E. Milani

Extreme ultraviolet single-crystal diamond detectors by chemical vapor deposition

High-quality single-crystal diamond films, homoepitaxially grown by microwave chemical vapor deposition, have been used to produce diamond-based photodetectors. Such devices were tested over a very wide spectral range, from the extreme ultraviolet (UV) (20 nm) up to the near IR region (2400 nm). An optical parametric oscillator tunable laser was used to investigate the 210–2400 nm spectral range in pulse mode. In this region, the spectral response shows a UV to visible contrast of about 6 orders of magnitude. A time response shorter than 5 ns, i.e., the laser pulse duration, was observed. By integrating the pulse shape, a minor slow component was evidenced, which can be explained in terms of trapping–detrapping effects. Extreme UV gas sources and a toroidal grating vacuum monochromator were used to measure the device response down to 20 nm in continuous mode. In particular, the extreme UV He spectrum was measured and the He II m, 30.4 nmand He I 58.4 nm emission lines were clearly detected. The measured t...

Growth of diamond films: General correlation between film morphology and plasma emission spectra

We have studied the emission spectra from plasmas excited in several compositions of three different gas mixtures useful for the growth of diamond films, namely CH4‐H2 (the classical one), CH4‐CO2 and C2H2‐CO2 (not previously reported by other authors). In all three cases we find the same correlation between the quality of the obtained diamond films and some spectral features: in particular, the yield of the best diamond films corresponds to the presence of the emission line at wavelength 431 nm ascribed to CH, in the absence of the emission band at 505–517 nm attributed to C2. The appearance and the progressive increase of the latter corresponds to a gradual worsening of the diamond film quality. We propose such spectral features of the plasma as a general and practical gauge of the diamond film growth conditions, for any gas mixture used.

High-quality diamond grown by chemical-vapor deposition: Improved collection efficiency in α-particle detection

Diamond films were grown on silicon by microwave chemical-vapor deposition using a CH4–H2 gas mixture. The crystalline quality of the films was assessed through their α-particle detection performance, a property highly sensitive to film quality, by using a 5.5 MeV 241Am source. A maximum collection efficiency η of 70%, 50% being the average value, was obtained in a 115-μm-thick sample after β-particle irradiation (“priming effect”). Unprimed efficiency η=50% maximum, 30% average, was also obtained on other samples. The dependence of the efficiency and the resolving power on the external electric field was studied as well. The results are interpreted by means of a Monte Carlo analysis of the α-particle detection process. It is concluded that, in the priming process, a saturation occurs of deep defects limiting the as-grown detector performance, and charge collection distance is only limited by grain boundaries located close to the substrate side. Therefore, there is indication that further improvement can ...

Dosimetric characterization of a synthetic single crystal diamond detector in clinical radiation therapy small photon beams

PURPOSE To determine the potentialities of synthetic single crystal diamond Schottky diodes for accurate dose measurements in radiation therapy small photon beams. METHODS The dosimetric properties of a diamond-based detector were assessed by comparison with a reference microionization chamber. The diamond device was operated at zero bias voltage under irradiation with high-energy radiotherapic photon beams. The stability of the detector response and its dose and dose rate dependence were measured. Different square field sizes ranging from 1 × 1 cm2 to 10 × 10 cm2 were used during comparative dose distribution measurements by means of percentage depth dose curves (PDDs), lateral beam profiles, and output factors. The angular and temperature dependence of the diamond detector response were also studied. RESULTS The detector response shows a deviation from linearity of less than ±0.5% in the 0.01-7 Gy range and dose rate dependence below ±0.5% in the 1-6 Gy/min range. PDDs and output factors are in good agreement with those measured by the reference ionization chamber within 1%. No angular dependence is observed by rotating the detector along its axis, while ∼3.5% maximum difference is measured by varying the radiation incidence angle in the polar direction. The temperature dependence was investigated as well and a ±0.2% variation of the detector response is found in the 18-40 °C range. CONCLUSIONS The obtained results indicate the investigated synthetic diamond-based detector as a candidate for small field clinical radiation dosimetry in advanced radiation therapy techniques.PURPOSE To determine the potentialities of synthetic single crystal diamond Schottky diodes for accurate dose measurements in radiation therapy small photon beams. METHODS The dosimetric properties of a diamond-based detector were assessed by comparison with a reference microionization chamber. The diamond device was operated at zero bias voltage under irradiation with high-energy radiotherapic photon beams. The stability of the detector response and its dose and dose rate dependence were measured. Different square field sizes ranging from 1 × 1 cm(2) to 10 × 10 cm(2) were used during comparative dose distribution measurements by means of percentage depth dose curves (PDDs), lateral beam profiles, and output factors. The angular and temperature dependence of the diamond detector response were also studied. RESULTS The detector response shows a deviation from linearity of less than ±0.5% in the 0.01-7 Gy range and dose rate dependence below ±0.5% in the 1-6 Gy∕min range. PDDs and output factors are in good agreement with those measured by the reference ionization chamber within 1%. No angular dependence is observed by rotating the detector along its axis, while ∼3.5% maximum difference is measured by varying the radiation incidence angle in the polar direction. The temperature dependence was investigated as well and a ±0.2% variation of the detector response is found in the 18-40 °C range. CONCLUSIONS The obtained results indicate the investigated synthetic diamond-based detector as a candidate for small field clinical radiation dosimetry in advanced radiation therapy techniques.

Chemical vapor deposition diamond based multilayered radiation detector: Physical analysis of detection properties

Recently, solid state photovoltaic Schottky diodes, able to detect ionizing radiation, in particular, x-ray and ultraviolet radiation, have been developed at the University of Rome “Tor Vergata.” We report on a physical and electrical properties analysis of the device and a detailed study of its detection capabilities as determined by its electrical properties. The design of the device is based on a metal/nominally intrinsic/p-type diamond layered structure obtained by microwave plasma chemical vapor deposition of homoepitaxial single crystal diamond followed by thermal evaporation of a metallic contact. The device can operate in an unbiased mode by using the built-in potential arising from the electrode-diamond junction. We compare the expected response of the device to photons of various energies calculated through Monte Carlo simulation with experimental data collected in a well controlled experimental setup i.e., monochromatic high flux x-ray beams from 6 to 20 keV, available at the Diamond Light Sour...

High performance Li6F-diamond thermal neutron detectors

Fabrication reproducibility and high performance reliability were obtained in fissile-material-free thermal neutron detectors based on chemical vapor deposited diamond in a multilayered p-type/intrinsic/metal design. Under α particle irradiation, all the detectors (more than ten) have shown 100% charge collection efficiency and approximately 1.5% energy resolution. A Li6F layer was deposited on the detector surface as converting material for thermal neutrons through the Li6(n,α)T nuclear reaction. Both the 2.73MeV tritium and the 2.06MeV α peaks are detected and clearly resolved. Stable performance and excellent linear behavior of the count rate versus the incident neutron flux were observed.

Thermal and fast neutron detection in chemical vapor deposition single-crystal diamond detectors

Recently, a compact solid-state neutron detector capable of simultaneously detecting thermal and fast neutrons was proposed [M. Marinelli et al., Appl. Phys. Lett. 89, 143509 (2006)]. Its design is based on a p-type/intrinsic/metal layered structure obtained by Microwave Plasma Chemical Vapor Deposition (CVD) of homoepitaxial diamond followed by thermal evaporation of an Al contact and a L6iF converting layer. Fast neutrons are directly detected in the CVD diamond bulk, since they have enough energy to produce the C12(n,α)B9e reaction in diamond. Thermal neutrons are instead converted into charged particles in the L6iF layer through the L6i(n,α)T nuclear reaction. These charged particles are then detected in the diamond layer. The thickness of the L6iF converting layer and the CVD diamond sensing layer affect the counting efficiency and energy resolution of the detector both for low- (thermal) and high-energy neutrons. An analysis is carried out on the dynamics of the L6i(n,α)T and the C12(n,α)B9e reactio...

Characterization of a synthetic single crystal diamond Schottky diode for radiotherapy electron beam dosimetry

PURPOSE To investigate the dosimetric properties of synthetic single crystal diamond based Schottky diodes under irradiation with therapeutic electron beams from linear accelerators. METHODS A single crystal diamond detector was fabricated and tested under 6, 8, 10, 12, and 15 MeV electron beams. The detector performances were evaluated using three types of commercial detectors as reference dosimeters: an Advanced Markus plane parallel ionization chamber, a Semiflex cylindrical ionization chamber, and a p-type silicon detector. Preirradiation, linearity with dose, dose rate dependence, output factors, lateral field profiles, and percentage depth dose profiles were investigated and discussed. RESULTS During preirradiation the diamond detector signal shows a weak decrease within 0.7% with respect to the plateau value and a final signal stability of 0.1% (1σ) is observed after about 5 Gy. A good linear behavior of the detector response as a function of the delivered dose is observed with deviations below ±0.3% in the dose range from 0.02 to 10 Gy. In addition, the detector response is dose rate independent, with deviations below 0.3% in the investigated dose rate range from 0.17 to 5.45 Gy∕min. Percentage depth dose curves obtained from the diamond detector are in good agreement with the ones from the reference dosimeters. Lateral beam profile measurements show an overall good agreement among detectors, taking into account their respective geometrical features. The spatial resolution of solid state detectors is confirmed to be better than that of ionization chambers, being the one from the diamond detector comparable to that of the silicon diode. A good agreement within experimental uncertainties was also found in terms of output factor measurements between the diamond detector and reference dosimeters. CONCLUSIONS The observed dosimetric properties indicate that the tested diamond detector is a suitable candidate for clinical electron beam dosimetry.

Compositional and spectroscopic study of the growth of diamond films from several gaseous mixtures

A critical experimental test of the empirical unifying scheme recently proposed by Bachmann, Leers, and Lydtyn [Diamond and Related Mater. 1, 1 (1991)] about the gas compositions useful to grow diamond films by plasma‐assisted chemical‐vapor deposition has been performed. The data confirm the main concept of the Bachmann scheme, namely, the existence of a single compositional ‘‘diamond domain’’ in a C‐O‐H triangular diagram, in which the overall gas compositions are plotted; however, quantitatively, the shape and the borders of such a diamond domain are rather different from those assumed in the work of Bachmann and co‐workers. For all the gas mixtures investigated, the changes in the plasma optical emission spectra consequent to crossing the border from the ‘‘no‐growth zone’’ into the ‘‘diamond domain’’ have been studied. These changes show universal features, which point to the presence of the same growth mechanisms over all the diamond domain, independent of the identity of the initial chemical species.

Single-crystal diamond detector for time-resolved measurements of a pulsed fast-neutron beam

A fast-neutron detector for time-resolved beam measurements at spallation neutron sources is presented. The device features a p-type/intrinsic/metal Schottky barrier structure where the active (intrinsic) detection layer is a 150 μm thick single-crystal diamond obtained by chemical-vapour deposition. Coupling to fast front-end electronics preserves the excellent timing properties of the device as demonstrated in tests performed at the ISIS spallation neutron source in UK. The device represents a novel approach in the field of pulsed fast-neutrons spectroscopic techniques. It will find immediate application in localized (mm resolution) fast-neutron fluence measurements required by neutron irradiation experiments at ISIS also envisaging its use for spectrum measurements.