METHOD AND APPARATUS FOR READING THERMOLUMINESCENT PHOSPHORS

Abstract: An apparatus and method for rapidly reading thermo­ luminescent phosphors to determine the amount of lumines­ cent energy stored therein. The stored luminescent energy is interpreted as a measure of the total exposure of the thermoluminescent phosphor to ionizing radiation. The thermoluminescent phosphor reading apparatus uses a laser 10 to generate a laser beam 16. The laser beam power level is monitored by a laser power detector 20 and controlled to maintain the power level nearly constant. A shutter 26 or other laser beam interrupting means can be used to control exposure of the thermoluminescent phos­ phor to the laser beam. The laser beam is equalized such as by using an optical equalizer 31 so that the laser beam has an approximately uniform power density across the beam. The heated thermoluminescent phosphor emits a visible or otherwise detectable luminescent emission which is mea­ sured by an emission detector 45 and used as an indication of the radiation exposure of the thermoluminescent phos­ phors. Also disclosed are preferred signal processing 62 and control circuits 70, 75.

Description: Technical Field The technical field of this invention includes apparatus and methods for reading thermoluminescent phosphors to determine the level of excitation which has occurred from exposure of the phosphors to ionizing radiation. Background of the Invention It is well known in the art that certain materials called thermoluminescent phosphors can be irradiated with high energy radiation, and then subsequently stimulated using heat, to produce a luminescent emission. Thermoluminescent phosphors are in widespread use in radiation dosimeters used to measure the amount of incident radiation to which people, animals, plants and other things are exposed. Thermoluminescent dosimeters are widely used by workers in the nuclear industries to provide a constant monitor for measuring exposure to radiation. Thermoluminescent phosphors are excited by energetic radiation such as ultraviolet, X-ray, gamma, and other forms of radiation. Such ionizing radiation causes electrons within the thermoluminescent material to become highly energized. The nature of thermoluminescent materials cause these high energy electrons to be trapped at relatively stable higher energy levels. The electrons stay at these higher energy levels until additional energy, usually in the form of heat, is supplied which releases the trapped electrons thereby allowing them to fall back to a lower energy state. The return of the electrons to a lower energy state causes a release of energy primarily in the form of visible light which is ordinarily termed a luminescent emission. The use of thermoluminescent phosphors in personal dosimeters has led to demand for a large number of dosimeters which must be read on a routine basis in order to monitor the possible exposure of persons or other objects to ionizing radiation. Because of the substantial numbers, the job of reading dosimeters becomes time-consuming and costly. There are four commonly known methods of heating thermoluminescent material in order to release the trapped electrons and provide the luminescent emission which is measured as an indication of the amount of ionizing radiation to which the dosimeter was exposed. The first and most common method for heating thermoluminescent phosphors is by contact heating. The second method is heating using a hot gas stream which is impinged upon the phosphor. The third method uses radiant energy in the form of infrared beams which heat the luminescent phosphor. The fourth method uses infrared laser beams to provide the necessary heat for luminescent emission. Contact heating is the most widespread, but is also the most time-consuming and unreliable. Conventional contact heating occurs using a hot finger or contacting probe which produces a highly non-uniform temperature distribution in the phosphor being heated. This is particularly true when thin layer or film type dosimeter configurations are used. Contact heating has also been found unacceptable for reading dosimeters used to measure relatively low energy beta ray doses. Heating of phosphors using a hot gas stream has proven to be faster and more uniform than contact heating. Unfortunately, this method of heating requires rather large scale heat exchanger equipment and is fairly costly to implement. Heating with infrared beams has provided improved results over contact and hot gas heating. One example of such a thermoluminescent phosphor reading apparatus is shown in U.S. Patent No. 4,204,119 to Yasuno et al. The Yasuno patent shows an apparatus using an infrared emitting incandescent lamp which rapidly heats the back surface of a substrate. A small amount of thermoluminescent powder is attached to the substrate on the opposite side from the surface exposed to the infrared beam. Yasuno thus shows a configuration where the substrate is heated directly, and through conduction the heat is transferred to the thermoluminescent powder. United States Patent No. 3,531,641 to Weissenberg teaches the manufacture of thermoluminescent dosimeters containing thermoluminescent phosphors held in a synthetic material. The synthetic material must be capable of surviving the heat and heating rays since it is used as a conductor of heat to the thermoluminescent phosphor which is contained therein. The dosimeters and heating method described by Yasuno et al and Weissenberg have provided useful technology, but unfortunately are limited and have been found not entirely satisfactory for thermoluminescent dosimetry used in medical research, radiation therapy, and personal and environmental monitoring. Such applications preferably use or require very small dosimeters using in many cases less than one milligram of thermoluminescent phosphor in the form of a small discrete dot or very thin layer. In the case of thin layer dosimeters, it is desirable to have less than ten milligrams of thermoluminescent phosphor per square centimeter of dosimeter area. Using such small amounts of thermoluminescent material requires that the material be heated in a very rapid manner in order to provide sufficient luminescent energy emission so that detectable levels of emission can be measured without being obscured by the electronic noise in the measuring equipment. Rapid heating of such small amounts of luminescent phosphor to about 400°C is severely slowed when a substrate material must be initially heated in order to conduct heat to the thermoluminescent phosphor. Heating of thermoluminescent phosphors by heat conduction through a substrate also severely limits the rate at which the material can be heated for another reason. If too much heat is applied very rapidly, the substrate material itself tends to incandesce thereby creating luminous emissions which are sensed by the luminescent emission detection equipment and translated into an erroneous reading of the dosimeter. Accordingly, the time required for reading thermoluminescent phosphors using conduction techniques has been severely limited and the best known times are approximately one half second. U.S. Patent No. 3,729,630 to Yamashita et al discloses a thermoluminescent readout instrument utilizing an infrared laser source which is used to heat a thermoluminescent dosimeter element. Instruments constructed according to Yamashita et al have been found lacking in that the luminescent emission glow curve resulting from laser stimulation does not provide the characteristic glow peak which is desirable for easy, accurate determination of the radiation levels to which the dosimeter was exposed. The current invention has identified that nonuniformity in the laser beam power density and also instabilities over time in the power output of the laser contribute to a lack of speed and accuracy in reading thermoluminescent phosphors. The nonuniformity in laser beam power appears to be a characteristic of carbon dioxide lasers used in this type of application. Carbon dioxide lasers typically produce a laser beam exhibiting a Gaussian or bell-shaped curve when power or intensity is graphically plotted as a function of beam cross-sectional position. This nonuniform beam power profile causes localized intense heating at the center of the beam because of the substantially higher amounts of power being released thereat. This nonuniform beam power profile causes temperatures to vary greatly, even across the small distances from the center to the outside of the laser beam. Instabilities in the power output of the laser with time have also been identified in the current invention as a contributing factor making the resulting glow curves from the luminescent phosphors less capable of accurate interpretation. It has been previously known that laser power can be made more constant over time using temperature stabilizing devices, or closed loop controlled piezoelectric pushers which move the laser mirrors relative to one another in quick response to the thermally induced motions of the laser components. The temperature stabilization techniques for stabilizing laser power output are not totally effective, in part because of the very slow response times. The piezoelectric pushers are very expensive and accordingly have not been widely used. It is an object of this invention to provide thermoluminescent phosphor reading apparatus which can very rapidly stimulate thermoluminescent phosphors in a manner that allows accurate and reliable measurement of the resulting luminescent emissions. It is another object of this invention to provide methods by which thermoluminescent phosphors can be very rapidly stimulated using a laser source adapted to provide a uniform beam profile to produce very accurate and repeatable luminescent emissions which are indicative of the amount of ionizing radiation to which the original thermoluminescent material was exposed. These objects are achieved by the invention as claimed. Other advantages of this invention will be apparent from the description given herein. Brief Description of the Drawings Preferred embodiments according to the invention are illustrated in the accompanying drawings, in which: Fig. 1 is a diagrammatic representation of a preferred embodiment thermoluminescent phosphor reading apparatus according to this invention; Fig. 2 is a schematic representation of electronic circuitry which can be used to construct the laser power detection and laser power signal enhancer portions shown in Fig. 1; Fig. 3 is a schematic representation of electronic circuitry which can be used to construct the modulation control circuit and power reference signal generator portions shown in Fig. 1; Fig. 4 is a schematic representation of electronic circuitry which can be used to construct the central control and operator control portions shown in Fig. 1; Fig. 5 is a graph showing two different luminescent glow curves; Fig. 6A is a graph showing a triangle wave and two laser control level signals; Figs. 6B and 6C are graphs showing two different laser modulation signals used to modulate the laser power supply; Fig. 7 is a timing diagram showing the relationship between the detector signal and control timing signals; Fig. 8 is an alternative beam equalizing means; and Fig. 9 is a still further alternative beam equalizing means. Fig. 10 is a still further alternative beam equalizing means. Detailed Description of the Preferred Embodiments A preferred form of apparatus according to this invention is shown in Fig. 1 and includes a laser beam source means 10 to provide the laser radiant energy for heating thermoluminescent phosphors being read. Laser beam source means 10 includes a laser head 12 having a laser cavity 11, and preferably a laser cooling and temperature stabilization unit 13 attached thereto for helping to stabilize the temperature and intensity level of laser cavity 11. Laser beam source means 10 preferably emits a laser beam 10 which is preferably in the infrared spectrum so that the visible light luminescent emissions can be distinguished therefrom. Laser head 12 is powered by a laser power supply 15. Examples of acceptable instruments for constructing the laser beam source means 10 include a radio frequency carbon dioxide laser available from Laakmann Electro-Optics, Model RF125. The laser cooling and temperature stabilization unit 13 can advantageously be a Laakman Electro-Optics Model RF165 cooling unit and Model TC16000 temperature controller. Laser power supply 15 can advantageously be a radio frequency power supply such as Laakman Model RF250. Such a radio frequency power supply can advantageously be operated at 44 megahertz or other suitable frequencies depending on the system employed. Such a combination of components produces a laser beam 16 having a nominal power of 15 watts and a nominal beam diameter of 1.8 millimeters. An alternative laser configuration could include a carbon dioxide wave guide laser with a direct current high voltage excitation such as Model 82-40000 from California Laser. Such a laser has the nominal power output of 45 watts. Using such a model eliminates the need for the radio frequency power supply since the Model 82-40000 has a high voltage direct current power supply and also uses water cooling for temperature stabilization. Laser power supply 15 must be capable of being modulated to regulate the output power of the power supply and hence the output power of the laser beam 16 which is emitted from laser cavity 11. In the case of a radio frequency power supply, the laser power supply 15 can be modulated by varying the amplitude of the radio frequency voltage. In the case of a direct current high voltage power supply the discharge current of the high voltage DC power supply is modulated to regulate the laser beam power. Modulation of the laser power supply 15 may be possible using other alternative variations. Laser beam 16 is emitted from laser beam source means 10 and is advantageously, but not necessarily, directed against a reflective means such as infrared mirror 17. Reflected laser beam 16 then encounters a laser beam splitting means 18 which allows most of the beam to be transmitted directly therethrough and the remaining beam to be reflected toward a laser power detector 20. Laser beam splitting means 18 can advantageously be a thin sheet of material which is transparent to the 10.6 micrometer wavelength radiation produced by a typical CO₂ laser. Examples of suitable materials include germanium, barium fluoride, or zinc selenide. Thin flat pieces constructed of these and other suitable materials form windows which are usually antireflectively coated to provide high transmissibility. Orientation of the beam splitting means at a particular angle allows only a small percentage of the beam to be reflected, thereby forming detector beam 21. Laser beam 16 is preferably polarized either within laser cavity 11 or at some point between laser beam source means 10 and infrared mirror 17. Polarization of beam 16 is achieved using a polarization means which preferably provides time independent or fixed polarization. Fixed polarization is needed because the reflective elements such as mirror 17 and beam splitter 18 have varying reflectiveness depending upon the polarization orientation of the beam. Changes of polarization experienced in some lasers cause changes in the intensity of the reflected detector beam 21. This leads to erroneous power adjustments solely due to polarization changes. An appropriate polarization means 118 can comprise a thin film of polarizing material well known in the art applied upon an optically flat substrate which can advantageously be zinc selenide. The composite is oriented with the incidence beam at the Brewster angle of approximately 67 degrees from a line perpendicular to the surface of the polarizer. Other alternative polarization means will be apparent to those skilled in the art. As an alternative to the lasers and polarizer described above, it may also be possible to use a laser having a polarizer integrated or included in the laser cavity. An example of such a laser is Model No. LS35-P from Directed Energy, Inc., Irvine, CA which can be equipped with an intracavitary polarizer. Detector beam 21 from beam splitting means 18 is directed to a laser power detector 20. A beam interruption means such as chopping wheel 23 can be used to limit the time during which detector beam 21 strikes laser power detector 20. Chopping wheel 23 is provided because a preferred form of laser power detector is not capable of continuously reading the power of detector beam 21. Where alternative laser power detectors capable of continuous monitoring are used, then such a beam interruption means will be unnecessary. Laser power detector 20 is preferably capable of responding very quickly to Variations in the power of beam 16 and the resulting power of detector beam 21. A preferred laser power detector 20 is a pyroelectric type detector. An example of an acceptable pyroelectric detector is the Series 350 leod-zirconate-titanate detector manufactured by Barnes Engineering. Laser power detector 20 monitors the power during the time that it is exposed to detector beam 21 when the aperture 24 of chopper wheel 23 is properly positioned. The chopper wheel is necessary because such pyroelectric detectors are not capable of continuously measuring the laser power output. The chopper wheel 23 is advantageously synchronized with the operation of the overall system so that beam 21 is allowed to pass through chopper wheel aperture 24 at times appropriate for control of the primary laser beam power. Preferably a number of power readings are taken during the period of time during which the luminescent phosphor sample 40 is being heated. If the sample exposure time is 100 milliseconds, then chopping wheel 23 would be turned at a rate of approximately 100 revolutions per second, or some integer division thereof if more than one aperture 24 is provided. In addition to the pyroelectric detector and chopping wheel configuration shown in Fig. 1, it is also possible to use alternative laser power detectors which are capable of continuously monitoring laser beam power. One example of an alternative detector which is capable of continuous monitoring is the photoconductive mercury-cadmium-telluride detectors that are known in the art. These detectors are more costly but do provide the benefits of continuous monitoring. In such a case the beam interruption means 23 can be eliminated. Other components relating to the electronic control of the system can also be eliminated in such an alternative embodiment. Laser beam splitter 18 is designed and oriented so that the bulk of laser beam 16 passes directly therethrough. The continuing laser beam 16 is directed to a shutter means 26 for controlling exposure of the thermoluminescent phosphor sample 40 to the laser beam. Shutter means 26 can be selected from a variety of standard shutter's available for high-powered lasers. Shutter 26 is preferably electrically actuated and controlled by an electrical control signal coming from the central control circuitry 75. Shutter means 26 is preferably oriented at an oblique angle so that the reflected laser beam 16a (in phantom) is directed into a beam dump 28. Beam dump 28 is designed to stop the laser beam and to dissipate the energy of the laser beam. Beam dump 28 can be of a variety of different types, with a heat resistant ceramic having provision for heat dissipation being the preferred type. Opening shutter means 26 allows laser beam 16 to continue therethrough and the beam can advantageously be redirected by reflecting it off of a second reflective means such as infrared mirror 30. Laser beam 16 is then preferably fed into a beam equalizing means 31. Beam equalizing means 31 can include a lens means 32 which preferably is a convex, hemispherical or other infrared lens which disperses the parallel laser light into a dispersed beam 35 which is directed into an optical channel means 33. Optical channel means 33 has an optical channel 34 extending therethrough. Optical channel 34 is an elongated passageway or channel for reflecting, aligning, and equalizing the dispersed laser beam 35 as it passes therethrough. Optical channel 34 can have a square, round or other cross-sectional shape. The inner walls of optical channel 34 are preferable highly polished and coated with a reflective substance, such as gold, to provide almost complete reflection of the dispersed laser beam 35. Dispersed laser beam 35 is emitted from optical channel means 33 as an equalized laser beam 36 and preferably beamed directly onto the luminescent phosphor sample 40 being read. Fig. 8 shows an alternative beam equalizing means 150. Beam equalizing means 150 reflects and disperses laser beam l6 from a convex reflector 151 and into an optical channel 152. Optical channel 152 is highly reflective and can be similar to optical channel 33. The dispersed laser beam 153 is beamed onto the concave surface of a parabolic or other focusing reflector 154. The dispersed beam 153 is focused by parabolic reflector 154 onto an appropriate phosphor sample 155. The relative position of sample 155 with respect to reflector 154 is preferably adjustable so that the degree of focusing can be adjusted. The beam equalizing means of Fig. 8 provides a nearly uniform power and intensity profile across the beam at the point of impingement upon sample 155. Fig. 9 shows a further alternative beam equalizing means 160. Incoming laser beam 16 is beamed through a convex lens 161 to disperse the beam into an optical channel means 162. The resulting dispersed laser beam 163 is gathered by a focusing lens 164 and focused to a point near phosphor sample 165. The relative position of sample 165 is advantageously adjustable so that the size and intensity of the impinging beam can be adjusted as needed. Fig. 10 shows a still further alternative beam equalizing means. Incoming laser beam 16 is focused through a convex lens 219 onto a solid infrared transmissive fiber optic light guide 220. Internal reflections within the optic fiber 220 produce an equalized beam similar to optical channel 34. Fig. 10 also shows a thermoluminescent phosphor sample 221 positioned at the end opposite from lens 219. The thermoluminescent emission from sample 221 can be transmitted through a light guide similar to that shown in Fig. 1 at numeral 47 and then detected by a detector similar to detector 45. This alternative beam equalizing means has the advantage of allowing the laser beam to be directed to remote locations to perform readings in place at the remote measurement location. The equalized laser beam 36 has an approximately uniform beam intensity and power profile. Prior art laser beam heating techniques typically employed carbon dioxide or other laser beams which have an approximately Gaussian beam intensity profile. The Gaussian beam intensity profile causes the central areas of the beam to more rapidly heat the substrate or thermoluminescent material positioned at the center of the beam than at the edges. Such nonuniformity in the heating laser beam causes localized incandescence to occur in the substrate material if power is increased. Such incandescense substantially changes the characteristics of the glow curve produced by the thermoluminescent sample 40. Even if incandescense does not occur, heating with a laser beam having a Gaussian power profile also causes the phosphor to be heated nonuniformly. The temperature threshold for thermoluminescence accordingly propagates from an inner core outwardly, thus releasing the thermoluminescent emission over a period of time. This delayed release greatly reduces the height of the measured luminescent emission glow peak. The height of the glow peak as well as the integral under the glow curve are used in analyzing the overall amount of ionizing radiation to which the phosphor has been exposed. Fig. 5 shows two glow curves produced in response to heating of a thermoluminescent sample with a carbon dioxide laser beam. Curve 91 shows a typical thermoluminescent emission occurring when a sample is heated using an unequalized laser beam having an approximately Gaussian beam intensity. Curve 91 rises rapidly, usually to a maximum, and then begins dropping off as the thermoluminescent temperature threshold propagates outwardly. Curve 92 shows a theoretical glow curve which occurs when the thermoluminescent phosphor is heated uniformly such as with the apparatus of this invention. Glow curve 92 includes a glow peak 93 which is clearly defined in comparison to the glow curve 91. One of the advantages of the intensified glow peak 93 is that the signal to noise ratio is increased substantially. This is particularly important when the amounts of thermoluminescent phosphor materials are small as indicated for the preferred thin film and discrete dot dosimeter configurations. The thermoluminescent phosphor films used in this invention are preferably very thin having approximately less than 10 milligrams of phosphor per square centimeter of film. The discrete dots preferably use less than 1 milligram of thermoluminescent material. It is also possible to heat localized areas of thermoluminescent phosphor layers. A large number of different types of thermoluminescent phosphors known in the art can be used with this invention. Examples of acceptable types of thermoluminescent phosphors include calcium fluoride doped with manganese, lithium fluoride doped with titanium and magnesium, calcium sulfate doped with thulium, and others known in the art and hereafter developed. It is desirable that the thermoluminescent phosphors used in this invention be supplied in a form which does not interposition a substrate material between the equalized laser beam 36 and the thermoluminescent phosphor being heated. Elimination of the interpositioned substrate allows for increased heating rates of the thermoluminescent material without slowing the heating process and without the troublesome incandescent emissions which can occur when substrate materials are heated very rapidly. Luminescent phosphor sample 40 is preferably held in position for beaming equalized laser beam 36 thereon using a phosphor holding or positioning means (not shown). The phosphor holding or positioning means can be of any type suitable surface or bracket serving to hold the specific dosimeter configuration or other phosphor sample 40 in position to receive equalized beam 36. It is well known that thermoluminescent materials heated to thermoluminescent threshold temperatures will produce luminescent emissions which are usually in the visible light range with the specific wavelength(s) of light produced being dependent upon the particular phosphor chosen. Thermoluminescent threshold temperatures usually range between 100-500°C depending upon the particular phosphor. Heating of the phosphor sample 40 by equalized beam 36 causes such a luminescent emission to occur. The luminescent emission can be measured by a thermoluminescent emission measuring means such as photodetector 45. The thermoluminescent emission measuring means is preferably an amplifying device such as a photomultiplier tube well known in the art. Such amplifying photomultiplier tubes require a high voltage power supply 48. Other alternative means for measuring the intensity of emission from luminescent phosphor 40 may be possible either with current or future technology. Photodetector 45 can receive the luminescent emission directly or using a light guide 47 which is helpful in gathering and transmitting the thermoluminescent emission from phosphor 40 to the thermoluminescent emission measuring means 45. An infrared filter means 46 is preferably included somewhere between phosphor sample 40 and the photodetector 45. Filter means 46 filters any infrared light caused by any minute incandescence of sample 40 or any scattered infrared light from the laser beam or other sources. The intensity or other relevant output signal from thermoluminescent emission measuring means 45 is sent to a luminescent emission signal processing and memory unit 52. Appropriate signal processing and memory units are well known in the art and can provide amplification, digitalization, visual display, computer analysis, graphical printouts, and memory storage capability. It is highly desirable that the signal be amplified and recorded so that glow curve configurations can be carefully examined to determine the amount of ionizing radiation which struck the thermoluminescent phosphor being read. It is also possible in signal processing unit 52 to provide computerized integration or other empirical analysis to calculate the amount of ionizing radiation to which the thermoluminescent phosphor sample 40 was subjected. One embodiment of the invention performs further analysis to compare the amount of ionizing radiation to a threshold value and provide a simple output in the form of acceptable versus nonacceptable. The apparatus of this invention is capable of reading thermoluminescent samples using heating times which are greatly reduced as compared to the prior art. It has been found that heating times in the range of 5 to 500 milliseconds are sufficient to heat and read the thermoluminescent phosphors. This must be compared with prior art techniques which at best claim reading times down to 500 milliseconds. This reduction in the amount of time necessary to heat the thermoluminescent phosphors can be translated into greatly reduced times and increased throughput for machinery constructed according to this invention designed for automated processing of dosimeters. The greatly reduced heating times are made possible in part by equalizing the laser beam intensity profile so that a nearly uniform beam strikes the thermoluminescent material being heated. This allows the phosphors to be uniformly and rapidly heated at a maximum rate without causing incandescense of the substrate or phosphor due to localized overheating. Another advantage of the uniform beam profile is that it allows a well defined portion of the thermoluminescent material to be read and read with accuracy without stimulating peripheral portions at reduced power which causes unwanted emissions to be read. Thus, it becomes possible to read a plurality of points on a single very small sample of thermoluminescent foil or other dosimeter configurations. Reading discrete dots on the same sample is limited only by conductive heating effects which may cause minute thermoluminescent emissions to occur about the boundary of the phosphor being heated directly by the equalized laser beam 36. In addition to beam equalization this invention also allows for much greater power stability in the laser beam used to stimulate phosphor 40. Stable beam power with time is accomplished using an electronic closed loop feedback control. A variety of systems are possible with alterations and alternatives being necessary depending upon the specific equipment and configuration being used. In general the laser power control system includes laser power detector 20, control circuitry for taking the power detector output signal and providing a modulated signal for controlling laser power supply 15, and a laser power supply 15 which is capable of varying its power output in response to the modulated control signal. Laser power detector 20 provides a laser power signal which is sent to a laser power signal enhancer 62. Fig. 2 shows in greater detail one preferred form of laser power signal enhancer. The signal from the preferred lead-zirconate-titanate pyroelectric laser power detector 20 produces a current varying signal which is connected to a first amplifier A1 as shown in Fig. 2. A first resistor R1 and first capacitor C1 having approximate values of 100 kilohms and 470 picofarads are connected in parallel with amplifier A1. Values of various electronic components are shown in Table 1 near the end of the Detailed Description. The output from amplifier A1 is passed through resistor R2 and to the negative input of a second amplifier A2. The positive inputs of amplifier A1 and A2 are set to zero volts by grounding. A third resistor R3 is connected in parallel with amplifier A2. Amplifier A1 and resistor R1 convert the current signal of detector 20 into a voltage varying equivalent. The current is converted to a voltage by a conversion factor that is equal to R1. Amplifier A2 amplifies the output of A1 to provide a signal in the range of 1-10 volts. Capacitor C1 limits the frequency response to A1 preventing response to high frequency detector signals such as the 100 KHz modulation frequency discussed below or radiated high frequency noise. The output of amplifier A2 is processed by amplifiers A3, A4, A5, and A6, resistors R4-R8, diode D1 and capacitor C2 to produce an output signal from amplifier A3 which is positive and referenced to zero. The resulting enhanced laser power signal is subsequently used in the modulation control circuit 70. A peak detection circuit formed by amplifier A6a, diode D2, capacitor C3 and resistor R9 can also advantageously be included to further process the output from laser power detector 20 so that peak values can be held and be more easily identified. Amplifiers A1-A6a can advantageously be type LF 356, LF353 or LF347 manufactured by National Semiconductor. The output from amplifier A6a is advantageously connected to a sample and hold circuit which includes an integrated circuit IC1 which preferably is a type LF 398. Integrated circuit IC1 is connected in the manner indicated with pin 1 being identified by the dot. Other integrated circuits are also referenced to pin 1 using the dot. Output 110 from integrated circuit IC1 is connected to the modulation control circuit 70 shown in greater detail in Fig. 3. Modulation control circuit 70 is designed to control the power of laser beam 16 by providing an output or modulation signal which is used to control laser power supply 15. The modulation control circuit will necessarily be governed in its specific design by the type of laser power supply used and many alternative modulation circuits are theoretically possible. With the preferred embodiment radio frequency laser power supply, It is advantageous to modulate the power of laser beam 16 by regulating the length of time during which power is supplied to the laser. This intermittent control of voltage amplitude is preferably done at a high frequency relative to the exposure time of the phosphor samples and any beam interruption means frequency which is used. Frequencies for the modulation of the laser power supply of between 10 kilohertz to 1 megahertz have been found useful when using a 44 megahertz laser power supply. A 100 kilohertz frequency is preferred. The radio frequency power from the radio frequency power supply is accordingly turned off and on at a rate of 100 kilohertz with the length of time during which the power remains on during each cycle being changed to control total laser beam power. Fig. 3 shows a preferred form of modulation control circuit 70. Modulation control circuit 70 receives an enhanced laser power signal from the output of signal enhancer 62 via connector 110. This output signal comes from IC1 and is connected to a difference amplifier A8. Difference amplifier A8 also receives input from a power reference signal generator 63. The gain of difference amplifier A8 is determined by RI0 and R9a. Power reference signal generator 63 is preferably constructed to provide an adjustable 0-5 volt output voltage range. Diodes D5-D8 and IC4 provide a stable power supply in that voltage range. Integrated circuit IC4 is preferably a type LM 336-5 by National Semiconductor. A variable potentiometer 89 serves as an adjustable operator power level control 89 (Fig. 1). The output of amplifier A8 is connected through resistors R45 and R44 to a positive current source 83 and a negative current source 80, respectively. Current source 83 includes two resistors R11 and RI2 connected to a positive 15 volt supply. Transistors Q1 and Q2 are connected to resistors R11 and R12 as indicated in Fig. 3. Current from positive current supply 83 is switched by transistor Q3 which is controlled by a timing signal TS-2, so that power adjustment occurs during timing period T2, immediately after a laser power measurement has been made (see Fig. 7). Negative current source 80 is similarly controlled by a parallel timing signal TS-1. Timing signals TS-1 and TS-2 originate in the central control 75. Current source 80 includes resister R13 and a variable resister R14 connected to a -15 volt supply. Transistors Q4 and Q5 control current supply. Timing signal TS-1 from central control 75 permits current to flow only during timing pulse T2. The collectors of transistors Q2 and Q4 are the outputs of current sources 83 and 80, respectively. Both outputs are connected to capacitor C5 which is also connected to the positive (+) input of amplifier A9. The output of amplifier A9 is fed back to the negative (-) input thereof, so that the amplifier gain is equal to 1. Current source 80 discharges capacitor C5 decreasing the voltage on capacitor C5 when the measured power represented by enhanced laser detector signal 110 is higher than the desired power level determined by the power reference signal from power reference generator 63. Current source 83 charges capacitor C5, increasing the voltage on capacitor C5 when the measured power is lower than the power reference signal. The voltage applied to capacitor C5 is buffered through amplifier A9 to provide a control level or control voltage. The control voltage from amplifier A9 is connected to the positive input of a comparator CP1. The negative input of comparator CP1 is connected to a triangle or other periodically increasing and decreasing wave generator having an output which is less than +15 volts and operated at a frequency which determines the modulation frequency, preferably at 100 kilohertz. Comparator CP1 provides an output which is a modulation signal used to modulate the radio frequency laser power supply 15. Fig. 6A shows a line 120 which represents the output voltage of triangle wave generator 85. Comparator CP1 compares the output voltage of generator 85 to the power control voltage from amplifier A9. Fig. 6B represents the output of comparator CP1 when the control voltage is at voltage level 122. The modulation signal has an on period P122. If this amount of laser power is insufficient as determined by laser power reference signal generator 63, then difference amplifier A8 causes current source 83 to increase the control voltage supplied to comparator CP1. An increase in control voltage is represented by line 121 in Fig. 6A. At control voltage level 121 there is an associated mdulation signal having an on period P121 which is longer than period P122. Such increased period increases the amount of time laser power supply 15 is on thus increasing the power in laser beam 16 and adjusting the power of laser beam 15 until the power is equal to the predetermined set point established by power signal reference generator 63. Downward adjustment of beam 16 power is accomplished in similar but opposite manner from that increase just described. The thermoluminescent phosphor reading apparatus of this invention also includes a central control 75. Central control 75 can be any one of many different control circuits which perform the necessary and any optional functions desired. A preferred circuit for central control 75 is shown in Fig. 4. The central control circuit of Fig. 4 includes a comparator CP2 which is provided with an adjustable reference voltage and capacitance at the negative input. The enhanced laser power detector signal is connected to the positive input terminal of comparator CP2. The output of comparator CP2 is inputted into a first integrated timing circuit IC2 which is advantageously a National Semiconductor type LM556 dual timing unit. Integrated timing circuit IC2 creates timing signals T1 and T2. A number of resistors and capacitors are connected to IC2 to provide the desired timing periods. Additional capacitance is provided to the circuit by capacitors C12-C15. The value of preferred components are given in TABLE 1. Timing signal T1 is initiated by the onset of the rise in the laser power detector signal and its effect on comparator CP2. This is illustrated in Fig. 7. Timing signal T1 is sustained until the laser power detector signal decreases. Timing signal T2 is initiated upon termination of timing signal T1. Timing signals TS-1 and TS-2 are generated from and based upon timing signal T2 using comparators CP3 and CP4. The differences in signals T2, TS-1 and TS-2 being the voltage thereof. The central control also includes a second timing integrated circuit IC3 which also is a dual timer type LM 556. Integrated timing circuit IC3 is used to time the startup, sequence and the period of exposure of phosphor sample 40. The read or exposure cycle is initiated by closing start switch 130 which sets latch L1 and starts second integrated timing circuit IC3 through a startup procedure which turns on laser 10 and adjusts the laser power to the desired level. After this startup is completed then a second cycle is started which determines the period of exposure of phosphor 40 to the equalized laser beam. The period is adjustable using an exposure time control, which is advantageously variable resistor 131. The second cycle also opens shutter means 26 using the output from transistor Q15. AND gate 109 generates a laser on signal which acts through comparator CP5 so that laser modulation signal from modulation control circuit 70 is grounded out unless a read cycle has been started. Indicator lights LED 1, LED 2, and LED 3 provide indication of power on, read cycle activated, and laser on, respectively. Latch L2 is used to prevent the shutter from operating during the timing signal T2 so that spurious electromagnetic noise does not affect the laser power detector signal. It will be apparent to a person of ordinary skill in the art that other alternative central control circuits 75 can be used to operate the apparatus of this invention. Apparatuses according to this invention are operated by first supplying electrical power to the system as a whole or the various components if separate power supplies are used. A previously exposed thermoluminescent phosphor is positioned in a phosphor positioning means (not shown) adjacent the optical equalizer means 31 for exposure to the equalized laser beam 36. The operator closes the start switch 130 and the central control 75, modulation control 70 and remaining circuitry is initiated and the power of laser beam 16 is adjusted automatically at a set point adjustable by power level control 89. Reading of the phosphor then automatically occurs, controlled by the control circuitry. The equalized laser beam strikes the phosphor sample 40 and heats it to the threshold temperature in the range of 100-500°C. The phosphor then produces a thermoluminescent emission which is advantageously collected by light guide 47 and converted by photodetector 45 into an electrical signal measuring intensity. The resulting intensity versus time information is advantageously received, recorded, printed and analyzed by a suitable luminescent emission signal processing and memory unit 52 well known in the art of scientific data analysis. The method of this invention comprises a new method for rapidly reading thermoluminescent phosphors to accurately determine the amount of ionizing radiation to which the phosphors have been exposed. This method first involves emitting a laser beam from a source of appropriate laser light. The laser beam is preferably an infrared beam emitting from a carbon dioxide laser such as described above as laser beam source means 10. The method also involves measuring the power contained in the emitted laser beam and controlling the power level of the laser beam to provide a very nearly constant laser beam power level with time. It is preferable to control the laser beam power within a variance range equal to approximately 1% or less of the total power contained in the laser beam. The method of this invention also includes equalizing the power profile of the laser beam with an optical equalizer such as 31 to produce a nearly uniform power density across the full cross section of the laser beam used to stimulate the thermoluminescent phosphor sample being read. The method of this invention further involves controlling the amount of time the thermoluminescent phosphor sample is exposed to the equalized laser beam and the power of the beam so that rapid uniform heating to approximately 100-500°C is obtained. This is necessary in order to prevent overheating of the thermoluminescent phosphors so that erroneous readings and damage do not occur. Control of exposure time and power of the thermoluminescent phosphors is also advantageous in that it allows different phosphor samples or different areas of phosphors to be exposed for reading at different times so that discrete portions or samples of the thermoluminescent phosphors can be read independently. Control of exposure time can be accomplished using shutter means 26 described above. The method of this invention also advantageously includes the step of measuring the luminescent emission occurring as a result of exposure of the thermoluminescent phosphor to the equalized laser beam. This is accomplished using the thermoluminescent emission measuring means. The resulting measurements of thermoluminescent emission intensity, time interval and peak height can be theoretically analyzed and checked by calibrated specimens to provide standard values or measurements which are accurate as a measure of total radiation exposure. The improved methods according to this invention provide increased speed and accuracy in heating and reading thermoluminescent phosphors.

Claims: A thermoluminescent phosphor reading apparatus having a laser beam source means (10) for generating and emitting a laser beam (16), a phosphor positioning means for holding a thermoluminescent phosphor (40) in position for exposure to the laser beam, a shutter means (26) positioned to control exposure of a thermoluminescent phosphor (40) to the laser beam, and a thermoluminescent emission measuring means (45) for measuring the intensity of any thermoluminescent emission occurring when a thermoluminescent phosphor is heated by the laser beam; characterized by a laser beam power detection means (20) for measuring the power of the laser beam, and a laser power control means (62,70,75) connected to the laser power detection means and the laser beam source means for controlling the power output of the laser beam source means to control the power of the laser beam to a desired power. The thermoluminescent phosphor reading apparatus of claim 1 further characterized by a laser beam optical equalizing means (31) for equalizing the cross-sectional intensity of the laser beam; the optical equalizing means being positioned to equalize the laser beam before the laser beam strikes a thermoluminescent phosphor positioned by the phosphor positioning means. The thermoluminescent phosphor reading apparatus of claim 2 further characterized in that the laser beam power detection means comprises a fast pyroelectric laser detector and a beam interruption means for periodically allowing at least a portion of the laser beam to strike upon the pyroelectric laser detector. The thermoluminescent phosphor reading apparatus of claim 2 further characterized in that the laser beam power detection means comprises a fast photoconductive laser detector. The thermoluminescent phosphor reading apparatus of claim 2 further characterized in that the laser beam optical equalizing means comprises a lens means for dispersing the laser beam into and through an adjacent optical channel having reflective channel walls. The thermoluminescent phosphor reading apparatus of claim 2 further characterized by a focusing reflector means for focusing the laser beam onto the thermoluminescent phosphor. The thermoluminescent phosphor reading apparatus of claim 2 further characterized by a focusing lens means for focusing the laser beam onto the thermoluminescent phosphor. The thermoluminescent phosphor reading apparatus of claim 2 further characterized in that the optical equalizing means comprises an optic fiber light guide. A method for rapidly reading a thermoluminescent phosphor to accurately determine the amount of ionizing radiation to which the phosphor has been exposed, emitting a laser beam from a laser beam source means; controlling exposure of the thermoluminescent phosphor to the laser beam so that the phosphor is heated for a predetermined amount of time; and measuring the luminescent emission occuring as a result of exposure of the thermoluminescent phosphor to the laser beam, characterized by measuring the power of the laser beam; controlling the laser beam source means to maintain a desired nearly constant laser beam power level. The method of claim 9 further characterized by the phosphor being heated for a predetermined period of time in the range between 0.1-500 milliseconds. The method of claim 9 characterized by the step of equalizing the laser beam with an optical equalizer means to produce an equalized laser beam having approximately uniform power density. The thermoluminescent phosphor reading apparatus of claim 1 characterized in that the laser power detection means is adapted to measure the power of the laser beam repeatedly during a phosphor exposure period. The thermoluminescent phosphor reading apparatus of claim 1 characterized by a beam splitter (18) for producing a detector leaser beam (21) which is detected by the laser beam power detection means (20). The thermoluminescent phosphor reading apparatus of claim 13 characterized by a beam interruption means (23) for interrupting the detector laser beam.

Inventors: BRAUNLICH, PETER F., TETZLAFF, WOLFGANG
Applicants: INTERNATIONAL SENSOR TECHNOLOGY, INC.
Application year: 1985
Main classification: G01T1
IPC codes: G01T1