[Gate-users] Defining source energy: Mono(energy) vs Discrete Spectrum

[Devir-Wolfman, Ayeleth (GE Healthcare)]

Hi all,

I'm trying to add a I-131 source to a SPECT system, and I've tried to simulate it in three different ways:

  1.  I added a source with energy type Mono, i.e. mono-energy of 364keV.
  2.  I added a UserSpectrum energy type, with a Discrete Spectrum file, that contains the emission in 364keV (written in the file as 0.364).
  3.  I added a UserSpectrum energy type, with a Discrete Spectrum file, that contains all the emissions of I-131 (written similarly to the previous file, with all the lines probability weighting).

The first two simulations give me a very similar energy spectrum (both in the output window, and in the whole spectra), but I detect half the amount of photons with the UserSpectrum compared to the mono-energy.
On the other hand, when I compare the detected photons ratio between the full emissions simulation to the mono-energy simulation, the ratio is 0.89 (which makes much more sense).

Have anyone ever encountered this issue?

Thank you,
Ayeleth Devir-Wolfman.
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