The Radiaton Physics Group at Universitat Autònoma de Barcelona (UAB) uses Poly-Allyl-Diglicol-Carbonate (PADC) based passive dosimeters in order to measure the neutron component in general mixed radiation fields, including those encountered in proton radiotherapy. Essentially, the whole dosimeter is constituted by a layer of PADC and several layers of diverse materials acting as neutron converters, in which incident neutrons produce protons. These protons originate submiscroscopic damage (latent tracks) in the PADC, which can be afterwards enhanced through an electrochemical process that allows visualising and counting the tracks and, therefore, quantifying the neutron field. The physics and working principles of this dosimeter are explained elsewhere . One great advantage of this dosimeter is that it is not sensitive to the photon component, so its use is of specific interest in photon-neutron mixed fields. Our group participates in the task of characterizing the radiation field present in proton therapy installations in the frame of WG9 (Radiation dosimetry in Radiotherapy) of EURADOS (The European Radiation Dosimetry Group). One of the aims, is to determine the out-of-field dose equivalent due to neutrons at patient’s radiological organs of interest. For this purpose, water slab and anthropomorphic phantoms are filled with several types of dosimeters, which need accurate and specific characterization.
We consider a mixed radiation field in a small volume constituted by neutrons and protons (let us disregard photons as the PADC dosimeter is intrinsically insensitive to them). In proton radiotherapy, protons can come from the scattered beam or from, essentially, “non-local” neutrons (far neutrons or neutrons not reaching the neutron converters). As the calibration coefficient of the entire dosimeter is obtained in, basically, a pure field of neutrons in calibration facilities, whereas in the real situation we can find protons reaching the PADC itself, therefore producing damage (etchable tracks), leading to an overestimation of the neutron dose equivalent. A methodology to deal with this problem is proposed and applied to results from a recent intercomparison of the response of several passive detectors in an irradiation campaign at CERF (The CERN-EU high-energy Reference Field) , in the frame of EURADOS-WG9, where a mixed radiation field of, at least, neutrons, photons and protons is present . To employ this methodology, it is necessary to use Monte Carlo simulations to model the facility, as well as to know the probability of a neutron or proton reaching the dosimeter and the PADC layer. It is found that under this situation and under this model, up to 30 % of the track density of the PADC layer is coming from protons or, probably, non-local neutrons.
 Domingo C., et al. Estimation of the response function of a PADC based neutron dosimeter in terms of fluence and Hp(10). Radiation Measurements 50 (2013) 82-86.
 Pozzi, F., Silari, M. The CERN-EU high-energy Reference Field (CERF) facility: New FLUKA reference values of spectral fluences, present and newly proposed operational quantities. Nuclear Inst. and Methods in Physics Research, A 979 (2020) 164477.