Protean Instrument Corporation
112
Service MPC
900
Contamination
Background counts may increase abruptly or gradually over a long term due to
contamination. In either case, it will be necessary to decontaminate the instrument.
The following items may be source and should be cleaned:
•
Sample Carriers – Use the recommended liquid detergent and dry thoroughly.
•
Sample Drawer – Clean with methanol.
•
Detector
–
Replace the window.
Electronic
Noise
An increase in background counts can be caused by a noisy high voltage supply,
preamplifier, main amplifier, or cables. As a simple test check these components,
Disconnect the sample detector from the sample amplifier by removing the coax cable
from the PREAMP-IN connector of the APA-10-D-DP. Run a trending background
check. The counts due to electronic noise are typically less than 0.1 cpm.
Performance
Changes
–
Efficiency
Changes in efficiency may be caused by an inadvertent change in counting geometry or
detector changes. A continually increasing or decreasing trend in a beta control chart
requires investigation.
Geometry
For alpha and low energy beta radiation the efficiency may decrease by 10% to 15%
using 5/16 planchet as compared to a 1/8” planchet. Verify the calibration sources are
counted geometrically the same as the reference used to establish the instrument
efficiency.
Gas
Quality
Insure that the efficiency change is not associated with a recent gas cylinder change or
changes in the gas supply system.
Verify proper flow rate. Nominal flow is 60 cc/min. If a greater flow rate is required to
sense “Good Flow” you may have a tear or pin hole in the window and it should be
replaced.
