Measurements with the new PHE Neutron Survey Instrument

Transcription

1 Measurements with the new PHE Neutron Survey Instrument Neutron Users Club Meeting National Physical Laboratory 16 th October 2013 Jon Eakins, Rick Tanner and Luke Hager Centre for Radiation, Chemicals and Environmental hazards (CRCE) Public Health England (PHE), Chilton, OXON, UK

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 Measurements with the new PHE Neutron Survey Instrument

3 The Ideal Instrument Survey instruments typically calibrated in terms of H*(10) The ideal instrument is: Light and easy to use Well-characterized Flat angle- and energy-dependence of response across the range from thermal to MeV neutrons Appropriately sensitive 3 Measurements with the new PHE Neutron Survey Instrument

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 (aside from being heavy) exhibit one common problem... typically, response characteristics either: and / or under-respond at low energies (~ mev scale) and high energies (~ MeV scale) over-respond at intermediate energies (~ kev scale) 4 Measurements with the new PHE Neutron Survey Instrument

5 A Solution? Surround a central detector with moderating and attenuating layers, penetrated by air-filled guides* Moderator: Attenuator: Air Guides: Slows high-energy neutrons Suppresses response to intermediate-energy neutrons Channel thermalized neutrons to detector, raising low-energy response But, each of these features impacts other components Investigate and optimize design using Monte Carlo modelling * International patent PCT/EP2008/ Measurements with the new PHE Neutron Survey Instrument

6 Monte Carlo Optimization Monte Carlo code MCNP5 used for modelling Aim was to optimize: The radii of each of the layers The number, shape, size, depth and extension of the guides Other factors, e.g. Overall size Mass Ease of manufacture Likely cost Etc. 6 Measurements with the new PHE Neutron Survey Instrument

7 Final Design Central 3 He-filled SP9 PC Inner polyethylene moderator Borotron attenuator layer Outer polyethylene moderator 6 air-filled steel tubes (1cm radii) along cardinal axes 5 hemispherical plugs Al-faced built-up region to affix electronics 7 Measurements with the new PHE Neutron Survey Instrument

8 Modelled Response Modelled H*(10) response characteristic of final design of instrument in isotropic neutron field Compared against: Leake 2004 LB6411 Studsvik 2202D 8 Measurements with the new PHE Neutron Survey Instrument

9 Modelled Relative Response Modelled relative H*(10) response characteristic of final design of instrument in isotropic neutron field, normalized to response at 1 MeV Compared against: Leake 2004 LB6411 Studsvik 2202D 9 Measurements with the new PHE Neutron Survey Instrument

10 Prototype Construction Prototype of instrument manufactured by Sherwood Nutec Scientific Ltd. Prototype tested at both PHE and the National Physical Laboratory (NPL) 10 Measurements with the new PHE Neutron Survey Instrument

11 Calibration at PHE Initial calibration of the prototype performed using the 241 Am-Be neutron source available at PHE Corrections for source geometry and room scatter made using algorithm developed by PHE Metrology group Two orientations considered: 0, i.e. with the guide that is opposite the stem of the SP9 pointing directly at the source 45, i.e. with the device rotated horizontally by 1/8 th of a turn Also exposed instrument to intense 60 Co gamma source to check photon discrimination in the SP9 PC 11 Measurements with the new PHE Neutron Survey Instrument

12 Calibration at PHE With gain of 120 and operating voltage of 805 V, multichannel analysis of 241 Am-Be response confirms signal to be in the 1 to 5 V range o o < 1 V taken to be background > 5 V assumed spurious Find fluence response of (0.094) cm 2 giving H*(10) response of 2.4 (0.2) nsv -1 (c.f nsv -1 from MCNP5) 0 and 45 results agreed to < 1% For 60 Co, find fluence response of 6.70 (0.20) 10-5 cm 2 and H*(10) response of (0.003) nsv -1 Instrument responds negligibly to photons 12 Measurements with the new PHE Neutron Survey Instrument

13 Calibration at NPL Calibration of the prototype using the primary standard facilities offered by the National Physical Laboratory (NPL) Four ~monoenergetic exposures: 144 kev, 565 kev, 5 MeV and 16.5 MeV Both 0 and 45 orientations again considered Located in centre of large room to reduce scatter contribution Shadow cone correction also made Suitably long exposures performed to ensure good counting statistics Results compared against MCNP5 data at the four energies 13 Measurements with the new PHE Neutron Survey Instrument

14 Calibration at NPL H*(10) response rel. to mean Measured - 0 degs Measured - 45 degs MCNP - 0 degs MCNP - 45 degs Energy (MeV) NPL and MCNP datasets normalized to their respective average H*(10) responses over the four energies for the 0 exposures, i.e nsv -1 and 1.48 nsv Measurements with the new PHE Neutron Survey Instrument

15 Workplace Fields Important also to consider the response of the device in the types of field likely encountered in the workplace: Will contain lower energy neutrons than the calibrations, including thermalized components A number of difficulties associated with such fields: Dosimetric, e.g. limited data on their characterization Pragmatic, e.g. physically gaining access to those types of facility Simple solution: Use MCNP5 to design a set-up that produces a neutron energy-distribution a workplace environment Response of instrument in field then determined No substitute for real thing... plan proper series of field trials in the future 15 Measurements with the new PHE Neutron Survey Instrument

16 A Simple Workplace Field Aim: Generate a field with energy distribution resembling a workplace field using 241 Am-Be neutron source available at PHE Three stage approach: 1) Create realistic model of PHE Metrology Laboratory 2) Introduce water tanks between source and point-of-test to moderate fast 241 Am-Be neutrons 3) Characterize fluence-energy distribution that results Response of instrument in that field can then be determined and compared against measurements 16 Measurements with the new PHE Neutron Survey Instrument

17 1) Neutron Metrology Laboratory 17 Measurements with the new PHE Neutron Survey Instrument

18 1) Neutron Metrology Laboratory 10 0 Raw 241 AmBe Room Scattered N. Fluence per Ln(Bin Width) Energy (MeV) 18 Measurements with the new PHE Neutron Survey Instrument

19 2) Moderated 241 Am-Be Field Introduce water tanks to moderate the field Several configurations considered, with thicknesses up to 60 cm Final arrangement used 2 tanks of dimension cm 3 (which matched stock already available at PHE) 19 Measurements with the new PHE Neutron Survey Instrument

20 2) Moderated 241 Am-Be Field N. Fluence per Ln(Bin Width) Water Moderated EVIDOS: NF EVIDOS: BN Energy (MeV) Compares well with workplace fields characterized in EVIDOS project ( though angle dependency not considered ) 20 Measurements with the new PHE Neutron Survey Instrument

21 3) Field Characterization Determine fluence-to-h*(10) conversion coefficients for field in subsequent MCNP5 simulation Use moderated energy-distribution as a source in a plane-parallel exposure of the ICRU sphere Simulation run in neutron-photon mode, with the dose calculated in a 0.1 mm radius sphere centred at a depth of 10 mm Dose equivalent determined by summing photon kerma and neutron dose equivalent [f4:n fluence tally multiplied by kerma factors and the appropriate Q(L) relationship] N. Fluence per Ln(Bin Width) 10 0 Water Moderated EVIDOS: NF EVIDOS: BN Energy (MeV) Normalized to applied fluence, to give (3.5) psv cm 2 21 Measurements with the new PHE Neutron Survey Instrument

22 NSM in Artificial Workplace Field Neutron survey instrument added to Laboratory + Water Tank model Capture events in SP9 estimated using f4:n fluence tally modified by (n,p) reaction tally multiplier Find fluence response of (0.005) cm 2 Using above conversion coefficient, gives H*(10) response of 1.84 (0.07) nsv Measurements with the new PHE Neutron Survey Instrument

23 Measurements in Artificial Field Modelled configuration recreated in Metrology Laboratory Exposures performed for 1 hour, for both 0 and 45 orientations At 0, find fluence response of (0.023) cm 2 in the 1 to 5 V channel Corresponds to H*(10) response of 2.14 (0.23) nsv -1 Statistically irresolvable from the 45 results Agree with modelled response to within ~15 % 23 Measurements with the new PHE Neutron Survey Instrument

24 Summary Measured response data determined for a prototype design of neutron survey instrument, using facilities at PHE and NPL In general, results demonstrated good angular invariance, and agreed well with data obtained by Monte Carlo modelling Confidence raised in the accuracy of the response function predicted for the device Very simple artificial workplace field also developed and characterized, and performance of device assessed in it Reasonable agreement between measured and modelled results suggests device would behave as expected in real workplace fields Relative H*(10) Response 10 1 Isotropic Plane parallel (x) To be investigated further Energy (MeV) 24 Measurements with the new PHE Neutron Survey Instrument

25 Acknowledgements Special thanks to Tim Daniels, Jan McClure and colleagues in the PHE Metrology Group David Thomas and his team at NPL John Leake and Bob Mason at Sherwood Nutec Scientific Ltd. The InterAct Proof of Concept fund 25 Measurements with the new PHE Neutron Survey Instrument

26 26 Measurements with the new PHE Neutron Survey Instrument