Inside the Nucleus Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106 Atomic Physics The branch of physics concerned with the study and understanding of the atomic nucleus, including its composition and the forces which bind it together, is called nuclear physics. Understanding the huge variety of nuclei, and the forces holding them together, not only helps scientists to understand the evolution of the universe and the conditions leading to life, but also provides the underpinning knowledge needed to exploit nuclear properties in new technologies. Extreme nuclei are important because they are thought to form the transient stepping stones in the nuclear reactions by which the heavier elements are built up in stellar processes, particularly supernova explosions. The elements then spread out into space, to form the construction materials for subsequent generations of new stars and planets – and eventually life. About 7000 different combinations of protons and neutrons in nuclei are thought possible, most of which are short-lived. Researchers explore the outer edges of stability where nuclei with extreme ratios of neutrons or protons can behave in exotic ways. In very neutron-rich nuclei, for example, the neutrons may form extended ‘halos’ around a dense core. ‘Super heavy’ nuclei are of great interest because theorists predict that some of them, which do not exist naturally, could be quite stable. Nucleus of atom Atom as the fundamental building block of matter is made up of electrons revolving around a nucleus. Nucleus in any atom consists of protons equal to number of electrons and with varying number of neutrons. The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickly developed by Dmitri Ivanenko and Werner Heisenberg. Almost all of the mass of an atom is located in the nucleus,
proton neutron electron
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The relative mass(m) and the relative charge (q) of the three main sub–atomic particles.
with a very small contribution from the electron cloud. Protons and neutrons are bound together to form a nucleus by the nuclear force. The diameter of the nucleus is in the range of 1.75 fm (1.75×10−15 m) for hydrogen (the diameter of a single proton) to about 15 fm for the heaviest atoms, such as uranium. These dimensions are much smaller than the diameter of the atom itself (nucleus + electron cloud), by a factor of about 23,000 (uranium) to about 145,000 (hydrogen). The subatomic particles in a nucleus are called nucleons. The two types of nucleons are protons and neutrons. The proton is a positively charged particle. It carries a charge of +e, where e is equal to 1.6 × 10-19 C. The neutron carries no charge. The neutron has approximately the same mass as the proton. The number of protons in the nucleus of an atom is known as the proton number, Z. The total number of protons and neutrons in the nucleus of an atom is known as nucleon number, A or mass number. Then number of neutrons, N = A – Z. A nuclide is a type of atom with a particular nucleon number. This term is also used for a type of nucleus. The nuclide notation of an atom gives the symbol of the elements, the proton number and the nucleon number of the atom. 1
Isotopes: Isotopes are atoms of the same elements with the same numbers of protons but different number of neutrons. Isotopes have the same proton number but different nucleon numbers. All isotopes of an element have the same chemical properties because their electrons are arranged in exactly the same way. Their physical properties such as densities, boiling points and melting points are different. Some elements in nature such as oxygen, carbon, and bromine consist of a mixture of isotopes. Some isotopes of an element are stable while some are unstable. The unstable isotopes or radio-isotopes. Radioisotopes will undergo spontaneous decay to emit radioactive rays such as alpha, beta and gamma rays. After radioactive decay, the proton number and nucleon number of the radioisotope may be changed. Inside the nucleus Protons and neutrons each consist of three fundamental particles called quarks. They are combinations of the two lightest quarks; there are also four, heavier quarks that form unstable nuclear particles. Quarks can also bind in nuclear pairs called mesons, some of which played a significant role in the universe’s evolution. Particles such as quarks and electrons also exist in antimatter forms with opposite charge. The nucleons sit in ‘shells’ with discrete quantum energies to form a diverse range of structures (just as electrons are arranged in shells in atoms). So-called magic nuclei, with spherical shells of protons and neutrons, are very stable. However, some heavier nuclei can adopt more unusual shapes, with different shell arrangements. Very light nuclei sometimes behave like clusters of nucleons, while very heavy species are described as liquid drops of nuclear matter that can rotate and deform. Theorists and experimentalists work together to test these concepts. Scientists are trying to understand how quarks became confined in protons and neutrons, as well as how they behave in the cooler but unbelievably high pressure environment of a neutron star. It is very interesting to know, how the forces do the binding of nuclei, and where does the mass and spin of the nucleus come from. The forces holding nuclei together operate in an extremely complex way that is still not well understood. For example, the constituent quarks of a proton account for only one-fiftieth of its mass, with the rest arising from strong force interactions. The proton has three parts, two up quarks and one down quark and the gluons which these three quarks exchange, which is how the strong (nuclear) force works to keep them from getting out. The proton’s world is a totally quantum one, and so it is described entirely by just a handful of numbers, characterizing its spin (the proton’s spin is 1/2), electric charge (+1 e, or 1.602176487(40)×10-19 C), iso-spin (also 1/2), and parity (+1). These properties are derived directly from those of the proton parts, the three quarks; for example, the up quark has an electric charge of +2/3 e, and the down -1/3 e, which sum to +1 e. Another example, color charge: the proton has a color charge of zero, but each of its constituent three quarks has a non-zero color charge – one is ‘blue’, one ‘red’, and one ‘green’ – which ‘sum’ to zero (of course, color charge has nothing whatsoever to do with the colors we see with our eyes). Murray Gell-Mann and George Zweig independently came up with the idea that the proton’s parts are quarks, in 1964 (though it wasn’t until several years later that good evidence for the existence of such parts was obtained). Gell-Mann was later awarded the Nobel Prize of Physics for this, and other work on fundamental particles. The quantum theory which describes the 2
strong interaction (or strong nuclear force) is quantum chromodynamics, QCD for short (named in part after the ‘colors’ of quarks), and this explains why the proton has the mass it does. The up quark’s mass is about 2.4 MeV (mega-electron volts; particle physicists measure mass in MeV/c2), and the down’s about 4.8 MeV. Gluons, like photons, are massless, so the proton should have a mass of about 9.6 MeV (= 2 x 2.4 + 4.8). But it is, in fact, 938 MeV. QCD accounts for this enormous difference by the energy of the QCD vacuum inside the proton; basically, the self-energy of ceaseless interactions of quarks and gluons. Quarks are spin 1/2 particles. Gluons are spin zero carriers of the so-called 'Strong Force', the force that binds neutrons and protons inside the nucleus. Proton: Along with neutrons, protons make up the nucleus, held together by the strong force. The proton is a baryon and is considered to be composed of two up quarks and one down quark. It has long been considered to be a stable particle, but recent developments of grand unification models have suggested that it might decay with a half-life of about 1032 years. Experiments are underway to see if such decays can be detected. Decay of the proton would violate the conservation of baryon number, and in doing so would be the only known process in nature which does so. The nature of quark confinement suggests that the quarks are surrounded by a cloud of gluons, and within the tiny volume of the proton other quark-anti-quark pairs can be produced and then annihilated without changing the net external appearance of the proton.
Neutron: Along with protons, neutrons make up the nucleus, held together by the strong force. The neutron is a baryon and is considered to be composed of two down quarks and one up quark. A free neutron will decay with a half-life of about 10.3 minutes but it is stable if combined into a nucleus. The decay of the neutron involves the weak interaction as indicated by Feynman. This fact is important in models of the early universe. The neutron is about 0.2% more massive than a proton, which translates to an energy difference of 1.29 MeV. The decay of the neutron is associated with a quark transformation in which a down quark is converted to an up by the weak interaction. The average lifetime of 10.3 min/0.693 = 14.9 minutes is surprisingly long for a particle decay that yields 1.29 MeV of energy. This decay is steeply "downhill" in energy and would be expected to proceed rapidly. It is possible for a proton to be transformed into a neutron, but we have to supply 1.29 MeV of energy to reach the threshold for that transformation. In the very early stages of the big bang when the thermal energy was much greater than 1.29 MeV, such transformation between protons and neutrons was proceeding freely in both directions so that there was an essentially equal population of protons and neutrons.
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