Deeply trapped electrons in imaging plates and their utilization for extending the dynamic range

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Abstract

The absorption spectra of deep centers in an imaging plate (IP) made of BaFBr0:85I0:15:Eu2+ have been studied in the ultraviolet region. Electrons trapped in deep centers are considered to be the cause of unerasable and reappearing latent images in IPs over-irradiated with X-rays. Deep centers showed a dominant peak at around 320 nm, followed by two small peaks at around 345 and 380 nm.

By utilizing deeply trapped electrons, we have attempted to extend the dynamic range of an IP. The IP was irradiated by 150-kV X-rays with doses from 8.07 mGy to 80.7 Gy. Reading out the latent image by the stimulation of Eu2+ luminescence with a 633-nm He–Ne laser light from a conventional Fuji reader showed a linear relationship with irradiated dose up to 0.8 Gy, but then becoming non-linear. After fully erasing with visible light, unerasable latent images were read out using 635-nm semi-conductor laser light combined with a photon-counting detection system. The dose–response curve so obtained gave a further two orders of magnitude extending the dynamic range up to 80.7 Gy. Comprehensive results indicate that electrons supplied from deep centers to the F centers provided the extended dynamic range after the F centers became saturated. Based on these facts, a model of the excitation of deeply trapped electrons and PSL processes is proposed.

Introduction

Since photostimulated luminescent (PSL) materials were developed as an imaging plate (IP) for X-ray imaging [1], excellent properties of PSL materials have been reported such as wide linearity, high spatial resolution, simplicity of the PSL statistics, and high confidence of stored image information in practical use [2], [3], [4]. Besides, IPs can be used repeatedly by exposing them to visible light for 20–30 min between uses. Because of these advantages, the IP has frequently been utilized in medicine for computed radiology and a number of other fields [5], [6], [7]. Through these various experiences, some unexpected effects have become known, i.e., the phenomena of unerasable and reappearing latent images, particularly for IPs irradiated with a high dose and cumulative over-irradiated IPs. In these IPs, quite stable latent images remain unerasable after fully erasing with visible light and latent images reappear after a while even when they seem to have been completely erased. These have been called unerasable and reappearing latent images, respectively. The level scheme of the PSL material has been reported by Iwabuchi et al. [8]. In the sensitive layer of an IP, ionizing radiation creates a large number of F centers, which are anion vacancies trapping electrons and record information about the deposited energy and its position. Stimulating the IP optically with a He–Ne laser (633 nm) from the image reader provides the energy necessary to liberate the trapped charges, thereby causing the occurrence of PSL (390 nm) from the positions of the F centers. However, in practice, the irradiation creates additional complex centers in the material, which may cause unerasable and reappearing PSL signals in over-irradiated IPs. In our previous study [9], we obtained the following results by using BAS-TR type IPs: (1) unerasable PSL latent images almost disappeared when the IP was held at a temperature of 120 °C for 28 days; (2) ultraviolet light of wavelength around 290 nm promoted the reappearance of PSL signals; and (3) the amount of PSL signal reappearance increased in proportion to the irradiation dose. It was considered that electrons are trapped in deep centers, they can be excited optically by 290 nm ultraviolet light and transferred towards the longer wavelength of around 633 nm, producing F centers and they cause latent images to reappear. The number of deeply trapped electrons will also increase in proportion to the irradiation dose. The deep centers might be defect centers induced by the radiation and/or oxide impurity [10].

In this study, the absorption spectra of deep centers in an over-irradiated IP were studied in the ultraviolet region. By utilizing deeply trapped electrons, we have attempted to extend the dynamic range of an IP readout under a higher irradiation dose. Deep centers are assumed to have somewhat smaller capture cross-sections than those of F centers because unerasable latent images are observed in over-irradiated IPs and the number of deeply trapped electrons would increase in proportion to the irradiation dose.

Section snippets

Measurements of luminescence inducing spectra

Diffracted ultraviolet light from a deep ultraviolet lamp (UXM500SX: USHIO INC.) was used as a light source for excitation of electrons in deep centers. Fig. 1 shows a schematic illustration of the diffracted ultraviolet light irradiation system, in which ultraviolet light passing through a 1.0-mm slit is separated into its component wavelengths with a 50-mm2 diffraction grating having 600 grooves/nm and a blaze wavelength of 400 nm (Edmund Optics Inc.). The complete system is installed in a dark

Luminescence inducing spectrum

Luminescence inducing spectra from 200 to 500 nm of the IP sample irradiated with a 30 Gy dose and one not irradiated are shown in Fig. 4. In the irradiated IP sample, a dominant peak is observed at around 320 nm, followed by two small peaks at around 345 and 380 nm. However, no intense peak is seen in the non-irradiated sample. This result indicates that several deep centers were created in the irradiated IP sample in the ultraviolet region. Our previous study indicated that deep centers in BAS-TR

Conclusion

In over-irradiated IP samples, measurement of the luminescence inducing spectra in the ultraviolet region reveals a dominant peak at around 320 nm, followed by two small peaks at around 345 and 380 nm. These deep centers may act as competitive trap centers to the F centers in the PSL material. Based on these results, a model of the excitation of deeply trapped electrons and PSL processes by stimulation with 630-nm light is described in the level scheme of the PSL material.

By utilizing deeply

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