Hi there,

without knowing the details of your simulation I will say that this is a deep penetration problem. Meaning that the strong attenuation does not let many photons to reach your tallies. In fact, this is the reason why the tally uncertainty increases with distance. Related to that, MCNPX does not account for background "noise" or radiation; everything has to be modeled. If you search for "variation reduction" techniques you will get some ideas how to decrease the uncertainties (meaning weight windows, source biasing, importances). As a first step, you should look at the F5 tallies and the "flux-to-dose" conversion factors, included in MCNP. The latter will give you a more meaningful dose calculation for use in radiation protection. I hope this helps.

Good luck,

dimitris

MCNPX does not calculate the background, unless it is included in the model, which would be quite a heavy work.

The large uncertainty is presumably due to a too small number of simulated photon histories. Besides variance reduction techniques, increasing the number of simulated photons should be the main part of the solution.

In MCNP you need to know the source strength. The background radiation as a source must be evaluated experimentally. Or easily can be neglected in most of the problems.

Hello, Sir Dimitrios Kontogeorgakos, François Tondeur, Farshid Tabbakh. Thank you for the answer, it helps a lot. Here I attached the code I used for this simulation. I use F6 tallies with multiplication factors that convert the output unit from MeV/gram to mSv/hour. Thank you so much for your attention and participation.

A very rough calculation based on the radpro calculator http://www.radprocalculator.com/Gamma.aspx

for a Co60 source in a U shield of 12 cm, indicates that probably only

Dear Rinaldi Hadisaputra, it is great that you have made progress and got the results you were looking for. One thing I would like to suggest is testing the use of the dose energy (DE) and dose function (DF), described in section 5.6.8 of the MCNPX2.6 manual.

Best of luck,

Dimitris

Dear François Tondeur. Based on that website, for a Co60 source in a U shield of 12 cm, the dose-rate result I have got is 4.742246E+006. Then how do you know it can indicate that probably only

With the online calculator, for 1 Ci Co60 at 1 m

No shielding 11350 µSv/h

10 cm water (similar to soft tissue) 10300 µSv/h , i.e. 10 cm water stop only about 10% of Co60 radiation

12 cm U 0.024 µSv/h thus 12 cm U reduce Co60 radiation by a factor 473000.

With the very rough assumption that the dose reflects the intensity of the primary Co60 radiation, thus neglecting the build-up (not too bad in U, but not very good in water)

from 2 billion photons, only 2000000000/473000 go through U = 4230

at 1 m they cross a sphere of 4*3.14*100*100 = 125600 cm2

a detector of 10*10 = 100 cm2 with be hit by 4230*100/125600 = 3.3 photons

If the detector is water, only 10% = 0.3 photons will be stopped on the average by the detector.

As indicated, this is a very rough estimation, but it should give the correct order of magnitude.

For a better evaluation, put an additional tally in your MCNPX model, that will give you the number of photons, e.g. a flux tally.