“There is nothing more deceptive than an obvious fact.” Sherlock Holmes Radiation physics can be dealt with quite naturally in two parts: atomic physics and nuclear physics. The basic concepts upon which these rest were developed over a period of about 50 years that began just as the 19th century was ending.
This short period saw a remarkable series of discoveries which quite literally changed the world forever.
Physical laws for ordinary phenomena were well understood by 1890. The dynamics of everyday objects were precisely described by Newton’s laws of motion. The experimental facts of static electricity and magnetism were incorpo-rated in Maxwell’s famous equations which were well used and accepted. Electric fields were reasoned to have a magnetic field equal and opposite to each and vice versa, and light had been shown to consist of electromagnetic waves. The first and second laws of thermodynamics were known in the form now used for heat engines of all kinds. Matter was thought to consist of atoms, but physicists believed that the recent kinetic theory of gases could be used to describe their motion similar to that of tiny billiard balls. Physics was thought to be intact with the possible exception of more precise measurement of the basic constants.
The following discoveries, which occurred in a period of less than 20 years, shook the foundation of this confidence and set a new course for physics as well as humankind:
. Roentgen’s discovery of x-rays (1895)
. Becquerel’s discovery of radioactivity shortly thereafter (1896)
. Thomson’s discovery of the electron (1897)
. Planck’s basic radiation law (1900)
. Einstein’s theory of special relativity (1905)
. Rutherford’s alpha scattering experiments (1911)
. Bohr’s model of the atom (1913).
These phenomena and the information they provided made it clear that atoms were not solid little balls, but had structure and properties that required new and elaborate means to describe. We pick up three major threads in this tapestry of
Physics for Radiation Protection: A Handbook. James E. Martin Copyright 6 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-40611-5
what is called modern physics: discoveries that determined the substructure of atoms and their energy states; the fundamentals of quantum emissions of radia-tion; and combinations of these to describe the structure and dynamics of the atom system. The insight required to formulate fundamental theories to resolve conflicts between observed phenomena and prevailing ideas in physics not only explained the phenomena but provided new discoveries of the laws of nature that led to even greater discoveries. These insights are presented as a means of under-standing physics concepts that are basic to radiation science and their importance to physics as well.
2.1
Great Discoveries from Simple Tools
Einstein’s “gedanken experiments” on the laws of physics in different inertial frames led to the monumental, but simple, relation between mass and energy. It has been suggested that Einstein’s inspiration for the concepts of special relativity came from his daily observations of a large clock tower and surrounding buildings as he rode the train, a moving inertial system, to and from his work in the tele-graph office. Einstein’s principles, which were developed before any model of the atom was known, are directly applicable to describing the energetics of atoms as bound systems.
The discovery of x-rays, radioactivity, and the electron were prompted by the study of gases. Since it was necessary to enclose a gas to study it, two simple tools were used: glassblowing techniques and the vacuum pump which had been recently developed. Such experiments eventually took the form of creating an elec-tric potential across electrodes imbedded in closed glass tubes; these studies led to Roentgen’s discovery of x-rays in 1895 and Thomson’s discovery of the electron in 1897. In simplest form these experimental tubes are as shown in Figure 2-1.
Fig. 2-1 Cathode-ray tube.
When a cathode-ray tube is filled with a gas, a discharge will occur, emitting light containing wavelengths characteristic of the gas in the tube. But when such a tube is evacuated to very low pressure (made possible with a good vacuum pump), the glow changes and diminishes, and at a pressure of about 10–3atm, the tube produces a luminous glow filling the tube, as in a neon sign. Below 10–6atm, 2 Major Discoveries in Radiation Physics
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the negative electrode, or cathode, emits invisible rays that propagate through the nearly empty space in the tube. These emanations were quite logically called cath-ode rays. Although the rays by themselves are invisible, they make their presence known by producing a blue-green phosphorescence in the walls of the glass tube.
The rays have notable characteristics:
. They travel in straight lines, projecting a glow on the end of the tube.
Obstructions in their path cast a shadow.
. The rays can penetrate a small thickness of matter. If a “window” of thin alu-minum or gold foil is built into the end of the tube, the rays produce lumi-nous blue streamers in the air outside.
. They carry a negative electric charge as evidenced by an electroscope placed in their path becoming negatively charged.
. They are deflected by both electrostatic and magnetic fields.
. They have considerable kinetic energy; metal obstacles put in the path of the rays glow brightly.
2.1.1
Discovery of X-rays (1895)
Two other simple tools, photographic plates and light-emitting phosphors such as zinc sulfide and barium-platinocyanide, contributed to the discovery of x-rays.
These tools in combination with a highly evacuated cathode ray tube (called a Crooke’s tube) and the prepared mind of Wilhelm Conrad Roentgen set the stage for this major discovery.
One evening in December 1895, in an attempt to understand the glow produced in such a tube, he covered it with opaque paper. Then one of those events that triggers great minds to discovery happened: he had darkened his laboratory to bet-ter observe the glow produced in the tube, and in the dim light he noticed flashes of light on a barium-platinocyanide screen that happened to be near the appara-tus. Because the tube (now covered with black paper) was obviously opaque to light emitted from the tube, he realized that the flashes on the screen must be due to emissions from the tube because they disappeared when the electric poten-tial was disconnected.
Roentgen called the emissions x-rays to denote their “unknown” nature, and in a matter of days went on to describe their major features. The most startling prop-erty was that the rays could penetrate dense objects and produce an image of the object on a photographic plate as shown in Figure 2-2, the classic picture of the bones in his wife’s hand.
2.1 Great Discoveries from Simple Tools 31
2 Major Discoveries in Radiation Physics
Fig. 2-2 X-ray photograph of Frau Roentgen’s hand.
He also discovered that x-radiation could produce ionization in any gas through which it passes, a property that is used to measure x-ray intensity. The x-rays could be reflected, refracted, and diffracted; they are a form of electromagnetic radiation like light, only of much shorter wavelength.
Very few discoveries have been as important to human existence as Roentgen’s x-rays. The rays were used almost immediately in diagnosis and treatment of disease, uses that are even more common a century later and new applications continue. Roentgen never considered patenting this remarkable invention; his was truly a gift to science and humankind.
X-rays will be discussed more completely in Chapter 15, but at this stage it is important to highlight their major role in discoveries so basic to radiation physics.
2.1.2
Discovery of Radioactivity
The discovery of x-rays appears to have triggered the discovery of radioactivity.
Since x-rays appeared to emanate from the portion of the Crookes tube that glo-wed the brightest, Becquerel postulated that fluorescence or phosphorescence might be the source. To test the theory, he placed a salt of potassium uranyl sulfate on a tightly wrapped photographic plate and exposed it to sunlight to determine if the induced fluorescence perhaps contained Roentgen’s x-rays. When the plate was developed the outline of the crystal was clearly visible, which he assumed was 32
2.1 Great Discoveries from Simple Tools due to fluorescence induced in the crystal. Apparently he prepared several other photographic plates to repeat the experiment, but since the sky was cloudy for several days he placed the plates in the back of a drawer with the uranium crystals still attached. Even though the crystals had not been exposed to light, he developed the plates anyway (perhaps for use as a reference comparison for his earlier observation) and discovered that it had the same clear outline of the crystal on the plate as before.
Aha! The images were much like those produced by Roentgen’s rays, but since the crystals had not been exposed to sunlight they could not be due to fluorescence or phosphorescence. Becquerel decided that the radiations affecting his plates origi-nated from within the uranium salt itself and was spontaneous. Luckily, he had cho-sen a crystal that contained uranium; had he used a different fluorescent crystal among the many available, he would not have observed the image and likely would have just concluded that phosphorescence was not the source of Roentgen’s rays.
Marie S. Curie and her husband Pierre studied this new property of materials and named it radioactivity. Her discovery of radium and polonium and her work on describing radioactivity far surpassed the work of Becquerel. Nevertheless, Becquerel is credited with the discovery of radioactivity, an intrinsic property of certain substances. These events are discussed in further detail in Chapter 6.
Three major types of radioactive emissions were determined, as shown in Figure 2-3, by placing a well-collimated radium source in an evacuated chamber and observing the patterns produced on a photographic plate by varying a magnetic field perpendicular to the path of the emitted “rays.” If a weak magnetic field is applied, an undeviated spot is observed in the center of the phosphoric plate due to neutral rays (referred to as gamma rays), and a second spot to the right which exhibits the same deflection as negatively charged electrons (these were called beta rays). If the weak magnetic field is replaced by a strong one in the same direction, the undeviated spot is eagain found; however, no spot for the beta rays is observed since the strong field deflects them clear off the plate. But the strong field produces a third spot slightly deflected to the left, indicating positively charged particles that were called alpha rays.
The strong field is required because the mass of the positively charged particles is much greater than that of the negatively charged particles. Clearly, the alpha particles (later established to be helium ions) were much more massive than beta particles, which were shown to be high-speed electrons. These radiations are not emitted simul-taneously by all radioactive substances. Some elements emit alpha rays, others emit beta rays, and gamma rays may or may not be emitted.
Marie Curie in a series of comprehensive experiments on the nature of radioac-tivity established that the acradioac-tivity of a given material is not affected by any physical or chemical process such as heat or chemical combination, and that the activity of any uranium salt is directly proportional to the quantity of uranium in the salt.
Further work by Rutherford and Soddy clearly established that radioactivity is a subatomic phenomenon (see Chapter 6). And since helium nuclei and electrons were emanated from radioactive atoms, all atoms must be made up of smaller units. They could no longer be considered little round balls, and a theory of atomic structure that incorporated these discoveries was needed. Formulation of an adequate theory required two fundamental concepts not yet discovered:
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2 Major Discoveries in Radiation Physics
Fig. 2-3 Three radiations from radioactive materials and their paths in a magnetic field perpendicular to the plane of the diagram.
Planck’s quantum theory of electromagnetic radiation and Einstein’s equivalence of mass and energy (as discussed in Chapter 1).
2.1.3
Discovery of the Electron
In other work with cathode rays J. J. Thomson established that they are in fact small “corpuscles” with a negative electric charge. Thomson used a specially de-signed cathode-ray tube to investigate and quantitate the deflection (“deflexion”
per Thomson) of the rays by electric and magnetic fields as shown in Figure 2-4.
Fig. 2-4 J. J. Thomson’s apparatus for measuringe/m of cath-ode rays by establishing the electric field required to exactly cancel the deflection (shown by a scale glued onto the end of the tube) induced by the magnetic force field.
34
2.1 Great Discoveries from Simple Tools A narrow beam of cathode rays was directed between two plates which con-tained a uniform vertical electric field. A uniform magnetic field was established in the horizontal direction by an external electromagnet. The magnetic field deflected the rays vertically; however, the electric field could be adjusted to exactly cancel this “deflexion”, yielding a net vertical force of zero. Thomson’s work succeeded because he was able to establish a good vacuum in the tube. Hertz in 1883 (some 14 years earlier) had subjected the “rays” to an electric field but did not observe a deflection because the “rays” ionized gas still in the tube and the ions quickly neutralized the electric field between the plates.
The measured values of the field strengths allowed Thomson to calculate the ratio e/mein terms of known and measurable quantities, although his value was too small by a factor of about two. The best modern value for the charge-to-mass ratio of an electron is
e
me¼ 1:758805 · 1011C=kg
The same value of e/m was measured for cathode rays no matter what element was used for the cathode suggesting that the “rays” were an intrinsic component of all elements. Thompson compared his measured values of e/m to those of ion-ized hydrogen, the lightest element. The values suggested that the charge on cath-ode rays was either very large or that the mass of the hydrogen atom was about 1800 times larger than the mass of the cathode rays. Thomson chose the latter;
i.e., that the “rays” contained a unit charge and were individual “corpuscles” of matter that are approximately 1836 (the modern value) times smaller than hydro-gen. He called these “corpuscles” electrons.
Deflection experiments with electrons in electric or magnetic fields could only determine the ratio e/me, but not e or meseparately. If either is known, however, the other could be calculated from Thomson’s measurements. Accurate determi-nation of the unit charge of the electron is of most value because of its wide use in atomic phenomena.
2.1.4
The Electron’s Charge
In 1909 R. A. Millikan performed a landmark experiment to measure the charge on the electron. He observed the motion of individual droplets of oil in an appara-tus like the one shown in Figure 2-5. Millikan used oil droplets rather than water to avoid uncertainties introduced by partial evaporation during the measurement.
Oil droplets from an atomizer settle through a small hole in the plate into a uni-form electric field, which can be adjusted to modify the fall of a given droplet. The oil drops, illuminated by a light beam, look like tiny bright stars when viewed through a telescope. The droplets move slowly under the combined influence of their weight, the buoyant force of the air, and the viscous force opposing their motion. The oil droplets usually have a negative charge from the atomizer; thus,
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2 Major Discoveries in Radiation Physics
the electric field will affect their motion. Additional ions, both positive and nega-tive, can be produced in the space with x-rays (another value of Roentgen’s dis-covery).
Fig. 2-5 Millikan’s oil-drop experiment. Illuminated drops could be held in space by adjusting the electric field between the plates P1and P2; the drops appeared as starlike dots when viewed by an external telescope.
When a spherical body falls freely in a viscous medium, it attains a terminal velocity vgsuch that the gravitational settling force mg is just equal to the viscous force kvg
mg = kvg
where k is a constant of proportionality from Stoke’s law and the mass of the drop-lets was calculated using the measured free-fall velocities.
The experiment allowed Millikan to isolate an oil droplet in free fall. He would then switch the electric field on which usually caused the droplet to undergo accel-eration upward and attain a new terminal velocity, which was measured. By care-ful adjustment of the electric field strength, the droplet could be held in mid-air, which corresponded to the settling force and was a function of known and mea-sured quantities. Millikan found that the charges of a large number of droplets could be expressed as integral multiples of a unit of charge, which he identified as the magnitude of the charge of a single electron. The best result obtained by Milli-kan was e = 1.592 L 10–19C because he had used the wrong value for the viscosity of air (even the most meticulous researcher can make a mistake, but he still received a Nobel prize for this most important determination). The best modern result is
e = 1.60217653 L 10–19C 36
2.2 First Concept of the Atom This charge is the fundamental quantum of charge; electric charges of all particles found in nature are always 0, –e, –2e, –3e, etc. Millikan could also now calculate the mass of the electron from measured values of e/me. The modern value for the mass of the electron is
me= 9.1093897 L 10–31kg 2.2
First Concept of the Atom
Atoms were first proposed by philosophy, not physics, as indivisible units. Early Greek philosophers had reasoned that if a small piece of matter were continually subdivided that one would eventually obtain a particle so small that it could not be divided any further and still have the same properties. They called these “pieces”
atoms. Some 2000 years later, John Dalton proposed a similar atomic theory of matter in an attempt to consolidate observations that chemicals combined in mul-tiples of whole numbers, or definite proportions. These observations suggested to Dalton that all matter is made up of elementary particles (atoms) which retain their identity, mass, and physical properties in chemical reactions. He visualized that these “atoms” linked up in chemical reactions in a way that resembled a hook and eye connection (Figure 2-6) that could of course be “hooked” and “unhooked”
when materials combined or disassociated. Dalton’s formulation is an important landmark in the development of modern atomic physics.
Fig. 2-6 Dalton visualized that atoms linked up in ways that resembled the hooks and eyes used on clothing.
J. L. Gay-Lussac added additional evidence of the discreteness or atomic nature of matter with his law of combining volumes: when two gases combine to form a third, the ratios of the volumes are ratios of integers. For example, when hydrogen combines with oxygen to form water vapor, the ratio of the volume of hydrogen to that of oxygen is 2 to 1.
In 1811 an Italian physicist, Amedeo Avogadro, proposed three remarkable hypotheses which, though not accepted for some time, eventually provided the foundation for the atomic theory of chemistry. Avogadro assumed that:
. Particles of a gas are small compared with the distances between them.
. The particles of elements sometimes consist of two or more atoms stuck
. The particles of elements sometimes consist of two or more atoms stuck