Radiophotoluminescence Research Articles

A review of the optically and thermally stimulated luminescence properties of aluminosilicates

This paper reviews the main literature describing models for thermoluminescence (TL), optically stimulated luminescence (OSL) and radiophotoluminescence (RPL) in aluminosilicate materials, namely natural feldspar minerals and synthetic glasses. The work examines the different models proposed to explain the various luminescence phenomena and compares them with each other. The models include thermally and optically stimulated excited-state tunneling, band-tail state hopping, and ionization and transportation through the band-tail states and/or the conduction band. Temperature and stimulation wavelength are critical parameters, with one model or another dominating over different temperature and/or wavelength ranges. Some recommendations for future research are noted.

Online and offline Radiation-Induced Attenuation measurements on FD-7 radiophotoluminescence dosimeters irradiated at high X-ray doses

Radiophotoluminescence (RPL) FD-7 glass dosimeters find applications in low to high dose radiation environments. This work presents an experimental characterization of RPL glass dosimeter, irradiated with 100 kV X-ray tubes at room temperature at doses ranging from 1.3 kGy to 0.47 MGy, much higher of their common use range. In this study, a customized set-up has been developed, allowing the online investigation of the glass transmission changes with radiation, known as Radiation-Induced Attenuation (RIA), as well as the recovery and post-mortem characterizations up to 2 months after irradiation. Multi-wavelength analysis was performed, focusing on the range between 200 nm and 800 nm. At 700 nm and 800 nm, RIA increases progressively with dose up to about 5 kGy, and tends to approach saturation (2–3 dB/mm) for doses higher than 50 kGy. Higher attenuation is reported at lower wavelengths: 445-nm light transmission reduces to only about 1% of its initial value after 2 kGy. RIA recovery after irradiation was observed, up to 6% at 700 nm wavelength within 3 h from the irradiation conclusion and up to 26% 2 months after, especially at doses in the kGy range. Both online RIA and its recovery are highly dependent on the selected wavelength and on the total absorbed dose. This information is crucial for the extension of the use of these dosimeters up to high doses, complementary to the RPL signal, traditionally used alone for the determination of doses up to the Gy range.

Tissue-equivalent radiophotoluminescence dosimetry materials based on production of luminescent molecules via radiation chemical reactions

Radiophotoluminescence (RPL) has been a key phenomenon in dosimetry. Most materials exhibiting RPL are inorganic single crystals, glasses, and ceramics. Recently, similar phenomena (i.e., fluorescence after irradiation) have been realized in soft matters, such as liquids, gels, and organic solids, on the basis of the radiation-induced production of fluorescent molecules. Dosimeters showing such phenomena are attractive from the viewpoints of their tissue equivalence, flexibility, scalability, and workability. The objective of this paper is to present an overview of such dosimeters with emphasis on the radiation chemical reactions used in the materials. Moreover, the sensitivity and the measurable range is introduced.

Dosimetric response of radiophotoluminescent glass detectors in a scanning pencil proton beam

The dose and linear energy transfer (LET) dependence of the radiophotoluminescence (RPL) response was evaluated for RPL glass detectors, taken from the environmental dosimeter type SC-1, using a clinical 100 MeV and 200 MeV scanning pencil proton beam. The measurements were performed at different positions along the Bragg curves. The fluence-averaged (LETf) and dose-averaged (LETd) LET at the positions of the detectors were calculated using two Monte Carlo codes (MCNP and PHITS). Before measurements in proton beams, the linearity of the dose response for the dose range of interest (up to 10 Gy) was checked and confirmed in a Co-60 gamma ray field, since the FGD-202 reader for SC-1 dosimeters is usually used for doses up to 1 Gy and we found no data for higher doses in the literature. For proton beams, the linear dose response was determined for the doses investigated from 0.5 Gy to 8.8 Gy. For the LET range, which corresponds to the LETd from 0.47 keV/μm to 4.70 keV/μm, a decrease in response with an increase in LET was observed.

Preliminary analyses of radiophotoluminescence centers in a recently developed Ag-doped alkali-phosphate glass (SAPANS)

The mechanism behind radiophotoluminescence (RPL) emission in Ag-doped alkali-phosphate glass remains unclear. To contribute to resolving this issue, the authors measured and analyzed the temporal change of electron spin/paramagnetic resonance (ESR) spectrum of a recently developed, weather-resistant RPL glass, named “SAPANS,” after irradiation with X-rays. As a result, a continuous gradual increase in Ag2+-derived signals was observed even at four weeks post-irradiation, which corresponded well with the time build-up pattern of the RPL of SAPANS. In contrast, the signals derived from phosphorous-oxygen-hole centers (POHC) rapidly decreased over a few days. The peaks of Ag0 and Ag2+ were not successfully separated from the ESR spectra because of the possible overlap of unidentified peaks, which implies a complex mechanism of RPL-center formation in Ag-doped alkali-phosphate glass that requires further investigation.

Combinatorial characterization of metastable luminous silver cations

Thermodynamically metastable glasses that can contain metastable species are important functional materials. X-ray absorption near-edge structure (XANES) spectroscopy is an effective technique for determining the valence states of cations, especially for the doping element in phosphors. Herein, we first confirm the valence change of silver cations from monovalent to trivalent in aluminophosphate glasses by X-ray irradiation using a combination of Ag L3-edge XANES, electron spin resonance, and simulated XANES spectra based on first-principles calculations. The absorption edge of the experimental and simulated XANES spectra demonstrate the spectral features of Ag(III), confirming that AgO exists as Ag(I)Ag(III)O2. A part of Ag(I) changes to Ag(III) by X-ray irradiation, and the generation of Ag(III) is saturated after high irradiation doses, in good agreement with conventional radiophotoluminescence (RPL) behaviour. The structural modelling based on a combination of quantum beam analysis suggests that the local coordination of Ag cations is similar to that of Ag(III), which is confirmed by density functional theory calculations. This demonstration of Ag(III) in glass overturns the conventional understanding of the RPL mechanism of silver cations, redefining the science of silver-related materials.

Radiophotoluminescence properties of Bi-doped NaCaBO3

Bi-doped NaCaBO3 exhibits radiophotoluminescence (RPL) properties, where the Bi2+ centre is formed through X-ray irradiation. As a result of RPL, a broad emission band appears around 800 nm in the photoluminescence (PL) spectrum, attributed to X-ray irradiation. The intensity of the PL signal is proportional to the irradiation dose, making it useful for measuring accumulated radiation dose. The sensitivity depends on the doping concentration of Bi, and the optimal concentration to maximize sensitivity is approximately 0.005%. With a laboratory-constructed reader system, the optimal sample enables measurement of X-ray dose as low as 25 mGy. Following irradiation, the Bi2+ signal slightly increases by about 13%, but it is significantly reduced to around 7% of the initial value after heat treatment at 500°C for 100 s. The formation and annihilation of the Bi2+ centre can be reproduced multiple times. Therefore, RPL of Bi-doped NaCaBO3 is attractive for radiation dosimetry applications.

Radiophotoluminescence properties of Eu-doped Li2SrSiO4 for radiation detectors

To search for new radiophotoluminescence (RPL) materials, Eu-doped Li2SrSiO4 was synthesized by the solid-state reaction and then studied its RPL properties for radiation detector applications. All the samples were identified to be Li2SrSiO4 by X-ray diffraction. The as-prepared sample shows photoluminescence (PL) due to Eu3+ while an additional PL band appears due to a formation of Eu2+ after X-ray irradiation. This indicates that the Eu-doped Li2SrSiO4 has RPL properties. The sensitivity of RPL (or PL intensity of Eu2+) to X-ray dose is the highest when the Eu concentration is 0.1%, and the dynamic range is 5–10 000 mGy. In addition, the generated Eu2+ is so stable that only 5% of the signal degrades over 20 min after irradiation. The RPL signal notably decreases to about 30% of the initial value by heating at 500 °C for 100 s, and it is reproducible multiple times even after the heat treatment.

Radiophotoluminescence properties of Ag2O–Cs2O–BaO–Al2O3–P2O5 glasses

In this study, we synthesized Ag2O–Cs2O–BaO–Al2O3–P2O5 glasses with different Ag concentrations (0.1, 0.3, 1.0, and 3.0%) using the melt-quenching method and investigated the optical properties and the spatial resolution as an imaging plate. These glasses showed luminescence at around 340 nm upon 230 nm excitation before X-ray irradiation and luminescence at around 620 nm upon 340 nm excitation after X-ray irradiation. The lifetimes for the 340 and 620 nm emissions were 7.2–32.2 μs and 15.6–19.7 ns, respectively. Based on the emission wavelength and lifetimes, the luminescence at 340 and 620 nm was considered to originate from Ag+ and Ag2+, respectively. In other words, the results suggested that these glasses showed the radio-photoluminescence (RPL) phenomenon. The emission intensity of Ag2+ increased with annealing at 100–200 °C and disappeared by annealing at 600 °C in all the Ag-doped samples. The 3.0% Ag-doped sample indicated the highest emission intensity of Ag2+ and the spatial resolution of 10 LP/mm.

Effect of Al-doping on dosimetric properties of CaF2

In this study, the effect of Al-doping on dosimetric properties of CaF2 was investigated. Al-doped CaF2 showed a radiophotoluminescence (RPL) peak at 750 nm after X-ray irradiation. The RPL intensity decreased with the increase of Al concentration and was less than that of undoped CaF2. The minimum detectable dose as the RPL dosimeter of 0.01% Al-doped CaF2 was 42 mGy. Al-doped CaF2 also indicated thermally stimulated luminescence (TSL) glow peaks at 225, 285, 387, and 436 °C. The wavelengths of TSL were 390 and 500 nm. The minimum detectable dose as the TSL dosimeter of 1% Al-doped CaF2 turned out to be 0.43 μGy which is comparable to commercial dosimeters.

Relative efficiency of radiophotoluminescent glass dosimeters in a scanning pencil proton beam

The increasing role of hadron therapy, and particularly of proton therapy, in cancer treatment requires dosimetric characterization of luminescent detectors in heavy charged particle beams. The ionization density, which is related to the linear energy transfer (LET), is changing when heavy charged particle is penetrating through a material. Therefore, it is crucial to evaluate how efficiency of the used dosimeters depends on the change in LET.The aim of this study was to evaluate relative efficiency of radiophotoluminescent (RPL) glass dosimeters (type GD-352M) in a clinical scanning pencil proton beam of 100 MeV and compare it to the results obtained with thermoluminescent (TL) dosimeters (type TLD-100). LETf and LETd at the selected dosimeter's position were evaluated and compared by two Monte Carlo codes (MCNP and PHITS). Linear dose response is precondition for the evaluation of dosimeter efficiency and first, dose response in proton beam was evaluated and compared with those in 60Co gamma ray field.Linear dose response up to 9 Gy was confirmed for RPLs in proton beam, and gamma ray field. For TLDs, supralinearity from about 5 Gy was observed. A constant relative efficiency is observed along the plateau (corresponding to LETf from 0.7 keV/μm to 1.7 keV/μm) of non-modulated proton beam with an average value 1.00 ± 0.02 for RPL and 0.97 ± 0.02 for TL dosimeters. Relative efficiency decrease of 10 % is observed for RPLs within the Bragg peak with LETf = 2.3 keV/μm. Future work will be focused on the evaluation of the efficiency for higher LET values using more energetic proton beams and precise positioning along the Bragg peak.

Development of organic RPL dosimeters based on fluorescent products of the radiation-induced reaction of 2′,7′-dichlorofluorescein diacetate

In this study, we achieved a shift in the fluorescence wavelength of the organic molecules by X-ray irradiation and fabricated organic materials for radiation dosimetry, focusing on the radiation-induced radical oxidation of 2′,7′-dichlorofluorescein-diacetate (DCFHDA). We doped DCFHDA into processable and transparent polymers to develop a novel tissue-equivalent dosimeters with 3D measurement capabilities. Photoluminescence spectra of the organic materials were measured before and after X-ray irradiation, and their sensitivities to X-rays were compared based on the changes in their fluorescence intensity. Fluorescent molecules were produced from DCFHDA upon X-ray irradiation, and the material was sensitive to X-rays at doses of up to 6 kGy. Furthermore, trichloroacetic acid doping enhanced the dose sensitivity of the material by promoting the radical reaction, which indicates that it can be used as a sensitizer. These results indicate that the fluorescence wavelengths of DCFHDA switched to those of another fluorescent molecule, i.e., the achievement of radiophotoluminescence (RPL). We achieved an organic RPL (ORPL) and showed the possibility of ORPL that can be utilized in dosimetry in radiotherapy.

CERN Super Proton Synchrotron Radiation Environment and Related Radiation Hardness Assurance Implications

The Super Proton Synchrotron (SPS) is the second largest accelerator at CERN where protons are accelerated between 16 GeV/c and 450 GeV/c. Beam losses, leading to the mixed-field radiation of up to MGy magnitude, pose a threat to the reliability of the electronic equipment and polymer materials located in the tunnel and its vicinity. Particularly in the arc sectors, where both main magnets and radiation sensors are periodically arranged, the Total Ionizing Dose (TID) is of concern for the front-end electronics of A Logarithmic Position System (ALPS). The SPS is equipped with multiple radiation detection systems such as Beam Loss Monitors (BLM), RadMons, and as of 2021, the Distributed Optical Fibre Radiation Sensing (DOFRS), that combined all together provide a very comprehensive picture of both the TID spatial distribution and its time evolution. Within this study, the overview of measured 2021 and 2022 TID levels is presented, together with the demonstration of capabilities offered by the different radiation monitors. The DOFRS, supported by the passive Radio-Photo Luminescence (RPL) dosimeter measurements, is used to assess the TID values directly at the electronic racks, which turned out to be reaching several tens of Gy per year, potentially affecting the ALPS lifetime.

Electronics Irradiation With Neutrons at the NEAR Station of the n_TOF Spallation Source at CERN

We study the neutron field at the NEAR station of the neutron time of flight (n_TOF) facility at CERN, through Monte Carlo simulations, well-characterized static random access memories (SRAMs) and radio-photo-luminescence (RPL) dosimeters, with the aim of providing neutrons for electronics irradiation. Particle fluxes and typical quantities relevant for electronics testing were simulated for several test positions at NEAR and compared to those at the CERN high energy accelerator mixed-field facility (CHARM), highlighting similitudes and differences. The SRAM detectors, based on single event upset (SEU) and single event latch-up (SEL) counts, each one with a different energy response, and RPL dosimeters were tested in a reference position and the results were benchmarked to FLUKA simulations. Finally, the neutron spectra at NEAR are compared to those of the most well-known spallation sources and typical environments of interest, for accelerator and atmospheric applications, showing the potential of the facility for electronics irradiation.

Relative efficiency of radiophotoluminescent glass detectors in low energy ion beams

In heavy charged particle dosimetry, the luminescence efficiency strongly depends on linear energy transfer (LET) and characterization of the detectors' efficiency as a function of the particle LET is very important. Available literature data for the efficiency of radiophotoluminescent (RPL) glass detectors are scarce and especially for low energy range no data was found. In this study, detectors made of the most widely known RPL material (silver activated phosphate glass) were exposed to various low energy ion beams (1H, 7Li, and 12C). For every ion beam, the linear dose (fluence) response of the RPL glass detectors was confirmed for the investigated dose (fluence) range and the RPL efficiency relative to 60Co gamma rays was evaluated. The relative efficiency decreased as LET increased but not as a unique function of LET, where the relative efficiency (expressed for doses in RPL glass) decreased from 0.85 ± 0.06 for the 5 MeV 1H ion beam (LETw = 8.1 keV/μm) to 0.017 ± 0.001 for the 15 MeV 12C ion beam (LETw = 673.8 keV/μm). More experimental data are still needed but results obtained in this study may motivate the application and testing of microdosimetric models for RPL glass detectors.

Estimation of elapsed time after an unnoticed radiation exposure using the weathering resistant radiophotoluminescence glass (SAPANS)

A method using a newly developed radiophotoluminescence (RPL) glass for estimating the elapsed time after an unnoticed radiation exposure was proposed and tested. Cuboid-shaped samples of a silver-activated glass composed of P2O5–Al2O3–Na2O–Ag2O, named “SAPANS”, were irradiated with 137Cs-source γ-rays at five dose levels (1.0, 2.0, 3.5, 5.5 and 8.5 Gy) and the time changes of the RPL intensities emitted from the samples stimulated by the UV pulses were observed for up to 97 days after irradiation. It was found that the RPL intensities continuously increased with the same build-up patterns keeping linear dose responses at all dose levels, and reached to about 86% of those after preheating at 160 °C for 1 h at 97 days after irradiation. According to this finding, it was proposed to use the SAPANS for estimation of the elapsed time after an accidental high-dose exposure as well as for the determination of radiation doses. The practicality of this approach was examined in a blinded experiment wherein five sets of the SAPANS samples were irradiated with unknown γ-ray doses at unknown timings in the period of 53 days. As results, the errors of estimated elapsed times were −25% for 4.5 h, 18% for 3.3 days, 70% for 10 days, 30% for 30 days and 0% for 45 days after irradiation. These findings indicate that the proposed method could be applied to retrospective dose assessment even when a few months have passed after an unnoticed radiation exposure.

Visible Radiophotoluminescence of Color Centers in Lithium Fluoride Thin Films for High Spatial Resolution Imaging Detectors for Hard X-rays

Passive solid-state detectors based on the visible radiophotoluminescence (RPL) of stable aggregate F2 and F3 + color centers in lithium fluoride (LiF) are successfully used for X-ray imaging and advanced diagnostics of intense X-rays sources. Among their advantages, these detectors offer a wide dynamic range and simplicity of use. They can be read non-destructively using a fluorescence microscope, enabling high spatial resolution over a large field of view. Optically transparent LiF films, of three different increasing thicknesses, were grown by thermal evaporation on glass and silicon substrates and subsequently irradiated with monochromatic 7 keV X-rays at several doses from 1.3 × 101 to 4.5 × 103 Gy at the SOLEIL synchrotron facility. For all the LiF films, the RPL response was found to depend linearly on the irradiation dose, with films grown on Si(100) substrates exhibiting up to a 50% higher response compared to those grown on glass. A minimum dose of 13 Gy was detected, despite the low thickness of the irradiated films. The limited thickness of the homogeneously colored LiF film allowed to obtain a spatial resolution of (0.44 ± 0.04) μm in edge-enhancement imaging experiments conducted by placing an Au mesh in front of the samples.

Storage and encryption of submicron spatial resolution X-ray images based on Ag-doped phosphate glass

The ultrastable encrypted storage of X-ray image with quantitative and ultrahigh-spatial resolution are crucial for a wide range of applications in radiation imaging, and it can be achieved by using the radio-photoluminescence (RPL) material, Ag-doped phosphate glass (Ag-PG), which was fabricated via the melt-quenching method. Under X-ray irradiation, Ag-PG produces a luminescence center, obvious 650 nm emission peak, which can be repeatedly excited by ultraviolet light without intensity attenuation. Quantitative X-ray image storage was realized based on the linear relationship between RPL intensity and X-ray irradiation time. X-ray image with a submicron spatial resolution of 0.7 µm was achieved, which could be stably stored without attenuation for more than 300 days. This submicron level X-ray image can be written into the Ag-PG as an encrypted information, and can only be visualized by an ultraviolet light. Additionally, heat treatment (400 °C for 2 h) could eliminate the influence of X-ray radiation on the Ag-PG, and restore the state of the Ag-PG before X-ray irradiation, which allows the recyclability and reusability of the Ag-PG. This research is of great significance for promoting the encrypted storage of X-ray images based on Ag-PG or other RPL materials.

Radio-photoluminescence phenomenon in Cs2O-SrO-Al2O3-P2O5 glasses doped with different concentrations of Ag2O

In this study, Ag2O–Cs2O–SrO–Al2O3–P2O5 glasses with different Ag concentrations (0, 0.1, 0.3, 1.0, and 3.0 mol.%) were made by the melt-quenching method. In addition, the photoluminescence (PL) properties, the radio-photoluminescence (RPL) phenomenon, and spatial resolution on the X-ray imaging by the RPL were studied. The RPL was observed by irradiating with X-rays as an appearance of a new PL emission at around 590 nm. According to the shape of the PL spectrum and lifetime, the luminescence was confirmed to be due to Ag2+ with a lifetime constant of 14.7–45.5 ns. From the X-ray imaging resolution test, the 3.0% Ag-doped sample showed a spatial resolution of at least 10.0 LP/mm.

Radio-photoluminescence phenomenon in Ag-doped Cs2O–CaO–Al2O3–P2O5 glasses

In this study, Ag2O–Cs2O–CaO–Al2O3–P2O5 glasses with different Ag concentrations (0, 0.1, 0.3, 1.0, and 3.0 mol.%) were made by the melt-quenching method. The photoluminescence (PL) properties, the radio-photoluminescence (RPL) phenomenon, and the spatial resolution of X-ray imaging based on RPL were studied to aim for the X-ray imaging application. The RPL phenomenon was observed as an appearance of a new PL emission at around 620 nm after X-ray irradiation. According to the shape of the PL spectrum and lifetime (13.1–16.7 ns), the luminescence was confirmed to be due to Ag2+. In addition, the 3.0% Ag-doped sample had the highest emission intensity of Ag2+. From the X-ray imaging resolution test, the 3.0% Ag-doped sample showed a spatial resolution of at least 10.0 LP mm−1.