F. Fizzotti

Review of the development of diamond radiation sensors

Abstract Diamond radiation sensors produced by chemical vapour deposition are studied for the application as tracking detectors in high luminosity experiments. Sensors with a charge collection distance up to 250 μm have been manufactured. Their radiation hardness has been studied with pions, proton and neutrons up to fluences of 1.9×10 15 π cm −2 , 5×10 15 p cm −2 and 1.35×10 15 n cm −2 , respectively. Diamond micro-strip detectors with 50 μm pitch have been exposed in a high-energy test beam in order to investigate their charge collection properties. The measured spatial resolution using a centre-of-gravity position finding algorithm corresponds to the digital resolution for this strip pitch. First results from a strip tracker with a 2×4 cm 2 surface area are reported as well as the performance of a diamond tracker read out by radiation-hard electronics with 25 ns shaping time. Diamond pixel sensors have been prepared to match the geometries of the recently available read-out chip prototypes for ATLAS and CMS. Beam test results are shown from a diamond detector bump-bonded to an ATLAS prototype read-out. They demonstrate a 98% bump-bonding efficiency and a digital resolution in both dimensions.

IBIC investigations on CVD diamond

A 3 MeV proton microbeam has been used for the first time both to investigate the transport properties and to probe the electrical field in a CVD diamond sample of detector grade. Qualitative results concerning the spatial distribution of charge collection efficiency and, consequently, of the collection length are obtained. Collection length seems to be of the order of several tens of microns only in small regions of the same dimensions, characterized probably by a good crystalline behaviour. A strong polarization due to the creation of a local internal electric field and ascribed to the space-charge accumulated in the grain boundaries, makes the measurements very difficult and reduces the statistical validity of the data. However, imaging of physical quantities like collection length is concluded to be a viable technique and it will constitute a useful and powerful method of investigating homogeneity of nuclear detectors and of other similar devices.

Micromachining of silicon with a proton microbeam

Abstract In the recent years the fabrication of sensors and actuator devices on a microscopic scale and their integration with electronic devices and micro-electromechanical systems (MEMS) has become an area of considerable commercial and technological interest, with huge development potentialities. High energy ion microbeam is a suitable tool for such purposes. In this paper we present an alternative way to exploit the lithographic properties of micro ion beams based on the selective damage of silicon to produce porous silicon microstructures. We used a 2 MeV proton microbeam to irradiate definite areas of silicon samples in order to produce damaged layers localised at the end of the proton trajectories. By performing an electrochemical etching in a suitable HF solution, a porous silicon pattern, complementary to the irradiated one, is always formed. The main effect of the damage on the porous silicon formation is to reduce the velocity of formation. To interpret this, such dead layers can be seen to be more or less opaque to the migration of free holes. Consequently the patterned region can be more or less revealed according to the formation time. The procedure allows for the production of microstructures of porous silicon whose unique properties are of great interest for applications. Preliminary results obtained on silicon samples, with different doping levels (p+, p, n+) and irradiating regions with different areas (from 200×200 μm 2 to 25×25 μm 2 ) are presented in order to evaluate the most suitable range of exposure and aspect-ratio of the microstructures.

Grain size effects in CVD diamond detectors

CVD diamond nuclear detectors have been investigated with 241Am α particles and with 90Sr β particles in various conditions of bias polarization and irradiation geometry. It is proved that the parameter indicated as collection distance in the literature is an average value defined over a broad distribution which should be related to the grain size distribution inside the detector and, likely, also to fluctuations of the electrical field inside the grains. The sampling techniques used to detect the pulses normally select the largest values of the collection distances, as underscored by results obtained from our measurements with β particles by using these techniques; results have been found to be in good agreement with literature data. The collection length distribution has been found to be compatible with a superposition of two log-normal distributions, which are typical of grain dimension distributions in polycrystalline materials. It can be argued that grain size is a limiting factor for the collection distance and contributes with its fluctuations to the broadening of the pulses height spectrum of a minimum ionizing particle (MIP) in CVD diamond detectors. It can also be argued that sampling techniques suffer from pulse counting losses, which should be partly due to grain boundary effects.

Theory of ion beam induced charge collection in detectors based on the extended Shockley–Ramo theorem

Abstract An analysis of the charge collection process induced by focused MeV ion beams in semiconductor devices is presented. It is based on the extended Shockley–Ramo theorem that provides a rigorous mathematical tool for the calculation of the induced charge and current under the assumption of a quasi-steady-state operation of the semiconductor device. A complete description of the theory and underlying assumption is given as well as a simple application of the method aimed to evaluate the main transport properties of fully depleted semiconductors from the analysis of frontal and lateral ion beam induced charge collection (IBICC) measurements.

Pulse height distribution and radiation tolerance of CVD diamond detectors

The paper reviews measurements of the radiation tolerance of CVD diamond for irradiation with 24 GeV/c protons, 300 MeV/c pions and 1 MeV neutrons. For proton and neutron irradiation, the measured charge signal spectrum is compared with the spectrum calculated by a model. Irradiation by particles causes radiation damage leading to a decrease of the charge signal. However, both the measurements and the outcome from the model shows that for tracker applications this drawback is at least partly counterbalanced by a narrowing of the distribution curve of the charge signal. In addition, we observed after proton irradiation at the charge signal spectrum a decrease of the number of small signals. As a result, the efficiency of a CVD diamond tracker is less affected by irradiation than the mean charge signal. (11 refs).

Ionoluminescence in CVD diamond and in cubic boron nitride

Abstract Using the new ion beam-induced luminescence (IBIL) apparatus in National Legnaro Laboratories, Italy, a series of measurements concerning both wide-area luminescence spectra and monochromatic luminescence maps with a space resolution of a few μm has been carried out on several CVD diamond and c-BN samples. Protons of 2 MeV with a penetration depth of approximately 25 μm have been used in order to investigate the materials in the bulk. These measurements have been correlated with particle-induced X-ray emission (PIXE) and EPR data. The measurements have been performed at increasing proton doses in order to also investigate the radiation hardness of luminescence peaks. The results indicate that ionoluminescence of CVD diamond is dominated by three bands at approximately 2, 2.4 and 2.9 eV, with the intermediate band being very radiation-hard, and the other two radiation-weak. The band at 2 eV is correlated with N content, and is particularly high in samples with poor electronic properties. IBIL in c-BN is also dominated by three bands, one at approximately 2 eV, and the other two at higher energies with respect to CVD diamond. All these three bands seem to be relatively radiation-hard with respect to CVD diamond, and to be related to defects induced by doping.

Effects of light on the primed' state of CVD diamond nuclear detectors

Abstract Diamond radiation detectors produced by Chemical Vapour Deposition (CVD) have been extensively studied as nuclear detectors both for the application as tracking detectors in high energy physics experiments and for applications in nuclear industry domain. A meaningful measurement to characterise the performance of such devices is the evaluation of the charge collection distance (CCD), i.e. the mean distance the charge carriers travel before being trapped. It is well known that CCD increases with the absorbed dose (priming or pumping process). The priming of diamond is usually explained by the saturation of active traps in the diamond bulk, which are filled by charge carriers generated by the ionisation. However, such a primed state is metastable, i.e. the exposure to light can de-pumps diamond to its initial state. The analysis of ‘pumping’ and ‘de-pumping’ processes is then useful to explain trapping mechanisms and polarisation effects, which limit the performances of diamond detectors. This paper deals with an investigation of the ‘pumping’ process on detector grade CVD diamond samples exposed to some doses of irradiation (X-rays or beta particles). Photoconductivity measurements carried out during monochromatic illumination as well as optical bleaching spectra of thermoluminescence glow curves highlight the passivation effects of some trap levels due to irradiation. A possible interpretation of these effects is presented and discussed, taking into account charge collection efficiency measurements carried out using Am-241 alpha particles.

Status of the R&D activity on diamond particle detectors

Chemical Vapor Deposited (CVD) polycrystalline diamond has been proposed as a radiation-hard alternative to silicon in the extreme radiation levels occurring close to the interaction region of the Large Hadron Collider. Due to an intense research effort, reliable high-quality polycrystalline CVD diamond detectors, with up to 270μm charge collection distance and good spatial uniformity, are now available. The most recent progress on the diamond quality, on the development of diamond trackers and on radiation hardness studies are presented and discussed.

CVD diamond microdosimeters

Abstract CVD diamond detectors have been evaluated for use as X-ray dosimeters. The response both at radiological diagnostic and at therapeutic energies of one “detector grade” commercially available parallelepiped CVD sample has been measured and compared with the performances of a “needle shaped” CVD diamond detector grown by the hot-filament CVD technique on a tungsten tip. All the detectors were demonstrated to be very suitable for bio-medical application: they are linear in dose rate, stable and relatively fast, with a signal-to-noise ratio higher than 1000.