How should I interpret the output of USRTRACK or USRBDX as a function of energy? Since USRTRACK is targeting a region, under what circumstances is the value of energy determined?
I simulated a simple photon model with two concentric spheres. The inner is vacuum, from which the source particles are emitted. The shell material is sodium iodide, which is the area being probed. I used both USRBDX and USRTRACK for recording, and the relevant settings can be seen in the accompanying input file.
I plotted the curve using the data in the *tab.lis file, and the results are shown below. The count of USRBDX at 0.662 MeV is close to 1, but USRTRACK is only 0.5, what is the reason?
Can I interpret the data in the third column of the *tab.lis file as the count rate? That is, the ratio of the count value of one energy bin to the total number of simulated particles. At the same time, the third column of data does not add up to 1. Does this mean that not all particles can be recorded by the count card, after all, I set up the sphere?
My question is a bit trivial, thanks for your reading. I would be grateful for your suggestions!
It is important to understand which quantity you are plotting with your cards :
USRBDX: a boundary crossing estimator
USRBDX 111 PHOTON -22 SOURCE TARGET 0.125664Surface
USRBDX 0.0006620.00000001 512 1 &
111 : → Linear binning in energy and solid angle. / Two-way scoring / Fluence scoring (inverse cosine-weighted).
Source to target : → you check the photon fluence through your inner surface 0.125664cm2
1 & :-> you are scoring the fluence over your entire surface : your results are normalized to cm^{-2}.GeV^{-1}.sr^{-1} per incident primary
Multiply by 4Pi to have your results normalized to cm^{-2}.GeV^{-1}
USRTRACK : track-length scoring in a given region.
The results of are always given as differential distributions of fluence in energy, in units of cm-2 GeV-1 (or cm GeV-1) per incident primary unit weight.
To obtain integral binned results (fluence in cm-2 or tracklength in cm per energy bin per primary) one must multiply the value of each energy bin by the width of the bin
It gives the average dΦ/dE.
USRTRACK 1 PHOTON -23 TARGET 4.18879E9 512Cell
USRTRACK 0.000662 0 &
1 :-> Linear binning in energy.
Target : → You are scoring the differential distributions of fluence amongst your entire Target, your results are normalized to [GeV^{-1}cm^{-2}] per primary as you provided the Region volume.
In your example
You can directly plot those estimators using flair (using both _sum.lis with USR-1D plots ), you can see in this high statistic simulation the effect of the two-way scoring with USRBDX which correspond to your tail at lower energies.
As the photons will get attenuated very quickly in your volume (you can observe the attenuation with the USRBIN card), the average fluence given by USTRACK over your entire volume will of course be way lower than the one scored at the boundary of your source.
So to answer your question: in your plot you are not scoring the same things. One will be the average of 1cm^3 (USRTRACK) whereas with USRBDX it will be the average crossing your surface. All in all, it depends on the quantity you plan to investigate.
You can find all the details about those cards in the FLUKA manual:
USRBDX: 7.22.86. USRBDX — FLUKA Manual
USRTRACK 7.22.92. USRTRACK — FLUKA Manual
