X-Ray Studies of Synthetic Radiation-Counting Diamonds.
- PDF / 1,465,657 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 104 Downloads / 157 Views
X-RAY STUDIES OF SYNTHETIC RADIATION-COUNTING DIAMONDS. ANDREW YACOOT*, MORETON MOORE- AND ANTHONY MAKEPEACE*
*Royal Holloway, London University, Dept. of Physics, Egham, Surrey TW20 OEX, UK.
"*BristolUniversity, Department
of Physiology, Bristol BS8 1TD, UK.
ABSTRACT Synthetic diamonds with a nitrogen content less than 100ppm may be used as radiation dosimeters in a conduction counting mode, and are especially useful in medical applications. Crystal imperfections, revealed by X-ray diffraction topography, were found to affect counting performance. The best quality diamond gave the highest photocurrent 1 (5OOnA at 50 V mm- and 2.75 Gy min-l). Diamonds containing dislocations had lower photocurrents but had the advantage of shorter settling times (seconds rather than minutes). Placing contacts on two opposite cube {100} faces gave a higher photocurrent than on a pair of octahedral {111} faces. Higher photocurrents were also achieved when the majority of dislocations were perpendicular rather than parallel to the electric field. Some recommendations for selecting synthetic diamonds for dosimeters are given. INTRODUCTION. Radiotherapy requires the accurate measurement of doses of ionizing radiation received by biological tissue. These doses are usually measured with a tissue equivalent material whose radiation absorption characteristics are similar to those of the human body. Since the absorption of low-energy photons in biological tissue occurs mainly by a photoelectric process, the absorption coefficient for such a material is proportional to the fourth power 1 of the atomic number . The effective atomic numbers of fat, muscle and lung are 5.92, 7.42 and 7.64 respectively2 . Diamond (Z=6) matches these closely. For over forty years it has been known that some diamonds can 'count' ionizing radiation 3 . If an electric field is placed across a counting diamond, then, in the presence of ionizing radiation, an ionization current will be observed. The magnitude of this current, typically a few nano-amps, is proportional to the radiation dose rate. Only 2% of natural diamonds are Type Ila (i.e. contain less than 100ppm of nitrogen 4 impurity) and it has been noted that only 80% of these count radiation. Furthermore, many natural counting diamonds suffer from space charge and polarization effects: after counting radiation for a short time an electric field is set up within the specimen in opposition to the external electric field, thereby reducing the counting efficiency. These factors, together with the fact that no two natural diamonds perform in an identical fashion, have hindered the commercial development of natural diamond radiation-counting dosimeters. 5 Recently synthetic diamonds suitable for radiation dosimetry have been developed . These diamonds are grown by the reconstitution techniques and have a paramagnetic nitrogen content of less than 100ppm. This paper describes work undertaken to image 7 defects (mainly dislocations and inclusions) using X-ray diffraction topography and ex8 plains how they have been
Data Loading...
