LASER DRIVEN NUCLEAR ASTROPHYSICS study at ELI-NP

Given its nature, the plasma state is characterized by a complexity that vastly exceeds the one exhibited by the solid, liquid, and gaseous states. Correspondingly, the physical properties of nuclear matter (structure, lifetimes, reaction mechanisms etc.) may change inside the plasma. Thus, the study of these properties represents one of the most far ranging, difficult and challenging research areas today. Implications can cover also others fields, from quantum physics to cosmology, astrophysics etc. One of the crucial topics related to nuclear reactions in the ultra-low energy regime is the electron screening, which prevents a direct measurement of the bare nucleus cross section at the energies of astrophysical interest. Since in the laboratory interacting particles are in the form of neutral atoms, molecules or ions, in direct experiments at very low beam energy, electron clouds partially screen the nuclear charges thus reducing the Coulomb suppression. This results in an enhancement of the measured cross section compared with the bare nucleus one. The electron screening effect is significantly influenced by the target conditions and composition. In this context, it is of particular importance the measurement of cross-sections at extremely low energetic domains including plasmas effect, i.e. in an environment that under some circumstances and assumptions can be considered as “stellar-like” (for example, the study of the role played by free/bounded electrons on the Coulombian screening can be done in dense and warm plasmas). A further key point is connected with the fact that in such environment nuclear reactions can be triggered also by the excited states of the interacting nuclei. Thus, determining the appropriate experimental conditions that set the role of the excited states in the stellar environment can strongly contribute to the development of nuclear astrophysics. The study of direct measurements of reaction rates in plasma offers this chance. The future availability of high-intensity laser facilities capable of delivering tens of peta-watts of power into small volumes of matter at high repetition rates will give iv Word Template by Friedman & Morgan 2014 the unique opportunity to investigate nuclear reactions and fundamental interactions under extreme plasma conditions, including also the influence of huge magnetic and electric fields, shock waves, intense fluxes of X and γ-rays originating during plasma formation and expansion stages. A laser is a unique tool to produce plasma and very high fluxes of photon and particle beams in very short duration pulses. Both aspects are of great interest for fundamental nuclear physics studies. In a plasma, the electron–ion interactions may modify atomic and nuclear level properties. This is of prime importance for the population of isomeric states and for the issue of energy storage in nuclei. Nuclear properties in the presence of very high electromagnetic fields, nuclear reaction rates or properties in hot and dense plasmas are new domains of investigation. Furthermore, with a laser it is possible to produce electric and magnetic fields strong enough to change the binding energies of electronic states. If nuclear states happen to decay via internal conversion (IC) through these perturbed states, a modification of their lifetimes will be seen. The excitation of nuclear levels by means of energy transfer from the atomic part to the nuclear part of an atom is the subject of a large number of investigations. Their goal is to find an efficient mechanism to populate nuclear isomers in view of further applications to energy storage and development of lasers based on nuclear transitions. In addition, other new topics can be conveniently explored such as three-body fusion reactions as those predicted by Hoyle. Several Laser facilities are under construction around the world to push the physics beyond the actual level of knowledge. Among of these, the Extreme Light Infrastructure for Nuclear Physics at Magurele (Bucharest) in Romania, will be the only one devoted to nuclear physics studies. ELI-NP will be made up of a very high intensity laser system, consisting of two 10 PW laser arms able to reach intensities of 1023 W/cm2 and electrical fields of 1015 V/m, and very short wavelength γ beams with very high brilliance (1013 γ/s) and energy up to 19.5 MeV. This combination allows for three types of experiments: stand-alone high power laser experiments, stand-alone γ beam experiments and combined experiments of both facilities. Here the low repetition rate (1/min) of the high power laser requires the same low repetition rate for the γ beam in combined experiments. While the standalone γ beam will be used with typically v Word Template by Friedman & Morgan 2014 120 kHz, the low repetition mode requires few very intense γ pulses. With the high power laser we do not plan to interact with nuclear dynamics directly, but we use the laser for ion acceleration or to produce relativistic electron mirrors followed by a coherent reflection of a second laser beam in order to generate very brilliant X-ray or γ beams. We plan to use these beams later to produce exotic nuclei or to perform new γ spectroscopy experiments in the energy or time domain. The production of heavy elements in the Universe, a central question of astrophysics, will be studied within ELI-NP in several experiments. In this Ph.D. thesis some of the activities, related to the project of study of nuclear astrophysics at ELI-NP will be reported and discussed. The Thesis is organized as follows; Chapter I: a general introduction is given to present the main open problems on nuclear astrophysics and the opportunity offered by the laser matter interaction scheme. Chapter II: the physics of laser matter interaction is discussed. Chapter II: a short presentation of the Laser facility around the world is given with special attention to the ELI-NP. Chapter III: the research project et ELI-NP is presented. Chapter IV: the studies performed to prepare the future activities at ELI-NP are discussed: simulations, laser matter interaction test, R&D activities on plasma and nuclear detectors. Chapter V: the results of the tests performed on the detector prototypes are presented and discussed. More in detail, in Chapter III the idea of using a colliding plasma suitable for nuclear physics studies and the proposed schema of interaction is presented. A first laser pulse imping on a primary solid target producing plasma through the TNSA (Target Normal Sheath Acceleration) acceleration scheme. The rapidly streaming plasma impacts on a secondary plasma, prepared through the interaction of a second synchronized laser pulse on a gas jet target. The produced ions expand along a cone, whose axis is normal to the target surface, with a relatively low emittance, while the properties of the secondary plasma vi Word Template by Friedman & Morgan 2014 (working as a “plasma target”) can be modified or tuned, depending on the energetic domains one wants to explore. By using femtosecond pulses, secondary plasma temperatures lie in the tens of eV range. For reactions with fully thermalized plasmas at medium-high ion temperatures, the duration of the secondary laser beam can be extended in the nanosecond domain. Simulations about the two plasmas interaction have been performed with different models. Such work has been focalized to evaluate the total reaction rate and further the possible information, which could be extracted on the reaction cross-sections. Chapter IV is dedicated to the TNSA studies. Target Normal Sheath Acceleration is the key mechanism for the production of the primary plasma using a high power - femtosecond laser beam imping on a solid thin target (1-20 μm). TNSA was intensively studied in the last years; experiments and models show that this acceleration scheme works very well in the intensity domain between 1018-1020 W/cm2. The observed ion energy distributions have an exponential shape with a high-energy cut-off, linearly depending on the laser intensity and scaling with the atomic number. These experimental observations are well described and predicted by theoretical models. A further fine-tuning can be done acting on other parameters such as the laser incident angle or polarization, the structure of the target surface, or the target thickness. In this respect, we performed several experimental campaigns in order to refine the information on TNSA. The activity was conduced at the Intense Laser Irradiation Laboratory (ILIL), in Pisa in the area of CNR-INO. By using the available 10 TW/10Hz system, we carried out a systematic experimental investigation to identify the role of target properties on TNSA, with special attention to target thickness and dielectric properties. It has focused on the results obtained using a Thomson Parabola Spectrometer (TPS). During the experiment several targets have been used. These targets have been selected to study the acceleration mechanism and its dependence on the surface/bulk contribution or on the possible dependence of a metal layer deposited on the irradiated surface, etc. Chapter V is focussed on the R&D activity performed on the prototypes of a highly segmented detection system for neutrons and charged particles which will be realized and installed at ELI-NP for the conduction of the experiments. The segmentation is required for the reconstruction of the reaction’s kinematics. vii Word Template by Friedman & Morgan 2014 The “ideal” neutron detection module for these studies must have high efficiency, good discrimination of gammas from neutrons, good timing performance for Time of Flight neutron energy reconstruction. In addition, it must be able to work in hard environmental conditions, like the ones established in the laser-matter interaction area. All these requirements can be fulfilled by a configuration based on PPO-Plastic scintillators plus a SiPM readout and a totally digital acquisition of the multi-hit signals. The charged particle detectors must be able to work in plasma environment and then must be insensible to visible light and experience high resistance to radiation damage. SiC detectors have been recently proven to have excellent performance in this respect joint to the high energy and time resolution.