报 September 17, 2015 Efficiency Linearity 17 S : energy spectrum of incident neutron σ(E) : Cross section with energy : Flux of incident neutron, : Reaction rate, η: Conversion efficiency n : Atomic density of 10 B, d: Thickness of 10 B Energy Linearity: Uniform d: if energy spectrum S(E) is uniform in 2- dimensional cross section, Conversion η is constant. 10 B △d△d d Cathode plate Spatial Linearity: Intensity Linearity: Determined by counting rate of detector and readout speed of electronics

18. Page 散裂中子源进展汇报 September 17, 2015 Performance Test 18

19. Page 散裂中子源进展汇报 September 17, 2015 Prototype Chamber 19 The enrichment 92% of boron-10 (mass thickness 1mg/cm 2 ) is coated on one surface of aluminum cathode plate as the neutron convertor with electrophoresis in Beijing 261 Factory so as to obtain the maximum conversion efficiency for irradiation tests. The signal collection plane is a printed circuit board with 96 square pads with each area 8mm×8 mm. Active area 100mm*100mm

20. Page 散裂中子源进展汇报 September 17, 2015 Irradiation tests Neutron signal 20 241 Am(Be) A channel directly connected to charge sensitive amplifier (made in IHEP) with sensitivity 500mV/pc. The majority of amplitude is about 100mV. The minority is above 200mV. Noise <3mv ~1.5μs

21. Page 散裂中子源进展汇报 September 17, 2015 Signal induced by α 239 Pu α source (1.0×10 6 Bq, 5.2 MeV α-ray) 21 One channel with a current preamplifier (FBPANIK). pulse ~150ns wide. noise <30mV. ~150ns

22. Page 散裂中子源进展汇报 September 17, 2015 Image Test by scanning 239 Pu source 22 A block of 3mm thick steel with a hole of diameter 2mm is used to collimate the emitted α. It is hoped that the spatial resolution about 10mm (FWHM) can be obtained easily with each pad 4mm×4mm Pad Pre-amplifier ScalerCAMAC DAQ PC Discriminator

23. Page 散裂中子源进展汇报 September 17, 2015 23 New Prototype Chamber

24. Page 散裂中子源进展汇报 September 17, 2015 HV supply 24 1 、 Resistance HV 2 、 R-C Filter HV

25. Page 散裂中子源进展汇报 September 17, 2015 Prospects Ⅰ GEM Monitor –Spatial resolution: ~1mm –Active area: custom size. 100mm*100mm, >10 4 channels –ASIC readout –Insensitive to γ –Calibration for energy, intensity and location –Stable for long time –Native GEM, THGEM 25

26. Page 散裂中子源进展汇报 September 17, 2015 Prospects Ⅱ 26 Efficiency(1.8A) : 30%, resolution~1mm S. Uno, KEK, 2007 Efficiency(1.8A) : 50%, resolution~1mm M. Klein, University of Heidelberg,2004

27. Page 散裂中子源进展汇报 September 17, 2015 THANKS! 27