[Gate-users] Strange pile up effects with Cylindrical PET system

[K B]

Hello Everyone,

 I am noticing a strange thing when using cylindrical PET. I am
simulating a very high activity using mono-energy 511 keV source(and
therefore expect pileups) and when my detector is thinner than 16 mm
(radial thickness), I see events past the 511 keV photopeak as expected
and this is a continuous spread past 511 keV. However, when I change my
crystal thickness to 17 mm or above, there the continuous spectrum is
replaced by one small peak at 681 kev (511 + 170.3). This seems wrong to
me. There cannot exist a hard threshold for detection. This is a singles
energy spread.

  An example of the spectrum with 16 mm and 17 mm crystals is attached
and is online at http://img15.imageshack.us/img15/4026/imageshackzw.png

 I am using a cylindrical PET and the readout is set at depth 1. The
same digitizer is used for both the 16 mm and 17 mm crystals.

Thanks to anyone who can shed some light on this.

KB
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