nuclear-physics

"Maurice Goldhaber has emphasized that the situation with respect to possible nuclear resonances in (γ,n) or (γ,fission) reactions was quite unclear at the time of George C. Baldwin and G. Stanley Klaiber’s papers on these reactions. ... This was because the rapid rise of their yield to a prominent peak with increasing energy, followed by a slower fall off was then thought to have been due to the competition between the rapidly rising density of nuclear states and the eventual domination of other reaction channels at higher energies. Goldhaber realized, however, that there could be an analogy between a possible collective nuclear resonance and the restrahl resonance (essentially the transverse optical phonon mode) in polar crystals. Goldhaber sought out Teller because of his paper with Russell Lyddane and Robert Sachs, ... relating the restrahl frequency to the asymptotic behavior of the crystal’s dielectric function. Goldhaber and Teller, in their paper together, went on to predict universal, giant photo-nuclear resonances. ..."

"A powerful method to study the properties of a system is to subject it to a weak external perturbation and to examine its response. For the atomic nucleus subjected to the absorption of a photon or to the scattering of a particle (electron, proton, etc.) the response is ... a function of the energy and linear momentum transferred to the system. ... Up to about 10 MeV the nucleus responds through the excitation of relatively simple states often involving only one or a few particles. In the energy range between 10 and 30 MeV the system response exhibits broad resonances. These are the giant resonances ... Giant resonances correspond to a collective motion involving many if not all the particles in the nucleus. The occurrence of such a collective motion is a common feature of many-body quantum systems. In quantum-mechanical terms the resonance corresponds to a transition between the ground state and the collective state and its strength is described by a transition amplitude. Intuitively it is clear that the strength of the transition will depend on the basic properties of the system such as the number of particles participating in the response and the size of the system. This implies that the total transition strength should be limited by a sum rule which depends 'only' on ground-state properties. If the transition strength of an observed resonance exhausts a major part, say greater than 50%, of the corresponding sum rule we call it a giant resonance."

"Nuclei interact with the external environment through a number of different fields—electromagnetic, weak and hadronic. The collective excitations induced by these interactions are known as giant resonances. The best known example is the giant dipole resonance, which is stimulated when the electric field of an incident gamma ray exerts a force on the positively charged protons in a nucleus, moving them relative to the uncharged neutrons ... Other giant resonances that have been studied are the monopole, quadrupole and spin-isospin modes of oscillation. The spin-isospin mode involves charge-changing processes, in particular beta decay. The quadrupole and monopole giant resonances are most easily seen with fields that act equally on neutrons and protons, because in these modes the neutrons and protons oscillate in the same mode. The giant resonances are collective oscillations and the various modes of oscillation depend on specific aspects on the nuclear force to sustain them. In the monopole mode, the motion is radial and the frequency depends on the compressibility of the nucleus. In the dipole and spin-isospin resonances, the protons and neutrons are excited out of phase, and the proton-neutron interaction provide the restoring force."

"The spectrum of gamma-radiation emitted by a highly excited nucleus can be calculated in two ways. In the first method the transition probability for gamma emission is related to the photon absorption cross-section by detailed balance. The second method relies on the fact that an excited hot nucleus has thermal fluctuations. In particular it has a fluctuating dipole moment which produces thermal radiation. The two methods are closely related and in both cases the spectrum of the radiation emitted is dominated by the giant dipole resonance. The equivalence of the detailed balance and thermal radiation theories can be demonstrated explicitly for a coupled oscillator model of the giant resonance."

"Perhaps the best-known giant resonance in nuclei is the giant dipole resonance (GDR). The GDR is described in classical hydrodynamics as a class of nuclear motion in which the neutrons and protons within a nucleus move collectively against one another, providing a separation between the centers of mass and charge, thus creating a dipole moment."

"The first results from the magnet in 1931 and 1932 were dramatic and completely unexpected. An approximately equal number of particles of positive and negative charge were observed, whereas, according to the theories known at the time, one would expect to see only ordinary electrons (all of negative charge). The presence of such an abundance of particles of positive charge was perplexing—something new and mysterious must be ocurring."

"Then on 2 August 1932 there came along the discovery of the by C. Anderson. ...For Dirac it meant the satisfaction that his equation predicted the situation correctly as he had hoped. His work had also provided the first example in the history of physics where the existence of a new particle was predicted on a purely theoretical basis."

"joined me... and I assigned him to the task of continuing the curvature measurements... As more data accumulated... practically all of the low-velocity cases of positive charge were particles... whose mass seemed to be too small to permit their interpretation as s. The alternative explanations... were that these particles were either ordinary electrons (of negative charge) moving upward, or some unknown lightweight particles of positive charge moving downward. In the spirit of scentific conservatism I tended... toward the former... [[Robert Andrews Millikan|[T]he chief]]... repeatedly pointed out that cosmic ray particles travel downward, and not upward, except in extremely rare circumstances, and that these... must be downward-moving protons. This point of view was difficult for me to accept... since in nearly all cases the density of the... tracks... was too low for particles of proton mass. To resolve this apparent paradox, a plate was inserted across the center of the ... [A] fine example was obtained in which a low-energy lightweight particle of positive charge was observed to traverse the plate... This particle came in from the bottom of the chamber, passed through the lead plate and went out near the top of the chamber. ...[I]ts track... was more curved above the plate... this meant it was going slower... therefore, it must have passed through the plate traveling upward."

"We can visualize that neutral spinless maximons (or photons) are produced at t < 0 from contracting matter having an excess of antiquarks, that they pass "one through the other" at the instant t = 0 when the density is infinite, and decay with an excess of quarks when t > 0, realizing total of the universe. All the phenomena at t < 0 are assumed in this hypothesis to be CPT reflections of the phenomena at t > 0."

"It appears that the strong interactions and electromagnetic interactions are invariant with respect to C, P, and T separately, while the weak interactions do not conserve P or C. All experimental results are consistent with the assumption the T invariance holds true for all interactions; consequently, from the CPT theorem, weak interactions must be invariant under CP. One could not, then, determine if the photographed scene were a scene of particles viewed normally, or a scene of antiparticles projected in a mirror."

"I felt that writing this paper on the electron was not so difficult as writing the paper on the physical interpretation."

"It thus appears that we must abandon the identification of the holes with protons and must find some other interpretation for them. A hole, if there were one [in the world], would be a new kind of particle, unknown to experimental physics, having the same mass and opposite charge to an electron. We may call such a particle an anti-electron. We should not expect to find any of them in Nature, on account of the rapid rate of recombination with electrons, but if they could be produced experimentally in high vacuum they would be quite stable and amenable to observation. An encounter between two hard γ-rays (of energy of at least half a million volts) could lead to the creation simultaneously of an electron and anti-electron. This probability [of the creation of a pair] is negligible, however, with the intensities of γ-rays at present available."

"A few hundred heavy nuclei of antideuterium, antitritium, and antihelium-3 have been observed... Sadly, they have been unable to keep these antimatter fragments under control long enough to add positrons and make neutral antiatoms..."

"The word "antimatter"... Strictly speaking, it's not... accurate... Antimatter is not "negative matter." It does not have negative mass, or negative spin, or negative (anti-) gravity (...scientists are... running experiments to see if antiprotons have the same kind of gravity as protons). ...One researcher has suggested replacing the... "anti-" with "co-,"... co-matter, co-protons,... Another... suggested... "exo-"... "Exo" in Greek means "outside." Other suggestions... "ob-" (obmatter, obproton) and "contra-" (contramatter and contraproton). None... ever caught on... Hannes Alfven in... Worlds-Antiworlds... said... let's coin a new word for "ordinary" matter... the word koinomatter... after the Greek word koinos, meaning common or well-known. ..."Matter will remain "matter" and "antimatter"... "antimatter"... However... "mirror matter" is the most accurate and unbiased term."

"The strong violation of the baryon charge during the superdense state and the fact that the baryons are stable in practice do not contradict each other. ...The baryon charge is violated if the interaction... is supplemented with a three-boson interaction leading to virtual processes ...we find the decay probability ...The lifetime of the proton turns out to be very large (more than 1050 years), albeit finite."

"It seems probable that the interactions between elementary particles can be completely described by symmetry properties and s and by dimensionless numbers representing interaction strengths. Similarly, we might expect that the elementary particles, as quanta of these interactions, may be described in the same in terms... At the present... however, our description... must also include the , and in some cases, the magnetic moment, although in principle these are probably derivable from interaction strengths and symmetries. ...Symmetries usually result in conservation laws. ...Invariance under space inversion results in ...the conservation of parity. Let us also consider invariance under time reversal, and invariance under charge conjugation, the change of particles to antiparticles."

"Invariance of interactions with respect to space inversion restricts observables to those which do not differentiate between a left-handed and a right-handed coordinate system. Time reversal invariance allows only observables which do not depend on the direction of time, and invariance under charge conjugation restricts observables to those which remain unchanged when all particles are changed to antiparticles."

"A positron, Feynman has written, can... be thought of as an electron moving backwards in time! An electron doing such... would be indistinguishable from an electron moving with a positive charge. This would also be essentially true for any other mirror matter particle or object."

"We make use of the exclusion principle of Pauli... there can be only one electron in any state of motion. We... make the assumptions that in the world as we know it, nearly all the states of negative energy for the electrons are occupied... any unoccupied negative-energy state, being a departure from uniformity, is observable and is just a ."

"These three invariances are not independent. In the framework of local field theory, invariance under proper leads to the invariance of all interactions under combined operations CPT, where C is the charge conjugation operator, changing particles to antiparticles, P, the parity space inversion operator, changing \overline{r} to -\overline{r}, and T is the time reversal operator, changing t to -t. The equality of the masses and lifetimes of the particles and their antiparticles follows from this theorem."

"The theory of the expanding universe, which presupposes a superdense initial state of matter, apparently excludes the possibility of macroscopic separation of matter from antimatter; it must therefore be assumed that there are no antimatter bodies in nature, i.e., the universe is asymmetrical with respect to the number of particles and antiparticles (С asymmetry). In particular, the absence of antibaryons and the proposed absence of baryonic neutrinos implies a nonzero baryon charge (baryonic asymmetry)."

"We wish to point out a possible explanation of С asymmetry in the hot model of the expanding universe... by making use of effects of CP invariance violation... To explain , we propose in addition an approximate character for the baryon conservation law."

"An unoccupied negative-energy state, or hole... will have a positive energy, since it is a place where there is a shortage of negative energy. A hole is... just like an ordinary particle, and its identification with the ... the most reasonable way of getting over the difficulty of... negative energies..."

"[W]e should expect an ordinary electron, with positive energy, to be able to drop into... and fill up this hole, the energy being liberated in the form of . This would mean... an electron and a positron annihilate one another. The converse... creation of an electron and a positron from electromagnetic radiation, should also be able to take place. Such... appear to have been found experimentally, and are... being more closely investigated..."

"It was an imperfection of the theory and I didn't see what could be done about it. It was only later that I got the idea of filling up all the states."

"I felt right at the start that the negative energy electrons would have the same rest mass as the ordinary electrons ...I hoped that there was some lack of symmetry somewhere which would bring in the extra mass for the positively charged ones. I was hoping that in some way the Coulomb interaction might lead to such an extra mass, but I couldn't see how it could be brought about."

"[C]reation and annihilation concepts antedate quantum mechanics. The concept of annihilation of pairs of oppositely charged, elementary particles... dates from the turn of the twentieth century. It became important in astrophysics about 1924... The annihilating pairs were first positive and negative electrons, later protons and electrons, and finally, starting in 1931, electrons and anti-electrons. ...In Dirac's "hole" theory of 1930... pair annihilation was neither novel nor central. Dirac's object was to deal with a difficulty... that the theory allowed electrons to make transitions to . ...interpreting electrons in states of negative energy as unobservable, and empty negative-energy states, or "holes" as protons. As a by-product, when an electron jumped into a vacant negative-energy state, an electron and a proton disappeared together into radiation. Since pair annihilation was already an accepted concept, this... was admissible."

"Since every particle needed to make up atoms has its antiparticle, it is conceivable... to combine s and s to make ... Then one could use s to make heavier forms of antihydrogen such as antideuterium (an antinucleus containing one antiproton and one antineutron, with a positron in orbit) and anti (one antiproton and two antineutrons)."

"Dirac's... paper, "A Theory of Electrons and Protons," makes it clear that his primary purpose was to deal with the negative energy difficulty, and his secondary purpose... was to present a theory of protons. ...[T]he chief novelty ...was the identification of the proton with the absence of the electron, whereas the concept of pair annihilation was not a novelty ...He began by stating the difficulty: relativistic theories of the electron all yield solutions in which the electron has a negative total energy, and quantum mechanical relativistic theories... permit the electron to make transitions from states of positive energy to these states of negative energy. He then argued... that these states, and the transistions to them, cannot be disregarded as nonphysical..."

"When a matter particle and its mirror antimatter twin are brought into contact, the two annihilate each other. The mass of both is totally converted into energy. The amount of energy... Einstein's E=mc^2... The annihilation of a gram of matter and antimatter would produce the energy of a 20-kiloton nuclear bomb, the size... dropped on Japan."

"After Dirac's publication of the electron wave equation in 1928, many people took up its study."

"On this view the positron is just a mirror-image of the electron, having exactly the same mass and opposite charge. This has already been roughly confirmed by experiment. The positron should also have similar spin properties to the electron, but this has not yet been confirmed..."

"Mirror matter is, first and foremost, matter. ...[A]ll mirror matter is still matter."

"[I]t is probable that negative protons can exist, since as far as the theory is yet definite, there is a complete and perfect symmetry between positive and negative electric charge, and if this symmetry is really fundamental in nature, it must be possible to reverse the charge on any kind of particle. ...[N]egative protons would... be much harder to produce... since a much larger energy would be required, corresponding to... larger mass."

"We must regard it rather as an accident that the Earth (and presumably the whole solar system), contains a preponderance of negative electrons and positive positrons. It is quite possible that for some of the stars it is the other way about... built up mainly of s and negative protons. ...[T]here may be half the stars of each kind. The two... would both show exactly the same spectra... there would be no way of distinguishing them..."

"The interpretation of these tracks as due to protons, or other heavier nuclei, is ruled out on the basis of range and curvature. Protons or heavier nuclei of the observed curvatures could not have ranges as great as those observed. The specific-ionization is close to that for an electron of the same curvature, hence indicating a positively-charged particle comparable in mass and magnitude of charge with an electron."

"In his theory of beta reactivity Fermi introduced a new type of interactions among elementary particles, which today we call "weak interactions". Many new manifestations of weak interactions, which could be interpreted using Fermi's 1933 theory, were found in the following decades. The study of weak interactions has led to surprising discoveries, among which the violation of specular symmetry (known as parity symmetry or P symmetry), and the violation of time reversal symmetry (T symmetry) and of the symmetry between matter and antimatter (CP symmetry)."

"Dear Millikan, I have just received a letter from Rutherford which contains some of Blackett's work which may interest you and Anderson. It is that they have capitulated on the question of positive electrons and agree with Anderson that there are present in large numbers among the tertiary or quartinary (or whatever they are) ionizing particles seen in a Wilson photograph of the effects particles of positive charge and electronic mass. ...I take it that Blackett has collected so many photographs of such tracks as those earlier ones of Anderson that he can no longer resist this devastatingly interesting conclusion. Blackett's photos will come out in P.R.S. (Proceedings of the Royal Society) in March. I have a lecture to deliver."

"The annihilation of positrons with electrons from biological tissues constitutes the basis of Positron Emission Tomography (PET)... widely used in ... [S]ubstances called radiotracers and radiopharmaceuticals are injected into the patient. These are chemical compounds in which one or more atoms have been replaced by short-lived, positron-emitting, radioisotope of elements that are abundant in the body, like Carbon-11... ... Oxygen-15... and Fluor-18... the latter... for the localization and monitoring of tumors... Since these isotopes are short-lived... they must be produced just before being injected... To do this, the corresponding [common] elements are bombarded with protons... from a small accelerator. ...[I]nside the PET scanner ...a series of detector rings ...record the gamma radiation emitted when the positrons are annihilated inside the body. ...[T]he recorded signals are used to make a series of slices that combine to for a 3-D image. ...[T]hey allow doctors to assess the condition of organs and tissues as they can monitor blood flow and many bodily and metabolic processes, including neuronal transmission."

"Kirk: Like Lazarus. Identical, yet both Lazarus. Except one is matter and the other antimatter. If they meet... Spock: Annihilation Jim. Total, complete, absolute annihilation."

"If the Standard Model describes the world successfully, how can there be physics beyond it, such as supersymmetry? There are two reasons. First, the Standard Model does not explain aspects of the study of the large-scale universe, cosmology. For example, the Standard Model cannot explain why the universe is made of matter and not antimatter, nor can it explain what constitutes the of the universe. Supersymmetry suggests explanations for both of these mysteries. Second, the boundaries of physics have been changing. Now scientists ask not only how the world works (which the Standard Model answers) but why it works that way (which the Standard Model cannot answer). Einstein asked "why" earlier in the twentieth century, but only in the past decade or so have the "why" questions become normal scientific research in particle physics rather than philosophical afterthoughts."

"In later years, the advent of a new elementary particle would scarcely ruffle the intellectual sensibilities of the world's physicists; in 1932, Anderson's announcement of the ran into a wall of resistance. If the had resolved many long-standing difficulties of nuclear theory, the positron seemed to complicate matters. It is said that Neils Bohr dismissed Anderson's finding out of hand, and when in the fall of 1932 Millikan discussed the positron in a lecture at the Cavendish, various members of the audience suggested that Anderson had doubtless become tangled in some fundamental interpretive error. But not all of Rutherford's physicists were prepared to ignore Anderson's claims, especially not the resident Cavendish expert on s, Patrick M. S. Blackett."

"The Doctor: Here on Zeta Minor is the boundary between existence as you know it and the other universe which you just don't understand. From the beginning of time it has existed side by side with the known universe. Each is the antithesis of the other. You call it "nothing", a word to cover ignorance. And centuries ago scientists invented another word for it. "Antimatter", they called it. And you, by coming here, have crossed the boundary into that other universe to plunder it. Dangerous."

"It was fortunate that Alan Guth did his work at the same time that another idea came into fashion, which was the theory that we could understand why the universe contains matter and not antimatter in terms of some asymmetry, some favoritism for matter over antimatter in the early universe; it's no good having a scheme that can inflate the universe to enormous dimension of it's not possible to create matter to fill that large universe."

"The new quantum mechanics, when applied to the problem of the structure of the atom with point-charge electrons, does not give results in agreement with experiment. The discrepancies consist of "duplexity" phenomena, the observed number of stationary states for an electron in an atom being twice the number given by the theory. ...It appears that the simplest Hamiltonian for a point-charge electron satisfying the requirements of both relativity and the general transformation theory leads to an explanation of all duplexity phenomena without further assumption."

"The wave equation... refers equally well to an electron with charge e as to one with charge -e. If one considers for definiteness the limiting case of large quantum numbers one would find that some of the solutions of the wave equation are wave packets moving in the way a particle of charge -e would move on the classical theory, while others are wave packets moving in the way a particle of charge e would move classically. ...the electron suddenly changing its charge from -e to e ...has not been observed. The true relativity wave equation should thus be such that its solutions split up into two non-combining sets, referring respectively to the charge -e and the charge e. ...The resulting theory is therefore still only an approximation, but it appears to be good enough to account for all the duplexity phenomena without arbitrary assumptions."

"On August 2, 1932, during the course of photographing cosmic-ray tracks produced in a vertical Wilson chamber (magnetic field of 15,000 gauss) designed in the summer of 1930 by Professor R. A. Millikan and the writer, the tracks... seemed... interpretable only on the basis of the existence in this case of a particle carrying a positive charge but having a mass of the same order of magnitude as that normally possessed by a free negative electron."

"In the course of the next few weeks other photographs were obtained which could be interpreted logically only on the positive-electron basis, and a brief report was then published with due reserve in interpretation in view of the importance and striking nature of the announcement."

"[O]ur equations allow of two kinds of motion for an electron, only one of which corresponds to what we are familiar with. The other corresponds to electrons with a very peculiar motion such that the faster they move, the less energy they have, and one must put energy into them to bring them to rest."

"[W]e find from the theory that if we disturb the electron, we may cause a transition from a positive-energy state of motion to a negative-energy one, so that, even if.. all.. electrons in the world.. started.. in positive-energy states, after a time some... would be in negative-energy states. ...[B]ehaviour of these states in an electromagnetic field shows that they correspond to the motion of an electron with a positive charge ...a . One might... assume that electrons in negative-energy states are just positrons, but ...observed positrons ...do not have negative energies."