BNC 970 Portable Multi-Channel Analyzer (MCA) - Manual (Page 45)

Model 970 Instruction Manual 42
6.4 Perform a Shape Calibration
Prerequisites
This procedure assumes that the detector is attached to the Model 970, the 970 is switched on,
SNAP-MCA software has been successfully started, a radioactive source of known isotopic
composition is on the detector, the 970 Hardware Settings have been set appropriately, and an
Energy Calibration has been performed.
Discussion
For NaI (Tl) detectors, the Full Width at Half Maximum (FWHM) is proportional to the square root
of the centroid energy as is the overall peak area. The Shape Calibration helps determine the
exact multipliers for the FWHM for the specific detector.
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 Shape Calibrate… The Shape Calibration window will appear.
The Shape Calibration window can be moved around the screen by clicking on the blue bar
along the top and dragging it to the desired location.
3. On the Spectrum Display, double click near the center of a well-defined peak. This will put a
red line in the centroid of the peak as well as calculate and display the FWHM multiplier. Two
additional red lines will also be displayed estimating the bounds of the peak area.
4. Use the group of four buttons to adjust the outer red lines to accurately reflect the boundaries
of the peak.
5. Click the Shape Cal button.
The Shape Calibrations are now complete.