Slow counting rates with cosmic muons, coupled with the requirement of
high statistics for accurate tomography analysis, result in CMT
run durations that are impractical for treating all 240 CsI detector
elements. Therefore, an alternative method
has been developed to tomograph the CsI crystals. The Radioactive Source
Tomography Apparatus (RASTA) utilizes a 
Cs source instead
of cosmic muons as the tomography probe. The source, which emits 0.66 MeV
photons, is scanned over
the length of the crystal in 2 cm steps. A diagram of the RASTA
set-up is shown in Figure 3.5.
Figure: Radioactive Source Tomography Apparatus (RASTA). A
Cs source, collimated by a 6 mm diameter hole bored through a 5 cm
thick lead brick, is scanned along the surface of the crystal by a step motor,
in 2 cm increments.
The trigger rate for this operation is roughly 3 kHz, which is much higher than the 1 Hz rate for the CMT apparatus. Consequently, each crystal can be tomographed by RASTA in about one hour, as compared to one week in the CMT apparatus.
Like the CMT apparatus, the RASTA apparatus is a dark box with feed-through connectors for the signal and high voltage cables from the electronics outside the box. The RASTA apparatus can accomodate one CsI detector at a time. The trigger for one event consists of a signal from the CsI detector. The electronics logic is shown in Figure 3.6.
Figure: Schematic of the electronics the RASTA apparatus. The trigger
for one event consists of a signal from the CsI detector.
The number of photoelectrons per MeV is measured with the RASTA apparatus using the same technique as in the CMT box, which is described in Section 3.2.1.