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A TLD method for the determination of X-ray quality and output in the diagnostic range
Technical notes
442
Results and Discussion Count rates for 20 mg samples of KCr(MIDA)2 of varying specific activities are shown in Fig. 1. The effect of using varying weights of samples of fixed spe.cific activity is detailed in Fig. 2 where the plot of count rate is seen to decrease in a non-linear fashion with increasing sample weight. A plot of specific activity against the reciprocal of the weight of the solid sample is in Fig. 3 and is seen to be linear at lower sample weights and to deviate only slightly from linearity at higher weights. This plot (which is analogous to that of Ross and YERICK (7)) affords a useful method of making quench corrections to specific activities measured using less than the standard 20 mg of sample. Sensitivity to quenching in the case under study •as large, probably due to similarity of ?'max for me sample (363 nm) and the wavelength (370 nm) of the radiation emitted by the scintillator P.P.O.
Acknowledgement--We thank Dr. D. J. Spedding for helpful discussions. S. D U T T A - C H A U D H U R I *
A. L. ODELLt
Urey Radiochemical Laboratories University of Auckland New Zealand
Reierences 1. SAITO K. et al. Bull. Chem. Soc. Japan 41, 384 (1968). 2. FUNT B. L. Nucleonics 14, 83 (1956). 3. WHITE C. G. and HELF S. Nucleonics 14, 46 (1956). 4. O'rr G. D. et al. Nucleonics 17, 106 (1959). 5. BIRKS J. B. In A n Introduction to liquid scintillation, issued by Koch-Light Laboratories Ltd., p. 20. 6. HERBERG R. J. Analyt. Chem. 32, 1468 (1960). 7. Ross H. R. and YER1CK R. E. Analyt. Chem. 35, 794 (1963). 8. WEYH J. A. and HAMM R. E. Inorg. Chem. 7, 2431 (1968). 9. BERSHET G. J. In Organic Syntheses Vol. X V I I I (Ed. FusoN R. C.), p. 56. John Wiley & Sons, Inc, New York (1938). *Present address: Catalyst Department, Planning and Development Division, FCI Ltd., Sindri, Bihar, India. t T o whom correspondence may be addressed.
International Journal of Applied Radiation and Isotopes, 1977, Vol. 28, pp. 442-444. Pergamon Press. Printed in Northern Ireland
A TLD Method for the Determination ot X-ray Quality and Output in the Diagnostic Range (Received 3 May 1976; in revised form 15 September 1976)
Summary A SIMPLE TLD method is described for the determination of HVT and output of diagnostic X-ray machines. Thermoluminescent Li2B407:Mn phosphor sealed in polythene bags and attached to four 0.7 mm thick aluminium sheets as a step wedge was used as a compact packet and exposed to various energies of X-rays. The TL output was measured and plotted against thickness of Al and the HVT determined. Comparisons made with the usual aluminium wedge and X-ray dosimeter data showed agreement within :t:5% in the range of 40-140 kV. The HVT-TL-dosimeter wedge is convenient for handling and suitable especially for making measurements remotely by mail. The International Commission on Radiological Protection (1) recommends for diagnostic X-ray machines a minimum total filtration of 1.5 mm Al up to 70 kV, 2 mm A1 up to 100 kV and 2.5 mm A1 above 100 kV. It also recommends that the table top exposure for fluoroscopic machines should be less than 5 R/min. Since there are many X-ray units in vogue without the adequate filtration, and in order to ensure radiation protection, a simple method to find the total filtration as well as output is needed. If the quality of the X-ray beam is known at any particular kV, the inherent filtration can be estimated. The conventional method of HVT determination using aluminium wedge and X-ray dosimeter is cumbersome although it gives accurate and on-thespot data. Several TLD methods (2-4) were reported in literature based on the ratio between the TL output of an energy dependent phosphor and an energy independent one. Most of these methods are suitable in the region of 50-100 keV where the ratio varies steeply with energy. The above system is not suitable for diagnostic region as the same TL ratio may represent two different HVT values thus causing confusion. This can be seen from the work of Busr~oNG eta/. (3) where the TL ratios in the HVT region 0.1-4 mm A1 are same as in the region 4 m m A l - l . 5 mm Cu. A wedge method using a simple integrating dosimeter as described here is
443
Technical notes
convenient and accurate. An energy independent TL dosimeter such as Li2B407:Mri is well suited for this purpose. The TL reader system used for the measurements is described elsewhere. (5) Thermoluminescent lithium borate activated with 0.2 mole atom percent Mn (0.23% by wt) was prepared in our laboratory and its sensitivity compared with commercial samples was reported earlier. (5) The energy dependence characters of the phosphor were found to be in agreement with the data of CrmISTENSEN(6~. A Siemens " T R I D O R O S 4" radiography and fluoroscopy combined unit having a total filtration of 2 mm A1 was used for irradiation. An ion-chamber type dosimeter with a charge measuring unit (Electronic Instruments Limited, London, U.K.) and an A1 wedge was used for HVT measurements. Victoreen condenser R-chambers which were calibrated against primary standards were employed for exposure calibration purposes. Li2B4OT:Mn dosimeters were calibrated using a 6°C0 teletherapy beard, and this calibration was assumed for the entire X-ray region. However, for output measurements a factor of 0.95 for energy dependence was used. The TL exposures for the HVT experiments were ensured to be within the linear region. Thermoluminescence was recorded 2 days after exposure to minimize the effect of its low temperature glow peak. The method adopted for HVT measurements is as follows. Firstly, the HVTs were determined at several kV settings using the X-ray dosimeter and aluminium wedge. About 80 mg of Li2B4OT:Mn
phosphor powder sealed in polythene bags of dimensions 1 c m x 1 cm x 0.5 mm was used as the TL dosimeter. Four 0.7 mm thick aluminium sheets of size 2 cm x 8 cm held by tape were arranged as a step wedge and the dosimeters were attached, one front and four behind the steps with an adhesive tape. These package units were exposed to beams of the Siemens diagnostic unit in air at a distance of 50 cm for the various kV settings. The TL output was plotted against the thickness and the HVT was determined. A separate dosimeter was used for the measurement of table top exposure. Table 1 summarizes the results obtained on HVT measurements. Each of the T L - H V T values were obtained by plotting the TL output of the phosphor vs aluminium step thickness and drawing a smooth curve passing through the five points. First HVTs were read off from such curves obtained for various kV settings. It can be seen that the agreement is reasonably good and the maximum deviation abuve 60 kV is 6% although the curves are extrapolated in some cases. For values below 60 kV, it was observed that the aluminium wedge and X-ray dosimeter data were not very much reproducible. Hence, it is assumed that the present method is reasonably adequate for the HVT measurement after ensuring that thin samples of phosphor are used to avoid self shielding. The accumulated exposures given to the TL wedge gave TL output equivalent to about 15 R. In actual practice, this much exposure is not required since the minimum measurable exposure is of the order of 100 mR. In most cases one or two exposures of 100 mA's is
TABLE 1. Results of HVT measurements using the TL wedge dosimeter and the X-ray dosimeter
kV
No. of exposures mAs accumulated
140 120 110 100 90 80 75 70 65 60 55 50 45
90 100 100 100 100 150 200 200 300 300 400 500 500
5 5 5 5 6 7 6 8 5 8 5 5 5
HVT mm A1 deduced from TL dosimeters
Actual HVT mm A1 measured using X-ray dosimeter
4.9 4.2 3.9 3.4 3.15 2.85 2.6 2.45 2.2 2.1 2.05 1.95 1.7
4.9 4.0 3.8 3.5 3.0 2.8 2.75 2.35 2.30 2.05 1.9 1.7 1.5
Percentage deviation 0 5 3 3 5 2 6 4 4.5 2.5 7 15 13
Technical notes
444
adequate. The H V T - T L dosimeter wedge is convenient for handling and is specially suited for making measurements remotely by dispatching the dosimeter by mail. KOMANDURI AYYANGAR* BHUWAN CHANDRA MOHINDER PAL)
Division of Radiological Protection Bhabha Atomic Research Centre Trombay, Bombay-400 085 India
Relerences 1. International Commission on Radiological Protection, LC.R.P. Publication 15, Pergamon Press, Oxford (1970). 2. AYYANGAR K., PRADblAN A. S., MURTHY g. K. S. and KURUP P, G. G. Indian d. Radiol. (in press). 3. BUSHONG S. C., GRAY P. M., PRASAD N. and GLAZE S. A. Hlth Phys. 26, 358 (1974). 4. D i x o n R. L. and WATT F. C. Phys. Med. Biol. 17, 81 (1972). 5. AYYANGAR K., LAKSHMANAN A. R., BHUWAN CHAV4DRA and RAMADAS K. Phys. Med. Biol. 19, 665 (1974). 6. CHRISTENSEN P. Proc. 2rid Int. Conf. on Luminescence Dosimetry, Gatlinburg, CONF680920, U.S.A.E.C., p. 90 (1968).
* Present Address: Stein Research Center, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania. ) Present Address: Institute of Geomagnetism, Colaba. Bombay, India.
International Iournal of Applied Radiation and Isotopes, 1977, Vol. 28, pp. 444-445. Pergamon Press, Printed in Northern Ireland
Leakage Testing of ~26Ra Sources by a Mailed Detector Method (Received 29 June 1976) NEEDLES, tubes and plaques containing 226Ra have been in use in the field of brachytherapy for the past several decades. 226Ra is one of the most radiotoxic materials and its replacement by appropriate substitutes has been recommended by several workers. The medical use of 226Ra is likely to
continue for some more years for several reasons. Firstly there is a large body of clinical experience with 226Ra. Operational difficulties in keeping track of 6°C0 needles and tubes of different nominal activities led one institution to go back to 226RaJt/ Many institutions are in possession of radium sources and their replacement by substitutes may involve an unacceptable financial burden. From time to time such sources have to be leak tested and if a source is leaking radon, it is a warning that it may go bad soon. m Several reliable methods of leak testing such as the jar method (2~ charcoal method (3~ and the electrostatic method (4~ are reported in literature. All these methods make use of nuclear particle counting instruments, by trained personnel. It is desirable to develop a simple method which can be routinely employed by relatively less qualified persons. A mail order technique (5~ which was available in U.S.A., is practicable only for a small region. This involved assessing the amount of radon if any, which leaked out of the source into an absorbent liquid. Despatching and collecting liquid containers is considered to be inconvenient. Based on published information ~6~the National Council on Radiation Protection has suggested that radon leak tests requiring immersion of the source in a liquid should be avoided J 7) Solid state track detectors are popularly used for measuring very small amounts of alpha radioactivity. Foils of cellulose nitrate irradiated by alpha particles register the track which may be enlarged chemically by etching the foil in a solution of sodium hydroxide. These magnified tracks may be counted microscopically. The feasibility of using solid state track detectors for detecting leakage of radon was established with the help of a leaky radium needle. The alpha particles from 226Ra, 222Rn, 218po and 214po impinge on the foil and produce etchable tracks. Leakage of radium salt is seldom observed. The damage to the foil due to beta particles and gamma rays does not produce etchable tracks. The foils were etched at room temperature (23°C) in a 6 N solution of NaOH for 4 hr. When the time of irradiation was increased the number of tracks also increased proportionately. The number of tracks due to background alpha activity was established to be negligible by etching an unirradiated sample of the same foil. In some preliminary studies in connection with the development of track detectors as radon monitors, the sensitivity of the foil for airborne radon was re~orted to be 28 tracks/cm e per working level hr. ( ~ This sensitivity is adequate for the purpose of leak testing of radium sources. We believe that with this device, we can detect the
442
Results and Discussion Count rates for 20 mg samples of KCr(MIDA)2 of varying specific activities are shown in Fig. 1. The effect of using varying weights of samples of fixed spe.cific activity is detailed in Fig. 2 where the plot of count rate is seen to decrease in a non-linear fashion with increasing sample weight. A plot of specific activity against the reciprocal of the weight of the solid sample is in Fig. 3 and is seen to be linear at lower sample weights and to deviate only slightly from linearity at higher weights. This plot (which is analogous to that of Ross and YERICK (7)) affords a useful method of making quench corrections to specific activities measured using less than the standard 20 mg of sample. Sensitivity to quenching in the case under study •as large, probably due to similarity of ?'max for me sample (363 nm) and the wavelength (370 nm) of the radiation emitted by the scintillator P.P.O.
Acknowledgement--We thank Dr. D. J. Spedding for helpful discussions. S. D U T T A - C H A U D H U R I *
A. L. ODELLt
Urey Radiochemical Laboratories University of Auckland New Zealand
Reierences 1. SAITO K. et al. Bull. Chem. Soc. Japan 41, 384 (1968). 2. FUNT B. L. Nucleonics 14, 83 (1956). 3. WHITE C. G. and HELF S. Nucleonics 14, 46 (1956). 4. O'rr G. D. et al. Nucleonics 17, 106 (1959). 5. BIRKS J. B. In A n Introduction to liquid scintillation, issued by Koch-Light Laboratories Ltd., p. 20. 6. HERBERG R. J. Analyt. Chem. 32, 1468 (1960). 7. Ross H. R. and YER1CK R. E. Analyt. Chem. 35, 794 (1963). 8. WEYH J. A. and HAMM R. E. Inorg. Chem. 7, 2431 (1968). 9. BERSHET G. J. In Organic Syntheses Vol. X V I I I (Ed. FusoN R. C.), p. 56. John Wiley & Sons, Inc, New York (1938). *Present address: Catalyst Department, Planning and Development Division, FCI Ltd., Sindri, Bihar, India. t T o whom correspondence may be addressed.
International Journal of Applied Radiation and Isotopes, 1977, Vol. 28, pp. 442-444. Pergamon Press. Printed in Northern Ireland
A TLD Method for the Determination ot X-ray Quality and Output in the Diagnostic Range (Received 3 May 1976; in revised form 15 September 1976)
Summary A SIMPLE TLD method is described for the determination of HVT and output of diagnostic X-ray machines. Thermoluminescent Li2B407:Mn phosphor sealed in polythene bags and attached to four 0.7 mm thick aluminium sheets as a step wedge was used as a compact packet and exposed to various energies of X-rays. The TL output was measured and plotted against thickness of Al and the HVT determined. Comparisons made with the usual aluminium wedge and X-ray dosimeter data showed agreement within :t:5% in the range of 40-140 kV. The HVT-TL-dosimeter wedge is convenient for handling and suitable especially for making measurements remotely by mail. The International Commission on Radiological Protection (1) recommends for diagnostic X-ray machines a minimum total filtration of 1.5 mm Al up to 70 kV, 2 mm A1 up to 100 kV and 2.5 mm A1 above 100 kV. It also recommends that the table top exposure for fluoroscopic machines should be less than 5 R/min. Since there are many X-ray units in vogue without the adequate filtration, and in order to ensure radiation protection, a simple method to find the total filtration as well as output is needed. If the quality of the X-ray beam is known at any particular kV, the inherent filtration can be estimated. The conventional method of HVT determination using aluminium wedge and X-ray dosimeter is cumbersome although it gives accurate and on-thespot data. Several TLD methods (2-4) were reported in literature based on the ratio between the TL output of an energy dependent phosphor and an energy independent one. Most of these methods are suitable in the region of 50-100 keV where the ratio varies steeply with energy. The above system is not suitable for diagnostic region as the same TL ratio may represent two different HVT values thus causing confusion. This can be seen from the work of Busr~oNG eta/. (3) where the TL ratios in the HVT region 0.1-4 mm A1 are same as in the region 4 m m A l - l . 5 mm Cu. A wedge method using a simple integrating dosimeter as described here is
443
Technical notes
convenient and accurate. An energy independent TL dosimeter such as Li2B407:Mri is well suited for this purpose. The TL reader system used for the measurements is described elsewhere. (5) Thermoluminescent lithium borate activated with 0.2 mole atom percent Mn (0.23% by wt) was prepared in our laboratory and its sensitivity compared with commercial samples was reported earlier. (5) The energy dependence characters of the phosphor were found to be in agreement with the data of CrmISTENSEN(6~. A Siemens " T R I D O R O S 4" radiography and fluoroscopy combined unit having a total filtration of 2 mm A1 was used for irradiation. An ion-chamber type dosimeter with a charge measuring unit (Electronic Instruments Limited, London, U.K.) and an A1 wedge was used for HVT measurements. Victoreen condenser R-chambers which were calibrated against primary standards were employed for exposure calibration purposes. Li2B4OT:Mn dosimeters were calibrated using a 6°C0 teletherapy beard, and this calibration was assumed for the entire X-ray region. However, for output measurements a factor of 0.95 for energy dependence was used. The TL exposures for the HVT experiments were ensured to be within the linear region. Thermoluminescence was recorded 2 days after exposure to minimize the effect of its low temperature glow peak. The method adopted for HVT measurements is as follows. Firstly, the HVTs were determined at several kV settings using the X-ray dosimeter and aluminium wedge. About 80 mg of Li2B4OT:Mn
phosphor powder sealed in polythene bags of dimensions 1 c m x 1 cm x 0.5 mm was used as the TL dosimeter. Four 0.7 mm thick aluminium sheets of size 2 cm x 8 cm held by tape were arranged as a step wedge and the dosimeters were attached, one front and four behind the steps with an adhesive tape. These package units were exposed to beams of the Siemens diagnostic unit in air at a distance of 50 cm for the various kV settings. The TL output was plotted against the thickness and the HVT was determined. A separate dosimeter was used for the measurement of table top exposure. Table 1 summarizes the results obtained on HVT measurements. Each of the T L - H V T values were obtained by plotting the TL output of the phosphor vs aluminium step thickness and drawing a smooth curve passing through the five points. First HVTs were read off from such curves obtained for various kV settings. It can be seen that the agreement is reasonably good and the maximum deviation abuve 60 kV is 6% although the curves are extrapolated in some cases. For values below 60 kV, it was observed that the aluminium wedge and X-ray dosimeter data were not very much reproducible. Hence, it is assumed that the present method is reasonably adequate for the HVT measurement after ensuring that thin samples of phosphor are used to avoid self shielding. The accumulated exposures given to the TL wedge gave TL output equivalent to about 15 R. In actual practice, this much exposure is not required since the minimum measurable exposure is of the order of 100 mR. In most cases one or two exposures of 100 mA's is
TABLE 1. Results of HVT measurements using the TL wedge dosimeter and the X-ray dosimeter
kV
No. of exposures mAs accumulated
140 120 110 100 90 80 75 70 65 60 55 50 45
90 100 100 100 100 150 200 200 300 300 400 500 500
5 5 5 5 6 7 6 8 5 8 5 5 5
HVT mm A1 deduced from TL dosimeters
Actual HVT mm A1 measured using X-ray dosimeter
4.9 4.2 3.9 3.4 3.15 2.85 2.6 2.45 2.2 2.1 2.05 1.95 1.7
4.9 4.0 3.8 3.5 3.0 2.8 2.75 2.35 2.30 2.05 1.9 1.7 1.5
Percentage deviation 0 5 3 3 5 2 6 4 4.5 2.5 7 15 13
Technical notes
444
adequate. The H V T - T L dosimeter wedge is convenient for handling and is specially suited for making measurements remotely by dispatching the dosimeter by mail. KOMANDURI AYYANGAR* BHUWAN CHANDRA MOHINDER PAL)
Division of Radiological Protection Bhabha Atomic Research Centre Trombay, Bombay-400 085 India
Relerences 1. International Commission on Radiological Protection, LC.R.P. Publication 15, Pergamon Press, Oxford (1970). 2. AYYANGAR K., PRADblAN A. S., MURTHY g. K. S. and KURUP P, G. G. Indian d. Radiol. (in press). 3. BUSHONG S. C., GRAY P. M., PRASAD N. and GLAZE S. A. Hlth Phys. 26, 358 (1974). 4. D i x o n R. L. and WATT F. C. Phys. Med. Biol. 17, 81 (1972). 5. AYYANGAR K., LAKSHMANAN A. R., BHUWAN CHAV4DRA and RAMADAS K. Phys. Med. Biol. 19, 665 (1974). 6. CHRISTENSEN P. Proc. 2rid Int. Conf. on Luminescence Dosimetry, Gatlinburg, CONF680920, U.S.A.E.C., p. 90 (1968).
* Present Address: Stein Research Center, Thomas Jefferson University Hospital, Philadelphia, Pennsylvania. ) Present Address: Institute of Geomagnetism, Colaba. Bombay, India.
International Iournal of Applied Radiation and Isotopes, 1977, Vol. 28, pp. 444-445. Pergamon Press, Printed in Northern Ireland
Leakage Testing of ~26Ra Sources by a Mailed Detector Method (Received 29 June 1976) NEEDLES, tubes and plaques containing 226Ra have been in use in the field of brachytherapy for the past several decades. 226Ra is one of the most radiotoxic materials and its replacement by appropriate substitutes has been recommended by several workers. The medical use of 226Ra is likely to
continue for some more years for several reasons. Firstly there is a large body of clinical experience with 226Ra. Operational difficulties in keeping track of 6°C0 needles and tubes of different nominal activities led one institution to go back to 226RaJt/ Many institutions are in possession of radium sources and their replacement by substitutes may involve an unacceptable financial burden. From time to time such sources have to be leak tested and if a source is leaking radon, it is a warning that it may go bad soon. m Several reliable methods of leak testing such as the jar method (2~ charcoal method (3~ and the electrostatic method (4~ are reported in literature. All these methods make use of nuclear particle counting instruments, by trained personnel. It is desirable to develop a simple method which can be routinely employed by relatively less qualified persons. A mail order technique (5~ which was available in U.S.A., is practicable only for a small region. This involved assessing the amount of radon if any, which leaked out of the source into an absorbent liquid. Despatching and collecting liquid containers is considered to be inconvenient. Based on published information ~6~the National Council on Radiation Protection has suggested that radon leak tests requiring immersion of the source in a liquid should be avoided J 7) Solid state track detectors are popularly used for measuring very small amounts of alpha radioactivity. Foils of cellulose nitrate irradiated by alpha particles register the track which may be enlarged chemically by etching the foil in a solution of sodium hydroxide. These magnified tracks may be counted microscopically. The feasibility of using solid state track detectors for detecting leakage of radon was established with the help of a leaky radium needle. The alpha particles from 226Ra, 222Rn, 218po and 214po impinge on the foil and produce etchable tracks. Leakage of radium salt is seldom observed. The damage to the foil due to beta particles and gamma rays does not produce etchable tracks. The foils were etched at room temperature (23°C) in a 6 N solution of NaOH for 4 hr. When the time of irradiation was increased the number of tracks also increased proportionately. The number of tracks due to background alpha activity was established to be negligible by etching an unirradiated sample of the same foil. In some preliminary studies in connection with the development of track detectors as radon monitors, the sensitivity of the foil for airborne radon was re~orted to be 28 tracks/cm e per working level hr. ( ~ This sensitivity is adequate for the purpose of leak testing of radium sources. We believe that with this device, we can detect the