semiconductor detectors for gamma/neutron security imaging · carried out by the detector physics...

Paul Sellin, Department of Physics

Semiconductor detectors for gamma/neutron security imaging

Paul SellinDepartment of Physics

University of SurreyGuildford, UK


Overview

A summary of two case studies related to security applications, carried out by the Detector Physics group at Surrey:

High Energy X-ray detection (DSTL) – PIPSS/IPS project Use of Mercuric Iodide (HgI2) for high energy X-ray detection:

300-500 keV bremmstrahlung Eg. X-ray backscatter using

a portable X-ray generator 7.5 MeV photons penetrate

300mm steel or 1m of Concrete

SiC for fast neutron detection (AWE) – STFC CASE studentship Investigation of Silicon Carbide (SiC) as a direct fast neutron

detector – for use in very intense neutron/gamma environment Need a detector that is gamma blind, and radiation hard SiC offers greater fast neutron scatter cross section than silicon


Some challenges in X-ray Spectroscopy

High resolution X-ray (& low energy gamma) spectroscopy is a powerful tool for identification of special nuclear materials, eg U/Pu ratios

Selectivity is maximised by using the highest resolution detector systems

Even HPGe detectors struggle to resolve Pu and U X-rays

Spectrum shows ultra-high resolution spectroscopy from a cryogenic Transition-Edge detector operating at 1K.

http://qdev.boulder.nist.gov/817.03/whatwedo/quantumsensors/quantumsensors.htm


Low energy gammas from Uranium

Typical spectrum from Highly Enriched Uranium (HEU)235U contains a characteristic gamma line at 185 keV, that is resolvable

with a HPGe detector, in the absence of Compton background from higher energy lines

Unfortunately, the backscatter peak from 137Cs is at ~180 keV…


Imaging of gamma rays

Gamma imaging methods are being adopted in security applications, eg. stand-off imaging, which borrow techniques from astronomy and medical imaging applications

Truck-mounted coded aperture imaging arrays are being developed in theUS which combine the imaging of NaI ‘pixel’ detectors with the gamma selectivity of high resolution Ge detectors:

L. J. Mitchell et al, Mobile Imaging and Spectroscopic Threat Identification (MISTI), paper SD1-2, IEEE Nuclear Science

Symposium Dresden 2008


Coded aperture imaging detectors

Key elements of the coded aperture imaging system:1. lead tiles for coded aperture mask2. Ge detector linear area for selectivity (gamma spectroscopy)3. 2D array of NaI detectors for maximum imaging sensitivity4. LN2 dewars for Ge detector cooling

L. J. Mitchell et al, Mobile Imaging and Spectroscopic Threat Identification (MISTI), paper SD1-2, IEEE Nuclear Science

Symposium Dresden 2008


Motivation for high-Z detector materialsMany groups are developing alternative ‘room temperature’ detectors: high detection efficiency at gamma ray energies high Z, ideally >32 room temperature operation wide band gap semiconductor large active volume high quality semiconductor

Photon energy (keV)50 100 150 200 250 300 350 400 450 500

Det

ectio

n E

ffici

ency

(%)

1

10

100

SiGaAsCdTeHgI2TlBr

Calculated for 500m thick material


Semiconductor pixel detectors

There has been a lot of recent activity in transferring bump-bonded pixel detector technology from Silicon to High-Z materials (eg CZT)

Semiconductor pixel detectors providemuch higher spatial resolution thanscintiallator-based imaging arrays

Various groups have developed CZT pixel detector demonstrators eg. 20x20 pixel array, 250m pitch

full spectroscopy 20x10x5 mm pixellated CZT with 300m pixel pitch

CZT

Epoxy on pad

Stud on pad

ASIC

The stud-epoxy flip chip bond process

Surrey and RAL are partners in the Basic Technology ‘HEXITEC’ project to develop fully spectroscopic CZT pixel arrays RAL now has capability for silver-loaded epoxy bump bonding at <300m pitch, which is required for fragile II-VI materials


PIPSS/IPS collaboration with DSTL

A 3 year PIPSS/IPS project has been carried out with DSTL to develop mercuric iodide as a semiconductor detector for high energy X-rays

The Fort Halstead group at DSTL have wide ranging interests in detection of ionising and non-ionising radiation

This project aimed to develop HgI2 as the most promising ‘very high-Z’ semiconductor detector materials:

Material Z density mobility EG resistivity Iodides: g/cm3 cm2/Vs eV cm HgI2 80/53 6.4 50 2.1 1013 PbI2 82/53 6.2 53 2.5 1012 BiI3 83/53 5.8 48 1.7 1012 Bromides: TlBr 81/35 7.6 75 2.7 1012 PbBr 82/35 - - 2.5-3.1 - Oxides: PbO 82/8 9.5 - 1.9 -


www.realtimeradiography.com

Mercuric Iodide – polycrystalline and single crystalHgI2 is the most promising Z~80 detector material – available as either

single crystal or as a thick-film Thick film HgI2 has been used as an X-ray photoconductor coating for

amorphous silicon X-ray imaging panels – direct coating of polycrystalline HgI2 using low temperature physical vapour deposition

Single crystal HgI2 has much better charge transport properties, and significantly higher energy resolution

HgI2 is a soft material which tends to sublime at room temperature, and special handling is required

single crystal HgI2


Digital pulse shaping with HgI2 detectors

HgI2 shows very low electron and hole mobility, eg ~40 cm2/Vs for electrons

The optimum pulse shaping time is therefore very long, eg. 30sDigital pulse processing methods are used, with software corrections:

137Cs gamma, before correction 137Cs gamma, after correction


High resolution spectroscopy using ‘electron only’electrodes

High quality gamma ray spectrum of single crystal HgI2 using ‘Frisch Grid’ electrodes

L.J. Meng et al, “Spectroscopic Performance of Thick HgI2Detectors”,TNS 53 (2006) 1706-1712


Escape peaks in HgI2All high-Z detectors suffer from photoelectric ‘escape peaks’ – satellite peaks

formed at a fixed energy below the main photopeak due to escape of K-shell X-rays from the detector material:

Hg K-shell energies:

K1 = 68.9 keV (46%)K2 = 70.8 keV (28%)

K1 = 80.2 keV (11%)K2 = 82.5 keV (4%)K3 = 79.8 keV (6%)


Development of a linear array

1.4 mm

0.4 m3.8 m

Schematic top view of the strip detector – 13 strips on 2mm pitch

Cross-sectional view of the HgI2 strip module, mounted on a PCB substrate

The final phase of the project has been to extend the individual HgI2 detector technology to a linear 1D array of ‘short’ strips

The aim is to produce an imaging prototype without the cost of a full 2D pixel array

Detector tests are in progress, eg. with a focussed X-ray or synchrotron beam


Strip detector demonstrator system

We are using the RAL multi-channel spectroscopy NUCAM ASIC which is optimised for low noise X-ray measurements

The motherboard can accommodate up to 4 plug-in detector boards, also suitable for use with CZT detectors


Inelastic neutron imaging

Inelastic neutron scatter imaging is a valuable inspection method for munitions and other special materials.

The emitted gamma ray is characteristic of the chemical species in the item, eg. distinguishing between high explosive, and chemicals such as blister agents and nerve gas

The techniques required a

portable neutron source such as

252Cf, and a high resolution gamma ray spectrometer


Neutron detection for security applications

Many different modalities are under investigation for neutron interrogation of nuclear materials and cargo

Fissile Interrogation using Gamma Rays from Oxygen (FIGARO) uses 6-7 MeV gamma rays for active interrogation of actinides

A fluorinated target (e.g. PTFE) is bombarded with 3-5 MeV protons. F(p, γ)O reaction generates gammas at energies of 6.13, 6.92 and 7.12 MeV At 6-7 MeV these photons generate fissions within actinides such as 239Pu, 235U and 238U Produces a clean delayed neutron signature following a gamma burst

BJ Micklich et al: “FIGARO: detecting nuclear materials using high energy gamma-rays”, Nucl Instr Meth Phys Res A 505, 466-469 (2003)


Carbon-based neutron detectors

New semiconductor detector technologies are being developed for fast neutron detection – optimised for high flux environments

The deposited energy from fast neutron elastic scattering is strongly dependent on the mass of the scattering atom:

NMAX EAAE 2)1(

4

For neutron interrogation techniques carbon-based detectors record ~2x more deposited energy than Si

The figure shows the maximum deposited energy for different detector materials, irradiated with 14 MeV neutrons


High resolution SiC alpha detectors

SiC is an interesting detector material for neutrons and alpha particlesEpitaxial SiC detectors show alpha particle resolution comparable to

silicon, and can operate at temperatures >100C (bandgap = 3.25 eV)

F. Ruddy et al, IEEE Trans Nucl Sci 53 (2006) 1713 -1718

Intrinsic resolution calculation:SiC: 3.06 keV @ 5486 keV (W = 7.7 eV, F = 0.04)Si: 3.47 keV @ 5486 keV (W = 3.6 eV, F = 0.11)High resolution operation requires thin entrance windows, eg. <50nm Schottky contacts


Silicon Carbide neutron detectors

We are currently investigating bulk SiC for use as a fast neutron detectorin collaboration with AWE

Fast neutron response is complex, due to elastic and inelastic reactionsEg. all the reactions above are possible with 14 MeV D-T neutrons

Although the absolute efficiency for fast neutron detection is low, SiC is very ‘gamma blind’ and radiation hard

F. Ruddy et al, IEEE Trans Nucl Sci 53 (2006) 1666-1670


12C(n,)9Be reaction in SiC

Example neutron spectrum from a SiC detector – the dominant peak is the 12C(n,) reaction groundstate at 8.30 MeV

90m thick detector


Conclusions

New developments in gamma and neutron detector technologies are required by security applications

There is considerable potential for knowledge transfer from the High Energy Physics and Nuclear Physics research areas to security problems

High-Z gamma detectors are required for good energy resolution & room temperature operation –CZT and HgI2 continue to develop as promising candidates

Carbon-based detectors such as diamond and SiCoffer good potential for neutron detection in security interrogation applications


Acknowledgements

Our thanks to STFC and to many collaborators who have contributed to this work:

University of SurreyPeter Bryant, Sandeep Chaudhuri,

Annika Lohstroh, Veeramani Perumal

RALPaul Seller, Matthew Veale, Matthew Wilson

DSTLIan Jupp

AWEMark Ellis, John O’Malley, Stacey Robertson

semiconductor detectors for gamma/neutron security imaging · carried out by the detector physics...

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