BNC SAM 935 Portable Gamma Spectroscopy System - Manual (Page 42)

38 SAM 935™ Instruction Manual
If you are operating with a 5-second sample time (the default), a background of 180 seconds or longer is
desirable.
Once the background spectrum is taken, you are given the opportunity to review the report generated or to
print it in long or short form. Check the gross CPM displayed on the report screen carefully. Under normal
conditions, you should expect this number to be between 3000 CPM and 4000 CPM.
Two of the most likely causes for high backgrounds are radioactive materials near the counter in the form
of samples or standards, or high concentrations of natural radioactive material such as K40 in the materi-
als of the building. High background levels are sometimes associated with concrete or cement block con-
struction. High radon levels are also a possibility. The SAM 935 has the unique ability to operate well in a
wide range of background environments.
5.6 Calibration Mode
Accurate calibrations are essential for the SAM 935 to give accurate results. The calibration routines pro-
vided with the SAM 935 give you the tools to accurately calibrate and to maintain the calibrations easily.
5.6.1 Manual Coarse Adjust with Cs137
The primary purpose of the coarse adjust procedure is to adjust the high voltage, coarse gain, and fine
gain of the SAM 935 to put the reference peaks (32.89 and 661.66 keV) of Cs137 into known channels of
the MCA.
The coarse adjust algorithm first adjusts the high voltage of the SAM 935 and then the coarse and fine
gains to achieve this goal. When complete, the 661.66 keV line for Cs137 will be in the correct channel to
within 0.8 percent. This is close enough so that calibrations can be maintained across adjustments of the
electronics.
If the SAM 935 has been previously calibrated either by you or at the factory, a screen will appear to ask if
you want to initialize the hardware parameters. If the SAM 935 has been previously adjusted and has not
been subjected to large swings in temperature, select N
O (F2). When N O is selected, the SAM 935 begins
its adjustment at the existing values for the high voltage and coarse and fine gains. All parameters will be
adjusted if necessary to achieve proper calibration.
If there have been large temperature changes or this is the first time you have adjusted the SAM 935,
select Y
ES (F3). When Y ES is selected, the SAM 935 resets all parameters to their minimum values and
goes through the entire algorithm. Both will result in the same calibration; however, starting where you
were last time can be much faster. In rare circumstances such as a change of detectors, the operation may
succeed when parameters are reset when it fails keeping the original values.
During each acquisition sequence, the spectrum being measured will be displayed as it develops, and if
everything is connected correctly, you should be able to watch the spectrum resolve. The vertical scale is
shown above the spectrum, and in the message window below the spectrum, the current hardware param-
eters (high voltage, coarse gain, fine gain, zero) will be displayed. A vertical line (cursor) is positioned at
the target location of the 661.7 keV Cs137 line.
Note: Before your SAM 935 left the factory, it was carefully calibrated using a set of cali-
bration sources. These calibrations (stored in the battery-backed-up memory of the sys-
tem) were made with your detector and should be correct. Under normal circumstances,
you will be able to simply use the one-button quick calibration (F2 in the main display).