T.L. Nam

Synthetic diamonds as in vivo radiation detectors

Synthetic diamonds with controlled amounts of impurity atoms can be manufactured so that, as thermoluminescent dosimeters, they can be made to have sensitivities at least as good as presently available commercial thermoluminescent dosimeters. They also exhibit, for radiations normally found in therapy situations, a linearity of response that extends from less than 0.01 Gy (1 rad) to over 10 Gy (1000 rad). Their physical size and form, crystals which can have volumes of less than 1 mm3, make them ideal candidates for in vivo monitoring of radiation fields, particularly electron fields where high-resolution measurements are essential for accurate isodose line determinations. Aspects of dose response from gamma-ray beams in relation to the type and concentrations of the impurity atoms within the diamond are discussed, and some experimental values for gamma, x-ray, and electron beams are presented.

Diamond radiation detectors

Abstract A review to date is given of the status of diamond as a detector of ionizing radiations. The use of diamond as a thermoluminescence device, as an ionization chamber (pulse and d.c. modes), as a scintillation counter for α-particles and for high dose applications using radiophotoluminescence is discussed. Some of the data obtained have led also to a better understanding of physics and crystallographic structures of the diamond lattice. Appropriate examples and references are given. Particular attention is paid to the role played by the impurity atoms contained within the lattice.

The detection of ionizing radiations by natural and synthetic diamond crystals and their application as dosimeters in biological environments

Abstract Experimental results demonstrate that synthetic diamond crystals, with impurity concentrations carefully controlled during the synthesizing process, are excellent ionizing radiation detectors in instances of in vivo radiation dosimetry. Emphasis is laid on the fact that because of their size and physical properties, synthetic diamond crystals would lend themselves particularly well to electron beam dosimetry measurements in radiotherapy units. With slight modifications to the synthesizing process, the diamonds can be used equally well as thermoluminescence dosimeters using a standard reader or, after attaching leads and applying a polarizing voltage, as ionization chamber dosimeters monitoring either the induced d.c. or the pulses that can be digitized for lower dose rates. The nontoxic, tissue-equivalent diamond crystal dosimeters are robust and can be reused many times. They should provide not only an outstanding addition to the range of presently available commercial dosimeters but, in certain situations, will have unique and desirable advantages over other tissue-equivalent materials.

Miniaturized radiation detector with custom synthesized diamond crystal as sensor

Abstract A miniaturized detector consisting of three custom built hybrid circuits, a counter and a miniature high voltage power supply was designed to operate with custom synthesized Type Ib diamond crystals as sensors. Thick-film technology was incorporated in the circuit design. With a crystal having a volume of approximately 10 mm 3 and containing approximately 60 ppm paramagnetic nitrogen, the detector was capable of measuring γ-ray dose-rates as low as 7.5 μ Gy h −1 . The response characteristic was linear up to 1 cGy h −1 .

Synthetic diamonds used as pulse-counting gamma-ray detectors

Type Ib synthetic diamonds were used as pulse-counting radiation detectors. When used as detectors to measure γ-rays, dose rates as low as 1 μGy h−1 were recorded, and the response was linear over five orders of magnitude. For best response, the diamonds were first primed with a large γ-ray dose and were then not exposed to light as exposure returned the diamonds to an “un-primed” state. The response was also shown to decrease with increasing the single substitutional nitrogen concentration. A boron doped layer, made by ion implantation to the surface of the diamond and used as the positive contact was necessary to eliminate count rate decreases due to space charge effects.

Trapping levels in pulse-counting synthetic diamond detectors

Abstract Type Ib synthetic diamonds were used as pulse-counting radiation detectors. Special ion-implanted contacts to the diamond were used. These contacts were shown to suppress the development of space charge within the diamond, and to cause no significant voltage drop across the contacts. An initial large γ-ray pre-irradiation dose markedly improved the pulse counting response of the diamond. It is suggested that this improvement is due to relevent and involved traps being filled to saturation by charge carriers whereas the same traps in an under-populated state actively inhibit the charge collection for pulse formation. The traps are depopulated by exposure to ambient light and, using this depopulation effect, the effective energy level of these involved traps was measured to be 2.2 eV. Shallow trapping levels are postulated and are believed to be responsible for the trapping of the carriers which gives rise to space charge effects.

Detection of nuclear radiation by scintillation counting using synthetic diamond

Abstract The scintillation characteristics of boron activated synthetic diamonds, produced from an iron free solvent/catalyst, when subjected to nuclear ionizing radiations have been investigated. Despite the fact that the optical coupling between the crystal and the surface of the photomultiplier which was used in the experimental arrangement was far from ideal, the crystals, when exposed to irradiation by α-particles, exhibited good energy resolution and linear dose rate response characteristics.

A flexible kilocurie cobalt-60 facility for multi-purpose irradiation applications

Abstract The design of a shielded holder large enough to contain an activity of up to 3 × 10 3 Ci of 60 Co is described. It incorporates the unusual feature of allowing the source to be moved to positions outside the shield. The safety features incorporated in the design are presented and examples of the use of the “bomb” for atomic excitation and nuclear resonance scattering studies, and for applications such as the sterilization of human bones and tissues are given.

The presence of defects and their influence on the performance of CVD diamond as an α-particle radiation sensing element

Three types of diamonds produced by chemical vapor deposition (CVD) and broadly classified as detector grade, optical grade and single crystal were evaluated in terms of their response to alpha-particle radiation when used as detection elements. It is well known that the presence of defects in diamond, including CVD specimens, not only dictates but also affects the response of diamond to radiation in different ways. In this investigation, tools such as electron spin resonance (ESR), thermo-luminescence (TL), Raman spectroscopy and ultraviolet (UV) spectroscopy were used to probe each of the samples, which were then graded on their performance as alpha-particle radiation detectors. The presentation discusses the presence of defects identifiable by the techniques used and correlates the radiation performances of the three types of crystals to their presence.

Radiation dosimetry based on the radiophotoluminescence of synthetic diamond

Abstract A dosimetric method based upon the radiophotoluminescence of synthetic diamond crystals of low paramagnetic nitrogen concentration has been developed. By using light of 404 nm, it is shown that the thermally accessible charge-trapping states which are responsible for the thermoluminescence characteristics of the crystal can be de-excited to give a dosimetric technique with a response that is linear to radiation doses up to 10 Gy. When an excitation wavelength of 296 nm is used to empty the thermally inaccessible charge-trapping states, radiation doses up to 20 kGy are linearly measured. It is concluded that synthetic diamond crystals, with such favourable physical properties as near tissue equivalency and chemical inertness, have potential application as photoluminescence dosimeters in medical physics, radiobiology and radioprocessing.