Gamma and Cosmic Ray Astrophysics

 
 
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Position Sensitive Photomultiplier Tube

The development of imaging gamma cameras is crucial to achieving a wide variety of future scientific goals in high energy astrophysics, as well as in medical and diagnostic fields.  The  Position Sensitive Photomultiplier Tube (PSPMT) provides a very attractive approach to fabricating large, moderately priced position sensitive arrays with good imaging performance.  There are a variety of PSPMTs on the market.  We have tested several 3 inch square PSPMTs (Hamamatsu R2487) coupled to a a variety of scintillators. 

The device pictured above has a thin NaI(Tl) scintillator.  It was designed in support of a proposed NASA mission  to find the positions of gamma ray bursts with arc second accuracy (BLAST).  The photograph is of a prototype detector module which has undergone full electrical and mechanical testing for space flight.  It has been vibration tested to space flight qualification levels with no change in performance. 

The key measures of performance in the BLAST mission concept are: 

    a) use of the full photocathode area. 
    b) spatial resolution better than 2 mm rms. 
    c) reasonable energy resolution. 
A monolithic scintillator crystal was chosen since it provides good energy resolution over the active area.  It also provides larger active area than segmented scintillator arrays since there are typically small gaps between the segments which do not exist in a monolithic crystal.  Segmented scintillators have been tested with good results and have some advantages in imaging performance.  Further study and development of segmented scintillators is planned. 


The Figure shows the position resolution we achieved with a monolithic CsI(Tl) crystal (65 x 65 x 4 mm) using 122 keV gamma rays and a pin-hole aperture.  The pin-holes are spaced 8 mm apart.  To achieve this performance, data from each of the PSPMT anode wires are digitized (34 channels).  The distribution of the signals on the wires indicates the position of a gamma ray absorbed in the scintillator.  An efficient fitting algorithm is used to compute this position. The flight system will use a Digital Signal Processor (DSP) running this algorithm to compute x-y positions in real-time as the data is collected. 

We have successfully adapted the 16 channel ASIC developed for NASA's Advanced Composition Explorer (ACE) mission to readout the PSPMT (photo).  Two ACE chips provide a 32  readout channels.  Each channel has a preamplifier, double correlated sampling, 12-bit ADC, and two level discriminators.  The ACE is a low power (14 mW/ch), compact chip suitable for space flight. 
Note: two pairs of anode wires are tied together to reduce the number of signals from the R2487 PSPMT to 32.  This has a small effect, degrading the position resolution near the center of the PSPMT only slightly. 

A paper on our testing of PSPMTs is included in our preprints list. 

Back to Position Sensitive Detectors Project
last updated: 30-Sep-1998