Thermal neutron scintillators using unenriched boron nitride and zinc sulfide John McMillan CLASP meeting, May2015
Neutron detectors 1970-2008 3 He proportional tubes were the industry standard Greatest efficiency achieved by slowing neutrons to thermal energy where interaction cross-sections are highest.
Helium-3 The shortage of He-3 is an international crisis. Due to the shortage of Helium-3, the US Dept of Homeland Security has put on hold all installations of radiation portal monitors at ports and borders as of Nov 2009. He-3 is used in virtually all portal monitors in thermal neutron detectors. The (US) annual demand is estimated at 65000 litres, There is essentially no source that can meet this demand. Supply is dwindling due to reduced use of tritium. Price has risen from $100 to $2000/litre in recent years. see "The 3 He Supply Problem", R.L.Kouzes, PNNL-18388
Thermal neutron detectors which don't use Helium-3 my PhD on the "Barton detectors" (Polytechnic of North London - University of Leeds) Layered ZnS- 6 LiF scintillators with wavelength shifter readout Pulse-counting neutron discrimination
Detector design
detector
Detector design
detector
Pulse counting discrimination in ZnS Pulses in time gate counted Caines P.J., M Phil Thesis, University of London, 1972 Davidson P.L. Rutherford Laboratory Report RL-77-106A, 1977
Features of the PNL-Leeds detectors Active volume 90 x 14.4 x 14.4cm 37% efficient for 252 Cf fission neutrons (8 detectors surrounding source) Totally insensitive to gammas and muons Robust, stable operation over many years over a range of temperatures in harsh environments Woodhead Railway Tunnel, Yorkshire Holborn Underground station, London Boulby Potash mine, Yorkshire (1km depth)
Thermal Neutron Detectors for Portal Applications large-area, square metres needed for portals unambiguous, good signal-to-noise ratio, high efficiency, low background real-time signal discrimination (not compute-intensive post processed) deployable reasonably robust stable over many years in harsh environments transportable minimal health & safety implications must use easily available materials
Improvements to existing design Choice of capture material 6 LiF is a controlled material and increasingly expensive Can we make worse (but very much cheaper) detectors using boron compounds? Capture cross-section higher - but releases less energy Can probably use natural rather than isotopically enriched material.
Usable thermal neutron capture reactions
abundances
Boron Nitride Need inert boron compound needs to be white or colourless easily available with controlled grain size Hexagonal boron nitride is available in ~5um platelets for cosmetic applications Cubic boron nitride is available in controlled sizes as an abrasive
Geometric improvements PNL-Leeds detectors were optimized for volume configuration (maximum efficiency, lowest background ) Portal applications need to optimize effective area per unit cost Smaller or less efficient detectors can still win if they are very much cheaper!
Geometric optimization MCNPX simulations Four best layers Contribute ~76% of the efficiency Can re-deploy the other four to double the area.
Optical optimization Redesign optical configuration for planar detector New waveshifting materials and techniques
Choice of thickness Pulsed LED light, 460nm, shone right through layer
Neutron capture efficiency Capture efficiency of BN-ZnS(Ag) screens. MCNPX simulations.
Improved production of layers Capture compound + Scintillator + Binder ~ 220 ± 10 microns Minimize wastage, avoid aggressive solvents Original detectors used spreading technique Considered spray painting, powder coating, serigraphy, ink-jet systems but went back to spreading.
Binder / solvent Kraton G1652; linear triblock copolymer based on styrene and ethylene/butylene (SEBS) dissolves in light mineral oils forming a gel Solvent "white spirit"
Pulse height distributions in ZnS(Ag) screens
Light output from ZnS(Ag) screens BN gives ~0.43 of the light produced by 6 LiFmaterial. BN costs ~1000 less than 6 LiF!!
Wavelength shifting lightguides Low cost technique using BBQ dye disperse dyed into surface of clear acrylic sheet Tests with small samples suggest that this 1.2 times better than original Plexiglas GS2025 material
Neutron discrimination Original pulse counting system used hard-wired TTL Depends on choice of capture compound, scintillator, waveshifter and optical collection New computer based monitoring system controlling hardware decisions
Current status Working with ET-Enterprises, Ludlum, Eljen to produce a commercial version. Presentations at Applications of Novel Scintillators for Research and Industry to be published in Journal of Physics: Conference Series (JPCS) Paper on layer production ready to go to NIMPR. Paper on detectors started Paper on discrimination started. Research funded by UK Home Office Scientific Development Branch and STFC
Optimising the neutron environment of Radiation Portal Monitors: a computational study Glasgow, Culham, Sheffield
The neutron detection component of Radiation Portal Monitors consisted of helium-3 proportional tubes in a box. Can the background be reduced by modifying the immediate environment of the monitor? Can threat recognition be improved by judicious shielding, collimation and attention to moderator thickness?
Modifying the environment Negligible background from environmental U,Th. Cosmic induced neutron background is observed to be reduced over water. Can we artificially reduce the background by surface modification of the roadway?
NO! "Footprint" of a neutron detector is ~ 150m radius. Desilets,D. & Zreda, M., Water Resources Research, 49, 3566 3575, 2013 It is not feasible to treat such a large area.
Improve RPM design... Mark R. Gilbert, Zamir Ghani & computational study using MCNPX Lee W. Packer, CCFE
Results, Yes! Providing shielding of 20 cm or more behind the detector reduced the response to the background while improving back-scattering of neutrons from the threat. Combined with a 5cm thick layer of moderator and a 5 cm thick collimator with an exposed surface about 70% occupied by cylindrical holes, a factor of ~2.6 improvement over the unmodified RPM. Paper submitted to Nucl. Instrum. Meth.
Future Can we get portal manufacturers to adopt these ideas? Can they be applied to non- 3 He detectors?
Questions? e-mail j.e.mcmillan@sheffield.ac.uk