emanation (218).
88 Radium (226), thorium X (224), mesothorium (228), actinium
X (222).
90 Thorium (232), radiothorium (228), ionium (230), uranium Xi
(234), uranium Y (230), radioactinium (226).
It will be seen that many of the radioactive elements are isotopic
with known chemical elements. These radioactive isotopes
differ not only in atomic weight but also in radioactive properties.
The isotopes of lead are of special interest as they include the
end-products of the uranium, thorium and actinium series a
question that will be discussed more fully later. It has been
found that the X-ray spectrum of the y rays from radium B is
identical with that given by ordinary lead exposed to cathode
rays in a vacuum tube, a result to be anticipated from the
identity of their atomic number. It is of interest to note that
polonium is a new type of chemical element which has no
counterpart among the ordinary inactive elements.
Transformation of Uranium. In 1900 the late Sir W. Crookes
found that the /3-ray activity of ordinary uranium could be removed
by a single chemical operation and concentrated in an active res-
idue. This is due to the separation of the product uranium X, of
period 24 days, which emits /3 and 7 rays. A complete analysis of
the transformations of uranium has been a matter of much difficulty.
Boltwood showed that the o-ray activity of uranium was about
twice as great as that of a corresponding a-ray product in the urani-
um-radium series, indicating that uranium contained two successive
a-ray products: This was confirmed by Geiger, who showed that
the a rays from uranium consisted of two groups with ranges 2-5
and 2-9 cms. respectively. These two a-ray substances, called
uranium I. and uranium II., are isotopic, with atomic weights
238 and 234 respectively. The latter, whose period is estimated at
about 2 million years, exists in relatively very small quantity com-
pared with uranium I. Following the generalization connecting the
radiations and chemical properties of the series of radioelements,
Fajans predicted the presence of a new product with properties
analogous to tantalum, and promptly succeeded in isolating it
experimentally. The new product uranium X 2 , sometimes called
brevium, has a period of 1-15 minutes and emits swift rays. The
series of changes is thus:
Ur. !.-> Ur. X ,- Ur. X r-> Ur. II.-> Ionium.
We have seen that Antonoff discovered another /3-ray substance
called uranium Y, separated with uranium Xi, which has a period
of 24-6 hours. This exists in too small quantity to be in the main
line of succession, but is to be regarded as a branch product of ura-
nium Xi and is believed to be the first element of the subsidiary
actinium series.
Rutherford and Geiger found the number of a particles emitted
per gram of uranium per second to be 2-37 X IO 4 . From this the
period of uranium is calculated to be about 6,000 million years.
Thorium. The first product observed in thorium was the emana-
tion of period 54 seconds, and this gives rise to the active deposit,
which has been shown to consist of at least four successive products
called thorium A, B, C, D. The emanation, after the emission of an
a particle, changes into a product of very short life emitting a rays.
Its period was found by Geiger and Moseley to be about i/io
second. The succeeding product, thorium B, emits only weak j3 and
7 rays with a period of 10-6 hours, changing into thorium C of
period one hour. We have seen that thorium C breaks up in a com-
plex way, emitting three distinct groups of particles. Thorium D is
readily separated from C by the method of recoil. It emits pene-
trating /3 and 7 rays with a half-period of 3 minutes. The active
deposit as a whole decays ultimately with the period of thorium
B, viz. IO'6 hours.
A special interest attaches to the product thorium X, first sep-
arated by Rutherford and Soddy, since experiments with it laid the
foundation of the general theory of radioactive transformations.
A close analysis of thorium has led to the discovery by Harm of a
number of other important products. When the thorium X is
separated from a thorium mineral or old thorium preparation, there
appears with it another product called mesothorium I, of period 6-7
years, which is transformed with the emission of weak /3 rays into
mesothorium 2, of period 6 hours, which emits swift /3 particles and
penetrating 7 rays. This changes into an a-ray product, radio-
thorium, of period 2 years, which is transformed into thorium X.
Radiothorium is an isotope of thorium, while mesothorium I
is an isotope of radium. The radiothorium can readily be separated
from a solution of mesothorium and obtained in a concentrated
form. Mesothorium when first separated would show a very weak
activity, but in consequence of the growth of its subsequent product
radiothorium, its activity would increase for several years. After
reaching a maximum it would ultimately decay with the period of
mesothorium, viz. 6'7 years.
Actinium. Actinium of period about 20 years is believed to emit
weak /3 rays changing into radioactinium, an a-ray product of
period 19 days, first separated by Hahn. This changes into actinium
X, an a-ray product of period 1 1 days, discovered by Godlewski.
Then follows the actinium emanation of period 3-9 seconds, which
gives rise to four further products named actinium A, B, C, D.
Actinium A has the shortest life of any product whose rate of trans-
formation has been directly determined. Its period, as determined
by Geiger and Moseley and Fajans, is -002 second. After emitting
an a particle, A changes into B, a product of period 36 minutes
emitting weak /3 and 7 rays, analogous to thorium B. Actinium C
of period 2-16 minutes undergoes a complex transformation, giving
rise to two distinct groups of a particles. The main branch gives
rise to actinium D of period 4-8 minutes, which is readily isolated
by the recoil method. Actinium D, which emits /3 and 7 rays, is
analogous in all respects to thorium D.
In the discussion above on branch products it has been shown
that the parent of actinium, called protoactinium, has been recently
isolated by Hahn and Soddy. This substance emits a rays and has
an estimated period of 10,000 years. We have seen that the actin-
ium series is believed to have its origin in a dual transformation of
uranium X. The first branch product, representing about 4 % of the
total, is believed to be uranium Y, a /3-ray product of period one
day. This is directly transformed into protoactinium.
While very active preparations of actinium have been made, it
has not been found possible to separate it entirely from the rare
earths with which it is mixed. Protoactinium exists in much larger
amounts and should be ultimately obtained in a pure state.
End-products of the Transformations (re-stated). After the
radioactive transformations have come to an end, each of the
elements uranium, thorium and actinium should g ; ve rise to an
end or final product, which may be a known element or an
unknown element of very slow period of transformation. Since
the expulsion of an a particle lowers the mass of the atom by four
units, and there are eight a-ray products, the atomic weight of
the end atom should be 2388X4 = 206. The atomic weight of
radium by this rule should be 2383X4 = 226, a result in good
accord with experiment. The atomic weight of the end-product
of uranium is close to that of lead, viz. 207, and Boltwood early
suggested that lead was the end-product of radium. Since in
222
RADIOACTIVITY
old minerals the transformations have been in progress for inter-
vals measured by millions of years, the end-product should
collect and be an invariable companion of the radioelement.
Boltwood showed that lead is always present in old radioactive
minerals and in amount to be expected from their uranium con-
tent and geologic age.
In recent years this problem has been definitely attacked in the
light of the chemical generalization already given. It was clear
from this that the end-products of uranium, thorium and
actinium should all be isotopes of lead but with atomic weights
206, 208 and 206 respectively. In other words, uranium-lead if
uncontaminated with ordinary lead should show a smaller
atomic weight than ordinary lead (207), while thorium-lead
should give a higher value. By the work of Richards, Soddy and
Honigschmid, these conclusions have been definitely confirmed.
The lowest value for uranium-lead is 206, and the highest for
thorium-lead 207-7.
Since any admixture with ordinary lead tends to give a value
nearer 207, these results may be considered as a definite proof of
the nature and atomic weight of the end-products. In minerals
containing both uranium and thorium the atomic weight of the mix-
ture of the isotopes will depend on the relative amounts of these two
elements and their relative rates of transformation. In unaltered
minerals the determination of the amount of lead coupled with its
average atomic weight allows us to determine the amount of ura-
nium-lead even if some ordinary lead be present. In this way it
should be possible to make a reliable estimate of the age of selected
minerals and thus indirectly the age of the geologic strata. The
amount of helium in the mineral gives a minimum estimate of its
age, for, except in the most compact minerals, some of the helium
must undoubtedly escape.
Nature and Properties of the a. Rays (re-stated). Although the
o rays from active substances are of small penetrating power
compared with the /3 or 7 rays, they are responsible for most of
the energy evolved by radioactive substances and contribute
most of the ionization. Rutherford showed in 1903 that the
o rays were deflected in a powerful magnetic and electric field
and consisted of positively charged particles projected with
high velocity. From the first it seemed probable that the a
particle was an atom of helium and this was subsequently
confirmed in a number of ways. The value of e/m the ratio of
the charge on the particle to its mass and the velocity can be
determined from observations on the deflection of the pencil of
rays by a magnetic field and electric fields. In this way Ruther-
ford and Robinson showed that the o particle, whether from the
radium emanation, radium A or C, gave a value of e/m = 4820
e.m. units, while the electrochemical value of e/m = 48 26, assuming
that the mass of the helium atom is 4-00 and that it carries two
unit positive charges. The magnitude of the charge carried by
each particle was measured by Regener and Rutherford and
Geiger and found to be twice that carried by the electron. The
velocity of the a particles expelled from radium C (of range 7-06
cms.) was found to be 1-92X10' cm. per second, or about Vis
the velocity of light. From this result the velocity of expulsion
of all a particles can be calculated from the relation found by
Geiger, that V 3 =KR where V is the velocity of the particle and
R its range in air. The evidence indicates that the a particles
from active products are in all cases atoms of helium. The a
particles from a given product are all emitted with constant
velocity which is characteristic for that product. We have
already mentioned that the velocity of expulsion appears to be
connected with the period of transformation of the element. The
laws of absorption of the a particle were first worked out by
Bragg and Kleeman. On account of their great energy of motion,
the a particle travels in nearly a straight line through the gas,
producing intense ionization along its track. The effects produced
by the a particle, whether measured by ionization, phosphores-
cence or photographic action, vanish suddenly after the a particle
has traversed a definite amount of matter. This definiteness
of the end of the range of the a particle of given velocity is
remarkable. The range of the a particle is usually expressed in
terms of cms. of air traversed at 15 C. and 760 mm. pressure.
On account of its great energy of motion the effect due to a single
a particle can be detected in a variety of ways. Sir William Crookes
first noted that the o rays produce scintillations when they fall on a
screen of phosphorescent zinc sulphide. It is now known that each
of these scintillations is due to the impact of a single a particle.
The number of scintillations can be counted with the aid of a suit-
able microscope, and this method has proved of great utility in
many investigations. Scintillations due to o rays are observed in
certain diamonds, but they are usually not so bright as in zinc
sulphide. Kinoshita has shown that a single a particle produces a
detectable effect on a photographic plate. When the a rays fall on
a plate nearly horizontally the track of the a particle is clearly
visible under a high-power microscope. By the expansion method
developed by C. T. R. Wilson, the track of the a particle through
the gas is made visible by the condensation of the water on each of
the ions produced. In a similar way the track of a ft particle can be
easily shown. The photographs of these trails bring out in a striking
and concrete way not only the individual existence of o and ft par-
ticles, but the main effects produced in their passage through matter.
Properties of ft andy Rays (re-stated). We have seen that the /3
particles, which are emitted by a number of radioactive products,
consist of swift negative electrons spontaneously liberated during
the transformation of active matter. The velocity of expulsion
and the penetrating power of /3 rays vary widely for different
products. For example, the rays from radium B are much more
easily absorbed by matter than the swift /3 rays from radium C.
Moseley showed that in the case of these two products each
disintegrating atom gave rise on the average to one j3 particle.
There is undoubtedly a close connexion between ft and y rays,
and swift ft rays are usually accompanied by penetrating y rays.
For example, radium C, which emits very swift ft rays, some of which
reach a velocity more than 0-98 of the velocity of light, gives rise
to the most penetrating y rays observed in the uranium-radium
series. There is one very notable exception, viz. radium E, which
emits swift ft particles but weak y rays. Gray has shown that rays
in passing through matter give rise to y rays, and that these in some
cases correspond to the characteristic X radiations observed by
Barkla. The absorption of the y rays has been determined by the
electrical method. Radium B has been found to emit several groups
of y rays which differ widely in penetrating power. The greater
part of the rays from radium C consist of penetrating y rays which
are exponentially absorbed by matter. The ionization in an elec-
troscope falls off according to the equation I/Io = e~Md where d
is the thickness of matter traversed and /* the coefficient of absorp-
tion. When lead is used as an absorbing material the value of
jj=o-5 for the most penetrating y rays from radium C. The ab-
sorption coefficient for different kinds of matter is roughly propor-
tional to the density, indicating that the absorption depends only
on the mass of matter traversed.
The general evidence indicates that the y rays consist of types of
characteristic radiations which are excited by the passage of the ft
rays through the electronic system of the atom, but the y rays
from radium C are far more penetrating than any type of charac-
teristic radiation observed in X rays generated in a vacuum tube.
Rutherford and Andrade have determined the spectrum of the
y rays from radium B and C by reflection from rock-salt. The
most intense lines due to radium B are identical in wave-length with
the X-ray spectrum of lead. This is to be expected, since radium B
is an isotope of lead. The lines due to the " K " characteristic radia-
tion are also observed. General considerations, however, indicate
that the wave-length of the most penetrating y rays is much too
short to resolve or detect by the crystal method. In order to excite
such rays in an X-ray tube potential differences of the order of two
million volts will be necessary.
When the y rays from a product like radium B or radium C are
bent by a magnetic field and fall on a photographic plate, a kind of
magnetic spectrum is obtained. Superimposed on the continuous
spectrum clue to particles of all velocities (between certain limits)
certain sharp lines are observed, each of which represents a definite
group of ft rays which are emitted at the same speed. The velocity
corresponding to each line in the spectrum has been determined for
a number of /3-ray products by Hahn and Miss Meitner. The mag-
netic spectrum of radium B and radium C was examined in detail
by Rutherford and Robinson and more than 50 lines were observed,
representing ft particles projected over a wide range of velocity.
The appearance of these lines in the spectrum appears to be connected
with the emission of y rays and is believed to be due to the conver-
sion of the energy of the y ray of definite frequency into the energy
of an electron according to the quantum relation. When a thin layer
of absorbing material is placed over the source, the primary ft rays
diminish in velocity and the lines become broad and diffuse. At the
same time, however, new groups of ft rays are formed by the con-
version of y rays into ft rays in passing through the absorbing ma-
terial, and these give well-marked bands on the photographic plate,
occupying very nearly the same position as those due to the primary
ft rays before absorption. Results of this kind have an important
bearing on the general problem of radiation, and give us indications
of the facts to be accounted for in dealing with the conversion of
swift ft rays into y rays of high frequency, and vice versa.
RADIOTHERAPY
223
Production of Helium. It was stated in the earlier article
that, since the particle is an atom of helium, all radioactive
matter which emits a particles must produce helium. This has
been found to be the case for every a-ray product that has
been examined. The rate of production of helium by radium in
equilibrium has been measured with accuracy by Dewar, Bolt-
wood and Rutherford. In terms of the International Radium
Standard, the rate of production of helium by one gram of
radium in equilibrium with its three a-ray products has been
found to be 164 cub. mm. per year. This value is in excellent
accord with that calculated from the rate of emission of a
particles, viz. 163 cub. millimetres. The rate of production of
helium by the radium emanation, ionium and polonium has been
found by Boltwood to be in fair agreement with calculation.
Soddy has observed the production of helium by purified uranium,
while Strutt showed that the rate of production of helium in
uranium and thorium minerals accorded with calculation.
Strutt has made a systematic examination of the amount of
helium present in many minerals and rocks which contain minute
quantities of radium and has utilized the results to estimate the
age of the geological deposits. On account of the tendency of
the helium to escape from minerals in the course of geologic
ages, this method gives only a minimum estimate of the age of
the mineral, except in the case of very dense and compact
specimens. The measurement of the lead content should ulti-
mately prove a more reliable method of estimating the age.
Heat Emission of Radioactive Matter As was stated earlier,
there is no doubt that the evolution of heat by radium and
other radioactive matter is mainly a secondary phenomenon,
resulting mainly from the energy of the absorbed radiation.
Since the particles have a large kinetic energy and are easily
absorbed by matter, all of these particles are stopped by the
radium itself or by the envelope surrounding it and their energy
of motion is transformed into heat. The evolution of heat from
any type of radioactive matter is thus proportional to the
energy of the expelled a particles, together with the energy of
the /3 and 7 rays absorbed in the envelope. The energy supplied
by the recoil of the radioactive atom after the expulsion of an
a particle is about 2 % of the energy of the a particle.
These conclusions have been confirmed by the measurements of
Rutherford and Robinson, who found that each of the a-ray prod-
ucts gave a heating effect proportional to the energy of the a particle
and absorbed /3 and y rays. The emanation and its products when
removed from radium were responsible for three-quarters of the
heating effect of radium in equilibrium. The heating effect of the
radium emanation, radium A and radium C decayed at the same rate
as their activity. From their measurements they found that the
total heating effect of radium in equilibrium surrounded by sufficient
material to absorb the a rays was 134-7 gram-calories per hour per
gram. Of this, 123-6 gram-calories were due to the a particles,
4-7 to the /3 rays and 6-4 to the y rays. The energy of the /3 and y
rays comes from radium B and radium C, but on account of their
great penetrating power it is difficult to measure the /3 energy with
accuracy. The results, however, show that the energy of the y
rays is even greater than that of the /3 rays, and the two together
are equal to about 28% of the energy of the a particles from radium C.
Measurements have been made of the heating effect of radium,
uranium and thorium and of uranium and thorium minerals. In
each case the evolution of heat is of about the magnitude to be
expected from the energy of the radiations.
Radioactivity of Ordinary Mailer. Apart from the well-
known radioactive elements of high atomic weight, only two
other elements have been shown to exhibit the property of
radioactivity to a detectable degree, viz. potassium and rubid-
ium. Campbell showed that these elements emit only ft rays
and in amount small compared with uranium. This property
appears to be atomic, but no evidence has been obtained of any
subsequent changes. If the ft particle comes from the nucleus
of the atom, potassium should be transformed into an isotope of
calcium, and rubidium into an isotope of strontium.
Radium and thorium have been found to be distributed, but
in very minute amount, in the surface rocks and soil of the earth.
The emanation from the soil diffuses into the atmosphere and
causes a small ionization which can be readily measured. A
penetrating 7 radiation, no doubt due to the presence of radium
and thorium in the earth's crust, has been observed near the
earth's surface, but becomes very small over a lake or the sea.
BIBLIOGRAPHY. Mme. Curie, Traite de Radioactivite (2 vols.
1910); E. Rutherford, Radioactive Substances and their Radiations
(1913) ; St. Meyer and E. V. Schweidler, Radioaktivitat (1916) ; F.
Soddy, Chemistry of the Radioelements, parts I. and II. (1914-5);
see also under " Radioactivity " in annual Reports of the Chemical
Society. (E. Ru.)
RADIOTHERAPY. Since 1910 there have been notable
developments, extending the practice of X-ray treatment (see
28.887) m t the wider field now included in radiotherapy, a term
which had not then come into general use. Strictly speaking,
under this term should be included treatment by all kinds of
rays; thus treatment by heat, by sun's rays, by ultra-violet
rays, by X-rays and by the rays of radio-active substances,
all come under the etymological term of radiotherapy. In
practice, however, it is restricted to the application of ultra-
violet rays, X-rays and radium rays. Amongst radiologists,
the term has undergone an even sharper definition, so that
radiotherapy is applied by them to treatment with X-rays
alone, the terms radiumtherapy (or, in France curietherapy,
in honour of the discoverer of radium) being applied to treat-
ment with the rays of radium and other radio-active substances.
Treatment by means of high frequency currents and diathermy
are included rather under the term electrotherapy.
Ultra-violet Rays. These rays to a large extent are the es-
sential feature of those forms of medical treatment which depend
upon exposure to sunlight (heliotherapy). Probably this is
not the whole story. Even though heat rays may also play
some part, experience of the treatment of wounds by sunlight
in France during the World War indicated that a degree of
benefit arises from exposure to sunlight which cannot be entirely
attributable to warmth and ultra-violet rays. On the other
hand, in the Finsen light treatment of lupus and in the treat-
ment of tuberculosis at high altitudes, ultra-violet rays probably
play a predominant part. It is uncertain how these rays act;