Ludlum Measurements 3100, Руководство пользователя

Страница: 29 / 57

4-5
Figure 4-4. Air Flow Diagram.
4.4 RADIATION DETECTION.
The Model 3100 has two ionization chambers – one to measure tritium and the other to subtract out background
interference from the tritium measurement. These two chambers are mounted into the bottom of the instrument can,
which has marks on the outside to identify the center of each chamber. The mark on the instrument can closest to the
front of the instrument identifies the center of the gamma chamber. The mark on the instrument can closest to the
back of the instrument identifies the center of the tritium chamber. The chambers are mounted together on an
aluminum plate, with the electrometer circuit board mounted on the top of the plate. These two chambers and the
associated electrometer board (commonly called an electrometer) are the basis for the radiation detector of Model
3100.
4.4.1 Chambers.
The cylindrical chamber housings, approximately 250 cc in volume, are constructed from drawn
aluminum and are powder-coated black on the outside wall. Chamber wall thickness is 0.032 in. (0.081 cm). The
wall of the tritium chamber, which is electrically isolated, is maintained at approximately -75 VDC, while the wall
of the gamma chamber, which is also electrically isolated, is maintained at approxi75 VDC. The center
electrodes of both chambers are connected electrically together at the electrometer board. Since gamma background
affects both chambers, this parallel subtracting arrangement provides the gamma compensation so that the resulting
electrometer signal is due solely by the additional ionization within the tritium chamber.
The electrodes of both ionization chambers are connected to the electrometer board input and are maintained at
guard potential by the electrometer.
For air (at standard temperature and pressure: 22 °C and sea level), the average energy W to ionize a single ion pair
is 34 eV/ion pair. The expected ionization current per µCi (37,000 disintegrations per second) of tritium (identified
as “I” in the following equation) in the chamber, given a W of 34 eV in air, the charge of an electron equaling
1.60207 x 10
-19
C, and an average beta tritium energy of 5.65 keV (assuming negligible wall losses since the
maximum range in air of tritium is approximately 5 mm) is:
I = (5.65x10
3
eV) x (37,000 disintegrations per second) x (1.60207 x 10
-19
C) / 34 eV