Paul Sellin

Detector Research at the University of Surrey

Dr Paul Sellin

Centre for Nuclear and Radiation PhysicsDepartment of Physics

University of Surrey, Guildford

Paul Sellin

CdTe/CdZnTe are the most mature high-Z bulk materials for X-ray and gamma ray detectors:X-ray photon detection efficiency

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

High-Z materials for room temperature gamma ray detectors

single crystal HgI2

CdTe and CdZnTe offer good spectroscopic performance for X-rays and gammasFor higher photon energies very high-Z materials such as mercuiric iodide (HgI2) are actively being developed

www.realtimeradiography.com

Paul Sellin

Cadmium Zinc Telluride for X-ray/gamma spectroscopy

CdZnTe material quality has improved recently, with various new suppliers in the US/Canada, Europe and Japan:

wafers of large single-crystal areas are available, with excellent charge transport High resistivity =3x1011cm, and e=1.8x10-2 cm2/V

4x4 pixellated devices have shown very good resolution 1.35% FWHM at 662 keV

Paul Sellin

‘Hexitec’ Basic Technology project

Surrey is one of the major members of the recent Hexitec Basic Technology project – a £3M 4 year project

Project aim – development of CdZnTe pixel detectors for X-ray imaging

Manchester – detector applications Durham – CdZnTe material growth Surrey – Material and detector characterisation and prototyping CCLRC RAL – Pixel detector fabrication

Paul Sellin

Synthetic diamond for detector applications

Single-crystal natural diamonds have been studied in the past for detector applications – excellent electronic properties are seen in a tiny number of gem stones.

P. Bergonzo et al, Dia Rel Mat 10 (2001) 631-638

Paul Sellin

Single-crystal CVD diamond detectors

Specialist applications of diamond detectors: as tissue-equivalent rad-hard detectors, eg megavoltage therapy

beams, neutrons detectors for very high temperature, high radiation environments

True single-crystal material removes charge trapping associated with grain boundaries: 100% CCE demonstrated from alpha particles Extreme radiation hardness High mobility very fast signals (~10ps timing resolution)

silicon detector diamond detector

Ref: M. Pomorski et al, phys stat sol a 203 (2006) 3152-3160

Paul Sellin

Semiconducting polymer – a new radiation detector material?

Various candidate semiconducting polymers are of interest for radiation detection applications, eg:

Poly-acetylene PPV (poly-phenylene-vinylene)

These materials have the following properties: A ‘band gap’ energy low enough to give semiconducting properties

(eg. ~1.5 eV) High resistivity combined with reasonable charge transport:

Electron and hole mobilities are low Carrier concentrations are low (<1014cm-3) good depletion

thickness Sufficient radiation hardness

Potential Benefits: large area, low cost sensor technology fabrication onto flexible, complex geometry, substrates tissue equivalent for dosimetry applications

Paul Sellin

X-ray response from the PFO detector

X-ray response photocurrent was measured from the PFO device for dose rates up to 18.5 mGy/s

-50 -40 -30 -20 -10 0

Applied Vo ltage (V)

-7

-6

-5

-4

-3

-2

-1

0

Ph

oto

curr

ent

(nA

)

-7

-6

-5

-4

-3

-2

-1

0

No Dose0.99 mGy/s1.92 mGy/s3.79 mGy/s7.52 mGy/s13.16 mGy/s18.5 mGy/s

-50 -40 -30 -20 -10 0

Photocurrent vs bias voltage shows an approximately linear increaseDark current is ~0.8 nA @ -50V

Photocurrent vs dose rate gives the detector sensitivity. For a 10m thick device:~0.06 nC/mGy @ -10V~0.24 nC/mGy @ -50V

0 4 8 12 16 20X-ray Dose (mGy/s)

0

1

2

3

4

Co

rre

cte

d P

ho

toc

urr

en

t (n

A)

-50V

-10V

X-ray Dose Rate (mGy/s)

Paul Sellin

Other plastic dosimetry materials

Various organic and plastic bulk materials have been investigated since Fowler’s work in the 1950’s

Early measurements investigated X-ray induced photocurrents, and showed long time transients

Recent data at Surrey on newer material have studied dosimetry response of thick (~mm) plastic layers to 50 kV X-rays: signal-background ratio of ~500x ‘fast’ X-ray response of <100 ms

Paul Sellin

New MSc course in Radiation Detection and Instrumentation

A new MSc programme for September 2008 – exploring the operation, characterisation and development of radiation detectors and instrumentation

Application areas will include medical physics, dosimetry, particle physics, synchrotrons, space science and homeland security

Core programme modules: Radiation Physics and Laboratories Radiation Measurement Detector Instrumentation Interfacing and Signal Processing Imaging and Remote Sensing

Applications modules: Particle Physics Detector Technology Astronomy Detector Technology Radiation Protection Medical applications of Ionising Radiation

www.ph.surrey.ac.uk/msc/rdi