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X-ray and UV photovoltaic Schottky photodiodes based on single crystal diamond were recently developed at Rome 'Tor Vergata' University laboratories. In this work, different rectifying metallic contact materials were thermally evaporated on the oxidized surface of intrinsic single crystal diamond grown by chemical vapor deposition. Their impact on the detection performance in the extreme UV and soft x-ray spectral regions was studied. The electrical characterization of the metal/diamond Schottky junctions was performed at room temperature by measuring the capacitance-voltage characteristics. The diamond photodiodes were then tested both over the extreme UV spectral region from 10 to 60 eV by using He-Ne DC gas discharge as a radiation source and toroidal vacuum monochromator, and in the soft x-ray range from 6 to 20 keV at the Diamond Light Source synchrotron x-ray beam-line in Harwell (UK). In both experimental setups, time response and spectral responsivity were analyzed for all the investigated Schottky contact materials. A good agreement between the experimental data and theoretical results from Monte Carlo simulations is found [ABSTRACT FROM AUTHOR]

A study on the effect of secondary electron emission, which strongly affects the detection of extreme-UV radiation, was performed on diamond detectors. Two different structures were compared: interdigitated contacts and a transverse Schottky diode configuration. Both devices were electrically characterized by I-V measurements and their responsivity was measured in the extreme UV spectral region (20-120 nm) by using He-Ne gas discharge radiation sources and a toroidal grating vacuum monochromator. Through an ad-hoc measurement configuration, the contributions of the internal photocurrent and of the photoemission current have been analyzed and separately evaluated. The results showed that secondary electron emission, which clearly depends on the experimental conditions (e.g., external electric field, pressure, etc.), is one of the most relevant processes affecting the spectral responsivity in the extreme UV band. In particular, for interdigitated devices, extreme care must be taken in order to obtain an absolute value of their responsivity, while detectors in the transverse configuration can be shielded in such a way to avoid secondary electron current contribution and therefore provide a more correct and reliable response. [ABSTRACT FROM AUTHOR]

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 Source synchrotron in Harwell (U.K.). [ABSTRACT FROM AUTHOR]

High performance neutron detectors sensitive to both thermal and fast neutrons are of great interest to monitor the high neutron flux produced, e.g., by fission and fusion reactors. An obvious requirement for such an application is neutron irradiation hardness. This is why diamond based neutron detectors are currently under test in some of these facilities. In this paper the damaging effects induced in chemical vapor deposition (CVD) diamond based detectors by a neutron fluence of ∼2×1016 neutrons/cm2 have been studied and significant changes in spectroscopic, electrical, and optical properties have been observed. The detectors are fabricated using high quality synthetic CVD single crystal diamond using the p-type/intrinsic/Schottky metal/6LiF layered structure recently proposed by Marinelli et al. [Appl. Phys. Lett. 89, 143509 (2006)], which allows simultaneous detection of thermal and fast neutrons. Neutron radiation hardness up to at least 2×1014 n/cm2 fast (14 MeV) neutron fluence has been confirmed so far [see Pillon et al., (Fusion Eng. Des. 82, 1174 (2007)]. However, at the much higher neutron fluence of ∼2×1016 neutrons/cm2 damage is observed. The detector response to 5.5 MeV 241Am α-particles still shows a well resolved α-peak, thus confirming the good radiation hardness of the device but a remarkable degradation and a significant instability with time of charge collection efficiency and energy resolution arise. Symmetric, nearly Ohmic I-V (current-voltage) characteristics have been recorded from the metal/intrinsic/p-doped diamond layered structure, which before neutron irradiation acted as a Schottky barrier diode with a strong rectifying behavior. The nature and the distribution of the radiation induced damage have been deeply examined by means of cathodoluminescence spectroscopy. A more heavily damaged area into the intrinsic diamond at the same position and with the same extension of the 6LiF layer has been found, the increased damage being ascribed to the highly ionizing particles produced in the 6LiF layer by thermal neutrons through the nuclear reaction 6Li(n,α)T. [ABSTRACT FROM AUTHOR]

Diamond exhibits many properties such as an outstanding radiation hardness and fast response time both important to design detectors working in extremely radioactive environments. Among the many applications these devices can be used for, there is the development of a fast and radiation hard neutron detector for the next generation of fusion reactors, such as the International Thermonuclear Experimental Reactor project, under construction at Cadarache in France. A technology to routinely produce electronic grade synthetic single crystal diamond detectors was recently developed by our group. One of such detectors, with an energy resolution of 0.9% as measured using an 241Am α particle source, has been heavily irradiated with 14.8 MeV neutrons produced by the Frascati Neutron Generator. The modifications of its spectroscopic properties have been studied as a function of the neutron fluence up to 2.0×1014 n/cm2. In the early stage of the irradiation procedure an improvement in the spectroscopic performance of the detector was observed. Subsequently the detection performance remains stable for all the given neutron fluence up to the final one thus assessing a remarkable radiation hardness of the device. The neutron damage in materials has been calculated and compared with the experimental results. This comparison is discussed within the nonionizing energy loss (NIEL) hypothesis, which states that performance degradation is proportional to NIEL. [ABSTRACT FROM AUTHOR]

The efficiency and charge collection distance (CCD) of nuclear particle detectors based on high quality diamond films grown by chemical vapor deposition (CVD) have been systematically studied as a function of the methane content in the growth gas mixture and for varying film thickness. The effects of preirradiation with β particles (pumping) have been thoroughly studied. The results fully support a recently proposed model [Marinelli et al., Appl. Phys. Lett. 75, 3216 (1999)] discussing the role of in-grain defects and grain boundaries in determining the charge collection spectra of CVD diamond films both in the normal and in the pumped state. The model allows us to quantitatively explain the dependence of CCD and efficiency on film thickness, giving a microscopic picture of the effects of preirradiation with ionizing radiation in CVD diamond films. The highest average CCD obtained is 145 μm in a 160 μm thick detector (corresponding to about 50% average efficiency), while the maximum value (about 70% efficiency) is close to 370 μm. In addition, CCD is shown to be higher than film thickness and to monotonically increase with thickness, indicating margins for further improvements. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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 12C(n, α)9 Be reaction occurring between neutrons and C nuclei in the detector. The measured energy resolution (2.5% FWHM) meets the requirements for neutron spectroscopy applications in deuterium-tritium plasmas. [ABSTRACT FROM AUTHOR]

Measurements of the response of LaBr3(Ce) to 2.5 MeV neutrons have been carried out at the Frascati Neutron Generator and at tokamak facilities with deuterium plasmas. The observed spectrum has been interpreted by means of a Monte Carlo model. It is found that the main contributor to the measured response is neutron inelastic scattering on 79Br, 81Br, and 139La. An extrapolation of the count rate response to 14 MeV neutrons from deuterium-tritium plasmas is also presented. The results are of relevance for the design of γ-ray diagnostics of fusion burning plasmas. [ABSTRACT FROM AUTHOR]

Notwithstanding the advances of the past decades, significant developments are still needed to satisfactorily diagnose 'burning plasmas.' D-T plasmas indeed require a series of additional measurements for the optimization and control of the configuration: the 14 MeV neutrons, the isotopic composition of the main plasma, the helium ash, and the redistribution and losses of the alpha particles. Moreover a burning plasma environment is in general much more hostile for diagnostics than purely deuterium plasmas. Therefore, in addition to the development and refinement of new measuring techniques, technological advances are also indispensable for the proper characterization of the next generation of devices. On JET an integrated program of diagnostic developments, for JET future and in preparation for ITER, has been pursued and many new results are now available. In the field of neutron detection, the neutron spectra are now routinely measured in the energy range of 1-18 MeV by a time of flight spectrometer and they have allowed studying the effects of rf heating on the fast ions. A new analysis method for the interpretation of the neutron cameras measurements has been refined and applied to the data of the last trace tritium campaign (TTE). With regard to technological upgrades, chemical vapor deposition diamond detectors have been qualified both as neutron counters and as neutron spectrometers, with a potential energy resolution of about one percent. The in situ calibration of the neutron diagnostics, in preparation for the operation with the ITER-like wall, is also promoting important technological developments. With regard to the fast particles, for the first time the temperature of the fast particle tails has been obtained with a new high purity Germanium detector measuring the gamma emission spectrum from the plasma. The effects of toroidal Alfven eigenmodes modes and various MHD instabilities on the confinement of the fast particles have been determined with a combination of gamma ray cameras, neutral particle analyzers, scintillator probe, and Faraday cups. From a more technological perspective, various neutron filters have been tested to allow measurement of the gamma ray emission also at high level of neutron yield. [ABSTRACT FROM AUTHOR]

Chemical-vapor-deposited (CVD) diamond films are considered as neutron detectors for nuclear fusion devices because of their radiation hardness. Data about the radiation hardness of polycrystalline CVD diamond films exposed to 14 MeV neutron are missing in literature so the actual capability of CVD diamond detectors to withstand fusion device conditions must be truly demonstrated. In this work a polycrystalline CVD diamond detector, 101 μm thick, was irradiated for the first time with 14 MeV neutrons produced by the Fusion Neutron Source of the Japan Atomic Energy Research Institute with the goal to study its radiation hardness. The 14 MeV neutron fluence was 8×1014 n/cm2. The film performances were studied before and after the 14 MeV neutron irradiation by using 5.5 MeV α from 241Am source, both in the pumped and the “as-grown” state. A comparison with previous measurements performed in more soft neutron spectra (mean neutron energy of 1–2 MeV) is reported pointing out the more damaging effects of the 14 MeV neutrons. It was found that annealing at 500 °C and redeposition of the gold contact followed by a proper pumping procedure will restore more than 70% the initial working conditions of the irradiated detector. An analysis of the neutron field expected in the neutron camera of the International Thermonuclear Reactor fusion tokamak was also performed, showing the capability of CVD diamond detector to withstand the 14 MeV neutron fluence expected in about one year of operation. [ABSTRACT FROM AUTHOR]

A polycrystalline chemical vapor deposited (CVD) diamond detector was installed on a JET tokamak in order to monitor the time dependent 14 MeV neutron emission produced by D–T plasma pulses during the Trace Tritium Experiment (TTE) performed in October 2003. This was the first tentative ever attempted to use a CVD diamond detector as neutron monitor in a tokamak environment. Despite its small active volume, the detector was able to detect the 14 MeV neutron emission (>1.0×1015 n/shot) with good reliability and stability during the experimental campaign that lasted five weeks. The comparison with standard silicon detectors presently used at JET as 14 MeV neutron monitors is reported, showing excellent correlation between the measurements. The results prove that CVD diamond detectors can be reliably used in a tokamak environment and therefore confirm the potential of this technology for next step machines like ITER. [ABSTRACT FROM AUTHOR]

Reports on the measurement of the absolute neutron yield of the 14 MeV Frascati neutron generator by means of the associated alpha-particle method with a silicon surface barrier detector. Characterization of the neutron source in terms of absolute intensity and angle-energy distribution of emitted neutrons.

This paper describes the in situ calibration of the BF3 detector system used for measuring the total neutron yield from the FTU tokamak. Two different neutron sources, 252Cf (7.23×107 n/s) and AmBe (2.18×106 n/s), were used. The calibration was performed by placing the two sources at different radial and toroidal positions inside the vacuum vessel. Data on the response of the different detectors as a function of the source position are presented. The effect of the different spectra of the neutron sources is pointed out. Finally, the procedure for the computation of the relationship between the local flux at the detectors and the total neutron yield from an extended plasma is presented. [ABSTRACT FROM AUTHOR]

Thermal neutron flux monitors were fabricated using chemical vapor deposited single crystal diamond in a p-type/intrinsic/metal/6LiF layered structure. They were placed 80 cm above the core midplane of a 1 MW research fission reactor, where the maximum neutron flux is 2.2×109 neutrons/cm2 s. Good stability and reproducibility of the device response were observed over the whole reactor power range. A 150 000 counts/s count rate was measured at the maximum reactor power with no degradation of the detector signal. As the multiple pile-up process due to the slow readout electronics is accounted for, an excellent linearity of the diamond response is observed. [ABSTRACT FROM AUTHOR]

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 6LiF layer was deposited on the detector surface as converting material for thermal neutrons through the 6Li(n,α)T nuclear reaction. Both the 2.73 MeV tritium and the 2.06 MeV α peaks are detected and clearly resolved. Stable performance and excellent linear behavior of the count rate versus the incident neutron flux were observed. [ABSTRACT FROM AUTHOR]

The growth conditions and the detection properties of a homoepitaxial diamond film, deposited in Roma “Tor Vergata” University Laboratories by microwave chemical vapor deposition on a high-pressure high-temperature single-crystal substrate are reported. An energy resolution as low as 1.1% was achieved when irradiating the device with 5.5 MeV 241Am α-particles. The dependence of the charge collection efficiency and the energy resolution on the applied voltage are reported as well. A clear saturation plateau was observed in both curves. Preliminary results with 14.8 MeV neutron irradiation are reported, showing a well separated 12C(n,α0)9Be reaction peak. [ABSTRACT FROM AUTHOR]

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 time response of 0.2 s is totally due to the instrumental readout time constants. In both experimental setups an extremely good stability and reproducibility of the device response were obtained, whereas no persistent photoconductivity nor undesirable pumping effects were observed. [ABSTRACT FROM AUTHOR]

Defects limiting the movement of charge carriers in polycrystalline chemical vapor deposition (CVD) diamond films are located within the grains or in grain boundaries. Their geometrical distribution in the sample is different and is usually unknown. We present here a method to quantitatively evaluate the concentration and distribution of in-grain and grain-boundary located active carrier traps. Since the impact of these two kinds of defects on the performance of CVD diamond based devices is different, it is possible to obtain the defect distribution by measuring the response of diamond alpha particle detectors as a function of film thickness. The Hecht theory, describing the efficiency of a semiconductor particle detector, has been modified to take into account the polycrystalline nature of CVD diamond. This extended Hecht model was then used to fit experimental data and extract quantitative information about the defect distribution. [ABSTRACT FROM AUTHOR]

A method is presented to selectively characterize the traps involved in the pumping procedure of diamond films. The pumping process strongly reduces the concentration of active carrier trapping centers, leading to an enhancement of electronic properties of such material, and is obtained by irradiating the diamond films with ionizing radiation. Since the improved transport properties lead to an increased efficiency when diamond films are used as radiation detectors, valuable information on this process can be obtained by analyzing the response of diamond based particle detectors. For this purpose a high-quality diamond film was grown by microwave chemical vapor deposition and a particle detector was realized. Its response to a 5.5-MeV [sup 241]Am α-particles was measured after successive annealing steps performed at different temperatures in the 180–228 °C range. Before each annealing curve at a given temperature, the detector was driven to the pumped state through β-particle irradiation. The efficiency versus annealing time curves evidence a thermally induced detrapping, confirming the pumping mechanism as a filling and consequent passivation of defects. The analysis of the decay time of the detector efficiency as a function of temperature allows the determination of the activation energy of these defects, which is E[sub a]=1.62±0.15 eV. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A simple technique for measuring the electron and hole mean drift distance in chemical vapor deposition polycrystalline diamond in the as-grown and in the so-called pumped state obtained by 90Sr β-particle irradiation is presented. To this purpose, the efficiency h of a diamond-based particle detector was measured using a 5.5-MeV [SUP241]Am α-particle source. In particular, two different experimental setups were specifically designed and realized in order to perform a systematic study of the device efficiency as a function of the a-particle penetration depth, both in the positive and negative bias polarization. In the first setup, air is used as an absorbing layer in order to change the energy of the impinging a particles, while in the second one, the measurements were performed in vacuum and the incidence angle was varied in the 0°-80° range. The advantages of the latter setup are evidenced. The theoretical formula for the mean drift distances of carriers is derived using a properly modified Hecht model, and fitted to the data, allowing a separate evaluation of the charge collection distances of each carrier type (λ[SUBe] = μ[SUBe]τ[SUBe] E and λ[SUBh] = μ[SUBh]τ[SUBh]E). The obtained results)unambiguously show that the pumping process is much more effective on hole conduction, λ[SUBh] being much greater than λ[SUBe] in the pumped state. [ABSTRACT FROM AUTHOR]

Examines the crystalline quality of synthetic diamond thin films grown by chemical vapor deposition (CVD). Parameters in assessing the quality of CVD diamond films; Columnar nature of CVD diamond growth; Influence of applied electric field on the response of particle detectors.

A neutron spectrometer based on both the track position identification and the energy measurement of recoiling protons from a hydrogenous radiator is proposed. The expected performance limits of this spectrometer with regard to energy resolution, efficiency, and counting rate are evaluated for five different configurations. The results show the possibility of deriving an optimized spectrometer design with an energy resolution up to 1% at 14 MeV. In particular, the application of the spectrometer for the ion temperature measurement on the Next European Torus is presented. [ABSTRACT FROM AUTHOR]

A benchmark experiment has been performed for the absolute calibration of the FTU tokamak neutron activation system with the purpose of comparing experimental and numerical absolute reaction rates. Indium samples have been activated by a [sup 252]Cf neutron source located on the torus axis at three different toroidal angles with respect to the irradiation ends. The same experimental arrangements have been simulated with the Monte Carlo neutron and photon transport code MCNP, in order to reproduce numerically the experimental results. The comparison was intended to assess the accuracy of the numerical simulation and of the modeling of the FTU device. The activation technique will be routinely used as a complementary method to the use of neutron counters for the determination of the absolute neutron yield. An agreement within 22% between experiment and calculation has been found, showing that the model used in MCNP provides an acceptable representation for FTU. [ABSTRACT FROM AUTHOR]

The time-resolved neutron yield is routinely measured on the JET tokamak using a set of fission chambers. At present, the preferred technique is to employ activation reactions to determine the neutron fluence at a well-chosen position and to relate the measured fluence to the total neutron emission by means of neutron transport calculations. The delayed neutron counting method is a particularly convenient method of performing the activation measurement and the fission cross sections are accurately known. This paper outlines the measurement technique as used on JET. [ABSTRACT FROM AUTHOR]

The [sup 28]Si(n, p)[sup 28]Al reaction is shown to be ideal for monitoring the emission of 14 MeV neutrons from d-t fusion plasmas. The daughter nuclide, [sup 28]Al, has a 2.246 min half-life and so is suitable for studying discharges of up to 1 min duration; the decay gamma radiation has an energy of 1.779 MeV and is easily detected with a NaI (Tl) detector. The same sample can be recycled for successive JET discharges and the data taking is fully automatic. Absolute neutron yields are obtained by calibrating with the standard [sup 63]Cu(n, 2n)[sup 62]Cu reaction. The effective threshold energy of 5 MeV completely discriminates against 2.5 MeV d-d neutrons, making this reaction also highly appropriate for the study of the 14 MeV neutrons emitted during the burnup of the 1.0 MeV tritons produced in the second branch of the d-d reaction. Some results from this application will be presented. [ABSTRACT FROM AUTHOR]