Why Are MCNP Photons Getting Lost?

[MadGander]

Hello,

I've been running into some frustrating issues with my MCNP deck. Photons are getting lost which is terminating the run file prematurely. When consulting the output file there seems to be some sort of geometry issue, but there are no fatal errors that I can see so I'm lost on how to resolve this problem. Any help is appreciated. The deck is listed below.

c cell card
1 1 -14.36 -1 imp:p 1     $ sample
2 2 -1.43 -2 1 imp:p 1     $ Kapton tape
3 2 -1.43 -3 imp:p 1
4 2 -1.43 -4 imp:p 1
5 2 -1.43 -5 imp:p 1
6 2 -1.43 -6 imp:p 1
7 4 -1 -7 imp:p 1     $ Scotch tape
8 0 -8 imp:p 1     $ tally cells
9 0 -9 imp:p 1
10 0 -10 imp:p 1
11 0 -11 imp:p 1
12 0 -12 imp:p 1
13 0 -13 imp:p 1
14 0 -14 imp:p 1
15 0 -15 imp:p 1
16 0 -16 imp:p 1
17 0 -17 imp:p 1
18 0 -18 imp:p 1
19 0 -19 imp:p 1
20 0 -20 imp:p 1
21 0 -21 imp:p 1
22 0 -22 imp:p 1
23 0 -23 imp:p 1
24 0 -24 imp:p 1
25 0 -25 imp:p 1
26 0 -26 imp:p 1
27 0 -27 imp:p 1
28 0 -28 imp:p 1
29 3 -0.001205 -29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
     19 20 21 22 23 24 25 26 27 28 imp:p 1     $ bounding sphere
30 0 29 imp:p 0     $ graveyard

c surface card
1 RPP 33.02 33.1  -0.556 0.556  -0.556 0.556     $ sample 0.08 cm thick
2 RPP 33.017 33.103  -0.556 0.556  -0.556 0.556     $ Kapton tape 0.003 cm thick
3 RPP 33.1 33.103  -0.9525 0.9525  -0.953 -0.556     $ lower edge of kapton
4 RPP 33.1 33.103  -0.9525 0.9525  0.556 0.953     $ upper edge of kapton
5 RPP 33.1 33.103  -0.953 -0.556  -0.9525 0.9525     $ left edge of kapton
6 RPP 33.1 33.103  0.556 0.953  -0.9525 0.9525     $ right edge of kapton
7 RPP 33.0168 33.017  -3 3 -15 15     $ Scotch tape
8 RPP 58.42 59.42  -0.278 0.278  -0.278 0.278     $ center tally cell
9 RPP 58.42 59.42  0.278 0.556  0 0.278     $ center/right/top
10 RPP 58.42 59.42  0.278 0.556  -0.278 0     $ center/right/bottom
11 RPP 58.42 59.42  -0.556 -0.278  -0.278 0     $ center/left/bottom
12 RPP 58.42 59.42  -0.556 -0.278  0 0.278     $ center/left/top
13 RPP 58.42 59.42  0 0.278  0.278 0.556     $ center/top/right
14 RPP 58.42 59.42  -0.278 0  0.278 0.556     $ center/top/left
15 RPP 58.42 59.42  0 0.278  -0.556 -0.278     $ center/bottom/right
16 RPP 58.42 59.42  -0.278 0  -0.556 -0.278     $ center/bottom/left
17 RPP 58.42 59.42  0.278 0.556  0.278 0.556     $ center/northeast
18 RPP 58.42 59.42  0.278 0.556 -0.556 -0.278     $ center/southeast
19 RPP 58.42 59.42  -0.556 -0.278  0.278 0.556     $ center/northwest
20 RPP 58.42 59.42  -0.556 -0.278  -0.556 -0.278     $ center/southwest
21 RPP 58.42 59.42  -0.834 0.556  0.556 0.834     $ top off edge tally
22 RPP 58.42 59.42  0.556 0.834  -0.556 0.834     $ right off edge tally
23 RPP 58.42 59.42  -0.566 0.834  -0.834 -0.556     $ bottom off edge tally
24 RPP 58.42 59.42  -0.834 -0.556  -0.834 0.556     $ left off edge tally
25 RPP 58.42 59.42  -0.834 0.834  0.834 1.834     $ top open beam tally
26 RPP 58.42 59.42  -0.834 0.834  -1.834 -0.834     $ bottom open beam tally
27 RPP 58.42 59.42  0.834 1.834  -0.834 0.834     $ right open beam tally
28 RPP 58.42 59.42  -1.834 -0.834  -0.834 0.834     $ left open beam tally
29 SO 68

c data card
mode p
nps 1000000000
SDEF POS 0 0 0 PAR=P ERG=d1
SI1 H 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 0.0045
     0.005 0.0055 0.006 0.0065 0.007 0.0075 0.008 0.0085
     0.009 0.0095 0.01 0.0105 0.011 0.0115 0.012 0.0125
     0.013 0.0135 0.014 0.0145 0.015 0.0155 0.016 0.0165
     0.017 0.0175 0.018 0.0185 0.019 0.0195 0.02 0.0205
     0.021 0.0215 0.022 0.0225 0.023 0.0235 0.024 0.0245
     0.025 0.0255 0.026 0.0265 0.027 0.0275 0.028 0.0285
     0.029 0.0295 0.03 0.0305 0.031 0.0315 0.032 0.0325
     0.033 0.0335 0.034 0.0345 0.035 0.0355 0.036 0.0365
     0.037 0.0375 0.038 0.0385 0.039 0.0395 0.04 0.0405
     0.041 0.0415 0.042 0.0425 0.043 0.0435 0.044 0.0445
     0.045 0.0455 0.046 0.0465 0.047 0.0475 0.048 0.0485
     0.049 0.0495 0.05 0.0505 0.051 0.0515 0.052 0.0525
     0.053 0.0535 0.054 0.0545 0.055 0.0555 0.056 0.0565
     0.057 0.0575 0.058 0.0585 0.059 0.0595 0.06 0.0605
     0.061 0.0615 0.062 0.0625 0.063 0.0635 0.064 0.0645
     0.065 0.0655 0.066 0.0665 0.067 0.0675 0.068 0.0685
     0.069 0.0695 0.07 0.0705 0.071 0.0715 0.072 0.0725
     0.073 0.0735 0.074 0.0745 0.075 0.0755 0.076 0.0765
     0.077 0.0775 0.078 0.0785 0.079 0.0795 0.08 0.0805
     0.081 0.0815 0.082 0.0825 0.083 0.0835 0.084 0.0845
     0.085 0.0855 0.086 0.0865 0.087 0.0875 0.088 0.0885
     0.089 0.0895 0.09 0.0905 0.091 0.0915 0.092 0.0925
     0.093 0.0935 0.094 0.0945 0.095 0.0955 0.096 0.0965
     0.097 0.0975 0.098 0.0985 0.099 0.0995 0.1
     $ histogram upper boundaries (200 energy bins)
SP1 D 0
     0
     5.11347E-06
     0.000121555
     3.08604E-05
     0
     0
     8.31027E-06
     8.7771E-05
     0.000359376
     0.000946079
     0.001877169
     0.003548272
     0.005150231
     0.006742632
     0.016790123
     0.014374974
     0.012801486
     0.024634899
     0.011637658
     0.015204991
     0.0107191
     0.008945965
     0.009214431
     0.015634477
     0.009179067
     0.009152242
     0.00943935
     0.010478518
     0.010235272
     0.010345676
     0.010531003
     0.010234591
     0.01073557
     0.010859254
     0.011212683
     0.011140717
     0.011089523
     0.010867856
     0.010987345
     0.011155824
     0.010809004
     0.011037909
     0.010752565
     0.010937409
     0.010729171
     0.010745011
     0.010583666
     0.010481057
     0.010277613
     0.010390617
     0.010158208
     0.010171552
     0.009960702
     0.010035846
     0.009819078
     0.009828614
     0.009517127
     0.009333436
     0.009406346
     0.009269727
     0.008867548
     0.008940091
     0.008521914
     0.008432146
     0.008507657
     0.008371625
     0.008354725
     0.00794921
     0.007963582
     0.007768016
     0.007666614
     0.00774671
     0.007514888
     0.007287714
     0.007189879
     0.007027001
     0.006843111
     0.006814986
     0.006970499
     0.006564386
     0.006477514
     0.006288494
     0.006329008
     0.006155955
     0.006046507
     0.005918102
     0.005944737
     0.00578743
     0.005658994
     0.00563903
     0.005555042
     0.005472806
     0.005113881
     0.005375275
     0.005027333
     0.005019937
     0.005089112
     0.004891952
     0.004766316
     0.004586465
     0.004527969
     0.004351737
     0.004400257
     0.004311621
     0.004322532
     0.004149311
     0.004076936
     0.003974694
     0.004101935
     0.003906201
     0.003932123
     0.003898879
     0.003951311
     0.003648835
     0.008549116
     0.003395487
     0.012448897
     0.003362305
     0.003395445
     0.003248954
     0.003229609
     0.003147635
     0.002916915
     0.002987517
     0.002910967
     0.002877974
     0.00273077
     0.002851213
     0.002961867
     0.002615342
     0.002670858
     0.002596836
     0.005511989
     0.002374152
     0.002347264
     0.002388104
     0.002970019
     0.002251517
     0.001893168
     0.001914275
     0.002046383
     0.001923139
     0.001900081
     0.00176021
     0.002097179
     0.001679893
     0.001536802
     0.001690877
     0.002112264
     0.001642358
     0.001356783
     0.00140141
     0.001483782
     0.001486027
     0.001390132
     0.001416076
     0.001205739
     0.001301917
     0.001234389
     0.001154041
     0.001219586
     0.00121325
     0.001145995
     0.001201994
     0.001040079
     0.001012389
     0.000876481
     0.001004057
     0.000925759
     0.000918481
     0.000809436
     0.000821415
     0.000789026
     0.000813591
     0.00068724
     0.000703664
     0.000756499
     0.000682395
     0.000586809
     0.000578912
     0.00049449
     0.000509462
     0.000464834
     0.00045288
     0.000404728
     0.000394017
     0.000356244
     0.000352201
     0.000281882
     0.000271873
     0.00022954
     0.000237268
     0.000188778
     0.000170614
     0.000129775
     9.31903E-05
     6.61896E-05
     2.2146E-05
     $ probabilities per bin
F14:p 8
F24:p 9
F34:p 10
F44:p 11
F54:p 12
F64:p 13
F74:p 14
F84:p 15
F94:p 16
F104:p 17
F114:p 18
F124:p 19
F134:p 20
F144:p 21
F154:p 22
F164:p 23
F174:p 24
F184:p 25
F194:p 26
F204:p 27
F214:p 28
E0 0.02 0.1    $ 2 energy bins
EM0 0 1     $ Removing particles with <20 keV
M1 73000 4 6000 3     $ Ta4C3 MXene film
M2 6000 22 1000 10 7000 2 8000 5     $ C22H10N2O5 Kapton tape
M3 6012 -0.000124 7014 -0.755267 8016 -0.231781
            18040 -0.012827     $ Dry air (sea level)
M4 6000 76 1000 114 8000 49     $ Scotch tape (cellulose acetate)

 

[Alex A]

Please upload as an attachment with .txt added in future. Or you can use bbcode CODE tags which may keep most of the formatting for short pastes. I didn't see any obvious errors on reading so I fixed the formatting and ran mcnp with ip to plot the geometry.

mcnp ip inp=scotch
(let it do a default plot)
(clicked the plot> button)
px 33.101
extent 1.1,1.1

Which shows the Kapton tape, I think. The dotted lines suggest a geometry error in the corners. If this is four sections of tape those corner bits would be doubled up.

 

[MadGander]

Please upload as an attachment with .txt added in future. Or you can use bbcode CODE tags which may keep most of the formatting for short pastes. I didn't see any obvious errors on reading so I fixed the formatting and ran mcnp with ip to plot the geometry.

mcnp ip inp=scotch
(let it do a default plot)
(clicked the plot> button)
px 33.101
extent 1.1,1.1

Which shows the Kapton tape, I think. The dotted lines suggest a geometry error in the corners. If this is four sections of tape those corner bits would be doubled up.

Recoded the Kapton so that there wasn't any overlap and I'm still getting an issue. I think it has to do with cell 29 (my bounding sphere). Here's the output file. As you can see, none of the tally cells are recording any particles (in either energy bin).

 

[Alex A]

First a typo that causes another geometry error,

23 RPP 58.42 59.42  -0.566 0.834  -0.834 -0.556     $ bottom off edge tally

566 should be 556, but that didn't solve the issue.

RPP shapes looks like nice closed surfaces like spheres but they are actually a macrobody that is translated into six infinite, flat, plane surfaces. Some of your tally cells have 0 as one parameter and that means there are plane surfaces passing through the origin where your source is. Never have a point source coincident with a surface. The cause took me a full day to figure out and I already know that rule! It also doesn't seem to make much sense, this is not a cell crossing boundary! It's still in the same cell! But moving the source location ten micrometers in each of the axes (x was probably not needed) seems to fix the problem.

SDEF POS=0.001 0.001 0.001 PAR=2 ERG=d1

Some suggestions, you removed the extra corner bits, but if they exist on the real thing you could add them on top. The efficiency of the problem is very low and this could be improved by putting the source closer to the foil or using a beam instead of an isotropic source.

Anyway, best of luck.

 

[MadGander]

First a typo that causes another geometry error,

23 RPP 58.42 59.42  -0.566 0.834  -0.834 -0.556     $ bottom off edge tally

566 should be 556, but that didn't solve the issue.

RPP shapes looks like nice closed surfaces like spheres but they are actually a macrobody that is translated into six infinite, flat, plane surfaces. Some of your tally cells have 0 as one parameter and that means there are plane surfaces passing through the origin where your source is. Never have a point source coincident with a surface. The cause took me a full day to figure out and I already know that rule! It also doesn't seem to make much sense, this is not a cell crossing boundary! It's still in the same cell! But moving the source location ten micrometers in each of the axes (x was probably not needed) seems to fix the problem.

SDEF POS=0.001 0.001 0.001 PAR=2 ERG=d1

Some suggestions, you removed the extra corner bits, but if they exist on the real thing you could add them on top. The efficiency of the problem is very low and this could be improved by putting the source closer to the foil or using a beam instead of an isotropic source.

Anyway, best of luck.

Really appreciate the help Alex! Thanks a ton. I have another similar deck (420 keV) that is giving me a different fatal error ("entries must be integers"). Is my scientific notation syntax triggering this error, or is there something else potentially going on? I attached the deck below.

Thanks again

 

[Alex A]

You've wrapped around a comment on line 1125, so you need another $ or whatever. When that is fixed it moans about the number of multipliers not matching the number of energy bins.

Edit, You have two more multipliers than energy bins.

 

[MadGander]

You've wrapped around a comment on line 1125, so you need another $ or whatever. When that is fixed it moans about the number of multipliers not matching the number of energy bins.

Edit, You have two more multipliers than energy bins.

Silly mistakes on my part! I have one final question for you. I edited the original input deck to remove surfaces that pass through the origin point, and shifted the point source back to 0,0,0 thinking that it would be fine to do so. However, I'm again getting lost particles for some reason. Could you take a look?

 

[Alex A]

I don't get any lost particles on short runs. The full problem would be something like a thousand hours on my laptop. You have reintroduced the kapton four corners overlapping geometry error. If you fix that and still get lost particles you may need to post an output file. All the best!

 

[MadGander]

I don't get any lost particles on short runs. The full problem would be something like a thousand hours on my laptop. You have reintroduced the kapton four corners overlapping geometry error. If you fix that and still get lost particles you may need to post an output file. All the best!

My lost particle issues have been resolved! I have another unrelated question that pertains to F6 tallies. I'm getting an interesting fatal error that states simply "tally volumes or areas were not input nor calculated." I did some digging around online and in the MCNP manual, but can't seem to get any concrete answers why this is happening and how to fix it. I'm using GadOx detector material to compare the different tally cells. The issue automatically resolves itself when I change the tally type over to F4, so the problem is directly tied to the F6 feature. I've specified the tally cells geometrically and have a defined density... so the error doesn't really make any sense to me. Appreciate any insight you could provide. Thanks

 

[Alex A]

If F4 isn't broken something weird is going on. testt runs for me on several versions of the program. Check it's not a typo or something on the command line or you aren't running the wrong input file and if not post your output file please.

 

[MadGander]

If F4 isn't broken something weird is going on. testt runs for me on several versions of the program. Check it's not a typo or something on the command line or you aren't running the wrong input file and if not post your output file please.

Got it cleared up. One more thing... Why is the tally cell directly behind the shielded sample reading a higher MeV/g than those around it? In theory shouldn't it be the opposite, or am I not correcting for a certain factor?

 

[Alex A]

The source isn't isotropic in that input file, it has VEC=1 0 0 DIR=1, so it's a beam and a lot probably goes straight through the 0.8mm sample. I'm not completely sure what RAD is doing, line source side on?

The problem is mode p, and I'm not sure how that affects things either, in the real world most of these interactions are governed by the photoelectric effect and the actual energy loss is through electrons. Maybe its just tallying the photons as totally absorbed where they interact.

This sounds like progress though, well done and best of luck for finishing it!

 

[MadGander]

The source isn't isotropic in that input file, it has VEC=1 0 0 DIR=1, so it's a beam and a lot probably goes straight through the 0.8mm sample. I'm not completely sure what RAD is doing, line source side on?

The problem is mode p, and I'm not sure how that affects things either, in the real world most of these interactions are governed by the photoelectric effect and the actual energy loss is through electrons. Maybe its just tallying the photons as totally absorbed where they interact.

This sounds like progress though, well done and best of luck for finishing it!

Hi Alex,

I'm experimenting with a surface plane source instead of a point source and I'm running into the lost particle issue again. The same deck ran fine with a point source, but now is losing particles with a surface source. Any thoughts about why this is happening? The geometry is identical.

 

[Alex A]

The short answer is I don't know, but the source needs fixing first.

30.1 is a macrobody surface. MCNP doesn't deal in objects, it deals in surfaces and it knows where it is according to surface tests. RPP translates into 6 infinite surfaces, so while you think you've defined a finite edge, very logical attempt btw, MCNP doesn't work like that.

You have also made that surface the edge of a cell. That is fine, probably. Surface sources are not isotropic, shouldn't be a problem. How to ensure you are emitting from the correct side of a surface I don't know. Half of the surfaces for RPP are explicitly upside down/inside out. If one side fails, try the other one!

So how does MCNP sample a plane? If fed SUR=<plane surface>, it will sample a circle of radius RAD. That is on a circular line, not an area and it needs feeding POS= with a point on the plane in addition to RAD. If you need a uniform, flat, disc source make RAD a power distribution with a=1.

So adapted from the MCNP Primer, the missing entries are something like this;
POS=0 0 0 AXS=1 0 0 EXT=0 RAD=d2
SI2 0 3 $ radial sampling range: 0 to Rmax (=3cm)
SP2 -21 1 $ radial sampling weighting: r^1 for disk

I imagine you can also use distributions to construct square or rectangular degenerate volume sources. A degenerate source is a volume source in which one or more degrees of freedom that would normally be assigned a distribution have been set to a constant value. So by setting EXT=0 a cylinder source becomes a disc, or by setting the radius value to zero, RAD=0, a line source along the axis of what was the cylinder etc.

Fixing the source did not fix the problem and I tried 30.1 and 30.2. I can't see a problem with the cell description. I took away the source cell and replaced the source surface with px 0, which I assume was the 30.1 surface. It runs. I may try to investigate why later.

Congrats on finding some really strange errors and best of luck!