"10 particles lost" warning in MCNPX

[Nour_wahban]

TL;DR Summary

hello! i'm working on a landmine detection using neutron technique and the model involves using a ring detector i did my best to understand its part but i'm getting this error and the tally counts are not correct. I attached the input file if someone could help!

 

[Alex A]

Welcome to PhysicsForums @Nour_wahban,

The source is at 0 53 0. Surface 20 is defined as py 53. Never make a point source coincident with a surface. Even surfaces made by a macrobodies and even when they extend beyond the cell they define. Changing to POS=0 52.5 0 solves that error for me. I do not know how ring detectors work, if the tally is still wrong post more information and hopefully someone will know.

Suggest you define the void as "3 0 23", just because it's simpler.

Edit, btw your BPE doesn't surround your PE moderator, how big are the BPE and PE supposed to be?

 

[Nour_wahban]

Welcome to PhysicsForums @Nour_wahban,

The source is at 0 53 0. Surface 20 is defined as py 53. Never make a point source coincident with a surface. Even surfaces made by a macrobodies and even when they extend beyond the cell they define. Changing to POS=0 52.5 0 solves that error for me. I do not know how ring detectors work, if the tally is still wrong post more information and hopefully someone will know.

Suggest you define the void as "3 0 23", just because it's simpler.

Edit, btw your BPE doesn't surround your PE moderator, how big are the BPE and PE supposed to be?

Thank you for the reply, I edited the part of the source, and it works now but still gives me a tally warning "
warning. tally 4 tfc bin did not pass 2 of 10 statistical checks.
warning. 1 of 1 tallies did not pass all 10 statistical checks.
warning. 1 of 1 tallies had bins with large relative errors. "
for the part of BPE it should 5 cm radius with 2.5 cm height while PE is 2.5 cm radius and 5 cm height.

The model goal is to count back scattered neutrons on the ring detector and I'm still struggling how to model this properly.

EDIT, I changed the tally to f4 and added a ring detector cell since the output of f5 is not working properly. but unfortunately, it's not giving me a correct output also. I attached an image with the curve i'm trying to plot to validate this model and the new input file.

 

[Alex A]

"17 cy 5 $PE
18 cy 2.5 $BPE"
These definitions are backwards, swap 17 and 18. This probably isn't the cause of your problems.

Statistical errors are usually an indication too few particles are making it to the detector. Having multiple rings in the problem and running a higher nps might help to get a lot of results with the same compute and better stats.

What are your results and how are you working out the answer?

 

[Nour_wahban]

For the part of BPE and PE I switched them in my reply. They are written correctly in the input file and the image I attached.

I want to see the change in the flux with and without the TNT, but in the output flux doesn't change. It gives me 1.5*10^-3 with the TNT and 1.49*10-3 without it. I cannot understand what's the reason for this output it seems like it cannot see the TNT or it is counting the flux coming from the source not the backscattered.
EDIT: (I'm trying to calculate signal to background ratio, and it should be near 8 but with this output it is about 0.1)
Btw I'm trying to add the output files but their extensions are not acceptable here.

 

[Alex A]

A text file needs to be changed to have a .txt extension. Then you should be able to attach, so long as it isn't too big.

"17 cy 5 $PE
18 cy 2.5 $BPE"
Still seems to be the wrong way round to me, 18 is BPE but it should be 5, 17 is PE but it should be 2.5.

The paper says this "The EBS neutrons can be detected by a suitable detector capable of differentiating the EBS neutrons according to their energy and their flux.", which suggests that the detector is perhaps a bit more clever than most real world detectors. It hints that they have determined what counts as a thermal neutron by what gives the best result. Do you have a plot of flux versus energy with and without the mine?

 

[Nour_wahban]

Actually, it seems to be wrong for me as well, but I'm following the procedures to validate the model.

For the part of detector my conclusion was that he used ring detector tally f5 instead of defining it in a separate cell.
another thing is the EBS neutrons the flux doesn't differentiate between elastic and inelastic scattered neutrons and i think this is one of the drawbacks of my input file the counts are not for EBS neutrons only.

I have added a new tally CF4 to count only the neutrons after they reach cell 1 or 2 (soil and TNT) I don't know other ways to differentiate them in MCNP. I will attach the updated input file with the output files and the plots I got of flux versus energy. If you have any comments or conclusions on them, please let me know.
Thank you so much for your assistance.