ИСТИНА

There is effective generation of gamma-radiation (with photon energies above 1 MeV), and acceleration of protons and heavier ions at energies up to 100 MeV per unit charge in the interaction of laser radiation with sub-picosecond duration plasma in a mode of relativistic intensities (1018-1020 W/cm2). This complies with the basic requirements to the characteristics of the particle flows for the tasks of the nuclear laboratory astrophysics. Estimates conducted for some promising reactions show that proton-induced reaction 7Li(р, n)7Ве and 9Ве(р, n)9В are the most perspective for the tasks of the nuclear laboratory astrophysics from the point of view of obtaining directed flows of neutrons with energy less than 1 MeV. Optimization of the number and the energy spectrum of neutrons is possible dew to duration and intensity of the laser pulse. The use of «long» laser pulses (up to 1 ps) increases not only the total number of high-energy particles, but also, in certain modes, the «temperature» of the forming spectrum. The contrast of the laser radiation is also important for the realization of the approach developed as use of a thin (less than 1 mkm) foil is necessary for improving mechanisms of protons and heavy ions acceleration. In general, the contrast of the laser radiation should be from 106 to 1010 and more depending on characteristics of a stream of particles or quanta generated and intensity of the laser radiation used. The experiments were performed on the Neodim 10 TW picosecond laser setup. The parameters of the laser pulse are as follows: energy, up to 10 J, wavelength, 1,055 μm; and duration, 1,5 ps. The focusing system based on the off-axis parabolic mirror with a focal length of 20 cm concentrates no less than 40 % of the laser-beam energy to a spot with a diameter of 15 μm, so that the mean intensity on the target is 1018 W/cm2 and the peak intensity is 2×1018 W/cm2. The laser radiation that is generated in the Neodim setup is characterized by the prepulses of two types (with picosecond and nanosecond durations). The first prepulse with a duration of 1,5 ps and a relative intensity of less than 10–8 (relative to the main pulse) precedes the main pulse by 14 ns. The second prepulse is the pulse of the amplified spontaneous emission. The FWHM of the second prepulse is 4 ns and its intensity relative to the main pulse is less than 10–8. For the analysis of the initiation of the (p, n) nuclear reactions, we choose the 7Li(p, n)7Be reaction with a threshold energy for protons of 1,88 MeV, the 63Cu(p, n)63Zn reaction with a threshold energy of 4,1MeV, and the 48Ti(p, n)48V reaction with a threshold energy of 5 MeV. When the laser radiation is focused on the front surface of the Al-foil target with a thickness of 10 μm, the beam of accelerated protons is generated on the back surface. The proton beam initiates the (p, n) nuclear reactions on the LiF, Cu, and Ti secondary activation targets. Neutrons are detected in the experiments with all types of secondary targets, which indicates the initiation of the above (p, n) reactions. The irradiation of targets with high-power laser pulses leads to the generation of the femto- and picosecond laser plasma, which serves as the table-top pulsed microaccelerator or nuclear microreactor. We present the experimental results on the initiation of various (p, n) nuclear reactions in the picosecond laser plasma. At a laser intensity of 2×1018 W/cm2, the following (p, n) nuclear reactions can be initiated in the picosecond laser plasma: 7Li(p, n)7Be, 63Cu(p, n)63Zn, and 48Ti(p, n)48V with the proton threshold energies ranging from 1.88 to 5 MeV. For the measurement of the number of the initiated (p, n) nuclear reactions, we employ the method based on the detection of neutrons by the detectors with the 3He counters. The measured yields of the (p, n) nuclear reactions range from 5×101 to 105 reactions per laser pulse. This work was supported by the Russian Foundation for Basic Research (project nos. 12-02-00489, 13-02-00494 and 13-02-00878).