APPARATUS AND METHOD FOR DETECTING AND MONITORING RADIATION

FIELD OF THE INVENTION The present invention relates to an apparatus for detecting and monitoring radiation. In particular, the present invention relates to an apparatus for detecting and monitoring personal radiation levels on a subject and _. in particular a human. In particular embodiments of the present invention, there is provided personal dosimetry readers for detecting and monitoring radiation, imaging readers and methods of production and uses thereof.

There is also provided a method of detecting and monitoring radiation levels in a subject and in particular a human, animal or part thereof. In another preferred aspect, there is provided a use of the apparatus in the detection and monitoring of radiation in a subject or in imaging. In particular, there is also provided an apparatus and method for scientific and medical imaging using imaging plates comprising a photoluminescent radiation storage phosphor.

The apparatus, methods and uses of the present invention may be used in conjunction with phosphors and more particularly photoexcitable (photoluminescent) X ray storage phosphors. In particular, the photoexcitable X ray storage phosphors of interest may be based on the reduction of one or more rare earth metals from the trivalent +3 oxidation state to the divalent +2 oxidation state upon exposure to radiation where "radiation" may comprise UV, X-ray, gamma, beta and cosmic radiation and neutron irradiation. More particularly, the rare earth metal may be selected from the group consisting of samarium, dyspropium, europium and gadolinium. In a further particular embodiment, the rare earth metal may be samarium.

The rare earth metal ion may be embedded in a wide-bandgap semiconductor material or insulator, in particular the host material may be an alkaline earth metal halide.

Definitions

The following part of the specification provides some definitions that may be useful in understanding the description of the present invention. These are intended as general definitions and should in no way limit the scope of the present invention to those terms alone, but are put forth for a better understanding of the following description.

Unless the context requires otherwise or specifically stated to the contrary, integers, steps, or elements of the invention recited herein as singular integers, steps or elements clearly encompass both singular and plural forms of the recited integers, steps or elements.

Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated step or element or integer or group of steps or elements or integers, but not the exclusion of any other step or element or integer or group of elements or integers. Thus, in the context of this specification, the term "comprising" is used in an inclusive sense and thus should be understood as meaning "including principally, but not necessarily solely".

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations or any two or more of said steps or features.

Throughout this specification, unless the context clearly indicates otherwise, the term "light" refers to electromagnetic radiation in the visible, ultraviolet or near infrared region.

Throughout this specification, unless the context clearly indicates otherwise, the term "photoluminescence" refers to a light emission that occurs from the excited states of an optical centre upon its direct photoexcitatioπ unlike photostimulation where emission is excited indirectly.

BACKGROUND

It should be understood that any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.

Dosimeters are useful for measuring a radiation dose equivalent to the human body. In particular, personal radiation dosimeters include a thermoluminescent dosimeter (TLD) which comprises a TLD phosphor. The TLD phosphor (eg CaSO ₄ : Dy) is considered to suffer from the disadvantage that all information is erased when reading out the dose with TLD phosphors. Thus TLD based dosimetry cannot be used in an accumulative mode. There is also considered to be a disadvantage in that the readers for TLD phosphors are rather expensive and hence a centralized readout service is desirable.

Accordingly, the present invention seeks to overcome these problems in the prior art or to at least provide an alternative to the prior art. In particular, the present invention may in one aspect provide a cost-effective apparatus that may be used for a decentralised and daily/hourly personal radiation monitoring system. However, the apparatus of the present invention may also be used in a centralised radiation badge reader system. Currently, imaging plates based on photostimulable radiation storage phosphors, such as BaFBr(l):Eu ²⁺ are used in computed radiography. The readers for these plates are usually based on the so-called "flying-spot" technique, where the focused laser beam (e.g. Helium Neon laser) is scanned across the imaging plate. Disadvantages of this technology include the fact that each pixel is erased upon the readout process, limiting the signal-to-noise ratio, and the time-demanding sequential readout process.

Accordingly, the present invention seeks to overcome these problems by using a reader for imaging plates based on photoluminescent radiation storage phosphors or to provide an alternative apparatus, uses and methods to the prior art.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided an apparatus for detecting radiation stored in a photoluminescent storage phosphor comprising a light source which provides excitation light, one or more elements which modulate the excitation light to provide a suitable incident wavelength for a phosphorescent material located on a holding element, one or more elements to filter the emission from the phosphorescent material and a detector, wherein there is located between the holding element and the detector at least one gating element which acts to reduce or eliminate background light (e.g. excitation light) from the phosphorescent emission.

In one embodiment, the apparatus for detecting radiation is a personal radiation dosimeter reader for personal radiation monitoring or imaging reader or for monitoring of radiation in radiation therapy.

In one embodiment, the gating element may be a light chopper element which allows the emission from the phosphorescent material to pass to the detector whilst not allowing light from the light source to enter the apparatus. In another embodiment of the invention, there may be more than one gating element such as a light chopper wheel and an optical filter.

In another embodiment, the gating element may be a light chopper wheel which allows light from the light source to enter the apparatus whilst not allowing the emission from the phosphorescent material or the excitation light to pass to the detector.

In another embodiment, the gating element may be a light chopper wheel which may be a rotatable disc with a number of apertures spaced equally along the circumference of the disc. The number of apertures in the disc may be an odd number and the number of apertures may be selected from one, three, five, seven, nine, eleven, thirteen, and fifteen apertures. In an example of the apparatus of the invention the number of apertures may be nine.

In another embodiment, the gating element may be a pulsed laser arranged to provide pulses synchronized with the gating of the detector by either a chopper

wheel, optical shutter or by electronic means, at suitable wavelengths for the phosphorescent material.

In another embodiment, the gating element may comprise an electronically gated and synchronized detector. In another embodiment, the apparatus may comprise a detector which may be a CMOS or CCD camera which is capable of recording 2-dimensional latent images. The apparatus may also comprise a holding element which is an imaging plate. The apparatus may further comprise at least one gating element comprising a light chopper wheel together with an electronic controller element which synchronises the excitation light from the light source with the detector so as to reduce or eliminate background light, e.g. excitation light, from the phosphorescent emission. In this embodiment, the apparatus may be used for imaging and may be used in particular for X ray imaging.

In another embodiment of the present invention, the apparatus reads out latent images from imaging plates comprising photoluminescent radiation storage phosphors. This apparatus can be used for medical imaging, e.g. dental X-ray diagnostics, mammography etc, or scientific imaging including 2-dimensional X-ray diffraction experiments or for monitoring of radiation in radiation therapy.

The phosphorescent material may be a phosphor. The phosphor may be a photoexcitable or photostimulable storage phosphor. The photoexcitable storage phosphor may comprise at least one rare earth element in the trivalent +3 oxidation state and wherein upon irradiation by the suitable incident wavelength(s) may be reduced from the +3 oxidation state to the +2 oxidation state. The rare earth element may be selected from the group consisting of cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dyspropium, holmium, erbium, thulium, ytterbium, lutetium and mixtures or one or more of the aforesaid rare earth elements.

In one embodiment, the rare earth element may be selected from the group consisting of samarium, dyspropium, europium, gadolinium and mixtures thereof. In a particular example, the rare earth element may be samarium.

The photoexcitable storage phosphor may comprise at least one halogen element. The at least one halogen element may be selected from the group consisting of fluorine, chlorine, iodine and bromine and mixtures thereof.

The photoexcitable storage phosphor may further comprise an alkaline earth metal. The alkaline earth metal may be selected from the group consisting of barium, calcium, strontium or mixtures thereof.

The phosphorescent material may be a photoexcitable storage phosphor selected from the group consisting of BaFCIiSm ³⁺ , CaFChSm ³⁺ and SrFCkSm ³⁺ .

The light source may be any suitable light source including a laser, pulsed or continuous wave (CW), or other suitable light source which will provide the necessary suitable incident wavelength for the particular storage phosphor, which in this particular example is a X-ray storage phosphor. In one example, the light source may be a light emitting diode (LED) and in particular, a blue light emitting diode (LED).

In another aspect, the present invention relates to a method for detecting and monitoring radiation comprising the steps of: providing a light source which emits excitation light at suitable wavelengths for a phosphorescent material, modulating the excitation light, filtering the emission from the phosphorescent material; and detecting the phosphorescent emission wherein before the step of detecting the emission from the phosphorescent material, there is at least one gating element which reduces or eliminates background light from the light of the phosphorescent emission.

In another aspect, the present invention relates to a method of measuring personal radiation levels or imaging comprising using an apparatus of the invention on a subject.

In another aspect, the present invention relates to a use of an apparatus of the invention in the measurement of personal radiation levels on a subject. The use may also be for applications including but not limited to reading latent images from

imaging plates comprising photoluminescent radiation storage phosphors. The use includes medical imaging, e.g. dental X-ray diagnostics, mammography etc, or scientific imaging including 2-dimensional X-ray diffraction experiments.

The subject may be a human, animal or part of a human or animal. In particular, the subject may be a human or animal breast.

BRIEF DESCRIPTION OF THE FIGURES

A detailed description of the invention will now be provided, by way of example only, with reference to the accompanying drawings. It should be appreciated, however, that the drawings should not be construed as limiting the scope of the invention as defined in the claims in any way. Referring to the drawings:

Figure 1 illustrates a schematic representation of an apparatus for detecting and monitoring radiation in accordance with a first embodiment of the present invention;

Figure 2 illustrates a schematic representation of an apparatus for detecting and monitoring radiation in accordance with a second embodiment of the present invention;

Figure 3 illustrates a schematic representation of an apparatus for detecting and monitoring radiation in accordance with a third embodiment of the present invention where in this embodiment the light source is a laser that is synchronised with the chopper blade by using an optical switch such as is illustrated as reference numeral (34) e.g. Fairchild H22L;

Figure 4 illustrates a schematic representation of an apparatus for detecting and monitoring radiation in accordance with a fourth embodiment of the present invention;

Figure 5 illustrates a schematic representation of an apparatus for detecting and monitoring radiation in accordance with a fifth embodiment of the present invention wherein the gating element is a synchronized shutter;

Figure 6 illustrates a schematic representation of an apparatus for detecting and monitoring radiation in accordance with a sixth embodiment of the present invention wherein the gating element is an electronically gated and synchronized detector;

Figure 7 illustrates an example of a light chopper wheel comprising nine apertures which is used as a gating element for the apparatus as shown in Figures 1 to 4;

Figure 8 illustrates the response of Cs-137 exposed dosimeters as measured in the apparatus as shown in Figure 1 using BaFChSm ³⁺ dosimeters; Figure 9 illustrates the response of dosimeters exposed to 60 kVp. X-ray radiation as measured in the apparatus as shown in Figure 1 using BaFCIiSm ³⁺ dosimeters;

Figure 10 illustrates the response to exposure to140 keV Tc-99m gamma rays as measured in the apparatus (10) described in Figure 1. BaFCIiSm ³⁺ dosimeters used;

Figure 11 illustrates the energy dependence of response of apparatus (10) of Figure 1 and BaFCIiSm ³⁺ based dosimeters;

Figure 12 shows a top view of an implementation of the apparatus (10) according to Figure 1 ;

Figure 13 shows a perspective view of an implementation of the apparatus (10) according to Figure 1 ; Figure 14 shows a top view of an implementation of the apparatus (10b) according to Figure 2;

Figure 15 shows a perspective view of an implementation of the apparatus (10b) according to Figure 2;

Figure 16 is a schematic representation of an imaging plate reader for medical or scientific imaging plates, using at least one gating element as shown in Figures 1-6.

Figure 17 shows a view of an imaging plate reader according to Figure 16.

Figure 18 is another view of a reader for medical or scientific imaging according to Figure 16.

Figure 19 shows an X-ray image taken by using a BaFCIiSm ³⁺ imaging plate and the reader shown in Figures 16-18.

DETAfLED DESCRIPTION OF THE INVENTION

A) Apparatus for personal radiation monitoring (dosimetry) or radiation therapy

In Figure 1, there is described an apparatus (10) for detecting and monitoring levels of personal radiation comprising a light source (12), a first gating element in the form of a light chopper wheel (14), one or more lenses (16a, 16b, 16c, 16d), a beam splitter (18), a first detector (20), a sample holder (22), a second detector (26) to measure the excitation light level, a mirror (28), a motor (30) to drive the chopper wheel, and a second gating element which in this particular embodiment is an optical filter (24). The apparatus (10) for this embodiment of the invention is a personal radiation dosimeter reader for personal radiation monitoring.

In this embodiment, the apparatus (10) may use a very sensitive light detector as the first detector (20) for the emitted light from the sample. In this particular embodiment, the apparatus (10) is a personal radiation dosimeter reader for personal radiation monitoring. The sample used in the sample holder (22) of this embodiment is a samarium based X ray storage phosphor and is in particular a photoexcitable (photoluminescent) X-ray storage phosphor. In a particular example, the X ray storage phosphor is BaFCkSm ³⁺ .

The apparatus (10) is suitable for such phosphors and also for other phosphors which have a long excited state lifetime which is approximately 100 microseconds or greater. In this particular example, the phosphor used is nanocrystalline BaFCIiSm ³⁺ X-ray storage phosphor.

In Figure 2, there is shown an apparatus 10b for detecting and monitoring levels of personal radiation comprising a light source (12), a first gating element in the form of a light chopper wheel (14), one or more lenses (16a, 16b, 16c, 16d), a beam splitter (18), a first detector (20) for the excitation light level, a sample holder (22), a motor (30) to drive the chopper wheel, and a second gating element which in this particular embodiment is an electronic controller 32 and an optical switch (34). The apparatus (10) for this embodiment of the invention is a personal radiation dosimeter reader for personal radiation monitoring. In Figure 3, there is shown an apparatus 10c for detecting and monitoring levels of personal radiation comprising a light source (12), a first gating element in

the form of a light chopper wheel (14), one or more lenses (16c and 16d), a beam splitter (18), a first detector (20) for the excitation light level, a second detector (26), a sample holder (22), an optical filter (24) and a motor (30) to drive the chopper wheel, and a second gating element which in this particular embodiment is a electronic controller 32 and an optical switch (34). The apparatus (10) for this embodiment of the invention is a personal radiation dosimeter reader for personal radiation monitoring.

In Figure 4, there is shown an apparatus (10d) for detecting and monitoring levels of personal radiation comprising a light source (12), a first gating element in the form of a light chopper wheel (14), one or more lenses (16c and 16d), a beam splitter (18), a first detector (20), a second detector (26) for measurement of the excitation light level, a sample holder (22), an optical filter (24) and a motor (30) to drive the light chopper wheel (14). This embodiment is similar to that shown in Figure 1 except that the light source (12) comprises a temperature controlled light laser. The apparatus (10d) for this embodiment of the invention is a personal radiation dosimeter reader for personal radiation monitoring.

In Figure 5, there is shown an apparatus (10e) for detecting and monitoring levels of personal radiation comprising a light source (12), a first gating element in the form of an electronic controller for synchronizing the light source (12) with an optical shutter (36), one or more lenses (16a, 16b, 16c and 16d), a beam splitter (18), a first detector (20), a second detector (26) for measurement of the excitation light level, a sample holder (22), and an optical filter (24). This embodiment is similar to that shown in Figure 1 except that the light chopper wheel (14) is replaced by an electronic controller (32) and a synchronized optical shutter (36). The light source (12) in this embodiment may be a LED or a laser. When a laser is used, the lenses 16a and 16b are not required. The apparatus (10d) for this embodiment of the invention is a personal radiation dosimeter reader for personal radiation monitoring.

In Figure 6, there is shown an apparatus (10f) for detecting and monitoring levels of personal radiation comprising a light source (12), a first gating element in the form of an electronic controller (32) for synchronizing the light source (12) with the detector (20), one or more lenses (16a, 16b, 16c and 16d), a beam splitter (18), a

first detector (20), a second detector (26) for measurement of the excitation light level, a sample holder (22), and an optical filter (24).

This embodiment is similar to that shown in Figure 1 except that the light chopper wheel (14) is replaced by an electronically gated and synchronized detector which is synchronized by the electronic controller (32). The light source (12) in this embodiment may be a LED or a laser. When a laser is used, the lenses 16a and 16b are not required. The apparatus (10d) for this embodiment of the invention is a personal radiation dosimeter reader for personal radiation monitoring.

In this embodiment, the gating element is an electronic controller (32) which achieves gating by directly (electronically) switching off the detector (20) whilst the excitation light source (12) (LED ₁ laser etc) is switched on.

The individual elements of the preferred embodiments of the apparatus (10, 10a, 10b, 10c, 10d, 10e and 1Of) will now be described in more detail to further illustrate the invention. Light Source (12)

The light source (12) may be any suitable light source including a laser, pulsed or CW, or other suitable light source which will provide the necessary suitable incident wavelength for the particular storage phosphor, which in this particular example is a X-ray storage phosphor. In this preferred example, there is utilised as the light source (12), a high power blue light emitting diode (LED) from Phillips Lumileds or other suitable manufacturer as would be appreciated by the skilled person in the field. The Phillips Lumileds has a peak wavelength which may fall within the range of between 490 and 400 nm. In the apparatus (10) as shown in figure 1, a 3 W 470 nm LED with approximately 500 mW light output is used.

In order to facilitate constant light output, the light source (12) may be temperature stabilised. Further, in order to achieve ultimate stability, a servo loop may be implemented using the photodiode (26) which measures the light level of the

LED or the laser or any other light source. In this way, a constant power output of the LED, the laser or any other light source can be achieved for the apparatus (10).

As stated above, the LED as the light source (12) may be replaced by a blue or violet laser diode (semiconductor laser) with an ideal wavelength of between 407 to 415 nm. This embodiment of the invention is schematically illustrated in Figure 4 which shows an apparatus (10d) that uses a laser as the light source (12). The laser may be temperature and current stabilized as well to ensure constant output. In another embodiment of the present invention, a pulsed laser may be used as the light source (12). The pulse frequency and length of the laser should be synchronized with the phase and frequency of the light chopper wheel (14) in order to avoid laser light reaching the detector (20). A schematic diagram of this embodiment is illustrated in Figure 3 where an optical switch (34) and an electronic control unit (32) are used to synchronize the laser light pulses and the opening of the detector aperture by using an electronic control circuit. If the optical switch (34) is in the position of the excitation light aperture of apparatus (10) of Figure 1 , synchronization is automatically achieved as long as the rise and fall time of the laser pulse are substantially shorter than the frequency of the chopper blade.

Yet, in another embodiment of the apparatus of the present invention, the laser in the apparatus as shown in Figure 3 is replaced by the LED or another light source as is shown in Figure 2. The light source is then synchronized with the opening of the detector aperture (180 degrees phase shift) by using the optical switch

(34) and a control unit.

Light chopper wheel (14)

The light chopper wheel (14) may contain an uneven number of apertures (e.g. slots, holes) which are preferably symmetrically arranged around one diameter of the light chopper wheel (14). In one embodiment, the number of apertures may be three, five, seven, nine, eleven or other suitable number of uneven apertures for the light chopper wheel (14). In this particular example as shown in Figure 7, there is shown nine apertures (14a) in the light chopper wheel (14). Figure 7 also shows an enlarged view of an aperture (14a) in the light chopper wheel (14). The duty cycle has to be such that when the aperture for the light source (12) is open, the aperture for the detector (20) is closed i.e. the duty cycle has to be less than 50% and its

precise value depends on the diameter of the apertures on the wheel in relation to the radius.

Lenses (16)

The apparatus (10) may comprise one or more lenses (16) which act to coHimate the light of the light source (12) which is shown in Figure 1 as a LED into a parallel beam. However, it should be appreciated that the one or more lenses (16) are not required in the apparatus (10) if a laser source is used as the light source

(12). These modified embodiments are shown in Figures 3 and 4. When the one or more lenses are used in the apparatus (10), the one or more lenses may be anti- reflection coated for the blue region of the spectrum on the excitation side of the apparatus (10). This provides optimisation of the collimation of the blue light with minimal loss.

In this particular example, the second lens (16b) focuses the collimated light beam so that the LED is imaged on phosphor sample retained in the sample holder 22. In other words, the focal length of the lens (16b) is equal to the distance between lens (16b) and the sample/dosimeter. The second lens (16b) is again anti-reflection coated for the blue region of the spectrum to minimize reflection losses.

The third lens (16c) collimates the emitted phosphorescence from the radiation storage phosphor. The focal length of the third lens (16c) is equal to the distance to the phosphor on the dosimeter. The emitted light is then refocused on the entrance aperture of the photodetector by a fourth lens (16d).

Beam splitter (18)

The apparatus (10) may also comprise a beam splitter (18). The beam splitter (18) diverts a few percent of the collimated light as shown in Figure 1. The beam splitter (18) diverts the coliimated light by being arranged at a suitable angle e.g. 45 degrees with respect to the beam axis. Accordingly, the diverted excitation light is measured by a detector (26) which may be in the form of a photodiode. This allows the monitoring of the level/power of the excitation light.

First Detector for emitted phosphorescence (20)

The apparatus (10) in Figure (1) comprises a very sensitive light detector as the first detector (20). The first detector (20) may be a self-contained photomultiplier unit with built in high voltage supply, photon counter and interface. The first detector (20) may also be any other kind of photomultiplier-signal processor combination or sensitive light detector such as photon-counting avalanche photodiode or the like. The first detector (20) may also be any kind of CCD or CMOS camera.

It is envisaged that the signal from the first detector (20) is captured by a suitable electronic data storage means such as a computer and thus can be normalized with the power signal of the photodiode. A preferred method of calibrating the dosimetry reader as described in the accompanying figures is set out below.

Sample Holder (22)

The apparatus (10) further comprises a sample holder (22) and a dosimeter (24) that fits into the sample holder (22) so that the active photoluminescent material faces the incoming/exciting blue light beam. On the active photoluminescent material an image of the LED junctions is visible. A third lens (16c) and a fourth lens (16d) are necessary in the apparatus (10), even if a laser or a LED is used as the light source (12). The third lens (16c) collimates the emitted light from the photoluminescent X- ray storage phosphor located in the sample holder (22). The third and fourth lenses may also be anti-reflection coated for the red region of the light spectrum so that a maximum possible number of photons in the red region is collimated with minimal reflection loss.

The apparatus (10) may also comprise a mirror (22) which may be gold coated. The gold coating serves two purposes: a) the red emitted light from the X- ray storage phosphor is in a wavelength region where maximum reflection is achieved by the gold coating b) the gold coated mirror is not very effective in the blue region of the spectrum and hence introduces some significant attenuation of unwanted reflected blue excitation light.

The fourth lens (16d) may also be provided in the apparatus (10). The lens (16d) may act to slowly refocus the collimated beam of emitted red light onto the aperture in front of the detector (20).

In use, the focussed light enters the apparatus (10) and passes through the light chopper wheel (14) 180 degrees out of phase with the excitation light i.e. as described above if the light chopper wheel (14) is in the open position for the detector, the excitation light (12) is in a closed position and vice versa. See figure 2.

5 Likewise, in the embodiments of Figures 2 and 3, the light source (12) is switched on when the detector (20) is closed by the light chopper wheel (14) and the light source (12) is switched off when the light chopper wheel (14) is in an open position for the chopper wheel (14). The light chopper wheel (14) rotates at a suitable revolutions, per minute (RPM) speed which is set at a value which opens the detector0 aperture (not shown) before the luminescence has decayed e.g. if the phosphor has a lifetime of 2 ms (simple exponential decay usually) the time between fully closing the excitation light aperture or switching off the light source (12) (See Figures 2 and 3) and opening the detector aperture should be about equal to or less than 1 ms in order to maximize the output signal. 5 In other embodiments of the present invention, the light chopper wheel (14) is replaced by a shutter (36) (see Figure 5) which is synchronized (180 degrees phase shift) with the excitation light source (12) e.g. LED, laser etc.

Yet in another embodiment as shown in Figure 6, the gating element is an element which achieves gating by directly (electronically) switching off the detector0 (20) whilst the excitation light source (12) (LED, laser etc) is switched on.

Detector for excitation light power (26)

The apparatus (10) may further comprise a second detector (26) which may be a photodiode. In Figure 1, the apparatus (10) is a personal radiation dosimeter which uses a photodiode as the second detector (26). In the application for5 dosimeters, measurement of a particular excitation light level is required together with appropriate calibration of the apparatus (10). The calibration procedures for the apparatus (10) are described below in further detail.

As indicated above, this signal can also be used in feedback electronics that stabilizes the power of the LED (although if the temperature control of the LED (orCr laser) is well done, this may not be necessary).

Other elements which may be present in the apparatus (10) includes a second gating element which in figure 1 is shown as a filter (24). The filter (24) may be located between the light chopper wheel (14) and the first detector (20). In an example of this invention, the focussed emitted light is passed through a narrow band pass interference filter (24) that is optimised for a sharp emission line of Sm ²⁺ or similar luminophore e.g. 687.9 nm ( ⁷ F ₀ - ⁵ D ₀ transition for Sm ²⁺ in BaFCI, for example) filter with 1 nm band pass for the BaFChSm ³⁺ phosphor.

Table 1 summarizes the response of apparatus (10) as shown in Figure 1 with BaFCkSm ³⁺ dosimeters to radiation of various energies

Table 1 provides data which illustrates that the response of the apparatus (10) of Figure 1 is suitable for personal radiation monitoring across the range of electromagnetic radiation from UVB to gamma rays.

Calibration procedure for the dosimetry reader

In order to ensure that the output of the reader unit described above can be used to determine radiation exposures, the following calibration procedures may be undertaken.

Primary calibration

This procedure needs to be done only once for a particular storage phosphor material. Dosimeters/samples are exposed to known doses of ionising radiation of

different energies and a primary calibration is undertaken by measuring the output (photon counts/unit time) at the various energies with a well-defined excitation light level.

At this stage, the output is measured of a sample/mock dosimeter that contains a stable non-bleaching material that provides photoluminescenee in the red region of the spectrum upon blue excitation. This serves as a traceable calibration standard that can be used for secondary calibration. The requirements for this material is that it must be stable i.e. should not undergo any photochemistry/degradation or even ionising radiation induced variations. That is a material with photoluminescenee that can be measured repeatedly more than once without any variations. There are many systems that fulfil this condition.

For example, lab grown emerald of very low chromium (III) concentration can be used. Another standard could be nanocrystalline or microcrystalline LiGasOg doped with ca 0.1 % Fe ³⁺ . Yet another possibility would be fluorescent dyestuff doped polymer films. It is observed that the inorganic materials are usually more stable though and hence are more reliable.

Secondary standard

The secondary standard of calibration which may be used to calibrate the dosimetry reader (see description above) by setting the LED (or laser) light level to a value that yields an output of the reader unit as has been defined in the primary calibration procedure. For example, in the primary calibration the secondary standard gave a signal of 500000 cps. Also, from the primary calibration it can be calculated that exposure to 0.1 mGy ionizing radiation at e.g. 50 kV results in an output of the reader unit of e.g. 270000 cps. A dosimeter reader described above can be arranged with the secondary standard and an excitation light level set such that a 500000 cps output is obtained. A sample can then be obtained that has been exposed to 50 kV radiation. Since the exposure X is determined by the equation:

X = output of reader x 0.1 mGy/270000, a radiation level and reading can be obtained for the dosimeter.

It is observed that these calibration procedures lead to highly reliable dosimetry and that the dosimeter readers can easily be calibrated against the secondary standard.

The Cs-137 (662 keV) exposures illustrated in Figure 8 indicate that the BaFCI:Sm ³⁺ dosimeters' response is independent on the dose rate and fairly linear up to the maximum exposure of ca 10 mGy. At this energy the dosimeters in conjunction with the apparatus illustrated in Figure 1 display a readout of ca 10 cps/μGy. All data has been corrected for background radiation (control dosimeters).

B) Apparatus for medical or scientific imaging

Figures 16 to 18 illustrate an apparatus that can be used for medical or scientific imaging or measurement of radiation doses in radiation therapy. In this apparatus the detector (20) is replaced by a highly sensitive (e.g. single photon detection capacity, Andor Luca or similar) CCD or CMOS camera (46). The lens system (42) projects an image of the imaging plate onto the CCD camera. The lens system (42) may be a multielement telecentric lens or similar with an aperture (40). Illumination of the imaging plate (44) is conducted by a light source (38) consisting of one or several LEDs or lasers with appropriate wavelengths. The light source (38) is synchronized with the chopper blade (14) or shutter by an electronic controller (32) so that the excitation light is switched off whilst the aperture to the camera (46) is open. The apparatus shown in Figure 16 can be modified by using the various gating elements outlined in the schematic diagrams for the dosimetry readers shown in Figures 1-6. For example, the chopper wheel (14) can be replaced by a camera shutter or the CCD camera can be gated electronically. Increasing the integration time on the camera (46) facilitates the readout of high quality images based on the latent images recorded on photoluminescent radiation storage imaging plates.

Figure 19 shows an example of a X-ray image of an integrated circuit taken by using an imaging plate (dental size 2) based on the photoluminescent radiation storage phosphor BaFCIiSm ³⁺ and using the apparatus illustrated in Figures 16 to 18.

In medical or scientific imaging based on computed radiography, an object, eg. person, teeth in intraoral examination, is exposed to radiation. Behind the object an imaging plate is typically arranged and thus through the radiation exposure a latent image of the object/person is formed on the imaging plate. The imaging plate is then placed in a reader as shown in Figure 16 to 18 and the latent image shown in figure 19 is the readout by the apparatus of Figures 16 to 18.

Figure 19 shows an example of such an image. In this example, an integrated circuit was placed on the imaging plate, comprising the BaFCIiSm ³⁺ phosphor, and was exposed to a standard intraoral examination dose (ca. 5 microSievert, 75 keV radiation). The image clearly indicates the printed circuit and the IC within the housing. The size of the imaging plate was dental size 2 in this case.

Further, it can be seen in this example that the apparatus and reader for imaging of the present invention provides suitable resolution images which can be read multiple times due to the photoexcitable or photoluminescent phosphor in the imaging plate.

The foregoing describes the invention including preferred forms thereof. Alterations and modifications as will be obvious to those of skill in the art are intended to be incorporated in the scope hereof as defined by the accompanying claims.