ABSTRACT
The fission probability P(f) of highly excited targetlike nuclei produced in reactions of 2.5 GeV protons on Au, Bi, and U was studied as a function of excitation energy E* whereby E* is deduced eventwise from the multiplicity of evaporated light particles. At the highest E* of 1000 MeV P(f) amounts to approximately 30% with all 3 target nuclei irrespective of the initial fissility. Statistical-model calculations satisfactorily reproduce the observed evolution of P(f) with E*--provided that no extra transient delay is introduced. Fission thus is decided upon very fast and early in the long deexcitation chain towards scission which comprises as much as approximately 80% of all evaporated alpha particles.
ABSTRACT
An efficient acceleration of energetic ions is observed when small heavy-water droplets of approximately 20 microm diameter are exposed to ultrafast (approximately 40 fs) Ti:sapphire laser pulses of up to 10(19) W/cm2 intensity. Quantitative measurements of deuteron and neutron spectra were done, allowing one to analyze the outward and inward directed deuteron acceleration from the droplet. Neutron spectroscopy based on the D (d,n) fusion reaction was accomplished in four different spatial directions. The energy shifts of those fusion neutrons produced inside the exploding droplet reflect a remaining deuteron acceleration inside the irradiated droplet along the axis of the incident laser beam. The overall neutron yield of the microdroplets is relatively small as a result of the dominant outward directed acceleration of the deuterons with 1200 neutrons/shot. Relying on the "explosion-like" acceleration of such spherical droplet targets we have developed a spray target consisting of heavy-water microspheres with diameters of 150 nm . Both the high deuteron energies of up to 1 MeV resulting from the irradiation intensity of approximately 10(19) W/cm2 as well as the collisions between the deuterons and the surrounding spray delivered about one order of magnitude more neutrons than the single-droplet system. The approximately 6 x 10(3) neutrons per laser pulse from the spray can be attributed to an efficient deuteron release from a significantly smaller laser excited volume as from deuterium-cluster targets.
ABSTRACT
Detailed neutron energy spectra were measured for the D(d,n)3He reaction induced in solid (CD2)(n) targets by irradiation with 50-fs 2 x 10(18) W/cm(2) light pulses from a 10-TW Ti:Sapphire laser. The neutrons were observed at two angles 5 degrees and 112 degrees relative to the incident laser beam. The neutron spectra at the two angles are characterized by peaks with large widths of about 700 keV full width at half maximum and a shift of 300 keV between them. Neutron energies of up to about 4 MeV were observed indicating that deuterons are accelerated up to an energy of 1 MeV in the laser produced plasma. Simulation calculations can describe qualitatively the neutron spectra by assuming isotropic deuteron acceleration and a reduction of the reaction probability by a factor of 1/3 for deuterons emitted from the front of the target. These calculations indicate in particular that it is necessary to assume deuterons moving both into and out of the front of the target in order to describe the neutron energy spectra at the two angles. The highest recorded mean neutron yield was about 10(4) neutrons per pulse. The neutron yield increases with the number of electrons emitted from the front of the target and with the intensity of the prompt gamma flash induced by the bremsstrahlung of energetic electrons.
