What Could Be Causing Unexpected Results in the LET Analysis with MCNP6.2?

[lucasfaraujo]

I'm dealing with a specific situation: I'm analyzing Linear Energy Transfer (LET) in a cylindrical sample. According to the definition of LET in the manual, we have:

“The linear energy transfer (LET) special tally option allows track length tallies to record flux as a function of stopping power instead of energy. When the FTn LET option is specified, the values provided in the energy bins are interpreted as stopping power values with units of MeV/cm. This option can only be applied to charged particle tallies.”

As an example, the manual uses energy bin that are interpreted as stopping power values, presented as follows:
"fc4 Proton flux LET
f4:h 77e4 1e-2 99ilog 6e4
ft4 LET

This example is a tally that records the proton flux in cell 77 for a LET tally. The tallyresults are recorded in 100 bins of stopping power from 0.01 to 60000 MeV/cm."

In my case, I'm using an Ir-192 source, so I've set the stopping power "bin" to the range of the Ir-192 photon spectrum, and in my case I'm asking for the flow of electrons. Mine is set like this:

"fc4 Electron flux LET
f4:e 623 624 625
e4 1.01e-5 599ilog 1.38
ft4 LET"

The problem is that in this first bin, defined as "1.01e-5", it computes the largest LET contribution, which seems to be arbitrary, because if I put 0 or 1E-3 in place of this bin value, it computes practically the same thing in this bin, while in other bins it computes in an extremely small order of magnitude compared to the value found in this first bin.

I was wondering if anyone could explain what could be causing this?

 

[Alex A]

Hi @lucasfaraujo

Welcome to physicsforums. I don't know the answer and I've never used LET but I see something I don't understand and maybe that will help. Rather than setting energy bins you should have stopping power bins, so your numbers read very low to me. What is in your tally cell, and is it a gas?

If you are able to share your input file, or your output file you could change them to end with .txt and attach to a post.

 

[PSRB191921]

Hi,
The your result is logical. 1E-5 is in MeV/cm i.e. 100 keV/µm but for electrons the LET is much lower than this value: all results are in the bin 0 - 1E-5 therefore also between 0 and 1e-3.
you should sample between 0 and less than 10 keV/µm

 

[Alex A]

@PSRB191921 , respectfully, I think you have a math snafu. 1e-5 MeV I think is 10ev. The high value of 1.38 MeV/cm I make to be 138ev/um. I did a sanity check with total stopping power of silicon because I had it open for another problem, and it didn't dip below about 2MeVcm2/g, which is around 500ev/um for the elemental solid.

So the top bin is about 50'000 times lower than the example bins for protons and the highest frequency tally result is 3 or more orders below. So I'm wondering if the tally cell is air or empty.

 

[PSRB191921]

oups you are right !

 

[lucasfaraujo]

@PSRB191921 , respectfully, I think you have a math snafu. 1e-5 MeV I think is 10ev. The high value of 1.38 MeV/cm I make to be 138ev/um. I did a sanity check with total stopping power of silicon because I had it open for another problem, and it didn't dip below about 2MeVcm2/g, which is around 500ev/um for the elemental solid.

So the top bin is about 50'000 times lower than the example bins for protons and the highest frequency tally result is 3 or more orders below. So I'm wondering if the tally cell is air or empty.

Hey guys, first, thank you for your response and greetings.

Second, to simplify the analysis of the problem, I inserted a small water sphere less than 2 cm away from the Ir-192 source. I used this tally to calculate the LET. The observed behavior is consistent: it is counted in the smallest bin, and then it shows a zero value or an extremely small value. I put a large bin range, but it gave the same result

 

[Alex A]

I don't know why the program behaves in this way.

There are issues in the input file, energy cut off for electrons is 1kev so a lot of the photon flux from the source can't generate transportable electrons. I also don't understand the PHYS card but that is not the problem. Tables are loading, the transport works but at the last step to look up the LET is always 0 for any energy of electron. This is very puzzling, I hesitate to say it but maybe you have found a bug!

I do question why you want LET. It can be very useful in designing shielding, but your source is not a beta source, it's a photon source. I'm quite confused what LET means under these circumstances.

You can run the input file using energy bins and then convert to LET manually of course.

 

[lucasfaraujo]

Actually, I wanted to generate the LET from secondary electrons produced by this photon source. Although there are other analyses available, I came across this tool in the manual and wanted to test it. However, I encountered a problem. Since this tool is not commonly used, I was hoping to find someone who is familiar with it and can provide assistance.

I don't know why the program behaves in this way.

There are issues in the input file, energy cut off for electrons is 1kev so a lot of the photon flux from the source can't generate transportable electrons. I also don't understand the PHYS card but that is not the problem. Tables are loading, the transport works but at the last step to look up the LET is always 0 for any energy of electron. This is very puzzling, I hesitate to say it but maybe you have found a bug!

I do question why you want LET. It can be very useful in designing shielding, but your source is not a beta source, it's a photon source. I'm quite confused what LET means under these circumstances.

You can run the input file using energy bins and then convert to LET manually of course.