Basics of Nuclear Astrophysics

Physics is a resourceful subject having many branches and with the advancement of time many new ideas, observations, theories, and experimental facts still adding newer branches to it. Nuclear astrophysics is the study of nuclear‐level processes that occur naturally in space. Notably, this includes understanding the chain of fusion events, or nucleosynthesis, that occurs in stars, and how this can be detected from a distance by measuring the radiation these processes produce. Thus the hybridization of the ideas of astrophysics with those of nuclear physics led to the development of this new branch of physics which we call as nuclear astrophysics. It is an interdisciplinary branch of physics involving close collaboration among researchers in various subfields of nuclear physics and astrophysics, with significant emphasis in areas such as stellar modeling, measurement and theoretical estimation of nuclear reaction rates in the stellar environment, cosmology, cosmochemistry, gamma‐ray, optical and X $ {\mathbf{X}} $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_1.tif"/> ‐ray astronomy, advancing our knowledge towards nuclear lifetimes and masses. Generally, nuclear astrophysics aims i) to understand the origin of the chemical eleme nts and production of energy in stars by focusing on the explosive stellar events (known as supernova) and the associated nuclear phenomena, ii) to construct theoretical models for supernovae, nova, x $ {\text{x}} $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_2.tif"/> ‐ray, and gamma‐ray bursts, iii) to understand the creation of new atomic nuclei and their contribution in the formation of galaxies, new stars and planets, and iv) to explore the possible way to explain how the study of the stellar events and their nuclear products, that 12 form much of ourselves and our world, could connect us to the beginning of the Universe. Figure 1.1 tells about the major subject areas of nuclear astrophysics. The environment where we live in always triggers us in many ways so that we may understand it. For instance, when we are exposed to sun rays in hot summer days we are being invited by the nature to learn about the source of the radiant energy of the sun, which is far away from our earth. In this way, nature itself has been insisting us to search for the answer of many fundamental questions: How the present Universe came into existence? What existed at the beginning of space and time? How elements are synthesized? How stars are formed? How galaxies are formed? Why stars shine? How stars die? How supernova works? How neutron stars are produced? How black holes or indexwarm holewarm holes are formed? How old is the universe? And the list continues. Since the inception of modern science, brilliant mathematicians and scientists have sought the answers to these fundamental questions. Among those Copernicus, Galileo, Newton, Einstein, Hubble, and others used direct observation, reasoning, used mathematics, and new technologies to overturn ideas about cosmology that 13 were believed to be fundamental truths. Their breakthroughs reshaped the scientific understanding of the nature and structure of the universe. Their work, together with works of other well‐known cosmologists, not only provided new explanations of the underlying facts about the universe, but also raised seemingly ironic questions. Did the enormous variety and mass of matter that make up the cosmos evolve from nothing but energy? If that is true, then where did the energy that produced all of the matter in the universe come from? A literature survey on the issue reveals that: the basic principles of explaining the origin of the elements and the energy generation in stars were laid down in the theory of nucleosynthesis which came together in the late 1950 s $ 1 9 5 0 {\text{s}} $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_3.tif"/> from the seminal works of Burbidge, Burbidge, Fowler and Hoyle and independently by Cameron. Among them, Fowler is largely credited for initiating the collaboration among astronomers, astrophysicists, and experimental nuclear physicists to synthesize the subject which is what we now know as nuclear astrophysics. He won the Nobel Prize in 1983 for his remarkable contribution in nuclear astrophysics. The basic requirement of experimental nuclear and particle physicists is the projectile beam of appropriate energy and intensity together with appropriate well maintained reaction/event friendly environment. To some extent the required beam can be made available by artificially designed particle accelerators and reactors. However, to nuclear and particle physicists, the early universe represents the ultimate particle accelerator as well as nuclear reactor in which energy density and particle density are both beyond the values expected to achieve in artificially designed accelerators and reactors on the planet we are residing. Although in the early universe reactions occurred with incomprehensible rates and varieties, still there is hope, to successfully explain many aspects of those reactions including their underlying processes by careful study of their end products. Above all, the most challenging job one has to encounter in the study of nuclear astrophysics is the understanding of the mechanism of formation of heavy elements in the heart of the stars by fusion and neutron capture reactions. This is because the nuclear physics, the mechanics and thermodynamics involved in such environment is very complicated to be understood. The principal source of stellar informatory evidences are the data accumulated by astronomical observations with the latest equipments like γ $ {{\gamma }} $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_4.tif"/> ‐ray spectrometers carried in orbiting space vehicles in addition to the conventional optical telescopes. The golden age of nuclear physics started accidentally in 1896 with the discovery of natural radioactivity by Henri Becquerel by his famous observation of the event of blackening of the photographic plates kept in the vicinity of uranium‐sulfide crystals which brought to him the Nobel Prize for Physics in 1903. Further advancement of 14 the understanding of many characteristics of radio‐active substances was put forward by Pierre and Marie Curie by chemical isolation of different radioactive elements produced in the decay processes of uranium. In 1899 Ernest Rutherford performed a number of brilliant experiments leading to his discovery of atomic nucleus in 1911. In this chain of knowledge some points should also been reserved for Tycho Brahe and his fellow Johannes Kepler who two observed and studied radioactive phenomena in bright stellae novae, i.e. new stars in 1572 (Brahe) and in 1603 (Kepler). Such supernovae are now believed to be the explosions of aged stars at the termination of their normal lives. The post‐explosion energy source of supernovae is the decay of radioactive nickel (⁵⁶ Ni, half‐life 6.077 days) and then cobalt ( 56 Co , $ (^{ 5 6} {\text{Co}}, $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_5.tif"/> half‐life 77.27 days). Brahe and Kepler observed that the luminosity of their supernovae decreased with time at rate that could have been determined by the nuclear lifetimes. Like Becquerel, Brahe and Kepler did not realize the importance of what they had seen. In fact, the importance of supernovae dwarfs and that of radioactivity are much precious because they are the culminating events of the process of nucleosynthesis. This process started in the cosmological “big bang” where protons and neutrons present in the primordial soup condense to form hydrogen and helium. Later, when stars are formed, the hydrogen and helium are processed through nuclear reactions into heavier elements. These elements are then ejected into the interstellar medium by supernovae. Later, some of these matters condense to form new stellar systems, sometimes containing habitable planets made of the products of stellar nucleosynthesis. Nuclear physics has allowed us to understand the mechanisms of the processes by which elements are formed and also to determine their relative abundances. The distribution of nuclear abundances in the Solar System reveals the fact that most of the ordinary matter is in the form of hydrogen (∼ 3/4 by mass) and helium ( ∼ 1/4) . Very small fraction ( ∼ 1/50) of the solar system material is in the form of heavy elements, especially carbon, oxygen and iron. The unknown cosmological “dark matter” lives near a hydrogen burning star and are made primarily of elements like hydrogen, carbon and oxygen. The existing theory of nucleosynthesis yields some particularly mesmerizing results that the observed mix of elements is due to a number of slight inequalities of nuclear and particle physics. These inequalities include the fact that i) the neutron is slightly heavier than the proton; ii) the neutron‐proton system has only one bound state while the neutron‐neutron and proton‐proton systems are unbound; iii) the 8 Be $ ^{8} {\text{Be}} $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_6.tif"/> nucleus is slightly heavier than gross of two 4 He $ ^{4} {\text{He}} $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_7.tif"/> nuclei and the second excited state of 12 C $ ^{12} {\text{C}} $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_8.tif"/> is slightly heavier than sum of three 4 He $ ^{4} {\text{He}} $ https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429492457/81d7a6a9-6244-41c1-b4c5-33001449500a/content/inline-math1_9.tif"/> nuclei. Modification of any of these conditions would result in a radically 15 different distribution of elements. For instance, making the proton heavier than the neutron would make ordinary hydrogen unstable and none would have survived in the primitive age of the Universe. The extreme sensitivity of nucleosynthesis to nuclear masses has generated a considerable amount of debate about its interpretation. It hinges upon whether nuclear masses are fixed by the fundamental laws of physics or are accidental, perhaps taking on different values in inaccessible regions of the Universe. Nuclear mass depends on the strengths of the forces between neutrons and protons, and yet we do not know whether the strengths are uniquely determined by fundamental physics. If they are not, we must consider the possibility that the masses in “our part of the Universe” are as observed because other masses give mixes of elements that are less likely to provide environments leading to intelligent observers. Whether or not such “weak‐anthropic selection” had a role in determining the observed laws of nuclear and particle physics has a question that is appealing to some and irritating to others. Resolving the question will require better understanding of the origin of observed physical laws. The chain connections among the basic elements of nuclear, particle and astrophysics are summarized in Figure 1.2. An illustration of different scales for measurements of the dimensions of natural objects are illustrated in Figure 1.3. In the following sections brief introduction of some of the basic features of nuclear, particle and astrophysics are included for better understanding of the subject.