H. Maezawa

X-ray detection characteristics of gold photocathodes and microchannel plates using synchrotron radiation (10 eV–82.5 keV)

Abstract X-ray detection characteristics of gold photocathodes, and microchannel plates (MCP) have been investigated using synchrotron radiation in the energy range from 12 eV to 82.5 keV. The quantum efficiency of gold is compared with the published data which were reported at some discrete energies, and we add several new data points in the 12–35.9 eV. The detection response of MCP has been represented for photons from the VUV regime of 12 eV to the hard X-ray region of 82.5 keV along with its incident angle dependence. This MCP response has demonstrated the following remarkable sensitivity in this wide energy range: The energy response of MCP remains within about one order of magnitude through this broad range. This comes from the contribution of the combination of various ingredients of MCP; namely, C, O, Si, Ba and Pb. For the energy response, these ingredients, in turn, make some jumps and humps at their edge energies. Another useful feature of the MCP response we obtained is a rapid decrease in the MCP response for the energies less than about ten eV; this is desirable for avoiding the effect of visible light on the X-ray data. Also, the MCP response is compared with the detection efficiency of a silicon surface barrier detector for the application to plasma X-ray diagnostics.

EFFECTS OF NEUTRONS ON SEMICONDUCTOR X-RAY DETECTORS INCLUDING N-TYPE JOINT EUROPEAN TORUS AND P-TYPE GAMMA 10 TOMOGRAPHY DETECTORS

Characterization experiments have been carried out so as to investigate the effects of fusion-produced neutrons on the x-ray-energy responses of semiconductor detectors for x-ray tomography in the Joint European Torus (JET) tokamak (n-type silicon) and the GAMMA 10 tandem mirror (p-type silicon). Neutron effects on the x-ray-energy responses of these detectors are studied using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory. Changes in the material properties of the detectors have been investigated using an impedance analyzer to estimate neutron effects on x-ray-sensitive depletion thicknesses. A cyclotron accelerator is employed for well-calibrated neutron irradiation onto these plasma x-ray detectors; a fluence of 2–5×1013 neutrons/cm2 is utilized for simulating the effects of fusion-produced neutrons in JET. Modifications of the x-ray responses after neutron exposure due to fusion plasma shots in JET as well as cyclotron-produced neutron irradiations are found to have ...

Current response characteristics of microchannel plates x‐ray detector using synchrotron radiation (0.6–2 keV and 5–20 keV)

The characteristics of microchannel plates (MCPs) for detection of x rays have been investigated using synchrotron radiation in the energy ranges from 0.6 to 2.0 keV and from 5 to 20 keV. Microchannel plates are operated under the condition of an unsaturated pulse‐height distribution mode. The current response curve of MCPs is measured continuously with x‐ray energy variation for the first time. The experimental result of some discontinuous jumps in the response is obtained at the energies corresponding to the absorption edge of the MCP materials. In the low‐energy range (hν 5 keV), a weak dependence on θ is observed, and is attributed mainly to the penetration of x rays through multiple channels.

Quantum efficiency of gold photocathodes (2–8 keV) and EXAFS in its secondary electron yield and in the detection currents of a microchannel plate and a silicon surface barrier detector

The quantum efficiency of gold photocathodes has been investigated by using synchrotron radiation from 2–8 keV; in particular its detailed structure near the M absorption edge region is obtained. The secondary electron conversion efficiency of gold has been calculated by using the mass absorption coefficient given by a relativistic Hartree‐Slater model and by the semiempirical values of Henke et al., respectively. These data are compared with the published data at some discrete energies, and we add several new data points, especially for the gold M edge region (2–4 keV). X‐ray absorption fine structure (EXAFS) has been observed in the secondary electron current of the gold photocathode as well as in the detection current responses of a microchannel plate and a silicon surface barrier detector.

Detection characteristics of microchannel plates and gold photocathodes for plasma x‐ray diagnostics [near the absorption edges (2–8) keV)] (abstract)

Using undulator radiation from 2 to 8 keV, quantum efficiencies [QE(E)] of gold photocathodes, microchannel plates (MCP), and silicon surface barrier (SSB) detectors have been investigated. For the gold photocathodes, the detailed structure of QE(E) near the M absorption edges has been presented. Also, the secondary electron conversion efficiency of gold has been calculated using the mass absorption coefficient given by a relativistic Hartree–Slater model and by the semiempirical values of Henke et al., respectively. Extended x‐ray absorption fine structure (EXAFS) has been observed in the secondary electron current of the gold photocathode as well as in the detection current responses of an MCP and of an SSB detector. Furthermore, the new findings adding to our recent paper1 have been summarized as follows: (i) EXAFS above the Si‐K edge in the MCP response depends on photon incident angles, and (ii) a little upshift of the starting point energy of EXAFS in the MCP response is observed. These detailed cha...

Characterization of new semiconductor detectors for x-ray tomography in the ASDEX Upgrade Tokamak and its generalized physics interpretations

The energy response of a new semiconductor detector in the ASDEX Upgrade Tokamak for plasma x-ray tomography studies is characterized using synchrotron radiation from a 2.5 GeV positron storage ring at the National Institute for High Energy Physics in Japan. This international collaborating research clarifies a fairly good agreement between the x-ray energy response data and our recently proposed theoretical predictions for such a semiconductor x-ray-detector response. The x-ray response for several positions on the active area of the detector unit is studied; a good uniformity observed guarantees that the detector can employ any sized and shaped collimator for the x-ray tomography regardless of any correction factor coming from the response nonuniformity on the detector active area. Operational conditions of the detector for the ASDEX Upgrade plasma diagnostics are optimized using its capacitance measurements as a function of an applied bias as well as the numerical evaluations of the detector response; ...

New methods for semiconductor charge-diffusion-length measurements using synchrotron radiation

The extension of a new theory on the X-ray energy response of semiconductor detectors is carried out to characterize the X-ray response of a multichannel semiconductor detector fabricated on one silicon wafer. Recently, these multichannel detectors have been widely utilized for position-sensitive observations in various research fields, including synchrotron radiation research and fusion-plasma investigations. This article represents the verification of the physics essentials of a proposed theory on the X-ray response of semiconductor detectors. The three-dimensional charge-diffusion effects on the adjoining detector-channel signals are experimentally demonstrated at the Photon Factory for two types of multichannel detectors. These findings are conveniently applicable for measuring diffusion lengths for industrial requirements.

X-ray detector characterization and applications for plasma research

Abstract Using synchrotron radiation, the detection characteristics of microchannel plates, silicon surface barrier detectors and gold photocathodes are investigated; in particular, the detailed structures of the detection efficiency including an extended X-ray absorption fine structure (EXAFS), an X-ray absorption near-edge structure (XANES) and a chemical shift is reported along the with discussion on the physical mechanisms. Also, the applications of these detectors to X-ray tomography diagnostics for the observation of the cross-sections of fusion-oriented tandem-mirror plasmas are described. These data give information on the spatial distribution of the hot electrons which play an important role in the formation of thermal-barrier potentials φb. Also, the tomographic reconstructed X-ray data in the central cell show the effect of φb formation on the improvement in the central-cell electron confinement for the first time.

Characterization and interpretation of the quantum efficiencies of multilayer semiconductor detectors using a new theory

On the basis of a new theory of semiconductor X-ray detector response, a new type of multilayer semiconductor detector was designed and developed for convenient energy analyses of intense incident X-ray flux in a cumulative-current mode. Another anticipated useful property of the developed detector is a drastic improvement in high-energy X-ray response ranging over several hundred eV. The formula for the quantum efficiency of multilayer semiconductor detectors and its physical interpretations are proposed and have been successfully verified by synchrotron radiation experiments at the Photon Factory. These detectors are useful for data analyses under strong radiation-field conditions, including fusion-plasma-emitting X-rays and energetic heavy-particle beams, without the use of high-bias applications.

Photoelectron spectroscopy for plasma x‐ray measurements

A new x‐ray spectrometer using the principle of a photoelectron spectroscopy method is developed for the purpose of the observations of plasma‐electron temperatures in the range of a hundred of eV. X‐ray photoelectron spectroscopy is a widely utilized method for surface‐physics analyses; here, we employ a parallel‐plate energy analyzer. This new type spectrometer is calibrated using synchrotron radiation from a 2.5 GeV positron storage ring at the Photon Factory (KEK). Theoretical calculations for photoelectron spectra from plasmas with various electron temperatures are carried out using the calibration data. This spectrometer is set up at the central cell of the GAMMA 10 tandem mirror for the measurements of potential‐confined electron temperatures. Experimentally observed spectra are compared with the calculated spectra as well as the data from the other x‐ray detection method.