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Radiation detectors made from diamonds
Physica lllB (1981) 319-320 North-Holland Publishing Company LETTER TO THE EDITOR
RADIATION DETECTORS MADE FROM DIAMONDS E.A. B U R G E M E I S T E R Academic Hospital Utrecht, Department of Radiotherapy, Catharijnesingel 101, 3511 GV Utrecht, The Netherlands
Received 26 May 1981
It was reported in ref. [1] that specimens cut from a very pure natural diamond operate as radiosensitive resistors. When these specimens were subjected to T-rays, X-rays or electron beams, the resistance value stabilized within several seconds and was relatively low at the dose rates normally used in radiotherapy. This was attributed to a lifetime of free charge of about 10-Ss which is 1000 × longer than that of the well-known counting diamonds (see, for instance, refs. [2, 3]). Therefore, it was concluded that these resistors are quite suitable for clinical dosimetry, but such a diamond is unfortunately extremely rare in nature. It was recently found that six synthetic diamonds with low nitrogen-concentrations (less than 20 ppm) and with little birefringence (less than 10 -4) operate approximately as well as the very pure natural diamond. These specimens are about 0 . 9 m g and were subjected to 6°Co radiation at a dose rate of 2.5× 10 - 2 G y s -1. A few seconds after switching on this radiation, the currents of all six diamonds were between 0.2 and 0.3/~A at plus or minus 1 0 V bias. The current stabilized to values between 0.3 and 0.4/~A after about 100 s. When the radiation was switched off, residual currents with long decay times were observed. In subsequent radiations from the 6°Co source currents between 0.3 and 0.4/zA were again found. The stabilization occurred in a reproducible way in about 100 s when the residual current had died down, but in the presence of residual current this period was much shorter as could be expected. The c u r r e n t -
voltage characteristics of all six diamonds are nearly linear below 50 V bias (up to 15% deviation from linearity was observed). The sensitivities of these diamonds are about 10× higher than that of the very pure natural diamond. The lifetime of free charge in these pure synthetic diamonds is longer than 10 -4 s. It can be concluded that these pure synthetic diamonds are very suitable for clinical dosimetry. Graphite layers on two opposite faces of a specimen serve for ohmic contacts. Thin electrical leads are m o u n t e d on these layers by means of graphite paint and the assembly is subsequently encapsulated in teflon (PTFE). Such a p r o b e is flexible and even m o r e equivalent to tissue than the one with stainlesssteel shielding described in ref. [1]. A second p r o b e with a very pure synthetic diamond and having graphite as leads and encapsulation would be useful for standard dosimetry since it is made from carbon only. D u e to the atomic n u m b e r and its small size, this second p r o b e obeys the B r a g g - G r a y cavity theory for most media used in practical irradiations as can be inferred from ref. [4].
Acknowledgements
0378-4363/81/0000-0000/$02.75 © 1981 North-Holiand
I thank Dr. J. Schipper for discussions, Professor Dr. H.A. van Peperzeel for encouragement, and the "Koningin Wilhelmina Fonds" for financial support.
320
E.A. Burgemeister / Radiation detectors made from diamonds
References [1] E.A. Burgemeister, Phys. Med. Biol. 26 (1981) 269. [2] E.A. Konorova and S.F. Kozlov, Sov. Phys.- Semicond. 4 (1971) 1600.
[3] B. Planskoy, Phys. Med. Biol. 25 (1980) 519. [4] K.R. Kase and W.R. Nelson, Concepts of Radiation Dosimetry (Pergamon Press, New York, 1978).
RADIATION DETECTORS MADE FROM DIAMONDS E.A. B U R G E M E I S T E R Academic Hospital Utrecht, Department of Radiotherapy, Catharijnesingel 101, 3511 GV Utrecht, The Netherlands
Received 26 May 1981
It was reported in ref. [1] that specimens cut from a very pure natural diamond operate as radiosensitive resistors. When these specimens were subjected to T-rays, X-rays or electron beams, the resistance value stabilized within several seconds and was relatively low at the dose rates normally used in radiotherapy. This was attributed to a lifetime of free charge of about 10-Ss which is 1000 × longer than that of the well-known counting diamonds (see, for instance, refs. [2, 3]). Therefore, it was concluded that these resistors are quite suitable for clinical dosimetry, but such a diamond is unfortunately extremely rare in nature. It was recently found that six synthetic diamonds with low nitrogen-concentrations (less than 20 ppm) and with little birefringence (less than 10 -4) operate approximately as well as the very pure natural diamond. These specimens are about 0 . 9 m g and were subjected to 6°Co radiation at a dose rate of 2.5× 10 - 2 G y s -1. A few seconds after switching on this radiation, the currents of all six diamonds were between 0.2 and 0.3/~A at plus or minus 1 0 V bias. The current stabilized to values between 0.3 and 0.4/~A after about 100 s. When the radiation was switched off, residual currents with long decay times were observed. In subsequent radiations from the 6°Co source currents between 0.3 and 0.4/zA were again found. The stabilization occurred in a reproducible way in about 100 s when the residual current had died down, but in the presence of residual current this period was much shorter as could be expected. The c u r r e n t -
voltage characteristics of all six diamonds are nearly linear below 50 V bias (up to 15% deviation from linearity was observed). The sensitivities of these diamonds are about 10× higher than that of the very pure natural diamond. The lifetime of free charge in these pure synthetic diamonds is longer than 10 -4 s. It can be concluded that these pure synthetic diamonds are very suitable for clinical dosimetry. Graphite layers on two opposite faces of a specimen serve for ohmic contacts. Thin electrical leads are m o u n t e d on these layers by means of graphite paint and the assembly is subsequently encapsulated in teflon (PTFE). Such a p r o b e is flexible and even m o r e equivalent to tissue than the one with stainlesssteel shielding described in ref. [1]. A second p r o b e with a very pure synthetic diamond and having graphite as leads and encapsulation would be useful for standard dosimetry since it is made from carbon only. D u e to the atomic n u m b e r and its small size, this second p r o b e obeys the B r a g g - G r a y cavity theory for most media used in practical irradiations as can be inferred from ref. [4].
Acknowledgements
0378-4363/81/0000-0000/$02.75 © 1981 North-Holiand
I thank Dr. J. Schipper for discussions, Professor Dr. H.A. van Peperzeel for encouragement, and the "Koningin Wilhelmina Fonds" for financial support.
320
E.A. Burgemeister / Radiation detectors made from diamonds
References [1] E.A. Burgemeister, Phys. Med. Biol. 26 (1981) 269. [2] E.A. Konorova and S.F. Kozlov, Sov. Phys.- Semicond. 4 (1971) 1600.
[3] B. Planskoy, Phys. Med. Biol. 25 (1980) 519. [4] K.R. Kase and W.R. Nelson, Concepts of Radiation Dosimetry (Pergamon Press, New York, 1978).