WO2014161732A1

WO2014161732A1 - Apparatus and method for determining a dose of ionizing radiation

Info

Publication number: WO2014161732A1
Application number: PCT/EP2014/055783
Authority: WO
Inventors: Helmut VINCKE; Julia TRUMMER
Original assignee: Cern - European Organization For Nuclear Research
Priority date: 2013-04-04
Filing date: 2014-03-24
Publication date: 2014-10-09
Also published as: EP2981845A1

Abstract

An apparatus for determining a dose of ionizing radiation absorbed by a dosimeter (105) comprises a holder (110) for holding said dosimeter (105); a first light source (120), a second light source (130) and a measurement unit (140a, 140b). The first light source (120) irradiates a first light (121) on said dosimeter (105) when held by said holder (110). The second light source (130) irradiates a second light (131) on said dosimeter (105) when held by said holder (110) to excite radio photo luminescence centers in said dosimeter (105). The measurement unit (140a, 140b) is configured to measure a photo luminescence light (141) from said excited radio photoluminescence centers and to measure a transmission light (142) related to said first light (121) after passing said dosimeter (105), wherein said measurement unit (140) is further configured to determine said dose based on said measured photoluminescence light (141) and on said measured transmission light (142).

Description

CERN - European Organization for Nuclear Research

Apparatus and Method for Determining a Dose of Ionizing Radiation

Field of the Invention

The present invention relates to an apparatus and a method for determining a dose of ionizing radiation absorbed by a dosimeter and, in particular, to a readout system for radio photoluminescence (RPL) glass dosimeters for high level dosimetry purposes (e.g. for doses larger than 0.1 Gy).

Background and Relevant State of the Art

Luminescence is the emission of light by certain materials caused by exposing the material to a particular radiation. As for RPL the material is exposed to an ionizing radiation and by measuring the luminescence a dose of ionizing radiation can be determined making them thereby suitable for dosimetry. Such dosimeter may, for example, be used in personal dosimetry, environmental monitoring and medical applications and may go up to doses of 100 Gy (Gray) or slightly higher. Possible materials for dosimetry are, for example, compositions of metaphosphate glass having almost the same absorption and emission band maximum. For example, non- irradiated glasses may exhibit an absorption band around 240 nm and a photolumines- cence band at around 380 nm. However, after the exposure to ionizing radiation a new absorption band at around 340 nm may be produced with a radio-photoluminescence emission centered at around 620 nm (e.g. emitted as fluorescent orange light). The large difference between the absorption and the emission band facilitates the analysis of the RPL significantly.

For example, in silver-activated metaphosphate glass ionizing radiation creates several types of RPL centers associated with several silver excitations. The frequency of the centers depends on several parameters such as glass composition, silver content and temperature. Hence, it is likely to have several different RPL centers together in an irradiated glass dosimeter. However, at absorbed doses, for example above 30 Gy, color centers are formed as well and the glass may start to get a yellowish brown color. These centers absorb part of the incoming UV light and also part of the luminescence light leading to dependency of the RPL signal on the absorbed dose as shown in Fig. 6, wherein the dose is measured in Gy. The depicted dose-response-relation in Fig. 6 comprises a maximum for the RPL signal between a monotonically increasing branch at low doses and a monotonically decreasing branch at high doses defining a low dose branch and a high dose branch. The maximum of the RPL signal is in the range of around 1 kGy. The decreasing branch of the RPL signal (above 1000 Gy) is a consequence of the above mentioned increasing absorption of the UV light and the RPL signal.

Due to this signal characteristic, the dose can be derived from the RPL signal uniquely only in the low dose branch or in the high dose branch. In addition, when talcing into consideration the error bars (see Fig. 6) for the RPL signal, also around the maximum of the RPL signal it becomes difficult to derive the correct dose when measuring the RPL signal. Therefore, conventional RPL glass dosimetry allows merely determining the amount of absorbed radiation from the intensity of the emitted light when used either for low doses (e. g. where the dose-response relation is linear) or for high doses. In particu- lar, in the range between 400 Gy and 2000 Gy it is difficult to obtain the absorbed dose from the (orange) RPL intensity because there is almost no dependency between those two magnitudes on the absorbed doses (see Fig. 6). In addition, for identifying the correct doses it is needed to know whether the dose range is below the maximum (e.g. below 400 Gy) or whether the dose range is above the maximum (e.g. above 2000 Gy). Since the dosimeter become increasingly yellowish with the dose, this distinction is made in conventional systems by a visible examination to determine whether the dosimeter is transparent or increasingly yellowish.

One conventional system is disclosed in US 2010/0176308 Al which uses quantum dots as dosimeter and is based on semiconductor nanocrystals as dosimeter medium. Three operation modes are used, one of which uses the photoluminescence radiation, one the transmission and the other uses a scattering of light. Further conventional systems rely on pulsed UV laser light sources to excite the RPL centers to measure luminescence light, and phototransistors placed behind the dosimeter measure the attenuation of the pulsed laser light. In yet another conventional systems the RPL light excitation is trig- gered by a mercury lamp allowing analyzing dosimeters at a dose range between 0.1 Gy and 1 MGy. Also for these systems the applicable dose-to-readout relation has to be obtained by using the visual inspection of the dosimeter as set out before. This procedure is error-prone and causes frequently wrong dose analysis. Therefore, these conventional systems do not allow a reliable discrimination between the low dose branch and the high dose branch. Therefore, there is a need of having an apparatus for determining a dose of ionizing radiation which can determine reliably and automatically the dose absorbed by the dosimeter over a large dose range. Overview of the Present Invention

The aforesaid problems are solved by an apparatus according to claim 1 and a method according to claim 16. Claims 2 to 15 and 17 to 18 refer to specifically advantageous realizations of the subject matter of the independent claims.

Accordingly, the present invention provides an apparatus for determining a dose of ionizing radiation absorbed by a dosimeter. The apparatus comprises a holder for holding the dosimeter, a first light source for irradiating a first light (e.g. visible light) on the dosimeter when held by the holder and a second light source for irradiating a second light (e.g. ultra-violet, UV light) on the dosimeter when held by the holder to excite radio photoluminescence centers in the dosimeter. In addition, the apparatus comprises a measurement unit configured to measure a photoluminescence light from the excited radio photoluminescence centers and to measure a transmission light of the first light after passing the dosimeter, wherein the measurement unit is further configured to de- termine the dose based on the measured photoluminescence light and/or on the measured transmission light. Thus, the apparatus according to the present invention performs two independent measurements, one measurement for the photoluminescence light as, for example, caused by fluorescence, and another measurement for the transmission light (to measure the amount of absorption by the dosimeter). These two results yield a redundancy, which on one hand can be used to improve the accuracy, but more importantly can be used to determine whether the dose to be detected is in the low dose branch (monotonically increasing RPL signal) of the luminescence curve or whether the dose to be detected is in the high dose branch (monotonically decreasing RPL signal) of the luminescence curve. As consequence, no visible inspection is needed and the apparatus can be operated in a fully automated way. In addition, the combined measurement provides also results around the maximum of the RPL signal.

Although in the following the first light will mostly be denoted as visible light and the second light as UV light, it is understood that in further embodiment the visible light can be replaced by any light suitable for the transmission measurement (i.e. interact with the color centers) and the UV light can be any light suitable to excite the RPL centers.

In further embodiments the apparatus comprises optionally as first light source a blue light-emitting diode (LED) and as second light source an UV LED. In addition, the measurement unit may comprise a first photodiode for measuring the photoluminescence light and/or a second photodiode for measuring the transmission light. The use of light- emitting diodes for the first and second light source provides the effect that the apparatus is robust and can be operated cost-efficiently.

Optionally, the second light source is configured to irradiate continuously the UV light over a predetermined time period and wherein the measurement unit is configured to measure an accumulated photoluminescence light during the predetermined time period. In addition, the first light source may be configured to irradiate continuously the visible light during the predetermined time period or during another predetermined time period. By using a continuous UV light source it becomes possible to increase the sensitivity in that an integrated signal over the predetermined time can be generated. The predetermined time is, e.g., between 50ms to 1.5s or between 0.1 s to 1 s or about 0.3s. The dosimeter may comprise an elongated shape elongated in a longitudinal direction of the dosimeter and, optionally, the first light source may be configured to irradiate the visible light along the longitudinal direction. The second light source may be configured to irradiate the UV light perpendicular to the longitudinal direction of the dosimeter. The apparatus may further comprise an optional third light source for irradiating the dosime- ter with a third light (e.g. a further UV light) which may or may not be the same second light as irradiated by the UV light source, wherein the third light source may be configured to irradiate the dosimeter the further UV light along the longitudinal direction of the dosimeter. By irradiating the dosimeter with the UV-light from the smaller direction (perpendicular to the longitudinal or elongated direction) it may also be possible to measure the dose over a large range, because even for larger doses where the dosimeter becomes less transparent to UV light a large surface of the dosimeter is exposed to the UV source resulting in a stronger RPL excitation (in contrast to irradiation along the dosimeter axis). On the other hand, the transmission signal when using the elongated direction becomes very sensitive already for minor attenuations so that by irradiating the transmission signal along the elongated direction a high sensitivity can be obtained.

The apparatus may comprise an optional optical unit with a first input for receiving the visible light from the first light source and a second input for receiving the further UV light from the third light source, and an output. The optical unit is configured to provide a first optical path between the first input and the output, and to provide a second optical path between the second input and the output. Both paths may be open at the same time or one of them may be closed by a switch. As one example, the optical unit may be realized as a Y-shaped optical fibre. The photoluminescence is a function dependent on the received dose of the ionizing radiation, wherein this function may comprise one or more maximums for the photoluminescence values and thus an accurate value of the dose cannot be determined by only measuring the RPL signal. The additional transmission measurement with the first light source is used to obtain a unique dose for a measured RPL signal. According to yet another embodiment, the first light source is optimized with respect to the used dosimeter and/or the second light source so that the apparatus can easily distinguish whether the dose to be detected is within the range of an increasing RPL signal with doses or whether the RPL signal decreases with an increasing dose. This problem can be solved by selecting or configuring the first light source to irradiate the visible light such that the doses of the one or more maximal photoluminescence values are within a range where the dosimeter comprises a transmittance for the visible light within a range of, for example, 3% to 95% or of 10% to 90% of a maximal transmittance. Yet another problem is to enable a flexible automatic measurement applicable for different types of dosimeters which relate to different RPL and transmission signals-to-dose relations so that for the determination of the dose of a particular dosimeter a particular calibration curve has to be available. The calibration curve defines for a given dosimeter the dependence of the RPL and the transmission signals on the doses and is thus used to determine the doses. Therefore, the apparatus may further comprise an optional storage unit which is configured to store measurement results of the measurement unit and to store at least one calibration curve. Hence, the measurement unit can store measured values for both photoluminescence light and the transmission light in the storage unit and load the at least one calibration curve from the storage unit.

In further embodiments the apparatus may optionally be configured to switch between a calibration mode and a measurement mode, wherein in the calibration mode a calibration curve is updated, e.g. by using reference dosimeters (which are dosimeters calibrated with a higher accuracy). Therefore, the dosimeter can easily be calibrated to obtain the correct dose from the measurements. Optionally, the apparatus may further comprise a housing with cavity, the holder being arranged in that cavity, wherein the cavity comprises walls with a coating to reduce reflection at the walls. This optional housing may further improve the accuracy, because reflected signals at the side walls of the cavity may deteriorate the meas- ured signal and the coating or a painting can suppress such reflections. In addition, the exemplary fluorescence and/or luminescence light may pass first through an optional filter to cut off any light below and/or above the light measured by the measurement unit (e.g. the orange light measured by a silicon photodiode). The measurement unit may comprise an optional preamplifier unit (e.g. a current to volt- age transformer with a sensitivity of IV / ΙμΑ) to amplify the measured signals, or it measures the currents from the exemplary photodiodes directly.

In yet further embodiments the holder may be configured to hold multiple dosimeters and the apparatus may further comprise a drive unit to move the holder such that each of multiple dosimeters is subsequently positioned to receive the UV light and visible light accordingly. This multi-dosimeter holder may further be configured to hold the reference dosimeter with a stable predetermined RPL light emission after excitation. The measurement unit may further be configured to perform a reference measurement with the reference dosimeter. Since the RPL light emission of the reference dosimeter is known, the reference measurement may be used to calibrate the apparatus. The same reference dosimeter may also be used to calibrate the transmission branch of the apparatus via sending the visible light through the dosimeter. Such a reference dosimeter, being stable in its transmission and RPL parameters, allows for calibration of all measurement aspects of the apparatus. The holder may, for example, hold up to 100 or up to 50 dosime- ters. The drive unit allows an automatic processing of the dosimeters so that in a short time many dosimeters can be measured automatically with a high accuracy.

Further embodiments relate also to a method for determining a dose of ionizing radiation absorbed by a dosimeter. The method comprises the following steps: irradiating a visible light of the first light source on the dosimeter, which is held by a holder; irradiating a second light (e.g. a UV light) of a second light source on the dosimeter, which is held by the holder to excite photoluminescence centers in the dosimeter; measuring a photoluminescence light from the exited radio photoluminescence centers and/or measuring a transmission light of the first light after passing the dosimeter; and determining the dose based on the measured photoluminescence light and/or based on the measured transmis- sion light. The method may, optionally, further comprise the step of irradiating a further or the same (ultra-violet) light on the dosimeter to excite radio-photoluminescence centers from a different direction in the dosimeter. Although the RPL centers are the same for both irradiations, not all RPL centers may be accessible with each irradiation. Optionally, the step of irradiating UV light is carried out perpendicular to the longitudinal direction of the dosimeter when the dose to be detected is above a threshold value (e. g. above 3 kGy or above 10 kGy), and wherein the step of irradiating UV light is carried out along the longitudinal direction, when the dose to be detected is below a further threshold (e. g. 200 Gy or 300 Gy or 400 Gy). An advantage of irradiating the exemplary UV light along the longitudinal axis is that the dosimeter provides a smaller surface (it is only one end surface) for potential reflections resulting in lower background and thus clearer signal. In other words, the advantage of the longitudinal irradiation lies in the reduction of light reflexion on the surface of the dosimeter, and the advantage of the lateral exposure lies in the fact that at high doses the dosimeter becomes opaque to UV light and only a minor part of the RPL centres is excited when being irradiated longitudinally.

When the dose to be detected is between both threshold values, the step of determining the dose can be based only on the transmission light signal. The decision whether the dose is above the threshold (e.g. dosimeter is opaque) and/or whether the dose is below the further threshold (e.g. dosimeter is transparent), wherein the further threshold is smaller than the threshold, can be made based on the transmission measurement. In an alternative realization, the threshold and the further threshold can be transformed (e.g. using the calibration curve) to respective thresholds for readout signals. Again, the UV light may be any light suitable to excite RPL centers and the visible light may be any light suitable to interact with color centers. Detailed Description of Preferred Embodiments

The features and numerous advantages of the apparatus for determining a dose of ioniz- ing radiation according to the present invention will be best appreciated from the detailed description of the accompanying drawings, in which:

Fig. 1 depicts an apparatus for determining the dose according to a first embodiment;

Fig. 2 depicts the apparatus for determining a dose including further optional components;

Fig. 3 depicts a side view of a holder for multiple dosimeters and/or a reference dosimeter;

Figs. 4A,B depict schematically further embodiments of the present invention;

Figs. 5A,B depict curves for typical signals for the luminescence and transmittance as function of the dose; and

Fig. 6 depicts a curve for the luminescence as function of the dose.

Fig. 1 shows an apparatus for determining a dose of ionizing radiation absorbed by a dosimeter 105 (or more dosimeters), wherein the apparatus comprises a holder 110, a first light source 120, a second light source 130 and a measurement unit 140 with two components 140a, 140b. The holder 1 10 is configured to hold the dosimeter 105. The first light source 120 is configured to irradiate a first light 121 (e.g. visible light such as blue light) on the dosimeter 105 which is held by the holder 1 10. The second light source 130 is configured to irradiate a second light as, for example, an ultraviolet (UV) light 131 on the dosimeter 105 in the holder 1 10 to thereby excite radio photo lumines- cence centers in the dosimeter 105. The measurement unit 140 is configured to determine the dose based on the measured photo luminescence light 141 and, in addition, based on the transmission light of the first light 142. Therefore, the apparatus performs a combined measurement with different lights, a first light and a second light having a shorter wavelength than the first light, wherein both measurements can, for example, be performed in parallel at the same time or one after the other (e.g. using subsequent cycles). One measurement cycle may, for example, take Is.

The term light is used to cover all electromagnetic radiation in the range from about 100 nm to about 1000 nm and induces, in particular, UV-light. The light is selected such that it interacts with the luminescence centers and color centers created by the ionizing radiation in the exemplary glass dosimeter so that the luminescence centers, when excited by the second light (e.g. UV) source, emit e.g. orange light and the amount of orange light is correlated to the dose received by the dosimeter. The first light source is selected such that it shows a strong absorption change in the range of several hundreds to several thousands Gy. The color centers (responsible for the light absorption of the first light) are responsible for a darkening of the glass: transparent (low dose)-yellow-brown-black (high dose) and from this darkening one can infer the range of the dose to which the dosimeter was exposed (in conventional systems by visual inspections).

In the following description it will be assumed that the first light is a visible light and the second light is a UV light, although in further embodiments the UV light may be any light suitable to excite RPL centers and the visible light may be any light suitable to provide the aforementioned absorption criteria.

The present invention combines two independent measurements using two different light sources to measure on the one hand the transmitted light and, on the other hand, the luminescence light being excited by the UV light source 130. Therefore, the system provides two functionalities as given by the two subsystems, a measurement of RPL light emission 141 (e.g. orange light), which is caused by the exposure of the dosimeter 105 to the UV light 131 suitable to excite the RPL centers. In addition, a further meas- urement measures the transmission light 142 (e.g. blue light) which is sent through the dosimeter 105. As a result, doses can be measured throughout the range of 0.1 Gy to several MGy. Therefore, the present invention provides the advantage that the missing range can be covered reliably and a visual inspection of the color of the dosimeter is not needed anymore. Therefore, the present invention allows an automated system for reading out dosimeters and the measurement is independent of any human judgment and therefore is less error-prone.

Moreover, small RPL dosimeters, which may comprise a cylindrical shape and were exposed to ionizing radiation, can be analyzed with this system with respect to a dose received during the radiation process. The dosimeter may comprise an elongated shape with a longitudinal direction in the elongated direction (i.e. along the cylindrical axis, which is horizontal in Fig. 1). According to further embodiments the first light source 120 and the second light source 130 comprise light-emitting diodes, thereby providing a reliable light source at low cost. The use of LED makes the system, in particular, significantly cheaper than laser-based systems. Fig. 2 depicts a further embodiment with further optional components added to the embodiment of Fig. 1. In addition to the holder for the dosimeter 1 10, the measurement unit 140, the first and the second light sources 120 and 130a, the embodiment of Fig. 2 comprises a third light source 130b for irradiating a third light 132 (e.g. a further UV light) and, in addition, a storage unit 160. The storage unit 160 is configured to exchange data with the measurement unit 140a, b. For example, the storage unit 160 can store the measured results from the measurement unit 140a, b and, in addition, may store at least one calibration curve, which can be loaded by the measurement unit 140a, b to improve the measurement results. The third light source 130b may, for example, be provided at the same side as the first light source 120, wherein the first light source 120 and the third light source 130b are configured to couple to an optional optical unit 150, which is configured to receive the visible light from the first light source 120 and the further UV light 132 from the third light source 130b and directs the visible light 121 and the further UV light 132 onto the dosimeter 105. The further UV light 132 may or may not comprise the same wavelength as the UV light 131 from the second light source 130a. For example, the optical unit 150 may comprise a "Y"-shaped fiber to direct both lights in parallel onto the dosimeter 105.

In further embodiments the apparatus may comprise an optional display to show the measured results. Moreover, the values of the at least one calibration curves used for the analysis may, for example, be entered by hand using an optional input device or be loaded using a connection to a network.

The calibration curve is needed in order to obtain a dose value of an unknown dosimeter covering the whole dose range of interest (e.g. : 0.1 Gy - 1 MGy or higher). A calibration curve consists of the analysis of a certain number of dosimeters (e.g. 30 dosimeters), each of which irradiated to a well-defined dose value. When being exposed to the given light sources each of the various dosimeters provide to the well-known dose value a transmission signal and a RPL signal, resulting in two calibration curves: one for the dose-RPL light correlation and a second for the dose-transmission light calibration. When measuring a dosimeter with an unknown dose the apparatus is doing the following: it measures the transmission light and the RPL light. These two measurement results lead via interpolation of the existing calibration curves ("dose-voltage caused by RPL light" relation and "dose-voltage caused by transmission light" relation) to a dose rate of the dosimeter of interest.

When using the further UV light 132 along the elongated direction (longitudinal direction of the dosimeter) the measurement of the luminescence light may, for example, rely on light, which is emitted perpendicular to the direction of the irradiation of the third light source 130b. In further embodiments, the first, second and third light sources 120, 130 are operated in parallel so that the transmission of the visible light and the UV light along the elongated direction can be measured at the same time or one after the another, wherein the whole measurement process may take 1 ... 1.5s. If, for example, the dose to be detected is very low (increasing branch in Fig. 6), the third light source and the visible light source may be operated at the same time or one after another and/or from the same direction. The third light provides the following advantage. In contrast to first UV light 131 the photodiode will see less scattering from the dosimeter surface. Hence, it has a lower background signal. The third light 132 traverses the dosimeter and excites on its way the centres within the dosimeter. The second light 131 excites about the same amount of centres in the dosimeter but it will provide also a background due to scattering on the surface of the dosimeter 105. Hence, when using the second light 131 the background is higher than when using the third light 132. This advantage is important for low dose dosimeters where the signal background might be higher than the actual signal and a reduction of the background is needed to go to lower dose values. On the other hand, for high doses the irradiation with the second light 131 is better since the third light 132 is stopped or absorbed completely after a short distance in the dosimeter 105. Therefore, when irradiating with the second light 131 more RPL centers can be reached than with the third light 132 since we irradiate a larger surface of the dosimeter 105. The decision of whether the second or third light is used can be made based on the transmission measurement and, in particular, can be made automatically by the system without any user interaction.

The embodiment as shown in Fig. 2 further comprises a chamber or housing 170 with a cavity 175 wherein the holder 1 10 for the dosimeter 105 is arranged. The housing 170 may comprise windows for enabling the UV light 131, 132 and the visible light 121 to enter the cavity 175 and to allow the luminescence light 141 and the transmitted part of the visible light 142 to leave the cavity 175 and be transmitted to the measurement unit 140. In the optical paths between the dosimeter 105 and the measurement unit optional filters may be provided, in particular for the RPL signal, to filter a selectable spectral region of interest in the radiation received from the dosimeter (i.e. the luminescence light or transmission light) and to suppress the remaining spectral parts of the light received from the dosimeter or the surroundings. In further embodiments, the measure- ment unit 140 may comprise one or two components and the transmission light 142 and the luminescence light 141 may be directed to an optical device which transmits these light signals to the measurement unit 140. If the measurement unit comprises a first part 140a and a second part 140b, the first part 140a may be configured to receive the lumi- nescence light 141 from a direction perpendicular to the longitudinal direction, and the second part 140b may be configured to receive the transmission light 142 in the longitudinal direction. In further embodiments also the first part of the measurement unit 140a may be configured to measure the transmittance of the dosimeter (i.e. in a perpendicular direction to the longitudinal direction) and/or the second part of the measurement unit 140b may be configured to measure the luminescence in the longitudinal direction.

The optional housing 170 comprises walls defining the cavity 175 and the walls may be coated or painted by an absorbing coating 161 which prevents or suppresses reflection of light at the walls of the cavity 175. In addition, the holder 110 or the whole system may comprise a further coating 111 which is configured to prevent further reflections of light on the holder 1 10. The painting 161 of the interior of the RPL reader or the further coating 11 1 of the holder 1 10 may decrease the signal background caused by reflection of the light generated by the first, second and third light sources 120, 130. The paint 161 , 1 11 , for example, will provide a maximum light absorption of the surfaces.

Fig. 3 depicts an embodiment wherein the holder 110 comprises multiple holding portions for multiple dosimeters 105a, 105b, 105c (the embodiment of Fig. 3 depicts only three holders, wherein in further embodiments also more than three holders can be provided in the holder 1 10). In addition, the holder 1 10 may comprise a holding portion for a reference dosimeter 106 which can be held by the holder 110 in addition to the dosimeters 105 which shall be read out. The reference dosimeter 106 may comprise a reference crystal/material with constant, well-known RPL and transmission characteristics. Such reference dosimeters 106 may have never seen a dose. For example, they can be solids (e.g. crystals) having a natural tendency to provide RPL light when being exposed to UV light. They only need is to create the RPL light and to allow also for stable transmission of the first light. Then the reference dosimeter 106 can be used to calibrate both branches (for 3 light sources, all three branches) of the machine. The RPL light creation and the transmission must not change with time when being irradiated with the same light intensities. To achieve reliable and repeatable measurement results, the holder should only move the dosimeters in a same measurement position (for all dosimeters held by the holder) such that entrance hole and the exit hole, for example, of the visible light 121 and third light 132 are always the same. In addition, the embodiment as shown in Fig. 3 may comprise a drive unit 180 which is configured to move the holder 110, for example, in a direction perpendicular to the drawing plane of Figs. 1 and 2 or perpendicular to the longitudinal direction of the dosimeter to thereby expose the different dosimeters 105a, 105b or the reference dosimeter 106 to the UV lights 121 and 132 or the visible light 121. Therefore, in an automatic procedure, multiple dosimeters can be read out and, moreover, a reference measurement can be implemented by reading out the reference dosimeter. The holder 110 may hold an arbitrary number (for example, several dozens) of dosimeters.

A number of external factors influence the results of high-dose dosimetry (i.e. will alter the RPL signal-to-dose relation). Along these are the following: time-dependent instabilities (fading), dependence on radiation type and energy, geometrical factors and dose rate effects, environmental effects, temperature, humidity and light. Therefore, to ensure a high quality of the results a calibration process is advisable from time to time. For this purpose a reference material (e.g. the reference dosimeter 106) in the dosimeter holder 1 10 can be used to allow determining a change in the readout functionality and to allow discovering short or long-term changes of the system in terms of RPL light detection and transmission measurements. Thus, the reference readouts will be used to correct measurement results if needed according to the change of the readout system. This provides an additional quality control and will improve the reliability of the reader. The reference material shall provide long-term stability in the radio photoluminescence production when being exposed to UV light and shall provide stable light transmission properties over time.

Therefore, the apparatus may optionally be configured to switch to a calibration mode (from the measurement mode) to update a calibration curve for a particular dosimeter type by using a reference dosimeters. After calibration long term modifications are taken into account and correct doses can be determined from the measurements as set out above.

A further way to improve or to amplify the RPL signal is to modify the time analysis. In the conventional systems the time analysis of the RPL signal, was performed in the range between 0 and some tens of microseconds. According to further embodiments of the present invention an integral readout is implemented which is fully sufficient for high level dosimetry reasons. For example, the RPL glass dosimeter is exposed to ultraviolet light for less than 700 ms (or between 100 and 700 ms) during which the integral RPL response of the dosimeter is measured several times, leading to an average value of the measured light intensity (alternatively this predetermined time can be adapted to the requirement (e.g. desired sensitivity of the measurement). Therefore, the luminescence measurement and/or the transmission measurement may be adapted to measure an accumulated signal over a predetermined time period. By increasing the time period ac- cordingly, the sensitivity even for small signals can be further increased. However, there is an upper limit for this time period. For example, if the dosimeter is irradiated for more than a second the dosimeter may start losing (temporarily of even on a permanent base) RPL centres. Therefore, in further embodiment the predetermined time period is selected to compromise between the sensitivity and the loss of RPL centres.

Due to the extended time period (compared to micro seconds) embodiments of the present invention do not need to have a photomultiplier or phototransistor, because by selecting the predetermined time period in accordance to the expected amount of light it is possible to increase the sensitivity sufficiently without the need of an extensive amplifi- cation of the measured signals. However, as optional feature photomultipliers, photore- sistors or phototransistors can be employed, e.g., if the measurement shall be carried out only during a short predetermined time (for example, in order to limit the time needed to measure one dosimeter or to better observe the RPL time structure of the various RPL processes). In this case, however, the signal processing is replaced in order to take into consideration the additional amplification of the signal. Moreover, because of this inte- gral measurement, the laser light source used in conventional systems could be replaced by LED light sources.

Fig. 4A depicts a schematic view for an embodiment wherein the first and second light source 120, 130 use LED lights. For example, the first light source 120 comprises an LED irradiating light of, for example, 440 nm (or light with a wavelength between 400 and 500 nm). In addition, the second light source 130 may comprise an LED irradiating UV-light of, for example, 365 nm (or light with a wavelength between 300 nm to 400 nm). In the embodiment as depicted in Fig. 4A, the measurement unit 140 comprises a first part 140a with a first photodiode (e.g. a silicon photodiode) for the luminescence measurement and comprises a second part 140b with a second photodiode (e.g. a silicon carbon photodiode) for the transmission measurement.

By measuring the transmission through the exemplary RPL glass dosimeter it becomes possible to determine the dose range: low dose range (below a first dose threshold Λι), high dose range (above a second dose threshold Λ₂) or the region around the maximum of the RPL signal (between the first dose threshold Λ[ and the second dose threshold Λ₂; for A_\ <Λ₂) and based thereon the measurement unit may select or switch among three calibration regions depending on the amount of dose absorbed. Contrary to conventional systems this switching can be performed automatically. Alternatively, it is also possible to define the thresholds for the RPL signal or the transmission signal, in which case the low dose threshold translates into a high voltage (signal) threshold and vice versa.

The UV light source 130 is positioned perpendicular to the glass cylinder axis of the radio photo luminescence device (RPLD, which includes the holder 1 10 and the dosime- ter 105) instead of parallel, because the dosimeters 105 which have seen high doses become very opaque for UV light. Hence, when exposing the dosimeter 105 through its front face with ultraviolet light 131 only a very small part of the dosimeter sees the ultraviolet light 131 , resulting in a low RPL center excitation. This means that the light output is small for dosimeters 105 which have seen high dose values. Irradiating the dosimeter 105 with ultraviolet light 131 from the side has the advantage that a larger surface of the dosimeter is available for UV exposure and hence for emission of luminescence light (the UV light does not need to travel a long distance through the dosimeter resulting in its absorption). As a result, a larger RPL light output is possible for dosimeters which have absorbed high dose levels. In other words, when the dose is too high the dosimeter is opaque and the UV light cannot pass the whole dosimeter.

Fig. 4B depicts a further embodiment (similar to the embodiment as shown in Fig. 2) wherein a third UV light source 130b is arranged together with the visible light source 120 on the same side with respect to the dosimeter 105. The third light source 130b comprises in this embodiment the same LED light source as the second light source 130a (also irradiating an UV light of 365 nm, which may also be in the range between 200 and 500 nm). Moreover, the optical unit 150 as depicted in Fig. 2 is given in this embodiment by a Y-shaped fiber combining the further UV light 132 of the third UV light source 130b and the light 121 of the visible light source 120 to transmit both lights to the dosimeter 105.

Therefore, according to this embodiment the RPL dosimeter 105 can be irradiated with UV light 131, 132 either from the front side or from the lateral position by one of the two UV LED light sources 130a, 130b. UV irradiation through the dosimeter (front side) will result in a higher sensitivity for dosimeters being exposed to low dose, whereas the lateral irradiation increases the readout signal when analyzing dosimeters which have been exposed to dose values, for example, in the range of 10 kGy and above. The use of the exemplary blue light source 120 remains unchanged and the decision, which of the two UV light sources 130a, 130b is used for the RPL analysis depends on the analysis results concerning the optical density in the transmission measurement (blue light).

Fig. 5 A depicts exemplary values of both measurement results, wherein values 510 relate to transmission measurement and values 520 relate to luminescence measurement (RPL readouts). In addition, in this embodiment the first threshold Λ[ defining an upper limit for the low dose range (increasing branch of RPL signal) is exemplary between 400 and 500 Gy and the second threshold Λ₂ defining the lower limit of the high dose range (decreasing branch of the RPL signal) is exemplary between 3000 and 4000 Gy. The mid dose range between the first and the second threshold Λ_ΐ5 Λ₂ is related to the range around the maxima of the RPL signal in which case the RPL signal may not be used for determining the dose (instead the transmission measurement produces in this region better results).

Fig. 5B depicts equivalent measurement results (wherein a photoresistor instead of a photodiode has been used), wherein a first curve 510 shows a result of the transmission measurement and a second curve 520 shows the a result of the luminescence measurement (the dose-response-relation). Therefore, the measured values are connected to curves and, in addition, an error bar is added for each measurement value.

In general, the transmission curve 510 comprises one region which is usable for dose measurements (e.g. between the first threshold A_\ and the second threshold Λ₂) and two regions of which are only used to differentiate between the dosimeter being transparent or opaque (dose <Λι and dose >Λ₂). The two thresholds may be defined by a constant or almost constant functional dependence on the dose . The region which is usable for quantitative transmission measurements connects or interpolates between the two regions which are only used to define the transparency of the dosimeter in a qualitative manner. The transmission curve in the usable range is defined by a strong gradient (in Fig. 5B, e.g., between 500 Gy ~ Ai and 10.000 Gy ~ Λ₂), thereby providing a high sensitivity for dose changes.

On the other hand, for the luminescence measurement, there are two regions usable for obtaining quantitative results of dose values and a region which is not usable in that respect. The latter region is given by the region around the maximum or the multiple maxima of the function of the luminescence signal as function of the dose (between 400 Gy and 4000 Gy in Figs. 5A, B). The regions usable for quantitative measurements are given by the increasing/decreasing branches of this function.

Conventional available systems measured either only in the low dose range (up to an 5 upper dose of 500 Gy where the response is still linear) or in the high does range between 2000 Gy and several MGy. In particular, the critical dose range between 400 Gy and 2 kGy could not be resolved reliably.

However, when using the combined measurements according to the present invention l o also the critical region in the luminescence measurement (the missing range between the first and second threshold, e.g. between 400 and 2000 Gy) can be measured reliably. For example, the transmission measurement can determine whether the dosimeter can be regarded as transparent (e.g. when transmission signal in Fig. 5B is above 3000 mV associated with the first threshold Λι) or as opaque (e.g. when transmission signal in Fig. 15 5B is below 4 mV associated with the second threshold Λ₂). If both conditions do not hold, the dose to be detected is in the mid dose range wherein the RPL signal is not used for the determination of the dose, but instead the dose may be determined solely from the transmission signal (which is very sensitive in this region). Therefore, both measurements are combined and allow a precise dose calculation over the whole dose range.

20

Embodiments of the present invention relate also to optimization of the first light source 120 with respect to the used dosimeter and/or the second light source 130 so that the apparatus can easily distinguish whether the dose to be detected is within the range of increasing RPL signal or whether the RPL signal decreases. This optimization can be 25 achieved as follows.

Since the photoluminescence light caused by the irradiation with UV light is not a mono- tonically increasing or monotonically decreasing function, it is not possible to derive the dose unambiguously when measuring merely the photoluminescence light. To distin- 30 guish between these the two branches, the present invention uses a second measurement which measures the transmission light. Therefore, the light to be used for the transmis- sion measurement should be such that the transmission signal varies strongly with the dose in the range where RPL signal does not vary strongly (i.e. around the maximum). In this case, it can clearly differentiate between the two branches in the RPL signal-to-dose relation. This can, for example, be achieved in that the transmission signal falls rapidly within a range, where the photoluminescence signal does not vary significantly (for example, around the maximum value). Embodiments solve this problem by selecting or configuring the first light source to irradiate the visible light such that the doses of the one or more maximal photoluminescence values are within a range where the dosimeter comprises a strong change of transmittance for the visible light (e.g. changing from 95% to 5%) clearly distinguishable range, wherein the transmittance may be measured as a ratio of in-falling light intensity to transmitted light intensity.

In embodiments one may, for example, use 600 Gy for the start of the transmission measurement and 10 kGy as end of the transmission curve. Although the RPL signal is already usable above 2-3 kGy, the transmission signal shows a stronger dependency on the dose up to 10 kGy.

The results from luminescence and transmission measurements can thus be combined according to the present invention to provide also correct measurements within the re- gion between 400 to 2000 Gy, where the transmission curve 510 exhibits a strictly falling shape so that this curve is very sensitive in this region.

Embodiments of the present invention relate also to a method of determining the dose of ionizing radiation absorbed by the dosimeter comprising the steps of using the apparatus or system as described above. Following this method the measurements can be carried out as follows.

The system may start with the transmission measurement. Based on the transmission result there are three choices (chosen by the machine automatically): 1. In case a lot of light crosses the dosimeter (in Figs. 5 high transmission voltage on the left hand side) indicating the low dose branch, the UV light 132 from the third light source 130b is sent through the dosimeter 105 to obtain a dose reading.

2. In case almost no light crosses the dosimeter (in Fig. 5 low transmission voltage on the right hand side) indicating the high dose branch, the UV light 131 from the second light source 130a is sent from the lateral position on the dosimeter 105 to obtain a dose reading.

3. In case the light transmission signal 142 lies between 10 V (Fig. 5 A) and 0.1 V the transmission curve is used to provide a dose value, because the system is in the plateau of the UV curve so that irradiation with UV does not give sensitive results (around the maximum of curve 520 in Fig. 5).

The advantage of the lateral irradiation is the following: a higher readable range towards high dose can be expected. At very high dose values the UV light is already absorbed at the very beginning of the dosimeter. Hence, only a tiny fraction of the dosimeter is used for RPL creation. The higher the dose the smaller the region for RPL creation. When irradiating from the side we use a larger surface resulting at the end in a higher number of RPL centres excited.

This procedure can be followed with all dosimeters, only the levels from the readout (in voltage) are different. Moreover, in each measurement blue light irradiation and UV light irradiation are used. However, at the mid dose range much better information from the blue light irradiation is obtained than from the UV light (plateau of RPL response). Hence, blue light is used for the dose evaluation in this dose range around 1000 Gy, where no RPL light to dose correlation is possible.

In summary, the major aspects of the present invention can also be compiled as follows. Embodiments comprise two subsystems, wherein the first subsystem excites luminescence centers in the RPL dosimeter by an UV LED (for example of 365 nm) positioned perpendicular to the cylinder axis of the dosimeter (see Fig. 4A). The orange fluorescence light resulting from this excitation is read out, for example, by a silicon photodi- ode. This method allows reading out dose values between 0.1 and 600 Gy and 10000 Gy to several MGy. The second subsystem uses a second LED 130 irradiating light of, for example, 440 nm through the RPL dosimeter 105 along its cylinder axis to measure its transmission through the irradiated dosimeter. For example, a silicon carbon diode is used at the detector measuring the intensity of light passing through the dosimeter 105. The transmission coefficient of the glass dosimeter decreases with increasing dose received by the dosimeter prior to the analysis process. This method allows to cover the missing dose range between 600 Gy and 10000 Gy and it also provides information about the readout-to-dose conversion applicable (0.1 Gy to 600 Gy or 10 kGy to several MGy) in case the RPL method is applied.

For example, three dose ranges can be distinguished: a low dose range (up to several hundreds of Gy) wherein UV light through the front face is used; a mid dose range between several hundred of Gy and about 10 kGy wherein a blue light is used, and a high dose range of above 10 kGy wherein an UV light located laterally to the dosimeter is used. The dose limits used for this classification depends on the type of dosimeters used and therefore these values shall serve only as approximate values.

In further embodiments the system for automatic dose analysis comprises a display to display the dose value on the screen. The values of the calibration curves used for the analysis can, for example, be entered by hand. Alternatively, it is also possible to have an implementation, of an automatic saving possibility of the measured data. In addition to the final dose value, also the name of the dosimeter and additional information being either entered by the user or being measured by the system can be written to a file containing the saved measured data. Moreover, it may also be possible to implement the system such that a functionality allowing loading calibration values is possible, which can be used by the user for the dosimeter readout analysis. Finally, the system may also allow optionally that the apparatus is operated in a calibration mode or in a measurement mode, wherein the calibration mode is used for the production of a calibration curve (i.e. a dose-to-readout correlation) or is used in the analysis of the dosimeters in the measurement mode.

The RPL dosimeter glass cylinders exist, for example, in two sizes: (i) with a length of 6 mm and a diameter of 1 mm, and (ii) with a length of 8.5 mm and a diameter of 1.5 mm.

The described apparatus may be used for dose monitoring at nuclear power plants, high energy accelerators, spallation sources and irradiation facilities. A typical application can be found in the machine protection application of such facilities.

The embodiments described above and the accompanying drawing merely serve to illustrate the subject matter of the present invention and the beneficial effects associated therewith, and should not be understood to imply any limitation. The features of the invention, which are disclosed in the description, claims and drawings, may be relevant to the realization of the invention, both individually and in any combination.

Reference signs

105 dosimeter

1 10 holder

1 11 further coating

120 first light source

121 first light

130 second light source

141 second light

140a, 140b measurement unit

141 photoluminescence light

142 transmission light

150 optical unit

170 housing

171 coating

175 cavity

160 storage unit

180 drive unit

Λι first threshold

Λ₂ second threshold

Claims

CERN - European Organization for Nuc search

C31 851 WO

Claims

Apparatus for determining a dose of ionizing radiation absorbed by a dosimeter (105), the apparatus comprising: a holder (110) for holding said dosimeter (105); a first light source (120) for irradiating a first light (121) on said dosimeter (105) when held by said holder (110); a second light source (130) for irradiating a second light (131) on said dosimeter (105) when held by said holder (1 10) wherein said second light (131) is adapted to excite radio photoluminescence centers in said dosimeter (105); and a measurement unit (140a, 140b) configured to measure a photoluminescence light (141) from said excited radio photoluminescence centers and to measure a transmission light (142) related to said first light (121) after passing said dosimeter (105), wherein said measurement unit (140) is further configured to determine said dose based on said measured photoluminescence light (141) and on said measured transmission light (142).

Apparatus according to claim 1, wherein said first light source (120) comprises a blue light-emitting diode (LED) to emit a blue light as first light (121) and/or said second light source (130) comprises a UV LED to emit a UV light as second light (131). 3. Apparatus according to one of the preceding claims, wherein said measurement unit (140a, 140b) comprises a first photodiode for measuring said photolumines- cence light (141) and a second photodiode for measuring said transmission light (142).

Apparatus according to one of the preceding claims, wherein said second light source (130) is configured to irradiate continuously a UV light as second light (131) over a predetermined time period and wherein said measurement unit (140) is configured to measure an accumulated photoluminescence light during said predetermined time period, and wherein said first light source (120) is configured to irradiate continuously a visible light (121) during said predetermined time period and/or wherein said measurement unit (140) is configured to measure an accumulated transmission light coming from said first light source (120) after being attenuated when passing through the said dosimeter (105) during said predetermined time period.

Apparatus according to one of the preceding claims, wherein said dosimeter (105) is elongated in a longitudinal direction, and wherein said first light source (120) is configured to irradiate said first light (121) along said longitudinal direction and said second light source (130) is configured to irradiate said second light (121) perpendicular to said longitudinal direction when said dosimeter (105) is held by said holder (1 10).

Apparatus according to claim 5, further comprising third light source (130b) for irradiating a third light (132), said third light source (130b) being configured to irradiate said third light (132) along said longitudinal direction when said dosimeter (105) is held by said holder (1 10).

Apparatus according to claim 6, further comprising an optical unit (150) with an output and a first input for receiving said first light (121) from said first light source (120), and a second input for receiving said third light (132) from said third light source (130b), said optical unit (150) is configured to provide a first optical path between said first input and said output and to provide a second opti- cal path between said second input and said output.

Apparatus according to one of the preceding claims, wherein said measured pho- toluminescence light is a function dependent on said dose of said ionizing radiation, said function comprises one or more maximal photoluminescence values, wherein said first light source (120) being configured to irradiate said first light (121) such that said one or more maximal photoluminescence values relate to one or more doses where said dosimeter (105) comprises a transmittance for said first light (131) within a predetermined percentage range of a maximal transmittance of said dosimeter (105) for said first light (121).

Apparatus according to one of the preceding claims, further comprising a storage unit (160) which is configured to store measurement results of said measurement unit (140) and at least one calibration curve, wherein the measurement unit (140) is configured to store measured values for said photoluminescence light (141) and said transmission light (142) in said storage unit (160) and to load said at least one calibration curve from said storage unit (160).

Apparatus according to claim 9, which is configured to switch between a calibration mode and a measurement mode, wherein said calibration mode is configured to correct said at least one calibration curve and said measurement mode is configured to determine said dose based said at least one calibration curve or said corrected at least one calibration curve.

Apparatus according to one of the preceding claims, further comprising a housing (170) with cavity (175), said holder (1 10) being arranged in that cavity (175), and wherein said cavity (175) comprises walls with a coating (171) to reduce light reflection at said walls and/or said holder (110) comprises a further coating (111) to reduce further light reflection at said holder (110). Apparatus according to one of the preceding claims, wherein said holder (1 10) is configured to hold multiple dosimeters (105a, 105b, 105c), the apparatus further comprising a drive unit (180) configured to move said holder (1 10) such that each of said multiple dosimeters (105a, 105b, 105c) is subsequently positioned to receive said second light (131) and said first light (121).

Apparatus according to claim 12, wherein said holder (1 10) is configured to hold a reference dosimeter (106) providing stable radio photoluminescence light emission after exposure to said second light (131), said measurement unit (140) being configured to perform a reference measurement with said reference dosimeter (106) allowing for a calibration, wherein said reference dosimeter (106) is further usable to calibrate said transmission measurement and said holder (110) is further configured to place said reference dosimeter (106) for exposure with said first light (121) from the first light source (120) providing thereby a stable light transmission.

System for detecting of doses of ionizing radiation, the system comprising:

the apparatus according to one of the preceding claims; and

a dosimeter (105) adapted to absorb the dose of said ionizing radiation.

The system according to claim 14, wherein the dosimeter (105) comprises a doped glass, especially a silver doped glass.

Method for determining a dose of ionizing radiation absorbed by a dosimeter (105) which is held by a holder (110), the method comprising: irradiating a first light (121) of a first light source (120) on said dosimeter (105); irradiating a second light (131) of a second light source (130) on said dosimeter (105) to excite photoluminescence centers in said dosimeter (105); measuring a photo luminescence light (141) from said excited radio photo luminescence centers; measuring a transmission light (142) of said first light (121) after passing said dosimeter (105); determining said dose based on said measured photoluminescence light (141) and/or based on said measured transmission light (142).

Method according to claim 16, further comprising irradiating a third light (132) on said dosimeter (105) to excite radio photoluminescence centers in said dosimeter (105), wherein said third light (132) is irradiated along a direction that differs from a direction along which said second light (131) is irradiated on said dosimeter (105), wherein the measured photoluminescence light (141) comprises photoluminescence light originating from said radio photoluminescence centers. 8. Method according to claim 16 or claim 17, wherein the step of determining said transmission light (142) comprises determining whether the dose to be determined is below a first threshold (Λι) or whether the dose to be determined is above a second threshold (Λ₂), where the first threshold (Λι) is smaller than said second threshold (Λ₂), wherein, when the dose to be determined is below said first threshold (Λι), the step of determining the dose is based on said photoluminescence light (141) excited by said third light (132) which is irradiated along an elongated direction of said dosimeter (105), and wherein, when said dose to be determined is above said second threshold (Λ₂), the step of determining the dose is based on said photoluminescence light (141) ex- cited by said second light (131) which is irradiated along a direction perpendicular to said elongated direction, and wherein, when the dose to be determined is between said first threshold (Λι) and said second threshold (Λ₂), the step of determining the dose is based on the transmission light (142).