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 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 3He 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-6LiF 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 3He 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, 1972Davidson 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 252Cf 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, YorkshireHolborn Underground station, LondonBoulby 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 p1885-1899 (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 robuststable over many years in harsh environmentstransportableminimal health & safety implicationsoperable by Front Line Officers

must use easily available materials

Improvements to existing design

Scintillator: problems with ZnSopacity

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

6LiF 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 improvementsPNL-Leeds detectors were optimized for volume configuration

(maximum efficiency, lowest background…)

Security applications need to optimize effective area per unit cost

€

efficiency × area

price

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 theother 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 transmissionthrough scintillating layer

=> can probably work with 250micron layers

Layer uniformity checksExamination 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 discriminationOriginal 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 0.512m2

Experimental trials with 252Cf and gamma sources over next 6 months

Research funded by

UK Home Office Scientific Development Branch

AcknowledgementsJohn Barton – arXiv:astro-ph/0305155

Graeme Hitchen – SUEL, University of Sheffield

Ed Marsden – Corus Redeem, Rotherham

Dick Lacey – HOSDB

Questions?

e-mail j.e.mcmillan@sheffield.ac.uk