Development of intense, long-lived positron sources

In recent years slow positron beam physics has become a very active and diverse field [1] . Currently under consideration are several applications of slow positron beams which require intense beams (>_ 1 Ci of e + ) for their success. A partial list of such applications includes : the production and study of antihydrogen [2], the use of positrons in synchrotron light sources [3], positron microscopy [4], laser spectroscopy of positronium atoms [5], and the production of positron/ electron plasmas [6] . As a result there has been a great deal of interest m the production of very intense positron sources. Schemes for producing positron sources from several different isotopes have been considered, including 48V [7], 64Cu [8], 68Ga [9], and 126, [10] . In addition, methods of constructing intense positron beams using electron cooling [11] and bremsstrahlung from electron LINACs [12] have been investigated . For small-scale laboratory experiments requiring intense positron beams, radioactive sources are the only feasible option . An important parameter to consider when choosing an isotope for a positron source is the half-life. Isotopes with long half-lives are preferred because they need to be replaced infrequently, which minimizes radiation exposure during handling . Of the useful positron emitters the two with the longest halflives are 22 Na (2 .6 years) and S8Co (71 days). Two separate projects have been undertaken at the University of Michigan aimed at developing processes for the fabrication of intense positron sources from both of these isotopes . Positron sources made from 22 Na are very attractive for laboratory use because 22 Na combines a long halflife with high positron emission (f(ß')=90%) .Although other schemes for producing 22 Na have been