Neutron Detection for Security Applications John McMillan June 2010

Neutron detectors are not vertex detectors! Few pixels (1) Low fluxes, low rates, untriggered Low energies <10MeV Neutral particles (neutrons) No energy spectroscopic capability Extreme constraints on price and environment

Radiation Portal Monitors

Passive systems Typical RPMs use NaI or PVT for gammas combined with He-3 for neutrons Used for border security, control of sites where nuclear materials are used and screening of scrap metal prior to melting Neutron emitters are Uranium, Plutonium, AmBe {Active systems (Nuclear Carwashes) have similar detector requirements}

Helium-3The 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 He-3 is used in virtually all portal monitors in thermal neutron detectors. The (US) annual demand is estimated at 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

A requirement has emerged for 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

PNL- Leeds detectors Built for studying rare fission events "superheavy elements in nature" Rival groups had claimed positive results with 3 He tubes Low cost alternative based on previous work by Hornyak, Stedman,... Barton & Caines 8 detectors formed a neutron multiplicity detector Intended for low background underground sites with harsh environments Detectors now at the University of Sheffield or Boulby Mine

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 A, 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)

PNL-Leeds detectors described in "A novel neutron multiplicity detector using lithium fluoride and zinc sulphide scintillator" Barton, J.C., Hatton, C.J. & McMillan, J.E. J. Phys. G: Nucl. Part. Phys. 17 p (1991).

Thermal Neutron Detectors for Security 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 operable by Front Line Officers must use easily available materials

Improvements to existing design Scintillator: problems with ZnS opacity long decay time poor pulse height discrimination => charge or photon counting discriminators needs coating to avoid Radon progeny problems But thin layered scintillator gives gamma immunity But ZnS is the brightest low cost scintillator… => stick with ZnS

Improvements to existing design II 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…) Security 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

Improved production of layers Capture compound + Scintillator + Binder originally ~ 100 ± 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"

Light transmission through scintillating layer => can probably work with 250micron layers

Layer uniformity checks Examination using USB microscope

Wavelength shifting lightguides Low cost technique using BBQ dye loaded lacquer sprayed onto surface of clear acrylic sheet Tests with small samples suggest that this 1.2 times better than original Plexiglas GS2025 material W. Viehmann and R. Frost. Thin film waveshifter coatings for fluorescent radiation converters. NIM, 167:405–415, 1979.

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 Rebuilding original prototype cases with new materials to give four detectors covering 0.512m 2 Experimental trials with 252 Cf and gamma sources over next 6 months Research funded by UK Home Office Scientific Development Branch

Acknowledgements John Barton – arXiv:astro-ph/ Graeme Hitchen – SUEL, University of Sheffield Ed Marsden – Corus Redeem, Rotherham Dick Lacey – HOSDB Questions?