In early applications of X-ray imaging and radiation dosimetry, photographic emulsions were directly exposed to X-rays. However, silver based films are highly inefficient in the capture of X-rays and thus fluorescent screens were introduced. The use of fluorescent screens for imaging applications involve X-rays, which have passed through or radiate from an object, which are then converted to visible light by a radiation sensitive phosphor layer. The radiation sensitive phosphor layer is typically present on a fluorescent screen (or an X-ray conversion screen).
The visible light is recorded by a conventional silver based emulsion film or plate. Scintillators are used in intensifying screens which are required to be relatively strong absorbers of X-rays. The scintillators emit light in the wavelength region of the highest sensitivity of the silver based emulsion film. The resultant exposed photographic emulsions then require wet chemical processing.
A further method of recording X-ray images comprises employing a temporary storage medium, known as an imaging plate (IP). In contrast to the previous film-screen method, where the X-rays are directly converted into visible light by the scintillators, X-ray storage phosphors store the radiation image in proportion to the intensity distribution of the X-ray.
The advantages of this method have only been recently appreciated due to advanced laser and computing technologies. In this further method, an X-ray storage medium such as a radiation image conversion panel comprises a photostimulable phosphor.
A schematic “flow” diagram illustrating this method is depicted in FIG. 10. An imaging plate comprising a photostimulable storage phosphor is subject to X-ray irradiation 1010. The X-rays are converted into stored energy in the photostimulable storage phosphor 1020 for later readout of the latent image by photostimulation 1030. The latent image may be erased to recycle the imaging plate 1040.
The imaging plate comprising the photostimulable storage phosphor absorbs X-ray radiation which has passed through or radiated from an object. It is suggested that the photostimulable storage phosphor absorbs or stores the X-ray radiation due to the creation of metastable electron-hole pairs in the photostimulable storage phosphor.
In order to release the latent X-ray radiation energy stored in the photostimulable storage phosphor, the photostimulable storage phosphor is exposed to visible or infrared laser light. This is also known as the readout step. It is believed that the step of photo-stimulation leads to the recombination of the electron-hole pairs, which in turn leads to the emission of visible light. Ideally, the photostimulable storage phosphor stores as much of the incident X-ray energy as possible, and does not emit the stored X-ray energy (described as fading) until the photostimulable storage phosphor is exposed to visible or infrared laser light.
Presently, laser light in the wavelength range of 400 to 900 nm is used in the readout step. The photostimulable storage phosphors typically exhibit a photostimulated emission with a wavelength range of 300 to 500 nm. The stimulated emission is detected by photoelectric detectors which produce an electric signal whose amplitude is linearly proportional to the light level of the emission.
The electric signal produced from the emission is then converted into a digital format such that the radiation image can be displayed on a video screen. This type of recording of an image is called digital radiography or computed radiography. Photostimulable storage phosphors have the potential of enabling X-ray imaging (e.g. medical imaging) at much lower dosages, as compared to conventional film-screen radiography whilst still providing a sufficient, if not higher, level of information. However, in practice it appears that commercially available imaging plates still require dosages comparable to the film-screen method.
A third method employs all solid state digital detectors. In these detectors the X-rays are converted by a scintillator screen (containing Csl particles) to light which is subsequently detected by a silicon panel in the form of a photodiode/transistor array.
The electric signal of the photodiode/transistor array is then converted to a digital format and displayed on a video screen. The disadvantages of the use of digital detectors include low resolution (>100 μm) and considerably higher cost. Further, digital detectors cannot be used in certain applications with restricted space such as dental X-ray imaging which is an important application. It is generally believed that digital detectors and imaging plates are complementary and both will be required for many applications.
The photostimulable storage phosphors presently known contain centres for the capture of X-ray generated electrons and holes. It is believed that X-ray irradiation creates F centres in halide crystals. F centres are anion vacancies occupied by electrons where the F+ centre is the anion vacancy without a trapped electron. For example, in a BaBrF:Eu2+ storage phosphor where the europium in a divalent oxidation state, both the F+(Br−) and the F+(F−) defects can act as electron storage centres whereas the Eu2+ acts as a hole trap.
Upon X-ray irradiation of the typical BaBrF:Eu2+ X-ray storage phosphor electron-hole pairs are created. The electrons and holes are trapped at the F+ defects and the hole trap, Eu2+, respectively. However, at room temperature some electron-hole pairs recombine immediately after their creation without being trapped and lead to spontaneous emission (scintillation) which is undesirable. Upon photostimulation of the F centres at 2.1 eV or 2.5 eV for the F(Br−) and F(F−) centres, respectively, the electrons recombine with the holes and transfer excitation energy to the activator, Eu2+, which in turns leads to broad 4f65d-4f7 emission at about 390 nm.
Despite the developments in phosphor preparation and processing, the main problems in the prior art remains the same. In all these prior art activated storage phosphors the level of spontaneous emission of the phosphors continues to be high.
Also, the stored information is lost (erased) by the readout process.
Furthermore, the annealing process for the prior art storage phosphors suffers from the disadvantage that under normal conditions, the X-ray storage efficiency of the phosphor is low. In the prior art, in order to achieve a high efficiency of the storage phosphor, a higher annealing temperature must be used but the afterglow becomes more of a problem as a result of the large crystal size. Accordingly, in prior art annealing processes, the use of a firing temperature is in the range of 850° to 1100° C.
The final annealing process for the prior art phosphors also has to be performed in the presence of an atmosphere which is typically a nitrogen or an argon atmosphere. The use of atmospheres of hydrogen and hydrogen mixtures with other gases are avoided in the prior art annealing process.
Dosimeters are useful for measuring a radiation dose equivalent to the human body. In particular, the personnel dosimeters include a thermo luminescent dosimeter (TLD) which comprises a TLD phosphor. The prior art TLD phosphors (e.g CaSO4:Dy) suffer from disadvantages which do not allow the TLD phosphors to be readily and cost effectively used for personal monitoring applications. These disadvantages are essentially due to the light sensitivity and stability of the prior art phosphors.
In view of the above, there is therefore a need for a photoexcitable storage phosphor and processes of production thereof with greater efficiency than hitherto achievable, for applications such as X-ray diagnostics, high resolution imaging work in scientific imaging and dosimeters (badges), particularly in personal and environmental radiation monitoring and in radiation monitoring in radiation therapy.