Spectrometry Gamma With OpenMC

User Support
1

Dear All,

I have been using the open-access OpenMC gamma detector repository available at:

to conduct my simulations and found it very relevant to my current research. In particular, I was interested in the simulation results shown in Figure which presents the pulse height distribution using OpenMC. Which closely aligns with the goals of my simulation study.

Using that notebook, the pulse height tally spectrum I obtained showed a very high y-axis count, reaching up to 10¹⁰. This is significantly higher than the experimental results I aim to compare with, which are in the range of 10³ to 10⁴ counts. I am simulating a standard Cs-137 source placed 18 cm from the detector with an activity of 4⁵ Bq, and I intend to compare the simulation with experimental data. In addition, I would like to calculate the absolute efficiency of the detector, but I am not familiar with how to implement this in OpenMC. In addition, I would like to implement a collimator between the source and the detector, so that the tally being calculated corresponds to the gamma rays from the source that have passed through the collimator before reaching the detector.

If you would be willing to share any guidance related to my issues, I would be sincerely grateful. It would greatly assist my research.

Thank you very much for your time and consideration.

2

Hi Dila, welcome to the openmc community, sorry for the late response to your similar topics before.

Regarding the high calculated pulse height tally in this notebook, I think it came from the source.strength definition input. As you can see, the first source was defined as a direct 10^12 photon particle with energy 800keV

source_1.energy = openmc.stats.Discrete([800_000.0], [1.0])
source_1.strength = 1e12
source_1.particle = 'photon'

While other sources was defined using the number of isotopes which also correlate to the activity.

source_2_Ba137_m1.energy = openmc.data.decay_photon_energy("Ba137_m1")
source_2_Ba137_m1.particle = 'photon'
source_2_Ba137_m1.strength = np.sum(openmc.data.decay_photon_energy("Ba137_m1").p) * number_atoms_Ba137_m1

In your case, you may want to try to use your Cs-137 activity (4^5 Bq) to find the number of atoms for your source strength by converting the activity to the number of Cs137 atoms (activity= N.lambda). From that, you can find the number of Ba137 atoms using the same correlation as the example (lambda and branching ratio).
I haven’t try to validate this example notebook with experiment, so I hope it can show you some consistency with your detector counts.

3

Thanks a lot for your answer and the information! I try to work on your that and i get good results

Thanks again.

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