|Message: Appropriate physics list for scoring fluence differential in energy for low energy electrons||Not Logged In (login)|
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My question is related to using Geant4 to score fluence differential in energy - normalised by absorbed dose. For the definition of fluence differential in energy, see (http://www.npl.co.uk/science-technology/radiation-dosimetry/ionising-radiation-quantities-and-units).
The geometry that I am using is a cylindrical water (approx 1m in length and 1m in diameter) phantom with a 0.15MeV photon beam (0.15MeV Bremstrahlung beam) incident on one of the cylinder faces. The photon beam has a diameter of 5cm.
Within the water phantom I define another small cylinder (material is water and the size is 1cm length and 2cm diameter) which is my scoring volume. Within this volume I score deposited dose and also fluence differential in energy (I have written my own scorer to do this).
I have results obtained using the exact same geometry in EGSnrc, and given that this code is well validated for electrons and low energies I treat this as my benchmark. The problem that I see is that when I try and replicate the results obtained from EGSnrc using my Geant4 code, the fluence at low energies is not what I would expect to see. However, at high energies the agreement is much better (please see attached image)
The same effect of a significant underestimation of fluence at lower energies is evident in higher energy incident photon beams, as show in the 4MeV spectra attached.
Does anyone have any experience with this type of calculation, or could give advice on a suitable parameters to include in a physics list/perhaps could indicate where I am going wrong? In the past I haven't worked with low energy electrons so would appreciated advice.
I have dabbled with various physics lists (such as the one in the TestEm11 example) and also with G4emLivermorePhysics (which these images are produced from).
Attachment: http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2014/09/08/05.17-96814-PhotonBeam_4MeV.png http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2014/09/08/05.20-19946-PhotonBeam_0.15MeV.png
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