Search Results

Serious contradictions to the existence of electrons in nuclei impinged in one way or another on the theory of beta decay and became acute when Charles Ellis and William Wooster proved, in an experimental tour de force in 1927, that beta particles are emitted from a radioactive nucleus with a continuous distribution of energies. Bohr concluded that energy is not conserved in the nucleus, an idea that Wolfgang Pauli vigorously opposed. Another puzzle arose in alpha-particle experiments. Walther Bothe and his co-workers used his coincidence method in 1928–30 and concluded that energetic gamma rays are produced when polonium alpha particles bombard beryllium and other light nuclei. That stimulated Frédéric Joliot and Irène Curie to carry out related experiments. These experimental results were thoroughly discussed at a conference that Enrico Fermi organized in Rome in October 1931, whose proceedings included the first publication of Pauli’s neutrino hypothesis.

We once saw a science fiction movie in which a monster from outer space is first detected because it sets Geiger counters clicking furiously. We were reminded of that movie by the story of how radon in homes first came to wide public attention. A nuclear power plant was built in a town in Pennsylvania, and like all such plants was equipped with radiation detectors, both to protect the health of employees and to prevent anyone from removing nuclear fuel from the plant. A newly employed engineer at the plant registered a high radioactivity when he walked by the detectors. This was not only alarming but surprising: the plant was not yet operating, and there should not have been any radioactive material around. It was quickly established that the source of the radiation was not the plant but the engineer’s house in a nearby suburban community, which had levels of radioactivity almost a thousand times greater than federal standards permit in mines. The radioactivity came from radon gas seeping into the house from the ground. Cigarette smoking is responsible for about 90% of lung cancers, but 10% of the victims of this disease had never smoked. It was already known that miners exposed to radon gas in uranium mines suffered a high rate of lung cancer, and the question immediately arose: could radon gas in homes be another cause of lung cancer? Radon in homes is not a consequence of the atomic bomb or the building of nuclear power plants; it is one of the major sources of the natural background radiation we are all exposed to. It is present even in outdoor air, and at higher concentrations in homes, castles, peasants’ hovels, and caves as long as people have lived in them. It is a product of the decay of the element uranium. Uranium is present to some extent in all minerals, so we expect to find more radon in houses built of stone or mineral products like stone, concrete, and gypsum than in houses built of wood, and we expect to find more of it in basements than in attics.

Physical hazards involve the release of energy in various forms: 1) noise, the most common and widespread physical hazard, can be continuous noise or impulse that can cause damage to the ear or deafness. 2) Vibration, either whole-body vibration or segmental vibration, which occurs when a particular body part is affected by vibrations from tools. 3) Pressure above or below atmospheric pressure in the workers' surroundings is associated with health risks in certain occupations, such as undersea diving and aviation. Conditions in the workplace may expose the worker to unusually high or low pressures. Examples are decompression sickness and high altitude sickness. 4) Temperature extremes are found in many occupations. The human body regulates its own internal level of heat, or core temperature, within a broad range through a variety of mechanisms (including sweating) but cannot adjust to extreme variations outside that range or when the mechanisms of adaptation are not working. 5) Ionizing radiation, either electromagnetic ionizing radiation (gamma radiation), or particle radiation. The major concern with exposure to ionizing radiation is severe tissue damage at very high levels and a risk of cancer in the future at lesser levels. 6) Nonionizing radiation consists of electromagnetic radiation of longer wavelengths when the energy level is too low to ionize atoms but sufficient to cause physical changes in cells. Ultraviolet radiation is the most common form and causes sunburn and prolonged exposure over time causes cataracts and skin cancer. Keywords: physical hazards, noise, vibration, pressure, temperature extremes, ionizing radiation, nonionizing radiation, ultraviolet radiation, tissue damage, cancer

Henry and Eve have been locked behind bars by their captors. Eve recalls that Henry accidentally stepped into the focal area of multiple laser beams and found himself in her office, having gone through the wall! This effect is called “quantum tunneling”. Eve’s reminiscence makes Henry realize that enhanced tunneling through the jail bars is achievable by a periodic force at the tunneling resonance frequency. Tunneling underlies diverse processes: nuclear radioactive decay, transistor action, superconducting junction operation and ultracold gas dynamics. It may be explained as predominantly destructive interference between quantum wavepacket portions inside the barrier, and can be drastically enhanced or suppressed via interplay between environment and control effects. Tunneling exemplifies the shattering of spacetime concepts that may profoundly affect human existence. The appendix to this chapter discusses Schrödinger’s wavefunctions in tunneling.

This chapter takes a historical look at the experiments carried out by Ernest Rutherford to determine the age of the Earth. In 1903, Rutherford performed an experiment in which he separated the alpha and beta particles and measured the velocity of the alphas, finding that they travel at the fantastic speed of 24,000 kilometers per second, or about 54 million miles per hour. Rutherford was able to calculate the energy of the alphas, which he found to be far greater than that of the beta and gamma rays combined. By measuring the amount of uranium and helium in a mineral and by knowing the half-life of uranium, Rutherford had the amount of parent atom, the amount of daughter atom, and the rate at which parent changed to daughter: he had an hourglass. He presented the results of his experiments on radioactivity at the 1904 meeting of the Royal Society of London. The discovery of radioactivity both falsified Kelvin's calculations for the age of the Sun and provided the means of making a correct calculation of the age of the Earth.

Theories of the atom were reintroduced into science by John Dalton and were taken up and debated by chemists in the nineteenth century. As noted in preceding chapters, atomic weights and equivalent weights were determined and began to influence attempts to classify the elements. Many physicists were at first reluctant to accept the notion of atoms, with the tragic exception of Ludwig Boltzmann, who came under such harsh criticism for his support of atomism that he eventually took his own life. But around the turn of the twentieth century, the tide began to turn, and physicists not only adopted the atom but transformed the whole of science by performing numerous experiments aimed at probing its structure. Their work had a profound influence on chemistry and, more specifically for our interests here, the explanation and presentation of the periodic table. Beginning with J.J. Thomson’s discovery of the electron in 1897, developments came quickly. In 1911, Ernest Rutherford proposed the nuclear structure of the atom, and by 1920 he had named the proton and the neutron. All of this work was made possible by the discovery of X-rays in 1895, which allowed physicists to probe the atom, and by the discovery of radioactivity in 1896. The phenomenon of radioactivity destroyed the ancient concept of the immutability of the atom once and for all and demonstrated that one element could be transformed into another, thus in a sense achieving the goal that the alchemists had sought in vain. The discovery of radioactivity led to the eventual realization that the atom, which took its name from the idea that it was indivisible, could in fact be subdivided into more basic particles: the proton, neutron, and electron. Rutherford was the first to try to “split the atom,” something he achieved by using one of the newly discovered products of radioactive decay, the alpha particle.

Chapter 5 describes how the concept of quantization (discretization) was first applied to atoms. This was done in 1913 by Niels Bohr, using Ernest Rutherford’s paradigm-changing, solar-system model of atomic structure, wherein the positively charged nucleus occupies a tiny central space, much smaller than the known sizes of atoms. Bohr, postulating a quantized version of this model for hydrogen, was able to explain previously inexplicable experimental features of that atom. He did so via an ad hoc quantization procedure that discretized the single electron’s energy, its angular momentum, and the radii of the orbits it could be in around the nucleus, formulas forwhich are presented, along with a diagram displaying the quantized energies. Despite this success, Bohr’s model failed not only for helium, with its two electrons, but for all other neutral atoms. It left some physicists hopeful, ready for whatever the next step might be.

Questions

Regarding atomic structure:

‘Z’ is the number of protons in the nucleus.

‘A’ determines an element’s place in the periodic table.

A stable nucleus contains equal numbers of protons and neutrons.

Neutrons have a relative charge of +1.

The primary advantage of nuclear power is that a lot of energy can be generated from very little material—that too with no ongoing CO₂ production. The disadvantage is the problem of dealing with radioactive waste—in particular,¹³⁷Cs. Although somewhat challenging, it still appears to be manageable. Progress in nuclear fusion research has been slow, but the ultimate reward of almost unlimited energy would make it worthwhile to keep going.

The applied voltages that drive electrochemical processes (see Chapter 11) are only one of many energy sources that can be used to activate reactions in oxide molecules and materials. Another common energy source that drives many environmental and technological oxide reactions is light from the sun. Water plays a key role in many of these reactions. Imagine that you are on vacation floating in a warm ocean bathed by the sun. Many of the phenomena you experience, from your painful sunburn to the photosynthetic growth of the seaweed you see beneath you, are photoactivated processes. In this chapter, we highlight the roles that oxides play in photon-activated solar energy technologies. Also included are reactions stimulated by other nonthermal energy sources, including electrons in high-energy plasmas. Titanium oxide, found in common white paint, is the basis for much of the discussion, because this oxide is used in many photoelectrochemical energy storage technologies. The photochemistry of colloidal manganese- and iron-oxide particles suspended either in atmospheric droplets or in the upper photic zone of the ocean where the sunlight penetrates are discussed in Chapter 18. Such oxide reactions are important globally in the elimination of pollutants. Both industrial and environmental examples illustrate how oxides participate in a wide range of photoactivated chemical reactions, including the catalytic decomposition of water, photoelectrochemistry, and photoactivated dissolution and precipitation reactions. Before exploring excited-state reactions, we need to introduce the energy sources that provide such excitation. In most of this chapter, the excitation source of interest is light. Most of us are familiar with the electromagnetic spectrum, in which the energy of a photon is given by … E=hv=hc/λ=hcω (13.1)… Here, h is Planck’s constant (h = 6.6 ·10 ^–34 J/second), c is the speed of light (3 ·1010cm/second), ν is the frequency of light (measured in Hertz or per second), λ is the wavelength of light (in centimeters), and ω is the wavelength expressed as wave number (measured per centimeter in infrared spectroscopy).

This chapter explains in simple terms the background physics of imaging using standard X-rays, computed axial tomography (CT), nuclear medicine (including positron emission tomography-PET), and magnetic resonance imaging (MRI). It covers the basics of ionising radiation, and also discusses lasers, which are a form of non-ionising radiation (imaging using ultrasound is covered in Chapter 10). X-rays, CT, aspects of nuclear medicine, and lasers are covered briefly. MRI is examined in more detail as this is a newer modality that is often difficult to comprehend, and in any case often involves the presence of the anaesthetist. Some isotopes are naturally occurring but many of the radioactive nuclides used in medicine are produced artificially by a nuclear reactor or cyclotron. Each of these will provide isotopes that are useful for different purposes. Unstable radioactive nuclides achieve stability by radioactive decay, during which they can lose energy. This occurs in a number of ways. For example, atoms can lose energy by ejection of an alpha particle (an extremely tightly bound basic atomic structure of 2 protons and 2 neutrons, which is equivalent to a helium nucleus). This occurs if they have too many nucleons (protons or neutrons) and results in the atomic number being reduced by two and the atomic mass by 4. Other ways that unstable radionuclides decay include: emission of an electron (β−) from the nucleus if the atoms have an excess of neutrons, or by, either emitting a positron (β+) or capturing an electron if they are neutron deficient. Normally isotopes produced by a reactor will be neutron rich and decay by emitting an electron and the cyclotron will tend to produce isotopes that are proton rich and the decay will then be by emitting a positron. This is illustrated in Table 29.1. The new nuclide formed by the decay process (the daughter nuclide) may be left in an excited nuclear state and can release this excess energy by emission of gamma (γ) radiation as shown in Figure 29.1. This example is where the electron (β−) has been emitted. The situation is more complex when a positron has been emitted.