Experiments to enhance photoneutron production from the Ta (γ,xn) reaction with 100 TW picosecond laser pulses irradiating on gas-solid hybrid targets have been performed on the XingGuangIII laser facility at the Laser Fusion Research Center (LFRC) in Mianyang. The so-called gas-solid hybrid target was composed of a 1 mm thick N2 gas jet and a 2 cm thick Ta block. Picosecond laser pulses with intensities up to 1019 W/cm2 first interact with the tenuous gas to enhance the yields of high energy electrons through the direct laser acceleration (DLA) mechanism, and then through bremsstrahlung and subsequent (γ, n) reactions in a Ta converter, photoneutrons were enhanced effectively and the total number of neutrons was optimized by changing the gas density. A maximum neutron yield of 4 × 107/shot over 4π was achieved which is 200 times higher than that in the direct laser-solid interaction shot in our experiment. The spectrum of photoneutrons was measured which is in agreement with Monte Carlo (MC) simulation. From the size of the reaction area and the neutron pulse duration inferred from simulation, the corresponding flux was calculated to be 1.2 × 1016 n/cm2/s.

Radioisotopes are indispensable agents in medical diagnosis and treatment, among which 62, 64Cu and 68Ga are medical isotopes widely used in positron emission tomography (PET) imaging. Experiments of generating these radioisotopes via laser-induced photonuclear reactions were performed on the XingGuangIII laser facility of the Laser Fusion Research Center (LFRC) at Mianyang. Large-charge (Q_e ~ 40 nC) MeV electron (e–) beams were generated with 100 TW picosecond (ps) laser pulses. The e– beams then impinge on a metal stack composed of Ta foil and activation plates (natural Cu and Ga2O3), producing high-energy bremsstrahlung radiations and isotopes 62, 64Cu and 68Ga, respectively. The characteristic emissions of the produced 62, 64Cu and 68Ga were off-line detected and the production yields of 62, 64Cu and 68Ga were obtained to be the order of 106 per laser shot. The dependence of radioisotope production efficiency (per e–) on electron temperature (T_e) is investigated through Geant4 simulations. It is found that the production efficiency increases with the T_e and then reaches a saturation value of 8 × 10−5 for 62Cu and 10–5 for 68Ga at T_e ~ 10 MeV. The prospect of producing medically isotopes 62, 64Cu and 68Ga is further evaluated by using table-top femtosecond laser system of high repetition. Considering a repetition rate of 100 Hz, it is expected that the activity can reach 0.2 GBq for 62Cu, 0.1 GBq for 64Cu and 0.05 GBq for 68Ga, respectively. Such activity would meet the required dose for clinical PET imaging, indicating the great potential to produce medical radioisotopes with an all-optical, high-repetition laser system.

We report an experimental investigation of a laser-gas-converter approach for generating high-yield ultrashort MeV positrons. We observe that MeV electrons with a high charge of several tens of nC can be well generated by a ∼ 6 J , ∼ 40 f s laser interacting with a high-density gas jet. However, it is shown that the propagation of the highly charged electron beam is significantly inhibited because the electrons are reflected by the sheath potential in the density decreasing region of the gas target, thus leading to a low positron yield. Consequently, by using an integrated nozzle-converter design to eliminate the density falling ramp of the gas target such that the electron refluxing is inhibited, we observe a significant enhancement of positron yield (up to a factor of 15), finally reaching a positron yield of 5 × 10 8 s r − 1 . This high-yield ultrashort MeV positron may have great potential toward the simulation of astrophysical pair plasma.