Characterization and Monte Carlo simulations for a CLYC detector

Transcription

1 Characterization and Monte Carlo simulations for a CLYC detector A. Borella 1, E. Boogers 1, R.Rossa 1, P. Schillebeeckx 1 aborella@sckcen.be 1 SCK CEN, Belgian Nuclear Research Centre JRC-Geel, Joint Research Centre The 017 International Conference on Applications of 1Nuclear Techniques Crete, Greece June 11-17, 017

2 Outline The CLYC detector, general features Detector response to gamma-rays Measurements Modelling Data analysis Results Detector response to neutrons and future work

3 CLYC features CLYC = Cs LiYCl 6 (Ce) Elpasolite crystal Hygroscopic Scintillator ~0,000 ph/mev (e) First work 1999 UDelft Suppliers RMD ( nat Li, enr6 Li, enr7 Li) Scionix (95% 6 Li) Sensitive to both neutrons and gamma-rays 3

4 CLYC features - Reaction of interest for neutron detection 6 Li(n,α)t Q=4.78 MeV 35 Cl(n,p) 35 S Q=0.615 MeV Highest (n,p) reaction XS Significant (n,) 4

5 6 Li(n,α)t Q=4.78 MeV 3. MeV ee Light output for α, t n, separation Energy Pulse Shape Discrimination CLYC without 6 Li 35 Cl(n,p) 35 S Applications Security, Safeguards (Fast) neutron dosimetry Space ( Neutron radiography (Physics Procedia 69 ( 015) ) Count rate / cps CLYC features 137 Cs 5 Cf E d / MeV ee 5

6 Outline The CLYC detector, general features Detector response to gamma-rays Measurements Modelling Data analysis Results Detector response to neutrons and future work 6

7 Outline The CLYC detector, general features Detector response to gamma-rays Measurements Modelling Data analysis Results Detector response to neutrons and future work 7

8 Measured pulse height spectrum N(E d ) Detector response function N( E ) R ( E, E ) R ( E, E ) ( E ) de de d d e 1 e E e -ray fluence spectrum Φ E (E ) E, -ray energy E d, observed signal R 1 (E e, E ) describes the transfer of E in electron energy E e R (E d,e e ) relates E e to E d. Counts E d / Channel 8

9 Detector response function R 1 (E e, E ) Transfer of E into E e determined by -ray transport in detector material can be calculated by Monte Carlo simulations R (E d, E e ) Transfer of E e into E d depends on processes such as ionization scintillation photo-multiplication signal processing phenomenological 9 R 1 R E e / MeV E / channel

10 Detector response function R ( E, ( E )) 1 exp d d e R includes Energy calibration, ( E ( E )) e E e μ(e e ) conversion of E e into observed signal E d Resolution broadening σ μ =f(μ(e e )) Ideally, μ(e e ) and σ μ are directly proportional to E e Phenomenological determination 10

11 Outline The CLYC detector, general features Detector response to gamma-rays Measurements Modelling Data analysis Results Detector response to neutrons and future work 11

12 1 x 1 cylindrical CLYC detector, 6 Li 96% Hamamatsu R PMT HV -100 V Ortec 113 preamplifier with 00 pf input capacitance MCA57 unit from GBS-Elektronic trigger filter (+1,0,-,0,+1) shaping time 0.5 µs Experiments - equipment Flat Top time to 5 µs 5 s 1

13 Experiments - sources Main E kev Radionuclide A kbq Unc 1 sigma kbq Reference date YYYY/MM/DD HH:MM CET Am ± : Cd ± : Co 7.47 ± : Ce 40.9 ± : Sn ± : Sr ± :00 ADD FIGURE with source Cs ± : Mn ± : Zn ± : Na 4.00 ± : Co ± :00 13

14 Experiments - setup Aluminium structure Plexiglas holder concentric circles three distances between detector surface and source 0.67, 4.73 and cm Repetitive measurements with a 137 Cs at 4.73 cm distance impact of the source positioning vs uncertainty due to counting statistics 14

15 Outline The CLYC detector, general features Detector response to gamma-rays Measurements Modelling Data analysis Results Detector response to neutrons and future work 15

16 Modelling Monte Carlo simulations Monte Carlo model of the measurement setup modelled detector and materials around Crystal covered with PTFE reflector surrounded by Aluminum casing μ-metal shield covers the PM Design information verified by X-ray radiographies crystal length.44 cm diameter.46 cm 16

17 F8 tally (R 1 ) MCNP6 Response to monoenergetic -ray Tally, per simulated -ray MCNP-CP (patch of MCNP 4c) Response to decay statistical simulation of processes coming with the radioactive decay i.e. accounts for quasi-simultaneous emission of more than one gamma ray following radioactive decay Modelling Monte Carlo simulations All emitted nuclear particles tracked within the same history Tally, per simulated decay Same input, no VR, different source definition 17

18 Modelling Monte Carlo simulations Differences MCNP6 and MCNP-CP vanish as distance increases Main E 0.67 cm 4.73 cm cm = 100 ε MCNP6 ε 1 P MCNP CP kev % % % Am -0.1 ± ± ± Cd 0.1 ± ± ± Co 0.3 ± ± ± Ce 1.5 ± ± ± Sn 0.0 ± ± ± Sr 5. ± ± ± Cs -0.1 ± ± ±0.4 full energy peak efficiency P emission probability 85 Sr Nuclear Decay Data used by MCNP-CP? Mn 0.8 ± ± ± Zn 0.9 ± ± ± Na 5.1 ±0.. ± ± Co 4.6 ±0..3 ±0..0 ±0.3 18

19 Outline The CLYC detector, general features Detector response to gamma-rays Measurements Modelling Data analysis Results Detector response to neutrons and future work 19

20 Data analysis Measured pulse height spectrum N exp (E d ) Calculated N fit (E d ) N ( E ) C R ( E, E ) R ( E, E ) ( E ) de de fit d d e 1 e E e E R 1 from modelling 1 Ed R ( Ed, ( Ee )) exp ( ( E )) e E e Least square minimization: C,, ( N N ) exp,, n i fit i i1 sni, Counts Experiment Fit C is # decayed nuclei, if R 1 expressed per decay, N exp (E d ) in counts 0 Residual E d / channel

21 Results repetitive measurements 9 repetitive measurements 137 Cs source at 4.73 cm distance source removed and repositioned every time C, and for each case with uncertainty from fit (statistics) analysis to determine uncertainty components not due to counting statistics n i1 ( ) i s, i C 100 s,i / i (statistics) / s,i / i (other)

22 Energy calibration kev/ch ± 0.7% over months energy independent no offset Results all sources Ee 0.7 % - Long term stability of high voltage, PMT or electronics

23 Energy resolution N photons E e Nphotons N photons FWHM % Deviation from 1/E e Effects other than pure statistic Crystal Intrinsic PMT 1 E e FWHM / % Transfer variance (photon electron) Results all sources FWHM = b 0 E + b 1 E = b 0 E E e / MeV 3

24 Results all sources Absolute model benchmark N ( E ) C R ( E, E ) R ( E, E ) ( E ) de de fit d d e 1 e E e # of decayed nuclei Model response overestimated Trend with energy Same trend for all distances Nominal / Model E / MeV 4

25 Design variations Parameters affecting the efficiencies Detector-Source distance Crystal density Crystal Diameter Crystal Length Change in density consistent with observed trend Efficiency relative to reference case distance (+1 cm) density (-10%) diameter (-1.4 mm) length (-.4 mm) E / MeV 5

26 Response of a CLYC detector to rays Conclusions Measurements with calibrated gamma point-sources in a wellgiven and reproducible geometry CLYC response modelled with MCNP-CP used to analyse the data Results Linearity Energy resolution Presence on non statistical effect Efficiency Overestimated by model Design variations 6

27 Future work: testing the performance with neutron sources Data acquisition based on a CAEN 5730B digitizer ns sampling Pulse Shape Discrimination on board Neutrons 7