# Dosimeter sensitivity calculation

Dear Fluka experts,

I’m trying to study the sensitivity of a dosimeter with respect to initial photon energy. By definition, the sensitivity S describes as: S = r/D, where r is a detector response, D is a true dose equivalent created by the source near the point of interest. For modelling D I’m using USRTRACK for DOSE-EQ, but I have some problems with choosing of the value for detector response r. Is it correct to use the total charge generated by primary radiation as value r? If so, can Fluka calculate the total number of secondary electrons, generated by primary radiation in the specified volume?

No, FLUKA, as any other condensed history Monte Carlo code, cannot explicitly calculate the total number of secondary electrons, because a major fraction of these fall below the lowest transport threshold of 1 keV. However, by calculating the physical dose (Gy) deposited in the detector sensitive volume, one can get the generated charge by dividing by a characteristic w factor, representing the energy to be deposited in order to generate an electron-ion pair. This is under the assumption that no quenching effects apply. In fact, the actual detector response does not come out from an ideal simulation, but has to be embedded in the simulation as an external ingredient depending on the detector.
As for the true dose (equivalent?), this is actually what the simulation provides, but here I do not see the point of a differential scoring (USRTRACK), rather would calculate a dose spatial distribution (USRBIN).

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Dear @ceruttif,

Thank you for your reply. May I ask you one more question. Instead of calculating dose, can I calculate deposited energy in the detector volume by using USRBIN (Energy) and then calculate the number of electrons by dividing this deposited energy on ionization potential for detector material (w)?

Sure, but note that the USRBIN scoring of ENERGY actually gives energy density (GeV/cm3) as a Cartesian/cylindrical grid is used, while gives the total energy over the region as a region ‘grid’ is used, since FLUKA does not know the region volume to divide by. In this regard, the average dose over the region is just the total energy deposited divided by the region volume and the material density (i.e. the region mass). Instead, DOSE (GeV/g) and ENERGY (GeV/cm3) in a Cartesian/cylindrical bin just differ by the material density (g/cm3).
Also, w is the average amount of energy required to form an ion pair and is significantly larger than the ionization potential.
Moreover, if you use r=number of ion pairs=energy/w and D=dose, you will get by construction S=mass/w, which is a constant not telling you anything about the sensitivity variation with respect to the initial photon energy. As previously mentioned, you should get D from the simulation and r from the detector calibration.

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