Calibration of the GNU and HSREM neutron survey instruments

Transcription

1 Calibration of the GNU and HSREM neutron survey instruments Neutron Users Club Meeting National Physical Laboratory 20 th October 2015 J. S. Eakins 1, L. G. Hager 1, J. W. Leake 2, R. S. Mason 2 and R. J. Tanner 1 1 Centre for Radiation, Chemicals and Environmental hazards (CRCE), Public Health England (PHE), Chilton, OXON, UK 2 Sherwood Nutec Scientific Ltd., Kent, United Kingdom

2 Context for Current Work Essential that adequate radiological protection is provided at sites where individuals may be exposed to neutrons Accurately monitor dose rates from ambient neutrons Risk to personnel kept within acceptable limits Use neutron survey instruments Portable devices that can measure dose rates at different locations Dose map of the area of interest Shielding / other provisions hence checked (and modified ) 2 The GNU and HSREM neutron survey instruments

3 The Ideal Instrument The ideal instrument is: Light and easy to use Well-characterized Appropriately sensitive Flat angle- and energy-dependence of response across a very wide energy range of neutrons (thermal to GeV) Survey instruments typically calibrated in terms of ambient dose equivalent, H*(10) Drives requirement for isotropic response Is highly energy-dependent Taken together, competing factors make design of accurate neutron survey instruments particularly challenging 3 The GNU and HSREM neutron survey instruments

4 Real Instruments A number of neutron survey instruments are currently available (e.g. Leake, LB6411, Studsvik 2202D, etc.) Relative performances of devices differ But often exhibit common problems... typically, response characteristics: and / or under-respond at low energies (< ev scale) and high energies (>MeV scale) over-respond at intermediate energies (~ kev scale) Designs are often also rather heavy 4 The GNU and HSREM neutron survey instruments

5 Solution 1: The GNU Surround a central detector with moderating and attenuating layers, penetrated by air-filled guides* Moderator: Attenuator: Lead Layer: Air Guides: A Guided Neutron Unit Slows high-energy neutrons Suppresses response to intermediate-energy neutrons Enhanced detection of very high energy neutrons via (n,2n) Channel thermalized neutrons to detector, raising low-energy response Excellent energy-dependence of response from mev TeV range Design optimized using Monte Carlo modelling Mass ~10 kg (incl. electronics & batteries) * International patent PCT/EP2008/ The GNU and HSREM neutron survey instruments

6 Solution 2: The HSREM Integrate a sensitive LND detector into the lightweight moderator arrangement of the NGREM A High Sensitivity NGREM Replace SP9 of NGREM with larger LND detector much greater sensitivity Increased diameter of Borotron layer due to loss of polyethylene Overall dimensions of device unchanged from NGREM (radius ~10.4 cm) Mass of < 5 kg (excl. electronics) Compact, lightweight design with good sensitivity Design optimized using Monte Carlo modelling 6 The GNU and HSREM neutron survey instruments

7 The GNU Lead [black] Polyethylene [light grey] Borotron [dark grey] Stem of LND through guide on right Connects to inbuilt FST SensorTechnik electronics 7 The GNU and HSREM neutron survey instruments

8 The HSREM Polyethylene [light grey] Borotron [dark grey] Stem of LND exits to right Controlled through external FST SensorTechnik electronics Shape / size identical to NGREM 8 The GNU and HSREM neutron survey instruments

9 Modelled Absolute Response 100 Modelled H*(10) response characteristics of GNU and HSREM ) Compared against: NGREM LB6411 Studsvik 2202D Both isotropic and plane-parallel fields considered for GNU H *(10) Response 1 (nsv Energy (ev) GNU (ISO) GNU (PP) HSREM LB Studsvik NGREM 9 The GNU and HSREM neutron survey instruments

10 Modelled Relative Response Modelled H*(10) relative response characteristics of GNU and HSREM normalized to response at 1 MeV Compared against: NGREM LB6411 Studsvik 2202D 1 MeV Relative Response 10 1 GNU (ISO) GNU (PP) HSREM LB Studsvik NGREM Both isotropic and plane-parallel fields considered for GNU Energy (ev) 10 The GNU and HSREM neutron survey instruments

11 Testing of Instruments Monte Carlo modelling of GNU and HSREM demonstrates promising response characteristics Prototypes of instruments constructed by Sherwood Nutec Scientific Ltd. Nominal sensitivities of devices quoted as: GNU: 7.05 counts nsv -1 at 1 MeV ( 1.96 counts s -1 µsv -1 h) HSREM: 8.44 counts nsv -1 at 2 MeV ( 2.34 counts s -1 µsv -1 h) (~3 more sensitive than NGREM) Plan series of exposures to test performances of the devices Exposures performed at: Metrology laboratory at Public Health England Primary standard fields of National Physical Laboratory High-energy fields at CERN (for GNU) Energies from thermal to ~100 MeV considered Gamma rejection tests also performed 11 The GNU and HSREM neutron survey instruments

12 Exposures at PHE Raw 241 Am-Be source Devices placed 1 m from source in low-scatter environment Four orientations for GNU: 0, 45, 180 and Top HSREM exposed from front only (i.e. 0 ) Orientation Rel. Response Sensitivity (nsv -1 ) ± ± ± ± ± ± 0.73 Top 1.05 ± ± 0.81 Find ~7.85 ± 0.86 counts nsv -1 for the HSREM, implying a relative H*(10) response of ~0.93 ± The GNU and HSREM neutron survey instruments

13 Exposures at PHE Simulated workplace field 241 Am-Be source moderated by two cm 3 water tanks to produce simple simulated workplace field Well-characterized by MCNP modelling: H*(10)/Φ conversion coefficient of ± 3.5 psv cm 2 H*(10) dose rate of 10.8 ± 0.4 µsv hr -1 at 1 m point-of-test Fluence-energy distribution containing significant thermal peak HSREM: Relative dose response of 0.93 ± 0.11 at 0 GNU: Relative dose response of 0.99 ± 0.11 at 0 and 0.99 ± 0.11 at The GNU and HSREM neutron survey instruments

14 Exposures at PHE Cobalt-60 source Devices exposed to intense gamma source to check photon discrimination in LND proportional counters. 60 Co radionuclide (E mean = 1253 kev) with H*(10) rate of 9625 µsv h -1 at 1m. 1 hour exposures performed (anticipating low sensitivities ) Dose of µsv measured by GNU at 0 H*(10) response of Gamma rejection ratio better than 7000:1? BUT: Dose of µsv recorded when repeated with shutter CLOSED Negligible response to photons Similar results found for GNU at 45 and HSREM at 0 14 The GNU and HSREM neutron survey instruments

15 Exposures at NPL Monoenergetic sources Devices exposed to primary standards fields provided by NPL. Six neutron energies used: 0.033, 0.144, 0.565, 3.5, 5 and 16.5 MeV Background corrections made by repeating measurements with shadow cone Neutron fluences derived using a long counter, uncertainties of <~5 % 5.5 Relative H *(10) Response GNU, 0, Measured GNU, 45, Measured GNU, 0, MCNP HSREM, 0, Measured HSREM, 0, MCNP (All data normalized to respective 3.5 MeV results) Energy (MeV) 15 The GNU and HSREM neutron survey instruments

16 Exposures at NPL Thermalized neutron field Devices exposed to thermal neutrons using NPL s graphite pile facility Corrections for epi-cadmium neutrons made by repeating all measurements with and without a cadmium sheet covering the thermal column GNU exposed from below and 45, HSREM at 0 Device Orientation Rel. Response Sensitivity (nsv -1 ) HSREM ± 0.02 GNU ± 0.02 GNU Bottom ± The GNU and HSREM neutron survey instruments

17 Exposures at CERN Cosmic Ray Fields GNU was taken to CERN as part of EURADOS WG11 work programme, for exposures at CERF facility Produces very high energy neutrons that simulate conditions present in cosmic ray fields at aviation altitudes 120 GeV/c beam of positive hadrons stopped in copper target, with secondary particles moderated by large concrete shield overhead Instruments placed on top of shield in one of 16 squares (50 50 cm 2 ) Neutron fields at reference locations differ, but all possess a broad energy distribution peaked at the 100 MeV energy scale Fields well-characterized by FLUKA modelling 17 The GNU and HSREM neutron survey instruments

18 Exposures at CERN Cosmic Ray Fields GNU measurements made at positions T6, T7, T9, T11 and T15 ( 188 to 270 psv cm 2 ) All dose responses accurate to within 20% Measured and modelled relative data agree well 18 The GNU and HSREM neutron survey instruments

19 Summary / Conclusions Two novel neutron survey instrument recently developed GNU: Excellent energy dependence of response from mev to TeV HSREM: Sensitive and (comparatively) lightweight Designs of instruments optimized by extensive Monte Carlo modelling Prototypes built and tested in various fields: 241 Am-Be distribution Monoenergetic neutrons from 33 kev to 16.5 MeV Thermalized field Simple simulated workplace field 60 Co gamma source Cosmic ray fields (for GNU) Prototypes performed as predicted by MCNP expect same at other energies No deleterious angular dependence of response found for the GNU Performance checked against IEC 61005:2014 and ANSI N42.17A:2003 standards 19 The GNU and HSREM neutron survey instruments

20 Summary / Conclusions Can now begin full commercial exploitation of the instruments 20 The GNU and HSREM neutron survey instruments

21 Acknowledgements Special thanks to: Colleagues within PHE Radiation Metrology Group David Thomas, Paula Salvador Castineira and co-workers in the NPL Neutron Metrology Group Marco Silari, Giacomo Paolo Manessi and the rest of the CERF team Christoph Hoffmann at FST SensorTechnik The InterAct Proof of Concept fund 21 The GNU and HSREM neutron survey instruments