Chapter 18 NUCLEAR CHEMISTRY by E. BRoDA Radiochemical Department, Institute 0/ Physical Chemistry, University 0/ Vienna, Allstria

For the purpose of this Guide, "Nuclear Chemistry" will be taken as being concerned with the various ways in which the chemical properties of matter are affected by nuclear reactions and by events resulting from these reactions.

NUCLEAR CONSTITUTION The now accepted ideas ab out the internal structure of the atom were developed just before the first world war by two physicists, then collaborating in Manchester, the New Zealander Ernest Rutherford and the Dane Niels Bohr. According to their theory, every atom can be pictured as a kind of minute planetary system. In the centre of the atom, there is a positively charged nucleus. A number of negatively charged electrons circle around this nucleus. The force holding the electrons within the atom is electrical attraction (Coulomb force) while in the solar system the planets are, of course, held by gravity. The positive charge of the nucleus is entirely due to the protons contained in it. Each proton contributes I elementary unit of electric charge (1.6 . 10-19 coulomb); the number of unit charges (" charge number ") of a given nucleus indicates immediately the number of protons within the nucleus. The charge of each electron is identical in magnitude with that of a proton, but is negative. Therefore, an electrically neutral atom contains as many electrons as protons; the number of electrons in the neutral atom is determined by the charge number of the nucleus. It was later (1932) found by Chadwick in Cambridge that most nuclei contain, in addition to the protons, electrically neutral particles, so­ called neutrons, which have practically no interaction with electrons. A comprehensive term for the two kinds of particles occurring in nuclei, i.e. protons and neutrons, is "nucleons". The number of nucleons within an atom is called its " mass number ". The capacity of each neutral atom for chemical interaction with other atoms-i.e. its chemical properties-is determined to a large extent by the number of its electrons and therefore ultimately by the charge number of the nucleus. Because of its profound importance for the behaviour of the atom, the charge number is also known as " atomic number". Thus, there are as many different kinds of atoms behaving differently chemically, as there are different charge numbers of nuclei, or atomic numbers. (This statement will require qualification later.) For ex am pIe, all atoms with charge number I

335 (1 proton in the nucleus) behave identically, namely as atoms of the " element" hydrogen. Similarly, the element helium is formed by the atoms with charge number 2, iron has charge number 26, and uranium 92. Uranium is the natural element with the highest atomic number. Although it is not important to know such figures by heart, it is instructive to compare the masses of the ultimate constituents of the atoms. The masses of the two kinds of nucleons are nearly the same, the neutron being only 0.13 per cent heavier than the proton. However, the proton is 1836 times heavier than the electron. The mass of the atom, then, is concentrated to an overwhelming extent within the nucleus. Further, it is important to point out that the particles take up only a small fraction of the total volume of the atom. The radius of a nucleus lies (depending on the number of nucleons, i.e. its mass number) between about 1.4· 10-13 (proton) and 6.5 . 10-13 cm. This is to be compared with the radius of the atom as a whole which, e.g. in the ca se of hydrogen, amounts to 0.53 . 1O-8cm. Thus in hydrogen the radius of the atom is about 38,000 times larger than the radius of the nucleus, and the position is qualitatively similar in other elements. Consequently, all atoms may be considered as largely empty, though in the parlance of modern physical theory such a statement would not be regarded as quite exact. On the other hand, it follows that the density of nuclear matter is incredibly high. From the radius (1.4' 10-13 cm) and the mass (1.67 . 10-24 g) of the proton it is readily calculated that the density is about 1.4· 1014 times higher than that of the standard substance, water.

ELEMENTARY PARTICLES In addition to those already mentioned (proton, neutron, electron), more elementary particles-among them various kinds of " mesons "-have been discovered by the physicists since the nineteen thirties in experiments with cosmic rays and with man-made e1ectrical machinery. Each particle either carries positive or negative unit charge or it is neutral. The particles are short­ lived and transform spontaneously within a small fraction of a second with the ultimate production of stahle (protons, electrons, neutrinos and photons) or nearly stahle (neutrons) particles. At present, it is not known why the masses of the various elementary particles have the particular values shown in Table 1. TABLE 1. SO ME PROPERTIES OF IMPORTANT ELEMENTARY PARTICLES Relative Approximate Charge Partic1e Relative Mass* (Charge Half-Life (Mass 0/ Proton = 1) 0/ Proton = 1)

Proton ...... 1 1 stable Neutron ...... 1.001 3 0 17 min. Electron (negative) ...... 0.000544 7 -1 stable Electron (positive) ...... 0.0005447 1 stable n-Meson (neutral) ...... 0.378 2 0 < 10-15 sec. n-Meson (positive) ...... 0.3659 1 2.6 X 10-8 sec. Neutrino ...... probably 0 0 stable Photon ...... ! 0 0 stable

* The masses given refer to the ffee particles at rest. Einstein's theory of relativity shows that actually the mass depends on the velocity of the partiele.

336 The photons, i.e. the quanta of electromagnetic radiation (visible light, UV light, X-rays etc.) are also considered as elementary particles. They invariably move with one and the same velocity e (e 3 X 1010 cm/sec.). The charge of the photon is zero. The mass of the photon is correlated with its energy by the famous general Einstein relationship : E= me2 The energy in turn is detennined by the frequency ofthe radiation in accordance with the law discovered by Max Planck in Berlin in 1900: E hv Because frequency and wave length I, are universally connected by: e = VA the Planck law can also be written as: E = he/I, and the Einstein law for photons as: In = h/el' Thus the mass of the photon-moving with its characteristic velocity-has no unique value but is determined by its frequency, or, what comes to the same, by its wave length. The short-lived elementary particles cannot be normal constituents of atoms in the same sense as nucleons or electrons, and in general teachers of chemistry will not wish to discuss their nature or physical role.

NUCLEAR FORCES How is it that the nuclei keep together at all? What acts as " nuclear glue"? While the electrons are kept within the atoms by electrical (Coulomb) forces, obviously such forces cannot be invoked to explain the mutual attraction of the nucleons. Being neutral, the neutrons do not exert any electrical forces at all. The protons, which all carry charges of identical sign (positive), can only repel one another. Nevertheless, the attractive (binding) fOl·ces within the nuclei are very strong. Numerical values will be given below. It will be necessary for the teacher to emphasize strongly the existence and the strength of these specific nuclear forces and to underline their non­ electric nature. Though it will hardly be possible to explain their origin, the teacher should be clear about the principle involved. It is the view of theore­ tical physics that nuclear attraction (nuclear cohesion) is mediated by TC-mesons. All nucleol1s have a tendency to emit TC-mesons. A very short time after emission, the (positively charged or neutral) meson is reabsorbed by the parent nucleon, or else it is absorbed by a neighbouring nucleon. The transfer of a neutral meson does not change the nature of the nucleons involved, but on the loss of a positive meson a proton is converted into a neutron, and on the gain of such a meson a neutron is converted into a proton. Through this exchange between the nucleons, the mesons act as a kind of bridge or glue, and the calculations of the theoretical physicists show that a strong attractive force results from this type of interaction. The attraction is independent of the charge of the nucleon. It will be seen later that the Coulomb repulsion between the protons, which is superimposed on the specific nuclear force, may strongly affect the properties of nuclei, particularly nuclei of high charge.

337 NUCLEAR ENERGY UNITS The energy needed for the separation of an electron from an atom, i.e. for the removal of this electron, is often modest. This energy is called the " ionization energy" because in the process the atom is transformed into a positive ion. For instance, in the case of some atoms (rubidium or cesium) the relatively small energy of a photon of visible light suffices for ionization, and the phenomenon in this ca se is known as the photo-electric effect, and is used in photo-electric cells. In other cases, much more energy may be required for the removal of an electron. This is true in particular for electrons which circle in lower shells, i.e. in orbits ne ar the nucleus. Nevertheless, the binding energy of the electrons in atoms is always much less than the binding energy of nucleons in nuclei. That means that the attractive forces within the nuclei are strong indeed. To measure the energy needed for the removal of nucleons from nuclei (the binding energy of these nucleons), a suitable unit of energy is needed. In nuclear science, the unit in common use is the "electron-volt " (eV). This is defined as the energy gained by a particle carrying one elementary electric charge when it is accelerated through an electrical field of I volt potential difference. The nature of the particle is immaterial, as the energy gained depends on nothing else but the charge: the particle may be an electron or a proton, for example. The choice of this unit is understood when it is remem­ bered that quite often in experiments in atomic and nuclear science elementary particles are exposed to electrical fields. For instance, in a conventional X-ray tube electrons are accelerated between the negative pole (cathode) and the positive pole (anode or anticathode). The eV is, so to speak, an " atomic unit " of energy, as it is based on the acceleration of single atomic or sub-atomic particles. It follows that for the transition to macroscopic units of energy, based on the behaviour of visible and weighable amounts of matter (erg, kcal), rather large conversion factors are needed: 1 eV 1.6· 10-12 erg = 3.82' 10-23 kcal After introduction of Avogadro's (Loschmidt's) number, which gives the number of atoms in a mole, it follows that: 1 eVJatom = 3.82' 10-23 .6.02. 1023 = 23 kcaljmole Instead of the eV, multiple units are often used in practice, namely the keV (kilo-electron-volt, 1,000 eV) and the MeV (mega-electron-volt, 106 eV).

ENERGIES OF NUCLEAR REACTIONS The experiments show that the binding energy of individual nucleons within nuclei is often ab out 8 MeV although large deviations from this value are observed in many cases. For instance, a well-known nuclear reaction consists in the absorption of a neutron by the nucleus of an atom of ordinary hydrogen. (It williater be shown that in this reaction an atom of deuterium­ heavy hydrogen-is formed.) The energy of this reaction, which must equal the bin ding energy of the neutron within the nucleus of an atom of deuterium, is found to be 2.2 MeV/atom. The magnitude of this energy, though moderate by nuclear standards, may be appreciated by calculating the value per mole. Applying Avogadro's number (6.02' 1023), it is found that a binding energy of 2.2 MeV/atom cor-

338 responds to a binding energy of 1.3 x 1030 eV = 5 X 107 kcal per mole ofhydrogen atoms. When this is compared with the energies of the most powerful (exo­ thermic) chemical reactions, it is seen that the nuclear reactiolls are superior by many orders of magnitude. This is the fundamental reason for the extent both of the promise and of the threat of atomic (more correctly: nuclear) energy. For instance, hydrogen element as agas yields only 28.9 kcal per mole (l gram) of hydrogen atoms on chemical combination with oxygen to form water vapour (" combustion "ofhydrogen). Thus the combination of ordinary hydrogen with neutrons in a nuclear reaction yields (per gram of " fuel ") nearly two million times more energy than the combustion of the same amount of hydrogen in a chemical reaction. The heat offormation of deuterium (5,107 kcal) is equal to 5 X 107/2.4 X 10-11 18 r--> 2.1 . 10 erg. According to Einstein's law (page 337) this corresponds to 2 18 20 a "mass defect" ofE/c or 2.1 X 10 /9 X 10 r--> 0.0023 g. (about 0.1 per cent)­ a measurable quantity. On the other hand, the mass decrease in the combustion ofhydrogen, being 2 million times sm aller, is at present beyond any possibility of measurement.

NUCLEAR REACTION EQUATIONS

To formulate nuclear reactions, a special nomenclature is required. For this purpose, the ordinary chemical symbol of an atom is provided with an upper index, giving the mass number, and with a lower index, giving the charge number of the nucleus. (The latter index is really redundant as the nuclear charge number is in any case determined by the chemical nature of the element, and vice versa.) Thus, the atoms of ordinary light hydrogen (" protium ") are symbolised by \H, and those of heavy hydrogen (deuterium) by 21H. The absorption (capture) of neutrons by ordinary hydrogen (the reaction of the nuclei oflight hydrogen with neutrons) is described, then, by the equation:

\H + \n -')- 21H + y y signifies the photon (or photons) which carry away the reaction energy in this particular ca se, and may be omitted. A shorthand notation to describe the same reaction is: \H(n, y) \H Another example would be:

147N + \11 -')- 146C + \H or (what comes to the same):

147N(n, p)l\C with p symbolising the proton. In all equations of nuclear reactions the sum of the lower indices must be the same left and right. This is a consequence of the general physical principle of conservation of electric charge. The sum of the upper indices must also be the same left and right. This is an expression of the experience that nucleons (though mutually transformable) can neither be destroyed nor created in nuclear reactions in the energy range with which we are concerned. Thus mass number is also conserved.

339 Within stable nuclei, all neutrons must be stable, in contrast to the free neutrons (Table 1) or to the neutrons in some types of radioactive nuclei (see below, page 343). Within nuclei, protons may also be unstable (page 343).

IS0TOPY We have seen that nuclei (and therefore atoms) may exist which are identical in respect to nuclear charge number (lower index) but not in respect to mass number (upper index). This holds, for instance, for the atoms of protium and deuterium. We have said that the chemical properties are de­ termined, in a good approximation, only by the nuclear charge (i.e. only by the number of protons) and not by the number of neutrons within the nucleus. Therefore, such atoms, distinguished only by their mass numbers, must be very nearly identical in their chemical properties. Such atoms, then, belong to one and the same element in spite of the difference in the mass numbers. Now for each element there is one, and only one, place in Mendeleeff's Periodic Table of the elements, in which they are arranged in the order of increasing atomic numbers. Consequently, different atomic species, identical in charge number but differing in mass number, occupy one and the same place in the Periodic Table. For instance, there is only one place for hydrogen (protium and deuterium) and one place for helium (42He, which is the common species, and the rare species, 32He). The different species, which share one and the same place, are called " isotopes" (Greek: isos equal, topos = place). The name was coined by a colleague of Rutherford, the British chemist Frederick Soddy, who may be considered as the founder of radiochemistry. An important didactic point: nuclear species of the same charge number but different mass number are isotopic, or what comes to the same, they are isotopes of an element. However, it is a common mistake to use the term " isotope" as a synonym of " nuclear species". Thus, it is right to say that \H and 21H are isotopes, but it is wrong to say that \H (or 22H) is" an isotope" It is even more strikingly wrong to say that 22H, \H, 32He and 42He are all isotopes. A telling analogy: Menelaos and Agamemnon were brothers, but it would not be appropriate to call Menelaos " a brother " and quite wrong to say that Menelaos, Agameml1on, Paris and Hector were all brothers. If a new term is wanted for a single nuclear species, without reference to other species, the term "l1uclide" may be used. Nuclides are often specified by adding the mass number to the name or symbol of the element. For instance, Helium 4 or He 4. Many, but by 110 means all, natural elements are found to be composed of several isotopes. For instance, natural hydrogen contains some heavy hydrogen (abundance only about 0.02 per cent) and natural chlorine the iso­ topes 3517Cl and 3717Cl (abundances 75.5 and 24.5 per cent). Some elements consist of a fairly large nnmber of isotopes, e.g., tin (10 isotopes). On the other hand, some natural elements contain only one isotope, for instance iodine (127531). The abundances of the isotopes of a given element within that element are in general always the same, whatever the terrestrial source of the sample. (In special cases there are exceptions which are weH understood.) Even in the meteorites, the abundances within the elements are the same as on Earth. This identity of the abundances within the elements is a powerful argument for a common origin of the solar system. When the substances now composing

340 the solar system were formed by processes of which we are stilllargely ignorant, the different nuc1ides were formed in certain accidental proportions. Nuclides of identical chemical properties (isotopes) mixed in the subsequent chemical pro ces ses, and because of their essential chemical identity they had no chance of separating to any important extent later. Geochemical research and the analysis of meteorites, supported by spectroscopical data, lead to the conc1usion that the cosmic abundances of nuc1ei with even numbers of protons are mostly greater than those with odd numbers of protons and similar mass numbers. Among the nuc1ei of the former type, that sub-group is more abundant which contains an even rather than an odd number of neutrons. Stable nuc1ei with odd numbers of protons alm ost always contain even numbers of neutrons. Thus even numbers are mostly " preferred" to odd numbers of protons or neutrons. This preference no doubt reflects the ease with which the nuc1ei of these kinds were formed ini­ tially. Light hydrogen, which presumably was the starting point for nuclear synthesis, holds an anomalous position; it is more abundant in the cosmos than all other nuclides taken together. Naturally, because of segregation processes, the frequencies with which the different elements appear in the accessible parts of the Earth need not coincide with their frequencies in the cosmos or in the Earth as a whole.

ISOTOPE EFFECTS The reader of this Guide has 110ted the reservation made repeatedly that isotopes are" nearly " or " practically " identical chemically. Now it is time to speak of the (slight) chemical differences. The chemical properties of atoms are not determined exc1usively by the charge numbers, i.e. the numbers of electrons, but are affected to a small extent by the masses of the atoms, and therefore by their mass numbers (numbers of nuc1eons within the nuc1ei). Mass number is precisely the parameter by which isotopes are distinguished. According to the kinetic theory of matter, the average kinetic energy of the atoms of a given substance is determined by its temperature. From the elementm·y relationship of mechanics (valid for translational energy): E = mv2/2 it follows that at a given average energy E (and temperature) the velocity (v) of the atoms is inversely related to the root of their mass (m). Therefore, the mean translational velocity of the atoms of the heavier isotopes will be less than that of the atoms of the lighter isotopes. A similar relationship holds for the vibrational energy of atoms within molecules. Consequently, the different isotopes will show certain differences in phenomena where the velocities of the atoms are important, as in the speed of diffusion or in the rate of chemical reactions. Small differences between isotopes are also observed in the positions of chemical equilibria. The chemical differences between isotopes are known as " isotope effects ". The size of these effects is, of course, related to the difference in mass number between the isotopes concerned. For instance, the isotope effects are negligible with natural lead (mass numbers of the natural isotopes 204, 206, 207, 208) where the difference in mass does not exceed 2 per cent. On the other hand, the isotope effects are most pronounced with hydrogen where the atom of deuterium weighs twice as much as that of protium.

341 ISOTOPE SEPARATION For many purposes of science and also for certain practical applications, separation of isotopes is essential. Because of the chemical near-identity of the isotopes the processes employed are often difficult and expensive but the problem can be solved in the case of every element. It is a relatively easy task to separate heavy from light hydrogen by making use of chemical isotope effects. One method is the electrolytic decomposition of water. Using suitable electrodes, it is found that the water remaining with an (alkaline) electrolyte is much richer in deuterium than the hydrogen gas set free. Another method is based on a chemical exchange reaction. After equilibration of hydrogen gas with water vapour in the presence of certain catalysts which ensure the exchange of hydrogen atoms between the two substances (without a change in the proportion of hydrogen gas to water) the water is richer in deuterium than the free hydrogen; the equilibrium cons­ tant: K = [HDO] [H2] ! [H20] [HD] is greater than 1. In the equation, the special symbol D has been used for deuterium althotigh in most cases isotopes are distinguished by indices only. The enrichment factor K in such chemical reactions, even in the relatively favourable ca se of hydrogen, is insufficient to yield immediately a product of such an isotopic abundance as is required in practice. Therefore, it is usual to enrich the product in several or many successive stages until finally a sub­ stance of satisfactory composition is obtained. In the case of heavy elements, where the chemical differences between the isotopes are particularly small, physical methods of isotope separation are often preferred. For instance, the separation of the two important natural isotopes of uranium (mass numbers 235 and 238) is mainly accomplished by two physical methods. One method, wh ich again gives gradual enrichment in many stages, is based on the more rapid diffusion of the lighter isotope in the form of UFs. When this volatile fluoride diffuses through a large number of fine holes in a metallic membrane, the first portion of the diffused gas is richer in U 235 than the starting material. The second physical method needs one stage only. It uses the principle of the mass spectrometer. A narrow beam of uranium ions, rapidly moving in a vacuum, is exposed to a magnetic field. The ions may be considered as constituting an electric current. The partic1es of the current are deflected by the field, and the extent of this deflection is determined, other things being equal, by the velo city (and therefore by the mass) of the partic1es. In a suitable apparatus the different isotopes of uranium are then collected separately. So far we have spoken, though not explicitly saying so, only of stable nuc1ides, or of nuc1ides whose instability is unimportant in the context. How­ ever, most nuc1ides are unstable and are subject to radioactive decay. This phenomenon will now be dealt with.

RADIOACTIVlTY Radioactivity was discovered by Becquerel in Paris in 1896, and thoroughly investigated by Pierre Curie and his Polish wife, Marie Sklodowska-Curie, long before the theory of the nuc1ear atom was put forward. In fact, radio­ activity was the starting point for nuc1ear science.

342 In painstaking chemical work in difficult conditions the Curies extracted from the uranium mineral pitchblende, supplied to them by the Austrian Government, several previously unknown elements, primarily polonium and radium, which are distinguished by the spontaneous emission of intense radiation. The chemical operations were at first empirically directed, on the basis of the frequent measurement of the radiation, in such a way that the unknown active elements were concentrated. The active substances were termed " radioactive ". It later became clear that the radiations are emitted in nuclear reactions (transformations), and it is now the general practice to define as "radio­ activity" the phenomenon of spontaneous nuclear transformation (" dis­ integration "). The rays most commonly emitted in spontaneous reactions consist of fast electrons (ß-rays) or of fast helium ions (a-rays) or of photons (y-rays). While these rays all come from the nuclei, in many cases the emission of rays, e.g. X-rays, from outer parts of the atom is also observed as a conse­ quence of radioactive decay. The principles of the measurement of all these rays will be mentioned later, page 351. It is now known that the most general kind of radioactivity is ß activity. This consists in the transformation of a nuc1eon into another kind of nucleon (neutron into proton, or proton into neutron). Conservation of charge is ensured mostly by the generation of an electron (ß particle). This electron did not pre-exist within the nuc1eus. An example would be, for instance:

210S3Bi --'>- 210S4PO + 0 -le Bismuth 210 is found in nature, alld is also kllown as "radium E ", while polonium 210 is called " radium F ". In the nuc1eus of radium E, a neutron is converted into a proton, which indicates that the daughter nuc1eus is more stable. In other cases, stability increases when a proton transforms into a neutron; here a positive electron (" positron") may be emitted, for instance in the decay of the (artificial) radioactive nuc1ide 3015P:*

3015P --'>- 30I4Si + OIe The properties of the positrons are, apart from the sign of their charge, quite identical with those of the electrons. They are also stable in themselves, and in free space they would indeed remain unchanged. However, when electrons of opposite signs meet, they neutralize each other (annihilate) and disappear as corpusc1es. The energy contained in them, which is calculated as ab out 0.5 MeV for each of the two by the Einstein relationship (page 337), is conserved in the form of photons (electromagnetic radiation). Because negative electrons abound, the positrons disappear rapidly in practice. A third variant of ß-decay is " K-capture", or (more precisely) " orbital electron capture". This curious process occurs, often in competition with positron decay, whenever stability increases in the conversion of a proton into a neutron. In the case of K-capture no corpusc1e is emitted by the nuc1eus. Instead an orbital electron of the atom (usually from the lowest shell, the K-shell) is captured by the nuc1eus, for instance in:

40I9K + o_le --'>- 4°1SAr

* Here and in same other instances artificial radioactive nuclides will be used as examples although artificial radioactivity will be introduced as such on page 348.

343 The process is detected and its rate is measured through the X-ray photons which are emitted when an electron falls from outside into the shell which has a vacancy as a consequence of the loss of an electron. Cf.-decay is now known to be a more specialized phenomenon than ß-decay. It occurs almost without exception only among heavy (and therefore highly charged) nuclei. There the Coulomb repulsion between the many protons may be so powerful that protons are expelled. Note, however, that in (spont­ aneous) radioactive transformations the protons are never ejected alone, but always as particles consisting of two protons and two neutrons. This particle is of course nothing else but a helium nucleus, or doubly charged helium ion, 42He (Cf.-particle); it happens to be distinguished by very great cohesion (binding force). An example: 226 222 4 SsRa -+ s6Em + 2He Emanation (element 86) is an inert gas; the isotope here formed is radium emanation, or radon (half-life almost 4 days). Cf.-rays show much stronger interaction with matter than ß-rays, and therefore much less penetration. The range in air is a few cm.; ß-rays often have ranges of some metres. After the emission of an Cf.-ray or ß-ray the nucleus often, but by no means always, remains in an excited state and emits the excess energy subsequently­ without change in charge number or mass number-as a photon or as a cascade of photons. These photons coming from the nuclei are called y rays. They are so penetrating that they are used for the industrial radiography of thick pieces of steel, for instance of rails or tubes. As a rule, the emission of the y-rays follows that of the Cf. or ß-rays within an exceedingly short time interval. Thus it looks as if the Cf. or ß-active parent nucleus had emitted the y-ray as well, i.e. as if emission had been simul­ taneous. This is not true, however, For instance, after the emission of a ß-ray by radium D (lead 210) with energy 0.018 MeV, a y-ray of 0.047 MeV follows within a very short time; but strictiy speaking this is emitted by the daughter of radium D, namely radium E (bismuth 210). Much later the nucleus of radium E, now in its ground state, submits to its own ß decay (half-life 5 days). Occasionally the time interval between the production of the excited nucleus, capable of y-ray emission, and the actual emission is much longer, and mayamount to days or even months. An example is tellurium 123. It decays from the excited state to the ground state with a half-life of 4 months. It so happens that in this case, as often, the photon does not leave the atom but instead its energy is carried away by an electron from the outer shell (not bya nuclear electron, i.e. a ß-ray); this process is called " internal conversion ". The long-lived excited nucleus and the nucleus in the ground state are called " nuclear isomers". The justification of this name is that the two kinds of nucleus have identical COlllposition of protons and neutrons, and yet have different properties. The analogy with chemical isomerism is instructive and might be discussed. Radioactive disintegration leads from a well-defined initial state of the nucleus (and atom) to an equally well-defined final state. Therefore, one should expect the emitted partic1e also to be endowed with a well-defined energy. This is confirmed by experience in the cases of Cf.-rays and y-rays; in any kind of disintegration, they have sharp energies at the time of emission. It caused great surprise when the experiments gave a different result in respect of ß-rays. Their energy spectrum is always continuous, and not sharp.

344 This was later explained by Pauli (then in Zürich) and Fermi (Rome) to me an that a varying part of the available energy in ß-decay is carried away, not by the ß-particle (electron) but by an uncharged elementary particle, the "neutrino", which probably has zero rest mass and moves with the velocity of light. Because of its lack of mass or charge the neutrino has little interaction with matter, and its existence is hard to prove directly. Yet all experiments specially undertaken in recent years are consistent with the neutrino theory. It is, then, the upper limit of the energy of the ß-particles observed in a parti­ cular ca se of ß-decay which corresponds to the true difference in energy between the initial and the final state ofthe nucleus. When this limiting energy is carried by the ß-particle, the energy of the neutrino becomes zero.

THE RADIOACTIVE DECAY LAW Each radioactive species is characterized by a particular half-life. That of radium 226 is 1620 years, that of uranium 238 is 4.5 X 109 years and that of polonium 214 is 0.16 milli-seconds. This means that of any given amount of radium one half will have decayed after 1,620 years, of the remaining half, one half (that is, one quarter of the initial amount) will have decayed after a further 1,620 years (that is, after 3,240 years from the beginning), etc. It is important to avoid the error that after the double half-life the whole of the substance has decayed. The decay curve rather folIo ws an exponential function (see below). The curve never fully reaches the abscissa, which means-strictly speaking-that radioactive decay is never complete. In practice, of course, the decay law cannot be applied when only a few radioactive atoms are left. For instance, starting with three atoms, it cannot be expected that after one half-life one and a half atoms decay. The shape of the decay law is explicable on the assumption that each radioactive atom of a particular kind is characterized by adefinite prob ability of decay within the next unit of time; usually seconds are used for calculation. This probability is called the " decay constant". It is quite independent of the external circumstances such as pressure, temperature, chemical composition of the sample, etc., and therefore also independent of the number of identical atoms which decayed in the sample before. Thus the atoms of radium are characterized by a decay constant which will give a decay prob ability of one half for aperiod of 1,620 years, no matter how many radium atoms there were in the neighbourhood (the sample) initially. The approximate number of " surviving" radioactive atoms of a particular kind in apreparation can be estimated graphically. On the abscissa the time, and on the ordinate the relative numbers of the active atoms (percentage of initial number) are plotted. Then, at the times "Y2' 2,,%, 3"Y2'" the ordinates are 50, 25, 12.5 ... and a li ne drawn through these points represents the decay law. The exact shape of the decay law curve can be derived quite easily by the use of elementm'y calculus. The decrease per unit time of the nu mb er of radioactive atoms N in a sample, -dN/dt, is given by the product of the decay constant (probability of decay) " and the number of atoms present: -dN/dt = AN (1) This can be written as: 1 dN -A (2) NM

345 or: d -ln N = -I, (3) dt After integration, we have: In N - j,t + C (4) with C as an integration:constant. This is transformed to: N e-),t eC (5) Now at the beginning of the experiment, N TImst have the initial value No. Hence for t = 0: N= No eC (6) so that eC can be replaced by No in the decay law, which assumes the form: N = No e- i,t (7) -dN/dt (or A) is known as the " activity" of the sampIe. The dependence of A on time is obtained by substituting (7) in (1): A = ANoe- i-t (8) and can be written as: A Aoe- i,t (9)

The reciprocal value of A has the dimension of a time and is called Tm. It is the time in which the number of atoms and the activity both decay to the e-th part of their initial value. It can be shown by integration that Tm is the mean life of the atoms of the sampIe. In practice, powers of 2 are preferred to powers of e. Therefore we write instead of: N No e- At (7) N = No 2x (10) and obtain by dividing (7) by (10): 2x e- i,t (11) and: x In 2 = - At (12) and: I,t x = -ln 2 (13) and: H (14) N = N 2 -In2 o Clearly, (1n2)/I, is the time in which the sampIe has decayed to one half, and is therefore called the half-life (T%). We write, therefore, for practical application:

(15) N = No 2 't"!1:! and by analogy:

(16) A Ao 2 't"% Note that the half li fe is smaller than the me an life. T% = Tm • ln2 Tm • 0.693 (17)

346 The activity can be expressed in absolute terms as dpm (disintegration per minute) or dps (disintegration per second). Most counting instruments do not measure absolute activities directly, their yields being smaller than 100 per cent. The rate at wh ich the instrument indicates events (pulses) is often expres­ sed as cpm (counts per minute). Only when the yield 100 per cent does cpm dpm. The absolute activity can also be expressed in the unit "Curie". A preparation of 1 Curie (C) undergoes 3.7 . 1010 dps. The numerical value has been chosen so that 1 gram radium has the activity of very nearly 1 Curie. Sub-units are the milli-Curie (mC = 1/1000 C) and micro-Curie ([LC = 1O-0C). The nu mb er of Curies of apreparation by definition does not depend on the kind of disintegration or on the number or energy of the rays emitted in each disintegration. For instance, in the disintegration of radium E, beta-rays of 1.14 MeV, and in that of ordinary actinium (actinium 227), beta-rays of 0.045 MeV are emitted. Nevertheless the Curie value is the same provided equal numbers of nudei decay in unit time in the two preparations.

STATISTICAL FLucTuATloNs The shape of the decay curve is ultimately determined by the probability of decay, expressed as the decay constant. Now the concept of probability implies possibility, even inevitability of deviation from the most probable value. When the prob ability of" death-in-the-next-year " for the sexagenarians in the country " Anywhere" has a particu1ar value, it cannot be expected that the number that die in the next year corresponds exactly to the probability. Similarly, the actual rate of decay (activity) of a sampIe " fluctuates " irregularly around the most probable value, and the fluctuations are the more pronounced the smaller the absolute activity. Therefore: the greater the precision required in the measurement of an activity, the greater the number of disintegrations which must be registered. Expressed differently: for a given precision, the time needed for measurement increases with the inverse value of the activity of the sampie. Mathematical analysis shows that the standard deviation (probable error 0) in the activity A (dpm) (provided this is not too small) is given by the simple Poisson formula: v= VAt (1) The relative standard deviation (which is the important magnitude in practice) is given by: v/At = ± VI/At (2) An analogous equation holds when A (the absolute activity of the sampie) is replaced by the number of counts per unit time registered with an instrument of a yield smaller that 100 per cent (for instance, a Geiger counter), i.e. by the counting rate a (cpm): v/at= 1..Vl/a·t (3) It follows from (3) that with a measured count (at) of 1,000, the relative standard deviation is about + 3 per cent, and with a measured count of 10,000 it is l.. 1 per cent. It may be recalled that according to the theory of probability the likelihood of finding the experimental value in the range between (at + v) and (at - v) is about 68 per cent, and between (at + 2 v) and (at - 2 v) it is 95 per cent.

347 RADIOACTIVE EQUILIBRIUM Often a substallCe produced in a radioactive disintegration is itself radio­ active (parent-daughter relationship). The daughter of the daughter may again be radioactive, etc. In some cases a whole chain of disintegrations is initiated in this way by the decay of one substance. In the so-called natural radioactive series the chains may consist of about a dozen members. The most famous of these chains starts with uranium 238, and ends after a number of

7. and ß disintegrations with lead 206. Radium (226ssRa), radium emanation 222s6Em), radium D (210S2Pb), radium E (210S3Bi) and radium F (also known as ordinary polonium, (2lOS4PO) are among the members of this chain. Other natural chains are started by " ordinary " thorium (23290Th) or by uranium 235. It is, of course, a condition for the survival of a chain to the present day that the first member has a half-life which is not too small compared with the time elapsed since the formation of the nuclides. The primordial synthesis of the nuclides may have taken place some 5 or 6 . 109 years ago. Within a sampie containing the first member of a chain, for instance a piece of uranium ore, all members are in "secular equilibrium ", provided the sampie has not been disturbed~for instance, leached out by water. In secular equilibrium the activities of all members of the chain within the sam pie must be equal. The reason is easy to see: assume for a moment that one of the chain members had excess activity. Then it would disintegrate more rapidly than it would be re-formed, and as a consequence its quantity would decrease until equilibrium was re-established. Similarly, the quantity of the substance would grow if its activity were initially below normal. Secular equilibrium is also approached after chemical isolation of a member of aseries, provided the half-life ofthe parent much exceeds that of the daughter. For instance, after isolation of radium D (2lOszPb, half-life 19.4 years) the activities of radium E (ZlOS3Bi, half-life 5 days) and of radium F (ZlOS4PO, half-life 138 days) grow in the isolated sampie until they equal that of the radium D. All natural isotopes of the elements 84 to 92 and some natural isotopes (namely the radioactive isotopes) of elements 81 to 83 occur as members of the natural radioactive series. Isotopes of the elements 85 (astatine, At, a halogen) and 87 (francium, Fr, an alkali metal) were found in nature only recently. These isotopes of astatine and francium are short-lived. No element of atomic number 84 or more has a stable isotope.

ARTIFICIAL RADIOACTIVITY Up to 1934, only those radioactive nuclides which nature supplies were known. These natural radionuclides are in most cases members of the radio­ active series, that is, descendants of uranium 238 (for instance radium), of uranium 235, or of thorium (more precisely: thorium 232). In addition, some "isolated" radiOlmclides are found in nature. Rather surprisingly, ordinary potassium was found to contain not only the stable isotopes 39 and 41, but also a ß-active isotope 40 (see page 343). From the stand points of scientific research and of application, this restriction of radioactivity to a few elements was a serious obstacle. The position changed with one blow when Irene Curie (daughter of Pierre and Marie Curie) and her husband Frederic J oliot discovered artificial radioactivity (Paris, 1934). They found that in certain nuclear reactions radioactive species are produced.

348 As early as 1919, Rutherford (then in Cambridge) had discovered the first example of a non-spontaneous (artificial) nuclear reaction. On exposure of nitrogen to :x-rays fast protons originating in the reaction: 17 147N + 42He -+ 80 + \H were observed. Other reactions were discovered in subsequent years. However, it so happened that the nuc1ides produced were stable, and the reactions were not followed by any further (spontaneous) transformation. In contrast, the Joliots found that in the reaction:

2\3Al + 42He -+ 3015P + Ion (aluminium sheet bombarded with CI.-rays) a radioactive isotope of phosphorus is produced, which decays with the emission of a positron (half-life 2.55 mi­ nutes). The phosphorus can be identified chemically by dissolving the alu­ minium in nitric acid and precipitating the phosphorus with ammonium molyb­ date. All the activity is found in the precipitate. A search for further examples of articifial radioactivity was very successful. Nowadays radioactive isotopes of all elements are known, and the total number of radioactive nuc1ides listed in the handbooks exceeds a thousand. Unfortunately, not all elements have isotopes of practical utility. In some cases the half-lives are too short. It was Enrico Fermi who showed that slow neutrons are a very effective agent in the production of artificial radioactivity. Free neutrons were first obtained from "natural neutron sources ". A useful neutron source may consist, for instance, of an intimate mixture of an :x ray emitter (a radium salt) and beryllium metal. On bombardment of the latter with CI. rays the reaction:

94Be + 42He -+ 126C + Ion occurs, and neutrons are set free. The neutrons carry a lot of kinetic energy (mostly more than 1 MeV) and are, therefore, termed "fast neutrons". Their speed may be many thousands of kilometres per second, of a similar order of magnitude as that of the CI.-partic1es. Fermi observed that the neutrons can be slowed down (changed into " slow neutrons ") by introducing them into a material containing hydrogen, e.g., water 01' paraffin wax. The protons of the hydrogen nuc1ei have a mass which is practically identical with that of the neutrons. Now it is a basic theorem of c1assical mechanics that in a collision between two particles the average energy transfer is maximal when the two masses are identical. After a number of collisions, the neutrons have no more kinetic energy than the atoms of surrounding matter (water or wax). The slow neutrons slowed down as far as possible are known as "thermal" neutrons. Substances serving to slow down neutrons are called "moderators ". The general value of neutrons as transforming agents is due to their lack of charge: neutrons approach and penetrate nuc1ei easily whereas charged particles would be repelled by atomic nuc1ei. The particular value of slow neutrons is because they spend a relatively long time in the neighbourhood of each nuc1eus and thereby give it a good chance to absorb them. Examples:

\H + Ion -+ 31H

63Li + Ion -+ 42He + 31H 55 1 56 25M11 + 011 -+ 25Mn The hydrogen isotope with the mass 3 is ß-active with a half-life of 12.3 years and is also known as " tritium" (symbol T).

349 It is fortunate that nowadays neutrons can be made in enormous quantities in the nuclear reactors or " piles" which will be described later. In fact, olle of the main uses of these piles is that they serve as neutron factories. It is also possible to make neutrons with accelerators, for instaJ1Ce with cyclotrons. Using artificial transmutation, isotopes of the elements 43 and 61 have been made. These elements are not found in nature as they have no sufficiently long-lived isotopes. They are named now" technetium" and "promethium ". Technetium (Tc) stands in one column ofthe Periodic Table between manganese and rhenium, while promethium (Pm) is a rare earth element. For instance, the ß-active isotope technetium 99 may be obtained by the sequence:

9842Mo + Ion -+ 9942Mo (half-li fe 67 hours) 99 99 0_l 42Mo -+ 43Tc + e Being relatively long-lived (half-life 212,000 years), technetium may be prepared in weighable quantities and used for experiments by traditional chemical methods. Some artificially produced radioelements may be first members (parents) of chains (page 348). This is frequently so in the case of fission products (page 360).

ACCELERATORS In the production of artificial radioactivity with natural radioelements, clearly only those projectiles can be applied for bombardment which are supplied by the radioactive substances, mainly CI. and y rays. However, any kind of charged ,~tomic particle can be accelerated to high speeds and kinetic energies by using electrical machinery. The first machine of this kind was constructed by Cockcroft and Walton in Cambridge in 1932. Here hydrogen ions (that is, stripped protons) 01' other small ions are accelerated through an electriq field with a potential difference of several hundred thousand volts in a long evacuated glass tube so that the particles gain energies of several hundred thousand electron-volts. In the end, the fast ions hit a "target ", and in some cases may react with it. It is also possible to lead the beam of fast particles through a thin but gas-tight window (metal) into the open and do experiments there. Such machines allow the use of protons, deuterons (ions of deuterium), etc. as projectiles. Moreover, the intensities of the beams are far superior to that obtainable from radioactive substances, i.e. the number of projectiles per unit time and unit area is much higher-in practice thousands or tens of thousands times higher. An example of a nuclear reaction with fast ions is: 5324Cr + 21H -+ 5425Mn + Ion Accelerators are found now in all major centres of research and also in some important factories. Of course, it has been desirable to increase the energy of the ions in order to, obtain many types of reactions. Now in the linear accelerators of the Cockcroft-Walton type a practical limit is soon reached because of the diffi­ culties in the electrical insulation. But E.O. Lawrence in California invented a machine in which this difficulty is circumvented, the cyclical accelerator 01' " cyclotron ". The essence of this machine is that no particularly high voltage is applied. The ions are forced by a magnetic field to travel in the vacuum along a spiral path where they are exposed again and again, many times, to one and the same electric field. They gain kinetic energy every time, and in

350 the end they may have energies amounting to 10 or 20 MeV or more. More recently, machines-working on modified principles-have been built in some places (Geneva, Berkeley in California and Dubna in Russia) where energies still a thousand times higher are supplied to the ions (synchrotrons, bevatrons, phasotrons).

DETECTION AND MEAsuREMENT OF RADIOACTIVlTY The main interest attaches to the measurement of intensities. An intensity is defined as the nu mb er of rays per unit area per unit time. The nature of the radiation is irrelevant for the definition. In a given ex[erimental arrange­ ment, the intensity of the radiation is proportional to the strength (activity) of the source. Other things being equal, the intensity decreases with decreasing effective solid angle. The phenomenon of radioactivity was first detected through the diffuse blackening of a photographic emulsion, and photographic methods are still of great importance, The quality of the emulsions has been much improved, and in the special" nuelear emulsions" the paths even of individual rays can be followed by inspection with the microscope after development and fixing. The length of typical CI.-tracks, for instance, may be 20 or 30 microns in such emulsions. An important application of the photographic method, mainly in medicine, biology, mineralogy and metallurgy, is "autoradiography". In biology, for instance, the radioactive substance is introduced into the organ of an experimen­ tal animal before it is killed. A smooth surface of the specimen is produced, and this is brought into elose contact with a photographic plate. After develop­ ment, the distribution of the black silver indicates the distribution of the radioactive substance within the experimental object. This method was invented by Lacassagne in Paris who investigated the distribution of (CI.-active) polonium in the body of a rat after feeding it a compound of this element. The structure of alloys can be determined autoradiographically either after intro­ duction of a radioactive component into the melt of the met als or after selective activation of a component of the alloy with neutrons (see page 354). Inci­ dentally, a substance made radioactive by addition of a radioactive isotope, and in this way made detectable by instruments for the measurement of radiation, is caUed "labelled ". For the quantitative measurement of activities, the ionization chamber is a elassical instrument. It consists essentially of a box in which a uniform and constant electric field (a few hund red volts) is maintained. The rays to be measured may be emitted by a sample situated within the box, or they may enter the box from outside. In their interaction with gas atoms, the rays remove electrons and in this way ionize the gas. The electrons and ions are pulled to the electrodes and collected there. This flow of electric charge constitutes a current which is measured. Given certain precautions, the strength of the current is proportional to the intensity of the radiation. The familiar electroscope, explained in physics instruction, may be derived from the ionization chamber. Here the field is not kept constant by a con­ tinuous supply of electricity, but after charging up once, the rate of the decrease in the field strength due to the collection of the ions and electrons is measured. This rate is a function of the intensity of the radiation. The decrease may be followed visually by observing the approach of two movable objects initially kept apart by charges of equal sigl1. In the gold leaf electroscope the two

351 leaves are connected with the electrode which is not earthed. They are there­ fore both charged to equal potential and repel each other. As the charge disappears, the two leaves can follow gravity and approach one another. In the more sensitive Lauritsen electroscope a gold covered quartz fibre is repelled by the wall of the electroscope with which it is connected electrically. As the charge is neutralized, the elastic fibre returns to its rest position. The most frequently used measuring instrument is the Geiger counter, which is also derived from the ionization chamber. The counter consists of a cylindrical metal tube, a few cm. long, and an axial metal wire; the electric field lies between counter wall and wire. The rays are usually admitted into the counter through a thin mica window. In contrast to the ionization chamber, the field strength in the counter suffices for " multiplication". That means that the primary electrons produced in ionization are acceIerated to a sufficient degree in the direction of the wire to produce secondary ionization in collisions with gas atoms. Additional electrons are set free in the collisions, they are accelerated as weIl, ionize, etc. The final effect is that for each primary electron set free a very large number of secondary eIectrons (maybe 100 mil­ lion) reach the anode (the wire). Thus an electrical impulse of considerable size is obtained, which is easily registered with an amplifier of moderate power. These impulses are counted electro-mechanically, and the number of impulses per unit time is, in a given experimental arrangement, a measure of the activity of the sampie. When the activity of the sampie is very high, the electromechanical register, which necessarily contains heavy moving parts of considerable inertia, may not be able to follow. Therefore, modern measuring apparatus is always equipped with " scalers". These are electronic devices, which do not contain any heavy moving components, and are actuated by the impulses. For instance, the immediate effect of the impulse may be that a light beam in the device changes its position. Only after the electronic device has "absorbed" a predetermined number of impulses, is a signal forwarded to the electromechani­ cal register. In this case, the indication of the latter has to be multiplied by the scaling ratio, i.e. by the number of impulses needed by the electronic device to produce an outgoing signal. Since the last war, the Geiger counter has been supplemented by the scintillation counter. This instrument is particularly useful for the counting of y-rays. The trouble with y-ray counting with Geiger counters is that y-rays are penetrating and are not easily absorbed. Without absorption, they cannot ionize the gas within the counter. In the scintillation counter, absorption with considerable yield takes place in a rather big transparent body, often consisting of a suitable crystal, where some of the y-ray energy is converted into the energy of visible light. This light is directed on a photoelectric cell, the resulting electric pulses are amplified with an electron multiplier and registered electro­ mechanically through a scaler. For instance, the airplanes prospectil1g for uranium ores are equipped with scintillation counters which are actuated by the y-rays emitted by ore in the ground. In all measurement of radiations of low intensities, " background values " have to be considered. For the purpose, the instrument is worked for a time in the absence of a radioactive sampIe. The indication of the instrument is not zero and must be deducted from the value read in the presence of the sampie. The background, as it is briefly called, is mainly due to two sources. The first is the inevitable contamination of the instrument and surrounding matter with (mostly natural) radioelements. The second is cosmic radiation

352 which incessantly hits the Earth from outer space. The background can be much reduced in magnitude by various means including shielding with thick layers of heavy materials (lead, iron 01' the like). The reduction of the back­ ground is essential in " low level work ".

RADIOCHEMISTRY According to Paneth of Vienna, one of the great masters of the chemistry of radioactive substances, radiochernistry may be defined as the chemistry of the substances which are detected through their radiations. Now radioactive isotopes behave chemically-apart from any isotope effects-in the same way as stable isotopes. Certainly their activity has no infiuence whatever on their chemical properties before the actual disintegration. Therefore at first glance it is not clear what the specific features of radio­ chemistry-as distinct from ordinary chemistry-are. However, it must be remembered that as a consequence of their radioactivity, radioactive substances can be detected and measured in exceedingly small quantities, provided their " specific activity" is high. (" Specific activity" is a magnitude-expressed by different authors in diITerent units-which is a measure of" activity per unit weight ".) In certain conditions, it is even possible to detect a few atoms through the rays emitted by them. Therefore the radiochemist can observe the behaviour of matter in conditions of extreme dilution. The adsorption of atoms, moleeules or ions on solid surfaces from extre­ mely dilute solutions can be studied by radioactive measurements. Equa11y, the concentration of surface-active solutes in the interface water-air can be determined directly provided the solutes contain radioactive atoms within their molecules. The vapour pressure of metals of high boiling points, e.g. iron, can be followed down to fairly low temperatures although in these con­ ditions it is exceedingly sm all. For the purpose, the vapour from a given volume in equilibrium with the labelled metal is condensed on a cold surface, and the amount of substance in the condensate is determined through the radioactivity. For many radiochemical experiments it is necessary to introduce radio­ active atoms into molecules. This is done by "radiosynthetic" methods.

For instance, to obtain radioactive acetic acid, radioactive CO2 is reacted with a methyl Grignard reagent:

H3C . MgBr + 14C02 -+ H3C . 14COO . MgBr

H 3C . 14COO . MgBr -+- H 20 -)- H 3C . 14COOH -+- Mg(OH)Br In this case, a11 the radiocarbon is contained in the carboxylic group. When, on the other hand, radioactive methyl magnesium bromide is reacted with inactive CO2, acetic acid labelled exclusively in the methyl group is produced. It is seen, then, that different kinds of labelled compounds of identical chemical composition (" radioisomers ") exist, and it must be specified in a context which compound is meal1t. The two kinds of acetic acid, for example, behave differently in radiochemical experiments where the chain of the molecule is split and the carbon from the carboxyl group has a " fate " which is different from that of the methyl carbon. This fact is of extraordinary importance in the application of radioactive compounds to the study of the mechanism of organic reactions and in radiobiochemistry. On page 342 we have mentioned processes, in which atoms 01' groups of atoms are exchanged between molecules. The investigation of these processes is one ofthe main tasks ofradiochemistry. In fact, exchange cannot be detected

353 at all by other than isotope methods. For instance, when deuterium gas is mixed with water vapour in absence of a catalyst, and after a time the two substances are separated chemically, it is found that the water has not increased its deuterium content. Obviously, no exchange of hydrogen takes place in the given conditions between water and elementary hydrogen. The result is interpreted as being due to the strength of the link between the two hydrogen atoms in the elementary hydrogen. In contrast, hydrogen is exchanged rapidly between water and the hydroxyl group of alcohols. After mixing heavy water and light ethyl alcohol and then chemically separating the two substances, it is found that the ratio protium/deuterium is nearly the same in the water and in the hydroxyl group of the alcohol, but the ethyl group of the alcohol does not take part in the exchange reaction. (Complete equality of the protium/ deuterium ratio in water and in the hydroxyl group is prevented by the isotope effect which is noticeable in this particular case.) In general, the investigation of exchange reactions supplies information about the nature and strength of chemical bonds.

ISOTOPES IN ANALYTICAL CHEMISTRY The applications of radioactivity to analytical chemistry are very numerous. Generally the assay based on radioactivity is sensitive, simple and rapid. One group of applications is provided by " activation analysis". This was invented by the outstanding Hungarian radiochemist Hevesy and is much used in metallurgy, mineralogy and biology. The principle is that the sampIe is exposed to a current of atomic particles, usually slow neutrons, wh ich react with the nuclei of the element to be determined and convert one of its isotopes into a radioactive species. The radioactivity measured is-other things being equal-proportional to the amount of the element present, as the chance of the neutron" finding" an atom of this element is proportional to the number of its atoms in the sampie. The irradiation is usually carried out in a nuclear reactor but often the neutron flux from a natural source is sufficient. The measured activity is compared with the activity induced in a standard sam pie containing a known amount of the element, which is irradiated and measured in the same conditions. In favourable cases the analysis is non-destructive, i.e. the sampie need not be worked up chemically after irradiation but can be transferred to the measur­ ing instrument without treatment. This applies if in the given conditions no other element present in the sampie is activated to an appreciable extent, or-if this is not the case-if the radiations induced in other elements can be distinguished by their energy or by their half-li fe from the radiation to be measured. Whenever non-destructive analysis is impossible, the sampie is dissolved after irradiation, the element to be measured is isolated in a suitable chemical form and measured. The isolation may be facilitated by previous addition of a larger amount of the same element in an inactive form to act as a "carrier" so that in the chemical operations macroscopic amounts of the element can be handled. Naturally the carrier, being added only after irradiation, cannot influence the activity. Two examples may be mentioned. Very small amounts of mangane se can be determined in otherwise pure aluminium by irradiation with neutrons. Manganese undergoes the reaction given on page 349, and the radiomanganese emits hard (energy-rich) ß-rays with a half-life of 2.58 hours. Though a

354 ß-activity is also induced in aluminium, this decays quickly with a half-life of 2.3 minutes. Therefore, destruction of the sampIe is not necessary provided it is left "to cool down" for some time before measurement. For purposes of geochemistry and cosmochemistry, the determination of trace quantities of elements in meteorites is of interest. Thus small amounts oi gold were determined in iron meteorites. Here many other constituents, inc1uding the iron itself, are activated by neutrons, so it was necessary to dissolve the sampIe after irradiation with the addition of inactive gold carrier, and to isolate the gold for measurement. 10-11 grams of gold can be determined without great difficulty. Another group ofradiochemical methods is analysis by" isotope dilution ". This is applicable where several elements or compounds which are very similar to each other are present in the sampIe. For instance, it is not easy to separate, with quantitative yield, the rare earth elements, e.g. neodymium (Nd) from praseodymium (Pr). Here the following procedure may be applied: radioactive Pr of known activity is added, and some of the Pr is separated from the mix­ ture. Provided quantitative yield is not essential, a pure preparation, free from Nd, can be obtained without great difficulty. The activity of this pre­ paration is measured. The ratio between this activity and the initial activity (if necessary corrected for decay) must at the same time givc the ratio between the amount of Pr in the pure preparation and the initial amount of Pr. As the former can be determined by standard methods (weighing), the initial amount of Pr in the mixed sampIe can be calculated. This isotope dilution method can also be applied to problems in organic and biochemistry (assay of amino acids, of fatty acids, of carbohydrates or of different penicillins in mixtures).

ISOTOPES IN PHYSIOLOGY AND BIOCHEMISTRY Isotopes are also most useful in physiology and biochemistry. For instance, physiologists have long wanted to know the speed of distribution of elements 01' compounds within the animal and human body in vi)lo. How­ ever, in the ca se of elements which occur naturally within the body, this cannot be done by ordinary chemical analysis. How can the time which is needed for calcium from food to appeal' in various bones be guessed as long as this calcium cannot be distinguished from the large amount of calcium pre-existing in the body? This problem can now be solved with radioactively labelIed calcium. The rate at which radiocalcium appears in the different bones of animals after feeding radiocalcium is determined. Similarly, the rate with which water (radioactive water containing tritium) reaehes the different parts of the body after drinking or injection, i.e. the speed of circulation of the water, is now known. The rate of dissolution of pills in the stomach can be estimated by inc1uding a y-active element (radiosodium) and locating the radioelement from outside the body with a scintillation counter. Applications of isotopes to the elucidation of biochemical reaction mechanisms are very numerous. For instance, life as we know it would be impossible without the assimilation of carbon from the air by green plants with the aid of the energy of sunlight (photosynthesis). It has been known for a long time that in the assimilation and reduction of the CO2 carbohydrates (sugars, starch) are formed. However, these are rather complicated molecules, and it cannot be assumed that they are synthesized in one step. There must be simple primary products of photosynthesis which are only later converted

355 to carbohydrates and the other main contituents of plant cells. Again, the problem has been solved with isotopes. Illuminated plants (suspensions of algae) are fed radioactive CO2, the plants are killed, extracted with water or other solvents, the substances dissolved in these extracts are separated chemi­ cally (paper chromatography), and the radioactivity of the individual substances determined. In aseries of experiments by Calvin (U.S.A.) the time given to the plants to carry out" radiophotosynthesis " was shortened more and more so that the plant had less and less time to build up secondary products from the primary products. In the end, (i.e. at the shortest time of radiophotosyn­ thesis) only one product was found to be radioactive: phosphoglyceric acid. This compound is now believed to be the first product of assimilation. It is probably produced by the reaction:

C5Hs0 3 • (H2POJ2 + CO2 + H 20 ->- 2 C3H50 3(H2POJ ribulose diphosphate (RDP) Phosphoglyceric acid (PGA) The ribulose diphosphate is regenerated cyclically. No light is needed for the absorption ofthe CO2, but the RDP can be regenerated only with consumption of light energy. Although at the moment the interest in this research is acade­ mic, there can be no doubt that sooner or later the increased knowledge of the meehanism of photosynthesis will enable us to improve agriculture, and in this way will help solve the urgent problem of fee ding mankind.

ISOTOPES IN TECHNOLOGY Isotopes and radiations are also applied to many branches of technology. An instructive example is the use of isotopes for the estimate of the wear of metals. Moving parts in machinery are used up in friction. For instanee, in combustiol1 engines the pistons, the cylinder walls al1d the bearings lose substance during operation. It is a matter of the greatest importanee for engineers engaged in machine building to know the speed of wear of the differents parts. Traditionally this was determined by taking the engines to pieces at certain time intervals and measuring the dimensions of the various parts-a time consuming, tedious and expensive procedure. Now it is possible to hibel radioactively the various surfaces, and to determine the content of metal dust in the lubricating oil by its radioactivity. This is done intermit­ tently or continuously by carrying the circulating oil in a tube past a measuring instrument (scintillation counter) arranged outside the engine. By this sensitive method the wear can be measured as early as a few minutes after starting up. The contribution of the different parts can, if desired, be detennined separately by labelling these parts with different radioelements and distinguishing between the different radiations. Radioactive thickness gauges are in common use in many industries, particularly in the manufacture of sheet metal. The sheet leaves the metal rollers, between which it is produced, at speeds of many meters per second. Now the pressure of the rollers must be adjusted exactly so that the sheet has the correct thickness. But how can the thickness of the rapidly moving sheet be measured? It is very hard to do with instruments in contact with the sheet. Nowadays a y-ray source is arranged on one side and a measuring instrument (ionization chamber) on the other side of the sheet. The intensity of the penetrated radiation, and therefore the reading on the instrument, depends on the thickness of the sheet. The set-up responds immediately to

356 any changes, and the roller pressure can be at once adjusted accordingly. The adjustment may even be done automatically on the basis of the intensity of the radiation. The method is also used for plastic materials, paper, card­ board and similar industrial products.

RADIOACTIVE DATING Rutherford pointed out that radioactivity enables us to determine the absolute age of rocks. The geologists and pal

First Appearance Age Name of Epoch of (Million Years)

Cambrian ...... Trilobites 500 Ordovician ...... Fishes 360 Carboniferous ...... ; Reptiles 270 Triassic ...... ' Primitive Mammals . 160 Jurassie ...... Birds 120 Eocenium ...... Lemurs 60 Oligocenium ...... Monkeys 45 Miocenium ...... Apes 30 Pleistocenium (= Diluvium) ...... Man 1

Because of the slowness of the transformation of uranium, the uranium­ lead method is suitable only for the determination of great ages. Happily, Libby in the United States has invented a method eminently suitable for the determination of ages in the range of a few thousand years, up to about 50,000 years. This is the radiocarbon method. It may be assumed that the intensity of the cosmic radiation, which impinges on Earth from space, has been constant, at least in recent times. In the interaction of the cosmic rays with the terrestrial atmosphere, neutrons are set free from nuclei. Most of these neutrons react with atmospheric nitrogen (see page 339) to give radiocarbon, which is ~-active with a half-life of 5,700 years. This radiocarbon is present as carbon dioxide. Apart from a small isotope effect the ratio of active to inactive carbon (and therefore the specific activity ofthe carbon) is the same in allliving matter as in atmospheric carbon

357 dioxide. However, after the death of the individual or an organ (heart-wood of trees) assimilation stops, and the specific activity decreases with the half­ life of radiocarbon. By measuring the specific activity (because of its smallness, a difficult experimental task) the " age " of the object (more exactly: the time elapsed since its death) is fixed. The method was first tried out with objects of known age. On the one hand, objects from Egyptian tombs, dated by the historians, were used. On the other hand, pieces of old trees were tested, where the average age could be determined by counting rings. In both ca ses, very satisfactory results were obtained. Thereafter, the method has been used with confidence to date many objects of history and prehistory. The specific activity of charcoal from the French caves (Lascaux) adorned with the wall paintings of prehistoric man lead to an age of 15,500 900 years. Similarly, the linen wrapping of one of the Dead Sea Scrolls (book of Isaiah) proved to be 1,920 J.- 350 years old, and woven Indian sandals, from a " shoe shop" in Oregon, U.S.A., 9,050 350 years.

NUCLEAR FISSION At first, nuc1ear transformations on Earth were observed on a sma1r scale only. In terms of mass 01' weight, not much substance transformed spontaneously per unit time in preparations of radioactive substances. Even the accelerators are not efficient for large sc ale nuc1ear transformation because only a smaH fraction of the charged partieles hitting the target effect nuc1ear reactions. The situation changed dramaticaHy in 1939 after the discovery by Hahn and Strassmann in Berlin that uranium nuc1ei may be split into two big fragments on absorption of a neutron, and the subsequent discovery of Joliot, Halban and Kowarski in Paris that neutrons are emitted in this process. The splitting of the uranium nuc1eus (" nuc1ear fission ") was a previously unknown type of nuc1ear reaction. The possibility of fission, like that of CI.-decay, is ultimately due to the Coulomb repulsion of the protons within highly charged nuc1ei. Bohr showed a little later that it is the nuc1eus of uranium 235 which is capable of fission by slow neutrons (the more important case in peaceful applications) as weH as by fast neutrons, while the more abundant isotope uranium 238 is split by fast neutrons only. In each fission an amount of energy, tremendous even by nuc1ear standards, is set free-on an average about 150 MeV. But the main interest of fission lies in the emission of 2-3 neutrons per nuc1eus split. It was suggested by Joliot that these neutrons could effect a chain reaction. They might split fmther nuc1ei of uranium, more neutrons would be emitted, these would split more uranium etc. Provided the loss of neutrons in side reactions were kept down sufficiently, such a chain reaction would be self-sustaining, and finally convert a large part of the uranium initially present, i.e. transform nuc1ei on a large scale. About as much energy is released in the fission of 1 kg. of uranium 235 as in the combustion of 2,500 tons of coal or in the explosion of 13,000 tons of dynamite. The two fragments produced in fission are not vastly different in mass (typical mass ratio 2:3). The fragments are, of course, nothing other than smalleI' atomic nuc1ei surrounded by some orbital electrons, and they sub­ sequently capture more electrons until neutral atoms are formed. The chemical nature of these atoms varies within wide limits because the distribution of the initial nuc1ear charge (92 protons) and the uranium over the fission fragments varies. Among others, various isotopes of krypton, xenon, rubidium, barium,

358 cesium and iodine are formed. Almost without exception the atoms are ß• active. The mass equivalent of the energy released in fission is readily calculated from the Einstein law, i.e. E = mez. The energy supplied by 1 kg. of uranium 26 26 6 235, i.e. 1000/235 moles, or I""-' 6 . 10 /235 atoms, is about 6· 10 • 150· 10 / 32 20 2 235 (I""-' 3.8· 10 )eV, or 6.1 . 10 erg. Inserting into m = E/e , one obtains: 20 20 m 6.1 . 10 / 9 . 10 I""-' 0.68 g Thus in the fission of 1 kg. of uranium 235 about 680 mg. of material are converted into an equivalent amount of heat energy or radiation energy. It is instructive to compare this figure with the energy which would be gained if it were possible to convert 1 kg. of uranium (or of any other substance) entirely into heat or radiation energy. From Einstein's law we conclude: 2 20 23 9 13 E = me = 1000·9· 10 = 9 . 10 erg I""-' 3.2 . 10 kwh I""-' 2.2 . 10 kcal

NUCLEAR REACTORS A chain reaction was first realized by Fermi in Chicago in 1942. The nuclear reactor, or pile in this case, consisted of several tons of uranium and uranium oxide of the natural isotopic compositioll and an even larger amount of carbon (graphite). The latter served as a moderator. All materials must be of the utmost purity to prevent " parasitic " absorption of neutrons. The chain reaction was mediated by the slow neutrons and therefore developed fairly slowly. Its velo city could be controlled at will with rods consisting of absorbing material (boron, cadmium). Some other reactors, also using slow neutrons, have heavy water as a moderator instead of carbon. Even ordinary water may be used (although many neutrons are lost in capture by light hy­ drogen) provided the neutron balance is improved by employing uranium enriched with uranium 235. The system "enriched uranium-ordinary water" is applied for instance in a widely used type of reactor for purposes of research, the so-called " swim­ ming pool" reactor. Here plates of enriched uranium alloy are immersed in water. This serves at the same time as moderator, as coolant and as radiation shield for the protection of personnel. In typical cases the heat production in a swimming pool reactor is 2,000 kilowatts. The heat produced in reactors for power production mayaiso be carried away by water. The water may be evaporated and the steam used to drive a turbine or, alternatively, the cooling water may be circulated under pressure in a primary circuit and transmit its heat to water in a secondary circuit in a heat exchanger outside the reactor. Only in the secondary circuit is the water allowed to boil; the steam is then not contaminated by fission products. In a further type of power reactor the uranium is cooled by CO2 gas, and the hot gas is used to raise steam (Calder Hall and similar stations). A number of reactors with electric capacities between 100 and 1000 Mw are now in ope­ ration or construction in various countries. Two factors which at present limit the temperature to which the uranium may be heated, and therefore the power, are: 1) due to changes in the crystal structure, uranium tends to defonn and lose its shape at higher temperatures; and 2) the speed of corrosion of the uranium by coolant increases rapidly with temperature. Some protection against this attack is afforded by cladding, e.g. with certain alloys of magnesium or of zirconium.

359 In the nuclear re ac tors large quantities of radioactive fission fragments appear as by-products. The disposal of these dangerous substances is a major problem in atomic energy. Usually the radioactive waste is stored in steel containers. However, many of these cheap fission products can be put to good scientific, medical and technical uses. Examples are ~-active strontium 90 and ~-and y-active cesium 137, both long-lived. In addition, radioactive substances of other kinds can be made cheaply by introducing into the reactors elements which are activated by the free neutrons. Thus ß-active sodium 24 (half-life 15 hours), much used in research, is made by neutron capture according to: 23n Na + Ion ---+ 24n Na When uranium 235 or plutonium 239 (see below) is to be used as a nuclear explosive (weapon), fast, rather than slow, neutrons are employed to propagate the reaction chain as rapidly as possible. The atom bombs consist of the fissile element but contain no moderator. lt may be added without further discussion that in certain circumstances research or power reactors mayaiso be operated with fast neutrons.

TRANSURANIC ELEMENTS In addition to the fission fragments, transuranic elements are also inevitably produced in the piles. An important sequence of reactions is started by those slow neutrons which are captured by uranium 238 rather than by uranium 235 and therefore do not give fission:

23892U + Ion ---+ 23992U ~ 23992U -----+ 23993Np (neptunium) 23 min

~ 239 239 93Np 94PU (plutonium) 2.3 days Plutonium 239 is long-lived (half-life 24,400 years) and therefore accumulates in the pile. The chemical properties of neptunium and plutonium are different from those of uranium. For instance Pu is more easily reduced to the 4-valent and 3-valent state than U. Therefore, the plutonium can be separated chemically from the neutron-irradiated uranium and obtained in a pure state in large quantity. Like uranium 235, plutonium 239 can be used as a nuclear explosive or as concentrated nuclear fuel. Numerous other isotopes of elements 93 and 94 have also been made recently in piles or in the targets of accelerators. Moreover, one or more isotopes of elements 95 to 103 (americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, lawrencium; chemical symbols Am, Cm, Bk, Cf, Es, Fm, Mv, No, Lw) have been prepared. The isolation or at least identification of these nuclides is often quite difficuIt as they are present only in exceedingly small quantities, and their half-lives are usually very short. In particular, the more highly charged nuclei are subject to decay by spontaneous fission, a process discovered for uranium by Flerov and

360 Petrzhak in Moscow in 1940. The discovery of the last two elements (No and Lw) has not yet been generally accepted. The chemica1 properties of the transuranic elements show considerable similarity. All tend to appear as trivalent cations. They also resemble the trivalent ions of elements 89 to 92. For this reason, Seaborg in California (who together with the Italian Segre discovered most of the transuranic ele­ ments) suggests that the elements 89 to 103 form a group by themselves similar to the group of the rare earths, the " lanthanides ". The new group is called the "actinide" group after actinium (element 89), its first member. It is thought that the elements of the group are similar because the l1ewly added electrons enter an incomplete internal shell (the V shell) rather than the outermost shell. In the ca se of the lanthanides a deeper incomplete shell, the 4f shell, is filled. Element 104, if it can be prepared and investigated, is expected to be the first element outside the actinide group, and to resemble hafnium. However, in contrast to the lanthanides, the first members of the actinides are more similar to those elements which, if the actinide concept were 110t accepted, would share a vertical column of the periodic table with them, than they are to each other. For example, the chemistry of protactinium is stri­ kingly similar to that of tantalum, and the chemistry of thorium to that of zirconium and hafnium.

NUCLEAR FUSION It was mentioned on page 349 that the readiness of neutrons to enter nuclei and to react with them is due to their lack of charge which prevents Coulomb repulsion. This repulsion can be overcome by charged particles (ions) only if they have very high kinetic energy. Sufficient kinetic energy may be carried by ions which have just been formed in nuclear reactions and have not yet lost this energy in Coulomb interaction with electrons. In this way the reaction of et.:-particles with alu­ minium nuclei is explained (see page 349). Or else ions are accelerated elec­ trically until they have sufficient energy (see page 350). But in neither case is a self-sustaining nuclear chain reaction started since the chance of rapid energy loss in the interaction with electrons is enormous. Nuclear chain reactions mediated by charged particles are expected, however, as so on as the average kinetic energy of these ions is raised to a suffi­ ciently high level, that is, as soon as these particles are heated to a high enough temperature (millions of degrees Celsius). Such temperatures exist in the inner parts of fixed stars, so that " thermonuclear reactions " can and do proceed there. There is no doubt that these reactions supply the energy which is continuously radiated by the stars. For instance, the temperature near the centre of the sun is estimated as 20 million degrees. Thermonuclear reactions occur most readily with nuclei of low charge because he re the Coulomb repulsion is smallest. In the ca se of such nuclei the reactions supplying most energy (the most exothermic reactions) oftenlead to a combination of nuclei. A wen known reaction of this kind is the " fusion" of one nucleus of deuterium (a deuteron) with one nucleus of tritium (a triton) to give one nucleus of helium and a neutron:

21H + \H -+ 42He + \n The energy released in this process has the high value of 17.7 MeV. Thus in thermonuclear reactions, in contrast to fission, heavier nuclei are synthesized.

361 A system capable of a self-sustaining thermonuclear reaction must be ignited. The necessary temperature can be produced on earth through un­ controlled fission, i.e. in an atomic bomb. The hydrogen bomb therefore consists of at least two main components: a fission bomb containing uranium 235 or plutonium 239, and a system containing hydrogen isotopes (deuterium or tritium) and capable of a thermonuclear reaction. The explosion of a hydrogen bomb cannot be controlled. Much work has been carried out in recent years to initiate thermonuclear reactions which can be controlled and therefore exploited for the production of useful heat and electricity. However, the goal has not yet been reached. A main obstacle lies in the fact that no materials for containment exist which resist the reaction temperature.

RADIATION CHEMISTRY On page 353 et seq. we have treated radiochemistry as ifthe radiauons had no effects on the chemical systems containing the radioelements. This assump­ tion is justified as long as the intensity of the radiation acting on the chemical system is inappreciable, i.e. as long as the activity of the radioactive substance in the system is small. As a rule, the activities are ample for measurement at a level at which no chemical effects are noticeable. The chemical changes produced when the assumption does not hold are the subject of a new bral~ch of science-" radiation chemistry". The radiation chemist studies the chemical effects of ionizing radiations. Ionizing radiations are radiations which, in contrast to long wave radiation (radiowaves), to visible light and to ultra-violet light, ionize atoms during the passage through matter. The transfer of energy must be sufficient to remOve electrons from at least SOme atoms. In other atoms electrons will be raised to higher energy levels (orbits). These atoms are excited but not ionized. X-rays, y-rays, (X-rays, ß-rays and neutron rays belong to the ionizing radiations. It is true that neutrons, because of their lack of charge, do not ionize directly. But in the slowing-down of fast neutrons, nuclei (e.g. protons) gain a lot of kinetic energy so that they act as ionizing rays. In the absorption of slow neutrons by nuclei the binding energy is usually emitted in the form of photons -again an ionizing radiation. The photochemist studies the chemical effects of visible and ultra-violet light. It is an important feature of photochemistry that the action of the light is selective. Radiation of a particular wave length and frequency is chemically active only if 1) it is absorbed, and 2) the absorbed energy is sufficient for the given reaction. These two limitations do not appear in radiation chemistry. Ionizing radiation is always absorbed, though the amount of absorbed energy depends on the conditions. Moreover, because of the large energy carried by ionizing corpuscles and photons, the energy transfer to individual atoms has in practice no limit. Therefore, any re action is possible in radiation chemistry, provided, of course, the system does not spontaneously revert to its original state. For instance, dissolved sodium fluoride cannot undergo any change because any conceivable decomposition will reverse itself spontaneously. Many pure substances and aqueous solutions have been investigated. The yields are expressed through the "G-values". The G-value indicates the number of molecules decomposed per 100 eV of absorbed radiation energy. A most important example is that of air-saturated water. On irradiation with y­

rays, in addition to H 20 2 and H 2, free hydrogen atoms and hydroxyl radicals are formed with a G-value of 4.5 for the decomposition of the water.

362 The hydrogen atoms act as powerful reducing agents, the hydroxyl radicals i as powerful oxidizing agents, If the water contains as a solute an organic sub­ stance, this is oxidized by the hydroxyl radicals. On the other hand, if pure water is irradiated, the atoms and radicals react among themselves: they must ultimate1y disappear in reactions among themselves, or with the "mole­ cular products" H 2 and H 20 2• Already radiation chemistry has found a few technical applications. Organic free radicals are also formed by irradiation, and these radicals may serve as initiators in polymerizations by chain reactions. In this way, useful polymers are obtained. In other processes, the properties of pre-existing high polymers are modified by irradiation. Radicals of the high polymers are made, and these react to give bridges with adjoining molecules. The bridged (vulcanized) polymers may exceed the normal products with respect to mechanical or thermal resistance. The process has been applied to various rubbers and to plastics, including polyethylene. Irradiated polyethylene has improved heat resistance so that underground cables insulated by it, rather than by ordinary polyethylene, can carry more electric current. Rubber may also be vulcanized by radiation.

RADJA nON HAZARDS Living matter is surprisingly sensitive to ionizing radiation. For instal1Ce human beings are killed by a whole-body irradiation with so small a dose of y-rays or X-rays that the energy absorbed increases the body temperature by as little as one thousandth of a degree Celsius. More primitive animals and plants are somewhat less sensitive, and bacteria and viruses even less so. Some of the effects on humans are: damage to the intestinal mucosa so that food is not absorbed properly, impairment of the production of blood corpuscles, damage to the system producing y-globulins (anti-bodies-im­ portant in the defence against infections), damage to the reproductive system, damage to the lens of the eye, production of cancers, including leukemia. Generally, developing organisms (children, particularly unborn children) are more sensitive than adult organisms. Not only is the irradiated individual affected but even the progeny may suffer from the damage to the parental germ cells (genetic damage). The action of radiation on living things (" radiation biological " action) must ultimately be due to chemical action. The high sensitivity shows that the effective agents (probably hydroxyl radicals and similar substances) pre­ ferentially attack molecules of central importance to life. Many radiation biologists think that the attack is directed mainly against the nucleic acids which control the processes of life and are also responsible for heredity. From a practical point of view, external and internal irradiation may be distinguished. External radiation sources (sources outside the body) are effective mainly when they emit penetrating radiation (e1ectromagnetic and neutron radiation). Internal irradiation takes place after radioactive substances have been taken up by the body, for instance with food or water, and have been deposited (stored) in organs. In this case, even non-penetrating rays (CI. or ß rays) transfer all their energy to the body. The danger from internal deposition depends mainly on the properties of the radioactive substance. Firstly, the nature and the energy of the rays and the half-life of the substance are clearly important. Secondly, the extent to which the particular element is taken up by important radio-sensitive organs

363 is decisive. For instance radiosodium is not very dangerous as it is fairly quickly eliminated with the urine. An equal amount (in terms of radioactivity) of radiocalcium is much more dangerous as it is deposited along with non­ active calcium in the mineral substance of the bones and stays there for a long time. Radioiodine is extremely dangerous as it is concentrated in the thyroid gland, a very radio sensitive organ. To prevent radiation damage, rules have been worked out by an Inter­ national Commission of Radiation Protection (ICRP). "Tolerance doses" have been published which are permissible for individuals exposed professional­ ly to penetrating radiations. Maximum amounts of the various radioelements permitted within a human body have been elaborated; the concentrations of the radioactive nuclides in drinking water and in air (as a vapour or as dust), which lead to the maximum amounts in the body, have also been computed. The tolerance doses and tolerance concentrations applicable to persons not exposed professionally are lower, and the figures applicable to the general population are lower still. Particular care is needed, of course, in connection with nuclear reactors, and all measures must be taken to prevent their getting out of control. More­ over, after separation from the nuclear fuel the strongly radioactive fission products must be stored in safety. Storage tanks made of steel are generally used to contain the unwanted fission products and to let them decay. Caution is also indicated in laboratories and in hospitals using radioactive substances. In addition it must be emphasized that X-radiation is just as dangerous as radiation from radioactive substances, and that all unnecessary exposure to X-rays (for instance in shoe shops) should be avoided. The destruction of living matter by ionizing radiations is used in cancer therapy where it is essential to kill all cancer cells. The selective destruction of the malignant cells is often made easier by the fact that rapidly growing tissue is as a rule more sensitive to ionizing radiation than stationary tissue. Obviously the utility of this action for the body as a wh oie is not in contra­ diction to the rule that ionizing radiation is harmful to life. The death of the cancer cells is an advantage to the organism. Ionizing rays are also used for disinfection and sterilization. Harmful organisms (bacteria, worms, insects) are killed or at least rendered incapable of reproduction. This technique, which does not involve heating, may be applied to food, drugs or medical instruments.

BIBLIOGRAPHY

1. G.A. BOYD, Autoradiography in Bio1ogy and Medicine. New York 1955. 2. E. BRODA, Radioaktive Isotope in der Biochemie. Wien 1958. 3. E. BRODA, Radioactive Isotopes in Biochemistry. Amsterdam 1960. 4. E. BRODA and T. SCHÖNFELD, Die technischen Anwendungen der Radioaktivität. Berlin 1956; Leipzig 1962. 5. G.B. COOK and J.F. DUNcAN, Modern Radiochemical Practice. Oxford 1952. 6. W. FINKELNBURG, Einführung in die Atomphysik. Berlin 1962. 7. G. FRIEDLANDER and J.W. KENNEDY, Nuclear and Radiochemistry. New York 1955. 8. E. FÜNFER and H. NEUERT, Zählrohre und Szintillationszahler. Karlsruhe 1959.

364 9. M. HAISSINSKY, La chimie nucleaire et ses applications. Paris 1957. 10. F. HECHT and M.K. ZACHERL (Editors), Handbuch der mikrochemischen Methoden, Band 2: Radiochemische Methoden der Mikrochemie. Wien 1955. Authors: E. BRoDA and T. SCHÖNFELD; B. KARUK, T. BERNERT and K. LINTNER; H. LAuDA. 11. L. HERFORTH, H. KOCH, Radiophysikalisches und radiochemisches Grundpraktikum. Berlin 1961. 12. I. JOUOT-CURIE, Les radioelements natureIs. Paris 1946. 13. M.D. KAMEN, Isotopic Tracers in Biology. New York 1957. 14. V. KMENT, A. KUHN, Technik des Messens radioaktiver Strahlung. Leipzig 1960. 15. S.A. KORFF, Electron and Nuclear Counters. New York 1955. 16. L. MELANDER, Isotope Effects on Reaction Rates. New York 1960. 17. C.F. POWELL and G.P.S. OCCHIAUNI, Nuclear Physics in Photographs. Oxford 1947. 18. W. RIEZLER, Einführung in die Kernphysik. München 1959. 19. K. SCHMEISER, Radioaktive Isotope. Berlin 1957. 20. A.c. WAHL, N.A. BONNER (Editors), Radioactivity Applied to Chemistry. New York 1951. 21. F. WEYGAND and H. SIMON, Herstellung isotopenhaItiger organischer Verbindungen, in: Methoden der organischen Chemie (Houben-Weyl), Band IV/2. Stuttgart 1955.

365 FROM THE CATALOGUE

ABILITY AND EDUCATIONAL OPPORTUNITY Papers by J. Ferrez, J. Floud, T. Husen, K. Härnqvist. J. Vaizey, P. de Wolft, D. Wolfte (December 1961) 212 pages (15x21 cm) 15 s. $ 2.50 NF 10.00 Sw. fr.10.00 DM 8.30

NEW THINKING IN SCHOOL MATHEMATICS (May 1961) 248 pages (demy 8vo) 15 s. $ 2.50 NF 10.00 Sw. fr. 10.00 DM 8.50

SYNOPSES FOR MODERN SECONDARY SCHOOL MATHEMATICS 310 pages (demy 8vo) Free on request

SCHOOL MATHEMATICS IN O.E.E.C. COUNTRIES Summaries 116 pages (demy 8vo) Free on request

MATHEMATICS FOR PHYSICISTS AND ENGINEERS (December 1961) 224 pages (demy 8vo) Free on request

NEW THINKING IN SCHOOL CHEMISTRY (July 1961) 216 pages (demy 8vo) Free on request

A MODERN APPROACH TO SCHOOL PHYSICS 24 pages (crown 4to) Free on request

NEW THINKING IN SCHOOL BIOLOGY (June 1963)

SUPPLY TRAINING AND RECRUITMENT OF SCIENCE AND MATHEMATICS TEACHERS (October 1962) Free on request

TEACHING THROUGH TELEVISION (July 1960) 72 pages (demy 8vo) 5 s. $ 0.75 NF 3.00 Sw. fr. 3.00 DM 2.60

TELEVISION FOR SCHOOL SCIENCE Ashridge (July 1960) 182 pages (demy 8vo) Free on request

POLICY FOR SCHOOL SCIENCE (September 1961) 38 pages (crown 4to) Free on request