M. Girolami

Diamond Detectors for UV and X-Ray Source Imaging

This work reports on the realization and test of a compact beam-profiling system for UV and X-ray sources, based on polycrystalline CVD diamond detectors. Multistrip and pixel structures have been used for 1-D and 2-D photodetectors, respectively. A dedicated read-out electronic circuitry has been designed and used to independently sample the signal produced by each strip (or pixel), enabling a real-time beam profile reconstruction.

A diamond based neutron spectrometer for diagnostics of deuterium-tritium fusion plasmas

Single crystal Diamond Detectors (SDD) are being increasingly exploited for neutron diagnostics in high power fusion devices, given their significant radiation hardness and high energy resolution capabilities. The geometrical efficiency of SDDs is limited by the size of commercially available crystals, which is often smaller than the dimension of neutron beams along collimated lines of sight in tokamak devices. In this work, we present the design and fabrication of a 14 MeV neutron spectrometer consisting of 12 diamond pixels arranged in a matrix, so to achieve an improved geometrical efficiency. Each pixel is equipped with an independent high voltage supply and read-out electronics optimized to combine high energy resolution and fast signals (<30 ns), which are essential to enable high counting rate (>1 MHz) spectroscopy. The response function of a prototype SDD to 14 MeV neutrons has been measured at the Frascati Neutron Generator by observation of the 8.3 MeV peak from the (12)C(n, α)(9)Be reaction occurring between neutrons and (12)C nuclei in the detector. The measured energy resolution (2.5% FWHM) meets the requirements for neutron spectroscopy applications in deuterium-tritium plasmas.

Pixelated Single-crystal Diamond Detector for fast neutron measurements

Single-crystal Diamond Detectors (SDDs), due to their high radiation hardness, fast response time and small size, are good candidates as fast neutron detectors in those environments where the high neutron flux is an issue, such as spallation neutron sources and the next generation thermonuclear fusion plasmas, i.e. the ITER experiment. Neutron detection in SDDs is based on the collection of electron-hole pairs produced by charged particles generated by neutron interactions with 12C. Recent measurements have demonstrated the SDD capability of measuring the neutron flux with a good energy resolution and at high rates. In this work a novel detector based on a 12-pixels SDD matrix will be presented. Each pixel is equipped with an independent electronic chain: the fast shaping preamplifier coupled to a digitizer is able to combine the high rate capability and the good energy resolution. Two CAEN digitizers are compared and the possibility of performing good energy resolution measurements ( 1 MHz per channel) is described. Each pixel was characterized and calibrated using an 241Am source: the energy resolution was evaluated and gives a mean value of 1.73% at 5.5 MeV. The good energy resolution achieved and its uniformity between pixels are the demonstration of the capability of this novel detector as a spectrometer. This system will be installed during the next Deuterium-Tritium campaign on a collimated vertical line of sight at JET for 14 MeV neutron measurements.

Radiation-assisted Frenkel-Poole transport in single-crystal diamond

The measurement of the density of occupied states as a function of the applied electric field, performed on single-crystal chemical vapour deposition diamond by x-ray modulated photocurrent technique, is reported. Two regimes of non-linear charge transport were observed: a classical Frenkel-Poole (FP) process at high electric fields (>6800 V/cm), and a radiation-assisted transport mechanism at intermediate electric fields (2000 to 6800 V/cm), consisting of a double-step process in which the direct re-emission into the extended band occurs following multiple photo-induced FP-like hopping transitions.

X-ray diamond detectors with energy resolution

Polycrystalline diamond detectors with energy resolving capability of the impinging beam were realized and tested by using a miniature pyroelectric x-ray pulse generator. Microstrip structures were defined by photolithography aimed to reduce parasitic capacitances and to perform characterization measurements in a sandwich configuration. Leakage currents as low as 20pA at 500V were measured on a 270μm thick device. Pulse height distributions were carried out around TaLα (8.14keV) and CuKα (8.05keV) characteristic lines of the source. Energy resolution at 200V was found equal to 9% with an increase to 11% at 500 V. When the bias was increased to the maximum voltage the sample shows an Ohmic behavior.

First neutron spectroscopy measurements with a pixelated diamond detector at JET

A prototype Single crystal Diamond Detector (SDD) was installed at the Joint European Torus (JET) in 2013 along an oblique line of sight and demonstrated the possibility to carry out neutron spectroscopy measurements with good energy resolution and detector stability in discharges heated by neutral beam injection and radio-frequency waves. Starting from these positive results, within the Vertical Neutron Spectrometer project of the Joint European Torus, we have developed a pixelated instrument consisting of a matrix of 12 independent SDDs, called the Diamond Vertical Neutron Spectrometer (DVNS), which boosts the detection efficiency of a single SDD by an order of magnitude. In this paper we describe the main features of the DVNS, including the detector design, energy resolution, and data acquisition system for on-line processing. Preliminary spectroscopy measurements of 2.5 MeV neutrons from the present deuterium plasma at JET are finally presented.

Buried Boron Doped Layer for CVD Diamond Photo-Thermionic Cathodes

A buried boron (B) doped layer has been fabricated by ion implantation into the bulk structure of a chemical vapor deposition (CVD) diamond film engineered to act as a photo-thermionic cathode for high-temperature solar cells. The boron layer implantation is a fundamental step in order to obtain an efficient diamond-based solar cell. Implantation of boron ions has been performed by fixing the ion dose to a value of 1 × 1015 at/cm2, while varying the ion beam kinetic energy from 40 to 250 keV which localizes the doped layer at different depths (from tens to a few hundreds of nanometers) below the absorbing surface of the cathode. Characterization of the optical and photoelectronic properties of the different implanted layers has been carried out so as to evaluate their effectiveness within the cathode structure. An ion beam kinetic energy of 40 keV is found to significantly increase the quantum efficiency of the CVD diamond plate. This will be useful for the development of the diamond-based photo-thermionic cathode.

Defect engineering of diamond cathodes for high temperature solar cells

A cathode structure for photon-enhanced thermionic emission was designed for high temperature energy conversion in solar concentrating systems. Surface-hydrogenated diamond is one of the few semiconductors to show negative electron affinity and a work function as low as 1.7 eV if nitrogen-doped, that is connected to a significant thermionic emission at moderate temperatures (up to 800 °C). But diamond is transparent to solar radiation, consequently advanced techniques for preparing an efficient sunlight absorbing diamond are discussed.

Diamond device architectures for UV laser monitoring

The paper reviews the status of diamond detectors for UV laser monitoring and imaging. Single pixel detectors, position sensitive architectures, optically activated switches and sensor arrays for beam positioning and imaging are analyzed. The performances of natural diamond and synthetic diamond produced by chemical vapor deposition are compared to evaluate the suitability of such an outstanding material for the described applications.

Radiation hard imaging detectors based on diamond electronics

We report on novel radiation hard imaging detectors based on diamond electronics. The proposed detectors can be operated at room temperature and are able to detect deep UV photons, X-rays, gamma rays, charged particles and neutrons for a wide range of industrial and research applications as: particle tracking at CERN, beam conditions monitoring for synchrotrons and LINACS, radiotheraphy imaging, excimer laser beam diagnostics etc. State of the-art commercially available photon and particle beam imaging and position detectors are mainly based on silicon, despite its intrinsic limitations. Main limitation of silicon devices is in the analysis of high-power sources as high energy particle and photon beams: the maximum density of energy that can be transferred to the detector without radiation damage is quite low, and the detectors lifetime is limited. To limit radiation damage, a reduced signal to noise ratio and high leakage current, silicon detectors need to be cooled down and cannot operate at room temperature; moreover, silicon detectors are also forced to use attenuators and/or wavelength converters (i.e. fluorescent crystals), so introducing loss factors in terms of spatial resolution.