# Radiochromic Films for the Two-Dimensional Dose Distribution Assessment
## 4 Radiochromic Films Response to Relevant Physical Quantities  
Ionizing radiation induce the polymerization process into the RCF's active layer. As a consequence of this, RCFs are sensitive to almost all types of ionizing radiation and have therefore been used in photon, electron, proton, and heavy ion radiation environment in different dose ranges. Several works carried out to study the RCF dose rate dependence confirmed that RCFs can be considered dose rate independent over a wide dose range [89; 100; 101]. [PERSON] et al. reported on absolute calibration measurements for HD-V2 Gafchromic(tm) films in extremely high dose rate, approaching 7 \(\times\) 10\({}^{11}\) Gy/s, for laser-driven ion beams [102]. RCF radiation type and energy dependence has been deeply investigated over the last few years mainly for the RCF types used in medical dosimetry, such as Gafchromic(tm) EBT series and XRQA-2. The dose response of EBT-3 Gafchromic(tm) films is very weakly (less than 1%) dependent on photon and electron energies of clinical interest (6-16 MeV), as well as on \({}^{60}\)Co gamma rays, while, at very low x-ray energies (50 keV), the energy dependence is observed [9; 103; 104]. EBT-3 films show an under response at energies less than 100 keV and this under response increases as the energy decreases [105; 106]. [PERSON] et al. suggested that the energy dependence can be improved by modifying the chemical composition of the active element [105]. For charged particles, the energy dependence is more complicated. The dose \(D\) for a charged particle of kinetic energy \(E\) is given by:  
\[D=K\times(S/\rho)_{E}\times\Phi,\]  
where \(\Phi\) is the fluence in cm\({}^{-2}\); \(S/\rho\) is the mass stopping power, expressed in MeV \(\times\) cm\({}^{2}\)g\({}^{-1}\), and it is evaluated for the reference material (water for medical applications) at the energy \(E\); and \(K=1.6\times 10^{-10}\) is a conversion factor to express the dose in Gy. EBT-3 Gafchromic(tm) film's response to protons in the energy range 63-230 MeV has been observed to be not different from that of photons, within the uncertainties [37]. The response to low energy high LET charged particles, namely in the proximity of the Bragg peak, is characterized by a significant dose under response, also referred to as quenching effect. It is useful, as well as interesting, to understand the energy and LET range for which the quenching effect is observed. An analysis on the data from the referred works suggests that quenching effects for EBT series films exposed to protons are observed from about 5 MeV (79 MeV \(\times\) cm\({}^{2}\)g\({}^{-1}\)). Indeed, [PERSON] et al. observed the same calibration curves (within experimental uncertainty) for 7 and 10 MeV protons, and that these curves correspond to the one proposed by [PERSON] et al. for 148.2 MeV protons [107; 108]. The same calibration curve for EBT3 films exposed to 23, 50, and 200 MeV protons, as well as to low LET radiations, such as \({}^{60}\)Co gamma rays, was found by [PERSON] et al. [53]. Quenching effects have been observed for 4 MeV (94 MeV \(\times\) cm\({}^{2}\)g\({}^{-1}\)), as well as 5 MeV protons [109; 110]. Overall EBT-3 films response to high energy and low dE/dx (far from the Bragg peak) protons has proved to have weak energy dependence over a wide energy range and that it is comparable to that to gamma radiation. This implies that it is possible to use the same calibration, within experimental uncertainties, for EBT-3 films (from the same batch) to high energy and low LET protons and to gamma rays of the energy of, e.g., \({}^{60}\)Co. EBT-XD film response features similar characteristics as that of EBT-3 films both to photon and proton radiation [108]. Gafchromic(tm) film models used in diagnostic applications, such as XRQA-2 Gafchromic(tm) films, have been observed to have a significant dependence on photon energies which are typical of x-ray radiology diagnostics [111]. HD-V2 Gafchromic(tm) films feature a different structure from the EBT series Gafchromic(tm) films.