# The Full Wiki

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

# Encyclopedia

Radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter"). This is a stochastic process on the atomic level, in that it is impossible to predict when a given atom will decay,[1] but given a large number of similar atoms the decay rate, on average, is predictable.

The SI unit of activity is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of GBq (gigabecquerel, 1 x 109 decays per second) or TBq (terabecquerel, 1 x 1012 decays per second) are commonly used. Another unit of radioactivity is the curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with of one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 1010 Bq. The use of Ci is presently discouraged by the SI.

## Explanation

The trefoil symbol is used to indicate radioactive material.

The neutrons and protons that constitute nuclei, as well as other particles that may approach them, are governed by several interactions. The strong nuclear force, not observed at the familiar macroscopic scale, is the most powerful force over subatomic distances. The electrostatic force is almost always significant, and in the case of beta decay, the weak nuclear force is also involved.

The interplay of these forces produces a number of different phenomena in which energy may be released by rearrangement of particles. Some configurations of the particles in a nucleus have the property that, should they shift ever so slightly, the particles could rearrange into a lower-energy arrangement and release some energy. One might draw an analogy with a snowfield on a mountain: while friction between the ice crystals may be supporting the snow's weight, the system is inherently unstable with regard to a state of lower potential energy. A disturbance would thus facilitate the path to a state of greater entropy: the system will move towards the ground state, producing heat, and the total energy will be distributable over a larger number of quantum states. Thus, an avalanche results. The total energy does not change in this process, but because of the law of entropy, avalanches only happen in one direction and that is towards the "ground state" – the state with the largest number of ways in which the available energy could be distributed.

Such a collapse (a decay event) requires a specific activation energy. For a snow avalanche, this energy comes as a disturbance from outside the system, although such disturbances can be arbitrarily small. In the case of an excited atomic nucleus, the arbitrarily small disturbance comes from quantum vacuum fluctuations. A radioactive nucleus (or any excited system in quantum mechanics) is unstable, and can thus spontaneously stabilize to a less-excited system. The resulting transformation alters the structure of the nucleus and results in the emission of either a photon or a high-velocity particle which has mass (such as an electron, alpha particle, or other type).

## Discovery

Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel, while working on phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in cathode ray tubes by X-rays might be connected with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent salts on it. All results were negative until he used uranium salts. The result with these compounds was a deep blackening of the plate. These radiations were called Becquerel Rays.

It soon became clear that the blackening of the plate had nothing to do with phosphorescence, because the plate blackened when the mineral was in the dark. Non-phosphorescent salts of uranium and metallic uranium also blackened the plate. Clearly there was a form of radiation that could pass through paper that was causing the plate to become black.

At first it seemed that the new radiation was similar to the then recently discovered X-rays. Further research by Becquerel, Marie Curie, Pierre Curie, Ernest Rutherford and others discovered that radioactivity was significantly more complicated. Different types of decay can occur, but Rutherford was the first to realize that they all occur with the same mathematical approximately exponential formula (see below).

The early researchers also discovered that many other chemical elements besides uranium have radioactive isotopes. A systematic search for the total radioactivity in uranium ores also guided Marie Curie to isolate a new element polonium and to separate a new element radium from barium. The two elements' chemical similarity would otherwise have made them difficult to distinguish.

The danger classification sign of radioactive materials
Ionizing radiation hazard symbol (recently introduced).[2]
Alpha particles may be completely stopped by a sheet of paper, beta particles by aluminum shielding. Gamma rays can only be reduced by much more substantial barriers, such as a very thick layer of lead.
Different types of decay of a radionuclide. Vertical: atomic number Z, Horizontal: neutron number N

The dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when electrical engineer and physicist Nikola Tesla intentionally subjected his fingers to X-rays in 1896. He published his observations concerning the burns that developed, though he attributed them to ozone rather than to X-rays. His injuries healed later.

The genetic effects of radiation, including the effects on cancer risk, were recognized much later. In 1927 Hermann Joseph Muller published research showing genetic effects, and in 1946 was awarded the Nobel prize for his findings.

## Types of decay

As for types of radioactive radiation, it was found that an electric or magnetic field could split such emissions into three types of beams. For lack of better terms, the rays were given the alphabetic names alpha, beta and gamma, still in use today. While alpha decay was seen only in heavier elements (atomic number 52, tellurium, and greater), the other two types of decay were seen in all of the elements.

In analyzing the nature of the decay products, it was obvious from the direction of electromagnetic forces that alpha rays carried a positive charge, beta rays carried a negative charge, and gamma rays were neutral. From the magnitude of deflection, it was clear that alpha particles were much more massive than beta particles. Passing alpha particles through a very thin glass window and trapping them in a discharge tube allowed researchers to study the emission spectrum of the resulting gas, and ultimately prove that alpha particles are helium nuclei. Other experiments showed the similarity between beta radiation and cathode rays; they are both streams of electrons, and between gamma radiation and X-rays, which are both high energy electromagnetic radiation.

Although alpha, beta, and gamma are most common, other types of decay were eventually discovered. Shortly after discovery of the neutron in 1932, it was discovered by Enrico Fermi that certain rare decay reactions yield neutrons as a decay particle. Isolated proton emission was eventually observed in some elements. Shortly after the discovery of the positron in cosmic ray products, it was realized that the same process that operates in classical beta decay can also produce positrons (positron emission), analogously to negative electrons. Each of the two types of beta decay acts to move a nucleus toward a ratio of neutrons and protons which has the least energy for the combination. Finally, in a phenomenon called cluster decay, specific combinations of neutrons and protons other than alpha particles were spontaneously emitted from atoms on occasion.

Still other types of radioactive decay were found which emit previously seen particles, but by different mechanisms. An example is internal conversion, which results in electron and sometimes high energy photon emission, even though it involves neither beta nor gamma decay.

## Decay modes in table form

Radionuclides can undergo a number of different reactions. These are summarized in the following table. A nucleus with mass number A and atomic number Z is represented as (A, Z). The column "Daughter nucleus" indicates the difference between the new nucleus and the original nucleus. Thus, (A–1, Z) means that the mass number is one less than before, but the atomic number is the same as before.

Mode of decay Participating particles Daughter nucleus
Decays with emission of nucleons:
Alpha decay An alpha particle (A=4, Z=2) emitted from nucleus (A–4, Z–2)
Proton emission A proton ejected from nucleus (A–1, Z–1)
Neutron emission A neutron ejected from nucleus (A–1, Z)
Double proton emission Two protons ejected from nucleus simultaneously (A–2, Z–2)
Spontaneous fission Nucleus disintegrates into two or more smaller nuclei and other particles -
Cluster decay Nucleus emits a specific type of smaller nucleus (A1, Z1) smaller than, or larger than, an alpha particle (AA1, ZZ1) + (A1,Z1)
Different modes of beta decay:
Beta-Negative decay A nucleus emits an electron and an antineutrino (A, Z+1)
Positron emission, also Beta-Positive decay A nucleus emits a positron and a neutrino (A, Z–1)
Electron capture A nucleus captures an orbiting electron and emits a neutrino — The daughter nucleus is left in an excited and unstable state (A, Z–1)
Double beta decay A nucleus emits two electrons and two antineutrinos (A, Z+2)
Double electron capture A nucleus absorbs two orbital electrons and emits two neutrinos — The daughter nucleus is left in an excited and unstable state (A, Z–2)
Electron capture with positron emission A nucleus absorbs one orbital electron, emits one positron and two neutrinos (A, Z–2)
Double positron emission A nucleus emits two positrons and two neutrinos (A, Z–2)
Transitions between states of the same nucleus:
Isomeric transition Excited nucleus releases a high-energy photon (gamma ray) (A, Z)
Internal conversion Excited nucleus transfers energy to an orbital electron and it is ejected from the atom (A, Z)

Radioactive decay results in a reduction of summed rest mass, once the released energy (the disintegration energy) has escaped. The energy carries mass with it (see mass in special relativity) according to the formula E = mc2. The decay energy is initially released as kinetic energy of the emitted particles. Later these particles come to thermal equilibrium with their surroundings. The energy remains associated with a measure of mass of the decay system invariant mass, in as much as the kinetic energy of emitted particles, and, later, the thermal energy of the surrounding matter, contributes also to the total invariant mass of systems. Thus, the sum of rest masses of particles is not conserved in decay, but the system mass or system invariant mass (as also system total energy) is conserved.

## Decay chains and multiple modes

The daughter nuclide of a decay event may also be unstable (radioactive). In this case, it will also decay, producing radiation. The resulting second daughter nuclide may also be radioactive. This can lead to a sequence of several decay events. Eventually a stable nuclide is produced. This is called a decay chain.

Gamma-ray energy spectrum of 238U (inset). Gamma-rays are emitted by decaying nuclides, and the gamma-ray energy can be used to characterize the decay (which nuclide is decaying to which). Here, using the gamma-ray spectrum, several nuclides which are typical of the decay chain have been identified: 226Ra, 214Pb, 214Bi.

An example is the natural decay chain of 238U which is as follows:

• decays, through alpha-emission, with a half-life of 4.5 billion years to thorium-234
• which decays, through beta-emission, with a half-life of 24 days to protactinium-234
• which decays, through beta-emission, with a half-life of 1.2 minutes to uranium-234
• which decays, through alpha-emission, with a half-life of 240 thousand years to thorium-230
• which decays, through alpha-emission, with a half-life of 77 thousand years to radium-226
• which decays, through alpha-emission, with a half-life of 1.6 thousand years to radon-222
• which decays, through alpha-emission, with a half-life of 3.8 days to polonium-218
• which decays, through alpha-emission, with a half-life of 3.1 minutes to lead-214
• which decays, through beta-emission, with a half-life of 27 minutes to bismuth-214
• which decays, through beta-emission, with a half-life of 20 minutes to polonium-214
• which decays, through alpha-emission, with a half-life of 160 microseconds to lead-210
• which decays, through beta-emission, with a half-life of 22 years to bismuth-210
• which decays, through beta-emission, with a half-life of 5 days to polonium-210
• which decays, through alpha-emission, with a half-life of 140 days to lead-206, which is a stable nuclide.

Some radionuclides may have several different paths of decay. For example, approximately 36% of bismuth-212 decays, through alpha-emission, to thallium-208 while approximately 64% of bismuth-212 decays, through beta-emission, to polonium-212. Both the thallium-208 and the polonium-212 are radioactive daughter products of bismuth-212, and both decay directly to stable lead-208.

## Occurrence and applications

According to the Big Bang theory, stable isotopes of the lightest five elements (H, He, and traces of Li, Be, and B) were produced very shortly after the emergence of the universe, in a process called Big Bang nucleosynthesis. These lightest stable nuclides (including deuterium) survive to today, but any radioactive isotopes of the light elements produced in the Big Bang (such as tritium) have long since decayed. Isotopes of elements heavier than boron were not produced at all in the Big Bang, and these first five elements do not have any long-lived radioisotopes. Thus, all radioactive nuclei are therefore relatively young with respect to the birth of the universe, having formed later in various other types of nucleosynthesis in stars (particularly supernovae), and also during ongoing interactions between stable isotopes and energetic particles. For example, carbon-14, a radioactive nuclide with a half-life of only 5730 years, is constantly produced in Earth's upper atmosphere due to interactions between cosmic rays and nitrogen.

Radioactive decay has been put to use in the technique of radioisotopic labeling, which is used to track the passage of a chemical substance through a complex system (such as a living organism). A sample of the substance is synthesized with a high concentration of unstable atoms. The presence of the substance in one or another part of the system is determined by detecting the locations of decay events.

On the premise that radioactive decay is truly random (rather than merely chaotic), it has been used in hardware random-number generators. Because the process is not thought to vary significantly in mechanism over time, it is also a valuable tool in estimating the absolute ages of certain materials. For geological materials, the radioisotopes and some of their decay products become trapped when a rock solidifies, and can then later be used (subject to many well-known qualifications) to estimate the date of the solidification. These include checking the results of several simultaneous processes and their products against each other, within the same sample. In a similar fashion, and also subject to qualification, the rate of formation of carbon-14 in various eras, the date of formation of organic matter within a certain period related to the isotope's half-live may be estimated, because the carbon-14 becomes trapped when the organic matter grows and incorporates the new carbon-14 from the air. Thereafter, the amount of carbon-14 in organic matter decreases according to decay processes which may also be independently cross-checked by other means (such as checking the carbon-14 in individual tree rings, for example).

The decay rate, or activity, of a radioactive substance are characterized by:

Constant quantities:

• half life — symbol t1/2 — the time taken for the activity of a given amount of a radioactive substance to decay to half of its initial value.
• decay constant — symbol λ — the inverse of the mean lifetime.

Although these are constants, they are associated with statistically random behavior of populations of atoms. In consequence predictions using these constants are less accurate for small number of atoms.

Time-variable quantities:

• Total activity — symbol A — number of decays an object undergoes per second.
• Number of particles — symbol N — the total number of particles in the sample.
• Specific activity — symbol SA — number of decays per second per amount of substance. (The "amount of substance" can be the unit of either mass or volume.)

These are related as follows:

$t_{1/2} = \frac{\ln(2)}{\lambda} = \tau \ln(2)$
$A = - \frac{dN}{dt} = \lambda N$
$S_A a_0 = - \frac{dN}{dt}\bigg|_{t=0} = \lambda N_0$

where a0 is the initial amount of active substance — substance that has the same percentage of unstable particles as when the substance was formed.

### Activity measurements

The units in which activities are measured are: becquerel (symbol Bq) = number of disintegrations per second; curie (Ci) = 3.7 × 1010 disintegrations per second. Low activities are also measured in disintegrations per minute (dpm).

## Decay timing

The decay of an unstable nucleus is entirely random and it is impossible to predict when a particular atom will decay.[1] However, it is equally likely to decay at any time. Therefore, given a sample of a particular radioisotope, the number of decay events −dN expected to occur in a small interval of time dt is proportional to the number of atoms present. If N is the number of atoms, then the probability of decay (−dN/N) is proportional to dt:

$\left(-\frac{dN}{N} \right) = \lambda \cdot dt.$

Particular radionuclides decay at different rates, each having its own decay constant (λ). The negative sign indicates that N decreases with each decay event. The solution to this first-order differential equation is the following function:

$N(t) = N_0\,e^{-{\lambda}t} = N_0\,e^{-t/ \tau}. \,\!$

Where N0 is the value of N at time zero (t = 0). The second equation recognizes that the differential decay constant λ has units of 1/time, and can thus also be represented as 1/τ, where τ is a characteristic time for the process. This characteristic time is called the time constant of the process. In radioactive decay, this process time constant is also the mean lifetime for decaying atoms. Each atom "lives" for a finite amount of time before it decays, and it may be shown that this mean lifetime is the arithmetic mean of all the atoms' lifetimes, and that it is τ, which again is related to the decay constant as follows:

$\tau = \frac{1}{\lambda}.$
Simulation of many identical atoms undergoing radioactive decay, starting with either 4 atoms (left) or 400 (right). The number at the top indicates how many half-lives have elapsed. Note the law of large numbers: With more atoms, the overall decay is less random.

The previous exponential function generally represents the result of exponential decay. It is only an approximate solution, for two reasons. Firstly, the exponential function is continuous, but the physical quantity N can only take non-negative integer values. Secondly, because it describes a random process, it is only statistically true. However, in most common cases, N is an extremely large number (comparable to Avogadro's number) and the function is a good approximation.

### Half life

A more commonly used parameter is the half-life. Given a sample of a particular radionuclide, the half-life is the time taken for half the radionuclide's atoms to decay. The half life is related to the decay constant as follows:

$t_{1/2} = \frac{\ln 2}{\lambda} = \tau \ln 2.$

This relationship between the half-life and the decay constant shows that highly radioactive substances are quickly spent, while those that radiate weakly endure longer. Half-lives of known radionuclides vary widely, from more than 1019 years (such as for very nearly stable nuclides, e.g. 209Bi), to 10−23 seconds for highly unstable ones.

The factor of ln2 in the above relations results from the fact that concept of "half life" is merely a way of selecting a different base other than the natural base e for the life time expression. The time constant τ is the "1/e" life (time till only 1/e = about 36.8% remains) rather than the "1/2" life of a radionuclide where 50% remains (thus, τ is longer than t½). Thus, the following equation can easily be shown to be valid.

$N(t) = N_0\,e^{-t/ \tau} =N_0\,2^{-t/t_{1/2}}. \,\!$

Since radioactive decay is exponential with a constant probability, each process could as easily be described with a different constant time period which (for example) gave its "1/3 life" (how long until only 1/3rd is left) or "1/10 life" (a time period till only 10% is left) and so on. Thus the choice of τ and t½ for marker-times, are only for convenience, and from convention. They reflect a fundamental principle only in so much as they show that the same proportion of a given radioactive substance will decay, during any time-period that one chooses.

## Changing decay rates?

A number of experiments have shown that decay rates of naturally-occurring radioisotopes (for decay modes other than electron capture) are, to a high degree of precision, unaffected by external conditions such as temperature, pressure, the chemical environment and electric, magnetic or gravitational fields. Comparison of laboratory experiments over the last century, studies of the Oklo natural nuclear reactor, and astrophysical observations of the luminosity decays of distant supernovae (which occurred long ago as the light has taken a great deal of time to reach us), for example, strongly indicate that decay rates have been constant (at least to within the limitations of small experimental errors) as a function of time as well.

On the other hand, some recent results suggest the possibility that decay rates might have a very weak dependence (0.1% or less) on environmental factors. It has been suggested that measurements of decay rates of silicon-32, manganese-54 and radium-226 exhibit small seasonal variations (about 0.1%), proposed to be related to either solar flare activity or distance from the sun.[3][4][5] However, such measurements are highly susceptible to systematic errors, and a subsequent paper [6] has found no evidence for such correlations in a half-dozen isotopes, and sets upper limits on the size of any such effects.

An exception is the decay mode known as electron capture exhibited by a small number of nuclides. Chemical bonds can affect the rate of electron capture to a small degree (generally less than 1%) depending on the proximity of electrons to the nucleus. For example in 7Be, a difference of 0.9% has been observed between half-lives in metallic and insulating environments.[7] This relatively large effect is due to the fact that beryllium is a small atom whose valence electrons are in 2s atomic orbitals which penetrate the nucleus, and thus are subject to electron capture.

## Notes

1. ^ a b "Decay and Half Life". Retrieved 2009-12-14.
2. ^ This symbol is included in ISO 21482:2007. ISO International Standards are protected by copyright and may be purchased from ISO or its members (please visit www.iso.org for more information). ISO has not reviewed the accuracy or veracity of this information.
3. ^ The mystery of varying nuclear decay, Physics World, October 2, 2008 Physicsworld.com
4. ^ Perturbation of Nuclear Decay Rates During the Solar Flare of 13 December 2006, preprint available at arXiv.org e-Print archive
5. ^ J. H. Jenkins et al., Evidence of correlations between nuclear decay rates and Earth–Sun distance, Astroparticle Physics, Volume 32, Issue 1, August 2009, Pages 42-46. Preprint available at arXiv.org e-Print archive
6. ^ E. B. Norman et al., Evidence against correlations between nuclear decay rates and Earth–Sun distance, Astroparticle Physics Volume 31, Issue 2, March 2009, Pages 135-137, available online at University of California, Berkeley
7. ^ B.Wang et al., Euro. Phys. J. A 28, 375-377 (2006) Change of the 7Be electron capture half-life in metallic environments

# Study guide

Up to date as of January 14, 2010

## Contents

The nuclei of some atoms are spontaneously disintegrate from one form of isotope to another. until they reach stable form. These atoms emitparticles (alpha, beta, gamma) which are different in charge, size, penetrating power and ionization energy.

## Half life of the isotope

The half - life of isotope is the time, needed to decay the isotope so that half of it's mass is remaining. Here is an example. Lets say we have 50 grams of the radioactive isotope Carbon 14, which has a half-life of 5,730 years. After 5,730 years, 25 grams will remain. After another 5,730 years have passed only 12.5 grams remain and so on, until the parent isotope has completely decayed into a daughter isotope, in which either it will decay into another isotope, or remain the same, depending if it is stable or radioactive.

## Types of decay

There are three types of radioactive decay, these include Alpha, Beta, & Gamma Radiation Alpha and Beta decay are released in particles, while Gamma radiation is released in rays. Gamma radiation is much stronger than the other two types, as an alpha particle can be stopped by a thin piece of paper, and a beta particle can be stopped by a sheet of aluminum foil, gamma can penetrate both with ease. The most efficient way to stop gamma rays, are through the element lead (You may have wondered why you wear a lead bib or covering when you get a dental X-Ray).

Alpha particles are completely stopped by a sheet of paper, beta particles by an aluminum plate. Gamma rays however, can only be reduced by much more substantial obstacles, such as a very thick piece of lead.

Here are some common radiation emitters:

• Alpha

Americium-241

• Beta

Strontium-90

• Gamma

Uranium-238

## Example of alpha decay

• ${}^{226}_{88}Ra$
• ${}^{222}_{86}Rn$
• ${}^4_2He$

# 1911 encyclopedia

Up to date as of January 14, 2010

### From LoveToKnow 1911

RADIOACTIVITY. The subject of radioactivity deals with phenomena exhibited by a special class of bodies of high atomic weight of which uranium, thorium, radium and actinium are the best known examples. These substances possess the property of spontaneously emitting radiations of a special character which are able to penetrate through matter opaque to ordinary light. The beginning of this subject dates from 1896, and was an indirect consequence of the discovery of the X rays made a few months before by Röntgen. It was known that the production of X rays in a vacuum tube was accompanied by a strong phosphorescence of the glass, and it occurred to several investigators that ordinary substances made phosphorescent by visible light might emit a penetrating radiation similar to X rays. Following out this idea, H. Becquerel (1),1 a distinguished French physicist, exposed amongst other substances a phosphorescent compound of uranium, uranium 1 These numbers refer to papers noted under References (below).

In addition to radium and polonium, a number of other radioactive substances have been found in uranium minerals. With the exception of the radium emanation, none of these have yet been isolated in a pure state, although preparations of some of them have been obtained comparable in activity with radium itself. Debierne (9) found a radioactive substance which was separated from pitchblende with the rare earths and had chemical properties similar to those of thorium. This he called actinium. Giesel (10) independently noted the presence of a new radioactive substance which was usually separated with lanthanum and cerium from the minerals. It possessed the property of giving out a radioactive emanation or gas, the activity of which died away in a few seconds. For this reason he called it the emanating substance and afterwards emanium. Later work has shown that emanium is identical in chemical and radioactive properities with actinium, so that the former name will be retained.

We have already seen that Mme Curie gave the name polonium to a radioactive substance separated with bismuth. Later Marckwald found that a very radioactive substance was deposited from a solution of a radioactive mineral on a polished bismuth plate. The active matter was found to be deposited in the bismuth with tellurium, and he gave the, name radiotellurium to this substance. In later work, he showed that the new substance could be chemically separated from tellurium. By treating the residues from 15 tons of Joachimsthal pitchblende, Marckwald (I I) finally obtained 3 milligrams of intensely active material - far more active weight for weight than radium. It has been definitely settled that the active substance of Marckwald is identical with polonium. Both substances give out a type of easily absorbed a rays and both lose their activity at the same rate. The activity of polonium decays in a geometrical progression with the time and falls to half its initial value in 140 days. This law of decay, as we shall see, is characteristic of all radioactive products, although the period of decay is different in each case.

Mme Curie and Debierne (12) have described further experiments with polonium. The latter substance was extracted from several tons of pitchblende and purified until 2 milligrams of material were obtained containing about i/10 milligram of pure polonium. From a knowledge of the relative periods of transformation of radium and polonium, it can be calculated that the amount of polonium in a radium mineral is 1/5000 of the amount of radium, while the activity of pure polonium measured by the a rays should be 5000 times greater than that of radium.. As we have seen, polonium is rapidly transformed, and it is of great interest to determine the nature of the substance into which polonium changes. We shall see later that there is considerable evidence that polonium changes into lead.

All the radioactive substances possess in common the property of emitting radiations which darken a photographic plate and cause a discharge of electrified bodies. Very active preparations of radium, actinium and polonium also possess the property of causing strong phosphorescence in some substances. Bodies which phosphoresce under X rays usually do so under the rays from radioactive matter. Barium platinocyanide, the mineral willemite (zinc silicate) and zinc sulphide are the best known examples.

There are in general three types of radiation emitted by the radioactive bodies, called the a, 0 and y rays. Rutherford (2) in 1899 showed that the radiation from uranium was complex and consisted of (a) an easily absorbed radiation stopped by a sheet of paper or a few centimetres of air which he called the a rays and (b) a far more penetrating radiation capable of passing through several millimetres of aluminium, called the 0 rays. Later Villard found that radium emitted a very penetrating kind of radiation called the rays capable of passing before absorption through twenty centimetres of iron and several centimetres of lead.

Giesel and, later, Curie and Becquerel showed that the 0 rays of radium were deflected by a magnetic field. By the work of Becquerel and Kaufmann the /3 rays have been shown to consist of negatively charged particles projected with a velocity approaching that of light, and having the same small mass as the electrons set free in a vacuum tube. In fact the 0 rays are electrons spontaneously ejected from the radioactive matter at a speed on an average much greater than that observed in the electrons set free in a vacuum tube.

The very penetrating y rays are not deflected in a magnetic or electric field and are believed to be a type of radiation similar to X rays. The y rays are only observed in radioactive substances which emit /3 rays, and the penetrating power of the y rays appears to be connected with the initial velocity of expulsion of the /3 rays. Two general theories have been advanced to account for the properties of these rays. On one view, the y rays are to be regarded as electromagnetic pulses which have their origin in the expulsion of the /3 particle from the atom. On the other hand Bragg has collected evidence in support of the view that the y rays are corpuscular and consist of uncharged particles or "neutral doublets." There is as yet no general consensus of opinion as to the true nature of the y rays.

Under ordinary experimental conditions the greater part of the ionization observed in a gas is due to the a particles. This ionization due to the a rays does not extend in air at atmospheric pressure for more than 7 cms. from radium, and 8.6 cms. from thorium. If a screen of aluminium about oi cms. thick is placed over the active material, the a rays are completely absorbed, and the ionization above the screen is then due to the 0 and y rays alone. If a layer of lead about 2 mms. thick is placed over the active material, the 0 rays are stopped, and the ionization is then due almost entirely to the penetrating y rays. By the use of screens of suitable thickness we are thus able to sift out the various types of rays. These three types of radiations all set up secondary radiations in passing through matter. A pencil of 0 rays falling on matter is widely scattered in all directions. This scattered radiation is sometimes called the secondary 0 rays. The y rays give rise to secondary rays which consist in part of scattered y rays and in part electrons moving with a high velocity. These secondary rays in turn produce tertiary rays and so on. The impact of the a rays on matter sets free a number of slow moving electrons which are very easily deflected by a magnetic or electric field. This type of radiation was first observed by J. J. Thomson, and has been called by him the S rays.

In addition to their power of emitting penetrating radiations, the substances thorium, actinium and radium possess another very striking and important property. Rutherford (15) in 1900 showed that thorium compounds (especially the oxide) continuously emitted a radioactive emanation or gas. This emanation can he carried away by a current of air and its properties tested apart from the substance which produces it. A little later Dorn showed that radium possesses a similar property, while Giesel and Debierne observed a similar effect with actinium. These emanations all possess the property of ionizing a gas and, if sufficiently intense, of producing marked photographic and phosphorescent action. The activity of the radioactive gases is not permanent but disappears according to a definite law with the time, viz. the activity falls off in a geometric progression with the time. The emanations are distinguished by the different rates at which they lose their activity. The emanation of actinium is very shortlived, the time for the activity to fall to half value, i.e. the period of the emanation, being 3.7 seconds. The period of the thorium emanation is 54 seconds and of the radium emanation 3.9 days. This property of emitting an emanation is shown in a very striking manner by actinium. A compound of actinium is wrapped in a sheet of thin paper and laid on a screen of phosphorescent zinc sulphide. In a dark room the phosphorescence, marked by the characteristic scintillation, is seen to extend on all sides from the active body. A puff of air is seen to remove the emanation and with it the greater part of the phosphorescence. Fresh emanation immediately diffuses out and the experiment may be repeated indefinitely. The emanations have all the properties of radioactive gases. They can be transferred from point to point by currents of air. The emanations can be separated from the air or other gas with which they are mixed by the action of extreme cold. Rutherford and Soddy (16) showed that under ordinary conditions the temperature of condensation of the radium emanation mixed was - 150° C.

The emanations are produced from the parent matter and escape into the air under some conditions. Rutherford and Soddy (17) made a systematic examination of the emanating power of thorium compounds under different conditions. The hydroxide emanates most freely, while in thorium nitrate, practically none of the emanation escapes into the air. Most of the compounds of actinium emanate very freely. Radium compounds, except in very thin films, retain most of the emanation in the compound. The occluded emanation can in all cases be released by solution or by heating. On account of its very slow period of decay, hc emanation of radium can be collected like a gas and stored, when it retains its characteristic properties for a month or more.

## Induced Activity

Curie (18) showed that radium possessed another remarkable property. The surface of any body placed near radium, or still better, immersed in the emanation from it, acquires a new property. The surface after removal is found to be strongly active. Like the emanations, this induced activity in a body decays with the time, though at quite a different rate from the emanation itself. Rutherford (19) independently showed that thorium possessed a like property. He showed that the bodies made active behaved as if a thin film of intensely active matter were deposited on their surface. The active matter could be partly removed by rubbing, and could be dissolved off by strong acids. When the acid was evaporated the active matter remained behind. It was shown that induced activity was due to the emanations, and could not be produced if no emanation was present. We shall see that induced activity on bodies is due to a deposit of non-gaseous matter derived from the transformation of the emanations. Each emanation gives a distinctive active deposit which decays at different rates. The active deposits of radium, thorium and actinium are very complex, and consist of several types of matter. Several hours after removal from the emanation the active deposit from radium decays to half-value--26 minutes, for actinium half-value-34 minutes, for thorium half-valueo. 5 hours. The active deposits obtained on a platinum wire or plate are volatilized before a white heat, and are again deposited on the cooler bodies in the neighbourhood. Rutherford showed that the induced activity could be concentrated on the negative electrode in a strong electric field, indicating that the radioactive carriers had a positive charge. The distribution of the active deposit in a gas at low pressure has been investigated in detail by Makower and Russ.

## Theor y of Radioactive Transformations

This process of production and disappearance of active matter holds for all the radioactive bodies. We shall now consider some special cases of the variation of the amount of active matter with time which have proved of great importance in the analysis of radioactive changes.

(a) Suppose that initially the matter A is present, and this changes into B and B into C, it is required to find the number of atoms P, Q and R of A, B and C present at any subsequent time t.. Let X 1, X2, X3 be the constants of transformation of A, B and C respectively. Suppose n be the number of atoms of A initially present. From the law of radioactive change it follows: P = dQ/dt= A 1 P (I) dR/dt= A2Q-X3R. .. .. .. .. (2) Substituting the value of P in terms of n in (t), dQ/dl = A,ne Alt-A2Q; the solution of which is of the form Q=n(ae Alt+be-A20, where a and b are constants. By substitution it is seen that a= Al/(A2-Ai). Since Q=o when t=o, b= -Al/(A2 - A1) Thus Q = (eA 2 t e - Al t). (3) Similarly it can be shown that R =n(ae A i t +be A 2 t +ce - 1 3 1 ) (4) where a = AlA2 b = XX' X 2)(A 1 -A 3) (A2-A1)(A 2 A3) c= (A8-A1) (Aa-A2) It will be seen from (3), that the value of Q, initially zero, increases to a maximum and then decays; finally, according to an exponential law, with the period of the more slowly transformed product, whether A or B.

(b) A primary source supplies the matter A at a constant rate, and the process has continued so long that the amounts of the products A, B, C have reached a steady limiting value. The primary source is then suddenly removed. It is required to find the amounts of A, B and C remaining at any subsequent time t. In this case of equilibrium, the number n of particles of A supplied per second from the source is equal to the number of particles which change into B per second, and also of B into C. This requires the relation no =A1 =y2Q o = A3Ro where P °, Q„ R o are the initial number of particles of A, B, C present, and A lt A 21 A3 are their constants of transformation.

Using the same quotations as in case (t), but remembering the new initial conditions, it can easily be shown that the number of particles P, Q and R of the matter A, B and C existing at the time t after removal are given by P = ?° e A11, Q '_' o (At e ?2t -?lt Al - A2 R =n o (ae Alt where a=(A1- A3) , b c = A3(A1-A3)(A4-A3) The curves expressing the rate of variation of P, Q, R with time are in these cases very different from case (t).

(c) The matter A is supplied at a constant rate from a primary source. Required to find the number of particles of A, B and C present at any time t later, when initially A, B, and C were absent.

This is a converse case from case (2) and the solutions can be obtained from general considerations. Initially suppose A, B and C are in equilibrium with the primary source which supplied A at a constant rate. The source is then removed and the amounts of A, B and C vary according to the equation given in case (2). The source after removal continues to supply A at the same rate as before. Since initially the product A was in equilibrium with the source, and the radioactive processes are in no way changed by the removal of the source, it is clear that the amount of A present in the two parts in which the matter is distributed is unchanged. If P, be the amount of A produced by the source in the time t, and P the amount remaining in the part removed, then P i -FP = P o where P D is the equilibrium value. Thus P t /P o = I - P/Po.

The ratio P/P o can be written down from the solution given in case (2). Similarly the corresponding values of Q l /Q o, R1/R o may be at once derived. It is obvious in these cases that the curve plotted with P/P o as ordinates and time as abscissae is complementary to the corresponding curve with P 1 /P o as ordinates. This simple relation holds for all recovery and decay curves of radioactive products in general.

We have so far considered the variation in the number of atoms of successive products with time when the periods of the products are known. In practice, the variation of the number of atoms is deduced from measurements of activity, usually made by the electric method. Using the same notation as before, the activity of any product is proportional to its rate of breaking up, i.e. to X1P where P is the number of atoms present. If two products are present, the activity is the sum of two corresponding terms X I P and X2Q. In practice, however, no two products emit a or 0 particles with the same velocity. The difference in ionizing power of a single a particle from the two products has thus to be taken into account. If, under the experimental conditions, the ionization produced by an a particle from the second product is K times that from the first product, the activity observed is proportional to X 1 P-{-KX 2 Q. In this way, it is possible to compare the theoretical activity curves of a mixture of products with those deduced experimentally.

The analysis of the successive changes occurring in uranium, thorium, radium and actinium has proved a very difficult matter. In order to establish the existence of a new product and to fix its position in the scheme of changes, it is necessary to show (a) that the new product has a distinctive period of decay and shows some distinctive physical or chemical properties; (b) that the product under consideration arises directly from the product preceding it in the scheme of changes, and is transformed into the product succeeding it.

While in the majority of cases the products break up either with the emission of a or /3 particles, some products have been observed which do not emit any characteristic radiation and have been called "rayless products." For example, radium D and thorium A are changing substances which break up without emitting either penetrating a or /i rays. They appear to emit slow b rays which can only be detected by special methods. The presence and properties of a rayless product can be easily inferred if it is transformed into a product emitting a radiation, for the variation in activity of the latter affords a method of determining the amount of the parent product present. The distinction between a "ray" and a "rayless" product is not clear. It may be that the atom of a rayless product undergoes a re-arrangement of its constituent parts giving rise to an atom of the same mass but of different properties. In the case of an a ray or /3 ray product, the expulsion of an a or /3 particle affords an obvious explanation of the appearance of a new product with distinctive physical properties.

In the table a list of the known products of transformation is given. In each case, the half period of transformation is given and the type of radiation emitted. If the product emits a rays, the range of ionization of the a particle in air is given.

Table Oi' Radioactive Products In each of the groups under the heading uranium, thorium and actinium, each product is derived from the direct transformation of the product above it.

The radium emanation is to be regarded as a typical radioactive product or transition element which exists in a gaseous form. It is produced from radium at a constant rate, and is transformed into radium A and helium. Its half-period of transformation is 3.86 days. The emanation from radium has been purified by condensing it in liquid air, and pumping out the residual gases. The volume (26) of the emanation at normal pressure and temperature to be derived from one gram of radium in equilibrium is about o 6 cubic millimetres. This small quantity of gas contains initially more than three-quarters of the total activity of the radium before its separation. In a pure state, the emanation is ioo,000 times as active weight for weight as pure radium. Pure emanation in a spectrum tube gives a characteristic spectrum of bright lines (27). The discharge in the gas is bluish in colour. With continued sparking, the emanation is driven into the walls of the tube and the electrodes. Notwithstanding the minute volume of emanation available, the boiling-point of the emanation has been determined at various pressures. At atmospheric pressure Rutherford (28) found the boiling-point to be - 67° C., and Gray and Ramsay (29) 71° C. Liquid emanation appears colourless when first condensed; when the temperature is lowered, the liquid emanation freezes, and at the temperature of liquid air glows with a bright rose colour. The density of liquid emanation has been estimated at 5 or 6.

Approximate estimates of the molecular weight of the radium emanation were early made by diffusion methods. The molecular weight in most cases came out about zoo. In a comparison by Perkins of the rate of diffusion of the emanation with that of a monatomic vapour of high molecular weight, viz. mercury, the value deduced was 234. Since the radium atom in breaking up gives rise to one atom of the emanation and one atom of helium, its atomic weight should be 226 - 4 = 222. The emanation appears to have no definite chemical properties, and in this respect belongs to the group of inert monatomic gases of which helium and argon are the best known examples. It is partially soluble in water, and readily absorbed by charcoal.

## Actinium

The transformations observed in actinium are very analogous to those in thorium. Actinium itself is a rayless product which changes into radioactinium, an a ray product of period 19.5 days, first separated by Hahn (32). This changes into actinium X, of period io 2 days, first separated by Godlewski (33) Actinium X is transformed into the emanation which in turn gives rise to three further products, called actinium A, B and C. Although very active preparations of actinium have been prepared, it has so far not been found possible to separate the actinium from the rare earths with which it is mixed. We do not in consequence know its atomic weight or spectrum.

where T2 and T 1 are the half-periods of transformation of uranium and radium respectively. The work of Boltwood (34), Strutt (35) and McCoy (36) has conclusively shown that the ratio of radium to uranium in old minerals is a constant. Boltwood and Strutt determined the quantity of radium present in a mineral by the emanation method, and the amount of uranium by analysis.

Since the direct parent of radium must be present in radioactive minerals, one of the constituents separated from the mineral must grow radium. This was shown to be the case by Boltwood (38), who found that actinium preparations produced radium at a fairly rapid rate. By the work of Rutherford and Boltwood, it was found that the growth of radium was not due to actinium itself, but to a new substance separated in some cases with the actinium. This new substance, which emits a rays, was separated by Boltwood (38), and called by him "Ionium." It has chemical properties very similar to thorium. Soddy has shown that the period of ionium is probably not less than 20,000 years, indicating that ionium must exist in uranium minerals in not less than ten times the quantity of radium. It has not yet been directly shown that uranium produces ionium, but there can be no doubt that it does do so. Since ionium produces radium, Boltwood (38) has determined by direct experiment that radium is half transformed in 2000 years - a number in good agreement with other data on that subject. The constant relation between uranium and radium will only hold for old minerals where there has been no opportunity for chemical alteration or removal of its constituents by the action of percolating water or other agencies. It is quite possible that altered minerals of no great age will not show this constant relation. It seems probable that this is the explanation of some results of Mlle Gleditsch, where the relation between uranium and radium has been found not to be constant for some mineral specimens.

 Uranium 1 Radium B. . 0.04(?) Ionium 0.34 Radium C. . 0.91 Radium . 0.45 Radium F. . o 46 Emanation 0.62 Actinium and its Radium A 0.54 products. . 0.28

We have already seen that a number of slowly transforming radioactive substances, viz. polonium (radium F), radiolead (radium D) and ionium are linked up to the uranium-radium series of transformations. Boltwood (39), has made a systematic examination of the relative activity in the form of very thin films due to each of the products present in the uranium-radium family. The results are shown in the following table, where the activity of pure uranium itself is taken as unity: Total activity mineral, 4.64 times uranium.

Taking into account the differences in the ionization due to an a particle from the various products, the results indicate that uranium expels two a particles for one from each of the other a ray products in the series of transformations. This indicates either that two particles are expelled during the transformation of the atom of uranium, or that another a ray product is present which has so far not been separated from the uranium.

Although thorium is nearly always present in old uranium minerals and uranium in thorium minerals, there does not appear to be any radioactive connexion between these two elements. Uranium and thorium are to be regarded as two distinct radioactive elements. With regard to actinium, there is still no definite information of its place in the scheme of transformations. Boltwood has shown that the amount of actinium in uranium minerals is proportional to the content of uranium. This indicates that actinium, like radium, is in genetic connexion with uranium. On the other hand, the activity of actinium with its series of a ray products is less than that of radium itself or uranium. In order to explain this anomaly, Rutherford has suggested that at a certain stage of disintegration of the uranium-radium series, the disintegration is complex, and two distinct kinds of matter appear, one in much larger quantity than the other. On this view, the smaller fraction is actinium, so that the latter is a branch descendant of the main uranium-radium series.

## End Products of Transformation

It is now definitely established that the a particle expelled from any type of radioactive matter is an atom of helium, so that helium is a necessary accompaniment of radioactive changes involving the expulsion of a particles. After the radioactive transformations have come to an end, each of the elements uranium and thorium and actinium should give rise to an end or final product, which may be either a known element or some unknown element of very slow period of transformation. Supposing, as seems probable, that the expulsion of an a particle lowers the atomic weight of an element by four units - the atomic weight of helium - the atomic weights of each of the products in the uranium and radium series can be simply calculated. Since uranium expels two a particles, the atomic weight of the next ray product, ionium, is 238.5-8 or 230.5. The atomic weight of radium comes out to be 266.5, a number in good agreement with the experimental value. Similarly the atomic weight of polonium is 210.5, and that of the final product after the transformation of polonium should be 206.5. This value is very close to the atomic weight of lead, and indicates that this substance is the final product of the transformation of radium.

This suggestion was first put forward by Bolt wood (40), who has collected a large amount of evidence bearing on this subject. Since in old minerals the transformations have been in progress for periods of time, in some cases measured by hundreds of millions of years, it is obvious that the end product, if a stable element, should be an invariable companion of the radioelement and be present in considerable quantity. Boltwood has shown that lead always occurs in radioactive minerals, and in many cases in amount about that to be expected from their uranium content and age. It is difficult to settle definitely this very important problem until it can be experimentally shown that radium is transformed into lead, or, what should prove simpler in practice, that polonium changes into helium and lead. Unfortunately for a solution of this problem within a reasonable time, a very large quantity of polonium would be necessary. Mme. Curie and Debierne have obtained a very active preparacion of polonium containing about f i oth milligram of pure polonium. Rutherford and Boltwood and Curie and Debierne have both independently shown that polonium produces helium - a result to be expected, since it emits a particles.

## Production of Helium

In 1902 Rutherford and Soddy suggested that the helium which is invariably found in radioactive minerals was derived from the disintegration of radioactive matter. In 1903 Ramsay and Soddy definitely showed that helium was produced by radium and also by its emanation. From the observed mass of the a particle, it seemed probable from the first that the a particle was an atom of helium. This conclusion was confirmed by the work of Rutherford and Geiger (41), who showed that the a particle was an atom of helium carrying two unit charges of electricity. In order to prove definitely this relation, it was necessary to show that the a particles, quite independently of the active matter from which they were expelled, gave rise to helium. This was done by Rutherford and Royds (42), who allowed the a particles from a large quantity of emanation to be fired through the very thin glass walls of the containing tube. The collected particle gave the spectrum of helium, showing, without doubt, that the a particle must be a helium atom.

Since the a particle is an atom of helium, all radioactive matter which expels a particles must give rise to helium. In agreement with this, Debierne and Giesel have shown that actinium as well as radium produces helium. Observations of the production of helium by radium have been made by Ramsay and Soddy, Curie and Dewar, Himstedt and others. The rate of production of helium per gram of radium was first definitely measured by Dewar (43). His preliminary measurements gave a value of 134 cubic mms. of helium per year per gram of radium and its products. Later observations extending over a larger interval give a rate of production about 168 cubic mms. per year. As a result of preliminary measurements, Boltwood and Rutherford (44) have found a growth of 163 cubic mms. per year. It is of interest to note that the rate of production of helium by radium is in excellent agreement with the value calculated theoretically. From their work of counting the particles and measuring their charge, Rutherford and Geiger showed that the rate of production of helium should be 158 cubic mms. per year.

Properties of the a Rays. - We have seen that the rays are positively charged atoms of helium projected at a high velocity, which are capable of penetrating through thin metal sheets and several centimetres of air. Early observations indicated that the ionization due to a layer of radioactive matter decreased approximately according to an exponential law with the thickness of the absorbing matter placed over the active matter. The true nature of the absorption of the a rays was first shown by Bragg and by Bragg and Kleeman (45). The active particles projected from a thin film of active matter of one kind have identical velocities, and are able to ionize the air for a definite distance, termed the "range" of the a particle. It was found that the ionization per centimetre of path due to a narrow pencil of a rays increases with the distance from the active matter, at first slowly, then more rapidly, near the end of the range. After passing through a maximum value the ionization falls off rapidly to zero. The range of an a particle in air has a definite value which can be accurately measured. If a uniform screen of matter is placed in the path of the pencil of rays the range is reduced by a definite amount proportional to the thickness of the screen. All the a particles have their velocity reduced by the same amount in their passage through the screen. The ranges in air of the a rays from the various products of the radioelements have been measured. The ranges for the different products vary between 2.8 cms. and 8.6 cms.

## Heat Emission of Radioactive Matter

Experiments on the evolution of heat from radium and its emanation have brought to light the enormous amount of energy accompanying the transformation of radioactive matter where a particles are emitted. For example, the emanation from one gram of radium in equilibrium with its products emits heat initially at the rate of about 90 gram calories per hour. The total heat emitted during its transformation is about 12 ,000 gram calories. Now the initial volume of the emanation from one gram of radium is 6 cubic millimetres. Consequently one cubic centimetre of emanation during its life emits 2 X 10' gram calories. Taking the atomic weight of the emanation as 222, one gram of the emanation emits during its life 2 X 109 gram calories of heat. This evolution of heat is enormous compared with that emitted in any known chemical reaction. There is every reason to believe that the total emission of energy from any type of radioactive matter during its transformation is of the same order of magnitude as for the emanation. The atoms of matter must consequently be regarded as containing enormous stores of energy which are only released by the disintegration of the atom.

A large amount of work has been done in measuring the amount of the thorium and radium emanation in the atmosphere, and in determining the quantity of radium and thorium distributed on the surface of the earth. The information already obtained has an important bearing on geology and atmospheric electricity.

References.-1. H. Becquerel, Comptes Rendus, 1896, pp. 420, 501, 559, 689, 762, 1086; 2. Rutherford, Phil. Mag., Jan. 1899; 3. Mme Curie, Comptes Rendus, 1898, 126. p. t ioi; M and Mme Curie and G. Bemont, ib., 1898, 127. p. 1215; 4. Mme Curie, ib., 1907, 1 45. p. 422; 5. Thorpe, Proc. Roy. Soc., 1908, 80. p. 298; 6. Giesel, Phys. Zeit., 1902, 3. p. 578; 7. Giesel, Annal. d. Phys., 18 99, 6 9. p. 91; Ber., 1902, p. 3608; 8. Rutherford and Boltwood, Amer. Journ. Sci., July 1906; 9. Debierne, Comptes Rendus, 18 99, 12 9. p. 593; 1900, 130. p. 206; io. Giesel, Ber., 1902, p. 3608; 1903, p. 342; II. Marckwald, ib., 1903, p. 2662; 12. Mme Curie and Debierne, Comptes Rendus, 1910, 150. p. 386; 13. Boltwood, Amer. Journ. Sci., May 1908; 14. Rutherford, Phil. Mag., Feb. 1903, Oct. 1906; 15. Rutherford, ib., Jan. 1900; 16. Rutherford and Soddy, ib., May 1903; 17. Rutherford and Soddy, ib., Nov. 1902; 18. M and Mme Curie, Comptes Rendus, 18 99, 12 9. p. 714; 19. Rutherford, Phil. Mag., Jan. and Feb. 1900; 20. Rutherford and Soddy, ib., Sept. and Nov. 1902, April and May 1903; Rutherford, Phil. Trans., 1904, 204A. p. 169; 21. Russ and Makower, Proc. Roy. Soc., 1909, 82A. p. 205; 22. Hahn, Phys. Zeit., 1909, 10. p. 81; 23. Rutherford, Phil. Mag., Nov. 1904, Sept. 1905; 24. Meyer and Schweidler, Wien. Ber., July 1905; 25. Antonoff, Phil. Mag., June 1910; 26. Cameron and Ramsay, Trans. Chem. Soc., 1907, p. 1266; Rutherford, Phil. Mag., Aug. 1908; 27. Cameron and Ramsay, Proc. Roy. Soc., 1908, 81A. p. 210; Rutherford and Royds, Phil. Mag., 1908, 16. p. 313; Royds, Proc. Roy. Soc., 1909, 82A. p. 22; Watson, ib., 1910, 83A. p. 50; 28. Rutherford, Phil. Mag., 1909; 29. Gray and Ramsay, Trans. Chem. Soc., 1909, PP. 354, 1073; 30. Rutherford and Soddy, Phil. Mag., Sept. and Nov. 1902; 31. Hahn, Proc. Roy. Soc., March 1905; Phil. Mag., June 1906; Ber., 4 0. pp. 1462, 3304; Phys. Zeit., 1908, 9 PP. 245, 246; 32. Hahn, Phil. Mag., Sept. 1906; 33. Godlewski, ib., July 1905; 34. Boltwood, ib., April 1905; 35. Strutt, Trans. Roy. Soc., 1905A.; 36. McCoy, Ber., 1904, p. 2641; 37. Soddy, Phil. Mag., June 1905, Aug. 1907, Oct. 1908, Jan. 1909; 38. Boltwood, Amer. Journ. Sci., Dec. 1906, Oct. 1907, May 1908, June 1908; 39. Boltwood, ib., April 1908; 40. Boltwood, lb., Oct. 1905, Feb. 1907; 41. Rutherford and Geiger, Proc. Roy. Soc., 1908, 81A. p. 141; 42. Rutherford and Royds, Phil. Mag., Feb. 1909; 43. Dewar, Proc. Roy. Soc., 1908, 81A. p. 280; 1910, 8 3. p. 404; 44. Boltwood and Rutherford, Manch. Lit. and Phil. Soc., 1909, 54. No. 6; 45. Bragg and Kleeman, Phil. Mag., Dec. 1904, Sept. 1905; 46. Rutherford, ib., Aug..1906; 47. Geiger, Proc. Roy. Soc., 1910, 83A. p. 505; 48. Geiger, ib., 1910, 8 3 A. p. 492; 49. Rutherford and Geiger, ib., 1908, 81 A. pp. 141, 163; 50. Crookes, lb., 1903; 51. Regener, Verhandl. d. D. Phys. Ges., 1908, 10. p. 28; 52. Curie and Laborde, Comptes Rendus, 1904, 136. p. 673; 53. Schweidler and Hess, Wien. Ber., June 1908, 117; 54. Rutherford and. Barnes, Phil. Mag., Feb. 1904.

General treatises are: P. Curie, Ouvres, 1908; E. Rutherford, Radioactive Transformations, 1906; F. Soddy, Interpretation of Radium, 1909; R. J. Strutt, Becquerel Rays and Radium, 1904; W. Makower, Radioactive Substances, 1908; J. Joly, Radioactivity and Geology, 1909. See also Annual Reports of the Chemical Society. (E. Ru.)