Positron and gamma-photon production and nuclear reactions in cascade processes initiated by a sub-terawatt femtosecond laser

Numerous proposals to induce nuclear transformations by intense lasers ~see, e.g., Ref. 1! predicted exceedingly small output of ‘‘nuclear radiation’’ ~positrons, gammaphotons, neutrons, and fission fragments!, even for the laser intensity still out of reach. In fact, efficient production of nuclear radiation was not the subject of those proposals; as a result, optimal choice of processes and targets was not addressed. Recently, we have shown theoretically that already available laser intensities are sufficient for producing, through a specially arranged cascade of processes, practically useful ultrashort-pulse, high-flux nuclear radiation with possible applications in material science, medicine, and nuclear engineering. High nuclear radiation yield predicted in Ref. 2 presumes, however, laser power of tens of terawatt ~TW!, which is currently available only from a few unique systems. In the present letter, we concentrate on the opportunities provided by much more modest lasers, with the output power of ;1 TW. We demonstrate theoretically that even such, widely available lasers are capable of generating, through relativistic laser–plasma electrons interacting with optimal targets, noticeable amounts of nuclear radiation and could, therefore, be instrumental in proof-of-principle experiments for future practical laser-based sources of that radiation. Since direct electron–positron pair production or nuclear transformations by a laser field apparently require exceedingly high intensities, the only presently practical road to producing nuclear radiation by a laser is using MeV electrons present in laser plasma at already available laser fields. The efficient generation of such electrons occurs in plasma of subcritical density ~see below!; at the same time, efficient pair production and nuclear transformations require the highest density possible, in other words, solid targets. Moreover, cross sections of electron-induced processes of interest are orders of magnitude smaller than those of the respective photon-induced reactions, which calls for using a Bremsstrahlung converter. The above arguments bring us to a three-step cascade: ~i! generation of MeV electrons in subcritical laser plasma; ~ii! Bremsstrahlung conversion of MeV electron energy into MeV photons in a high-Z solid target; and ~iii! electron–positron pair production or photonuclear reactions. The energy thresholds of the third step determine