# An extremely basic question on MCNP

• Takvorian

#### Takvorian

TL;DR Summary
How to calculate the gamma dose rate from MCNP results ?
Hi there,

I have a very simple question about MCNP (6.2 to be precise) ... maybe someone here might enlighten me ...

Based on the (more than simple) MCNP input file below, which describes a sphere with R=200cm, filled with air and a point source in the center. There's a single ring tally on the X axis in a distance of 100 cm from the source, which should give the results in mSv/h/particle. The source is Co-60 with its 3 gamma photons defined.

When I run that file, the tally F15 gives me a value of 1.4657E-15 mSv/h/particle (error is 0.0008).

Question: How do I calculate the correct dose rate for a source with i.e. 1E+6 Bq from that tally result ?

Greetings and may that not too boring a question ... but that topic seems to be a bit neglected in the manuals and lectures ... :(

Takvorian

MCNP input:
MCNPX Visual Editor Version X_25
c     Created on: Wednesday, February 23, 2022 at 19:58
1   304 -0.001205 -1  imp:p=1
2     0         1  imp:p=0

c    Welt
1        so 200  $Welt mode p m304 6000 -0.000124$Air (Dry, Near Sea Level) Density: -0.001205
7014        -0.7525 7015     -0.0027668 8016       -0.23123
8017   -8.7866e-005 18036  -4.2793e-005 18038  -8.0671e-006
18040     -0.012776
sdef pos=0 0 0 ERG=D1
c
c source energies (Co-60)
si1 L 0.6938 1.1732 1.3325
sp1 D 1.6312e-4 1 1
c
f15x:p 100 1 -0.125
df15 IU 2 ic 10
stop F15 0.0005

Hi,
for my part to calculate dose equivalent rate, I use a function response (DE/DF).
The “de” card contains discrete energies and the “df” card provides the values of the conversion
factors from ICRP 74.
For example for photons in terms of H*(10) :
DE15 .01 .015 .02 .03 .04 .05 .06 .08 .1 .15 .2 .3 .4 .5 .6 &
.8 1 1.5 2 3 4 5 6 8 10
DF15 0.061 .83 1.05 .81 .64 .55 .51 .53 .61 .89 1.2 1.8 2.38 2.93 3.44 &
4.38 5.2 6.9 8.6 11.1 13.4 15.5 17.6 21.6 25.6

Your results are in pSv for one particle.
So for A=1E6 Bq you must calculate the number of photons :
N=1E6*( 1.6312e-4 + 1+ 1)=2E6 gamma/s
MCNP gives F5=4.7807E-05 (the unit is pSv per photon)
So H*(10)=4.7807E-05*1e-12*2E6*3600= 3.44E-7 Sv/h
I insert input and output files
This is well described in this book :

#### Attachments

• PF_dose.txt
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• outa.txt
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• • Astronuc, Takvorian and berkeman
Thanks - that resolved my question.

• berkeman