[fluka-discuss]: Simulation of coincidence summing effects in HPGe efficiency calibration

From: Andrea Mattera <andrea.mattera_at_physics.uu.se>
Date: Wed, 30 Mar 2016 13:04:16 +0200

Dear Fluka experts,

I am trying to simulate the detection efficiency of a HPGe; specifically
we are interested in the coincidence summing effects if a source with
multiple gamma lines is placed in close proximity of the detector. In
order to do this I am using a BEAM ISOTOPE and the HI-PROPE cards to
define a radioisotope as a source and I am detecting the energy in the
HPGe using a (series of) DETECT [input file attached].

We are comparing the Fluka simulation with measurements performed in
very similar geometries and with the same sources (152Eu, 54Co, 133Ba):
we could verify (see pag. 1 in the attached pdf) that the two efficiency
curves agree very well over a wide energy range if we are in 'ideal'
conditions (point-source placed at a relatively long distance to the
source). This gave us some confidence that no big mistakes are there in
the geometry and/or the scoring of particles.

However, once we reduce the source-to-detector distance - i.e. when we
expect the first effects of coincidence summing to appear - problems
start to arise. Our simulation seems to largely over-estimate the
effects of coincidence summing for most points (sometimes even by a
factor of 3 or more, compared to the measurement) - see pag. 2.
[OBS: another difference between the 'far point-source' vs 'close
geometry' is that, in the latter case (both in measurement and
simulations), the source is extended and mixed in concrete powder to
account for self-absorption effects in the source]

My first question to you is: is Fluka able to simulate these effects
with the settings above in the input file (ISOTOPE + DETECT)?
I would like to stress that we are not interested in (time-dependent)
pile-up effects on the efficiency (the activity of the sources and hence
the count-rate in our detector are very low). We only want to simulate
coincidences coming from gamma cascades in the same decay event. From
simple tests (e.g. with a 60Co source) it seems like Fluka shows a
summing peak, which makes me hopeful that this is a good path to explore
for our application [pag. 3].

But: am I missing something in the physics or in the implementation? If
our approach is in principle correct, do you have any guesses or hints
at where we should look for an error that could explain the differences
between simulation and measurements?

Thanks a lot in advance for your help!


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Received on Wed Mar 30 2016 - 15:36:34 CEST

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