Making relativistic positrons using ultraintense short pulse lasers

This paper describes a new positron source using ultraintense short pulse lasers. Although it has been theoretically studied since the 1970s, the use of lasers as a valuable new positron source was not demonstrated experimentally until recent years, when the petawatt-class short pulse lasers were developed. In 2008 and 2009, in a series of experiments performed at the Lawrence Livermore National Laboratory, a large number of positrons were observed after shooting a millimeter thick solid gold target. Up to 2×1010positrons∕s ejected at the back of approximately millimeter thick gold targets were detected. The targets were illuminated with short (∼1ps) ultraintense (∼1×1020W∕cm2) laser pulses. These positrons are produced predominantly by the Bethe–Heitler process and have an effective temperature of 2–4MeV, with the distribution peaking at 4–7MeV. The angular distribution of the positrons is anisotropic. For a wide range of applications, this new laser-based positron source with its unique characteristics may complement the existing sources based on radioactive isotopes and accelerators.