Single-crystal Chemical Vapor Deposition Research Articles

Simultaneous measurements for fast neutron flux and tritium production rate using pulse shape discrimination and single crystal CVD diamond detector

This paper presents the development of a simultaneous measurement method for fast neutron energy spectra and tritium production rates within mixed radiation fields using a single crystal chemical vapour deposition diamond detector combined with a lithium fluoride (LiF) foil. The method involves the separation of pulses with rectangular shapes and the determination of the depth position within the single crystal diamond (SCD) struck by fast neutrons or nuclear reaction products including recoil tritons from the LiF foil based on pulse width, extracting pulse events occurred at the specific bulk region and the surface region of the SCD. Subsequently, unfolding techniques were employed to analyse the energy deposition spectrum of pulses at the specific bulk region which are induced only by fast neutrons, allowing the deduction of the fast neutron energy spectrum. To evaluate the tritium production rate, the energy deposition spectrum of pulses from events occurring at the SCD surface facing the LiF foil was analysed. By estimating the energy deposition spectrum solely induced by fast neutrons striking the SCD surface and subtracting it from the energy deposition spectrum of events at the SCD surface, the contribution of energetic ions, such as recoil tritons generated by the 6Li(n,α)3H reaction in the LiF foil, was determined. The fast neutron flux and tritium production rate obtained through this study were consistent with particle transport calculations, demonstrating the successful development of a method suitable for performance testing of fusion reactor blankets.

High temperature X-ray and γ-ray spectroscopy with a diamond detector

A single crystal chemical vapour deposition diamond detector was electrically characterised and then investigated as a spectroscopic photon counting X-ray and γ-ray detector across the temperature range, T, −20 °C ≤ T ≤ 100 °C. The detector was connected to a custom-built charge-sensitive preamplifier of low electronic noise and illuminated in turn by 55Fe radioisotope X-ray and 109Cd radioisotope X-ray and γ-ray sources. In combination, the radiation sources provided characteristic soft (e.g. 5.9 keV) and hard (e.g. 22.16 keV) X-rays and 88.03 keV γ-ray emissions. The Mn Kα (5.9 keV) and Kβ (6.49 keV) X-ray emissions from the 55Fe radioisotope X-ray source were detected (and separated from the zero energy noise peak) at temperatures of −20 °C ≤ T ≤ 60 °C; the Ag Kα (22.16 and 21.99 keV) and Kβ (24.94, 24.99, and 25.46 keV) X-ray emissions and 88.03 keV γ-ray emissions were detectable across the entire temperature range investigated (−20 °C ≤ T ≤ 100 °C). The detector and the preamplifier were operated without cooling across the entire temperature range. The energy resolution (full width at half maximum) at 22.16 keV (Ag Kα1) was 4.57 keV ± 0.26 keV at 100 °C and 2.70 keV ± 0.18 keV at −20 °C. Noise analysis indicated that the combination of dielectric noise and any incomplete charge collection noise present dominated the other noise components at 5.9 keV. When the detector was illuminated with the 109Cd radioisotope X-ray and γ-ray source, a high count rate introduced parasitic effects (possibly baseline shift) which reduced the optimum shaping time of the spectrometer.

Influence of high fluence heavy ion irradiation on the performance of single crystal diamond detectors

Radiation detectors based on diamond are highly favored for particle physics research due to their superior radiation hardness. In this work, the performance of single crystal diamond detectors under high fluence heavy ion radiation has been investigated. After irradiation by Fe13+ and Xe20+ ion beams with a high particle fluence of 1.2 × 1015 cm-2, the properties of unirradiated and irradiated intrinsic single crystal chemical vapor deposition (CVD) diamond detectors are compared by means of the spectroscopic energy resolution, charge collection efficiency (CCE), mobility-lifetime (μτ) product and time response using a 241Am alpha (α) particle source. The experimental results of the unirradiated sample with respect to I-V dark current levels, showed an ohmic behaviour of the Al and Cr/Au contact, CCE of 100%, fast response with a rise time of 750 ps and a fall time of 600 ps as well as drift velocities of 6.67 × 104 m/s and 4.72 × 104 m/s for holes and electrons respectively. In the case of the irradiated samples, the CCE, μτ product and time response pulse amplitude of holes and electrons were all degraded, indicating that high density trappings for both kinds of carriers were created under the diamond crystal surface and the trapping probability of holes and electrons was equal, which was different from the previous reports in the literature where only higher trapping probability of holes were observed under heavy ions irradiation. In particular, the spectroscopic performance as well as μτ product of holes and electrons in Fe13+ irradiated sample decreased more than those in Xe20+ irradiated sample, which implied that the recombination of electrons and holes increased in Fe13+ irradiated sample with greater penetration depth than that in Xe20+ irradiated sample when radiation fluence was up to 1015 cm-2, despite the lower density of lattice vacancies in Fe13+ irradiated sample. Moreover, the rise time of holes was slightly different, but the drift velocity was basically the same as that of the unirradiated sample, and so was that of the electrons. Anyway, the presented results showed that the diamond detectors irradiated by heavy ions at a high fluence of 1015 cm-2 still retained their good spectroscopic and very fast time response properties.

A diamond double-sided strip detector for alpha-tagging in associated particle imaging

- In associated particle imaging, neutrons from deuterium-tritium fusion are used to interrogate objects of interest. The associated particle imaging technique utilizes an alpha-tagging sensor in the neutron generator to detect the alpha particle that is emitted antiparallel to the neutron, providing neutron birth time and direction. This technique of neutron imaging and induced fission imaging can provide higher contrast image reconstructions and better estimations of fissile material mass than passive measurement techniques. The performance goals of an alpha-tagging sensor in associated particle imaging are a 700-ps timing resolution, a 0.5 mm spatial resolution, high level of radiation hardness, low sensitivity to X-rays, high signal-to-noise ratio, and survivability in the high temperature bake out of the neutron generator. Currently employed YAP scintillators are overwhelmed by the intense X-ray field in the neutron generator, requiring a new alpha tagging sensor design to be developed. We have developed and tested a diamond double-sided strip detector as a proposed sensor for associated particle imaging. Diamond is radiation hard, has a low sensitivity to X-rays, fast timing capabilities, high signal-to-noise ratio, and can withstand the temperatures of bake-out. The spatial resolution can be accomplished with a charge division readout and double-sided strip design. We measured the timing resolution of a single crystal chemical vapor deposition (CVD) diamond with a Cividec C6 charge amplifier readout by measuring the time-of-flight of 241Am alpha particles between a thin plastic scintillator and the diamond. We also fabricated a polycrystalline CVD diamond double-sided strip detector with charge division readout to test the spatial resolution of the design. We found that the detector met both the 700-ps timing resolution and the 0.5 mm spatial resolution goals (both reported as FWHM resolution).

A direct experimental comparison of single-crystal CVD diamond and silicon carbide X-ray beam position monitors.

Single-crystal chemical vapour deposition (CVD) diamond detectors are an established transmissive synchrotron beamline diagnostic instrument used for beam position and beam intensity monitoring. A recently commercialized alternative is silicon carbide (4H-SiC) devices. These have the potential to provide the same diagnostic information as commercially available single-crystal CVD diamond X-ray beam position monitors, but with a much larger transmissive aperture. At Diamond Light Source an experimental comparison of the performance of single-crystal CVD diamond and 4H-SiC X-ray beam position monitors has been carried out. A quantitative comparison of their performance is presented in this paper. The single-crystal diamond and 4H-SiC beam position monitors were installed in-line along the synchrotron X-ray beam path enabling synchronous measurements at kilohertz rates of the beam motion from both devices. The results of several tests of the two position monitors' performance are presented: comparing signal uniformity across the surface of the detectors, comparing kHz intensity measurements, and comparing kHz beam position measurements from the detectors. Each test is performed with a range of applied external bias voltages. A discussion of the benefits and limitations of 4H-SiC and single-crystal CVD diamond detectors is included.

Measurement of 6Li burn-up reaction rate using a single crystal CVD diamond detector under fast neutron irradiation environment

The study aimed to develop a technique for measuring the 6Li burn-up rate using a single crystal diamond (SCD) detector (SDD) made by chemical vapor deposition (CVD) with a thin lithium fluoride foil. The method relied on pulse shape discrimination (PSD) to separate pulses by nuclear reaction products of 6Li from pulses due to fast neutrons and gamma-rays.By applying the PSD method, the rectangular shape pulses induced by events occurred at the specific depth positions of the SCD, i.e., the incident surface, the ballistic center region (BCR), and the rear surface, were extracted. Thereafter, pulses occurred each specific depth position of the SCD were discriminated according to the pulse width.Due to a short range of nuclear reaction products in the SCD, pulses at the incident surface should be induced by both fast neutrons and nuclear reaction products, although pulses at the BCR are caused by fast neutron only. The study demonstrated that quantitative measurements of nuclear reaction products in the 6Li burn-up reaction under fast neutron irradiation can be achieved using only one SDD by analyzing event rates at the incident surface and in the BCR region.

Improving the Performance of HPHT-Diamond Detectors for Pulsed X-Ray Dosimetry Using the Synchronous Detection Technique

A single-crystal diamond sample grown by high-pressure high-temperature (HPHT) technique was used for the fabrication of a metal–semiconductor–metal (MSM) photoconductor. The sample quality was evaluated by means of spectral photocurrent measurements highlighting the presence of a significant density of defect states within the diamond bandgap, responsible for trap-mediated conduction mechanisms. The photoconductor was fully characterized under 6 MeV pulsed X-rays, sourced by a medical linear accelerator (LINAC), in the 0.05–20 Gy and 0.017–0.100 Gy/s dose (D) and dose-rate (DR) ranges, respectively. Photocurrent measurements performed with a conventional precision electrometer showed that the detector performance is strongly affected by charge-trapping phenomena, resulting in a sublinear dependence with the DR (power-law dependence with an exponent of 0.86). Measurements were repeated in the same experimental conditions by coupling the detector to a specifically developed gated integrator, allowing for a synchronous integration of photocurrent pulses (limited to 40 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <inline-formula> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> </i> s) centered on each impinging X-ray pulse. In this case, the detector photoresponse showed an excellent linearity with both the D and the DR (power-law dependence with 1.0009 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 0.0004 and 1.009 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm$</tex-math> </inline-formula> 0.005 exponents, respectively). Significantly, the proposed synchronous integration technique is then able to mitigate the detrimental effect that defects have on detector performance. The proposed method paves, therefore, the way to the exploitation of HPHT diamond as a low-cost alternative to single-crystal chemical vapor deposition (CVD)-diamond for the fabrication of accurate and linear X-ray dosimeters.

Fusion product diagnostics based on commercially available chemical vapor deposition diamond detector in large helical device

Fusion product diagnostics based on four commercially available single-crystal chemical vapor deposition (s-CVD) diamond detectors are installed in the Large Helical Device (LHD) in order to understand energetic ion confinement. Characteristics of s-CVD diamonds were surveyed using alpha and D-T neutron sources. It is found that the energy resolutions of s-CVD diamonds for ∼ 5 MeV alpha particles and 14 MeV neutrons are 1%–3% and ∼ 1.7%, respectively. Moreover, the response of four s-CVD diamond detectors to alpha particles and D-T neutrons is almost identical. The installation positions of the diamond detectors in the vacuum vessel are searched for, based on the loss points of charged fusion products reckoned by Lorentz orbit calculations. Energy- and time-resolved measurement of fusion product flux will progress in further understanding of energetic ion confinement in LHD.

Development and signals simulation of a diamond detector neutral particle analyzer on Huanliu-2A (HL-2A) tokamak.

A new neutral particle analyzer (NPA) diagnostic based on single crystal chemical vapor deposition (sCVD) diamond detector that provides measurements of fast ions has been designed and installed on HL-2A tokamak. Diamond detectors have been applied in some magnetic confinement fusion devices due to their outstanding properties of compact size and radiation hardness. This DNPA can measure energies above 13.4keV. The line of sight (LOS) of the DNPA intersects with the NBI No. 2 with a tangency radius of 154.8cm. Due to the pitch angle defined by the LOS and geometry of the diagnostic, the DNPA is mainly sensitive to trapped ions. To interpret the energy spectrum and verify the feasibility of the design of the DNPA, a Monte Carlo code called FIDASIM, which is a synthetic diagnostic code that simulates fast ion D-alpha and NPA signals, is applied to model the neutral flux reaching the detector. The results show that the flux is mainly contributed by the low energy fast ions (E < 10keV) and it is mainly coming from the active components, the passive signal is dominant in the high energy region (E > 15keV). The modeling features the ability to distinguish between active and passive signals, and the simulated strong passive signals are suggested to come from charge exchange between cold neutrals and fast ions around the plasma edge. In addition, despite the large ratio of halo neutrals, essentially it has a limited contribution to the energy spectrum.

High-temperature performance of solid-state sensors up to 500 °C

The applicabilities of single-crystal chemical vapour deposition diamond sensors and 4H-SiC diodes for particle spectroscopy in high-temperature environments are investigated. The spectroscopic performance of the sensor materials is measured as function of temperature, from room temperature to 500 °C. Previously published measurements showed a stable spectroscopic response of sCVD diamond sensors up to 200 °C. In this follow-up experiment, the temperature range was extended up to 500 °C for sCVD diamond as well as 4H-SiC sensors.

Charge collection efficiency of scCVD diamond detectors at low temperatures

The charge collection efficiency (CCE) in electronic grade, single crystal chemical vapor deposition (scCVD) diamond detector was studied using different ions of the MeV energy range from room temperature down to 47 K. The measurements were performed by using the ion beam induced charge (IBIC) technique, with 0.8 MeV H+, 3 MeV He2+, 5.6 MeV Li2+, and 12.8 MeV C4+ ion beams. The total accumulated charge as a function of temperature was extracted from the measured pulse height spectra. A similar decreasing trend of the detector CCE with temperature was observed for all ion species. A plateau was observed from room temperature to 145 K followed by a sharp, step like, decrease in the temperature range of 65 K < T < 145 K. Below 65 K, the CCE reaches another plateau with small variations down to 47 K. The decrease in collected charge was observed in the same temperature window for each ion. However, the level of the CCE decrease varied. For He, Li and C, the CCE decreased to 24.7 ± 0.4 %, 23.6 ± 1.3 %, 25.9 ± 0.8 % respectively while 39.3 ± 0.8 % for H. The reduction of the CCE is in agreement with earlier reported results obtained by alpha particles which was associated with the changes in lifetime of excitons in diamond.

Recent development of neutron and energetic-particle diagnostics for LHD deuterium discharges

An integrated set of neutron diagnostics developed for the deuterium operation of the Large Helical Device (LHD) has been revealing behavior of energetic ions in three-dimensional plasmas, together with energetic-particle diagnostics. In order to obtain deeper understanding of physics related to energetic ions in the LHD, development of plasma diagnostics that can provide energy distribution of energetic ions is now being accelerated. Recent advances in development of the D-D neutron energy spectrometer, a neutral particle analyzer based on a single-crystal chemical vapor deposition diamond, and a tangential fast-ion Dα diagnostic for deuterium discharges of the LHD are described.

Self-powered solar-blind ultrafast UV-C diamond detectors with asymmetric Schottky contacts

Single-crystal chemical vapour deposition (CVD) diamond samples with asymmetric Schottky contacts were used for the fabrication of vertical metal-semiconductor-metal detectors for deep ultraviolet (UV–C) radiation. Spectral photoconductivity measurements, performed in the 190 ÷ 1100 nm wavelength range, returned an ultraviolet-to-visible (UV/Vis) rejection ratio of 6.8 × 103 under photovoltaic operating conditions, ensuring solar blindness and zero power consumption at the same time. When applying bias, UV/Vis rejection ratio reached excellent values (>104) even at very low electric fields (0.01 V/μm), thanks to the excellent structural quality and the superior charge transport properties of the diamond bulk, as inferred from Raman analysis and spectral photoconductivity measurements. Mobility-lifetime (μτ) product of the photogenerated carriers was indeed measured to be about 5 × 10−3 cm2/V in the case of holes, which is the highest room-temperature μτ value ever reported for CVD diamond, enabling a response speed in the nanosecond range even at zero-bias operating conditions.

Electron spectroscopy with a diamond detector

An electronic grade single crystal chemical vapour deposition diamond was investigated as a prototype high temperature spectroscopic electron (β- particle) detector for future space science instruments. The diamond detector was coupled to a custom-built charge-sensitive preamplifier of low noise. A 63Ni radioisotope source (endpoint energy 66 keV) was used to provide a spectrum of β- particles incident on the detector. The operating temperature of the detector/preamplifier assembly was controlled to allow its performance to be investigated between +100 °C and −20 °C, in 20 °C steps. Monte Carlo modelling was used to: a) calculate the β- particle spectrum incident on the detector; b) calculate the fraction of β- particle energy deposited into the detector; and c) predict the β- particle spectrum accumulated by the instrument. Comparison between the model and experimental data suggested that there was a 4.5 μm thick recombination region at the front of the detector. The spectrometer was demonstrated to be fully operable at temperatures, T, −20 °C ≤ T ≤ 80 °C; the results suggested that some form of polarisation phenomenon occurred in the detector at > 80 °C. This article presents the first report of an energy calibrated (≲ 50 keV) spectroscopic β- particle diamond detector.

Performance of CVD diamond detectors for single ion beam-tagging applications in hadrontherapy monitoring

In the context of online ion range verification in particle therapy, the CLaRyS collaboration is developing Prompt-Gamma (PG) detection systems. The originality in the CLaRyS approach is to use a beam-tagging hodoscope in coincidence with the gamma detectors to provide both temporal and spatial information of the incoming ions. The ion range sensitivity of such PG detection systems could be improved by detecting single ions with a 100ps (σ) time resolution, through a quality assurance procedure at low beam intensity at the beginning of the treatment session. This work presents the investigations that led to assessment of the Chemical Vapor Deposition (CVD) diamond detectors performance to fulfill these requirements. A 90Sr beta source, 68MeV protons, 95 MeV/u carbon ions and a synchrotron X-ray pulsed beam were used to measure the time resolution, single ion detection efficiency and proton counting capability of various CVD diamond samples. An offline technique, based on double-sided readout with fast current preamplifiers used to improve the signal-to-noise ratio, is also presented. The different tests highlighted Time-Of-Flight resolutions ranging from 13ps (σ) to 250ps (σ), depending on the diamond crystal quality and the particle type and energy. The single 68MeV proton detection efficiency of various large area polycrystalline (pCVD) samples was measured to be >96% using coincidence measurements with a single-crystal reference detector. Single-crystal CVD (sCVD) diamond proved to be able to count a discrete number of simultaneous protons while it was not achievable with a polycrystalline sample. Considering the results of the present study, two diamond hodoscope demonstrators are under development: one based on sCVD, and one of larger size based on pCVD. They will be used for the purpose of single ion as well as ion bunches detection, either at reduced or clinical beam intensities.

Simultaneous Measurements of Dose and Microdosimetric Spectra in a Clinical Proton Beam Using a scCVD Diamond Membrane Microdosimeter.

A single crystal chemical vapor deposition (scCVD) diamond membrane-based microdosimetric system was used to perform simultaneous measurements of dose profile and microdosimetric spectra with the Y1 proton passive scattering beamline of the Center of Proton Therapy, Institute Curie in Orsay, France. To qualify the performance of the set-up in clinical conditions of hadrontherapy, the dose, dose rate and energy loss pulse-height spectra in a diamond microdosimeter were recorded at multiple points along depth of a water-equivalent plastic phantom. The dose-mean lineal energy () values were computed from experimental data and compared to silicon on insulator (SOI) microdosimeter literature results. In addition, the measured dose profile, pulse height spectra and values were benchmarked with a numerical simulation using TOPAS and Geant4 toolkits. These first clinical tests of a novel system confirm that diamond is a promising candidate for a tissue equivalent, radiation hard, high spatial resolution microdosimeter in beam quality assurance of proton therapy.

Charge carrier properties of single-crystal CVD diamond up to 473 K

The drift behavior of charge carriers, generated by α-particles of a reference 241Am-source, in electronic grade, single crystal chemical vapor deposition (scCVD) diamond was investigated by the transient current technique (TCT) from room temperature up to ≈473K. Furthermore, the α-spectroscopic behavior was analyzed in terms of charge collection and spectroscopic resolution for the same temperature range. All conducted measurements revealed complete charge collection up to the maximum temperature. The electron–hole-pair creation energies were derived from the TCT as well as from the spectroscopic measurements. The herein presented results imply that high temperature α-spectroscopy with diamond-based semiconductor solid state detectors, using presently available scCVD sensor substrates, is feasible at least up to 473K. Only at the highest applied temperature, the conducted TCT measurements showed distorted signal traces, indicating a uniform positive space charge built-up.

Neutron-induced signal on the single crystal chemical vapor deposition diamond-based neutral particle analyzer.

A diamond-based neutral particle analyzer (DNPA) array composed of single-crystal chemical vapor deposition (sCVD) diamond detectors was installed on the Large Helical Device (LHD) for measuring the helically trapped energetic particles. In high neutron flux experiments, the unwanted neutron-induced pulse counting rate should be estimated using the neutron diagnostics because a diamond detector is sensitive to neutrons as well as energetic neutral particles. In order to evaluate the quantitative neutron-induced pulse counting rate on the DNPA, the response functions of the sCVD diamond detector for mono-energetic neutrons were obtained using accelerator-based D-D and D-7Li neutron sources in Fast Neutron Laboratory (FNL). As a result of the neutron flux estimation by the Monte Carlo N-Particle code at the NPA position in the LHD and the response function obtained in the FNL experiment, the counting rate of the neutron-induced signal was predicted to be 1.1 kcps for the source neutron emission rate of Sn = 1 × 1015 n/s. In the LHD experiment, the neutron-induced signals were observed by closing the gate valve during the plasma discharges. It is found that the counting rates of the neutron-induced signals proportional to Sn reached 1.1 kcps at Sn = 1 × 1015 n/s. As a result of the quantitative estimation of the neutron-induced signals on the DNPA using other neutron measurements, it has become possible to accurately measure energetic neutral particles in the high neutron flux experiment.

Adlayer-free large-area single-crystal CVD graphene growth on copper

Chemical vapor deposition (CVD) graphene growth on copper foil has the potential to produce high-quality graphene at the industrial scale to realize its applications in future electronics and optoelectronic devices. During the graphene growth process, various parameters such as catalytic substrate, temperature, flow rate of precursor gases (methane and hydrogen) and presence of oxygen on the Cu substrate, directly affect graphene grain size, thickness and crystallinity. Researchers have been able to achieve large-area single-crystal monolayer graphene on Cu substrate. However, small fractions of bilayer or multilayer (adlayer) regions also appear along with single-crystal monolayer graphene. In recent years, researchers have been studying the effect of various growth parameters on adlayer formation. In this review, we have discussed the role and optimization of these growth parameters to achieve adlayer-free large-area single-crystal graphene on copper foil.

Electronic Properties of a Synthetic Single-Crystal Diamond Exposed to High Temperature and High Radiation.

Diamond, as a wide band-gap semiconductor material, has the potential to be exploited under a wide range of extreme operating conditions, including those used for radiation detectors. The radiation tolerance of a single-crystal chemical vapor deposition (scCVD) diamond detector was therefore investigated while heating the device to elevated temperatures. In this way, operation under both high-temperature and high-radiation conditions could be tested simultaneously. To selectively introduce damage in small areas of the detector material, a 5 MeV scanning proton microbeam was used as damaging radiation. The charge collection efficiency (CCE) in the damaged areas was monitored using 2 MeV protons and the ion beam induced charge (IBIC) technique, indicating that the CCE decreases with increasing temperature. This decreasing trend saturates in the temperature range of approximately 660 K, after which CCE recovery is observed. These results suggest that the radiation hardness of diamond detectors deteriorates at elevated temperatures, despite the annealing effects that are also observed. It should be noted that the diamond detector investigated herein retained its very good spectroscopic properties even at an operation temperature of 725 K (≈2% for 2 MeV protons).