Nuclear physics

“ The nuclear force is so short-range, it barely reaches from end to end of a large nucleus like that of uranium. Left to itself, it holds together most of the time, but with the added energy of an entering neutron, we might imagine shock waves going through it and turning the nucleus into something like a quivering drop of liquid. Sometimes the uranium nucleus recovers, keeps the neutron, and then goes on to beta-particle emission. And sometimes the nucleus stretches to the point where the nuclear force doesn’t quite hold it together. ”

“ The fate of almost every neutron moving in a large piece of matter is eventual collision with a nucleus. [7] Neutrons are ideal nuclear projectiles, except for one thing: they are hard to get.
Protons and alpha particles are found abundantly in nature as the nuclei of hydrogen and helium atoms. Neutrons, however, are not found in nature, and in the past have been made in nuclear reactions that were themselves initiated by charged particles. For example ”

“ Recalling the disadvantages of charged particles as nuclear projectiles for alchemists, it must surely seem that neutrons would be ideal for this purpose. Being chargeless, they are neither electrically repelled by the nuclei nor constantly slowed down by energy-losing collisions with the electrons. ”

“ Gravitational attraction is extremely weak compared to the attraction between neutrons and protons, and is utterly negligible in the realm of nuclear phenomena. Neither can the nuclear forces be electrical in origin. The neutrons are electrically neutral; and the protons actually repel each other by virtue of their electrical charge. The nuclear forces are something entirely new. They are the strongest forces yet encountered, and they are without a counterpart in the macroscopic world. ”

“ In addition to the more or less orderly outward motion, considerable portions of the energy are found in the disorderly temperature motion, which has stripped most of the electrons off the nuclei and has transformed the atoms into a freely and chaotically moving assembly of charged particles. By this time many of the original nuclei have been transformed into nuclei of radioactive species, partly by the fission process and partly by the capture of neutrons in all sorts of atoms which had been originally present in the bomb materials. ”

“ Only one alpha particle in a million undergoes a nuclear reaction to produce a neutron. The neutron, of course, makes a nuclear reaction every time. Over-all, then, we obtain two nuclear reactions per million nuclear projectiles, instead of one per million. ”

“ When a neutron strikes the nucleus of uranium-235 (235U) or plutonium-239 (239Pu) , it may cause the nucleus to split into two roughly equal fragments, releasing neutrons and energy. This is the well-known process of neutron-induced fission, the method in which nuclear energy is produced in both reactors and bombs. [14] The most common uranium isotope, 238U, also breaks up by fission, but does so all by itself, without the need for any external neutrons. That process is spontaneous fission and it goes on at random, very much like radioactive decay. ”

“ One only has to add a neutron to common U238 to get radioactive U239. In the course of time this decays into plutonium. This element can be used like U235: It produces fission, a great amount of energy and enough neutrons to keep the process going. We shall also learn to extract energy from other nuclear fuels. Thorium acts like uranium, while deuterium can give energy by building up bigger nuclei rather than breaking them into smaller pieces. Therefore the source of energy will be universally available and quite inexpensive. ”

“ They are emitted when a nucleus shifts from one energy state to a lower energy state—the energy difference emerging as the gamma radiation. Gamma emission often accompanies alpha or beta emission, but the production of gamma rays does not itself alter the atomic number nor the mass number of the parent.
Nuclei also can decay by emission of a positron, which is a positively charged electron. When this occurs, the new nucleus has one more neutron and one less proton than its parent; in effect a proton has become a neutron as the nucleus loses a positive charge. ”

“ In that case, protons in virtual contact, as within the nucleus, would attract each other, but if the distance between them was increased sufficiently to place one outside the nucleus, the nuclear interaction would decrease in intensity to less than the electromagnetic repulsion. The proton would now be repelled by the positive charge of the nucleus and would go flying away. That is why atomic nuclei have to be so small; it is only when they are so tiny that the nuclear interaction can hold them together. ”

“ To shift the protons within the nucleus (“nuclear reactions”) and make nuclear energy available was a harder problem by far. The particles involved were much more massive than electrons and correspondingly harder to move. What’s more, they were buried deep within the atom. No temperatures available to the physicists of the 1920s could force atoms to smash together hard enough to reach and shake the nucleus. ”

“ Neutrinos are also emitted every time we release some nuclear energy. Among all the remarkable practical consequences of nuclear energy, the neutrinos have a unique distinction: they are never useful, and they are never harmful. ”

“ The point is that a neutron born in a nuclear reaction must travel on the average a certain distance before it can produce another reaction. If the size of the chain-reacting material is much less than this distance, practically all of the neutrons produced will be able to escape through the material surface, and no chain reaction will be possible. If the size of the material is large compared to this distance, the leakage loss becomes negligible, and the possibility of a chain reaction depends entirely on the magnitude of the first kind of loss, the wasteful captures in nuclei. ”

“ All natural radioactivity in the heavy elements proceeds by such a series of steps, and the series finally ends with a stable form of lead as its end product. In other words, any naturally radioactive heavy element eventually becomes nonradioactive lead.
The nucleus of every atom (except hydrogen) contains one or more neutrons and one or more protons. The instability of the nuclei of the heavy atoms is related to the ratio of the number of neutrons to the number of protons in the nuclei. Radioactive decay is, in fact, a way of adjusting these ratios. The adjustment can occur in various ways. ”

“ The positive electric charges within an atomic nucleus repel one another. In the most heavily charged nuclei this repulsion becomes so great that the nucleus can break into two parts, simultaneously releasing a considerable amount of energy. In the case of spontaneous nuclear fission the two parts are more or less equal in size. ”

“ Occasionally the random quivering of a nucleus would bring about a shape that the nuclear interaction could not bring back to normal and the nucleus would then break apart. In a gram of ordinary uranium, there is a nucleus undergoing such “spontaneous fission” every 2 minutes on the average. Therefore, enough uranium need only be brought together to surpass critical size and it will explode within seconds, for the first nucleus that undergoes spontaneous fission will start the chain reaction. ”

“ The mesotron causes the electric charge of the nuclear particles to be incessantly tossed back and forth between protons and neutrons. At one infinitesimal part of a second a given nuclear particle is a charged proton and the next an uncharged neutron. And these alternations of energy status are so unbelievably rapid that the electric charge is deprived of all opportunity to function as a disruptive influence. Thus does the mesotron function as an "energy-carrier" particle which mightily contributes to the nuclear stability of the atom. ”

“ If any one nucleus of a given ISOTOPE is stable, then all others are also stable, because what is true for one atom of a given kind is true for all others of the same kind. [5]
Some nuclides, both man-made and natural, are unstable, however. Their nuclei are in such violent turmoil that the nuclear forces cannot always hold them together, and various bits and pieces fly off. If we were to try to predict when one particular unstable nucleus would thus disintegrate, however, we could not succeed, because the instant any specific decay (or disintegration) event will occur is a matter of chance. ”

“ In 1932, during the early search for more efficient ways to bombard nuclei, James Chadwick discovered the neutron. This particle, which is neutral in charge and is approximately the same mass as a proton, has the remarkable quality of efficiently producing nuclear reactions even at very low energies. No one exactly knowns why. At low energies, protons, alpha particles, or other charged particles do not interact with nuclei because they cannot penetrate the electrostatic energy barriers. ”

“ But when a statistical quantity of such atoms are present—and any gross matter no matter how minute will contain a statistical quantity—there is always some number of atoms in the radioactive state of ejecting the excess energy. Some do it quickly; others take their time.
"In order to remove the excess energy all at once it is necessary to control the nuclear particles themselves."
"Which—up to now—has not been done," suggested Mangler.
"Right," beamed Velikof. "An instable atom can be considered as a billiard table with the balls in motion. The stable state consists of the balls at rest. ”

“ There is no reason why one should not use a cold target material, which is bombarded from the outside by energetic nuclear projectiles, for example protons or alpha particles. The projectiles, if they are energetic enough, can overcome the electrical repulsion of the target nuclei, and they actually can penetrate. ”

“ From the energy involved they could come only out of the nucleus. Since beta particles are electrons and since they come from the nucleus, it seemed to follow that there must be electrons within the nucleus to begin with.
The proton-electron theory of nuclear structure also seemed to account neatly for many of the facts of radioactivity.
Why radioactivity at all, for instance? The more complex a nucleus is, the more protons must be squeezed together and the harder, it would seem, it must be to keep them together. More and more electrons seemed to be required. ”

“ It is capable of doing great damage in the body.
Common to all three types of nuclear decay radiation is their ability to ionize (i.e., unbalance electrically) the neutral atoms through which they pass, that is, give them a net electrical charge. The alpha particle, carrying a positive electrical charge, pulls electrons from the atoms through which it passes, while negatively charged beta particles can push electrons out of neutral atoms. If energetic betas pass sufficiently close to atomic nuclei, they can produce X-rays which themselves can ionize additional neutral atoms. ”

“ Without a huge gravitational force we cannot simultaneously bring about sun-center densities and sun-center temperatures; one or the other must go.
On the whole, it would take much less energy to aim at the temperatures than at the densities and would be much more feasible. For this reason, physicists have been attempting, all through the nuclear age, to heat thin wisps of hydrogen to enormous temperature. ”

“ When the neutrons are first made in the fission process, many of them do have energies greater than one million electron-volts. But before they can cause a fission, they usually make a few nonfission collisions with uranium nuclei, giving part of their energy to the nuclei and escaping with the remainder. The nuclei are then left with too little energy to undergo fission and the neutrons with too little energy to cause fissions in their next encounters. Thus too few neutrons reproduce themselves and no chain is possible. ”

“ If the purpose of the nuclear accelerating machines were to produce cheap energy, they would not be of much value. A nuclear reaction may typically release five to 20 million electron-volts of energy. But to obtain this reaction, a million particles had to be accelerated to energies of several million electron-volts. The recoverable and useable energy will be only a minute fraction of the total invested. ”

“ In the most common type of activation analysis, the neutron bombardment of a sample is performed in a nuclear reactor where the neutrons that strike the target atoms have been slowed down so that they have very little energy of motion. In this case, the usual reaction between the target atoms and a neutron results in the capture of the neutron and this creates a nucleus with an atomic weight of one more unit than it started with. ”

“ This property of nuclei to decay by themselves is called radioactivity. Radioactive nuclei decay at constant rates regardless of temperature, pressure, chemical combination, or physical state. The process goes on no matter what happens to the atom. In other words, the activity inside the nucleus is in no way affected by what happens to the ELECTRONS circling around it. (Only in very special cases can outside disturbances affect the radioactivity of a nucleus and then only slightly. For all practical purposes, rates of radioactive decay are constant.) ”

“ VELOCITY OF ESCAPE OF HYDROGEN NUCLEI
We have seen that the experiments of Bieler on the scattering of α rays by aluminum and magnesium indicate that a powerful attractive force comes into play very close to the nuclei of these atoms. If this be the case, the forces of attraction and repulsion must balance at a certain distance from the nucleus. Outside this critical point the forces on a positively charged body are entirely repulsive. Certain important consequences follow from this general view of nuclear forces. ”