PCM-2 Manual.pdf - Voss Associates

Section 16
Detectors
General Description
The PCM-2 uses gas-flow proportional detectors (also referred to as "probes ")
operating on P-lO counting gas. The standard unit supports connection to an
external counting gas supply. Detector pulses, resulting from ionization of the
counting gas, are amplified and discriminated by pulse height in separate alpha
and beta counting channels for each detector zone. Nine (9) large, four (4)
medium and three (3) small size detectors are used in the standard PCM-2 unit for
a total of 34 detector channels (the large detectors have 3 individual channels
each). A 0.85 mg/cm 2 aluminized mylar is used for the face window of the
detectors, providing high sensitvity to low enegy beta particles. The PCM-2
detectors have the following physical and operational characteristics:
Large: 212 in 2 (1368 cm 2 ) window area
Medium: 113 in 2 (728 cm 2 ) window area
Small: 50 in 2 (325 cm 2 ) window area
Beta Efficiency (47r): 25 - 30% (contact efficiency to Tc 99 )
Alpha Efficiency (47r): 20% (contact efficiency to typical alpha emitters
such as Pu 239 , Am 241 and Th 230 )
Note: The overall efficiency of the bottom foot detector in the base of the unit
can be considerably less depending on the spacing of the detector's face away
from the bottom of the foot of the user, the open area of the footplate which
has been provided with the unit and the thickness of the protective
polypropylene or mylar film llsed on the detector ((f any).
Each of the three sizes of detector assemblies has been designed for internal
attachments of anode wires as close as possible to the detector chamber walls.
This design feature minimizes the insensitive regions which commonly exist at the
sides and/or ends of detectors of this type and reduces overall "dead area" of the
entire instrument when the detectors are placed closely together as in the PCM-2.
Another design feature of the PCM-2 detectors is a chamber faceplate which
presses onto the detector pan and secures the mylar window to the detector
PCM2. MAN! REV A! April 1995 16-1