Thermo Scientific SAM12, Manual

Page: 105 / 176

Issue: 1.0 Technical Description - Physics
Thermo Fisher Scientific SAM12 User Manual 6-1
Chapter 6
Technical Description - Physics
Performance Characteristics
The apparatus is designed to measure a wide variety of personal
articles and equipment for contamination so that they may be
removed safely from the active area or from the site. Other
potential uses include determining the gross contamination of
waste materials to determine their suitability for public disposal
or for further processing such as burial or incineration.
To achieve a good degree of measurement certainty, a high
uniformity of response throughout the sample is desirable. This
is achieved by surrounding the sample on all sides, by large
area detectors. Since the main requirement is to quickly prove
the absence of contamination, rather than to identify which
radio nuclides are present, large area Scintillation detectors,
having high gamma sensitivity, are the most robust, low
maintenance and cost effective solution.
The main problem with very large area gamma sensitive
detectors is the high sensitivity to background radiation. This
effect is minimised by shielding the detectors on all six faces
with 1" or 2" of lead. Low energy variants are fitted with an
additional 2 mm Copper shield to absorb the secondary x-rays
produced by gamma’s absorbed by the lead.
An attenuation factor facility exists to compensate for stable
directional background influences and for shielding effects of
known waste containers.
The background is measured accurately, by monitoring over a
long period. This is achieved by taking a series of 10 second
background counts and averaging over 100 seconds. The
background count is updated with a new count every 10
seconds to maintain a “rolling average”. Statistical checks on
each background value guard against sudden changes. The
average count is stored and used for background subtraction.
Software facilitates automatic calculation of monitoring time
and programmable probabilities of False Alarm and Detection,
Alarm levels and Efficiency Correction factors. A facility
known as Quickscan, when enabled, will identify whether the
article is either clearly clean or contaminated within a time
period very much shorter than the evaluated “worst case”
monitoring time. Five user definable discriminator thresholds
are available which allow the rejection of interfering high
Introduction