I am trying to calculate stopping power ratio for monoenergetic photon beam in air with respect to PMMA as it is defined in Attix (formula 10.6) and in the following work: doi: 0031-9155/30/10/004 (formula 1). The model I use is the following: a slab of PMMA material is irradiated by a photon pencil beam of square profile. I calculated the distance where there is Charge Particle Equilibrium exists. Then I placed the air chamber inside the PMMA slab at the distance which is slightly bigger than CPE distance. I calculated the dose in this air chamber. Then I filled this chamber with PMMA and again I calculated the dose ( I used built-in USRBIN card for REGION to calculate DOSE). Then I calculated the ratio Dair/Dpmma. But the result was completely wrong - the dose in the air chamber was 1.5 times bigger than the dose in chamber filled with PMMA, but the literature states that this coefficient should be 0.907. Can you please tell me - does USRBIN card with REGION and DOSE fit for such calculation, or I must choose another card and add some more conditions? I attached the input file to this message.
Hallo @Alexy, note that your input (that by the way does not feature a pencil beam, since as you say it has got a square size) gives a ratio quite close to 1 (and not 1.5) if run with the detector region set to air and PMMA (according to your material definitions), respectively. So it’s not clear how you obtained the completely wrong 1.5 result, assuming that your results were statistically meaningful, i.e. affected by a statistical error suitably low (more challenging to achieve with air).
Moreover, you should then correct your wrong PMMA material definition, since the NIST fractions you took refer to mass and not atom content. This way you will get a lower ratio compatible with literature values (one finds 0.92 in Phys. Med. Biol. 48 (2003) 2081–2099).
Finally, remember that region scoring is not normalized by the region volume. This is irrelevant if you look at the ratio, but it matters if you take for other purposes the absolute dose value (the USRBIN result has to be divided by the region volume in cm3 in order to obtain GeV/g per primary photon).
As a minor corollary on the side, note that “stopping power of photons” is a slight language abuse: “stopping power” is a term ordinarily reserved for and involving charged particles. What implicitly matters in your problem is the stopping power of electrons (set in motion by photon interactions).