Econstudentlog

Nuclear physics

Below I have posted a few observations from the book, as well as a number of links to coverage of other topics mentioned/covered in the book. It’s a good book, the level of coverage is very decent considering the format of the publication.

“Electrons are held in place, remote from the nucleus, by the electrical attraction of opposite charges, electrons being negatively and the atomic nucleus positively charged. A temperature of a few thousand degrees is sufficient to break this attraction completely and liberate all of the electrons from within atoms. Even room temperature can be enough to release one or two; the ease with which electrons can be moved from one atom to another is the source of chemistry, biology, and life.”

“Quantum mechanics explains the behaviour of electrons in atoms, and of nucleons in nuclei. In an atom, electrons cannot go just where they please, but are restricted like someone on a ladder who can only step on individual rungs. When an electron drops from a rung with high energy to one that is lower down, the excess energy is carried away by a photon of light. The spectrum of these photons reveals the pattern of energy levels within the atom. Similar constraints apply to nucleons in nuclei. Nuclei in excited states, with one or more protons or neutrons on a high rung, also give up energy by emitting photons. The main difference between what happens to atomic electrons relative to atomic nuclei is the nature of the radiated light. In the former the light may be in the visible spectrum, whose photons have relatively low energy, whereas in the case of nuclei the light consists of X-rays and gamma rays, whose photons have energies that are millions of times greater. This is the origin of gamma radioactivity.”

“[A]ll particles that feel the strong interaction are made of quarks. […] Quarks that form nuclear particles come in two flavours, known as up (u) or down (d), with electrical charges that are fractions, +2/3 or −1/3 respectively, of a proton’s charge. Thus uud forms a proton and ddu a neutron. In addition to electrical charge, quarks possess another form of charge, known as colour. This is the fundamental source of the strong nuclear force. Whereas electric charge occurs in some positive or negative numerical amount, for colour charge there are three distinct varieties of each. These are referred to as red, green, or blue, by analogy with colours, but are just names and have no deeper significance. […] colour charge and electric charge obey very similar rules. For example, analogous to the behaviour of electric charge, colour charges of the same colour repel, whereas different colours can attract […]. A proton or neutron is thus formed when three quarks, each with a different colour, mutually attract one another. In this configuration the colour forces have neutralized, analogous to the way that positive and negative charges neutralize within an atom.”

“The relativistic quantum theory of colour is known as quantum chromodynamics (QCD). It is similar in spirit to quantum electrodynamics (QED). QED implies that the electromagnetic force is transmitted by the exchange of massless photons; by analogy, in QCD the force between quarks, within nucleons, is due to the exchange of massless gluons.”

“In a nutshell, the quarks in heavy nuclei are found to have, on average, slightly lower momenta than in isolated protons or neutrons. In spatial terms, this equates with the interpretation that individual quarks are, on average, less confined than in free nucleons. […] The overall conclusion is that the quarks are more liberated in nuclei when in a region of relatively high density. […] This interpretation of the microstructure of atomic nuclei suggests that nuclei are more than simply individual nucleons bound by the strong force. There is a tendency, under extreme pressure or density, for them to merge, their constituent quarks freed to flow more liberally. […] This freeing of quarks is a liberation of colour charges, and in theory should happen for gluons also. Thus, it is a precursor of what is hypothesized to occur within atomic nuclei under conditions of extreme temperature and pressure […] atoms are unable to survive at high temperatures and pressure, as in the sun for example, and their constituent electric charges—electrons and protons—flow independently as electrically charged gases. This is a state of matter known as plasma. Analogously, under even more extreme conditions, the coloured quarks are unable to configure into individual neutrons and protons. Instead, the quarks and gluons are theorized to flow freely as a quark–gluon plasma (QGP).”

“The mass of a nucleus is not simply the sum of the masses of its constituent nucleons. […] some energy is taken up to bind the nucleus together. This ‘binding energy’ is the difference between the mass of the nucleus and its constituents. […] The larger the binding energy, the greater is the propensity for the nucleus to be stable. Its actual stability is often determined by the relative size of the binding energy of the nucleus to that of its near neighbours in the periodic table of elements, or of other isotopes of the original elemental nucleus. As nature seeks stability by minimizing energy, a nucleus will seek to lower the total mass, or equivalently, to increase the binding energy. […] An effective guide to stability, and the pattern of radioactive decays, is given by the semi-empirical mass formula (SEMF).”

“For light nuclei the binding energy grows with A [the mass of the nucleus – US] until electrostatic repulsion takes over in large nuclei. […] At large values of Z [# of protons – US], the penalty of electrostatic charge, which extends throughout the nucleus, requires further neutrons to add to the short range attraction in compensation. Eventually, for Z > 82, the amount of electrostatic repulsion is so large that nuclei cannot remain stable, even when they have large numbers of neutrons. […] All nuclei heavier than lead are radioactive.”

“Three minutes after the big bang, the material universe consisted primarily of the following: 75% protons; 24% helium nuclei; a small number of deuterons; traces of lithium, beryllium, and boron, and free electrons. […] 300,000 years later, the ambient temperature had fallen below 10,000 degrees, that is similar to or cooler than the outer regions of our sun today. At these energies the negatively charged electrons were at last able to be held fast by electrical attraction to the positively charged atomic nuclei whereby they combined to form neutral atoms. Electromagnetic radiation was set free and the universe became transparent as light could roam unhindered across space.
The big bang did not create the elements necessary for life, such as carbon, however. Carbon is the next lightest element after boron, but its synthesis presented an insuperable barrier in the very early universe. The huge stability of alpha particles frustrates attempts to make carbon by collisions between any pair of lighter isotopes. […] Thus no carbon or heavier isotopes were formed during big bang nucleosynthesis. Their synthesis would require the emergence of stars.”

“In the heat of the big bang, quarks and gluons swarmed independently in quark–gluon plasma. Inside the sun, relatively cool, they form protons but the temperature is nonetheless too high for atoms to survive. Thus inside the sun, electrons and protons swarm independently as electrical plasma. It is primarily protons that fuel the sun today. […] Protons can bump into one another and initiate a set of nuclear processes that eventually converts four of them into helium-4 […] As the energy mc² locked into a single helium-4 nucleus is less than that in the original four protons, the excess is released into the surroundings, some of it eventually providing warmth here on earth. […] because the sun produces these reactions continuously over aeons, unlike big bang nucleosynthesis, which lasted mere minutes, unstable isotopes, such as tritium, play no role in solar nucleosynthesis.”

“Although individual antiparticles are regularly produced from the energy in collisions between cosmic rays, or in accelerator laboratories such as at CERN, there is no evidence for antimatter in bulk in the universe at large. […] To date, all the evidence is that the universe at large is made of matter to the exclusion of antimatter. […] One of the great mysteries in physics is how the symmetry between matter and antimatter was disturbed.”

Some links:

Nuclear physics.
Alpha decay/beta decay/gamma radiation.
Positron emission.
Isotope.
Rutherford model.
Bohr model.
Spin.
Nucleon.
Nuclear fission.
X-ray crystallography.
Pion.
EMC effect.
Magic number.
Cosmic ray spallation.
Asymptotic giant branch.
CNO cycle.
Transuranium elements.
Actinide.
Island of stability.
Transfermium Wars.
Nuclear drip line.
Halo nucleus.
Hyperon/hypernucleus.
Lambda baryon.
Strangelet.
Quark star.
Antineutron.
Radiation therapy.
Rutherford backscattering spectrometry.
Particle-induced X-ray emission.

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June 5, 2017 Posted by | Books, Physics | Leave a comment