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

In the framework of R&D on high-intensity positron sources for linear accel erators, we have studied a possibility of using tungsten (W) single crystal as a target in the I GeV electron energy region. Positron production effic iencies 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 amorph ous plate (1.5 X-0, in total thickness). When the <1 1 1 > axis of the W cr ystal (0.34 X-0 thick) is aligned with the electron beam direction, positro n 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 spect ra. The obtained results are compared with simulations based on the coheren t bremsstrahlung (CB) process. The rocking curve, the electron-energy depen dence and the target-thickness dependence of the positron yields are explai ned predominantly by the CB process followed by the electromagnetic shower in this energy region. (C) 2001 Elsevier Science B.V. All rights reserved.