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Some physics from 550 BC to AD 1948

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Abstract This chapter outlines terminology and its origins. It traces the development of physics ideas from Thales of Miletus, via Isaac Newton, to the nuclear physics investigations at the beginning of the twentieth century. It also outlines the evolving technology required to make the discoveries that would form the basis of radiosurgery. Up to the 1920s, all experiments on atomic structure and radioactivity had involved the use of vacuum tubes and naturally occurring radioactive substances. There was a need to make useable subatomic particles to obtain better understanding of the interior structure of atoms. Because of this, machines that could make atoms move at high speed were invented, known as particle accelerators. A new era had dawned. There is a brief mention of the effect of radiation on living tissue and of the units used to measure it.

Keywords physics history, vacuum tube experiments, accelerators, units

1 INTRODUCTION It is a truism that radiosurgery could not be possible without understanding radiation. This chapter concerns the expanding knowledge of atomic structure and the radiation discovered during the research into this topic. This radiation is called electromagnetic. So where and how did this term originate? The importance of this for the current purpose lies in the way in which subatomic structure came to be understood before machines existed that were designed to split up the atom into its various components.

2 BEFORE ACCELERATORS 2.1 ANCIENT WORLD While modern nuclear physics uses mainly particle accelerators of different kinds in its research, there was a period prior to the invention of these machines when other methods had to be used. In a sense, knowledge about the relevant phenomena extends Progress in Brain Research, Volume 215, ISSN 0079-6123, http://dx.doi.org/10.1016/B978-0-444-63520-4.00002-8 © 2014 Elsevier B.V. All rights reserved.

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back to the time of the ancient Greeks, indeed to the first of the pre-Socratic philosophers, Thales of Miletus (ca. 624–546 BC). This sage is said to have predicted an eclipse. He measured the height of the pyramids using a method applied in modern times to measure the height of the mountains of the moon (Sagan, 1980). He also observed the attraction that lodestones, or loadstones, exert on iron. This stone contains Fe3O4 (magnetite), which is magnetic in its natural state. The name magnet comes from Magnesia in Thessaly—on the east side of mainland Greece—the location of deposits of magnetite (Da Costa Andrade, 1958). Magnesium, manganese, and milk of magnesia, that appalling peppermint-flavored concoction beloved by mothers whose children have indigestion, have the same root. The magnesia in this punishment for ill health is MgO, magnesium oxide, which is also considered to be a necessary component of the philosopher’s stone (Fig. 1). Thales also noted that if amber is rubbed with fur, it acquires the property of attracting small pieces of paper and other light articles (Semat and Katz, 1958). The ancient Greek word for amber was elektron, hence the name of electricity. Despite his genius, Thales would seem to have been an archetypal absent-minded professor. Writing over 150 years later, Plato put the following words into Socrates mouth: “Why, take the case of Thales, Theodorus. While he was studying the stars and looking upwards, he fell into a pit, and a neat, witty Thracian servant girl jeered at him, they say, because he was so eager to know the things in the sky that he could not see what was there before him at his very feet. The same jest applies to all who

FIGURE 1 A small map to illustrate the location of Magnesia.

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pass their lives in philosophy” (Fowler, 1921). Thus, curiosity about and knowledge concerning electricity and magnetism have been of interest for millennia. After this early work, little happened until the time of Isaac Newton (1643–1727).

2.2 NEWTON TO THE NINETEENTH CENTURY Following the time of Newton, there were several threads of research that would over time combine to give an understanding of the nature of electricity. Some of the research was directly related to electricity while some of it related to the nature of light. (Since light is today considered just one range of electromagnetic radiation, this is not a problem for us, but in the years succeeding the insights of Newton, this perception was impossible.) So the acquisition of understanding will be unavoidably fragmented. Firstly, let us consider research aimed at better understanding electricity itself. A device called a Leyden jar was invented in 1746 that could store a very considerable charge of static electricity. Such a device is called a capacitor. However, while this can release its electric charge, that discharge happens virtually instantaneously. A collection of these capacitors could provide a greater charge and Benjamin Franklin (1706–1790) used this arrangement calling such a collection a battery, taking a metaphor from a collection of military artillery. Other scientists, particularly Volta in 1800, invented a source of continuous electricity using a chemical cell. Thus, an electric current became available, rather than a discharge. All of these findings broadened the knowledge of some characteristics of electricity but not of its intrinsic nature. Earlier eighteenth-century work with static electricity had shown that sometimes electrified objects could either attract or repel each other. Various theories were proposed but it was Benjamin Franklin who suggested in 1747 that there was one kind of electricity that could be added or removed from objects making the objects charged. If there was too much electricity, then the object had a positive charge, and if there was too little, a negative charge. Positively charged objects would repel each other as would negatively charged objects but positive would attract negative. It remained to decide which kind was which. He considered that rubbed glass had an excess of electricity and was positive. He was wrong. Electricity in fact flows from negative to positive according to Franklin’s classification, and this convention has been maintained to this day. Thus, more characteristics have been learned. Electricity could be static or could flow. It was positive or negative but still the essence of the phenomenon remained obscure. Contemporary relevant research concerned the nature of light. The physicists of the time were faced with a problem. They knew that sound waves vibrated the air and that waves in water needed the water for their transmission. However, the nature of light was explained by two theories (particles according to Newton) (Newton, 1730) and waves (according to Huygens). Today, it is known that light has some properties of particles and some of waves, but that duality could not be known in the seventeenth or eighteenth century.

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The above description is a pre´cis of the development of relevant research about electricity or related topics from Newton to the beginning of the nineteenth century. During that century, further crucial advances were made. Michael Faraday (1791–1867) discovered that moving electric fields could induce magnetic fields and vice versa. Thus, they were seen to be two aspects of the same phenomenon. It was from this discovery that it was possible for Faraday to develop an electric motor and dynamo and permit the construction of machines that could easily generate electric current in a circuit over long periods. All this research reached a climax with the work of James Clerk Maxwell (1831–1879) who derived a set of equations that described all known behavior of electricity and magnetism. The radiation thus became known as electromagnetic. It may be noted that light, electricity, and magnetism have the common characteristic that they can pass through a vacuum. From a more modern point of view, it is understood that light is a form of electromagnetic radiation that is different only in that the frequency and energy of that radiation are perceptible to the visual apparatus of living organisms. The ability to cross a vacuum is a property of electromagnetic radiation in general and is not limited to any particular frequency of the radiation. It may seem a rather abstruse subject to present here but it will be seen that vacuum tubes came to be of central significance in terms of the early examination of subatomic structure (Fig. 2).

2.3 THE DEVELOPMENT AND APPLICATION OF VACUUM TUBES WITH ELECTRODES AT EACH END 1. The process started in 1855, when a glass blower Johann Heinrich Wilhelm Geissler (1814–1879) contrived a method for producing a much superior vacuum than had previously been possible. 2. At the request of the physicist Julius Plu¨cker, he made vacuum tubes with pieces of metal sealed into opposite ends. These could be connected to an electric current. The end considered to be positively charged was called the anode and the end that was considered to be negatively charged was called the cathode, from the ancient Greek words cathode meaning lower way and anode meaning upper way. It was Faraday who coined the terms.

Cathode rays Vacuum tube -

+ Electrodes

FIGURE 2 The arrangement of the vacuum tube with electric current that produces a green glow from the cathode.

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3. Passage of electricity through a partially evacuated vacuum tube containing some gas produced light, the color of which depended on the gas concerned. This is the underlying mechanism of the familiar neon lights. However, when there was a virtually total vacuum, Plu¨cker noted that there was a greenish glow at the cathode. A later physicist, Eugen Goldstein (1850–1930), showed that the glow was not dependent on either the gas evacuated to produce the vacuum or the substance of which the electrodes were made. Thus, he concluded the glow was associated with the current itself and he called this emission cathode rays. The tubes producing them came to be known as cathode ray tubes. 4. William Crookes (1832–1919) developed an even more thoroughly evacuated vacuum tube and demonstrated cathode rays more clearly. They traveled in straight lines and could even turn a little wheel. An object placed in the path caused a shadow to appear in the glow they produced. 5. The argument reemerged concerning whether cathode rays were waves or particles. If they were to be particles, they could carry a charge and would be bent in an electric field. If they were waves, then waves carry no charge and would not deviate. From the early 1880s, various experiments all suggested that they were waves as they did not deviate in electric fields. However, all the experiments suffered from technical difficulties that were finally overcome by Joseph John Thomson (1856–1940) who demonstrated that the rays were particles with a negative charge. 6. The degree of deflection of a particle in an electric field was proportional to the mass of the particle, the velocity of its movement, and the charge it carries. A similar deflection will occur in magnetic fields but in different ways. By comparing the two kinds of deflection, Thomson could calculate the relationship between charge and particle mass. He could thereby work out the mass of a single cathode ray particle for which he received the Nobel Prize in 1906. The particle came to be called the electron. (Thirty-one years later, his son shared the Nobel Prize in Physics for discoveries related to the diffraction of electrons.)

2.4 SUBATOMIC STRUCTURE Thomson discovered that the mass of an electron was considerably smaller than that of the smallest atom. This blew a huge crater in the accepted wisdom that the atom was the smallest possible component of matter. It became necessary to accept that atoms were made up of smaller structures. At this stage, nobody had any idea about the internal architecture of an atom. Indeed, Thomson’s own idea was that an atom was a featureless positively charged sphere into which electrons were embedded like seeds in a cake. Thus, for the time being, the nature of subatomic particles remained unclear. Nonetheless, since the electron has a negative charge and the atom was known to be electrically neutral, the search was on to discover what part of an atom contained this charge. In this part of the march of ideas, chance played an important part.

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2.5 EXPERIMENTS USING SPONTANEOUSLY RADIOACTIVE MATERIALS (ASIMOV, 1991) A naturally fascinating phenomenon is the emission of light by certain substances when exposed to light. There are two such phenomena. Fluorescence occurs when as substance exposed to light itself gives off light but ceases to do so the moment the stimulating external light is extinguished. Phosphorescence is similar but continues for a period after the external light source is extinguished. One physicist researching this interesting topic was Wilhelm Conrad Ro¨ntgen (1845–1923). He was investigating how not just light but cathode rays impinging on various chemicals could produce luminescence. He was using paper coated with a barium platinocyanide. He noticed that such paper, which was not in the pathway of the cathode rays, fluoresced when the cathode ray tube was turned on. He took this coated paper into the next room and turned on the cathode ray tube, and again, the paper fluoresced. He reasoned that the tube was emitting a radiation that was not just cathode rays and in view of its mysterious nature called the radiation X-rays. He received the very first Nobel Prize in Physics in 1901. Antoine Henri Becquerel (1852–1908) somewhat later was also investigating fluorescence. He used a known fluorescent substance, potassium uranyl sulfate, that contains one uranium atom. He was wondering if fluorescent substances gave off spontaneous radiation, and to test this, he used an elegant but simple experimental model. He wrapped some photographic plates in black paper, which sunlight could not penetrate. He placed this package in sunlight and placed a crystal of his fluorescent crystal upon the package. Sure enough, there was fogging of the plates suggesting radiation emanating from the crystal. To confirm the finding, he planned to repeat the experiments. However, as happens in northern Europe quite a bit, there was a run of cloudy days. During this period, a package of film plates with a crystal on top was kept in a drawer. It would seem that Becquerel could be impatient so he developed the film after a few days without exposing it to sunlight and discovered that the plates were strongly fogged in the absence of sunlight-induced fluorescence. He reasoned that the crystals must be giving off radiation independent of an external light source and he set about examining this phenomenon. He found that the culprit was the uranium atom in the potassium uranyl sulfate. Later, Marie Curie demonstrated that thorium, polonium, and radium and uranium had similar properties. From now on, the next steps of the research would continue using radioactive substances rather than a vacuum tube. After all, radioactive breakdown provided a spontaneous splitting of the atom into subatomic components. In 1899, the New Zealand physicist Ernest Rutherford (1871–1937) analyzed the radiation observing and quantifying its deflection and penetration and demonstrated two of its components as shown in Fig. 3. They were called alpha and beta particles. The alpha particles had poorer penetration than the beta. The beta particles were soon shown to be electrons. In the 1900s, Paul Ulrich Villard demonstrated the most penetrating of the rays, which were called gamma rays. This left a query as to the nature of the alpha particles. They were positively charged and more massive than

2 Before accelerators

FIGURE 3 The pattern of spontaneous radioactive decay as perceived by Rutherford. The proportion of the different components varies with different elements.

electrons. The research details do not matter here. Various models of the atoms had been proposed previously, but Rutherford provided convincing evidence that the majority of the mass of the atom lay at its center and that the majority of the volume of the atom was made up by circulating electrons. Because he provided quantitative evidence to underpin his concepts, he received the Nobel Prize in Chemistry in 1908 (Fig. 3). The central portion was called the nucleus, which means little nut in Latin. Interestingly, it could be argued that he did some of his best work after receiving a Nobel Prize, which is unusual. He devised a method of separating and accumulating alpha particles (using a specially adapted vacuum tube) showing them to be helium nuclei. He continued to search for the smallest positively charged particle to match the electron. He found nothing that small but instead found the smallest positively charged particle to have 1836.11 the mass of an electron. This he called the proton from the Greek word meaning first. The proton and electron had been demonstrated. However, it became clear that all atoms except hydrogen had a mismatch between charge and weight. For example, helium has two electrons and two protons. The electrons having a mass of 1/1837 of a hydrogen atom do not contribute to an atom’s mass. Yet a helium atom has a mass of four hydrogen atoms. There must be something else to account for that mass. A German physicist Walther Wilhelm Georg Bothe (1891–1957) bombarded beryllium with alpha particles from polonium in the hope of splitting this very light atom thereby releasing protons, which had not been done up to that time. Beryllium was attractive for the purpose being a very light element with very small nuclei. Instead, he produced very penetrating rays following this bombardment and assumed they were gamma rays, since alpha particles do not penetrate well. However, Fre´de´ric Joliot-Curie (1900–1958) and Ire`ne Joliot-Curie (1897–1956) repeated the Bothe

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CHAPTER 2 Some physics from 550 BC to AD 1948

FIGURE 4 Sir James Chadwick who provided convincing evidence for the existence and nature of neutrons.

experiment allowing the rays to strike paraffin. This caused protons to be ejected from the paraffin, an activity that did not happen with gamma rays. The stage was set for a new interpretation. This was undertaken by James Chadwick (1891–1974). He thought that because the radiation ejected a massive proton, it must be massive itself. Yet it had no charge, so it was not a proton. He suggested that this was the missing particle that accounted for the differences between atomic number and atomic weight and it was called the neutron. He received the Nobel Prize in 1935 for this work (Fig. 4).

3 THE NEED FOR NEW INSTRUMENTS Investigations into subatomic structure as outlined above had used vacuum tubes and natural spontaneous radioactive processes that impose a limit on what was achievable. To advance knowledge, more powerful instruments were needed that were specifically designed to examine the internal structure of the atoms of matter. The question was how to make such a machine. The first successful attempt was carried out in Cambridge in 1929 by John Douglas Cockroft (1897–1967) and Ernest Thomas Sinton Walton (1903–1995).

5 Units

4 A DIGRESSION We are now approaching the technology that would form the basis for radiosurgery. Before proceeding, a word or two are necessary on potentially confusing terms or concepts. Energy is a primary property of radiation be it electromagnetic radiation or streams of particles. For electromagnetic radiation, the energy increases as the frequency of the radiation increases and the wavelength decreases. Thus, in the visible spectrum, the energy increases from red to violet. No visible radiation can penetrate the tissue that much. Electromagnetic radiation of much higher energy and wave frequency is required and these rays are called X-rays or g-rays. It is important to remember that the intensity of radiation is not the same as the energy. For example, a bright red light does not contain radiation with a higher energy than a dim violet light. Intensity merely indicates the number of rays as opposed to their intrinsic energy. For electromagnetic radiation that moves at the constant speed of light, the ability to penetrate the tissue depends on its energy. The higher the energy, the more the radiation can penetrate matter. Only rays that penetrate matter can knock electrons off atoms and produce ionization, which is a damaging process. The damage is caused by the moving free electrons pass through the tissues and in the process bump into and damage large molecules, particularly DNA, which is made up of two long helical strands held together by bridges. If both these strands are damaged, the DNA can no longer be used to convey genetic information at cell division and the cell is effectively destroyed. For electromagnetic radiation, the degree of ionization is related to the energy of the rays. There are three ways in which rays can ionize tissue, but by far, the commonest in the context of radiosurgery radiation is called Compton scattering. It is illustrated in Fig. 5. For particles, the situation is different. Particles have different masses and move at different velocities. In this context, speed is a more important parameter for penetration than energy. Thus, while a particles due to radioactive breakdown have a high energy, they are slow and can be stopped by a sheet of paper. Electrons are much lighter but move much faster and can penetrate much further, although in terms of say the human head, not that far. Again, this behavior concerns the b particles produced by spontaneous radioactivity. Particles that penetrate matter are always ionizing. It follows that if a or b particles could be speeded up, they could acquire a higher energy and then be more penetrating.

5 UNITS A note about units is needed. Volts (V), kilovolts (kV), or megavolts (MV) are measures of a potential difference that may be used to obtain various effects. Electron volts (eV), kiloelectron volts (keV), and megaelectron volts (MeV) are measures of energy. They could be converted into SI units of joules, but for the sake of convenience, this is not done. Having said this, the situation is a little more complicated. For conventional linear accelerators, electrons are excited up to say 200 kV. They

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FIGURE 5 Compton scattering is the commonest mechanism of ionization in radiosurgery. An incident high-energy photon knocks an electron off the outer shell of an atom. This converts the atom to an ion (hence the term). The photon continues on its way, though with a lower level of energy. The free electron passes through the tissues and is the agent that damages the DNA producing cell death.

References

then get to strike tungsten releasing X-rays using the mechanism of bremsstrahlung. This means brake radiation. The idea is that by suddenly stopping the electrons (braking them), energy is released in the form of X-rays. The X-rays so produced will have an energy measured in eV or keV.

REFERENCES Asimov, I., 1991. Atom: Journey Across the Subatomic Cosmos. Truman Talley Books, New York. Da Costa Andrade, E.N., 1958. The early history of the permanent magnet. Endeavour 17 (65), 1–9. Fowler, H.N., 1921. Theaetetus Translation of Plato in Twelve Volumes. vol. 12, Harvard University Press, Cambridge, MA, Section 174a. Newton, I., 1730. Opticks or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light, fourth ed. William Innys at the West-End of St. Paul’s, London, p. 191. Sagan, C., 1980. Cosmos. Macdonald Futura Publishers, London, pp. 176–177. Semat, H., Katz, R., 1958. Electrostatics Chapter 22 in Physics. University of Nebraska, Lincoln, pp. 413–426.

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Some physics from 550 BC to AD 1948.

This chapter outlines terminology and its origins. It traces the development of physics ideas from Thales of Miletus, via Isaac Newton, to the nuclear...
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