Relativistic Positron Creation Using Ultraintense Short Pulse Lasers

We measure up to 2x10(10) positrons per steradian ejected out the back of similar to mm thick gold targets when illuminated with short (similar to 1 ps) ultraintense (similar to 1x10(20) W/cm(2)) laser pulses. Positrons are produced predominately by the Bethe-Heitler process and have an effective temperature of 2-4 MeV, with the distribution peaking at 4-7 MeV. The angular distribution of the positrons is anisotropic. Modeling based on the measurements indicate the positron density to be similar to 10(16) positrons/cm(3), the highest ever created in the laboratory.