Get difference dose rate among experiment, MCNP and GEANT4 simulation

Hello experts,
I have performed simple experiment using dosimeter. In which i have measured gamma dose rate using difference thickness of absorbers. I want to compare my experiment result with GEANT4 and MCNP simulation. But i get difference in dose rate by geant4 simulation.
In geant4 simulation, I have used application of code basic/exampleB1. Please tell me how can i improove my result in geant4 simulation.
Thank you.

The results shown in attached image.


More information is needed. What are the geometries for the experiment, the MCNP simulations and the Geant4 simulations? Are they the same? What energy gamma rays did you use?

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Thank you very much for the reply.
The experimental and MCNP geometry sent in attached images. Geant4 and MCNP geometry are same.
source to dosimeter distance is 4 cm.
lead collimeter length is 3 cm with 1.2 mm diameter hole.
An energy of gamma is 662 keV.
I have select QBBC physics list in Geant4 simulation. I have sent Geant4 simulation code in attached files.
Dosimeter consist of GM detector. (8.3 KB) (4.4 KB) (5.8 KB) (3.1 KB) (2.6 KB)
dosimeter_sim_geometry (2.9 KB) (3.8 KB)
CMakeLists.txt (2.4 KB)
B1PrimaryGeneratorAction.hh (2.7 KB)
B1SteppingAction.hh (2.3 KB)
B1RunAction.hh (2.4 KB)
B1EventAction.hh (2.3 KB)
B1DetectorConstruction.hh (2.4 KB)
B1ActionInitialization.hh (2.2 KB)
PhysicsList.hh (2.1 KB)

I ran your code with no modifications and got for 10^6 primary (source) gamma rays:
tPb Dose rms dose
0.18mm 598.003pGy 35.319pGy
0.36mm 609.27pGy 35.9464pGy
0.54mm 600.357pGy 35.7235pGy

Note that the latter is what your code outputs. Conversion to uSv/h requires knowledge of the source strength to get the time for 10^6 662 keV gamma rays to be emitted. I do not know your source strength, so I assumed 1microCuries and an 85.1% branching ratio for 662 keV gammas. That implies that 10^6 primary gammas are emitted in 31.75913s. This gives:

tPb Dose
0.18mm 0.067785 uSv/h
0.36mm 0.069062 uSv/h
0.54mm 0.068052 uSv/h

A discrepancy between your source strength and mine could explain the rough scale difference (~0.1/0.068), but the trend of mine with thickness is not the same as you have in your table.

What version of Geant4 are you running? What is your source strength?

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Thank you very much for the reply.

Version: geant4-10-07-patch-02
Present Activity : 50854 Bq
Why dose rates are not match with experimental data ?

Using your source strength, I get:
tPb Dose
0.18mm 0.1095 ± 0.0065 uSv/h
0.36mm 0.1115 ± 0.0065 uSv/h
0.54mm 0.1099 ± 0.0065 uSv/h
This agrees with your Geant4 values to within simulation statistical uncertainty. The dose values do not vary significantly with absorber thickness to within simulation statistical uncertainty. This may be reasonable, given the geometry, but I’ll have to look at it more closely.

What are the uncertainties for the MCNP run and the experiment?

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Relative error in MCNP is about 9 % and in experiment about 10%.


So the experiment, MCNP and Geant4 behave the same to within statistical error (i.e., variation with thickness). There is just a scale difference ~1.6 to 1.8.

I cannot say anything about MCNP, since I do not have the code and do not use it. (Maybe others can jump in on that.)

How is dose being measured in the experiment? Specifically, what is the detector and how good is its calibration?

For both MCNP and Geant4, have you checked how sensitive the results are to small changes in dimensions?

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I have used dosimeter PM1405. It is consist of GM detector. I have sent a mannual of dosimeter in attached file. I have measured gamma dose rate.
I have not checked last point as you mentioned.

dosimeter_mannual.pdf (1.6 MB)

I think the reason behind the discrepancies may be that your detector model is not a good approximation to a GM detector. You are representing the experiment’s Geiger Muller tube by a wall-less 3.2cm diameter x 2cm thick cylinder filled with an Argon/ethanol mixture at an unrealistic pressure. (The detector material has a density of 1.5842g/cm3 which would suggest a pressure of roughly 1000atm. You define argon with a density of 1.784g/cm3 which is 1000 times the density of argon at STP). Gas fills for typical GM tubes are ~0.1atm.

In a typical GM tube intended to detect gamma rays, there is a 1-2mm thick wall, often of steel, containing the gas. For 662 keV gamma rays, most of the gamma interactions that register as detections due to gas ionization occur in the walls. Furthermore, GM counters usually have dose compensation, using additional filters around the GM tube and/or software.

I looked at one study [1] of a GM tube and for their MCNP simulation they seem to input a sensitivity with units of (Cs-137 cpm at 1mR/h). That might explain why the MCNP values are closer to the experiment.

[1] “Development of Energy Compensated Geiger Muller Detector Based on the T2416A Canberra Co. GM Detector”, MSc Thesis, Omar Mohamed Noor, Faculty of Energy Systems and Nuclear Science Program University of Ontario Institute of Technology

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Thank you very much John for your reply. I understood which parameters affect my simulation result.