I have used LAM BIAS with INEPRI option which will bias inelastic hadronic interaction length. The incident particle will survive after the nuclear interaction with a reduced weight.
For example, let’s say incident particle = proton, target = thin target, produced particle = neutron + protons those have not interacted
Now my questions are
if we score USRBDX or USRTRACK of the incident proton as well as produced neutron and outgoing proton, are these reduced weight already been adjusted in the output fluence spectrum?
If we use USRBIN card to visualize the neutron profile around the target, will the color profile represent true situation or will it represent higher number since we have used BIAS? In USRBIN plots, we can visualise particle spectrum, but their weights cannot be seen from the plots. Is it so ??
Can you briefly explain how this adjustment due to bias is taken care of in the output fluence via some example? Or, if you can guide some slides on this topic.
Of course (otherwise, they would give a wrong result).
Also USRBIN takes properly into account the weights, which is the significant advantage of built-in scoring. On the other hand, USRBIN does not visualize at all particle spectra, since it’s not a scoring differential in energy. What you see from USRBIN, is a particle fluence spatial distribution, in cm^{-2}, which several particles of different weights can contribute to all together.
Look at the biasing lecture of the beginner course. If you reduce the inelastic interaction length of a proton by a factor 0.01, the resulting neutrons will get a statistical weight decreased accordingly and their scored fluence will reflect it.
Thank you for the prompt response. Let me explain in detail where I got stuck. I carried out two simulations:
I actually started with the lecture that you shared. I tried to do the simulation mentioned in slide 20 (100 MeV proton on Be). In the lecture, we can clearly visualize the effect of biasing, there are very smaller number of protons when biasing was not applied.
However, when I tried to do the simulation, I was unable to get the similar observation. To get the similar plot, I multiplied tab.lis values by the energy interval and then by the area.
In my case, although the statistical error is improved after applying biasing, but in the spectrum, although there is little variation in the numerical values, but that does not represent as given in the literature. So I assumed that may be I have to do some normalization for the biasing manually. Here is my input. First, I simulated as it is and then in the next simulation, I commented the biasing card.
In another simulation, I have considered 30 MeV proton on 1 mm Cu target. I tested with different lambda inelastic values and observed same number of neutron fluence. I think this is expected since biasing should not change the overall physics, but I was wondering how to visualize the effect of applying biasing in such cases. Is it reflected only through the improved statistical error part or can we visualize some where else ?
Here is the input for this case. test2.flair (2.7 KB)
May be I am missing something very trivial, but it will be helpful if you can guide.
Regards,
You did not get the same observation because your statistics in the absence of biasing was already enough.
Concerning the normalization and the comparison with literature (which literature specifically?), the target area - which is totally arbitrary in your simulation - should not enter the game (you put it in the scoring card, with no actual reason, and then you properly cancel it out by the result multiplication). As for the multiplication by the energy interval, I believe to have already caught the occasion to point out that, under normal circumstances, this is a basic conceptual mistake. When representing a population N as a function of a continuous variable A with a resolution dA, the distribution should be given as dN/dA (as automatically produced by the FLUKA scoring). If you multiply it back by dA, you get a number which scales with the arbitrary bin width dA and has no universal value. Here A is the kinetic energy and the spectrum should be given in /GeV (or /MeV).
Thank you for the explanation. By literature I meant slide 20 of
. Since it plotted number of protons, I multiplied the output by energy bin.
Apart from this, my query is, in a two step method, if we have to use USRBDX result, we can directly use the output obtained using with or without LAM bias, right?
In other words,
although we have used bias in the simulation, the fluence value that USRBDX prints has the weight 1 for all the energies. Is it so ?
Its unit - unfortunately not indicated - is actually /GeV, as you can guess from the top value at 0.1 GeV, which is 10^3 GeV^-1 over a 1 MeV wide bin, giving the expected integral of 1 (proton).
Yes.
The (differential) fluence is a physical quantity and as such has no associated statistical weight at all. The latter is associated to virtual particles that are simulated to calculate the fluence itself.