Geant4 Failing to Match with MCNP6 and NIST for Simple Test of Energy Deposited on a Cesium Iodide Cube

Hello, I have spent the past month and a half trying to get Geant4 to agree with a simple test done in MCNP6.I have been trying to recreate the first plot in the image given below, where the model is simple. A circular, monoenergetic photon source of radius .6 cm is placed .5 cm away from a 1 cubic centimeter Cesium Iodide cube, and the direction of the photons are normal to the surface of the cube. The energy deposited on the cube in MeV/photon is plotted versus the photon energy. I am using the QGSP_BERT_EMV physics list, my scoring volume is done via a macro and calculates eDep on the CsI cube. The issue is that Geant4 is giving me the following distribution which is the second plot in the image below. To get straight to the point, at high photon energies, Geant4 is underestimating the energy which is deposited on the cube. Now, I have tried numerous different physics lists which contain the standard electromagnetic processes, different methods of scoring, and playing with G4StepLimiter to change the maxStepLimit in the cube, but after a month I am still clueless as to what I am missing that is causing Geant4 not to match MCNP6 and get this nice linear relation, whether it is a blatant or subtle issue. I have also tried doing a simulation of mass-energy absorption coefficients of CsI to match with NIST at 20 MeV, and Geant4 again underestimated the energy deposited on the cube, however the absorption coefficient did match nicely with NIST at .6 MeV. Any insight that may help me identify what’s going on would be greatly appreciated!

Hi! I came to the same result, trying different physics lists and got nothing. But i my case the curve a little bit different from yours (in range up to 20 MeV). I used geant4.10.5 and QGSP_BIC pl for this plot. And to calculate the energy deposition I used sensitive detector as CsI volume.

Thanks for your observation! I am using Geant 10.2. Perhaps there is some sort of physical process that must be manually enabled that is causing this flattening in the curve.

Hi again! Maybe there should be included scintillation process to CsI material to get good comparision? Because by default scintillation process is not taken into account.

Hi, I think that is certainly a possibility. I will investigate this over the next few days, and post my findings here. Thanks for the suggestion!

Hi Aleksei,

I wanted to point out that I have tried conducting this same simulation but with lead, and am receiving similar results, where the energy deposited per photon at higher energies is still not linear. This leads me to believe that the issue may not be scintillation, and I am also aware that scintillation in Geant is not the most straight-forward process to implement. Lastly, I wanted to point out that the plot which was obtained in MCNP uses an F6 tally, which is a method that measures energy deposited based on collisional heating, which I do not think includes scintillation.

Hi! I agree with you. Yesterday I icluded scintillation process in that simulation and doposited energy from scintillation was very small and I think it is not worth to take into account this praction of energy. Could you please show the plot from MCNP of deposited energy in case of using 1 cudic centimeter lead to compare that with geant4?

Hi Aleksei, apologies for the delayed response. Unfortunately, I have no experience using MCNP. The MCNP plot which I am comparing with Geant was created a few years ago by another lab member. It may be worth noting that using the online NIST database for various attenuation coefficients, one can make a plot of the energy deposited per photon as a function of photon energy using the NIST coefficients for Cesium Iodide. I have attached one that I have made myself below. Note, E_0 is the photon energy, u is the mass-energy absorption coefficient from NIST at that energy, and x = 1 since I am interested in a cube of 1 cm thickness. NIST only has these coefficients up to 20 MeV on their website, so I have only gone up to 20 MeV, though it still seems that the “correct” distribution is in fact a linear one, at least for CsI. Furthermore, I recently read a paper comparing Geant and MCNP which stated that MCNP internally uses NIST coefficients for some of its energy calculations, which would explain why it matches nicely with the linear distribution. I am confident that Geant should be able to match this since it is used so widely, I just cannot seem to figure out what is missing.
nistplot

For anyone interested, i found this paper which matches Geant4 data to NIST coefficients successfully even at higher energies, but using Geant 6.2.

https://ieeexplore.ieee.org/abstract/document/1495783?casa_token=Lf-LmxdP-ecAAAAA:6MUzIwOKHXi-4yd6wtpVaX6OFDDVFtBVqnHy-_TSRhyn6oiisBZv7_sqJPibhqGuNXOK-Bph