Positron reemission properties of SiC were studied. Promising electric field assisted reemission results were observed with one sample. Without an applied field the diffusion length is 246±2 nm with y o = 21±0.5 % reemission at 1 keV incident energy. The reemission fraction increased to as much as 42.3±0.6% and 14.4±0.5% for incident positrons of 1 and 20 keV respectively after the application of an electric field. A field-assisted diffusion length of 1.91±0.14 µm and a branching ratio of y o =28.8±0.1% were extracted. These values lead to a reflection-mode moderator efficiency comparable to that of tungsten single crystal.The most promising technique to enhance the efficiency of moderators is to drift thermalized positrons across the moderator to the emitting surface by means of an external electrical field known as field-assisted moderators (FAM) [1]. While positron drifts have been observed in GaAs for example [2], all potential gains were lost at or in the thin contact layer used to apply the external field.A novel idea is demonstrated, that might well make FAM for positron beams practical and competitive with current moderators. The method eliminates the need for a contact layer on the positron-emitting surface that has been the stumbling block in previous attempts [3][4][5][6][7]. A state of the art wafer of semi-insulating SiC (s.i. SiC) was exposed to a broad electron beam. The electrons charge the surface of the SiC, thereby creating the required extraction field.All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Scite.
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