Investigation of a laser-driven neutron source with respect to different fields of application

• Main Author: Kleinschmidt, Annika
• Format: Others
• Language: en
• Published: 2017
• Online Access: https://tuprints.ulb.tu-darmstadt.de/6753/1/Dissertation_Kleinschmidt.pdf; Kleinschmidt, Annika <http://tuprints.ulb.tu-darmstadt.de/view/person/Kleinschmidt=3AAnnika=3A=3A.html> (2017): Investigation of a laser-driven neutron source with respect to different fields of application.Darmstadt, Technische Universität, [Ph.D. Thesis]

Due to their unique interaction with matter, neutrons are an interesting research and diagnostic instrument for various applications. To be able to utilize neutrons for the different applications, they have to be generated by nuclear reactions. This can for instance be done in accelerator-based spallation sources or fission reactors, which provide the possibility of generating high-flux neutron beams. However, they could be complemented by a novel and compact neutron source which is based on the conversion of laser-accelerated ions into neutrons inside a converter material, a so-called catcher. The angular distribution of the emitted neutrons is a superposition of an isotropic emission from generated compound nuclei in the catcher material and a forward directed neutron beam originating from special reactions such as pre-equilibrium emission and deuteron break-up. Neutrons from a laser-driven neutron source show an exponentially decaying energy spectrum with cut-off energies in the range of a few 10 MeV up to over 100 MeV. Nevertheless, for applications such as neutron resonance spectroscopy (NRS), neutrons in the epithermal energy range (0.1 eV - 100 keV) are preferable because many nuclei have distinct resonances in this regime. To maximize the neutron yield at epithermal energies, a moderating material is used to slow down the high-energy component of the neutron spectrum. The presented scientific thesis will focus on the applicability of a laser-based neutron source regarding established neutron applications in the high- and low-energy regime. In a first step, the for applications indispensable reproducability of such a source will be investigated. For that purpose, the neutron yield in the direction of the incoming ion's flight path (henceforth called "forward direction") will be increased and the total neutron yield will be measured for several shots. In a second step, the applicability of a laser-driven neutron source for neutron resonance spectroscopy on a static sample will be studied. As many elements have resonances in the epithermal region, the emitted neutron spectrum has to be moderated to increase the neutron flux in the desired energy range. Therefore, the study includes the analysis of the moderated neutron spectrum itself and its alteration after the NRS sample. The verification of reproducable neutron numbers and a detailed measurement of the angular distribution were conducted at the PHELIX laser (Petawatt High-Energy Laser for Heavy Ion EXperiments) at GSI Helmholtzzentrum für Schwerionenforschung GmbH in Darmstadt, Germany. The 200 J and 500 fs short-pulse laser beam was focused onto thin deuterated polymer foils with thicknesses between 400 and 1200 nm. During the experiment, intensities of the order of 10^20 W/cm^2 on target were achieved. The accelerated ions impinged a beryllium catcher yielding mean maximum neutron numbers of (5.25±0.77)·10^10 per shot. For a detailed measurement of the angular distribution, up to 30 bubble neutron dosimeters were used simultaneously. The result shows a forward pointing neutron beam with an opening angle of (100±2)° at full width half maximum. The highest measured neutron yield in the forward direction was (1.42±0.25)·10^10 neutrons per steradian which is an increase of 40% compared to the highest reported neutron numbers so far. The second key aspect of this thesis is the moderation of laser-driven neutrons and their subsequent application for a neutron resonance spectroscopy measurement on a static sample. This experiment was conducted at the Trident laser facility at Los Alamos National Laboratory (LANL), USA. The short-pulse configuration provides a maximum laser energy of 80 J on target within a pulse length of 500 fs, yielding maximum intensities above 10^20 W/cm^2. Ions were accelerated from thin polymer foils with thicknesses in the range of a few 100 nm. The catcher was surrounded by a block of high density polyethylene to slow down high-energy neutrons and thus maximize the epithermal yield. This could successfully be achieved by an increase of a factor 3 more neutrons in the energy range of the indium resonance compared to shots without a moderator. For the NRS measurement, a 5 mm indium sample was placed directly in front of the boron-doped microchannel plate (MCP) detector, which was used to measure the time of flight (ToF) transmission spectrum of the sample. The result of a single shot measurement shows a distinct resonance with a central energy of (1.61±0.19) eV and a width of (0.25±0.16) eV. These values are in good agreement with those of the indium resonance at 1.46 eV. During this experiment, we could sucessfully demonstrate a single shot neutron resonance spectroscopy measurement on a static sample. In summary, in the framework of the presented thesis it will be demonstrated, that a laser-driven neutron source satisfies the requirement of constantly high neutron fluxes, which is very important for significant and reliable measurements during applications. In addition, the effective moderation and the feasibility of laser-driven neutrons for neutron resonance spectroscopy on a static sample will be confirmed for the first time.