R.J. Keddy

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.

Relative electron dosimetry using a synthetic diamond probe

Abstract Implementation of Bragg–Gray Cavity Theory in electron dosimetry is complicated by the fact that most commercial detector volumes behave as small field inhomogeneities. Several correction factors are necessary to establish the absorbed dose at a particular point in a homogenous tissue-equivalent phantom. The energy dependence of air and the replacement effects introduced as a result of air ionization chambers’ construction and size, increase the uncertainty of electron beam calibrations. Preliminary relative dose measurements performed with a prototype synthetic diamond are presented here. Theoretically, the radiation response of diamond, synthetic or natural, has negligible energy dependence. The sensitivity and small size of the probe makes it an excellent candidate for measurements in fields of high dose gradient.

Ion beam analyses in relation to the physical properties of diamond

Abstract Diamond is a remarkable material with many unique physical properties, embracing optical, thermal, and electrical characteristics. The interplay of independent diagnostic measurements and nondestructive ion beam probes, forms the basis of these investigations. A gross subdivision of diamonds into four types can be made on the basis, predominantly, of optical properties. Other criteria, such as electrical and thermal measurements, endorse this subdivision. This type-categorization is due to the impurity characteristics. Important among these is nitrogen, which may be determined by (α, n) reactions, and questions on the aggregation of nitrogen can be broached through arguments based on thermal conductivity data. From multielemental analysis by neutron activation, it is concluded that all diamonds contain remnant inclusions of the parental magma. The distribution and aggregation of these magma inclusions are still open questions which are probed by selected nuclear reactions, by backscattering, and by proton induced X-rays. An important diamond genesis feature is the water-richness of the magma: hydrogen analysis and profiles are determined by the resonant reaction 19F, αγ). One of the four types of diamond is semiconducting: the elimination of impurities responsible for this property and the final determination of the responsible element as boron, by (p, α) reaction, are considered. Although diamond has been a subject of study for more than a century, the specific contribution of ion probes to the important open questions is relevant and timely.

A non-destructive nuclear method for the determination of nitrogen in solids

Abstract The nitrogen concentration in diamonds and in SiC was determined by charged particle activation analysis. The activation reaction used was 14N (α, n) 17F, the decay proceeding with the following characteristics: 17 F → 17 O + β + + ν (τ 1 2 = 66 sec ) . The annihilation radiation was detected. Nitrogen concentrations between ∼5 ppm and ∼2000 ppm were found in the samples.

Sub-ppm determination of boron: Identification of the acceptor centre in natural semi-conducting diamond

Abstract A sensitive proton activation technique is used for the detection of boron which is found to be present in natural semiconducting diamonds. The elemental concentrations obtained can be related to the acceptor concentrations determined by electrical measurements, leading to the conclusion that boron is the acceptor centre in this class of natural diamond.