Go Okada

Spatially resolved measurement of high doses in microbeam radiation therapy using samarium doped fluorophosphate glasses

The measurement of spatially resolved high doses in microbeam radiation therapy has always been a challenging task, where a combination of high dose response and high spatial resolution (microns) is required for synchrotron radiation peaked around 50 keV. The x-ray induced Sm3+ → Sm2+ valence conversion in Sm3+ doped fluorophosphates glasses has been tested for use in x-ray dosimetry for microbeam radiation therapy. The conversion efficiency depends almost linearly on the dose of irradiation up to ∼5 Gy and saturates at doses exceeding ∼80 Gy. The conversion shows strong correlation with x-ray induced absorbance of the glass which is related to the formation of phosphorus-oxygen hole centers. When irradiated through a microslit collimator, a good spatial resolution and high “peak-to-valley” contrast have been observed by means of confocal photoluminescence microscopy.

X-ray induced Sm3+ to Sm2+ conversion in fluorophosphate and fluoroaluminate glasses for the monitoring of high-doses in microbeam radiation therapy

Fluorophosphate and fluoroaluminate glasses doped with trivalent samarium were evaluated as sensors of x-ray radiation for microbeam radiation therapy at the Canadian Light Source using the conversion of trivalent Sm3+ to the divalent form Sm2+. Both types of glasses show similar conversion rates and may be used as a linear sensor up to ∼150 Gy and as a nonlinear sensor up to ∼2400 Gy, where saturation is reached. Experiments with a multi-slit collimator show high spatial resolution of the conversion pattern; the pattern was acquired by a confocal fluorescence microscopy technique. The effects of previous x-ray exposure may be erased by annealing at temperatures exceeding the glass transition temperature Tg while annealing at TA < Tg enhances the Sm conversion. This enhancement is explained by a thermally stimulated relaxation of host glass ionic matrix surrounding x-ray induced Sm2+ ions. In addition, some of the Sm3+-doped glasses were codoped with Eu2+-ions but the results show that there is no marked ...

Optically erasable samarium-doped fluorophosphate glasses for high-dose measurements in microbeam radiation therapy

Previous work has demonstrated that fluorophosphate (FP) glasses doped with trivalent samarium (Sm3+) can be used as a dosimetric detector in microbeam radiation therapy (MRT) to measure high radiation doses and large dose variations with a resolution in the micrometer range. The present work addresses the use of intense optical radiation at 405 nm to erase the recorded dose information in Sm3+-doped FP glass plates and examines the underlying physics. We have evaluated both the conversion and optical erasure of Sm3+-doped FP glasses using synchrotron-generated high-dose x-rays at the Canadian Light Source. The Sm-ion valency conversion is accompanied by the appearance of x-ray induced optical absorbance due to the trapping of holes and electrons into phosphorus-oxygen hole (POHC) and electron (POEC) capture centers. Nearly complete Sm2+ to Sm3+ reconversion (erasure) may be achieved by intense optical illumination. Combined analysis of absorbance and electron spin resonance measurements indicates that th...

Sm-doped CsBr crystal as a new radio-photoluminescence (RPL) material

Abstract Radio-photoluminescence (RPL) is a phenomenon seen in luminescent materials in which the appearance of new photoluminescence (PL) emission is induced by an incident ionizing radiation such as X-rays; and the signal is stable even after the irradiation and during the PL measurement. Since the induced PL intensity is proportional to the irradiation dose, the RPL can be used in radiation measurements. The distinct advantage of RPL over the conventional thermally- or photo-stimulated luminescence (abbreviated as TSL or PSL) dosimeters is the stability of response signal. With an RPL detector, it allows us to readout the signal multiple time without signal fading. In this work, we discovered that CsBr:Sm showed an RPL phenomenon by X-ray irradiation, and we characterized this new material as an RPL detector. While the sample showed PL emissions mainly in the visible range, after an X-ray irradiation additional emissions could be observed in the red to near-infrared range around 650–850 nm and 900–1000 nm and longer. The RPL response was fairly stable overall, but very interestingly the 650–850 nm signal slightly increased while the 900–1000 nm decreased during PL readout. The dynamic range was confirmed over 1–104 mGy with linear response.

ESR study of samarium doped fluorophosphate glasses for high-dose, high-resolution dosimetry

We have studied the effect of samarium doping concentration and thermal annealing on X-ray induced defect centers, including phosphorus-oxygen hole and electron centers (POHC and POEC), in Sm3+-doped fluorophosphate glasses towards developing a potential high-dose, high-resolution detector for microbeam radiation therapy. ESR measurements show that defect center formation is suppressed by increasing the Sm-dopant concentration with POECs more strongly influenced than POHCs. This can be explained by a model based on the competition between defect center formations and Sm3+ ⇆ Sm2+ interconversion. Thermal annealing at increasing moderate temperatures (TA = 100−300 °C) reduces the POHC related ESR and induced absorbance bands while those of POEC continue to survive. ESR measurements over a wider range show the trace of a very broad ESR signal in samples containing Sm2+ ions including those annealed at temperatures between 350 °C and glass transition temperature (Tg ≈ 460 °C). Finally, thermal annealing at 550 °C (> Tg) totally erases all the ESR signals and restores the sample to its original unirradiated state.

Optical and scintillation properties of Nd-doped SrAl2O4 crystals

Abstract Nd-doped SrAl2O4 (Nd:SrAl2O4) crystals were prepared by a floating zone (FZ) method with different dopant concentrations. The photoluminescence (PL) measurements under visible light excitations confirmed strong near infrared (NIR) emissions around 890 nm, which is due to the 4f-4f transitions of Nd3+, and the decay time constants were 300–400 μs. Furthermore, VUV excitations revealed two broad emissions around 250–450 and 500–700 nm, which were attributed to self-trapped excitons (STEs) perturbed by Nd3+. Moreover, the X-ray induced scintillation spectra showed some small peaks at around 380 nm in addition to NIR emissions at 1064 nm similarly seen in PL. The former scintillation decay time constants were 30–40 μs. In the undoped sample, these emissions mentioned above were not present but a weak broad emission around 450 nm appeared. In thermally stimulated luminescence (TSL) after X-ray irradiation, strong TSL glow peaks were observed in the Nd-doped samples at around 100 and 250 °C.

Thallium magnesium chloride: A high light yield, large effective atomic number, intrinsically activated crystalline scintillator for X-ray and gamma-ray detection

We report the luminescence and the scintillation properties of a newly developed thallium magnesium chloride (TlMgCl3) crystal. The crystal sample can be easily fabricated from the melt using the Bridgman method. The photoluminescence band appeared near the wavelength of 405 nm under excitation at 230 nm. An X-ray-induced scintillation spectrum showed an intense emission band near the wavelength of 405 nm. The decay time constant was estimated to be approximately 60 ns (~25%) and 350 ns (~75%) using a bi-exponential fitting. The scintillation light yield reached 46,000 photons/MeV with an energy resolution of 5% at 662 keV.

Luminescent and scintillation properties of Ce-doped Tb3Al5O12 crystal grown from Al-rich composition

A Ce-doped Tb3Al5O12 single crystal was successfully synthesized via the floating zone method in an Al-rich composition. In both photoluminescence and scintillation, the emissions were predominantly due to the 5d–4f transitions of Ce3+ peaking around 550 nm, but emissions due to defects and the 4f–4f transitions of Tb3+ were revealed by the time-resolved analyses. The crystal quality and luminescence intensity were sufficiently high such that the crystal was applicable for pulse-height spectrum measurement. Under 137Cs γ-ray exposure, a clear photoabsorption peak was observed, and the estimated light yield was 57,000 photons/MeV.

Radioluminescence properties of Sm-doped fluorochlorozirconate glasses and glass-ceramics

We have investigated X-ray induced radioluminescence (XL) properties of Sm-doped fluorochlorozirconate (FCZ) glasses and glass-ceramics. The FCZ glass is a modified ZBLAN glass which shows a very high optical transmission over a wide spectral range. The glass matrix includes Sm3+-doped nanocrystals of BaCl2 after heat-treatment at temperatures above 250 °C. The glass-ceramic emits red light under UV and X-ray exposure. Since conventional Si-based photodetectors, e.g., CCDs, have the highest quantum efficiency to red light in general, the Sm-doped FCZ glass-ceramic plate can be a good candidate as a scintillator material for indirect radiation detection. Moreover, a very broad emission is present in the glass-ceramic around 300–500 nm, which is attributed to a self-trapped exciton (STE) emissions. The temperature dependence of X-ray induced luminescence and photoluminescence are very similar. The XL light yield is linearly proportional to the X-ray exposure rate for rates higher than 20 mR/s. For low exposure rates, emissions by Sm2+ are more sensitive than others, leading to a nonlinear response.

Scintillation and optical properties of Ce-doped YAGG transparent ceramics

Abstract We investigated photoluminescence (PL) and scintillation properties of Ce 3+ -doped Y 3 Al 2 Ga 3 O 12 (Ce:YAGG) transparent ceramics synthesized by vacuum sintering with different Ce 3+ concentrations (mol.%) from 0.3% to 1% (0.3%Ce:YAGG, 0.5%Ce:YAGG, 0.8%Ce:YAGG and 1%Ce:YAGG). The samples were synthesized by the vacuum sintering technique. The obtained samples showed a very strong and broad PL emission by the 5d 1 -4f transition of Ce 3+ in the wavelength range from 470 to 600 nm. The PL decay profiles were approximated by a single exponential decay function with the time constants of around 33–39 ns. Furthermore, the scintillation spectra induced by X-rays showed similar features with those observed in PL. The scintillation decay time profiles followed a second-order exponential decay function. The fast component group (ranged 35–42 ns) were dominantly contributed by the 5d 1 -4f transition of Ce 3+ . The pulse height spectra utilizing the latter emissions showed a clear photoabsorption peak. Among those samples tested, 0.5%Ce:YAGG sample showed the highest scintillation light yield of 21,400 ph/MeV under 137 Cs γ-ray irradiation.