P. Polesello

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.

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).

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 tips as X-ray detectors

Abstract Diamond is a very attractive material for use as a radiation detector due to its inertness, radiation hardness and tissue equivalence. The development of diamond radiation detectors was limited by the availability of good and reproducible natural samples. The advent of chemical vapour deposition techniques has allowed these drawbacks to be overcome and applications of CVD diamond in X-ray and nuclear dosimetry to be explored by several researchers. In this paper, we report the fabrication and characterisation of miniaturised (0.1 mm3) X-ray detectors made from polycrystalline diamond films ( 10 20 μm thick) deposited by hot filament vapour deposition technique on metallic (tungsten) 300-μm-diameter tips. The detector response to X-ray flux produced by an X-ray generator with peak energies ranging from 50 to 250 keV and by electron linear accelerator used in radiotherapy with a peak energy of 6 MeV has been investigated. The linearity and the sensitivity of the detector as a function of dose rate at different photon energies has been evaluated, and the collection length has been estimated by analysing the behaviour of the photocurrent as a function of the applied bias voltage.

Investigation on the electric field profile in CdTe by ion beam induced current

Abstract The charge collection efficiency profile along the thickness of a CdTe detector was measured for first time by using a proton microbeam and a polished cross-section of the devices. Different samples were investigated and tests were carried out along different lines from cathode to anode, with different biases and polarities, and with different shaping times. The effects of all these parameters are evident in the experimental curves. By fitting these curves, the behaviour of the drift length for electrons and holes is obtained, if a constant ratio between them is assumed. If the mobilities and trapping times are uniform, the electric field profile is obtained. The profile has a minimum at the anode and a maximum towards the cathode, at least for large bias voltage.

A micro-IBIC comparison between natural and CVD diamond

Abstract Micro-IBIC (ion beam-induced current) by means of a proton microbeam is a powerful tool for mapping the distribution of charge collection lengths at the surface and in the bulk of diamond samples. Up to now, an evident correspondence between the morphological columnar structure of chemical vapour-deposited (CVD) diamond and the “stripy” shape of IBIC maps has been found. This points out the presence of a microcrystalline “electric” structure and the harmful effects of defects associated to grain boundaries on transport properties. In this work we present for the first time a comparison between micro-IBIC maps of cross-sections of a natural IIa diamond and maps obtained in CVD diamond samples. In natural IIa diamond, IBIC maps are homogeneous and the global IBIC spectrum is peaked at a mean collection efficiency of about 10% at an electric field of 7500 V cm −1 . Strong irradiation produces a uniform increase of the mean collection length in natural IIa diamond whereas, in CVD diamond, it produces an homogenisation of the space charge with an increase of the counting rate without any evident increase of the average collection length.

EVALUATION OF THE DIFFUSION LENGTH IN SILICON DIODES BY MEANS OF THE LATERAL IBIC TECHNIQUE

The transport properties of silicon p+–n–n+ diodes with a junction depth of about 47 μm have been investigated by means of the lateral IBIC technique. Cross section of the samples have been irradiated by a low intensity 5 MeV proton microbeam and the charge pulses have been recorded as a function of incident proton position. The charge collection efficiency (cce) profiles show broad plateaux with values close to 100%. The region where the electric field is absent shows exponentially decreasing cce profiles. The estimate of the decay rate allows the diffusion length of the minority carriers in the bulk of the device to be measured. The analytical method presented in this paper takes into account the dependence of the signal rise time on the incident proton position. The entire cce profile is in full agreement with the IBIC theory based on the extended Ramo’s theorem, which provides a very effective tool for quantitative estimates of transport parameters in semiconductor devices.