Diamond Photoconductive Detectors Research Articles

A Study of the Ultraviolet Radiation of Hybrid X-Pinches

The results of an experimental study of hybrid X-pinches (HXPs) as sources of vacuum ultraviolet (VUV) radiation within the wavelength range of λ < 2000 A are presented. A KING compact low-voltage generator (45 kV, 200 kA, 190 ns) was used to feed the X-pinches. The dynamics of forming the radiating plasma of HXPs of different materials were studied using pinhole cameras with radiation receivers based on microchannel plates with a time resolution of 5 ns and a spatial resolution of 150 µm. The time structure and energy characteristics of the radiation were studied using diamond photoconducting detectors (PCDs). The spectral composition of UV radiation was investigated using a spectrograph based on the transmission grating. It has been shown that the efficiency of conversion of the capacitor-stored energy to the VUV radiation may reach 5%, whereas the fraction of soft X-rays (SXR) with quanta energies above 800 eV is no more than 3%. In this case, the source brightness reaches 2 × 109 W/(sr cm2).

Understanding the Implications of a LINAC’s Microstructure on Devices and Photocurrent Models

The effect of a linear accelerator’s (LINAC’s) microstructure (i.e., train of narrow pulses) on devices and the associated transient photocurrent models are investigated. The data indicate that the photocurrent response of Si-based RF bipolar junction transistors and RF p-i-n diodes is considerably higher when taking into account the microstructure effects. Similarly, the response of diamond, SiO2, and GaAs photoconductive detectors (standard radiation diagnostics) is higher when taking into account the microstructure. This has obvious hardness assurance implications when assessing the transient response of devices because the measured photocurrent and dose rate levels could be underestimated if microstructure effects are not captured. Indeed, the rate the energy is deposited in a material during the microstructure peaks is much higher than the filtered rate which is traditionally measured. In addition, photocurrent models developed with filtered LINAC data may be inherently inaccurate if a device is able to respond to the microstructure.

Gamma ray measurements with photoconductive detectors using a dense plasma focus.

Photons in the MeV range emitted from the dense plasma focus (DPF) at the NSTec North Las Vegas Facility have been measured with both neutron-damaged GaAs and natural diamond photoconductive detectors (PCDs). The DPF creates or "pinches" plasmas of various gases (e.g., H2, D2, Ne, Ar., etc.) that have enough energy to create MeV photons from either bremsstrahlung and/or (n,n(')) reactions if D2 gas is used. The high bandwidth of the PCDs enabled the first ever measurement of the fast micro-pinches present in DPF plasmas. Comparisons between a slower more conventional scintillator/photomultiplier tube based nuclear physics detectors were made to validate the response of the PCDs to fast intense MeV photon signals. Significant discrepancies in the diamond PCD responses were evident.

South pole bang-time diagnostic on the National Ignition Facility (invited)

The south pole bang-time diagnostic views National Ignition Facility (NIF) implosions through the lower Hohlraum laser entrance hole to measure the time of peak x-ray emission (peak compression) in indirect-drive implosions. Five chemical-vapor-deposition diamond photoconductive detectors with different filtrations and sensitivities record the time-varying x rays emitted by the target. Wavelength selecting highly oriented pyrolytic graphite crystal mirror monochromators increase the x-ray signal-to-background ratio by filtering for 11-keV emission. Diagnostic timing and the in situ temporal instrument response function are determined from laser impulse shots on the NIF. After signal deconvolution and background removal, the bang time is determined to 45-ps accuracy. The x-ray "yield" (mJ∕sr∕keV at 11 keV) is determined from the time integral of the corrected peak signal.

Pinned, optically aligned diagnostic dock for use on the Z facility

The pinned optically aligned diagnostic dock (PODD) is a multi-configuration diagnostic platform designed to measure x-ray emission on the Z facility. The PODD houses two plasma emission acquisition (PEA) systems, which are aligned with a set of precision machined pins. The PEA systems are modular, allowing a single diagnostic housing to support several different diagnostics. The PEA configurations fielded to date include both time-resolved and time-integrated, 1D spatially resolving, elliptical crystal spectrometers, and time-integrated, 1D spatially resolving, convex crystal spectrometers. Additional proposed configurations include time-resolved, monochromatic mirrored pinhole imagers and arrays of filtered x-ray diodes, diamond photo-conducting diode detectors, and bolometers. The versatility of the PODD system will allow the diagnostic configuration of the Z facility to be changed without significantly adding to the turn-around time of the machine. Additionally, the PODD has been designed to allow instrument setup to be completed entirely off-line, leaving only a refined alignment process to be performed just prior to a shot, which is a significant improvement over the instrument the PODD replaces. Example data collected with the PODD are presented.

A diamond detector for X-ray bang-time measurement at the National Ignition Facility

An instrument has been developed to measure X-ray bang-time for inertial confinement fusion capsules; the time interval between the start of the laser pulse and peak X-ray emission from the fuel core. The instrument comprises chemical vapor deposited polycrystalline diamond photoconductive X-ray detectors with highly ordered pyrolytic graphite X-ray monochromator crystals at the input. Capsule bang-time can be measured in the presence of relatively high thermal and hard X-ray background components due to the selective band pass of the crystals combined with direct and indirect X-ray shielding of the detector elements. A five channel system is being commissioned at the National Ignition Facility at Lawrence Livermore National Laboratory for implosion optimization measurements as part of the National Ignition Campaign. Characteristics of the instrument have been measured demonstrating that X-ray bang-time can be measured with ±30 ps precision, characterizing the soft X-ray drive to +/- 1 eV or 1.5%.

(Cd,Mn)Te detectors for characterization of X-ray emissions generated during laser-driven fusion experiments

We present our measurements of (Cd,Mn)Te photoconductive detectors (PCDs), intending to characterize both the temporal and spectral dependence of X-ray emissions from laser-illuminated targets during inertial confinement fusion experiments. Our Cd1−xMnxTe (x=0.05) single crystals, doped with V, were grown using a vertical Bridgman method and annealed in Cd vapor for the highest resistivity of ∼1010Ωcm. The 1-mm-long and 2.3-mm-long detectors were placed in the same housing as two 1-mm-long diamond PCDs. Each device was preceded by a Be X-ray filter with 37% X-ray transmission at the 1keV cutoff energy. Energy of the incident OMEGA laser pulses varied from 2.3 to 28kJ. Using targets of empty plastic shells, we observed two X-ray emission events separated by 1.24ns: the first event was caused by heating of the shell that created a hot corona, while the second event was an X-ray emission from the fully compressed target. Experiments with targets with steel cores enabled us to analyze the time-resolved relaxation dynamics of photo-excited carriers in the (Cd,Mn)Te crystals. According to our calculations, the (Cd,Mn)Te material can very effectively absorb X-rays with energies of up to above 100keV and the (Cd,Mn)Te PCDs are likely to complement the diamond detectors currently used in laser-confinement fusion experiments.

Implosion dynamics and radiation characteristics of wire-array Z pinches on the Cornell Beam Research Accelerator

Experimental results are presented that characterize the implosion dynamics and radiation output of wire-array Z pinches on the 1-MA, 100-ns rise-time Cornell Beam Research Accelerator (COBRA) [J. B. Greenly et al., Rev. Sci. Instrum. 79, 073501 (2008)]. The load geometries investigated include 20-mm-tall cylindrical arrays ranging from 4to16mm in diameter, and consisting of 8, 16, or 32 wires of either tungsten, aluminum, or Invar (64% iron, 36% nickel). Diagnostics fielded include an optical streak camera, a time-gated extreme-ultraviolet framing camera, a laser shadowgraph system, time-integrated pinhole cameras, an x-ray wide-band focusing spectrograph with spatial resolution, an x-ray streak camera, a load voltage monitor, a Faraday cup, a bolometer, silicon diodes, and diamond photoconducting detectors. The data produced by the entire suite of diagnostics are analyzed and presented to provide a detailed picture of the overall implosion process and resulting radiation output on COBRA. The highest x-ray peak powers (300–500GW) and total energy yields (6–10kJ) were obtained using 4-mm-diameter arrays that stagnated before peak current. Additional findings include a decrease in soft x-ray radiation prior to stagnation as the initial wire spacing was changed from 1.6mmto785μm, and a timing correlation between the onset of energetic electrons, hard x-ray generation, and the arrival of trailing current on axis—a correlation that is likely due to the formation of micropinches. The details of these and other findings are presented and discussed.

Soft x-ray measurements using photoconductive type-IIa and single-crystal chemical vapor deposited diamond detectors

Photoconductive detectors (PCDs) are routinely used alongside vacuum x-ray diodes (XRDs) to provide an alternative x-ray flux measurement at laser facilities such as HELEN at AWE Aldermaston, UK, and Omega at the Laboratory for Laser Energetics. To evaluate diamond PCDs as an alternative to XRD arrays, calibration measurements made at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory are used to accurately calculate the x-ray flux from a laser-heated target. This is compared to a flux measurement using the Dante XRD diagnostic. Estimates indicate that the photoinduced conductivity from measurements made at Omega are too large, and calculations using the radiometric calibrations made at the NSLS agree with this hypothesis. High-purity, single-crystal, chemical vapor deposited (CVD) diamond samples are compared to natural type-IIa PCDs and show promising high resistivity effects, the corollary of which preliminary results show is a slower response time.

X-ray emission from an X-pinch and its applications

AbstractThe temporal and spatial X-ray emission from PPG-X, an X-pinch driven by pulsed power generator, was studied by using diamond photo-conducting detectors and pinhole cameras. It was found that the X-ray pulse usually consists of two sub-nanosecond peaks with a time interval of about 0.5 ns, these two X-ray peaks are consistent with two point sources of X-ray recorded with pinhole camera. The total X-ray energy changes from shot to shot and is averaged to be 0.35 J for hν &gt; 1.5 keV. The size of the X-ray point source is in the range from 100 µm to 5 µm, decreasing rapidly with the increase of the photon energy. The X-pinch was used as X-ray source for backlighting the electrical explosion of single wire and for phase-contrast imaging of a mosquito.

Development of a Silicon Calorimeter for Dosimetry Applications in a Water-Moderated Reactor

Abstract High fidelity active dosimetry in the mixed neutron/gamma field of a research reactor is a very complex issue. For passive dosimetry applications, the use of activation foils addresses the neutron environment while the use of low neutron response CaF2:Mn thermoluminescent dosimeters (TLDs) addresses the gamma environment. While radiation-hardened diamond photoconducting detectors (PCD) have been developed that provide a very precise fast response (picosecond) dosimeter and can provide a time-dependent profile for the radiation environment, the mixed field response of the PCD is still uncertain and this interferes with the calibration of the PCD response. In order to address the research reactor experimenter's need for a dosimeter that reports silicon dose and dose rate at a test location during a pulsed reactor operation, a silicon calorimeter has been developed. This dosimeter can be used by itself to provide a dose in rad(Si) up to a point in a reactor pulsed operation, or, in conjunction with the diamond PCD, to provide a dose rate. This paper reports on the development, testing, and validation of this silicon calorimeter for applications in water-moderated research reactors.

Diamond photodetector response to deep UV excimer laser excitation

Diamond photoconductive detectors have previously been shown to be suitable for detection of deep ultraviolet nanosecond-scale pulses from excimer lasers, which are to be used in next generation lithographic systems. Using simple test circuitry and low bias voltages easily measurable responses were observed when these detectors were illuminated with laser pulses at typical laser fluence levels. However, as the laser fluence is increased the detector response appears to broaden and change shape as well as increase in magnitude. In this paper we present analysis of the response of these detectors. This analysis shows that over the laser fluence range used the magnitude of the detector response exhibited a linear response. By approximating the temporal evolution of the laser pulse and simulating the transfer characteristics of the test circuit it can be seen that the pulse broadening and shape change seen with increasing fluence can be explained by the test circuit. The shape of the detector response, therefore, closely resembles the temporal shape of the laser pulse, demonstrating that these devices are fast enough to give a true representation of 10–15 ns laser pulses at a wavelength of 193 nm.

Temporal parameters of the X-pinch x-ray source

The X pinch has proved to be an excellent source of 2.5–10 keV radiation for point-projection radiography with spatial resolution of 2 μm or even better. The pulse duration of the x-ray bursts has been investigated for a wide variety of wire materials in the 1.5–10 keV energy range using a set of fast diamond photoconducting detectors with different filters, and using an x-ray streak camera to observe the source through four different filters on each pulse. All wires tested have intense continuum up to at least 6 keV, and the duration of the pulse is shorter for the harder radiation component for all materials. However, there are substantial differences between materials. For example, the pulse duration for Al with filtering for energy ⩾1.5 and ⩾5 keV are about 1 and 0.5 ns, respectively. By contrast, for Mo with filtering for energy ⩾2.5 and ⩾5 keV, the pulse durations are about 200 ps and ⩽80 ps, respectively.

Fabrication of CVD diamond photoconductive detectors

We present an investigation on the fabrication of synthetic diamond based photon detectors. These devices are made by depositing small gap interdigitated contacts on polycrystalline diamond films grown by Microwave Plasma Enhanced Chemical Vapour Deposition. Gold interdigitated contacts were deposited on the typically rough surface of these films by using both wet etching and unconventional lift-off lithographic processes. These detectors were tested through X-ray irradiation in order to compare their potentialities in obtaining higher performance diamond photoconductive detectors. A better resolution was observed in samples obtained through the lift-off process. In particular for these samples the gap between electrodes could be kept very small and of the order of the grain size, giving higher response due the lower contribution of grain boundary effects.

Achromatically filtered diamond photoconductive detectors for high power soft x-ray flux measurements

A 1-mm-square diamond photoconductive detector (PCD) has been installed on the LLNL Nova laser system, for use as a broadband soft x-ray power diagnostic. The PCD is installed behind an array of pinholes, which cast multiple, overlapping images of the source onto the diamond. This allows reduction of the x-ray intensity, to avoid saturation problems, while avoiding the spectral dependency of thin film filters. The diode current is read out on a 5 GHz bandwidth scope. The system is calibrated by comparison to an absolutely calibrated array of filtered vacuum x-ray photodiodes (“dante”). The time response of the PCD and its bias electronics have been characterized using the 5th harmonic (210 nm) of a short pulse (&amp;lt;1 ps) Ti: sapphire laser. The data show a fast rise, limited by the 5 GHz scope bandwidth, and a slower fall off, characterized by an RC time of order 200 ps.

Fielding and calibration issues for diamond photoconducting detectors

Diamond photoconducting detectors (PCDs) are routinely fielded as soft x-ray diagnostics on Sandia’s Saturn facility. We have developed an improved detector mount that provides a 200 ps time response, is easily cleanable, and is very rugged. In addition, we have developed a new, fast insertion unit to apply bias voltage to the detectors. Absolute calibration of the PCDs is carried out either at the Brookhaven National Synchrotron Light Source or on Sandia’s laser calibration facility. We are now fielding diamond elements that have the dimensions 1×3×0.5 mm and 1×1×0.5 mm. We are neutron damaging some of the 1×1×0.5 mm detectors to reduce their sensitivity. We can tailor PCD sensitivity by adjusting element size and neutron damage level.

Response of diamond photoconductors to soft x ray in the spectral range 125 Å to 240 Å

Due to the large bandgap of diamond, it is transparent to the visible spectrum, making it an attractive material for soft x-ray detection. Response of diamond photoconductive detectors fabricated using polycrystalline chemical-vapor-deposited (CVD) diamond to soft x-rays has been measured using x-rays emitted from a laser-produced plasma source in the spectral range 125 Å to 240 Å. These detectors have interdigitated electrode structure in order to increase the active area as well as detector sensitivity. Contribution to the detector sensitivity by the photoelectrons is discussed.

Diamond photoconducting detectors as high power z-pinch diagnostics (invited)

Diamond photoconducting detectors (PCDs) are one of the primary x-ray diagnostics for z-pinch experiments at Sandia National Laboratories’ Saturn facility. Diamond PCDs represent a significant advance in the state of the art for electrical x-ray detectors because of their stability of response, overall flatness of response, and ease of use. We have observed no aging effects in detectors used for years. Five PCDs on Saturn experiments where 1–3 TW of keV x-ray power is emitted in 5–20-ns pulses by z-pinch x-ray sources are routinely fielded. Typical PCDs used at Sandia have dimensions of 1 mm width ×3 mm long ×0.5 mm thick. The techniques used in the assembly, fielding, and calibration of diamond PCDs will be described.

A five-channel, diamond photoconducting x-ray detector array for z-pinch experiments

We have built a five-channel, x-ray detector array based on diamond photoconducting detectors (PCDs). The diamond elements have dimensions of 3 mm × 1 mm × 1 mm (or 0.5 mm). We use diamond PCDs for their stability, flat spectral response, and low leakage currents. The good time response of diamond PCDs is due to the 100-ps electron/hole recombination time. Filters were designed to give information in the 1–10-keV spectral region. Calibration of the diamond PCDs showed sensitivities between 4 and 7 × 10−4 A/W for a bias of 100 V. We shall present data from z-pinch experiments on Saturn.

Soft x-ray detection with diamond photoconductive detectors

Photoconductive detectors (PCDs) fabricated from natural IIa diamonds have been used to measure the x-ray power emitted from laser-produced plasmas. The detector was operated without any absorbing filters which distort the x-ray power measurement. The 5.5 eV band gap of the detector material practically eliminates its sensitivity to scattered laser radiation thus permitting filterless operation. Excellent agreement was achieved between a diamond PCD and a multichannel photoemissive diode array in the measurement of radiated x-ray power and energy.