High-Sensitivity Compton Imaging with Position-Sensitive Si and Ge Detectors Page: 4 of 6
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time order them through gamma-ray
tracking algorithms. These capabilities
in addition to using large-volume and
therefore efficient detectors provide the
basis for high sensitivity Compton
The concept of Compton imaging was
proposed many years ago [3,4],
however, only recent advances in
detector manufacture and compact
electronics allows us now to realize
instruments with competitive sensitivity.
Below, we will first briefly describe our
approach for Compton imaging and our
instrument. We will then present results
of angular resolution measurements we
obtained for a range of energies and
source locations as well as preliminary
results to image extended sources. We
have invested significant effort in
advanced data analysis ranging from
simple pulse-shape analysis to advanced
image reconstruction as well modeling
of a large variety of Compton imaging
implementations and scenarios which we
can't discuss here giving the scope of
The Si-Ge Compton imaging
We have performed extensive Monte-
Carlo simulations to determine a path to
demonstrate the potential capabilities of
Compton imaging for gamma-ray
energies ranging from about 150keV to
about 3 MeV, the range most interesting
for homeland security. Taking into
account fundamental physics and
technological limitations, such as the
intrinsic momentum of Compton
scattered electrons, or achievable
position and energy resolution of
specific detector materials and
implementations, we determined that a
semi-conductor detector system built of
low-Z and high-Z materials is currently
the most promising approach. Following
these calculations, we are developing a
hybrid Compton imager consisting of
low-Z Si detectors and higher-Z Ge
detectors. Figure 3 shows results of
Monte-Carlo simulations to estimate the
efficiency for different thicknesses of
each layer and gamma-ray energies. The
separation between each layer was 3 cm.
From this figure we can see that a Si
thickness of at least 20 mm and a Ge
thickness of 30 mm provide close to
maximum efficiency. For higher
energies the efficiency can be increased
with larger thickness, however, gamma
rays interacting only in Ge can be used
for efficient Compton imaging as well.
In addition, the drop in efficiency for
compared with the
energies is small
drop in collimator-
S, ay CI OSS[cml Ge 13yef Thck1 tcmj
Figure 1: Calculated coincident efficiencies for
hybrid Compton imager as a function of Si and
Ge detector thickness and incident gamma-ray
energy. Only gamma rays which interact exactly
once in the Si layer and deposit the full energy in
the instrument are considered.
As a result of these calculations we are
developing a hybrid Compton imaging
system which consists of two layers of
10 mm thick Si(Li) detectors in front of
two 15 mm thick HPGe detectors. We
are currently assembling such a system.
As a first step however, we have built
and demonstrated a hybrid system
consisting of one 64x64x10 mm3 Si(Li)
and one 76x76x11 mm3 HPGe detector.
The detectors mounted in individual
cryostats are shown in Figure 2. The
complete system including data
acquisition electronics and processing
and monitoring computer is mounted on
a cart to provide the ability to perform
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Vetter, K; Burks, M; Cork, C; Cunningham, M; Chivers, D; Hull, E et al. High-Sensitivity Compton Imaging with Position-Sensitive Si and Ge Detectors, article, May 19, 2006; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc834424/m1/4/: accessed September 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.