Problems with a simple disksource-model

[Ralf Eckert]

Hi,

i'am a novice-user and use MCNP5 to simulate/generate
pulse-high spectra.
Herefor i try with some simple experiments [end of this post]:
- NaI-Detector with a diameter 3cm
- Cs-137 disksource
i start the program with the following command:
mcnp n=nai o=naio tasks 4

Subsequently the three questions/problems:
[1] is my source definition correct?
SDEF RAD=d1 POS=0 0 0 AXS=0 0 1 PAR=2 ERG=0.66166
SI1 0 1.5
SP1 0 1

[2] while running, the program plotting endless lines of
the following messages:
number of points to plot is less than 2. no plot.
number of points to plot is less than 2. no plot.
number of points to plot is less than 2. no plot.
..and finally:
run terminated when 100000000 particle histories were done.
warning. 1 of 1 tallies had bins with large relative errors.
dump 2 on file nair nps = 100000000 coll = 12140587
ctm = 6.06 nrn = 605478702
mcrun is done

[3] moreover the generatet results don't give a reliable peak at 0.662 MeV

Can someone please help me and/or give me a hint, what
i'm doing wrong?

Thanks in advance!
Ralf

##$# input-file$###
c define cells
c
1 1 -3.67 -1 2 -3 imp:p=1 $detector
10 0 -100 #(-1 2 -3) imp:p=1 $define inside world
11 0 100 imp:p=0 $define outside world

c define surface
1 cz 1.5 $define detector
2 pz 1 $define detector bottem
3 pz 4 $define detector top
100 so 10 $define world

c define source
c Cs-137 (no x-rays)
SDEF RAD=d1 POS=0 0 0 AXS=0 0 1 PAR=2 ERG=0.66166
SI1 0 1.5
SP1 0 1
c define tallies
mode p $photon transport
f8:p 1 $ ? photons on surface 1
e8 0 1.e-5 0.1 1024i 1.0 $ 0.1Mev <-- 4096 channels --> 1.0 MeV
phys:p 100 0 0 0 0 $ [100] ?; [1] switch of e; [1] no Bremsstrahl.
c ***************** end of tallies
c define materials
m1 11000 0.15 53000 0.85 $NaI-detector
c
c define run time
c
mplot freq 50000
nps 100000000
c ctme 30

 

[Achy47]

Hello Ralf,

Thank you for sharing your MCNP5 input file and your questions. I am happy to help you with your simulation.

First of all, your source definition looks correct to me. You have defined a disk source with a radius of 1.5 cm and an energy of 0.66166 MeV. However, I would recommend using the "SRC" card instead of "SDEF" for your source definition. The "SRC" card allows you to specify a disk source with a defined energy spectrum, which would be more accurate for your simulation.

Secondly, the endless lines of messages you are seeing are normal output from MCNP5. These messages are just informing you of the progress of your simulation. The last few lines of the output indicate that your simulation has finished and your tally results have been dumped to a file called "nair".

Lastly, it is possible that your results are not giving a reliable peak at 0.662 MeV because of the low number of particles used in your simulation (100,000,000). I would recommend increasing the number of particles to at least 1 billion for better statistics. You can do this by changing the "nps" card in your input file.

I hope this helps you with your simulation. If you have any further questions, please don't hesitate to ask.