Experimental Tests of Screen Materials for High-Precision Transverse Beam-Size Measurements at the SuperKEKB Injector Linac

The SuperKEKB injector linac is required to deliver low-emittance electron and positron beams. Wire scanners are employed to measure Twiss parameters and to adjust beam optics conditions. Screen monitors also play important roles for single-shot measurements. However, the beam size became more than 10-times smaller compared with that of the KEKB injection. Beam tests have been performed in order to evaluate materials for high-precision transverse beam-size measurements at the injector. The main purpose of the beam tests is to quantitatively investigate the saturation effect of each screen material for generating the scintillation light, which is strongly depending on the beam-charge density. Several scintillating screen materials including YAG:Ce, LYSO:Ce, BGO and aluminum ceramic have been tested with high energy and high charge-density electron beams. The results are compared with that obtained by the OTR measurement. The saturation of the luminescence was confirmed for all crystals and evaluated in the charge density of 0.5-1.5 nC/mm2. The cause of the saturation and the effect to the measurement are discussed.