Thermo Scientific SAM12, Manual

Page: 141 / 176

Issue: 1.0 Setting Up Procedure
Thermo Fisher Scientific SAM12 User Manual 9-7
60
Co (1.2 MeV)
137
Cs (662 keV) SAM12
No. detectors Centre Centre
4C 40%
(36-44) 16%
(15-19)
6C 58%
(53-63) 28%
(23-33)
The above table includes variations for 1 and 2 door versions
and is independent of lead shielding.
4
Detector
Systems:
Individual detector efficiencies should be an
approximately equal proportion of the Total System
efficiency, (i.e. approx 10% each for
60
Co).
If one of an opposite pair of detectors is high, and the
other is low, then the source is probably off centre.
6
Detector
Systems: Individual top, bottom, left & right efficiencies
should be approximately equal.
The front and back detectors will be slightly lower
efficiency than the other four detectors due to
geometric effects – if the source is in geometric
centre of volume.
If the overall system efficiency or any individual detector
efficiency is too low (or too high), consider the source values at
the next HV step - review the background Criteria in Tests (1)
& (2) and the Source Criteria in Test (3). Repeat this process if
necessary.
NOTE:
The Minimum Detectable Activity (MDA) is
proportional to the square root of the Background – lower
background improves (reduces) the MDA. Therefore
‘Squeezing’ a few extra source counts at the expense of a
significant background increase, will be detrimental.
Variance Testing
When the system has been set-up, a background stability test,
usually referred to as a variance test, should be performed.
This is a test to measure the deviation of the distribution of
background from a perfect Gaussian distribution. This is
equivalent to a Chi-squared test.
See
Variance Test (page 5-17) for further information regarding
how to set up the variance test. The variance test result is the