Thermoluminescent Dosimeter Encoding And Readout Method

Abstract

A system and method for encoding personnel or other type of identification data from a computer center on a holder for dosimetric material, and automatically removing from the holder the material and reading out radiation dosage thereon while decoding the identification data on the holder, and transmitting the readout and decoded data to the computer center. The encoding is accomplished in such a manner as to substantially eliminate accidental or intentional errors, while the dosimetric material is removed by vacuum means and readout in a hot oxygen-free gas, such as nitrogen.

Description

BACKGROUND OF THE INVENTION

The invention described herein was made in the course of, or under, Contract No. W-7405-ENG-48, with the U.S. Atomic Energy Commission.

This invention relates to devices for the control and readout of dosimeters, and more particularly to a system and method for encoding with desired data, and after a desired period of exposure automatically reading out the dosage and deciphering the encoded data information for determining the radiation to which the dosimeter has been exposed. Dosimeters are useful and necessary in the measurement of radiation to which a body has been exposed, the body being either human or otherwise. As known, thermoluminescent material has been widely used in radiation sensitivity-measuring devices, and many prior art devices have been developed for the assembly and readout of this material. Primarily, hand processing has constituted the handling of these devices, thereby requiring a substantial amount of time, as well as being subject to human error and accuracy.

SUMMARY OF THE INVENTION

The invention provides a method composed of a system which utilizes a dosimeter encoded and controlled automatically, thereby substantially reducing costs in the replacement and dosage determination over the systems previously known. The inventive procedure for encoding and decoding of the dosimeter and comparison of the dosimetric material readout therewith substantially eliminates accidental or intentional errors. The dosimetric material is maintained in the dosimeter in a waterproof manner and removed from the dosimeter by vacuum means with the readout of the material being accomplished in a hot oxygen-free gas, such as nitrogen. The dosimeter provided by the system can be placed at any location for selected periods of time to provide a radiation readout for these areas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of one side of the dosimeter illustrating the identification indicia thereon;

FIG. 2 is a view of the opposite side of the FIG. 1 dosimeter showing the recesses for holding the dosimetric material and illustrating a selected identification sequence by the identification indicia; and

FIG. 3 is a diagrammatic view illustrating the inventive system utilizing the dosimeter of FIG. 1.

DESCRIPTION OF THE INVENTION

Broadly the invention produces a dosimeter encoding and readout method for personnel or other applications requiring the periodic measurement of accumulated radiation dosage. The dosimeter comprises a holder or disc having recesses therein for retaining dosimetric material and with identification indicia extending around the periphery thereof, and is automatically processed to first encode a desired identification in the indicia in accordance with data from an information storage system and position the dosimetric material in the recesses. After a predetermined period of exposure of the dosimeter, the continuing sequence of operations is accomplished by removing the material for readout while comparing the readout of the material with a decipher of the peripheral identification indicia on the dosimeter and recording same in the information storage system, and returning the dosimeter and dosimetric material for reuse.

Referring now to FIGS. 1 and 2, the embodiment of the dosimeter utilized for description of the inventive system comprises a holder or small disc 10 made of lightweight material, such as plastic (for example 13/8-inch diameter by 1/8-inch thick), with three spaced recessed areas 11 in the central region of one side (see FIG. 2) for holding a dosimetric substance 12 (only one shown), such as a thermoluminescent material detector (TLD), which is retained therein by a disposable cover or filter 13 of plastic or other suitable material, only two shown, constructed to be retained in the recessed areas 11 by a protruding lip or collar formed therein. As shown, the recessed areas 11 are each provided with a central countersink 14, the TLD 12 being positioned in the countersink 14 while the cover is secured in recess 11. The cover 13 may be 0.004 to 0.010-inch thick cellulose acetate and fits with sufficient tightness that removing is difficult and results in visual damage if tampered with. The type of TLD material utilized is dependent on the types of particles to be detected, the sensitivity desired, and the expected flux and spectrum. The TLD may constitute chips, power or a mixture with binder, and may for example be CaF.sub.2, natural isotopic abundance LiF, and LiF depleted in Li-6. The thermoluminescent materials utilized do not constitute part of this invention and thus need not be described in detail, but may, for example, be of the type described in U.S. Pat. No. 3,413,235, Donald E. Jones et al., issued Nov. 26, 1968. The TLD chips 12, for example, may have a thickness of 0.020-0.040 inch. Located about the periphery of the disc 10 is a region of identification indicia consisting in this embodiment of two rows of countersinks or cavities 15, each row consisting, in this embodiment, of 25 countersinks 15 (see FIG. 1). The disc 10 is encoded as shown in FIG. 2 by forcing the bottom out of selected countersinks 15 of FIG. 1 to form apertures 16 extending through the disc, this being done by encoding apparatus described hereinafter. An encoded disc consists of approximately 25 apertures 16 therethrough, each aperture 16 occupying one or two selected positions for each corresponding pair of countersinks 15 of each row, either radially closer or farther from the center of the disc. A selection of 25 such positions presents a possibility of 2.sup.25 of combinations to give a capacity of approximately 33.5 million different discs. Hence, the system has the potential to identify individuals of very large groups of people. The specific encoding system utilized in this embodiment is a BCD (Binary Code Decimal) encoding system which is able to detect substantially any error due to accidental or intentional punching or blocking of the holes, thereby substantially reducing the probability that both blocking and perforating holes in the same radial column will produce an undetected error. The identification coding will be described in greater detail hereinafter. Positioned radially inward from the rows of countersinks 15 is a personnel or other visual identification number and/or name, as indicated at 17 (see FIG. 1), which is imprinted by a machine or otherwise as described hereinafter. An aperture 18 is utilized in the processing the disc 10. The disc 10 as shown in FIGS. 1 and 2, except for the apertures 16 may be formed, for example, by injection molding, or other conventional methods. As shown in FIGS. 1 and 2 the basic encoding arrangement consists of six digits wherein the basic numbering system is a 1, 2, 4, 8 BCD code with the same information on the inner and outer rows of apertures 16. By using opposite parity for the two rows; i.e., an inner row blank is equivalent to an outer row aperture, the disc is made resistant to most errors or tampering.

Referring now to FIG. 3 wherein the overall inventive system is illustrated diagrammatically, it should be noted that the single lines indicate initial distribution while the double lines indicate recurring distribution. Also, two identically encoded dosimeters or discs 10 of different colors are actually utilized in the system, one being in use while the other is being readout and reassembled for period exchange with the one in use, but only the handling of one of the identical discs will be described hereinafter. While two colors of discs are utilized in the inventive system for the employees normally involved in activities which may expose them to radiation, other colors, if desired, could be used for visitors, etc., or all may be the same or different colors.

As shown in FIG. 3, a dosimeter disc or badge is formed by injection molding or the like as indicated at 20 and is placed as indicated by arrow 21 in a branding typewriter 22 which imprints or labels on the disc the desired employee information 14 (see FIG. 1) in accordance with the information received as indicated by arrow 23 from a card-reading section 24 of an IBM summary punch 25, the reading thereof being determined and provided on section 24 by a record keeping and data analysis computer center 26. The thus labeled disc or badge is then moved as indicated by arrow 27 into a solenoid-coded badge punch 28, described in detail with respect to FIG. 4. The employee information from computer center 26 forwarded via arrow 23 to the branding typewriter 22 is also directed via arrow 29 to an electronic Decimal to Binary/BCD circuit 30 wherein the employee information is converted to BCD information and the output signal is transmitted as indicated by arrow 31 to punch 28 wherein the various solenoids are actuated which position small punches or cutters for driving same through certain of the countersinks 12 of the disc 10 to produce the apertures 13 in accordance with the Binary code information thereby providing the identification code on the disc as shown in FIGS. 1 and 2. The thus labeled and identification-coded (ID) disc or badge is then sent as indicated by arrow 32 to decoder 33 for ID code verification which can be accomplished either visually or by a readout mechanism similar to that utilized and described hereinafter.

The ID verified disc or badge is forwarded via arrow 34 to a TLD loading station 35. At station 35 a TLD insertion operation 36 and the color-coded cover insertion operation 37 in the disc 10 are accomplished manually, but can be automated if desired. The TLD and cover insertion operations are accomplished by inserting a TLD of desired type into each of the three countersinks 11' of the recessed areas 11 and the cover or filter, constructed of plastic or other suitable material is inserted over each TLD by pressure application on the cover so as to snap the cover over the protruding lip portion of the recesses 11 which secures the TLD in a watertight condition. After the TLD and cover insertion operations 36 and 37 are completed, the disc or badge is visually inspected at station 35 to insure that the covers are properly inserted after which the unexposed badge is forwarded via arrow 38 to an employee indicated at 39 or another point of use. If an employee previously was utilizing a disc of the hereindescribed type the disc would be interchanged for the one previously exposed and the exposed badge or disc forwarded as at arrow 40 to a TLD reader system 41. The badge or disc, depending on the anticipated radiation dosage, may be carried by the employee or located at a desired point of use for varying time period of, for example, up to a year in cases where normal radiation dosage is substantially nil.

The TLD reader system 41 will be described in greater detail hereinbelow but functionally is composed of three separate sections or operations comprising a identification code readout section 42, a TLD cover removal section 43, and a TLD removal and dosage readout section 44. While each of the three TLD's in the disc is uncovered and readout in the same manner, only one sequence of operations will be described. As will be more readily seen from the description of FIG. 5, the disc 10 is selectively positioned on a rotatable table and the various operations indicated by sections 42, 43 and 44 of the TLD reader system 41 are accomplished at different locations as the table is controllably rotated from one to another of these sections. At section 42 the coded identification data on the periphery of the disc (apertures 16) is deciphered or read by mechanism described hereinafter. At section 43, the disc is positioned such that a heat source is applied to one of the covers in a recess 11 and upon heating same to a desired temperature a vacuum probe is placed over the cover and is removed from recess 11 thereby leaving the TLD thereunder exposed. At section 44 the TLD is removed from the countersink 11' of recess 11 by a vacuum probe means and is transferred into a hot nitrogen readout device which functions to heat the TLD and readout the radiation dosage thereof, and thereafter, return the TLD to a carriage which returns same to an exchange badge storage station 45 as indicated by arrow 46 for reuse and the disc is returned to the storage station 45 for future use as indicated by arrow 46. The readout information from sections 42 and 44 of the TLD reader system 41 is transmitted, as indicated by arrow 48 to an electronic Binary/BCD to Decimal circuit 49 which converts the Binary information to decimal such that the output signal therefrom is transmitted via arrow 50 to a card-punching section 51 of the IBM summary punch 25 for transmittal therefrom to the computer center 26 as indicated by arrow 52. In addition, the output signal from circuit 49 is fed as indicated by arrow 53 to a verification device 54 which verifies the proper TLD heating and ID readout information and upon an improper verification sends a signal, indicated by arrow 55 to the TLD reader system which stops the operation thereof and sounds an alarm or other signal to show an improper reading or an excessive radiation dosage, this of course being done prior to the forwarding of the TLD and disc to the storage station 45. Thus, the TLD and/or disc ID information can be rechecked to verify or disprove the previous readout.

At a selected point in time the computer center 26 forwards a request to the storage station 45, as indicated by arrow 56, that specified badges or discs should be changed and checked for radiation dosage. The selected discs or badges are ID code verified as indicated by arrow 57 from decoder 33 and upon verification TLD's and the selected discs are forwarded, as indicated by arrow 58, to the TLD loading station 35 whereafter the above described sequence for those discs is repeated. The readout information from section 44 of reader system 41 is also transmitted to a glow curve display 59 via line 60 which may be a count rate meter and recorder or a multichannel scaler.

To simplify the task of preparing the identification code for the large number of discs that were necessary, the automatic, solenoid actuated, encoding punch 28 was designed and built. While the details of the encoding punch 28 does not constitute part of this invention it generally constitutes a solenoid positioned punch assembly capable of simultaneously forming the apertures 16 through the remainder of the disc material forming the bottom of countersinks 15. An embodiment of the encoding punch is described and illustrated in detail in UCRL-50007-69-2 "Hazards Control Progress Report No. 34 (May-Aug. 1969)," available from the Lawrence Radiation Laboratory, University of California, Livermore, Cal.

To facilitate employee recognition of his badge, each disc 10 is labeled with his name and employee number indicated in FIG. 1 at 18. This was done by a branding typewriter indicated at 22 in FIG. 3, with a rotary indexing table. The rotary labeling allows both name and number to be typed on a single line without line spacing and simplifies typewriter input from the computer card 24.

The computer center 26 of FIG. 3 is programmed to provide a system of scheduling, cataloging, evaluating dose and maintaining radiation exposure records of employees, visitors, etc.

A major component of the overall system illustrated in FIG. 3 is the TLD reader system 41 that takes a stack of TLD discs 10 and (1) decodes the employee number 17 of each disc, (2) removes the plastic retaining cover 13 from the disc to be read, (3) lifts the TLD 12 from the disc and "reads" it out, and (4) transmits to computer cards 51 the identification, dose information, and the type of TLD read. A series of self-checks and automatic stops are incorporated into the reader system 4. These include verification of the correct employee number decoding, presence of the TLD in the readout chamber when reading, and proper heating of the TLD.

During the initial disc preparation, as described above, the binary coded decimal (BCD) employee number is placed upon the disc in a series of hole positions 16 around the disc periphery (apertures 16 through countersinks 16). The numbering system is redundant to allow for self-checking. Decoding is accomplished by rotating a pair of diodes over the backlighted disc and comparing the two signals. Plugged holes or punctured discs give an erroneous signal that stops the reader, lights an employee number void lamp, and gives an audible alarm. The operator may then remove the badge and determine the proper number.

After the cover 13 has been removed, a TLD 12 is removed from one of the recessed areas 11 of disc 10 by a vacuum applied to a hypodermic tubing that is pneumatically brought into contact with the TLD. The tubing tip with the TLD held thereon is then raised into a hot nitrogen gas stream to heat it, "reading out" the thermoluminescence. Occasionally, a rough surface on a TLD allows it to fall before the reading is completed. A temperature sensor in the vacuum line detects the missing TLD, alarms and stops the reader.

With a hot nitrogen gas reader of the type, for example, described in an article Hot Nitrogen Gas for Heating Thermoluminescent Dosimeters, published in the Proceedings of the Second International Conference on Luminescence Dosimetry, Gatlinburg, Tenn., Sept. 23-26, 1968, the combined reading of chamber background and photomultiplier tube (PMT) dark current is so low, that the presence of a TLD in the chamber gives a small but significant reading even after annealing. An adjustable electronic lower limit detector on the digital readout system is used to verify the presence and proper heating of a TLD in the reading chamber. If the preset lower limit is not reached, the reading sequence is interrupted, an alarm sounds and the low-dose lamp is lit.

In addition to the self-checks above, the reader system 41 also has a high-dose alarm and stop system. This is an adjustable upper limit on the digital readout similar to that used for detecting low readings.

The stopping of the reading sequence in each of the above cases is done to allow the operator to check the system and to remove the disc and TLD together for further examination if desired. Stopping is necessary because the TLD is automatically deposited into a receptacle for annealing and reuse after the reading is satisfactorily completed with the information punched upon the computer card 51. As only one disc and TLD are in the working part of the reader at a given time, there is no difficulty in associating a TLD with its disc.

While the details of the apparatus constituting the TLD reader system 41 does not constitute part of this invention, a general description thereof is set forth hereafter to provide a better understanding of the operation of sections 42, 43 and 44 of the system 41. A detailed description and illustration of an embodiment of the reader system 41 is set forth in the above referenced report UCRL-5007-69-2.

Reading a TLD disc 10 requires five separate operations that are performed at five stations in the reader system 41: (1) loading the badge into the reader, (2) decoding the employee number, (3) removing the TLD cover, (4) removing and reading the TLD, and (5) unloading the badge. It should be noted that the first and fifth station operation is not set forth separately in the FIG. 3 diagrammatic showing but would constitute arrows 40 and 48, respectively, therein.

The embodiment of the apparatus of the reader system 41 consists of two basic interrelated mechanisms: a rotating table that holds the TLD disc and stops in sequence at each of the five positions or stations, and a programmer that controls the readout operations. The table, for example, is an aluminum disc 12 inches in diameter and 1/2-inch thick driver by a 4 r.p.m. synchronous motor. A slip clutch and detent mechanism allows the table to step through the five stations on command from the programmer. Near the rim of the table is a recessed holder to accept the disc 10. Pins in the holder fit into the disc slot 18 for alignment. The holder may be rotated to any one of three positions, 120.degree. apart, to select which one of the three TLDs 12 in the disc recessed areas 11 is to be read. On the same radius as the holder and 72.degree. back is a receptacle cup to catch the used cover 13 and the TLD 12 after readout.

The programmer, in the embodiment utilized, consists of 16 cams which operate pneumatic valves and electrical switches controlling the readout functions at the five stations. A 1 r.p.m. drive motor has a mechanical clutch to allow the cams to be returned to the start position in the event of malfunction stops the reader in midcycle. A "manual" and "auto" mode allow either a single reading cycle or continuous operation.

Chronologically, a complete readout cycle of the TLD reader system 41 is as follows:

Station One: Reader loading operation.

Eighty discs 10 are stacked in a tubular holder with the disc slots 18 aligned by guide pins to assure proper orientation. The holder is slipped into its mount above station one. As the table stops at station one a pneumatically operated pickup head rises through the opening and by vacuum pulls the first bottom disc down into the table recess or holder. Three spring-loaded pins hold up the remaining discs. The table then carries the disc around to the next position.

Station Two: Employee number decoding.

As the disc enters station two (section 42) a lamp beneath the disc is turned on. Light passing through the apertures 16 in the periphery of the disc is detected by two photo diodes rotated above the two circular rows of apertures (see FIG. 2). A reference wheel having 25 holes is synchronized with the diodes to insure correct reading of the number. If the number is correctly read it appears in decimal form via electronics 49 on a nixic display 54 and is also punched on the computer card 51.

Station Three: Cover removal.

The table then moves the disc 10 to station three (section 43) where positioned above the disc is an infrared lamp, such as a commercial 750-watt unit, mounted in an elliptical reflector that focuses the heat on a cover 13 in one of the recessed areas 11 of the disc. A tip is mounted on the reflector that is designed to fit into the recess 11. The heat lamp is turned on for a preselected time to soften and shrink the cover 13. Vacuum is applied to the tip and the cover is picked up and discarded into the receptacle cup by a puff of compressed air as the disc moves to the next position. An autotransformer allows adjustment of the temperature as the amount of heat required varies with the material and color of the cover.

Station Four: Readout of TLD.

The disc is then moved to station four (section 44) where the TLD 12 is removed from the disc 10 and is read out in a highly polished hemispherical cavity machined in a block of aluminum that is mounted facing a photomultiplier tube. A stream of heated nitrogen passes horizontally through the center of the cavity or chamber. A pneumatically controlled 0.080-inch O.D. hypodermic tube passes down through the chamber to the badge, and with vacuum picks up the TLD 12 from recess 11 and returns to bring the TLD directly into the gas stream for heating. The chamber surrounding the TLD gathers and focuses the light on the PM tube. The gas is heated by a commercial heater consisting of a 1/4-inch diameter tube containing a nichrome element. Gas temperature is controlled to .+-.1 percent by an SCR proportioning controller with a sensing thermocouple placed in the gas stream near the TLD readout point. The chamber is water cooled while the PM tube is temperature stabilized to .+-.0.5.degree. C. by mounting in a commercial thermoelectric cooler. An infrared absorbing filter and a shutter mechanism are utilized. As the TLD is being read, the disc moves on to Station Five and the receptacle moves into Station Four to receive the read out TLD which is lowered to the receptacle and dropped therein by removal of the vacuum thereon. The TL light output is digitized by a unique current-to-frequency (CTF) converter. The integration of light output results by counting the CTF pulses on a scaler, and this output (counts) is illustrated on the glow curve display 59 and is printed on the computer card 51 at the end of the cycle. A new computer card is automatically initiated for each disc reading.

Station Five: Badge unloading.

When the table is rotated to station five, a pneumatically operated ram lifts the badge up into a tubular holder identical to that at station one. Thus the discs are restocked ready to have another type of TLD, contained in a different recess 11 of disc 10, read if necessary. When the reading is complete, the table with its empty recess or holder rotates into station one and is reloaded in preparation for the next cycle. As the receptacle cup (holding the discarded cover 13 and read out TLD 12) passes from station five to station one, a protruding arm dumps the contents into a container, whereafter the cover is discarded and the TLD moved to storage point 45 for reuse.

It has thus been shown that the present invention provides a method and system for encoding identification data from a computer center on a holder for dosimetric material, and automatically removing from the holder the material and reading out radiation dosage thereon while decoding the identification data on the holder, and transmitting the readout and decoded data to the computer center, thereby substantially reducing the time and expense expended in the utilization of previously known personnel dosimetry systems.

Although particular embodiments of apparatus have been described or illustrated for operating the inventive system and carrying out the method thereof, modifications and changes will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications and changes as come within the spirit and scope of the invention.

Claims

What we claim is:

1. A method for encoding and decoding a thermoluminescent dosimeter and reading out the radiation dosage of the thermoluminescent material comprising the steps of: encoding desired identification data on a thermoluminescent material holder by at least punching apertures therein in accordance with a selected code, encoding the holder in accordance with decimal information from a previously punched card, providing the decimal information by a computerized information storage center through a card-reading mechanism, securing selected thermoluminescent material in at least one recess located in the holder by inserting a suitable cover means thereover, exposing the thermoluminescent material to radiation for a selected period of time, decoding the identification data on the material by utilizing means responsive to light passing through the apertures therein, removing the cover means securing the thermoluminescent material in the recess of the holder by heating the cover means causing same to shrink and applying a vacuum thereto, removing the thermoluminescent material from the holder, reading out the radiation dosage on the thermoluminescent material by utilizing hot oxygen-free gas, recording the decoded identification data and radiation dosage by electronically transmitting same in decimal form to a summary punch mechanism and punching the transmitted information in the card means, and transmitting the recorded decoded identification data and radiation dosage to the computerized information storage center.

2. The method defined in claim 1, wherein the step of encoding desired identification data on the holder is accomplished by punching the apertures in accordance with a binary code and imbedding on the holder other identification data, the binary code being defined by selected apertures being located in a plurality of rows on the holder.

3. The method defined in claim 1, wherein the step of decoding the identification data is accomplished by directing light through the apertures in the holder and electronically reading out the encoded material defined by the apertures.

4. The method defined in claim 1, wherein the step of removing the thermoluminescent material from the holder is accomplished by applying a vacuum means thereto.

5. The method defined in claim 1, wherein the step of reading out the radiation dosage on the thermoluminescent material is accomplished by positioning the material in a stream of heated nitrogen passing through a highly polished cavity mounted so as to face a photomultipher tube means, the cavity functioning to collect and focus light on the photomultipher tube means, thereby providing a radiation dosage readout from the material.