Nobuyuki Nishimori

Nondestructive Detection of Heavily Shielded Materials by Using Nuclear Resonance Fluorescence with a Laser-Compton Scattering γ-ray Source

We perform a proof-of-principle experiment for a nondestructive method for detecting the elemental and isotopic composition of materials concealed by heavy shields such as iron plates with a thickness of several centimeters. This method uses nuclear resonance fluorescence (NRF) triggered by an energy-tunable laser-Compton scattering (LCS) γ-ray source. One-dimensional mapping of a lead block hidden behind 1.5-cm-thick iron plates is obtained by measuring an NRF γ-ray of a lead isotope 208Pb. We observe a 5512-keV γ-ray from 208Pb excited by the quasi-monochromatic LCS γ-rays with energies up to 5.7 MeV. The edge position of the lead block is consistent with the exact position within the uncertainty.

High-voltage testing of a 500-kV dc photocathode electron gun

A high-voltage dc photocathode electron gun was successfully conditioned up to a voltage of 550 kV and a long-time holding test for 8 h was demonstrated at an acceleration voltage of 500 kV. The dc photocathode electron gun is designed for future light sources based on energy-recovery linac and consists of a Cockcroft-Walton generator, a segmented cylindrical ceramic insulator, guard-ring electrodes, a support-rod electrode, a vacuum chamber, and a pressurized insulating gas tank. The segmented cylindrical ceramic insulator and the guard-ring electrodes were utilized to prevent any damage to the insulator from electrons emitted by the support-rod electrode.

Generation of a 500-keV electron beam from a high voltage photoemission gun

High-brightness, high-current electron guns for energy recovery linac light sources and high repetition rate X-ray free-electron lasers require an exit beam energy of ≥500 keV to reduce space-charge induced emittance growth in the drift space from the gun exit to the following superconducting accelerator entrance. At the Japan Atomic Energy Agency, we have developed a DC photoemission gun employing a segmented insulator to mitigate the field emission problem, which is a major obstacle for operation of DC guns at ≥500 kV. The first demonstration of generating a 500-keV electron beam with currents up to 1.8 mA is presented.

Performance of the LaBr 3 (Ce) scintillator for nuclear resonance fluorescence experiment

Performance of the LaBr3(Ce) scintillator for nuclear resonance fluorescence (NRF) experiment was investigated. Linearity and energy resolutions of the LaBr3(Ce) scintillator of 3 inch in diameter × 3 inch in thickness were measured using NRF scattering gamma ray from 208Pb (5.5 MeV and 7.3 MeV) and 24Mg (10.7 MeV). The linearity of the output for the gamma ray energy was less than 1 % and the energy resolution of the scintillator in keV was almost proportional to the square root of the gamma ray energy up to 10.7 MeV.

Development of an electron gun for an ERL based light source in Japan

We have developed a prototype DC photoemission electron gun for a future energy recovery linac (ERL) based light source in Japan. The prototype gun is operated at up to 250 kV and is designed to deliver up to 50 mA beam current. A diagnostic beam line for emittance and bunch length measurements has been constructed. We have also designed a new 500 kV DC gun which is capable of producing up to 10 mA electron current with emittance lower than 1 mm‐mrad. The high voltage terminal will be isolated by ten ceramics stacked in series. The new gun will satisfy performance required as an injector for the compact ERL, which will be constructed at KEK site as a prototype of the future ERL light source.

Frequency-resolved optical gating system with a tellurium crystal for characterizing free-electron lasers in the wavelength range of 10–30 μm

A second-harmonic generation frequency-resolved optical gating (SHG-FROG) system has been developed for the complete characterization of laser pulses in the wavelength range of 10-30 microm. A tellurium crystal is used so that spectrally resolved autocorrelation signals with a good signal-to-noise ratio are obtained. Pulses (wavelength approximately 22 microm) generated from a free-electron laser are measured by the SHG-FROG system. The SHG intensity profile and the spectrum obtained by FROG measurements are well consistent with those of independent measurements of the pulse length and spectrum. The pulse duration and spectral width determined from the FROG trace are 0.6 ps and 5.2 THz at full width half maximum, respectively.

Diffraction of γ-rays with energies of 1.17 and 1.33 MeV by a flat Si crystal

Diffraction of γ-rays by a flat Si crystal has been demonstrated using a high flux 60Co source with an intensity of 2.3 TBq. The diffraction intensities of the γ-rays with energies of 1.17 and 1.33 MeV have been measured as a function of the rotation angle of the crystal. Three peaks corresponding to the Si(440) and Si(220) diffractions for 1.17 MeV and the Si(440) diffraction for 1.33 MeV have been measured. The heights and shapes of these three peaks are well reproduced by taking into account Braggs law and the experimental geometry.

Development of a 500-kV Photocathode DC Gun for ERLS

We have developed a 500-kV, 10-mA photocathode DC gun for energy recovery linac (ERL) light sources. A segmented ceramic insulator with guard rings is employed to improve robustness at high voltage operation, because this structure can prevent field emission electrons from directly striking the ceramic surface. We have recently succeeded in applying 500 kV on the ceramics for eight hours without any discharge. This high voltage testing was performed with a simple configuration without NEG pumps, cathode and anode electrodes to mainly study the field emission from a tube supporting the cathode electrode. The same high voltage testing with a full configuration necessary for beam generation was carried out up to 380 kV where some increase of radiation was observed. Up-to-date status of our gun development is presented in detail.

Demonstration of Flying Mirror with Improved Efficiency

A strongly nonlinear wake wave driven by an intense laser pulse can act as a partially reflecting relativistic mirror (the flying mirror) [S. V. Bulanov, et al., Bulletin of the Lebedev Physics Institute, No. 6, 9 (1991); S. V. Bulanov, et al., Phys. Rev. Lett. 91, 085001 (2003)]. Upon reflection from such mirror, a counter‐propagating optical‐frequency laser pulse is directly converted into high‐frequency radiation, with a frequency multiplication factor ∼4γ2 (the double Doppler effect). We present the results of recent experiment in which the photon number in the reflected radiation was at least several thousand times larger than in our proof‐of‐principle experiment [M. Kando, et al., Phys. Rev. Lett. 99, 135001 (2007); A. S. Pirozhkov, et al., Phys. Plasmas 14, 123106 (2007)]. The flying mirror holds promise of generating intense coherent ultrashort XUV and x‐ray pulses that inherit their temporal shape and polarization from the original optical‐frequency (laser) pulses. Furthermore, the reflected radi...

Status of R&D efforts toward the ERL-based future light source in Japan

The energy recovery linac is a very promising synchrotron light source in future. We are contemplating to realize a ERL_based next generation light source in Japan, under a collaboration between KEK, JAEA, ISSP, and other SR institutes. To this end, we started R&D efforts on its key technologies, including a low-emittance photocathode gun and superconducting cavities. We also plan to assemble these technologies into a small test ERL, and to demonstrate their operations. We report our R&D status.