Thermal neutron scintillators using unenriched boron nitride and zinc sulphide. John McMillan ANSRI Jan 2015

Thermal neutron scintillators using unenriched boron nitride and zinc sulphide John McMillan ANSRI Jan 2015 Neutron detectors 1970-2008 3He prop...
Author: Rodger Allen
0 downloads 2 Views 5MB Size
Thermal neutron scintillators using unenriched boron nitride and zinc sulphide

John McMillan

ANSRI Jan 2015

Neutron detectors 1970-2008 3He

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 3He 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-6LiF 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 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, 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 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 improvements PNL-Leeds detectors were optimized for volume configuration (maximum efficiency, lowest background…) Portal 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 the

other four to double

the area.

Optical optimization Redesign optical configuration for planar detector New waveshifting materials and techniques

Choice of thickness



Average pulse height (mV)

800

600

400

200

0 0

50

100 150 Thickness of layer (µm)

200

250

Pulsed LED light, 460nm, shone right through layer

Neutron capture efficiency cumulative capture efficiency

0.4

0.3

0.2

0.1

0.0 0

100

200 300 layers, each 1µm thick

400

500

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"





500

400



BN 241

Am



counts

300

200 6

LiF

100

0 0

50

100

150

200

bin number

Pulse height distributions in ZnS(Ag) screens



Light output from ZnS(Ag) screens 100

241 6



Light output (A.U.)



Am

LiF

80

60 BN 40

20

0 0

1

2

3 Energy (MeV)

4

5

6

BN gives ~0.43 of the light produced by 6LiFmaterial.



BN costs ~1000 less than 6LiF !!

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. Paper on layer production almost ready to go. Paper on detectors started Paper on discrimination started.

Research funded by UK Home Office Scientific Development Branch and STFC

Questions? e-mail [email protected]

Suggest Documents