A. Brambilla

CVD diamond for radiation detection devices

CVD diamond is a remarkable material for the fabrication of radiation detectors. Radiation hardness, chemical resistance and high temperature operation capabilities of diamond explain its use in the fabrication of devices operating in hostile environments such as that encountered in the nuclear industry and in high energy physics. For this purpose, we have investigated the growth of high quality chemically vapour deposited (CVD) polycrystalline diamond as well as specific material and device processing. CVD diamond films were grown using the microwave plasma enhanced technique. Deposition processes were optimised according to the application requirements. This includes the synthesis of films with high sensitivity, with weak optical absorption in the UV-VIS domain or with short carrier lifetime. One inherent problem with diamond is the presence of defect levels altering the detection characteristics: these may be the cause of an observed instability of the device responses. We have found, however, that it was possible to moderate these trends through the fine-tuning of the growth conditions and of the device preparation steps. Films with thicknesses ranging from 5 to 500 μm have been used for detector fabrication. The role of post-growth treatments and the contact formation procedure was also extensively studied, leading to significant improvements of the detector characteristics. We present recent developments studied at CEA for material optimisation towards its use for specific applications, including radiation hard counters; X-ray intensity, shape and beam position monitors; solar blind photo-detectors, and high dose rate gamma-meters.

Neutron Detectors Made From Chemically Vapour Deposited Semiconductors

In this paper, the authors present the results of investigations on the use of semiconductors deposited by chemical vapor deposition (CVD) for the fabrication of neutron detectors. For this purpose, 20 {micro}m thick hydrogenated amorphous silicon (a-Si:H) pin diodes and 100 {micro}m thick polycrystalline diamond resistive detectors were fabricated. The detectors were coupled to a neutron-charged particle converter: a layer of either gadolinium or boron (isotope 10 enriched) deposited by evaporation. They have demonstrated the capability of such neutron detectors to operate at neutron fluxes ranging from 10{sup 1} to 10{sup 6} neutrons/cm{sup 2}.s. The fabrication of large area detectors for neutron counting or cartography through the use of multichannel reading circuits is discussed. The advantages of these detectors include the ability to produce large area detectors at low cost, radiation hardness ({approximately} 4 Mrad for a-Si:H and {approximately} 100 Mrad for diamond), and for diamond, operation at temperatures up to 500 C. These properties enable the use of these devices for neutron detection in harsh environments. Thermal neutron detection efficiency up to 22% and 3% are expected by coupling a-Si:H diodes and diamond detectors to 3 {micro}m thick gadolinium (isotope 157) and 2 {micro}m thick boron layers, respectively.

Diamond as a tool for synchrotron radiation monitoring: beam position, profile, and temporal distribution

Abstract Diamond polycrystalline films have been synthesised using the chemical vapour deposition technique in order to fabricate new types of photodetectors for the characterisation of X-ray light sources as encountered in synchrotron experiments. We present here new diamond-based devices for three different applications, including (i) semitransparent beam position monitors with high position resolution (

High collection efficiency CVD diamond alpha detectors

Advances in Chemical Vapour Deposited (CVD) diamond have enabled the routine use of this material for sensor device fabrication, allowing exploitation of its unique combination of physical properties (low temperature susceptibility (>500/spl deg/C), high resistance to radiation damage (>100 Mrad) and to corrosive media). A consequence of CVD diamond growth on silicon is the formation of polycrystalline films which has a profound influence on the physical and electronic properties with respect to those measured on monocrystalline diamond. We report the optimisation of physical and geometrical device parameters for radiation detection in the counting mode. Sandwich and co-planar electrode geometries are tested and their performances evaluated with regard to the nature of the field profile and drift distances inherent in such devices. The carrier drift length before trapping was measured under alpha particles and values as high as 40% of the overall film thickness are reported. Further, by optimising the device geometry, we show that a gain in collection efficiency, defined as the induced charge divided by the deposited charge within the material, can be achieved even though lower bias values are used.

Corrosion hard CVD diamond alpha particle detectors for nuclear liquid source monitoring

Abstract Corrosion hard alpha particle detectors have been fabricated from thin diamond layers grown on silicon using the microwave enhanced chemical vapour deposition technique (CVD). The devices can be directly immersed in radioactive liquid solutions and enable the real time and in situ measurement of their alpha activity. We report here measurements on devices immersed in a plutonium-239 radioactive source diluted in concentrated nitric acid. The tests have shown excellent reliability of the CVD diamond detectors which would be of great benefit in monitoring nuclear fuel assembly reprocessing. We believe that this is the first detection application of CVD diamond which relies on its corrosion hardness.

Thin film diamond alpha detectors for dosimetry applications

Abstract Diamond is a resilient material with rather extreme electronic properties. As such it is an interesting candidate for the fabrication of high performance solid state particle detectors. However, the commercially accessible form of diamond, grown by chemical vapour deposition (CVD) methods, is polycrystalline in nature and often displays rather poor electrical characteristics. This paper considers the way that this material may be used to form alpha particle dosimeters with useful performance levels. One approach adopted has been to reduce the impurity levels within the feed-stock gases that are used to grow the diamond films. This has enabled significant improvements to be achieved in the mean carrier drift distance within the films leading alpha detectors with up to 40% collection efficiencies. An alternative approach explored is the use of planar device geometry whereby charge collection is limited to the top surface of the diamond which comprises higher quality material than the bulk of the film. This has lead to collection efficiencies of greater than 70%, the highest yet reported for polycrystalline CVD material based detectors. Techniques for improving the characteristics of these devices further are discussed.

Geometrical non-uniformities in the sensitivity of polycrystalline diamond radiation detectors

Chemical vapour deposited (CVD) diamond is a remarkable material for the fabrication of photon and particle detectors. However, little is known about the perturbations induced by the polycrystalline nature of this material. For this purpose, we have used a micrometer size X-ray beam generated from a synchrotron light source to induce photocurrents in a CVD diamond-based detector. By comparing the measured currents in the device as the beam interaction position is moved on the sample with the topographical image of the surface observed using a scanning electron microscope (SEM), a significant non-uniformity has been observed that could be correlated with the grain structure.

Influence of the crystalline structure on the electrical properties of CVD diamond films

Abstract 〈100〉, 〈110〉 and non-textured polycrystalline diamond films were deposited by microwave plasma enhanced chemical vapour deposition. Resistive devices with coplanar and sandwich electrical contacts were made to characterise and compare these films towards their use as radiation detector. Current-voltage measurements in the dark and under UV light and X-ray radiation were carried out. The crystalline structure of the film is shown to have a significant influence on its electrical properties and on their sensitivity to UV and X-ray radiation. Film bulk resistivities range from 1 × 10 12 for the 〈100〉 textured films to 5 × 10 14 Ω cm for the 〈110〉 textured films. We found that the 〈100〉 textured film has the highest carrier drift length before trapping ( μτE , where μ and τ are the carrier mobility and lifetime and E is the applied electric field) while the non-textured film has the lowest μτE . The presence of nitrogen in a 〈100〉 textured film is shown to increase its resistivity by more than two orders of magnitude owing to compensation effect, but it reduces the carrier drift length before trapping by a factor of 2.

Electronic properties of CVD polycrystalline diamond films

Abstract Electronic properties of polycrystalline diamond films have been analysed using steady-state photoconductivity under UV (λ=190 nm) and red (λ=633 nm) illumination. We look at the effect of several post-deposition treatments: annealing in vacuum at 400°C, successive annealing under air and methane or the reverse, and X-ray exposure. These are shown to affect the photoresponses in different manners. Globally, the mobility–lifetime products of samples are found in the range 10 −7 –10 −6 cm 2 V −1 under UV illumination, and ratios of photoresponses in UV and red are in the range 10 3 –10 5 . First results obtained using the modulated photocurrent technique are also presented.

Influence of electrical defects on diamond detection properties

Abstract The remarkable properties of diamond, together with its radiation hardness, make it an attractive material for specific applications in X-rays and charged particle detection. Because electrically active defects are known to influence the detection properties, a thorough study of these defects is of great interest in order to improve the detector characteristics. In this work, the response of diamond devices to X-rays and alpha particles is investigated and the results are correlated with the presence of traps analysed using thermally stimulated current (TSC) methods. The results of TSC analysis on natural diamond have enabled the determination of the energy of three levels activated near room temperature at 0.7, 0.71 and 0.95xa0eV and of a main principal level on CVD diamond at 1.2xa0eV activated near 550xa0K. The comparative study of TSC measurements and time dependent X-ray sensitivity are investigated on natural diamonds. The results confirm the improvement of the detection properties after having filled deep trap levels, and show a detrimental effect of the trapping levels emptied at room temperature on the turn-off time of diamond. Further, the temperature dependence of the detector response under alpha particles was also investigated. Clearly, a temperature increase above a level activated at 375xa0K leads to the improvement of the detection properties.