PCM-2 Manual.pdf - Voss Associates

Shield Factors
Selecting Parameters
Response Check
Tuning The PCM-2
Problem Detectors
In this procedure, background rates are measured for all detectors with the
instrument unoccupied, then with a person in measurement position. The ratios
obtained are llsed to conect for background shielding effects when a user is in
position. Follow the instructions displayed; count times of at least 100 seconds
are reconunended to obtain accurate data at normal background levels.
Edit the INSTRUMENT PARAMETERS screen to specify the desired counting
mode (preset all, fixed time, or max sensitivity). Other settings on this screen,
such as enabling or disabling radon compensation and/or alpha sumzone alarms,
may be made as appropriate. RDA levels, count times etc. may be entered
through the SYSTEM PARAMETERS screen. An changes made will be saved on
the instrument's disk and will remain in effect until replaced.
Before attempting any measurements, place the PCM-2 in operating mode and
allow it to accumulate background count data for several minutes. Next, select
the SOURCE CHECK function from the CALIBRATION menu. Allow the
instrument to make several measurements with no source present and verify that
few (preferably zero) false alarms occur. Use a source to determine that each
detector responds adequately. If the results obtained are acceptable, the unit is
ready for service.
Once the PCM-2 has been set up, it may be desirable to fine-tune detector and
instrument parameters for optimum performance. The important fact to remember
when performing such adjustments is that changes which help reduce count times
and eliminate false alarms may also reduce th.e unit's sensitivity to genuine
contamination. An changes should be carefully evaluated to insure that overall
performance remains acceptable.
Regardless of how the unit is set up, there will always be one detector which, due
to high background or low efficiency, requires longer count times or provides
lower sensitivity than the others. A typical example is the detector located under
the foot platform; it is mounted horizontally, making it more sensitive to cosmic
radiation, its efficiency is reduced by the heavy metal grid in front of it, and it
may also be shielded by plastic film installed to protect it from dirt. These factors
combine to make this the least sensitive detector in the PCM-2, particularly for
alpha radiation. In order to detect a specific amount of contamination, basic
physics dictates that this detector will require a longer count time.
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PCM2.MAN/ Rev A/April 1995