1. A positron source comprising: an inner cylinder adapted to contain a liquid radioisotope as a source of positrons; an outer cylinder disposed coaxially with said inner cylinder so as to form an annular cavity therebetween, an inner surface of said outer cylinder being coated with a high atomic number material sufficient to stop positrons emitted from said inner cylinder while also providing a desired fraction of backscattered positrons; a pusher disk at a first end of said outer cylinder, said pusher disk being positively biased with respect to said inner cylinder to push low energy positrons emitted from said inner cylinder toward a second end of said outer cylinder; and an extraction grid at said second end of said outer cylinder, said extraction grid being negatively biased with respect to said inner cylinder so as to extract low energy positrons from said annular cavity.