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In recent years, radiophotoluminescent glass dosimeters have been increasingly used, especially for measuring doses in therapeutic proton beams. Reported data on measurements and characterization of RPL detectors are mainly related to photon and electron beams, while data for proton beams are limited and not always consistent. The increasing role of proton radiotherapy in cancer treatment requires detailed characterization and adaptation of dosimetry methods to charged particle measurements. In particular, the use of passive detectors in the charged particle field is challenging due to their energy and LET dependence and dose response. Moreover, the lack of mailed dosimetry audits in proton therapy centers in Europe foster detail characterization of the response of different passive detectors in various proton fields (beam modulation, range, etc.). In the framework of EURADOS WG9, the Ruđer Bošković Institute has participated in several measurement campaigns in different proton therapy centers with radiophotoluminescent (RPL) glass dosimeters type GD -352M and GD -302M. For the study of response in Spread out Bragg peak (SOPB) the detectors were positioned in a standard 30 cm × 30 cm × 30 cm PTW water phantom. The SOBP proton range (from 5 cm to 25 cm) and modulation width (from 5 cm to 20 cm) were gradually changed to determine the dosimeter response in 9 different SOBP configurations. Dose dependence was measured at the middle of the SOBP for doses ranging from 0.5 Gy to 120 Gy. The long-term fading of RPL detectors irradiated with proton beams was also determined under normal ambient conditions over a period of 1.5 years. To achieve high precision of measurements with RPL detectors in proton beams, it is of great importance to have detailed knowledge of their behaviour at different beam parameters (different ranges and modulations), LET spectra and dose levels. An overview of selected results in proton beams measured with RPL detectors from different measurement campaigns will be presented.

In charged particle dosimetry luminescence efficiency strongly depends on linear energy transfer (LET). Therefore, for correct interpretation of the measurements in hadron radiotherapy or space dosimetry it is important to characterize the detectors efficiency as a function of LET. Unlike for other luminescent detectors, available literature data for the efficiency of radiophotoluminescent (RPL) glass detectors are scarce and especially for low energy range no data was found. The aim of this study is to evaluate relative efficiency of RPL glass detectors in low energy proton beams. RPL glasses of environmental dosimeters (silver activated phosphate glass FD-7/SC- 1 ; AGC Techno Glass Co.) with dimensions 16 x 16 x 1.5 mm2 were used in this study. Measurement of the RPL signals were carried out with the FDG-202 reader. Detectors were exposed to proton beams in a high vacuum chamber at the microprobe experimental line of the RBI accelerator facility. A focused proton beams (1 x 1 mm2) were delivered using two electrostatic tandem accelerators: 1 MV and 6 MV accelerator for up to 2 MeV and 3-5 MeV proton beams, respectively. In all cases protons were stopped in the detectors. The range and LET of protons in water and RPL glass were calculated using SRIM code and used for absorbed dose calculations. Detectors were exposed to protons at the fluence range of 3.3x10^6 to 1.3x10^8 p/mm2 while doses absorbed in the irradiated volume, expressed as dose to water, ranged from 7.6 to 797.8 Gy. Firstly, fluence/dose response was investigated [1]. Secondly, RPL signals from the linear dose response range were used to evaluate efficiency relative to reference gamma radiation. Preliminary results show that in the linear fluence/dose response range the mean RPL signal normalized to the number of incident protons is decreasing exponentially with decrease of the beam energy. The relative efficiency (expressed for dose in water, relative to 60Co) is also decreasing with energy/LET from 0.6 for 5 MeV (8.1 keV/m) and 0.2 for 3 MeV (12.1 keV/m) to approximately 0.1 for 2 MeV and below.

Our aim was to study the dose response of radio- photoluminescent (RPL) glass dosimeters in low- energy proton beams. A 6 MV Tandem Van de Graaf accelerator connected to a vacuum chamber at the Ruđer Bošković Institute (RBI) was used to produce 3 MeV and 5 MeV proton beams with cross-sectional area 1x1 mm2. These beams were directed at RPL glasses (type FD-7) with dimensions 16x16x1.5 mm3 taken from environmental RPL dosimeters (type SC- 1). The sensitivity of the detectors in the batch was checked before and after the experiments in the 137Cs gamma field at the Secondary Standard Dosimetry Laboratory at the RBI. Standard annealing procedures were used and the dosimeters were preheated before dosimeter response values were measured with the FGD-202 reader. Detectors were exposed to protons fluence ranged from 3.3x106 to 3.3x107 p/mm2 and from 3.5x106 to 4.5x107 p/mm2 for 3 MeV and 5 MeV, respectively. RPL signals were normalised to the number of protons irradiating the detector. The linearity index was calculated as a ratio of normalised RPL signal and reference normalized RPL signal where reference signal originates from the linear dose response range. Linear dose response was confirmed for the whole investigated fluence range for 5 MeV and for the fluence range from 3.5x106 to 2.2x107 for 3 MeV. The corresponding dose linearity index was in the range of 0.95 – 1.05 whilst linearity slope is larger for 5 MeV compared to 3 MeV. For the fluence 4.5x107 p/mm2 of 3 MeV protons, linearity index of 0.59 was calculated suggesting sublinearity. Sensitivity of detectors was not changed after irradiations with proton beams and an average value ± standard deviation was 1.00 ± 0.02. The data collected in the linear dose response range was used to evaluate relative efficiency of RPL detectors to 3 and 5 MeV protons [1].

Cilj ovog rada bilo je proučavanje odziva radiofotoluminiscentnih (RPL) dozimetara u protonskim snopovima niskih energija. Korišteni dozimetrijski sustav sastoji se od radiofotoluminiscentnog stakla (tip FD-7; dimenzija 16 × 16 × 1.5 mm^2) izvađenog iz držača RPL dozimetara korištenih za zračenja u okolišu (model SC-1; AGC Techno Glass Co) i čitača signala FGD-202. Standardno, prije ozračivanja dozimetre smo anilirali (poništili), a prije samog mjerenja signala stavljeni su u peć na predgrijavanje. Samo ozračivanje odvijalo se na Institutu Ruder Bošković (IRB), gdje smo pomoću dva elektrostatska ubrzivača (akceleratora) (6.0 MV EN Tandem Van de Graaff i 1.0 MV Tandetron) dobili protonske snopove energija 1, 2, 3 i 5 MeV. RPL stakla su tijekom ozračivanja stavljena na prikladna postolja i pomoću držača smještena u vakuumsku komoru povezanu s ubrzivačem. Broj protona zaustavljenih u dozimetrima bio je u rasponu od 3.3 × 10^6 do 1.3 × 10^8 što je odgovaralo dozama u RPL staklu u rasponu od 14.6 do 1464.9 Gy. Rezultati pokazuju da u ispitanom području doza, odziv RPL dozimetra ovisi linearno o dozi do odredene vrijednosti. U linearnom području porast odziva s dozom je strmiji (veći nagib pravca) za veće energije što ukazuje na energijsku ovisnost odziva dozimetra. Izračunata relativna učinkovitost RPL dozimetara raste s porastom energije protona i dobivene vrijednosti su bile u rasponu od 0.103 za protonski snop energije 1 MeV do 0.54, za protonski snop energije 5 MeV. Our aim was to study the dose response of radiophotoluminescent (RPL) glass dosimeters in low-energy proton beams. The dosimetric system used consists of radiophotoluminiscent glass (FD-7 type; dimensions 16x16x1.5 mm3) taken from the casing of environmental RPL dosimeters (SC-1 model; AGC Techno Glass Co) and a FGD-202 reader. Standard annealing procedures were used before irradiation and the dosimeters were preheated in an oven before dosimeter response values were measured. The irradiation took place at Ruder Bošković Institute (RBI), where two electrostatic accelerators ( 6 MV Tandem Van de Graaf and 1.0 MV Tandetron) were used to produce 1, 2, 3 and 5 MeV proton beams. During irradiation, the RPL glasses were mounted on a suitable stand and with the help of a helve, placed inside a vacuum chamber connected to the accelerator. The number of protons stopped inside the dosimeter was in the range between 3.3 × 106 and 1.3 × 108, which corresponds to doses in RPL glass of 14.6 − 1464.9 Gy. The results in the investigated dose range imply the dose response was linear up to a certain value. In the linear range the response increases as the dose increases, more so (greater line slope) at greater energies, which points to an energy dependence of the dosimeter response. The calculated relative effectiveness of the RPL dosimeter increases with increasing proton energy and the obtained values were in the range between 0.103 for 1 MeV proton beam energy and 0.54, for the 5 MeV proton beam energy.