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

In the framework of R and 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 X0) and for combinations of a single crystal and an amorphous plate (1.5 X0 in total thickness). When the <1 1 1> axis of the W crystal (0.34 X0 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