Laser intensities available for experiments on laser-plasma interactions have increased rapidly during the past decade. Today, intensity in the range 1019-1020 W/cm2 is commonly available. These intensities result in fully relativistic electron motion and effective temperature in the MeV range. It is of great interest to characterise the properties of the fast particles, electrons and ions, emerging from the laser-plasma interaction. However, many of the techniques previously applied when particles energies reached only tens or hundreds of keV have proven insufficient at the highest intensities.
In this thesis, nuclear activation techniques have been developed for the study of the fast particles in laser-plasma experiments. These techniques have been applied in experiments carried out at the 50 TW VULCAN laser at Rutherford Appleton Laboratory. Simple nuclear reactions typically have reaction thresholds from a few MeV to tens of MeV. Hence, such reactions have two important properties: they provide energy sensitivity in the presently interesting energy range and they are completely insensitive to particles of low energy, removing the potential ambiguity between detecting many low-energy particles vs. a single energetic one.
For the detection of fast electrons, photonuclear reactions have been applied. This is an indirect process where the electrons first generate bremsstrahlung photons which subsequently induce activation in the detector materials. The reaction products are often positron-emitting. This allows sensitive detection of the induced activity by coincidence techniques. In laser-solid experiments, the photonuclear activation has been applied to get information on the angular distribution of the fastest electrons. In plasma accelerator experiments, the technique has been applied for the study of electron angular distribution, temperature and total yield. It has been shown that the electron spectrum is produced by plasma accelerator experiment is nearly an order of magnitude hotter than in laser-solid experiments at similar laser intensity.
Fast protons from the laser-solid interactions have been characterised by proton-induced reactions. Such reactions commonly produce positron-emitting products allowing the detection by coindicence techniques. Because of the continuous slowing-down and the short range of protons, spectral measurements could be carried out by using stacked foil detectors. With thin targets, these measurements showed that a much harder proton spectrum is generally observed behind the target than in the plasma blow-off direction. Nuclear reactions induced by heavier ions like 12C in the plasma blow-off were also detected in this work.reviewe
