Detector Research at the University of Surrey
Dr Paul Sellin
Centre for Nuclear and Radiation Physics
Department of Physics
University of Surrey, Guildford
Paul Sellin
High-Z materials for room temperature gamma ray detectors
CdTe/CdZnTeX-ray
are the
most
mature efficiency
high-Z bulk materials for X-ray and gamma ray detectors:
photon
detection
Detection Efficiency (%)
100
Calculated for 500m thick material
single crystal HgI2
Si
GaAs
CdTe
HgI2
TlBr
10
www.realtimeradiography.com
1
50
100
150
200
250
300
350
400
450
500
Photon energy (keV)
CdTe and CdZnTe offer good spectroscopic performance
for X-rays and gammas
For higher photon energies very high-Z materials such as
mercuiric iodide (HgI2) are actively being developed
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 r=3x1011Wcm, and te=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
Paul Sellin
Ref: M. Pomorski et al, phys stat sol a 203 (2006) 3152-3160
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
Photocurrent (nA)
-50
-40
-30
-20
-10
0
0
0
-1
-1
-2
-2
-3
-3
-4
-4
No Dose
0.99 mGy/s
1.92 mGy/s
3.79 mGy/s
7.52 mGy/s
13.16 mGy/s
18.5 mGy/s
-5
-6
-5
-6
-7
-7
-50
-40
-30
-20
-10
Applied Voltage (V)
0
Corrected Photocurrent (nA)
Photocurrent vs dose rate gives the
detector sensitivity. For a 10m thick device:
~0.06 nC/mGy @ -10V
~0.24 nC/mGy @ -50V
Photocurrent vs bias voltage shows
an approximately linear increase
Dark current is ~0.8 nA @ -50V
4
-50V
3
2
1
-10V
0
0
4
8
12 Paul
16 Sellin 20
X-ray
Dose
Rate
(mGy/s)
X-ray
Dose
(mGy/s)
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
Paul Sellin