LEAPR S(alpha,beta) and rho phonon frequency spectrum

[negatron1]

Hi everyone, hope everything is going well. I am new to NJOY environment but ı made a bit of progress by using njoy16 and end102 documents. I encountered something that caused confusion to me while producing s(a,b) for thermal scattering.

Firstly; To produce mf7 data for THERMR module, where can ı find alpha, beta and, rho phonon frequency spectrum values which are used by LEAPR to produce s(a,b) law? In fact, my aim is that finding mentioned data for not given temperatures. For example, I downloaded ENDF data for graphite which includes data for a few temperatures (296 400 500 600 700 800 1000 1200 1600 2000 K) but my target is a different temp, which is not available in the ENDF file. As given in the manual, values do not depend on temp for graphite meaning we can use them for other temperatures.; however, how about other moderators especially temperature is important for rho frequency distribution, for instance, h2o at other than the given temperatures?

Secondly, As ı understood, we have to produce s(a,b) law, which is necessary, in order to use it in thermr module and we create X section values for given materials at the desired temperature. If we already produced it while generating new X sections at the desired temperatures, why do we use s(a,b) again in the mcnp input file such as h2o at 650 K (h-h2o.56t)?

hope my questions and explanations seem to you clear, looking forward to hearing from you. best regards.

[whaeck]

The phonon spectrum used in the input for LEAPR can be obtained through experiments or DFT calculations, and this can actually be temperature dependent. As an example, this reference gives an overview of the evaluation process for H in H2O and a few other moderators and gives example input files for LEAPR that were used to obtain the resulting S(a,b) evaluations:

Mattes, M., Keinert, J., 2005. Thermal Neutron Scattering Data for the Moderator Materials H2O, D2O and ZrHx in ENDF-6 Format and as ACE Library for MCNP(X) Codes, INDC(NDS)-0470.

Unfortunately, we cannot provide you with the inputs for other moderator materials so you will have to perform a literature review to acquire the necessary information.

As for your second question, I'm not entirely sure what you mean. The processed thermal scattering data will give us the cross sections and energy-angle distributions for a material like h in h2o (the ACE files do not contain the S(a,b) functions). If you do not use an MTn card in the MCNP input file, you will only use the cross section data for the "free" nuclide (i.e. not bound in a molecule or crystal). The MTn card needs to be used to tell MCNP that certain nuclides are actually bound and that the free data below ~5 eV for these nuclides needs to be replaced with the bound data obtained from the S(a,b) data.

[negatron1]

Dear whaeck, thanks a lot for your beneficial explanations and when it comes to my point of view on your replies that follows as below: For your comment on my first question: As I understood referring to your statement, publication you shared, and njoy16 document;

Continuous frequency spectra and tabulated values considering Leapr examples for H2O in the njoy2016 document and publication you shared change a bit with respect to temperature. In the same publication, It was defined that for a specific temperature, interpolation for the continuous part ρ(ω) is done between the two limiting curves for example at 294 and 624 K given based on measurements by Haywood and Page. And, I checked the values by interpolating 3 different temperatures given in the example in the njoy2016 document, it works and ı am so happy :) thank you again.

As for your second statement, I wanted to compare works doing by LEAPR and THERMR, Obviously, Leapr generates thermal scattering data for various temperatures using ENDF format File 7; on the other hand, THERMR calculates pointwise thermal scattering data utilizing PENDF format File 3 and 6 but to calculate those by using endf tape for mf7 data in the THERMR input card1 under the nedf. So, If it is already used mf7 data to calculate pointwise thermal scattering cross-section, why do we have to give mf7 as the S(a,b) data in the MTn card, again?

Thanks in advance, looking forward to hearing from you.