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A method for reducing energy dependence of thermoluminescence dosimeter response by means of filters
Iurcntufioual Joumul qf Applied Rodrarim & lsoroprs. Vol. 31. pp. 448 to 449 0 Peraamon Press Ltd 1980. Printed in Great Britain 0020.708X/80/0701-0448M2.0010
A Method for Reducing Energy Dependence of Thermoluminescence Dosimeter Response by Means of Filters V. N. BAPAT* Area Materials Division. lnstituto de Energia Atomica. %o Paulo. Brasil (Received 16 Ocrober 1978; in revised form 27 September 1979)
Lms work describks the application of the method of partial surface shielding for reducing the energy dependence of the X-ray and y-ray response of a dosimeter containing a CaSO,: Dy thermoluminescent phosphor mixed with KCI. in pellet form. Results are given of approximate computation of filter combinations that accomplish this aim. and of experimental verifications. Incorporation of the described filter combination makes it possible to use this relatively sensitive dosimeter for environmental radiation monitoring. A similar approach could be applied to any type of dosimeter in the form of a thin pellet or wafer.
Introduction
(TL) dosimeters are becoming increasingly popular for use in radiation monitoring.“’ In environmental and personnel monitoring a wide range of unknown radiation energy spectra is encountered. Thus. radiation dosimeters with energy independent *response relative to air are preferred. where response is defined as glow-peak height. With the exception of such low atomic number phosphors as lithium fluoride. which gives a nearly energy independent response. most of the other TL phosphors. although comparatively more sensitive than lithium fluoride. show highly dependent response characteristics!2’ Various filters have been utilized in an attempt to reduce the energy dependence of the response of personnel dosimeters. A composite filter combination has been tried to obtain an energy independent response for photographic film dosimeters.‘3’ A composite dosimeter consisting of a heavily filtered. highly sensitive photographic film in combination with an unfiltered low sensitive film has also been used.(4’ Filtration over a portion of the surface of a phosphor was used with silver activated phosphate glass dosimeters.‘5’ More recently. a combination of two dosimeter films using the same phosphor has been used to produce an essentially energy independent response.@’ This work describes the application of the method of partial surface shielding to reduce the energy dependence of the response of a TL dosimeter. THERMOLUMINESCENCE
* Permanent address: Health Physics Division, Bhabha Atomic Research Centre, Bombay 400 085, India.
Principle
If a TL dosimeter is exposed to a beam of X- or y-radiation in such a way that a portion of the dosimeter area is exposed to unfiltered radiation while the remainder is covered by an appropriate filter. then the two areas of the dosimeter will respond differently. When the dosimeter is read. the TL output will represent the sum of the TL outputs from the two areas. It may be expected that with proper combination of open area, filter material and filter thickness. the response of the dosimeter may be made to be energy independent. A simple calculation of the dosimeter output under these conditions can be done by using the separate responses of the two areas and adding them in proportion to the area represented by each. Let the response of the dosimeter to the unfiltered radiation be RO. the response under the filter R,. and the fraction of the dosimeter area not covered by the filter f; then the TL output from the uncovered portion of the dosimeter will be proportional to fx& The response of the portion of the dosimeter covered by the filter will be proportional to (I - f)R, = (1 - f)R, exp(-G).x) where p(E) is the mass attenuation coefficient of the filter material as a function of photon energy E. and .Y is the filter thickness.“’ Toral response. R,,,,,. of the dosimeter, will be given by R ,0,51 = cf&)
+ (I - f)R,exp(-NW
The material and the thickness of the filter used in the case of high Z materials. such as CaSO,. normally causes the TL response to drop to negligible values below 50 keV. The photons below this energy, therefore. will contribute only to the TL output of the open portion of the dosimeter. A suitable value for J and a suitable filter material and thickness can then be selected in the following way: The fraction f is adjusted so that the dosimeter gives unity response with respect to its response to 6oCo y-irradiation. below 50 keV (i.e. below the filter cutoff energy). Based on this value off. the material and thickness of the filter is chosen to give approximately the same response as above in the remaining energy region as well. The energy dependence of the response of various TL dosimeters (R,) has been calculated theoretically by BA~SI et a/.‘*’ The energy dependence of the response of the dosimeter under various filters (R,) can be calculated approximately if one knows the unfiltered response of the dosimeter and the mass attenuation coefficient of the filter material.“’ Knowing these two responses one can then calculate the net response of the dosimeter under an otherwise identical filter with a central hole. These calculations can only give a guideline for the selection of the filter material and geometry. The final choice of the geometry must be based on the actual experimental determination of the dosimeter response. Experimental
Verification
The TL dosimeter used in this experiment consists of a cold-messed &let of 6 mm dia. and 1.5 mm thickness, consistin’g of a m’ixture of CaSO,: Dy (0.1% Dy,Oa by weight) and KCI in the ratio of 1:2. In normal field use this dosimiter is kept in a Perspex TLD badge with a wall thickness of 3mm. Filter studies were also carried out with similar Perspex covering as the same TLD badge was to be 448
449
Technical Note
I
01 001
I
I111111 IO
01
Photon energy(MeV)
FIG. 1. Calculated and experimental response of the CaSO,:Dy-KCI pellet dosimeter under various conditions of filtration. (a’) Response under Perspex 3 mm thick (exp.: x ). (b) Response under Perspex and I mm lead (talc.:.). (b’) Response under Perspex and 1mm lead (exp.: x). (c) Response under Perspex and I mm lead with 2 mm dia. hole (calc.:~). (c’) Response under Perspex and I mm lead with 2 mm dia. hole (exp.: x ). In all the above cases the response is normalized with respect to the unfiltered dosimeter response to 6oCo y-radiation. Inset: (L) I mm thick lead filter with a central hole of 2 mm dia. (P) 6 mm thick Perspex disc with a central depression 3 mm deep and 6 mm in dia. for the dosimeter. (D) Dosimeter pellet of CaSO,: Dy-KCI of 6 mm dia. and 1.5 mm thick.
used in the field. In this study a total of 25 dosimeters were used at a time. These dosimeters were arranged in a grid pattern of 5 x 5 cm, with dosimeter separation of 1cm, to achieve a uniform irradiation under the X-ray beam. The dosimeters are placed in the depressions of 6 mm dia. and 3 mm deep, in a Perspex block of 6 mm thickness. Filters with proper diameter holes are arranged on the other side of the Perspex block so that the holes match with the dosimeters. The dosimeters are exposed to X-rays of varying energy from “Stabilipan-300” (Siemens), with filters facing the X-ray beam. The cross section of the arrangment for one of the dosimeters is shown in the inset in Fig. 1. The response of the dosimeter pellet under 3mm of Perspex was determined experimentally and is used as R,,. Response of the dosimeter pellet under a I mm thick lead filter in addition to the above 3 mm Perspex and under I mm thick lead filter with a central hole of 2 mm dia. in combination with the above 3 mm Perspex. are calculated as described in the preceding section. The calculated responses together with the response of the dosimeter under the 3 mm Perspex filter are shown in the figure. The points around 100 keV lying away from the calculated response curve for 1mm lead filter correspond to the K edge. Experimental values of the response of the dosimeter under the above two filter combinations are given in the figure. The experimental values are the average of 25 dosimeter readings at each energy for each filter, and the standard deviation values for each set of readings are shown in the figure. Conclosiom The filter discribed here can be used to reduce the energy dependence of the response for any dosimeter containing a phosphor in the form of a pellet or a thin wafer. The same system can be modified to be. used with dosimeters of any fixed shape. The filter material and geometry required for a *.1.1.31/7-_o
particular dosimeter can be determined by performing the approximate calculations. Final fine adjustments of the filter geometry for optimum response characteristics can be achieved by slight changes in the perspex and metal filter thicknesses. With such a filter combination, sensitive high atomic number TL phosphors can be used for environmental dosimetry. If the metallit filter with the hole is held in close proximity to the dosimeter pellet, the directional dependence of the dosimeter with this filter combination will be minimum. Detailed study on this aspect is in progfeSS.
Acknowledgements-The author is grateful to all his colleagues at the Institute for their kind co-operation and to Dr Adelino Joti Pereira and his staff at the A.C. Camargo Hospital for their co-operation in using the X-ray facility. The author wishes to express his thanks to Dr Pieroni, Superintendent of the Institute, for providing an opportunity to work at this Institute.
Refereoces I. ATT~XF. H. Hlth Phys. 22. 282 (1972). 2. BECKERK. Solid Slate Dosimetry. p. 46. CRC Press. Cleveland, Ohio (1973). 3. STORME. and SHLAER_ S. Hlth Phys. 11, I127 (1965). 4. ALL~~YA. J. Radio/. Electrol. 36, 249 (I 955). 5. FOWLERJ. F. and ATTIX F. H. Radiation Dosimetry, 2nd edn. Vol. 2 (Edited by ATTIXF. H. and ROACH W. C.). Academic Press, New York (1966). 6. IGA K., YAMASHITAT.. TAKENAGAM., YA~UNO‘Y.. OONI~HIE. and IKEDAN. Hlth Phys. 33,605 (1977). 7. JOHNSH. E. and CUNNINGHAM J. R. Physics of Radiology. Tables A9-I 3. Thomas, Chicago, Illinois (1971). 8. B-1 P., BUSUOLIG. and RIMONDI0.. Inr. J. appl.’ Radiar. Isotopes 27, 29 1 (1976).
A Method for Reducing Energy Dependence of Thermoluminescence Dosimeter Response by Means of Filters V. N. BAPAT* Area Materials Division. lnstituto de Energia Atomica. %o Paulo. Brasil (Received 16 Ocrober 1978; in revised form 27 September 1979)
Lms work describks the application of the method of partial surface shielding for reducing the energy dependence of the X-ray and y-ray response of a dosimeter containing a CaSO,: Dy thermoluminescent phosphor mixed with KCI. in pellet form. Results are given of approximate computation of filter combinations that accomplish this aim. and of experimental verifications. Incorporation of the described filter combination makes it possible to use this relatively sensitive dosimeter for environmental radiation monitoring. A similar approach could be applied to any type of dosimeter in the form of a thin pellet or wafer.
Introduction
(TL) dosimeters are becoming increasingly popular for use in radiation monitoring.“’ In environmental and personnel monitoring a wide range of unknown radiation energy spectra is encountered. Thus. radiation dosimeters with energy independent *response relative to air are preferred. where response is defined as glow-peak height. With the exception of such low atomic number phosphors as lithium fluoride. which gives a nearly energy independent response. most of the other TL phosphors. although comparatively more sensitive than lithium fluoride. show highly dependent response characteristics!2’ Various filters have been utilized in an attempt to reduce the energy dependence of the response of personnel dosimeters. A composite filter combination has been tried to obtain an energy independent response for photographic film dosimeters.‘3’ A composite dosimeter consisting of a heavily filtered. highly sensitive photographic film in combination with an unfiltered low sensitive film has also been used.(4’ Filtration over a portion of the surface of a phosphor was used with silver activated phosphate glass dosimeters.‘5’ More recently. a combination of two dosimeter films using the same phosphor has been used to produce an essentially energy independent response.@’ This work describes the application of the method of partial surface shielding to reduce the energy dependence of the response of a TL dosimeter. THERMOLUMINESCENCE
* Permanent address: Health Physics Division, Bhabha Atomic Research Centre, Bombay 400 085, India.
Principle
If a TL dosimeter is exposed to a beam of X- or y-radiation in such a way that a portion of the dosimeter area is exposed to unfiltered radiation while the remainder is covered by an appropriate filter. then the two areas of the dosimeter will respond differently. When the dosimeter is read. the TL output will represent the sum of the TL outputs from the two areas. It may be expected that with proper combination of open area, filter material and filter thickness. the response of the dosimeter may be made to be energy independent. A simple calculation of the dosimeter output under these conditions can be done by using the separate responses of the two areas and adding them in proportion to the area represented by each. Let the response of the dosimeter to the unfiltered radiation be RO. the response under the filter R,. and the fraction of the dosimeter area not covered by the filter f; then the TL output from the uncovered portion of the dosimeter will be proportional to fx& The response of the portion of the dosimeter covered by the filter will be proportional to (I - f)R, = (1 - f)R, exp(-G).x) where p(E) is the mass attenuation coefficient of the filter material as a function of photon energy E. and .Y is the filter thickness.“’ Toral response. R,,,,,. of the dosimeter, will be given by R ,0,51 = cf&)
+ (I - f)R,exp(-NW
The material and the thickness of the filter used in the case of high Z materials. such as CaSO,. normally causes the TL response to drop to negligible values below 50 keV. The photons below this energy, therefore. will contribute only to the TL output of the open portion of the dosimeter. A suitable value for J and a suitable filter material and thickness can then be selected in the following way: The fraction f is adjusted so that the dosimeter gives unity response with respect to its response to 6oCo y-irradiation. below 50 keV (i.e. below the filter cutoff energy). Based on this value off. the material and thickness of the filter is chosen to give approximately the same response as above in the remaining energy region as well. The energy dependence of the response of various TL dosimeters (R,) has been calculated theoretically by BA~SI et a/.‘*’ The energy dependence of the response of the dosimeter under various filters (R,) can be calculated approximately if one knows the unfiltered response of the dosimeter and the mass attenuation coefficient of the filter material.“’ Knowing these two responses one can then calculate the net response of the dosimeter under an otherwise identical filter with a central hole. These calculations can only give a guideline for the selection of the filter material and geometry. The final choice of the geometry must be based on the actual experimental determination of the dosimeter response. Experimental
Verification
The TL dosimeter used in this experiment consists of a cold-messed &let of 6 mm dia. and 1.5 mm thickness, consistin’g of a m’ixture of CaSO,: Dy (0.1% Dy,Oa by weight) and KCI in the ratio of 1:2. In normal field use this dosimiter is kept in a Perspex TLD badge with a wall thickness of 3mm. Filter studies were also carried out with similar Perspex covering as the same TLD badge was to be 448
449
Technical Note
I
01 001
I
I111111 IO
01
Photon energy(MeV)
FIG. 1. Calculated and experimental response of the CaSO,:Dy-KCI pellet dosimeter under various conditions of filtration. (a’) Response under Perspex 3 mm thick (exp.: x ). (b) Response under Perspex and I mm lead (talc.:.). (b’) Response under Perspex and 1mm lead (exp.: x). (c) Response under Perspex and I mm lead with 2 mm dia. hole (calc.:~). (c’) Response under Perspex and I mm lead with 2 mm dia. hole (exp.: x ). In all the above cases the response is normalized with respect to the unfiltered dosimeter response to 6oCo y-radiation. Inset: (L) I mm thick lead filter with a central hole of 2 mm dia. (P) 6 mm thick Perspex disc with a central depression 3 mm deep and 6 mm in dia. for the dosimeter. (D) Dosimeter pellet of CaSO,: Dy-KCI of 6 mm dia. and 1.5 mm thick.
used in the field. In this study a total of 25 dosimeters were used at a time. These dosimeters were arranged in a grid pattern of 5 x 5 cm, with dosimeter separation of 1cm, to achieve a uniform irradiation under the X-ray beam. The dosimeters are placed in the depressions of 6 mm dia. and 3 mm deep, in a Perspex block of 6 mm thickness. Filters with proper diameter holes are arranged on the other side of the Perspex block so that the holes match with the dosimeters. The dosimeters are exposed to X-rays of varying energy from “Stabilipan-300” (Siemens), with filters facing the X-ray beam. The cross section of the arrangment for one of the dosimeters is shown in the inset in Fig. 1. The response of the dosimeter pellet under 3mm of Perspex was determined experimentally and is used as R,,. Response of the dosimeter pellet under a I mm thick lead filter in addition to the above 3 mm Perspex and under I mm thick lead filter with a central hole of 2 mm dia. in combination with the above 3 mm Perspex. are calculated as described in the preceding section. The calculated responses together with the response of the dosimeter under the 3 mm Perspex filter are shown in the figure. The points around 100 keV lying away from the calculated response curve for 1mm lead filter correspond to the K edge. Experimental values of the response of the dosimeter under the above two filter combinations are given in the figure. The experimental values are the average of 25 dosimeter readings at each energy for each filter, and the standard deviation values for each set of readings are shown in the figure. Conclosiom The filter discribed here can be used to reduce the energy dependence of the response for any dosimeter containing a phosphor in the form of a pellet or a thin wafer. The same system can be modified to be. used with dosimeters of any fixed shape. The filter material and geometry required for a *.1.1.31/7-_o
particular dosimeter can be determined by performing the approximate calculations. Final fine adjustments of the filter geometry for optimum response characteristics can be achieved by slight changes in the perspex and metal filter thicknesses. With such a filter combination, sensitive high atomic number TL phosphors can be used for environmental dosimetry. If the metallit filter with the hole is held in close proximity to the dosimeter pellet, the directional dependence of the dosimeter with this filter combination will be minimum. Detailed study on this aspect is in progfeSS.
Acknowledgements-The author is grateful to all his colleagues at the Institute for their kind co-operation and to Dr Adelino Joti Pereira and his staff at the A.C. Camargo Hospital for their co-operation in using the X-ray facility. The author wishes to express his thanks to Dr Pieroni, Superintendent of the Institute, for providing an opportunity to work at this Institute.
Refereoces I. ATT~XF. H. Hlth Phys. 22. 282 (1972). 2. BECKERK. Solid Slate Dosimetry. p. 46. CRC Press. Cleveland, Ohio (1973). 3. STORME. and SHLAER_ S. Hlth Phys. 11, I127 (1965). 4. ALL~~YA. J. Radio/. Electrol. 36, 249 (I 955). 5. FOWLERJ. F. and ATTIX F. H. Radiation Dosimetry, 2nd edn. Vol. 2 (Edited by ATTIXF. H. and ROACH W. C.). Academic Press, New York (1966). 6. IGA K., YAMASHITAT.. TAKENAGAM., YA~UNO‘Y.. OONI~HIE. and IKEDAN. Hlth Phys. 33,605 (1977). 7. JOHNSH. E. and CUNNINGHAM J. R. Physics of Radiology. Tables A9-I 3. Thomas, Chicago, Illinois (1971). 8. B-1 P., BUSUOLIG. and RIMONDI0.. Inr. J. appl.’ Radiar. Isotopes 27, 29 1 (1976).