Elsevier

Radiation Measurements

Volume 123, April 2019, Pages 54-57
Radiation Measurements

Luminescence efficiency of Al2O3:C,Mg radiophotoluminescence in charged particle beams

https://doi.org/10.1016/j.radmeas.2018.12.015Get rights and content

Highlights

  • 2D films and single crystals of Al2O3:C,Mg irradiated with charged particle beams.

  • Luminescence efficiencies curves.

  • 2D image along the Bragg curve is acquired with a proton beam.

Abstract

In hadron therapy, it is important to evaluate how sensitive a detector is upon radiation with varying ionization densities, i.e. linear energy transfer (LET), as the LET changes when penetrating through material. For solid-state detectors, it is therefore essential to characterize the luminescence efficiency (ηHT,γ) of the detector as a function of LET. In this work, we investigate the radiophotuminescence (RPL) response from Al2O3:C,Mg 2D films and crystals exposed to various high LET beams (1H, 4He, 12C, 28Si and 56Fe). The measured ηHT,γ curve from RPL films and crystals as function of the particle LET is compared with the ηHT,γ curve from Al2O3:C OSL samples. Furthermore, a 2D RPL image, from Al2O3:C,Mg films irradiated with a 61.3 MeV 40 mm diameter broad proton beam, depicts a 2D depth dose distribution of the Bragg peak and demonstrates similar LET dependence as from the luminescence efficiency curve. The ηHT,γ curves are consistent with Birks’ law, where we observe expected quenching for increasing LET.

Introduction

Complex dose distributions represent a dosimetric challenge, requiring detectors to verify two-dimensional (2D) dose distributions. 2D detectors are of great importance for radiotherapy because they provide dosimetric data that correlates with the fluence of the incident beam, which allows these detectors to measure dose outputs of irregular fields or to assess the quality of treatment plans trough comparative analysis of dose maps.

The clinical interest in hadron therapy (HT) resides in the fact that it delivers precision treatment of tumors, once hadrons deposit almost all of their energy in a sharp peak – the Bragg peak (BP) – at the very end of their path. In hadron therapy, the tumor receives a high dose, while the healthy tissue before the BP gets lower dose, and the tissue after the BP gets almost no dose, in comparison with conventional X-ray beams (Schardt et al., 2010).

Luminescence dosimetry has been studied in dosimetry for decades, such as thermoluminescent (TL) and optically stimulated luminescent (OSL) detectors (Akselrod et al., 1998; Gaza et al., 2006; Yukihara et al., 2004a). In recent years, radiophotoluminescence (RPL) detectors gained attention because of some advantages compared to TL and OSL; some RPL materials are non-destructive, which allows the estimation of dose multiple times; the dose measured is stable with time (no fading) and is not affected by exposure of the material to visible or ultraviolet light (light induced-fading). One of the RPL materials that gained attention recently is aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg), for its potential for volumetric optical data storage (Akselrod and Akselrod, 2006) and for imaging tracks of charged particles and for neutron dosimetry, at microscopic scale (Greilich et al., 2013). Al2O3:C,Mg films were successfully used to characterize MV photon beams and also 2D dose mapping using OSL 2D imaging techniques (Ahmed et al., 2016, 2017).

The use of the RPL signal of the Al2O3:C,Mg for HT dosimetry is still not fully exploited and the objective of this work is to characterize RPL films and crystals to different HT beam qualities. Furthermore, the use of the films for 2D dose mapping is explored for a 61.3 MeV proton beam.

Section snippets

Materials and methods

Al2O3:C,Mg films and crystals (Fluorescent Nuclear Track Detectors, FNTD) are provided by Landauer Inc. The films are composed of 47 μm thick layer of Al2O3:C,Mg powder mixed with a binder, deposited on a 75 μm thick of polyester substrate (Ahmed et al., 2014). The crystals are Al2O3:C,Mg single crystals cut in 6 × 4 × 0.5 mm3 rectangular detectors (Akselrod and Sykora, 2011) of which one side is polished, while the other is opaque.

Irradiation with heavy charged particles was performed at Heavy

Results

Films and crystals were irradiated with different energies, beam types and doses. First, we measured the RPL response of the films (Fig. 1) and crystals (Fig. 2) as function of absorbed dose to water. The full line in Figs. 1 and 2 represents the average measured signal (RPL0) from a chosen ROI, corresponding to ‘0’ Gy, or background dose, acquired from a non-irradiated film and crystal, respectively. The dotted lines represent the experimental standard deviation (±3σRPL0) from the same ROI,

Conclusions

In this study we compared the measured luminescence efficiencies from Al2O3:C,Mg films and crystals irradiated with various charged particle beams. Films presented higher background and decreased SNR, when compared to crystals. The luminescence efficiency curves (films, crystals and Al2O3:C OSL) present the same pattern, although films have lower values, caused by the combination of different batches of crystals composing the films.

Our results indicate that Al2O3:C,Mg films can be used to

Acknowledgements

We would like to thank Dr. Mark Akselrod (Landauer) for Al2O3:C,Mg films and crystals. Luiza Freire de Souza thanks CAPES, for the financial support.

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    Among these, RPL film detectors based on Al2O3:C,Mg show a non-destructive readout characterized by a stable signal measured over time (no fading) that allows the assessment of the absorbed dose multiple times [33]. Additionally, the films have low angular dependence, sub-mm spatial resolution, require no corrections for pixel blurring and there is no need to keep the detector in a dark environment, since the RPL material is light insensitive [26,34,35]. In this work, we explore the use of Al2O3:C,Mg RPL films for MLC QA and small field dosimetry.

  • Characterization of 2D Al<inf>2</inf>O<inf>3</inf>:C,Mg radiophotoluminescence films in charged particle beams

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    Different measurements of dose response, film uniformity and minimum detectable dose, were carried out to evaluate the properties that are of particular importance in relative dose measurements. Films presented better SNR, better homogeneity and lower minimum detectable dose compared to previous RPL samples (De Saint-Hubert et al., 2019; Nascimento et al., 2019). Also the dose response showed different non-linear behavior, which was regardless of beam type and energy.

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    Although SiPMs are becoming prevalent in radiation detection, only a handful of studies have been conducted using SiPMs as a photodetector for passive dosimetry application [16–18]. For instance, M. Saint-Hubert et al. measured radiophotoluminescence (RPL) of Al2O3:C,Mg films using a Multi Pixel Photon Counters (MPPCs) 2D reader [17]. Another study reported on the dose response of RPL and the reproducibility of optically stimulated luminescence (OSL) signals from Al2O3:C,Mg films by using an SiPM-based detector system [18].

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    On the other hand, such applications using a combination of a RPL dosimetry based on Al2O3:C,Mg single crystals and a CLMS have been successfully developed (Akselrod and Kouwenberg, 2018) as a FNTD, which was first developed by Akselrod et al. (2006), for HCPs, neutrons and even photons. In addition, experimental optically stimulated luminescence (OSL) efficiency versus LET for Al2O3:C detectors was investigated for a variety of HCPs with energies ranging from 7 to 100 MeV/u (Sawakuchi et al., 2008a) and the RPL response from Al2O3:C,Mg 2D films and crystals exposed to various high LET beams was also investigated (Saint-Hubert et al., 2019). The use of the RPL Ag-activated glass for various HCPs is still not fully developed, therefore the objective of this study is to investigate the dosimetric and physical characteristics of this material irradiated with 1.7, 4.5, and 160 MeV proton beams.

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