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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. Outline. Background & History Physical Design Performance Test Prospects. Development of Neutron Source. Reactor. Spallation. CSNS. 热中子通量. 年份.

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  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

  2. Outline • Background & History • Physical Design • Performance Test • Prospects

  3. Development of Neutron Source Reactor Spallation CSNS 热中子通量 年份

  4. China Spallation Neutron Source (CSNS) CSNS, with the power of 100kW and the pulse repetition frequency of 25Hz, is scheduled to begin operation in September, 2016.

  5. 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(~106Hz) High intensity (>108n/ cm2﹒s) Sample Neutron Beam Neutron Guide Neutron Beam Monitor Neutron Detector

  6. History

  7. 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

  8. The same work in KEK firstly H. Ohshita et al, KEK, 2009

  9. Physical Design

  10. Object for Detection • Beam monitor for Spallation or Reactor Neutron Source • Size of Beam: ~4cm*4cm • Flux at Guide Exit CSNS: ~108n/cm2.s(MS, HIPD), ~107n/cm2.s(SANS) Reactor: ~ 107n/cm2.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

  11. Structure neutron Gas ： Ar/CO2 = 70:30 n+10Bα+7Li+2.79MeV 7% Eα=1.78MeV ELi=1.0MeV n+10Bα+7Li* + 2.31MeV 93% τ=73fs α+7Li+γ+2.31MeV Eα=1.47MeV ELi=0.84MeV Cathode plane coatedwith 10B 7Li or α 10B: easy to get, cost less, chemical stability Gain: ~103 Charge input: 0.1~1 pC Pulse: ~100ns wide Drift -e GEM1 With the thin conversion layer, the time and the location of the emitted α or 7Li can be just treated as those of incident neutron. Transfer GEM2 Induction Active area: 50mm*50mm Readout pad: 4mm*4mm, 1mm gap

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

  13. GEM detector • Operation in flow mode with Ar/CO2 (70/30) mixtures at atmospheric pressure to avoid ageing effects . • 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. CERN Standard GEM: 50 µm Kapton 5 µm Copper 70 µm holes at 140 µm pitch

  14. Readout electronics • Pad readout In order to fulfill counting rate 1MHz/cm2 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 FPGA DAC DAC Counting individually Counting individually Pad Preamplifier +Shaper Discriminator Ethernet &USB Pad Preamplifier +Shaper Discriminator DAQ PC Timing counting Timing counting All channel All channel

  15. Readout PCB

  16. 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 7Li • Insensitive to γ background

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

  18. PerformanceTest

  19. Prototype Chamber Active area 100mm*100mm The enrichment 92% of boron-10 (mass thickness 1mg/cm2) 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.

  20. Irradiation tests • Neutron signal ~1.5μs 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 241Am(Be)

  21. Signal induced by α • 239Pu α source (1.0×106 Bq , 5.2 MeV α-ray) ~150ns One channel with a current preamplifier (FBPANIK). pulse ~150ns wide. noise <30mV.

  22. Image Test by scanning 239Pu source Discriminator Scaler CAMAC DAQ Pad Pre-amplifier PC 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

  23. New Prototype Chamber

  24. HV supply 1、 Resistance HV 2、 R-C Filter HV

  25. Prospects Ⅰ • GEM Monitor • Spatial resolution: ~1mm • Active area: custom size. 100mm*100mm, >104channels • ASIC readout • Insensitive to γ • Calibration for energy, intensity and location • Stable for long time • Native GEM, THGEM

  26. Prospects Ⅱ Efficiency(1.8A) : 50%, resolution~1mm M. Klein, University of Heidelberg,2004 Efficiency(1.8A) : 30%, resolution~1mm S. Uno, KEK, 2007

  27. THANKS!

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