Na-22 Gamma Spectrum

I hope this message finds you well. I am currently working on simulating the gamma spectrum of several various, and I have encountered a peculiar issue with Na-22.

I have noticed significant discrepancies between the simulated results and the expected spectrum.

The figures below show the Na 22 spectrum from the literature and the simulated one. Specifically, I am observing differences in the intensities of the peaks at 511 keV and 1274 keV.

In particular, the intensity of the 1274 keV peak in the simulated spectrum appears to be higher than expected. According to literature values, the 1274 keV peak should be less intense than obtained in the simulation.


DEAR,Perhaps,it is related to the detection efficiency of the detector at different energies.

I have indeed considered the NaI detector’s efficiency. I obtained accurate results when calculating the efficiency using a single particle source for both 511 keV and 1274 keV.

However, the problem arises when using the Na-22 isotope source, as the results seem to deviate unexpectedly, particularly at 511 keV.

Additionally, I replicated the gamma spectrum simulation using the GAMOS code for Na-22. The obtained spectrum is illustrated below:

It matches the expected spectrum.

NB: I have achieved satisfactory results for other isotopes such as Co-60, Cs-137, and Mn-54. I think the problem lies specifically with the Na-22 isotope.

In measurements or calculations, we do not consistently depend on 511 kilo electron volts due to it not being consistently unitary. Instead, it incorporates a component of the attenuation process.

In my personal opinion, I do not consider this complex line as a crucial factor in my calculations or attach much importance to it. This is because I am aware that it is a non-uniform single line.

As you know, In gamma spectroscopy measurements, the annihilation of a positron with an electron typically results in the production of two photons with an energy of 511 keV each. However, if you want to minimize this specific annihilation event and/or level, you can take the following steps in your simulations:

  1. Shielding: Use lead or another dense material to surround the gamma spectroscopy setup. This will help attenuate the amount of 511 keV photons that reach the detector.

  2. Coincidence measurements: Implement a coincidence measurement technique. In this method, you use two detectors positioned at a specific angle to detect the two 511 keV photons emitted simultaneously from the annihilation event. By requiring both detectors to register a signal within a specific time window, you can reduce the detection of random background events and further target the desired annihilation signal.

  3. Gamma energy discrimination: Use a detector that is capable of distinguishing different energy ranges of gamma photons. By setting the energy window of the detector to exclude or minimize the range around 511 keV, you can reduce the detection of annihilation events.

  4. Active background reduction: Incorporate active background reduction techniques, such as anti-coincidence shielding or background subtraction algorithms, to further minimize the detection of 511 keV photons resulting from annihilation.

Remember, it may not be possible to completely eliminate the detection of 511 keV photons resulting from positron-electron annihilation, but these steps can help in minimizing their occurrence in gamma spectroscopy measurements.

Dear Assia,

Na-22 decay will produce on average 1.8 511-keV gammas due to its decay being 90% b+ (10% EC). That decay data is faithfully reproduced in our decay database. You indicate having an issue with the amount of annihilation photons that you are scoring, can you send me your input file so that I can have a look?

If the issue is related to the detection of annihilation photons exclusively, it makes sense that you don’t notice anything in the case of Co-60 (B- only), Cs-137 (B- only) and Mn-54 (EC and B-).


dear Philippe,

I appreciate your clarifications. Attached is my input file.

NaI_Article1.flair (6.0 KB)

Dear Assia,

I’ll have a look and come back to you asap