International
Tables for
Crystallography
Volume C
Mathematical, physical and chemical tables
Edited by E. Prince

International Tables for Crystallography (2006). Vol. C, ch. 7.1, pp. 635-638

Section 7.1.8. Storage phosphors

Y. Amemiyaa and J. Chikawac

7.1.8. Storage phosphors

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A storage phosphor, called an `imaging plate', is a two-dimensional detector having a high detective quantum efficiency (DQE) and a large dynamic range. It was developed in the early 1980's for diagnostic radiography (Sonoda, Takano, Miyahara & Kato, 1983[link]; Kato, Miyahara & Takano, 1985[link]). The performance characteristics of the imaging plate was quantitatively evaluated in the mid 1980's (Miyahara, Takahashi, Amemiya, Kamiya & Satow, 1986[link]) and it was proved to be very useful also for X-ray diffraction experiments (Amemiya, Wakabayashi, Tanaka, Ueno & Miyahara, 1987[link]; Amemiya & Miyahara, 1988[link]). The imaging plate has replaced conventional X-ray film in many X-ray diffraction experiments.

The imaging plate (IP) is a flexible plastic plate that is coated with bunches of very small crystals (grain size about 5 µm) of photo-stimulable phosphor [previously BaFBr:Eu2+, recently BaF(Br,I):Eu2+] by using an organic binder. The photo-stimulable phosphor is capable of storing a fraction of the absorbed X-ray energy. When later stimulated by visible light, it emits photo-stimulated luminescence (PSL), the intensity of which is proportional to the absorbed X-ray intensity.

The mechanism of PSL is illustrated in Fig. 7.1.8.1.[link] When the IP absorbs incoming X-rays, some of the electrons in the valence band are pumped up to the conduction band of the phosphor crystals. (This corresponds to ionization of Eu2+ to Eu3+.) The electrons, in turn, are trapped in Br and F vacancies, which were intentionally introduced in the phosphor crystals during the manufacturing process, forming temporary colour centres, termed F-centres. Exposure to visible light again pumps up the trapped electrons so that they generate energy for luminescence, while returning to the valence band of the crystal. (This process corresponds to a recombination of electrons with Eu3+ ions, resulting in Eu2+ luminescence.) Because the response time of the PSL is as short as 0.8 µs, it is possible to read an X-ray image with a speed of 5–10 µs per pixel with high efficiency. The PSL is based on the allowable transition from 5d to 4f of Eu2+. The wavelength of the PSL (λ [\simeq] 390 nm) is reasonably separated from that of the stimulating light (λ = 632.8 nm), allowing it to be collected by a conventional high-quantum-efficiency photomultiplier tube (PMT). The output of the PMT is amplified and converted to a digital image, which can be processed by a computer. The residual image on the IP can be completely erased by irradiation with visible light, to allow repeated use. The IP is easy to handle, because it is flexible, like a film, and can be kept in light before its exposure to X-rays.

[Figure 7.1.8.1]

Figure 7.1.8.1 | top | pdf |

Mechanism of photo-stimulated luminescence.

The measured DQE of the IP is shown as a function of the X-ray exposure level together with that of a high-sensitivity X-ray film (Kodak DEF-5) in Fig. 7.1.8.2.[link] The advantage of the IP over X-ray film in DQE is clearly enhanced at lower exposure levels. This arises from the fact that the background noise level of the IP is much smaller than that of X-ray film. The background noise level of the IP corresponds to the signal level of less than 3 X-ray photons/100 µm2. This value compares favourably with the chemical `fog' level of X-ray film, which amounts to 1000 X-ray photons per equivalent area. The background noise level of the IP depends largely on the performance of the IP read-out system, and it can be smaller than that of a single X-ray photon with a well designed IP read-out system (Amemiya, Matsushita, Nakagawa, Satow, Miyahara & Chikawa, 1988[link]). The DQE of the IP decreases at higher exposure levels owing to `system fluctuation noise'. Fig. 7.1.8.3[link] shows the fluctuation noise of the IP and X-ray film as a function of the X-ray exposure level. It is shown that the noise fluctuation at high exposure levels is governed by system fluctuation noise, which amounts to about 1%. Fig. 7.1.8.4[link] shows the propagation of signal and noise in the IP system. The origins of the system fluctuation noise are non-uniformity of absorption, non-uniformity of the colour-centre density, fluctuation of the laser intensity, non-uniformity of PSL collection, and fluctuation of the high-voltage supply to the PMT. Although it might be possible to reduce the total system fluctuation noise from ∼1% to ∼0.5%, it is very difficult to reduce it down to [\lt] 0.1%. This means that the ultimate precision in intensity measurements with the IP is limited to the order of ∼0.5%.

[Figure 7.1.8.2]

Figure 7.1.8.2 | top | pdf |

Measured detective quantum efficiency (DQE) of the imaging plate and high-sensitivity X-ray film as a function of the exposure level. The circles correspond to the imaging plate (with the FCR 101 read-out system, Fuji Film Co. Ltd), triangles to the X-ray film (Kodak DEF-5). The filled symbols are for 8.9 keV and open symbols for 17.4 keV. The solid line indicates a noiseless counter of 100% absorption efficiency (ideal detector). The dashed line indicates a noiseless counter of 10% absorption efficiency (Amemiya & Miyahara, 1988[link]).

[Figure 7.1.8.3]

Figure 7.1.8.3 | top | pdf |

Fluctuation noise in the signal as a function of the exposure level. The circles correspond to the imaging plate and the triangles to the X-ray film (Kodak DEF-5). The filled symbols are for 8.9 keV and the open symbols for 17.4 keV. The dashed line indicates a noiseless counter of 10% absorption efficiency (Miyahara, Takahashi, Amemiya, Kamiya & Satow, 1986[link]).

[Figure 7.1.8.4]

Figure 7.1.8.4 | top | pdf |

Diagram showing a cascade of stochastic elementary processes during X-ray exposure and image read out of the imaging plate. The probability distribution of each stochastic process is described in parentheses together with the mean value. The numbers of the quanta, qi (i = 0, 5), are also shown. The noise elements of the upper line contribute to the background noise, which reduces the DQE at lower exposure levels. The noise elements of the bottom line contribute to the system fluctuation noise, which reduces the DQE at higher exposure levels (Amemiya, 1995[link]).

Compared to X-ray film, the dynamic range of the IP is much wider, of the order of 1:105 (Fig. 7.1.8.5[link] ). The response of the PSL is linear over the range from 8 to 40 000 photons/(100 µm2), with an error rate of less than 5%. It is shown that the dynamic range of an IP is extended towards the lower exposure levels of X-ray film, but not to the higher exposure levels. The dynamic range of the IP is practically limited to four orders of magnitude by that of the PMT during the read-out. Two sets of PMT's are used in some read-out systems in order to cover the entire dynamic range of the IP.

[Figure 7.1.8.5]

Figure 7.1.8.5 | top | pdf |

Dynamic range of the photo-stimulated luminescence of the imaging plate. The dynamic range of typical high-sensitivity X-ray films is also shown. O.D. refers to optical density (Amemiya, 1995[link]).

The spatial resolution of the standard IP with a 100 µm laser scanning pitch is 170 µm at the full width at half-maximum (FWHM). The spatial resolution is limited by laser-light scattering in the phosphor during the read-out. A high-resolution IP that includes blue pigments in the phosphor to minimize the laser-light scattering has been developed. A spatial resolution of 43 µm is obtained at a 25 µm laser scanning pitch with the high-resolution IP with the sacrifice of 30% of the amount of PSL. The active area sizes of the available IP range from 127 × 127, 201 × 252, 201 × 400 to 800 × 400 mm.

The IP response per incident X-ray photon is shown as a function of the X-ray energy in Fig. 7.1.8.6[link], together with the deposited energy per absorbed X-ray photon. The abrupt decrease in the energy deposition above the barium K-absorption edge is due to the energy escape in the form of X-ray fluorescence. This effect is preferable because it makes the IP response curve smoother by compensating for the abrupt increase of the absorption efficiency at the absorption edge.

[Figure 7.1.8.6]

Figure 7.1.8.6 | top | pdf |

Dependence of the IP response as a function of the energy of an X-ray photon. (i) is the IP response per an incident X-ray photon, and (a) the IP response per absorbed X-ray photon. The unit of the ordinate corresponds to the background noise level of the IP scanner (Ito & Amemiya, 1991[link]).

The image stored in the IP fades with the passage of time after exposure to X-rays. The fading rate depends on the type of IP and the temperature; it increases at higher temperature. But it does not depend on the exposure level or on the X-ray photon energy of the image. Fig. 7.1.8.7[link] shows the fading of an IP (type BAS III) as a function of time for two different X-ray energies at 293 K. The fading curve can be fitted well with three exponentials: [I(t)=A_1\exp(-k_1t)+A_2\exp(-k_2t)+A_3\exp(-k_3t).][1/k_1], [1/k_2], and [1/k_3] are 0.7, 18, and 520 h, respectively.

[Figure 7.1.8.7]

Figure 7.1.8.7 | top | pdf |

Fading of the IP signals as a function of time with two different X-ray energies (5.9 and 59.5 keV). Temperature 293 K, type of IP: BAS III (Amemiya, 1995[link]).

The non-uniformity of the response of the IP is about 1–2% over an active area of 250 × 200 mm. The distortion of the image is usually of the order 1%. It depends mainly on the type of IP read-out system.

Since the IP is an integrating-type detector, it is free from instantaneous count-rate limitations, which are accompanied by detectors operating in a pulse-counting mode. Therefore, the IP can make full use of a high flux of synchrotron X-radiation. Using synchrotron X-radiation, time-resolved measurements are possible by mechanically moving the IP (Amemiya, Kishimoto, Matsushita, Satow & Ando, 1989[link]). Caution has been paid not to irradiate extremely intense X-rays (more than 106 photons µm−2) on the IP; too intense X-rays create either non-erasable colour centres, or colour centres that are seemingly erasable but later reappear.

With minimum precautions, the IP yields reproducible results over a long period of repeated use, unlike X-ray film, whose performance is affected by slight changes in the development conditions. Various kinds of automated IP read-out systems are available, which permit on-site read-out in combination with an X-ray camera. The mechanical flexibility of the IP is also very important when it is used with a Weissenberg camera (Sakabe, 1991[link]).

References

Amemiya, Y. (1995). Imaging plates for use with synchrotron radiation. J. Synchrotron Rad. 2, 13–21.
Amemiya, Y., Kishimoto, S., Matsushita, T., Satow, Y. & Ando, M. (1989). Imaging plate for time-resolved X-ray measurements. Rev. Sci. Instrum. 60, 1552–1556.
Amemiya, Y., Matsushita, T., Nakagawa, A., Satow, Y., Miyahara, J. & Chikawa, J. (1988). Design and performance of an imaging plate system for X-ray diffraction study. Nucl. Instrum. Methods, A266, 645–653.
Amemiya, Y. & Miyahara, J. (1988). Imaging plate illuminates many fields. Nature (London), 336, 89–90.
Amemiya, Y., Wakabayashi, K., Tanaka, H., Ueno, Y. & Miyahara, J. (1987). Laser-stimulated luminescence used to measure X-ray diffraction of a contracting striated muscle. Science, 237, 164–168.
Ito, M. & Amemiya, Y. (1991). X-ray energy dependence and uniformity of an imaging plate detector. Nucl. Instrum. Methods, A310, 369–372.
Kato, H., Miyahara, J. & Takano, M. (1985). New computed radiography using scanning laser stimulated luminescence. Neurosurg. Rev. 8, 53–62.
Miyahara, J., Takahashi, K., Amemiya, Y., Kamiya, K. & Satow, Y. (1986). A new type of X-ray area detector utilizing laser stimulated luminescence. Nucl. Instrum. Methods, A246, 572–578.
Sakabe, N. (1991). X-ray diffraction data collection system for modern protein crystallography with a Weissenberg camera and an imaging plate using synchrotron radiation. Nucl. Instrum. Methods, A303, 448–463.
Sonoda, M., Takano, M., Miyahara, J. & Kato, H. (1983). Computed radiography utilizing scanning laser-stimulated luminescence. Radiology, 148, 833–838.








































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