Kensei Umemori

Enhancement of the characteristic X-ray yield from oriented crystal irradiated by high-energy electrons

Abstract In this report, results of a measurement of characteristic X-ray (CXR) yields from oriented tungsten crystal targets irradiated by 600–1000 MeV electrons are presented. Characteristic X-rays from tungsten are measured with a Si(Li) semiconductor detector placed at the backward direction with respect to the incident electron beam. We have observed an enhancement of the X-ray yield due to the K-shell ionization when the crystal axis 〈1 1 1〉 is oriented along the beam. The ratio of the K-line yield from the oriented crystal to the one from the disoriented crystal is about 1.6–1.9 for the target thickness of 1.2 mm at the electron energy of 1000 MeV. For L-line yields the enhancement is not appreciable. We demonstrated a possibility of using the orientation dependence of the CXR as a mean of aligning the crystal axis at the channeling condition to the beam.

Experimental study of positron production from crystal targets by 0.6–1.0 GeV electrons

Abstract In the framework of R&D on high-intensity positron sources for linear accelerators, we have studied a possibility of using tungsten (W) single crystal as a target in the 1 GeV electron energy region. Positron production efficiencies are measured for W crystal targets with different thicknesses (0.11, 0.34 and 0.63 X 0 ) and for combinations of a single crystal and an amorphous plate ( 1.5 X 0 in total thickness). When the 〈1 1 1〉 axis of the W crystal ( 0.34 X 0 thick) is aligned with the electron beam direction, positron yields are enhanced by 1.6–2.3 times compared to the non-aligned case at 0.6–1.0 GeV. In order to study the possible mechanism of the positron-yield enhancement in the aligned crystal, we have also measured the photon spectra. The obtained results are compared with simulations based on the coherent bremsstrahlung (CB) process. The rocking curve, the electron-energy dependence and the target-thickness dependence of the positron yields are explained predominantly by the CB process followed by the electromagnetic shower in this energy region.