BNC SAM 935 Portable Gamma Spectroscopy System - Manual (Page 43)

SAM 935™ Instruction Manual 39
After each adjustment attempt, a tone will sound to indicate that the SAM 935 is still adjusting itself. The
SAM 935 begins with the bias voltage at a minimum level and then increases it to achieve coarse agree-
ment with the desired peak location. Once coarse agreement is achieved (or the maximum bias voltage is
reached), the coarse gain and fine gain controls are used to achieve an energy match to within 0.8 per-
cent. A final step will adjust the zero offset. After the hardware parameters are set, a linear fit is done to the
32.7 keV and 661.67 keV lines. For some detectors, the initial parameters to the algorithm may not yield a
recognizable spectrum at first, but within a few iterations, a recognizable spectrum should appear.
The step-by-step instructions for performing a coarse calibration are in Section 3.2.1.
5.6.2 Fine Energy Calibration
The accuracy of the SAM 935’s energy calibration can be improved by using more than the two Cs137
points for fitting a quadratic equation to the channel versus energy relationship. An ideal standard for this
process would have evenly spaced peaks covering the entire range of 30 to 3000 keV. Unfortunately, no
such long-lived, single isotope standard exists. Eu152 provides a good set of peaks from 40 to 1408 keV.
Your SAM 935 has been calibrated using a multi-line standard at the factory. If you do not have a reason-
able standard to repeat this process, then we recommend not changing the factory calibration. If you
decide to use the two-point linear calibration from Cs137, some isotopes may not be identified accurately.
In some cases, they will show up as unidentified dosage or lines. The largest energy errors occur in the
range of 80 to 250 keV and above 1000 keV when a two-point energy calibration is used.
The step-by-step instructions for performing a fine energy calibration are in Section 3.2.3 above. If you
know the values, you can manually enter the fine energy quadratic coefficients via the Analysis Tools
Setup in the Utilities menu; see Section 6.8.