MEASUREMENTS OF ABSOLUTE CROSS SECTIONS AND ASTROPHYSICAL S FACTORS FOR PROTON INDUCED REACTIONS ON 10,11B ISOTOPES

Abstract

Proton induced reactions are of crucial importance to understand nuclear structure and reaction dynamics [1], nucleosynthesis of elements in the universe [2] and also elemental abundance in different astrophysical sites [3]. Proton induced reactions are also of great practical importance in production of radioisotopes used in radiobiology [4]. This thesis is devoted to the measurements of absolute, total cross sections of two different proton induced reactions, namely, capture and direct (p, n) reaction. Both these reactions are of considerable importance in nuclear astrophysics and nuclear structure studies. The capture reaction that is presented in this thesis is that of proton capture by 10B isotope. We report, for the first time, the absolute, total capture cross section and astrophysical S factor for 10B(p, γ)11C reaction at fifteen different beam energies in the energy region of 8 to 16 MeV. These measurements are important for multiple reasons. This reaction helps to probe the structure of the low lying states in 11C, the Giant Dipole Resonance (GDR) states based upon the low lying states etc. From nuclear astrophysics perspective, the 10B(p, γ)11C is of considerable significance for the production of mass-11 nuclei. The 11C nucleus produced from the capture of proton emits a series of γ-rays to reach the ground state. The 11C nucleus in its ground state undergoes β+ decay to produce stable 11B. It is now well established that the three light elements, Li, Be and B are primarily produced in Inter Stellar Medium (ISM) by spallation reactions. However, the capture reaction studied in the low to medium energy range in this work may be of some import for the overall abundance of 10B and 11B in the ISM. The proton induced direct reaction can be of different types, namely, elastic and inelastic scattering, pickup reaction, knock-out reaction, charge exchange reaction, etc. By studying these proton induced reactions spread over a wide range of projectile energy, we can have a better understanding of nuclear structure, reaction dynamics, and nucleon-nucleus and nucleon-nucleon interactions. A part of this thesis work is devoted to measurements of the absolute total cross section of 11B(p, n)11C reaction. Different groups have studied this reaction using various approaches, such as, the activation method and the neutron time of flight method. However, the results are widely in disagreement with one another. As a consequence, new measurements for this reaction are required to eliminate the existing discrepancies in the cross sections. The (p, n) reaction on 11B has been studied by us at nine different incident proton energies in the region of 8 to 16 MeV in steps of one MeV. In this thesis work the cross sections of both the above mentioned reactions have been measured by activation technique. The 11C nucleus produced from both the reactions undergoes β+ decay with half life of 20.4 minutes to produce stable 11B nucleus. The emitted β+ gets annihilated in the target to produce two 511 keV γ-rays. The number of the two 511 keV γ-rays, measured in coincidence, help us extract the absolute total production cross section of the 11C nucleus. In our measurements, two large volume cylindrical LaBr3:Ce detectors were used to detect the coincident γ-rays. The measurements show the utility of this novel and powerful technique for extracting absolute total cross sections for nuclear reactions which lead to the production of β+ emitters with suitable half lives. Many of the previous measurements had intrinsic limitation in extracting accurate (p, n) reaction cross sections, due to rather thick targets and also uncertainties associated with the measurements of neutrons. Our measurements are expected to produce very high quality cross section data in the energy region of 8 to 16 MeV, devoid of the uncertainties of the previous measurements. Other than the main experiments forming the kernel of this work, certain aspects of nuclear instrumentation are also addressed in this thesis. They include, studies in Sr co-doped LaBr3:Ce scintillation detector, design and fabrication of a cooled target ladder and fabrication of backed and self-supporting boron targets. In recent times, there are serious efforts to grow scintillation crystals with additional co-dopants with the hope of developing γ-ray detectors with even superior qualities. In this thesis work, we present our work on the complete characterization of a small volume (1.5′′ × 1.5′′) LaBr3:Ce crystal, co-doped with Sr, with gamma-rays up to 4.43 MeV. At 661.7 keV, the energy resolution of the co-doped crystal was found to be same as that of a regular LaBr3:Ce crystal of similar size. In contrast, timing resolution of the Sr co-doped crystal was found to be marginally poorer than that of a regular LaBr3:Ce crystal of equivalent volume. However, our results are somewhat at variance with few other authors who have reported improvement in energy resolution of LaBr3:Ce crystal with Sr co-doping. Our observation of deterioration in timing resolution with Sr co-doping is in conformity with previous authors. We have concluded that more measurements are required before Sr2+ co-doped LaBr3:Ce can be declared noticeably superior to LaBr3:Ce. The improvement observed in the energy resolution must be consistent regardless of the volume and type of PMTs or APDs used. The improvement in energy resolution should also be sufficiently better to compensate for any possible deterioration in timing [5]. Considerable efforts have been made, as part of this thesis work, to fabricate backed and self-supporting boron targets used for in-beam measurements. Fabrication of self-supporting boron targets is rather involved and needs special care and efforts at every stage. One of the chapters is fully devoted to the description of our efforts to prepare the targets using electron gun evaporation method. We also discuss in detail the design, fabrication, testing and commissioning of a chilled water cooled target ladder system. The design, test results and simulations carried out for this cooled ladder are discussed in detail in one of the chapters. Such a cooled ladder system is essential for measurements at low energies with high currents, often required for reactions of nuclear astrophysics.