BNC 970 Portable Multi-Channel Analyzer (MCA) - Manual (Page 41)

Model 970 Instruction Manual 38
It is worth mentioning that the Gain settings are arbitrary and not absolute. In other words, if you
put in a pulse of exactly 1000 mV with an overall Gain of x1, the pulse will not necessarily appear
in channel 1000. Similarly, a positive pulse of exactly 1000 mV will not necessarily appear in the
same channel of a negative but otherwise identical pulse. This is normal and is only mentioned
here so no surprise is experienced if you put a pulse generator on the 970. Relative Gain values
are reasonably accurate.
You should avoid using the extreme low end of the Fine Gain control to the extent possible. For
example, you can obtain an overall Gain of x3.391 with a Coarse Gain of x2 and a Fine Gain of
x0.848. You can obtain the nearly the same overall Gain value using a Coarse Gain of x125 and
a Fine Gain of x0.027. The latter settings will produce a distorted spectrum, however. As a
general rule, the lowest Coarse Gain setting possible should be used.
6.2.6 Shaping Time
Shaping Time is usually specific to classes of detectors. Virtually, all common scintillation and
CdZnTe semiconductor detectors use a Shaping Time of 1 µsec. If you are not sure, try various
Shaping Time settings until you get the best looking spectrum. Also, changing the Shaping Time
will usually necessitate changing the Gain settings as well.
6.2.7 Determining Initial Hardware Settings
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, and a
137
Cs of approximately 1 µCi is on the
detector. It is also assumed that the detector to be used is capable of seeing both the 32 and
662 keV peaks.
Discussion
Hardware Settings cannot be adjusted until the Allow Edit box is checked.
Generally speaking, most adjustments to expand the separation between peaks should be
performed using the HV (High Voltage) adjust. Start increasing the Gain only if the peak
separation remains too small even at the maximum HV recommended for the particular detector.
Increase the Gain until the peak separation is slightly wider than desired, and then reduce the HV
as needed. If the peaks are in a good location but appear distorted, try lowering the HV and
increasing the Gain.
Adjust the Threshold lower if too much of the low energy portion of the spectrum is being cut off.
Adjust the Threshold higher if noise is apparent in the lowest regions of the spectrum.
In using the Hardware Setting adjustments, the associated Set button will become enabled,
indicating that the value has been changed from those at which the 970 is currently set. For
example, if you change the HV the Set HV becomes enabled. Clicking the associated Set button
will (1) pause the current Acquisition if one is occurring, (2) set all Hardware Settings that have
been changed, not just the one the particular button is associated with, and (3) restart the
Acquisition (if one had been occurring) with the new settings without clearing the previous
spectrum.
Although it is completely up to the user (and subject to the characteristics of the detector being
used), it has been generally found that adjusting the Hardware Settings such that the 662 keV
peak is at about channel 1200 will give an overall energy range coverage to about 2,300 keV.
Hardware Settings are automatically saved and applied to a specific 970/detector combination
any time a Set button is pressed. If different sets of Hardware Settings are desired for the same
detector, create a “new” detector based on that detector, changing the description to reflect the
different purpose.