BNC 970 Portable Multi-Channel Analyzer (MCA) - Manual (Page 43)

Model 970 Instruction Manual 40
Discussion
SNAP-MCA uses a linear interpolation between Energy Calibration points. More sophisticated
modeling was not implemented because the wide range of characteristics in the detectors
supported by the 970 makes it impossible to accurately model for all possible energy response. It
was also determined that there were certain advantages to using this method, especially in the
low-energy range. In short, the more data points entered, the more accurate the Energy
Calibration will be. If there is a limited energy region in which you are interested, adding additional
data points in those areas will increase the accuracy in that area without skewing the Energy
Calibration in other regions of the spectrum. Be careful, however, of energies that are
extrapolated very far beyond calibrated points.
If an estimate of the energies for the lowest and/or highest energies on the spectrum (i.e.,
energies at channels 1 and/or 4094) can be made, it is a good idea to enter these as Energy
Calibration points to reduce extrapolation error. Additional Energy Calibration points, up to a total
of 14, can be added or altered at any time during the operation of SNAP-MCA, and will be
reflected in all subsequent analyses. At least two calibration points must be entered to perform an
Energy Calibration.
It is not necessary to stop a spectrum Acquisition to perform or alter an Energy Calibration,
although Acquisition may automatically pause briefly and restart. Calibration points may be
entered in any order; they will be arranged by order of energy when the Enrgy Cal! button is
pressed.
If this step is begun immediately following the Hardware Settings procedure above, and the final
137
Cs spectrum is displayed, that spectrum can be used instead of acquiring a new one (skipping
step 1 below).The spectrum can be zoomed in or out at any point as desired by using the mouse
to drag a box on the Spectrum Display.
Procedure
1. Acquire a spectrum with known peaks. It is usually a good idea to start with a relatively
simple spectrum, such as 137 Cs.
2. From the Utility menu, select Energy Calibrate… The Energy Calibration window will appear.
If this is the first Energy Calibration for this detector, the energies for the two
137
Cs peaks will
be present. The Energy Calibration window can be moved around the screen by clicking on
the blue bar along the top and dragging it to the desired location.