AU2010100473B4 - Smart card embedded personal radiation dosimeter and reading apparatus - Google Patents

Description

DESCRIPTION FIELD OF THE INVENTION The present invention generally relates to the field of zero power personal radiation detectors, and more particularly, to optically stimulated luminescence (OSL) radiation sensors based on phosphor materials with long luminescence times and a method for extracting information related to ionizing radiation dose measurement. BACKROUND OF THE INVENTION In the area of occupational health and safety applications, there are numerous ways of measuring doses from ionizing radiation. The most widespread purpose for accurate absorbed dose measurement is use in the area of personal dosimeters. All workers who are routinely exposed to radiation generating equipment or who come in contact with radioactive substances are required to have their absorbed radiation dose measured. Currently personal dosimeters are used worldwide in many industrial and service fields. A common form of commercially available devices is a personal badge which is issued to individuals for a period of 6-12 weeks. Such devices contain sensing plates covered with well know photographic emulsion or a variety of materials based on principles of Thermoluminescent Dosimetry (TLD) or Optically Stimulated Luminescence (OSL). All personal dosimeter badges are calibrated before use, given to employees for a period of time, and after that time, a final reading is provided by a brand specific dosimetry service. The major shortcoming of existing radiation dose monitoring systems is the requirement that all personal dosimeters must be returned to an officially approved service provider at the end of each time period. All measured doses are cumulative for the specific time interval and as such cannot provide data following an accidental radiation over exposure. On the other hand, daily monitoring is possible with existing electronic personal radiation detectors, but such devices are still comparatively large and fragile, and broad implementations can be expensive and difficult in routine monitoring or reporting tasks.

The application of OSL based personal dosimeters has been steadily increasing during recent years, however a major shortcoming is that a reading can only be performed once due to erasing effect of excitation light. All OSL radiation storage phosphors currently used for detection are characterized by a prompt fluorescence discharge effect. Prompt fluorescence emission is defined by the application of excitation illumination at the same time as emitted fluorescence is collected. Difficulties with prompt fluorescence include: minimal variations in emission must be detected while the system is exposed to excitation light; emission intensity level is less than the excitation light intensity level; and finally it is difficult to filter out all of the excitation so as to leave uncorrupted emission from the OSL medium. A new type of OSL phosphors have been recently discovered by W.A. Kaczmarek and H. Riesen and described in international patent application PCT/AU2005/001905 titled: "Radiation storage phosphor and applications ". Representative material with composition such as BaFCl:Sm 3 can be used as a radiation detecting medium. This phosphor material can be distinguished with exceptionally long luminescence lifetime. For such phosphors, the observed fluorescence intensity remains durable (no fading effect) and is insensitive to multiple periods of stimulation during dose measurements. Finally, the most important feature for radiation monitoring is the fact that an observed slow fluorescence signal is proportional with the intensity of stimulation up to a radiation saturation level for a particular value of accumulated radiation dose. Based on the recent discovery of phosphors with long luminescence, the radiation detection method has been described in W.A. Kaczmarek Australian Innovation Patent 2008101046 titled: "Apparatus and method for detecting ionizing radiation using storage phosphors with long fluorescence lifetime". The current patent describes the new application of storage phosphors with long fluorescence time in the form of smart card based personal dosimeters. The present invention proposes a smart card embedded personal dosimeter and compatible reader which can support daily monitoring of individual personnel over long periods of time with low implementation and overall running costs.

Patent application number W02004/077097 and U.S. Patent No. 7,573,048 to Patel, describes for the first time a radiation sensitive dosimeter in the form of smart card. Design incorporates radiation sensitive materials such as diacetylenes (R-C=C-C=C-R, where R is a monovalent group) in the form of coatings or a strip which is sandwiched between two plastic layers with a pressure sensitive adhesive (i.e. a well known molding technique). The main claim is related to self indication of radiation dose, which allows observation of a color change in the radiation monitoring component after irradiation, and the ability to estimate the dose exposure by comparing the color with a color reference chart. It is for these reasons that inventors refer to this device as a Self-indicating radiation alert dosimeter. However, in fact, the sensitivity and accuracy of such a device will be subject to significant errors, which make it inferior to the personal dosimeter and reader proposed in the current invention. U.S. Patent No.7,420,187,187 issued to Klemic, Bailey and Breheny describes "Citizen's dosimeter", a smart card radiation dosimeter with optically stimulated luminescent material (OSLM) incorporating a mechanical shutter sandwiched between a upper and lower card layer. The proposed dosimeter has been based on carbon doped aluminum oxide crystal (A1 2 0 3 :C), a well known OSL material used commercially in the InLight System manufactured by Landauer (and Panasonic). The main disadvantage of this type of OSL material is its sensitivity to external light, which can cause partial loss or clearing of stored radiation data. With the application of a mechanical shutter (i.e. stainless steel or tantalum) such a dosimeter card is lightproof or very close to lightproof. In contrast, the smart card dosemeter described here does not require any mechanical parts, as it does not need to be lightproof. This is due to the non fading response of the new OSL phosphor materials with long luminescence time (i.e. BaFCl:Sm). Furthermore, the two smart card dosimeters referred to above do not claim any applied integration between OSL radiation dose measurement and user authentication, device calibration or measured data storage performed by an integrated circuit (IC processor) and memory chip, all of which are proposed in the current patent disclosure.

DETAILS OF INVENTION On the basis of the new OSL phosphor, a method was developed that could measure the quantity of incident radiation by selectively detecting stimulated slow fluorescence at specified time intervals. By using an apparatus for radiation measurement that relies upon this technique of time-division multiplexing, a wide dynamic range of radiation doses from a very low level to a very high level can be covered with a single detector (instantaneous low or high-intensity X and y radiation doses can be measured). One of the important constituents of the present invention is the utilization of OSL phosphor materials which have an extended fluorescence lifetime (> 0.5 ms), such phosphors can be used as a radiation detection medium, but require a different measurement approach. The proposed invention adapts detection based on dichroic optical filters with or without gated optical detection of luminescence for radiation monitoring applications using new OSL radiation storage phosphors. The present invention is directed to long-term personal radiation dose monitoring using OSL-based sensors embedded in close vicinity of the smart card integrated circuit contact area. Figure 1 shows a cross-section diagram of the smart card with integrated OSL sensor in the form of a pastille. The OSL phosphor material has been dispersed in the transparent binding medium to form a flat and thick film, which is enclosed from both sides by scratch resistant foil and bonded with the body of the smart card. Referring to Figure 2a, the excitation light source (2) emits blue light (wavelength 350 nm - 500 nm) which after passing through a blue dichroic filter, arrives at the OSL sensor. The luminescent light from the irradiated OSL sensor arrives at the CCD detector after passing through a red dichroic optical filter (transmission wavelength in the region of 690 ± 5nm). Figure 2b shows a variation of the design in which two prisms (7) and (8) have been used to transfer light from the light source to the OSL sensor and detector. In such a modification, red (4) and blue (5) filters are supported in light filtering by application of dichroic mirrors located on the surface of the prisms (7) and (8).

The proposed invention also adapts to a detection technique based on gated optical detection, originally used in the Time Resolved Spectroscopy (TRS) method. A significant part of background contributions are absent or decay rapidly after termination of illumination. Novel OSL phosphors do not cease their emission immediately (characteristic long fluorescence), rather there is a specific period for an excited atom or molecule (varying from one to a couple of milliseconds) during which the OSL phosphor continues to emit after initial excitation has ended. During this period, the detector is turned on to observe the relevant fluorescence signal with zero background light. The use of gated or pulsed excitation light can be preferred because it can minimize the generation of background noise signals associated with continuous-wave sources. Gated or pulsed light sources can also allow for less stringent optical filtering as well as photo-detector auto calibration. When the OSL signal output is measured between exciting light pulses, optical filtering can be removed, contrary to the situation when measuring a luminescent signal simultaneously with the excitation light. An auto-calibration procedure can be performed directly before or during insertion of the smart card to the reader, and the procedure can be performed internally by measurement of excitation light pulse intensity. Figure 3 shows a cross-section diagram of a smart card with an integrated OSL sensor in a measurement position between two bistable LCD filters. In Figure 3a the excitation light source (2) emits blue light (wavelength 350 nm - 500 nm) which after passing through an open LCD filter, arrives at the OSL sensor. In the next half cycle, the second LCD filter opens and the luminescent light from an irradiated OSL sensor arrives at the CCD detector as shown in Figure 3b. A square wave DC generator has been used to drive the LCD switching and control passing light ON/OFF synchronization. For the most sensitive applications, highly efficient electro-optical filters based on Pockels effect (Pockels cells) can be used instead of LCD filters. Figures 4 and 5 show the reader design adaptations using focal plane shutters and rotary focal plane shutters respectively. The synchronized light switching can be controlled by the application of a processor controlled electronic square wave or pulse generator (Fig. 4), similar in design to those commonly used in computerized single-lens camera mechanisms, or can be performed by the rotation of two connected and overlapping round rotary focal plane shutters with cutout sectors (Fig. 5). OSL luminescence detection readout measurements can comprise of a luminescence response from a single exciting light stimulation pulse or an averaged luminescence response from a plurality of stimulation pulses. The ability to average multiple luminescence counts resulting from a plurality of excitation pulses can provide a direct way to increase sensitivity of dose monitoring. Figure 6 shows examples of a luminescent response in the case of two excitation light pulses, although the number of excitation light pulses can be extended to many more depending on the required sensitivity level specification. In the reading phase, the luminescence detector would perform readouts of the OSL sensor luminescence and transmit the associated data, which would include the amount of radiation exposure received by the sensor. A subsequent excitation signal would then initiate the second readout phase. The invented smart card radiation dosimeter with external OSL reading apparatus can be connected to a computer system containing a specific database application to track personal, location place, time and other data. It is apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by the way of example only, and is not limited as various improvements and modifications are possible within the scope of the present invention.

DESCRIPTION OF DRAWINGS Figure 1 Is a schematic diagram showing the cross-section assembly of the ionizing radiation sensor (1) as an integral part of the smart card (4). Parts (2) and (3) are scratch resistant and transparent caps covering the OSL phosphor sensor. OSL phosphor particles dispersed in the transparent binding medium form a thin pastille (1). Figure 2 Is a schematic drawing showing the vertical cross-section of the smart card radiation sensor (1) and dose reader assembly based on optical filters: (4) blue monochromatic dichroic filter to pass excitation light and (5) red monochromatic dichroic filter to pass luminescent light from OSL sensor. Parts (2) and (3) are the excitation light source and the luminescence detector respectively. Figure 2b shows a variation of the design presented in 2a. An optical prism with a mirror (7) is applied to reflect exciting light in the direction of the OSL sensor and a second optical prism with a mirror (8) is used to redirect luminescent light to the detector. The dashed lines with arrows show light direction to and from the OSL sensor. Figure 3 Is a schematic drawing showing the vertical cross-section of the smart card radiation sensor (1) and dose reader assembly based on bistable LCD filters (4) and (5). Synchronized ON and OFF switching is performed by a square wave generator (6). Figure 3a shows the situation in which the filter (4) is open and excitation light reaches the OSL sensor. At the same time, filter (5) is blocking any transmitted light from reaching the luminescence detector. Figure 3b shows the opposite situation, when only luminescent light from the OSL sensor is able to reach the light detector (3). For extremely sensitive applications, highly efficient electro-optical filters based on Pockels effect (Pockels cells) can be used instead of LCD filters. The dashed lines with arrows show the direction of light to and from the OSL sensor. Figure 4 Is a schematic drawing showing the vertical cross-section of the smart card radiation sensor (1) and dose reader assembly based on two focal plane shutters (4) and (5). Synchronized ON and OFF switching is performed by a square wave generator (6). Figure 3a shows the situation in which the shutter (4) is open and excitation light reaches the OSL sensor. At the same time, shutter (5) is blocking transmitted light from reaching the luminescence detector. Figure 3b shows the opposite situation when only luminescent light from the OSL sensor is able to reach the light detector (3). The dashed lines with arrows show the direction of light to and from the OSL sensor. Figure 5 Is a schematic drawing showing the vertical cross-section of the smart card radiation sensor (1) and dose reader assembly based on synchronized two rotary focal plane shutters (4). Synchronized ON and OFF switching is performed by the rotation of two connected overlapping round plates with cutout sectors over the OSL sensor (5). Figure 3a shows the situation when the cutout sector of the top plate (4) is open and excitation light reaches the OSL sensor. At the same time, the bottom shutter plate (4) is blocking transmitted light. Figure 3b shows the opposite situation in which luminescent light from the OSL sensor passes through the cutout sector of the bottom plate (4) to reach the light detector (3). The dashed lines with arrows show the direction of light to and from the OSL sensor. Figure 6 Is a schematic time-related arrangement of the illumination and detection cycle. This figure shows two illumination periods followed by luminescent light detection. As the intensity of luminescent light is reduced with time, the detection period can be extended and repeated to accommodate higher reading accuracy.

Claims (5)

1. A portable personal radiation dosimeter and the method for monitoring exposure to ionizing radiation by measuring optically stimulated luminescence (OSL) response from an x-ray storage phosphor sensor embedded in smart card body. Radiation dose measurement system uses a smart card personal dosimeter, comprising embedded radiation dose sensor, a microprocessor chip and non-volatile memory storage component, as well as an independent external OSL reader apparatus with compatible read/write optical and electrical interface for data access and storage. The preferred version of applicable smart card personal dosimeter can be in a credit card shape with either a contact-less or contact-based interface.

2. The method of claim 1, wherein the smart card dosimeter comprises of a sensor: a small plate, capsule or flat pastille made of OSL phosphor particles dispersed in a self-supporting binding medium, characterized by changes of physical properties when exposed to ionizing radiation (x-ray OSL storage phosphor). In particular, the sensor will contain fine particles of light excitable storage phosphors with distinctive slow fluorescence effect. OSL sensor particles have a composition selected from the group of nanocrystalline luminescent receptor materials consisting of rare earth 3+ activated barium fluorohalides (i.e. BaFCl:Sm with < 1% at. concentration Sm in 3+ oxidation state).

3. The method of claim 1, wherein the external reading apparatus comprises an optical channel incorporating a gap in the middle for the smart card radiation dosimeter with an OSL sensor. The light channel assembly of the reading apparatus includes a light source, passive dichroic filters and/or active light passing/blocking synchronized shutters, luminescent light detector and an optical screen which shields said radiation detector from ambient light. The preferred type of shutter used in the reading apparatus will be one with a characteristic short response time e.g. bistable LCD filter, Pockels effect cell, focal plane shutter or rotary focal plane shutter.

4. The method of claim 1, wherein during each individual radiation dose examination process multistep tasks will be performed. The process will incorporate user verification and authentication, sensor and reader calibration, OSL sensor accumulated radiation dose reading and final recording of measurement, as well as the transfer of reader operational status data to the smart card electronic non-volatile memory and/or central data management system.

5. The method of claim 1, wherein the excitation light source emits blue light (wavelength 350 nm - 500 nm) which after passing through a blue dichroic filter and/or open shutter, arrives at the OSL sensor. The luminescent light from the irradiated OSL sensor arrives at the photo-detector after passing through a red dichroic optical filter (transmission wavelength in the region of 690 ± 5nm) and/or open shutter with the excitation light shutter in the closed position.