I am running a simulation to obtain the nuclear de-excitation gamma-ray line spectrum produced by primary alpha-particles (with a power-law energy distribution of spectral index delta =- 4 in the range from 1 MeV to 200 MeV) injected into an Oxygen target. This is a continuation of our published work using FLUKA to interpret gamma-ray spectra from solar flares (Tusnski et al. 2019, Solar Phys. 294, 103). The spectrum obtained exhibits a line around ~1.6 MeV which is much stronger than expected from the cross-sections for the main reaction responsible for its production, namely the spallation reaction 16O(alpha,x )14N* (~14 mb). We are trying to understand why this line is so strong.
The alpha-capture reaction 16O(alpha,x)20Ne* can also produce photons at ~1.6 MeV. However, the cross-section for this reaction is even smaller (microbarns, according to the IAEA EXFOR database) so it is difficult to see how it would produce a greater flux in the line than the spallation reaction. However, by setting a RESNUCLE card in the simulation (with heavy fragment evaporation and coalescence activated) we noticed that a surprisingly large number of 20Ne nuclei is produced, which might be responsible for the strong line around 1.6 MeV – the first excited state of 20Ne is at 1.63 MeV giving a deexcitation line at this energy.
I will be grateful for any help understanding what is happening here: why are so many 20Ne nuclei being produced? Are they responsible for the gamma-ray line?
Sincerely,
Sérgio Szpigel
Centro de Rádio Astronomia e Astrofísica Mackenzie
Universidade Presbiteriana Mackenzie
Dear Sérgio @szpigel, most likely your explanation about the 20Ne origin of the 1.6 MeV line is correct. However, you are missing the fact that 20Ne is produced with a quite sizeable cross section (several hundred mb) on heavier oxygen isotopes (17O and 18O), which can compensate for their per mil abundance, as included in the natural composition you selected. (If needed, one can define a mono-isotopic material, see this post to make it right). Do not hesitate to come back in case of not convincing findings. Best
Thank you very much for your quick response. Indeed we missed the possibility that 20Ne could also be produced by reactions with 17O and 18O. Following the post you have indicated, I changed the input to define a 16O mono-isotopic material:
However, I still get the same result: a strong gamma-ray line around 1.6 MeV and a large number of 20Ne nuclei.
You are right: sorry, I had not checked the 20Ne production on 16O, which also turns out to be quite sizeable in FLUKA for alpha energies between 4 and 9 MeV (up to the opening of other channels). Now, this does not seem to be necessarily in contradiction with available data, since the EXFOR sub-microbarn ones you mentioned appear to be below 3 MeV (and the reaction cross section is then at hundred mb level above 10 MeV, when the 20Ne channel is already closed). Nevertheless, if you wish to exclude this contribution, you could set the alpha transport threshold at 10 MeV (by means of PART-THR). Keep in touch
Thank you very much again for your response. A sizeable cross section for 20Ne production on 16O by alpha-particles with energies between 4 and 9 MeV does explain the strong gamma-ray line we get at 1.6 MeV. We ran a simulation for alpha-particles injected into an Oxygen target with natural composition and by setting the alpha transport threshold at 10 MeV, as you suggested, the contribution from 20Ne to the 1.6 MeV line is indeed suppressed.
However, we are still puzzled about the FLUKA cross-sections for 20Ne production on 16O at lower alpha-particle energies, which seem to be much larger than the microbarn ones we get from ENDF.
We would appreciate any help to understand this issue.
Hi Sergio @szpigel. The ENDF/EXFOR plot you pasted displays data only below 3 MeV. The rest is from TALYS calculations. As for FLUKA, the complete fusion (20Ne) channel coincides with the total reaction cross section up to about 10 MeV, where data indicate values of the order of hundred mb for the latter, as another channel opens. If indisputable overestimation evidence for the preceding energy interval actually emerges, we can rather easily adjust the total reaction cross section onset (and so the 20Ne production yield accordingly).
Thank you very much for the explanation. It really clarified the issue about the cross sections and helped us to understand the results of our simulations.
