I am currently running a photon transport simulation on a slab geometry (water [1-4] cm/bone [4-6] cm/lung [6-13] cm/water [13-31] cm). Here, gamma source is a linear source.

I tried three dose calculation methods with Serpent. The first using

**dr -200 (Serpent's dose rate)**, the second using

**dr -26 (total heating cross section)**and the third using simultaneously

**dr -248 (for liquid water), dr -208 (bone cortical) and dr -228 (lung)**.

**How units were managed:**

1. I think that detector's response is D1= "Gy cm3/h". So I divided D1/volume/3600 = "Gy/s" ;

2. I think that detector's response is D2= "/s" (What I understand from the manual, may be I am wrong). So I divided (D2/Material_Density/Volume).*energy= "[(/s)*(cm3/kg)/cm3)]*J" = "J/Kg/s" = "Gy/s" ;

3. Exactly the same thing as in 1 for each material.

**Questions:**

Q.1. Can you, please, tell me if you agree with the mentioned units ?

Q.2. Can you, please, tell me why Methods 1 and 3 give a very, very, very close results (but not exactly the same), while there is a difference with respect to Method 2 (based on total heating cross section). These differences are important in the first water material and in the bone material. (Please, see Fig. 18MeV and become worse (catastrophic) for ultra-relativistic energy -- tested for 50 MeV and others -- ) ?

Q.3. All these methods will converge if the energy is decreased (please see Fig. 1MeV, become closer and closer for lower energies -- tested for lower energies --).

Q.4. If you want to give me some advice on the most fundamental method for dose calculations in Serpent, which one should I use and why ?

**Note**: Details on employed dose calculation methodology is very important for me. So if you want to add some details about how the dose rate is calculated by Serpent in each case, it is really appreciated.

Thank you so much for your time and efforts.