Automatic Gain Control Detectors
Each phoswich detector module has a calibration x-ray source mounted in the collimator immediately
above its entrance window, in the detector's field-of-view (Figure 5.2). The calibration source con
sists of a small quantity (activity level about 18 nCi) of the radionuclide 241Am suspended within a
solid cylindrical pellet of scintillation plastic (Bicron 440). The scintillation plastic is optically cou
pled to a 0.5-in photomultiplier tube. 241Am is chosen as the dopant in the scintillator since its pri
mary decay scheme yields a 59.6 keV x-ray in coincidence with an ~4 MeV a-particle. The 59.6 keV
photon exits the plastic and serves as a calibration reference line for the phoswich detector while the
a-particle, which is stopped within the plastic, generates a coincidence signal to indicate that the
event originated in the calibration source. Section 5.3.6 details how these AGC events are used in a
feedback loop and for continuous gain monitoring.
Even with the magnetic shielding around each detector, the gain (or pulse-height/energy relation) is
affected by the magnetic fields encountered throughout an orbit. Secular changes in the gain also
occur due to aging of the phototubes. To counteract these effects, the AGC system is designed to sta
bilize each phoswich detector's gain, such that photon events of a given energy will always produce
counts in the same PHA channels. X-rays of 59.6 keV energy from the 241Am source interact in the
NaI in coincidence with the associated alpha particle interaction in the gain control detectors
(described above). These NaI events are used to provide the gain control feedback signal that adjusts
the PHA conversion gain to preserve the energy/PHA channel relation. Coarse gain settings are
accomplished by selecting one of 256 high voltage steps (5% gain change per step) and fine steps can
be commanded by the AGC system within the range 0.2% to 0.0125%. The AGC updates the gain
every 0.5 s. In this manner short-term gain variations are kept to <1%.
All events detected in the phoswiches in coincidence with the a-particle events in the gain control
detectors are separately accumulated into a calibration spectrum which is acquired over 32 Instrument
Data Frames (8.5 minutes). The result is a very clean 241Am spectrum with a line at 59.6 keV, a blend
around 25 to 30 keV, and the L-shell blend around 17 keV (Figure 5.7). These spectral features will
provide constant monitoring of the gain and resolution for each phoswich detector throughout the
mission. Therefore, with this system meaningful comparisons, and co-additions of datasets, may be
made between spectral observations taken many months apart.
Phoswich Detectors HEXTE Clusters