Solid state silicon + 6 LiF thermal neutron detectors: GEANT4 simulations versus real data

Transcription

1 Solid state silicon + 6 LiF thermal neutron detectors: GEANT4 simulations versus real data P. Finocchiaro 1, S. Lo Meo 1,2, S. Caruso 3, L. Cosentino 1 1) INFN Laboratori Nazionali del Sud, Catania, Italy 2) ENEA, Research Centre Ezio Clementel, Bologna, Italy 3) NAGRA, National Cooperative for the Disposal of Radioactive Waste, Wettingen, Switzerland 1

2 material for thermal neutron conversion: 3 He new neutron detectors needed 3 H σ(0.025) 5330 b reasonably inexpensive n 3 He p available energy 0.76 MeV no gamma rays compact, rugged, simple to use reasonable detection efficiency perfect gas detector but... worldwide lack of 3 He expected good gamma/n discrimination 6 Li cross section: international standard σ(0.025) 940 b available E 4.78 MeV 2.73 MeV α 2.05 MeV t 2

3 Si detector + 6 LiF = SiLiF 4 He n 6 Li 7 Li 6 LiF converter Silicon detector 3 H Triton detected Alpha detected 4 He 6 LiF converter 7 Li 6 Li n 3 H low cost, technology (cheaper than 3 He?) low voltage (20-30 V) flat detector, compact, rugged, simple to use, easily handled position sensitivity (mm) easily achieved (strips) this sample 3cm x 3cm active area 10-7 neutron/gamma discrimination ( 60 Co) 3

4 the 6 LiF converter thickness plays a dominant role thin: perfect discrimination poor efficiency thick: worse discrimination better efficiency a trade-off is needed 4

5 the emission from the 6 LiF converter was simulated thin z thick z 1.6µm 16µm direction cosines direction cosines 5

6 why 16µm as thick layer? t endpoint neutron contribution rather flat alpha endpoint is clear allows simple energy calibration triton range 32µm (16µm = half range) > 16µm: delicate and tends to detach efficiency saturation at 16µm we gain in tritons we lose in alphas the acceptance cone shrinks 6

7 many configurations simulated and tested just 2 shown here 6 LiF (4x16µm) Si detector SiLiF1.6 SiLiF64 6 LiF (1.6µm) Aluminum box 2 Si detectors Aluminum box 7

8 SiLiF1.6 - simulation with 25.3 mev monochromatic n-beam n 7 Li α 6 Li t α 8

9 SiLiF1.6 - simulation with 25.3 mev monochromatic n-beam n 7 Li t 6 Li α α t 9

10 SiLiF1.6 - simulation with 25.3 mev monochromatic n-beam 10

11 Geant4 simulations reproduce the experimental spectrum shape both in flood and isotropic irradiation scheme flood (beam) isotropic (AmBe source) only 3.1% tritons below alpha endpoint 11

12 SiLiF16 simulation SiLiF64 flood simulation total triton alpha SiLiF64 exp data and simulation (renormalized ) SiLiF64 isotropic simulation 12

13 SiLiF absolute efficiency calibration at PTB with the Thermal Neutron Calibration Facility neutron exit window Cd plate moderator neutron exit window detector neutron detector laser alignment beams neutron spectrum neutron flux: 68.3 n/s/cm 2 uniform over > 10cm x 10cm with Cd plate: < 1 n/s/cm 2 13

14 SiLiF1.6 neutron detection efficiency: 0.5% with threshold at 60 Co: 0.6% 60 Co threshold suggested threshold agreement between data and simulation better than 2% (mainly systematic uncertainty) ratio Cd_OUT to Cd_IN flat above 1.5 MeV Cd plate OUT Cd plate OUT Cd plate IN Cd plate IN 14

15 SiLiF64 neutron detection efficiency: 8% with threshold at 60 Co: 10% agreement between data and simulation better than 4% (mainly systematic uncertainty) 60 Co threshold suggested threshold ratio Cd_OUT to Cd_IN flat above 1.5 MeV Cd plate OUT Cd plate OUT Cd plate IN Cd plate IN 15

16 SiLiF64 neutron efficiency vs discrimination threshold perfect agreement data-simulation in absolute units 16

17 SiLiF gamma/neutron discrimination tested with AmBe and 22Na (3 gammas: 511, 511, 1274 kev) 8.5 n/s and gamma/s on the detector rescaled to 1 neutron and 1 gamma this integral neutron efficiency 10-7 this integral gamma efficiency (1/3 was electronic noise and cosmics) in agreement with simulation 17

18 SiLiF64 tested with a neutron beam SiLiF64 comparison with 3 He tube on INES@RAL SiLiF64 18

19 n_tof neutron beam monitor Si detectors + 6 LiF Si Si Si Si triton alpha gamma 19

20 SiMon2D: neutron beam characterization the idea... Si detector + 6 LiF 6 LiF foil strip Si XY distribution the test with AmBe source 20

21 n_tof: neutron beam profiler 5cm x 5cm double-sided strip SiLiF detector 25 strips, 2mm x 5cm real beam profile 21

22 real beam profiles as a function of energy

23 6 LiF material: benefits vs drawbacks detection of 4 He and/or 3 H 4.78 MeV available energy 7 / g stable salt, easily evaporated not very high cross section natural abundance: 7% substrate: C-fiber, Al, glass,... substrate thickness 1µm 23

24 detector mechanical structure rugged, manageable large area = many detectors little non-detector material stackable 24

25 operating features & summary vacuum compatible low voltage operation quite stable easily handled (simple box) easily assembled/disassembled no physical/chemical agents on the converter (gas, high electric field,...) 25

26 current developments Monte Carlo GEANT4 sensitivity to changes inside SF casks? arrays of SiLiF to monitor spent fuel casks 24/7? test at ZWILAG next week 26

27 current developments back converters detectors placed front converters front+back converter back converter α t tile with 9 detectors, 2 x 1.6µm 6LiF layers in collaboration with S.Vaccaro, P.Peerani test at Geel with P.Schillebeeckx soon? 27

28 next step? SiLiF detector + electronics + WiFi all in the same box ongoing discussion with private companies (CAENsys,...) commercial agreement for SiLiF production? 28

29 conclusion solid state (Silicon + 6 LiF) low cost technology (cheaper than 3 He?) low voltage (25 V) compact, robust, manageable nice detection efficiency (5 10%) good gamma/n discrimination (10-7 ) tested and in use at neutron beam facilities ntof at CERN and ISIS at RAL very satisfactory agreement with simulations work partly supported by JRC Ispra 29

30 a few papers about this SiLiF technique P. Finocchiaro et al., Nucl. Instr. Meth. A885 (2018) S. Lo Meo et al., Nucl. Instr. Meth. A866 (2017) A.Pappalardo et al., Results in Physics 6 (2016) A.Pappalardo et al., Nucl. Instr. Meth. A810 (2016) 6-13 L.Cosentino et al., Rev. Sci. Instr. 86 (2015) D.Henzlova et al., "Current Status of 3He Alternative Technologies for Nuclear Safeguards", prepared for NNSA-DOE and Euratom P.Kavrigin et al., Nucl. Instr. Meth. A795 (2015) S.M.Carturan et al., EPJPlus 129 (2014) 212, Nuclear Physics News, 2014, v24, n3, (2014) 34 A.Pappalardo et al., Optical Engineering 53(4)047102, April 2014 M.Barbagallo et al., Rev. Sci. Instrum 84 (2013) , in Radioactive Waste: Sources, Types and Management, Nova Science Publishers,

31 acknowledgments We have to acknowledge the help, assistance, suggestion, cooperation, encouragement, support of several people. If I am forgetting somebody I deeply apologize!!! A.Pappalardo, C.Marchetta, A.Massara, S.Scirè, G.Vecchio, C.Scirè, C.Greco, S.Grillo, D.Bongiovanni, M.Barbagallo, S.Russo, P.Litrico, G.Passaro, C.Calì, M.Piscopo, B.Trovato, G.Vasta, S.DiModica, M.Tringale, R.Nolte, D.Radeck, C.Vasi, G.Ricco, M.Ripani 31

32 THANK YOU Catania and the Etna volcano 32