Experimental Verification of a Hand Held Electronically Collimated Radiation Detector and Compton Imaging
System
We are developing a hand held Compton-scatter gamma ray detector, direction localization and imaging. Feasibility results
from a partial prototype, using 5 mm thick cadmium zinc telluride (CZT) modules, are presented and compared to a simulation
model. The simulation model is being used for design studies of the detector system. The model allows for estimation of hole and
electron mobility from detector output alone and reasonably duplicates average detector energy response. Angle studies using the
model closely approximate the experimental data. Representative experimental results are presented as simple backprojection
images, angle scatter images and plots of median and mode average angles compared to simulation results.
Background
A hand held, electronically-collimated radiation detector (ECRD) is under development for locating gamma radiation sources
and for interoperative localization of sentinel lymph nodes. The design goals include compact size, portability and robustness,
wide field of view, and efficiency and accuracy in localization. The ECRD front end comprises an array of cadmium-zinc-telluride
(CZT) detectors, which provide position-sensitive gamma-ray detection with good energy resolution and room temperature
operation. The ECRD design provides high sensitivity to the presence of radiation because it lacks a physical collimator, and
simultaneously provides directional information by processing source photons that undergo Compton scattering in the detector to
calculate direction to the radiation source (electronic collimation). Unlike typical Compton cameras, the ECRD is concerned only
with identifying the direction to a radiation source in space, which is a less formidable computational task than mapping its 3D
activity distribution. Development of a directional algorithm has proceeded separately from the experimental work reported here
[2].
Methods and Materials
The prototype comprises four detector modules, mounted in pairs to form an L shape and modified to provide energy and
position information for coincident gamma ray interactions. Each monolithic CZT detector modules is 5 mm thick by 38 mm by
38 mm. Each module has a 16 by 16 array of 2.4 mm pitch pixels and ASICs for preamplification and multiple hit detection. The
current ASICs provide an output signal from only one pixel per module per gamma ray interaction. Additional amplification and
signal processing are performed by an Orbotech electronics boards and the position and energy signals are fed into a National
Instruments digital acquisition system. The device was placed in a light tight box.
The detector was energy calibrated with five low energy sources, Ba-133, Cd-109, Co-57, I-123, Tc-99m. Coincidence data
was collected from Na-22, Cs-137 and C-60 point sources and analyzed for sensitivity, energy deposition, measured scatter angle.
Because the prototype and source geometry is known, reported angles can easily be compared to assumed true angles. The
calculated Compton cones were also backprojected to visually demonstrate the potential for source location.
For our simulations we developed a model for charge collection in thick CZT. Thick detector CZT simulations must take into
account charge trapping, sharing and noise which reduce signal amplitude and broaden recorded energy distributions. Charge
trapping in the material reduces the charge that will be induced on collecting electrodes. Charge trapping, like ordinary charge
induction, is a function of depth of interaction. Charges produced between pixels can induce charges on more than one electrode;.
this kind of charge sharing is a function of depth and planar location. Statistical and electronics noise contribute further to
variations in the measured signals.
An infinite plane Hecht's relation was used to model charge trapping and a linear approximation was used for charge sharing
between pixels. Hecht's relation describes charge collection as a function of depth in a planar detector. The charges induced in the
detector's electrode are a function of the distance traveled, described by the Shockley-Ramo theorem. Electrons have greater
mobility than holes in CZT; typical values are 1350 cm2/V-s for electrons and 120 cm2/V-s for holes. Values of average electron
life time, have been reported between 100 nanoseconds and several micro seconds. Hole life
times vary between 50 and 300 nanoseconds[1]. Simulation results were compared to data
to select simulation parameters that resulted in average energy response comparable to the
calibration data was achieved. Angle and energy coincidence data from the prototype were
then compared for the high energy sources to the corresponding output from the simulation
model. Median and mode angle response were also determined.
Results
From the detector calibration, most pixels responded linearly; noisy or nonlinear were
disabled. The detector operates linearly from 80 keV to 200 keV. Sensitivity for Ba-133,
Figure 1: Prototype and simulation
Cd-109, Cs-137, Co-57, Co-60 and Na-22 was roughly equivalent to a common 1x1"
energy histograms.
NaI(Tl) scintillator at one meter. Calibrated energy histograms for Ba-133, Cd-109, Co-57,
I-123 and Tc-99m comparing the simulation model to the detector is shown in .
Typical simulation and prototype Compton scatter images are shown for Na-22 and Cs-137, Figure 2. These images plot each
measured angle from 0 to 90 degrees determined by energy response (y-axis) as a function of the angle determined by source and
pixel locations (x-axis), which is considered the true angle. The horizontal band of data represents the detector's angle limits, the
vertical band represents the detector's energy limits. Within this band, a perfect detector would show a unit slope line from corner
to corner. The simulation model shows this with broadening from detector tailing. The Na-22 simulation shows the consequence of
using the wrong energy for angle calculations, a line of incorrect response around the 1274 keV photon when 511 keV was
assumed. When the source and Compton angles are aligned for the detector, this line is distinct but blurred by tailing and
surrounded by scatter and electronic noise. The histogram of angular errors (the difference between the true and reported angles) is
typically peaked at zero degrees and has a FWHM of 25 degrees.
Plots of average and median angle for the detector and simulation model are compared in Figure
3. The results show an offset between the simulation results and the experimental data. This offset
may be partially a consequence of the simple physics model used in the simulation. We are currently
comparing the prototype data to the full GEANT4 model of the detector system that is being used for
design studies of the ECRD [3]. The simulation is plotted with two standard deviations and the data
is plotted on top. Different levels of collimation are are also shown for Na-22 averages. When the
Compton scatter interaction deposits energy in the detector's range, the most probable angle reported
by the detector closely tracks the actual scatter angle. Adding more detector modules and improving
the detector's energy response will increase the range and quality of measurable scatter angles.
Conclusions
Backprojections of the data clearly show a bright spot at the source location when the source
location is near the center of the field of view. For sources far off axis, the ability to identify the
source location worsens. This is due to the partial detector geometry implemented in the prototype as
well as the energy response limitations of the current detector modules. Implementing a full detector
geometry with improved position sensitive CZT detectors is a key step in developing the ECRD
system.
Figure 2: Cs-137 and Na22 scatter images,
simulation left, prototype
right.
References
[1] Knoll, Radiation Detection and Measurement, Third Edition, Wiley 2005.
[2] Lackie, Matthews et al., “An Electronically-collimated gamma-ray detector for localization of radiation sources”, IEEE NSSMIC 2006, session CF2-5.
[3] Smith, Matthews, et al., “An Electronically-collimated gamma-ray detector for localization of radiation sources”, IEEE NSSMIC 2006, poster N14-17.
Figure 3: Average and most probable angle comparison.