Quantum Beam Science Research

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Toward Nondestructive Assay of Fissile Materials－ Generation of 500 keV Electron Beam from DC Photoemission Gun －600

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LagrintteascachmCoTime (min)Fig.5-12 Generation of a 500 keV electron beam
A DC photoemission gun generates an electron beam uponlaser irradiation. The black arrows represent the periods wherea 500 keV electron beam was generated. The blue dashedline represents acceleration voltage; and the red solid line, thebeam current.

Fig.5-13 -ray source based on energy recovery linac
An energy recovery linac is under development at KEK.
The accelerated electron beam is delivered to the Compton
scattering chamber for γ-ray generation. A proof–of-principle
experiment for the proposed nondestructive isotope identification
will be conducted next year.

The quantification of fissile materials such as uranium-235(235U), plutonium-239 (239Pu), and minor actinides in spent fuelassemblies requires isotope-specific identification. We propose
the use of nuclear resonance fluorescence (NRF) to identify
the isotopic composition of sample materials nondestructively.
A mono-energetic γ-ray beam tuned to the resonance state of
the target nuclear isotope is injected into the spent fuels, andNRF γ-rays generated spherically from the fuels are detectedwith a γ-ray detector.
The proposed nondestructive isotope identification
system requires a high-intensity mono-energetic γ-ray beam.
Although a mono-energetic γ-ray beam can be generatedusing a conventional laser Compton scattering technique, thegeneration of a high-intensity γ-ray beam requires an electron
beam of unprecedentedly high brightness, which can begenerated with an advanced accelerator system known as theenergy recovery linac (ERL). A technological challenge of theERL system, which we have addressed, is the development of
a high-brightness, high-current electron gun.
The generation of high-brightness beam requires a gun
exit energy of ≧ 500 keV to avoid the beam degradationthat occurs due to nonlinear space charge effect. The gun
operational voltage has, however, been limited to 350 keV

because of a discharge problem. We have successfully
solved this problem by employing a segmented insulator andoptimizing the gun configuration.
Fig.5-12 shows the result of a beam generation test. The
electron beam is generated upon laser irradiation. The beam
current is proportional to the laser power. As shown in thefigure, we successfully generated a 500 keV electron beamwith a current of up to 2 mA.
The gun was recently moved to the compact ERL (cERL)
facility at KEK, where it is connected to an adjoining
superconducting accelerator, as shown in Fig.5-13. Monoenergetic γ-ray generation using Compton scattering will becommenced next year.
This work was supported by the Ministry of Education,Culture, Sports, Science and Technology of Japan (MEXT),
Quantum Beam Technology Program, “Development of anext-generation compact, high-brilliance light source using
superconducting accelerator technology” and partially
supported by the Japan Society for the Promotion of Science(JSPS) KAKENHI Grant-in-Aid for Scientific Research (C)No.23540353, “Study of micro-bunching instability using subpicosecond bunch measurement.”

Reference
Nishimori, N. et al., Generation of a 500-keV Electron Beam from a High Voltage Photoemission Gun, Applied Physics Letters, vol.102, issue 23, 2013,
p.234103-1-234103-4.

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JAEA R&D Review 2013