Computational study to enhance threat detection of radiation portal monitors:

Transcription

1 Computational study to enhance threat detection of radiation portal monitors: shielding, moderator, and collimator optimisation Mark Gilbert, Zamir Ghani, and Lee Packer United Kingdom Atomic Energy Authority October 20, 2015 This work was performed within the CLASP scheme grant entitled Optimising the neutron environment of Radiation Portal Monitors (RPMs). The United Kingdom Atomic Energy Authority was awarded funding through the United Kingdom Science and Technology Facilities Council STFC (Grant no. STK000152/1) in Neutron Users club 2015 CCFE is the fusion research arm of the United Kingdom Atomic Energy Authority

2 Introduction Radiation portal monitors (RPMs): used to identify smuggled fissile material in vehicles detect emissions of neutrons and gamma rays often sited at traffic choke points (ports, airports, etc) without consideration of local environment simple and judicious use of shielding and collimation may be able to improve signal-to-noise ratio thus reducing detection limits A joint CLASP project with Glasgow and Sheffield universities is aimed at demonstrating the possible simple and cost-effective improvements to the detector design John McMillan Val O Shea Challenge Led Applied Systems Programme, funded by STFC 2/18

3 RPM schematic Possible enhanced design (from original CLASP proposal): p Atmosphere π + π 0 n µ+ π µ γ n γ Collimator n, γ shielding Active detector volume n n n n Road crumb loaded with neutron absorbing material 2 Range of roadway materials 3/18

4 Talk outline Modelling approach Preliminary studies can simulations say anything useful about detector design? Detector-design parameter studies Computational optimisation 4/18

5 Modelling Approach Monte Carlo simulations performed with MCNP generic neutron RPM detector based on 3 He tubes embedded in polyethylene ( the base detector ) various different material, shielding, and collimation configurations can be added Calculate response (for each configuration) to: (1) bare 252 Cf neutron threat source moving at the expected 2 m s 1 velocity of vehicle motion (2) background source caused by cosmic-ray interactions with the atmosphere represented by a planar surface source of neutrons cos 2 θ directional distribution energy distribution calculated using EXPACS response multiplication factor to produce typical 100 n m 2 s 1 in between detectors 5/18 ver developed by T. Sato, Japan Atomic Energy Agency (2006) T. Sato et al. Radiat. Res. 170 (2008) R.T. Kouzes et al., Nucl. Instr. Meth. Phys. Res. A 584 (2008)

6 Preliminary Study Aimed at demonstrating feasibility of both design optimisation and modelling approach Response of the basic detector was compared to: configuration with 5 cm of boronated-polyethylene shielding around detector (on all sides except front face) configuration with shielding plus a simple, 5 cm thick boronated-polyethylene collimator added to the front of the detector (basic design with equally spaced cylindrical holes) Collimator holes moving threat Top view 166 cm 50 3 He tubes 2.3 cm radius collimation holes in an hexagonal arrangement 2.8 m 0 (cm) /18

7 Detector response (counts s -1 ) Preliminary study results (1) Background response as a function of ground material no shielding 5cm shielding 5cm shielding+collimator asphalt boron boron10 portland seawater steel water Ground material Asphalt is a good average most likely material in existing systems assumed in all subsequent simulations The shielding and collimator configuration reduces background by factor of 6 compared to unshielded system High neutron-absorbing materials, such as 10 B silicone rubber, can reduce response (compared to asphalt) even further but require some infrastructure changes at existing RPM locations 7/18

8 Cumulative response (counts) Preliminary study results (2) Integration of source response over 2 s measurement time cm shielding no shielding 5cm shielding+collimator alarm level - no shielding alarm level - 5cm shielding alarm level - 5cm shielding+collimator Time (s) The unshielded detector would not alarm in this case Alarm levels set using background response in equivalent time & Currie limit: B B Assumed source strength of n s 1 Even this non-optimised system demonstrated the potential benefit of simple modifications to detectors (and validates computational method) 8/18 Note that the basic shielding increases source response (due to back scattering)

9 Optimisation study Main study to optimise design of collimator and shielding using base model with boronated-polyethylene shielding and collimator Optimise for: collimator depth collimation hole profile & packing fraction extra solid shielding in front of detector Best configuration determined by maximum threat sensitivity: minimum source detectable B B 2R B is background counts per 2 s acquisition 2R is counts during 2 s for 252 Cf threat strength of 1 n s 1 Problem symmetry was utilised to reduce computation time reflecting planes so that only one detector was necessary moving source modelled as a line (i.e. only one simulation of 10 7 neutrons per configuration) 9/18 Note that unshielded+uncollimated system had a minimum detectable source response of approx n/s

10 Minimum alarmed threat vs collimator depth (δ) 7 8 cm is the optimal Investigations also show that an extra polyethylene shield of thickness κ can also help Results collimator depth Around 2 cm extra 1.5E+03 is most effective and then the original 5 cm thick collimator is 1.0E+03 optimal from threat Top view δ minimum source alarmed (n/s) 3.0E E E+03 original collimator depth δ (cm) 10/18 this 2 cm is used in all subsequent results presented here. Note that unshielded+uncollimated system had a minimum threat response of approx n/s

11 Minimum alarmed threat vs collimator depth (δ) 7 8 cm is the optimal Investigations also show that an extra polyethylene shield of thickness κ can also help κ Results collimator depth Around 2 cm extra 1.5E+03 is most effective and then the original 5 cm thick collimator is 1.0E+03 optimal from threat Top view δ minimum source alarmed (n/s) 3.0E E E+03 original extra shield depth κ 0 cm 1 cm 2 cm 5 cm 10 cm collimator depth δ (cm) 10/18 this 2 cm is used in all subsequent results presented here. Note that unshielded+uncollimated system had a minimum threat response of approx n/s

12 Influence of conical profile (cf. original cylinder) With a shallow, 2 cm collimator, the (marginally) optimal profile is strongly conical (i.e. more material) But for thicker collimators the original cylindrical profile remains optimal No real benefit of conical compared to (cheaper) cylindrical holes Results collimator profile minimum source alarmed (n/s) 3.0E E E E E+03 δ o r i r collimator depth δ 5 cm 10 cm 2.5 cm 6 cm 7.5 cm ratio of cone radii (i r /o r ) 11/18 with extra 2 cm of polyethylene shielding

13 Variation with cylindrical hole packing density Again, results determined by shielding balance For original, 5 cm-deep collimator wide holes are favoured and the original 70% packing ratio is optimal Face view Results collimator packing minimum source alarmed (n/s) 3.0E E E E E+03 71% open collimator depth δ 5 cm 10 cm 2.5 cm 6 cm 7.5 cm % open on collimator surface original 12/18 Probably no cost implication (for packing ratio) Might be a marginal cost benefit associated with having a thinner (cheaper) collimator and smaller holes with no change in detector sensitivity with extra 2 cm of polyethylene shielding

14 Variation with cylindrical hole packing density Again, results determined by shielding balance For a thinner, 2.5 cm collimator the optimisation produces extra shielding, in the form of narrower holes Face view Results collimator packing minimum source alarmed (n/s) 3.0E E E E E+03 30% open collimator depth δ 5 cm 10 cm 2.5 cm 6 cm 7.5 cm % open on collimator surface original 12/18 Probably no cost implication (for packing ratio) Might be a marginal cost benefit associated with having a thinner (cheaper) collimator and smaller holes with no change in detector sensitivity with extra 2 cm of polyethylene shielding

15 Optimisation summary Optimised configuration: an extra 2 cm polyethylene moderator in front of the bare detector 5 cm deep collimator with cylindrical holes occupying 70-80% of surface facing threat more than a factor of 2 improvement over bare, unmodified detector 13/18

16 γ 12cm β α Extensions detector unit optimisation γ to threat 4cm rear shielding β RRx RRx Cavity width γ (cm) 8cm rear shielding β Optimisation of air gap and polyethylene moderator around helium tubes in detector base units Optimised configuration: front moderator α = 8 cm; rear moderator β = 10 cm; air-gap thickness γ = 2 cm RRx RRx10 6 6cm rear shielding β Cavity width γ (cm) 10cm rear shielding β Cavity width γ (cm) Cavity width γ (cm) front moderator α 4cm 6cm 8cm 10cm 14/18

17 Extensions full optimisation revisited Re-optimisation using optimised 2 cm air-gap Additional rear shielding thickness λ optimisation suggests no upper-limit (but no significant improvement beyond 20 cm) κ λ δ New optimum for a rear polyethylene shield of λ = 40 cm: an extra κ = 5 cm polyethylene moderator in front of the detector 5 cm deep collimator with cylindrical holes (as before) occupying % of surface facing threat Producing an extra 15% improvement compared to the optimised configuration with no air gap 15/18

18 Published results Nuclear Instruments and Methods in Physics Research A 795 (2015) Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research A journal homepage: Optimising the neutron environment of Radiation Portal Monitors: A computational study Mark R. Gilbert a,n, Zamir Ghani a, John E. McMillan b, Lee W. Packer a a United Kingdom Atomic Energy Authority, Culham Science Centre, Abingdon OX14 3DB, UK b Department of Physics and Astronomy, University of Sheffield, Hicks building, Hounsfield Road, Sheffield S3 7RH, UK article info abstract Article history: Efficient and reliable detection of radiological or nuclear threats is a crucial part of national and Received 25 March 2015 international efforts to prevent terrorist activities. Radiation Portal Monitors (RPMs), which are deployed Received in revised form worldwide, are intended to interdict smuggled fissile material by detecting emissions of neutrons and 8 May 2015 gamma rays. However, considering the range and variety of threat sources, vehicular and shielding Accepted 24 May 2015 scenarios, and that only a small signature is present, it is important that the design of the RPMs allows Available online 3 June 2015 these signatures to be accurately differentiated from the environmental background. Using Monte-Carlo Keywords: neutron-transport simulations of a model 3 He detector system we have conducted a parameter study to Radiation Portal Monitors identify the optimum combination of detector shielding, moderation, and collimation that maximises Nuclear Security the sensitivity of neutron-sensitive RPMs. These structures, which could be simply and cost-effectively Neutron Detection 3 added to existing RPMs, can improve the detector response by more than a factor of two relative to an He detectors unmodified, bare design. Furthermore, optimisation of the air gap surrounding the helium tubes also improves detector efficiency. & 2015 The Authors. Published by Elsevier B.V. All rights reserved. 1. Introduction and locations. Measures to reduce the radiological and nuclear threat are many-faceted, but an important component includes In recent years, concerns about the threat of nuclear and the ability to detect, by direct means, attempts to illicitly transport radiological terrorism and of other malicious acts involving such radiological or nuclear material. Detection systems broadly fall materials have been raised at the international level [1]. Coordinated steps are being taken among states to minimise the threat, from the radioactive decay of the nuclear material itself; and into two categories: passive, which rely on detecting the emissions but over the next few decades there will be an inevitable global active, where an external stimulus is used to induce an enhanced expansion in nuclear technologies and energy. Whilst this will response from the threat. Here we focus on passive detection bring significant benefit to society, it will nonetheless result in systems for neutron emitting threats, such as weapons grade increased challenges from a nuclear security perspective. plutonium [5], mixed oxide (MOX) or spent fuel, or isotopic A significant number of incidents of trafficking of nuclear sources such as 241 Am Be, 210 Po Be, or 252 Cf. These systems, materials have been recorded in the IAEA Incident and Trafficking known as Radiation Portal Monitors (RPMs), measure the threat Database (ITDB) [2], including material which can be used in response from vehicles as they pass between two detector panels. radiological devices, and conceivably in nuclear weapons. For Commercially available RPM designs typically use gamma and/ example, a 2013 incident reported by the IAEA [3] included details or neutron detection technologies to scan vehicles, cargo and of a group of traffickers convicted in Moldova for attempting to sell people. They have been deployed extensively at front-line sites quantities of uranium and plutonium that had been transported in around the world, including ports, airports and traffic choke shielded lead canisters in an attempt to evade detection systems. points. Neutron detection systems are, for example, 6 Li-based Furthermore, evidence recently heard in the UK [4] refers to the scintillating glass fibers, 10 B-based pressurised gas tubes, or 3 He significant illicit transportation of 210 Po through European airports tubes. Similarly, gamma detection systems are based on, for and cities, which led to significant contamination of many people example, polyvinyltoluene (PVT) plastic scintillators, sodium iodide NaI(Tl) detectors, cadmium-zinc-telluride (CZT) or, at greater expense, high-purity germanium (HPGe) crystals. Here n Corresponding author. we evaluate neutron detecting RPMs by considering 3 He pressurised gas tubes as the detection technology, which are address: mark.gilbert@ccfe.ac.uk (M.R. Gilbert). used Complete simulation study recently published in NIMA 795 (2015) more than 3000 separate MCNP simulations /& 2015 The Authors. Published by Elsevier B.V. All rights reserved. 16/18

19 Summary A combination of shielding and collimation can improve the sensitivity of RPMs to radiological and nuclear threats hidden in vehicles Optimisation studies performed using Monte Carlo simulations of model systems indicate that additional attenuation in front of the detector and shielding behind it in the form of solid polyethylene can reduce the minimum detectable threat in conjunction with a 5 cm boron-doped collimator The detector sensitivity is less influenced by the collimator hole profile or packing density (under the optimal shielding/moderation conditions) Efficiency can be improved still further by the inclusion of an air gap immediately surrounding the helium tanks (to promote additional scattering and reflection of neutrons) 17/18

20 Future The modelling approach applied in this study is suitable for other nuclear detector systems Could be improved by developing a fully-automated code to optimise a system of variables for a given MCNP model & response function to handle a larger number of dependent variables would produce a more precise optimum parameter-set The RPM task could be extended to consider changes in conditions: alternative detector (e.g. 6 Li-based scintillators) different threat types ( 241 Am-Be, 137 Cs, 60 Co, shielded sources, etc. ) Experimental testing of mock-ups of the optimised RPM system would be desirable to check both the findings and the validity of the computational approach. 18/18