Presented at the WM’05 Conference, February 27 - March 3, 2005, Tucson, AZ |
Frazier L. Bronson CHP
Canberra Industries, Inc.
800 Research Parkway, Meriden CT 06450 USA
Valery Atrashkevich PhD
Verdansky Institute of Geochemistry and Analytical Chemistry
Moscow, Russia
ABSTRACT
Large NaI detectors are commonly used in gamma measurement systems where nuclide
identification and quantification is desired. These systems are used to measure
people, soil, drums, boxes, animals, and other things. For quantification,
an efficiency calibration must be performed, which becomes increasingly difficult
as the sources become large and complicated. Mathematical techniques can be
quite useful here, if they are easy enough to use, and accurate enough for
the application. A series of experiments was performed to show how accurately
the efficiency of large (3”x5”x16”) rectangular NaI detectors
can be computed with techniques that could be implemented within the commercially
available ISOCS mathematical efficiency calibration software. This software
assumes that the detector response function is cylindrically symmetric, which
certainly isn’t the case here. But, perhaps it is good enough for the
applications for which these large rectangular detectors are commonly used.
A series of mathematical experiments was performed comparing a normal 3x5x16
NaI detector with an optimized cylindrical NaI detector. The comparison was
done at 3 different energies: 100keV, 500keV and 2000keV. The first test was
done at 172 points from 1cm to 10m distance and radially out to 100 meters.
The second test was done for a series of discs, and the third test was done
for a series of lines. The final test was done to simulate a person standing
in a common whole body counter.
The tests revealed that if 20% accuracy is acceptable, most normal counting situations
can be adequately calibrated using this equivalent detector.
INTRODUCTION
Large NaI detectors are frequently used for high efficiency measurements of
low levels of gamma emitting radioactivity. The most common sizes of these
detectors are 4”x4”x16” and 3”x5”x16”.
The 4x4 detector was the initial replacement for large multi-tube cylindrical
detectors, and is commonly used for geological surveys, while the 3x5 detector
is normally used in Canberra systems, as it is similar in cost and background,
but approximately 25% higher in efficiency. At Canberra, our most common use
of these large rectangular detectors is in our Whole Body Counting systems.
The FastScan counter uses 2 of them, the Accuscan scanning bed counter can
use up to 4, and we have also built special in-vivo counters using up to 32
of them. Animal counters also used these large detectors as well as well as
counting systems for waste in drums, boxes, and trucks. These detectors are
also commonly used to survey large volumes of soil or building debris in D&D
and ER projects. This is commonly done in situ with fixed or moving
detectors, on conveyors with the sample moving passed a fixed detector, or
with the sample in large containers e.g. trucks. There is also much interest
in use of arrays of these large NaI detectors for Homeland Security portal
monitors for pedestrians and vehicles.
Large detectors are more sensitive than smaller detectors, and better able
to detect small levels of radioactivity. Traditionally large area gamma detectors
have been plastic scintillators. The advantage of NaI as compared to plastic
scintillators is that gamma spectroscopy can be performed to identify and to
quantify the radionuclides from the item or area being measured. But, before
this can be done, the system has to be calibrated for efficiency as a function
of gamma energy.
The normal way to perform efficiency calibrations is to construct a calibration
source that is the same physical size, and constructed in a radiologically
identical manner to properly simulate the item being measured. Then, a known
amount of radioactivity is distributed in the same manner as it will be in
the item to be measured. This must be done for a wide range of energies to
establish a calibration curve. For small water samples in a laboratory this
is relatively simple. But, as the samples get larger, or if they are not liquid,
the task becomes more complicated, more expensive, more time consuming, and
less accurate. For these situations mathematical efficiency calibrations are
especially attractive.
The code MCNP (Monte Carlo Neutron-Particle) is widely available for evaluating
radiation transport phenomenon. Canberra has previously shown that MCNP, when
properly applied, can create gamma spectroscopy efficiency calibrations that
are accurate to 5% [1]. But creating these models takes quite a bit of time
and experience, and running the computations takes much computer time, especially
for large samples at far distances. Computer time can be hours or even days.
The Canberra ISOCS (InSitu Object Calibration Software) mathematical efficiency
calibration software was developed to simplify and speed up this efficiency
calibration process [2]. It is capable of calibration accuracy in the 5-10%
range [3]. It was developed originally for Ge detectors but has been extended
to NaI detectors. A critical assumption in this calibration software is that
the detector is a right circular cylinder, and therefore that the radiation
response is radially symmetric about the detector axis. This, of course, is
not true for these rectangular detectors. But perhaps the ISOCS calibration
technique is still good enough to be useful for calibrations of adequate quality
in a limited spatial region around the detector.
This investigation was designed to answer that question. We know from extensive
testing that the ISOCS software can produce results that are within 2% agreement
of the MCNP results, for the specific shapes that are allowed within the various
ISOCS templates. Therefore, if this investigation done using MCNP shows acceptable
agreement, then so should the ISOCS process.
That does “good agreement” mean? A value of +/- 20% was subjectively
chosen. Gamma spectroscopy using NaI detectors is not nearly so easy as with
high resolution Ge detectors. Because of the much poorer peak resolution of NaI,
errors in determining the net peak area from other peaks in the spectrum can
easily occur. While laboratory users measuring simple spectra can get results
better than 20%, it is difficult with multiple nuclides in the spectra (e.g.
background containing radium, thorium, potassium), and also at low levels as
typically are encountered for the applications with these large detectors.
