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potassium sulphate, enveloped in paper beneath a photographic | of uranium. If the radioactivity were an atomic phenomenon, plate. A weak photographic effect was obtained. This was this could only be explained by the presence in these minerals shown to be due to a penetrating radiation capable of passing of another substance more active than uranium itself. Relying through sheets of matter opaque to ordinary light. Further on this hypothesis, Mme Curie made a chemical examination investigation showed that this photographic action was ex- of uranium minerals in order to try to separate this new radiohibited by all compounds of uranium and by the metal itself, active substance. In these experiments, the Austrian Governand had nothing to do with phosphorescence. It was shown ment generously provided Mme Curie with a ton of the residues equally if the uranium were kept in darkness and did not from the State manufactory of uranium at Joachimstahl, vary appreciably with time. Becquerel showed that the Bohemia. At that place there are extensive deposits of pitchrays from uranium like X rays were capable of discharging blende or uranite which are mined for the uranium. After a body whether positively or negatively electrified. A separation of the latter, the residues are three to five times as uranium compound brought close to the charged plate of a radioactive weight for weight as the uranium. From this gold leaf electroscope causes a rapid collapse of the gold leaves. residue Mme Curie separated a substance far more radioThis property of uranium, and also of the radioactive bodies active than uranium, which she called polonium in honour of in general, has supplied a delicate and quantitative method the country of her birth. This substance is usually separated of accurate comparison of the intensity of the radiations from with bismuth in the mineral, but by special methods can be substances under varying conditions. A modified form of partly separated from it. A further examination revealed the gold leaf electroscope has come into general use for comparison presence of a second radioactive substance which is normally of the radioactivity of substances. Rutherford (2) made a separated with the barium, to which the name “radium " systematic examination of the discharging effect produced by was given. This name was happily chosen, for in the pure the rays from uranium and showed that it was due to the pro state radium bromide has a very great activity--about two
in the volume of the gas million times as great as an equal weight of uranium. By through which the radiations pass. ' In an electric field, the | means of successive fractionations of the chloride, the radium positive ions travel to the negative electrode and vice versa, was gradually concentrated, until finally the radium was
obtained so that the barium lines showed very faintly. The 100
atomic weight was found by Mme Curie to be 225. In a recent redetermination, using a larger quantity of 0.4 grams of pure radium chloride, Mme Curie (4) found the atomic weight to be 2262. Thorpe (5) using a smaller quantity obtained a value 227. The spectrum of the purified sample of radium chloride obtained by Mme Curie was first examined by Demarçay. It was found to have a characteristic spark spectrum of bright lines analogous in many respects to the
spectra of the alkaline earths. Giesel (6) found that pure RADIUN EMANATION
radium bromide gives a brilliant carmine colour to the bunsen flame. The flame spectrum shows two broad bright bands in the orange-red. There is also a line in the blue-green and two weak lines in the violet. Giesel (7) has taken an active part in the preparation of pure radium compounds, and was the first to place preparations of pure radium bromide on the market. He found that the separation of radium from the barium mixed with it proceeded much more rapidly if the crystallizations were carried out using the bromide instead of the chloride He states that six to eight crystallizations are sufficient for an
almost complete separation. From the chemical point of view TIME IN DAYS
radium possesses all the characteristic properties of a new
element. It has a definite atomic weight, a well-marked and thus causing a discharge of the electrified body. If a suffi- characteristic spectrum, and distinct chemical properties. ciently strong field is used, the ions are all swept to the electrodes Its comparative ease of separation and great activity has before appreciable loss of their number can occur by recom- attracted much attention to this substance, although we shall bination. The rate of discharge then reaches a steady maximum see that very similar radioactive properties are possessed by a value which is not altered by a large increase in voltage. This large number of distinct substances. maximum current through the gas is called the saturation | Radium emits three distinct types of radiation, known as the current. The ions produced in gases by the rays from uraniuma, B and y rays, of which an account will be given later. It and other radioactive substances are in general identical with produces in addition a radioactive emanation or gas which is those produced by X rays, and the mechanism of conductivity about 100,000 times as active weight for weight as radium of the gas is very similar in both cases (see CONDUCTION, itself. The emanation released from 10 milligrams of pure ELECTRIC: $ Through Gascs). Es
radium bromide causes a glass tube into which it is introduced Some time after Becquerel's discovery, Mme Curie (3) made to phosphoresce brightly. A brilliant luminosity is produced a systematic examination of the electric method of a large in phosphorescent substances like zinc sulphide, willemite and number of chemical elements and their compounds to test barium platino-cyanide when introduced into a tube containing whether they possessed the “radioactive property of uranium. the emanation. The radium emanation, a more detailed account Only one other element, thorium, was found to show this effect of which will be given later, has proved of the greatest utility to a degree comparable with that of uranium-a result inde in radioactive experiments. The property of radium of propendently observed by Schmidt. Mme Curie examined the ducing the emanation has been utilized as a very delicate and activity of the various compounds of uranium and found that certain method, not only of detection but of estimation of small their radioactivity was an atomic property, i.e. the activity was quantities of radium. This “cmanation method ” depends proportional to the amount of the element uranium present, upon the introduction of the emanation, liberated from a suband was independent of its combination with other sub- stance by boiling or heating, into a suitable electroscope. The stances. In testing the activity of the minerals containing rate of discharge of the electroscope due to the emanation affords uranium, Mme Curie found that the activity was always four a quantitative measure of the amount of radium present. In to five times as great as that to be expected from their content this way, it is not difficult to determine with certainty the
presence of radium in a body which contains only 10-11 gram' Recently Boltwood (13) has separated another substance of radium. With care, 10-12 gram can just be detected. This from uranium minerals which he has called “ionium." This emanation method has been employed with great success in substance is sometimes separated from the mineral with actinium measuring the quantity of radium in minerals and in rocks. A and has chemical properties very similar to those of thorium. very simple method has been devised of determining the quantity Preparations of ionium have been obtained several thousand of radium present when it is not less than 1/100 milligram. The times as active as uranium. Ionium emits a rays of short tube containing the radium is placed some distance from an range and has a period of transformation probably much longer electroscope which is surrounded by a lead screen about 3 mms.than that of radium. Ionium has a special interest inasmuch thick. This cuts off the a and B rays and the effect in the as it is the substance which changes directly into radium. A electroscope is then due to the penetrating y rays. By com- preparation of ionium initially free from radium grows radium parison of the rate of discharge with that of a standard prepara at a rapid rate. Hofmann found that the lead separated from tion of radium at the same distance, the quantity of radium can uranium minerals and named it radiolead. The active conat once be deduced, provided the radium is in equilibrium with stituent in the lead is radium D, which changes into radium E its emanation. This is usually the case if the radium preparation and then into radium F (polonium). Both radium D and is one month old. This method is simple and direct, and has radium F are products of the transformation of radium. In the great advantage that the radium tube under test need not addition to these radioactive substances mentioned above, be opened, nor its contents weighed. We shall see later that the a large number of other radioactive substances have been amount of radium in an old mineral is always proportional to the discovered. Most of these lose their activity in the course of a amount of uranium present. Rutherford and Boltwood (8) few hours or days. The properties of these substances and their found that 3-4 parts of radium by weight are present in ten position in the radioactives million parts of uranium. Consequently an old mineral con Radiations from Radioactive Substances.-All the radioactive taining 1000 kilos of uranium should contain 340 milligrams of substances possess in common the property of emitting radiapure radium.
tions which darken a photographic plate and cause a discharge In addition to radium and polonium, a number of other of electrified bodies. Very active preparations of radium, radioactive substances have been found in uranium minerals. actinium and polonium also possess the property of causing With the exception of the radium emanation, none of these have strong phosphorescence in some substances. Bodies which yet been isolated in a pure state, although preparations of some | phosphoresce under X rays usually do so under the rays from of them have been obtained comparable in activity with radium radioactive matter. Barium platinocyanide, the mineral itself. Debierne (9).found a radioactive substance which was willemite (zinc silicate) and zinc sulphide are the best known separated from pitchblende with the rare earths and had chemical examples. properties similar to those of thorium. This he called actinium. There are in general three types of radiation emitted by Giesel (10) independently noted the presence of a new radio- the radioactive bodies, called the a, B and yy rays. Ruther. active substance which was usually separated with lanthanum ford (2) in 1899 showed that the radiation from uranium was and cerium from the minerals. It possessed the property of complex and consisted of (a) an easily absorbed radiation stopped giving out a radioactive emanation or gas, the activity of which | by a sheet of paper or a few centimetres of air which he called died away in a few seconds. For this reason he called it the the a rays and (b) a far more penetrating radiation capable of emanating substance and afterwards emanium. Later work has passing through several millimetres of aluminium, called the B shown that emanium is identical in chemical and radioactive rays. Later Villard found that radium emitted a very peneproperties with actinium, so that the former name will be trating kind of radiation called the y rays capable of passing retained.
before absorption through twenty centimetres of iron and We have already seen that Mme Curie gave the name polonium several centimetres of lead. to a . radioactive substance separated with bismuth. Later Giesel and, later, Curie and Becquerel showed that the B rays Marckwald found that a very radioactive substance was de.
substance was de- of radium were deflected by a magnetic field. . By the work of posited from a solution of a radioactive mineral on a polished Becquerel and Kaufmann the Brays have been shown to consist bismuth plate. The active matter was found to be deposited of negatively charged particles projected with a velocity, apin the bismuth with tellurium, and he gave the name radio- proaching that of light, and having the same small mass as the tellurium to this substance. In later work, he showed that the electrons set free in a vacuum tube. In fact the B rays are new substance could be chemically separated from tellurium. electrons spontaneously ejected from the radioactive matter at By treating the residues from 15 tons of Joachimsthal pitch- a speed on an average much greater than that observed in the blende, Marckwald (11) finally obtained 3 milligrams of intensely electrons set free in a vacuum tube.
re weight for weight than radium. The very penetrating y rays are not deflected in a magnetic It has been definitely settled that the active substance of or clectric field and are believed to be a type of radiation Marckwald is identical with polonium. Both substances give similar to X rays. They rays are only observed in radioactive out a type of easily absorbed a rays and both lose their activity substances which emit B rays, and the penetrating power of at the same rate. The activity of polonium decays in a geo- they rays appears to be connected with the initial velocity of metrical progression with the time and falls to half its initial expulsion of the B rays. Two general theories have been value in 140 days. This law of decay, as we shall see, is char advanced to account for the properties of these rays. On one acteristic of all radioactive products, although the period of view, the yy rays are to be regarded as electromagnetic pulses decay is different in each case.
which have their origin in the expulsion of the B particle from Mme Curie and Debierne (12) have described further experi the atom. On the other hand Bragg has collected evidence ments with polonium. The latter substance was extracted in support of the view that they rays are corpuscular and confrom several tons of pitchblende and purified until 2 milligrams sist of uncharged particles or “neutral doublets." There is as of material were obtained containing about 1/10 milligram of yet no general consensus of opinion as to the true nature of the pure polonium. From a knowledge of the relative periods of rays. transformation of radium and polonium, it can be calculated Rutherford (14) showed in 1903 that the a rays were deflected in that the amount of polonium in a radium mineral is 1/5000 of a powerful magnetic or electric field. The amount of deflection the amount of radium; while the activity of pure polonium is very small compared with the B rays under similar conmeasured by the a rays should be 5000 times greater than that ditions. The direction of deflection in a magnetic field is of radium. As we have seen, polonium is rapidly transformed, opposite to that of the B rays, showing that the a rays consist and it is of great interest to determine the nature of the substance | of a stream of positively charged particles. A pencil of rays into which polonium changes. We shall see later that there is from a thick layer of radioactive matter is complex and conconsiderable evidence that polonium changes into lead.
sists of particles moving at varying velocities If, however, a thin film of radioactive matter of one kind is taken, the phosphorescent action. The activity of the radioactive gases particles which escape without absorption are found to be homo- is not permanent but disappears according to a definite law geneous and consist of particles projected at an identicai with the time, viz. the activity falls off in a geometric prospeed. Observations of the velocity and mass of the particle gression with the time. The emanations are distinguished have been made by Rutherford. The general method employed by the different rates at which they lose their activity. The for this purpose is similar to that used for the determination emanation of actinium is very shortlived, the time for the of the velocity and mass of the electron in a vacuum tube. activity to fall to half value, i.e. the period of the emanation, The deflection of a pencil of rays in a vacuum is determined being 3.7 seconds. The period of the thorium emanation is for both a magnetic and electric field. From these observa- 54 seconds and of the radium emanation 3.9 days. This protions the velocity and value e/m (the ratio of the charge carried perty of emitting an emanation is shown in a very striking by the particle to its mass) are determined. The value of manner by actinium. A compound of actinium is wrapped in a elm has been found to be the same for the particles from all sheet of thin paper and laid on a screen of phosphorescent the types of radioactive matter that have been examined, zinc sulphide. In a dark room the phosphorescence, marked indicating that the a particles from all radioactive substances by the characteristic scintillation, is seen to extend on all sides are identical in mass. The value of cm found for the a particle from the active body. A puff of air is seen to remove the is 5.07 X 109 Now the value of elm for the hydrogen atom set emanation and with it the greater part of the phosphorescence. free in the electrolysis of water is 9660. On the assumption Fresh emanation immediately diffuses out and the experiment that the value of the charge e is the same for the a particle as may be repeated indefinitely. The emanations have all the for the hydrogen atom, the value would indicate that the a properties of radioactive gases. They can be transferred from particle has about twice the mass of the hydrogen atom, i.e. point to point by currents of air. The emanations can be has the same mass as the hydrogen molecule. If the charge separated from the air or other gas with which they are mixed on the a particle is twice that on the hydrogen atom, the value by the action of extreme cold. Rutherford and Soddy (16) of elm indicates that the a particle is a helium atom, for the showed that under ordinary conditions the temperature of latter has an atomic weight of four times that of hydrogen. condensation of the radium emanation mixed was - 150° C. It was difficult at first to decide between these and other hypo- The emanations are produced from the parent matter and theses, but we shall show later that there is now no doubt escape into the air under some conditions. Rutherford and that the a particle is in reality a helium atom carrying two Soddy (17) made a systematic examination of the emanating elementary charges. We may consequently regard the a rays power of thorium compounds under different conditions. The as a stream of helium atoms which are projected from a radio- hydroxide emanates most freely, while in thorium nitrate, active substance with a high velocity. The maximum velocity practically none of the emanation escapes into the air. Most of the a particle from radium is 2 X 10 cms. per second, or one of the compounds of actinium emanate very freely. Radium fifteenth of the velocity of light. Although the a rays are the compounds, except in very thin films, retain most of the emanaleast penetrating of the radiations, it will be seen that they tion in the compound. The occluded emanation can in all play an extremely important part in radioactive phenomena. cases be released by solution or by heating. On account of They are responsible for the greater part of the ionization and its very slow period of decay, the emanation of radium can be heating effects of radioactive matter and are closely connected collected like a gas and stored, when it retains its characteristic with the transformations occurring in them..
properties for a month or more Under ordinary experimental conditions the greater part Induced Activity.-Curie (18) showed that radium possessed of the ionization observed in a gas is due to the a particles. another remarkable property. The surface of any body placed This ionization due to the a rays does not extend in air at near radium, or still better, immersed in the emanation from atmospheric pressure for more than 7 cms. from radium, and it, acquires a new property. The surface after removal is 8.6 cms. from thorium. If a screen of aluminium about or cms. found to be strongly active. Like the emanations, this induced thick is placed over the active material, the a rays are com- activity in a body decays with the time, though at quite a pletely absorbed, and the ionization above the screen is then different rate from the emanation itself. Rutherford (19) due to the B and y rays alone. If a layer of lead about 2 mms. independently showed that thorium possessed a like property. thick is placed over the active material, the B rays are stopped, He showed that the bodies made active behaved as if a thin
onization is then due almost entirely to the penetrating film of intensely active matter were deposited on their surface. g rays. By the use of screens of suitable thickness we are The active matter could be partly removed by rubbing, and thus able to sift out the various types of rays. These three could be dissolved off by strong acids. When the acid was types of radiations all set up secondary radiations in passing evaporated the active matter remained behind. It was shown through matter. A pencil of B rays falling on matter is widely that induced activity was due to the emanations, and could not scattered in all directions. This scattered radiation is some- be produced if no emanation was present. We shall see that times called the secondary B rays. The y rays give rise to induced activity on bodies is due to a deposit of non-gaseous secondary rays which consist in part of scattered y rays and in matter derived from the transformation of the emanations.
rons moving with a high velocity. These secondary Each emanation gives a distinctive active deposit which decays rays in turn produce tertiary rays and so on. The impact of at different rates. The active deposits of radium, thorium and
atter sets free a number of slow moving electrons actinium are very complex, and consist of several types of which are very easily deflected by a magnetic or electric field. matter. Several hours after removal from the emanation the This type of radiation was first observed by J. J. Thomson, active deposit from radium decays to half-value- 26 minutes, and has been called by him the 8 rays.
for actinium half-value-34 minutes, for thorium half-value Emanations or Radioactive Gases.-In addition to their power 10.5 hours. The active deposits obtained on a platinum wire of emitting penetrating radiations, the substances thorium, or plate are volatilized before a white heat, and are again de. actinium and radium possess another very striking and im posited on the cooler bodies in the neighbourhood. Rutherford portant property. Rutherford (15) in 1900 showed that thorium showed that the induced activity could be concentrated on the compounds (especially the oxide) continuously emitted a negative electrode in a strong electric field, indicating that the radioactive emanation or gas. This emanation can be carried radioactive carriers had a positive charge. The distribution away by a current of air and its properties tested apart from of the active deposit in a gas at low pressure has been investi. the substance which produces it. A little later Dorn showed gated in detail by Makower and Russ. that radium possesses a similar property, while Giesel and Theory of Radioactive Transformations.-We have seen that Debierne observed a similar effect with actinium. These the radioactive bodies spontaneously and continuously emit emanations all possess the property of ionizing a gas and, if a great number of a and B particles. In addition, new types sufficiently intense, of producing marked photographic and l of radioactive matter like the emanations and active deposits
appear, and these are quite distinct in chemical and physical produced by the radium per second and N the number present properties from the parent matter. The radiating power is an after an interval 1, then dN/di=9-1N where is the radioatomic property, for it is unaffected by combination of the active constant of the emanation. It is obvious that a steady active element with inactive bodies, and is uninfluenced by the state will ultimately be reached when the number of atoms most powerful chemical and physical agencies at our command. of emanation supplied per second are on the average to the In order to explain these results, Rutherford and Soddy (20) | atoms which break up per second. If N. be the maximum in 1903 put forward a simple but comprehensive theory. The number, q=XN.. Integrating the above equation, it follows atoms of radioactive matter are unstable, and each second a that N/N.= I-el. If a curve be plotted with N as definite fraction of the number of atoms present break up'with ordinates and time as abscissae, it is seen that the recovery explosive violence, in most cases expelling an a or B particle curve is complementary to the decay curve. The two curves with great velocity. Taking as a simple illustration that an for the radium emanation period, 3.9 days, are shown in fig. 1, a particle is expelled during the explosion, the resulting atom the maximum ordinate being in each case 100. has decreased in mass and possesses chemical and physical This process of production and disappearance of active properties entirely distinct from the parent atom. A new type matter holds for all the radioactive bodies. We shall r of matter has thus appeared as a result of the transformation. consider some special cases of the variation of the amount of The atoms of this new matter are again unstable and break up active matter with time which have proved of great importance in turn, the process of successive disintegration of the atom in the analysis of radioactive changes. continuing through a number of distinct stages. On this view, (a) Suppose that initially the matter A is present, and this changes a substance like the radium emanation is derived from the into B and B into C, it is required to find the number of atoms P, Q transformation of radium. The atoms of the emanation are and R of A, B and C present at any subsequent time l. far more unstable than the atoms of radium, and break up at
Let , Ag, Ag be the constants of transformation of A, B and C
respectively. Suppose m be the number of atoms of A initially a much quicker rate. We shall now consider the law of radio- | present. From the law of radioactive change it follows: active transformation according to this theory. It is experi
P=ney mentally observed that in all simple radioactive substances,
dQ/dt = 41P-AQ. the tensity of the radiation decreases in a geometrical pro
Substituting the value of Pin termsof gression with the time, i.e. I/1. = 7*^' where I is the intensity the solution of which is of the form of the radiation at any time 1, I, the initial intensity, and a
Q=n(ac-di' +be-Agr), a constant. Now according to this theory, the intensity of the where a and b are constants. By substitution it is seen that radiation is proportional to the number of atoms breaking up a=1/(x-1). Since Q=0 when I=0,b=-11/(ag-a:) per second. From this it follows that the atoms of active
Thus Q- Le-dl-e-124) ........ matter present decrease in a geometrical progression with the
(3) time, i.e. N/No=(-dówhere N is the number of atoms present Similarly it can be shown that at a time i, N, the initial number, and X the same constant
R=n(de-a36+be-Az1 +ce-A3) . .. .. (4) as before. Differentiating, we have dN/d=-1N, i.e. repre
where = ? sents the fraction of the total number of atoms present which
(A.-12) (1, -13)' (12-21) (Az-as (13-11) (15-19) break up per second. The radioactive constant has a definite It will be seen from (3), that the value of Q, initially zero, increases and characteristic value for each type of matter. Since , is to a maximum and then decays; finally, according to an exponential
law, with the period of the more slowly transformed product, whether usually a very small fraction, it is convenient to distinguish
A or B. the products by stating the time required for half the matter (6) A primary source supplies the matter A at a constant rate, to be transformed. This will be called the period of the product, and the process has continued so long that the amounts of the
products A, B, C have reached a steady limiting value. The primary and is numerically equal to log -21. As far as our observation
source is then suddenly removed. It is required to find the amounts has gone, the law of radioactive change is applicable to all
of A, B and C remaining at any subsequent time l. radioactive matter without exception. It appears to be an In this case of equilibrium, the number 1. of particles of A expression of the law of probability, for the average number supplied per second from the source is equal to the number of particles breaking up per second is proportional to the number present.
which change into B per second, and also of B into C. This requires
the relation Viewed from this point of view, the number of atoms breaking
no = , P. =y:Q. = 1, R. up per second should have a certain average value, but the
where P., Q, R, are the initial number of particles of A, B, C number from second to second should vary within certain present, and 11, 12, 13 are their constants of transformation. limits according to the theory of probability. The theory of Using the samne quotations as in case (1), but remembering the this effect was first put forward by Schweidler, and has since
new initial conditions, it can easily be shown that the number of
particles P. O and R of the matter A, B and C existing at the time been verified by a number of experimenters, including Kohl
I after rernoval are given by rausch, Meyer, and Begener and H. Geiger. This variation in the number of atoms breaking up from moment to moment becomes marked with weak radioactive matter, where only a few atoms break up per second. The variations observed are in good agreement with those to be cxpected from the theory of probability. This effect does not in any way invalidate the law of radioactive change. On an average the number of where a= 7,-2) 0,-1, ,-) (4, -3) CO2, (01-) ,,3 atoms of any simple kind of matter breaking up per second is the curves expressing the rate of variation.of P, Q, R with time proportional to the number present. We shall now considerare in these cases very different from case (1). how the amount of radioactive matter which is supplied at al (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 Ć constant rate from a source varies with the time. For clear- |
present at any time I later, when initially A, B, and C were ness, we shall take the case of the production of emanation, by absent. radium. The rate of transformation of radium is so slow This is a converse case from case (2) and the solutions can be compared with that of the emanation that we may assume
obtained from general considerations. Initially suppose A, B and
Care in equilibrium with the primary source which supplied A at a without sensible error that the number of atoms of radium
constant rate. The source is then removed and the amounts of breaking up per second, i.e. the supply of fresh emanation, is A, B and C vary according to the equation given in case (2). The on the average constant over the interval required. Suppose source after removal continues to supply A at the same rate as that initially radium is completely freed from emanation. In
before. Since initially the product A was in equilibrium with the
source, and the radioactive processes are in no way changed by the consequence of the steady supply, the amount of emanation
removal of the source, it is clear that the amount of A present in present increases, but not at a constant rate, for the emanation is the two parts in which the matter is distributed is unchanged. If in turn breaking up. Let q be the number of atoms of emanation I Po be the amount of A produced by the source in the time 1, and P
products in generals for all recovery awith Pu/P. as ordinais comple
Rays. lin Air in
the amount remaining in the part removed, then PatP=P, where thorium A are changing substances which break up without Po is the equilibrium value. Thus
emitting either penetrating a or B rays. They appear to emit P./P. = 1 -P/P.. The ratio P/P. can be written down from the solution given in
slow 8 rays which can only be detected by special methods, case (2).
| The presence and properties of a rayless product can be easily Similarly the corresponding values of Q./Q., R./R, may be at once derived. It is obvious in these cases that the curve
inferred if it is transformed into a product emitting a radiation, plotted with P/P. as ordinates and time as abscissae is comple for the variation in activity of the latter affords a method of mentary to the corresponding curve with P1/P, as ordinates. This determining the amount of the parent product present. The simple relation holds for all recovery and decay curves of radioactive
distinction between a "ray" and a "rayless " product is n We have so far considered the variation in the number of atoms clear. It may be that the atom of a rayless product undergoes of successive products with time when the periods of the products a re-arrangement of its constituent par ng rise to an atom are known. In practice, the variation of the number of atoms is of the same mass but of different properties. In the case of an deduced from measurements of activity, usually made by the electric method. Using the same notation as before, the activity of any
a ray or ß ray product, the expulsion of an a or ß particle affords product is proportional to its rate of breaking up. ie. to me an obvious explanation of the appearance of a new product where P is the number of atoms present. If two products are present, with distinctive physical properties. the activity is the sum of two corresponding terms A.P and A.Q. In the table a list of the known products of transformation In practice, however, no two products emit a or p particles with l is given. In each case, the half period of transformation is the same velocity. The difference in ionizing power of a single a particle from the two products has thus to be taken into account. given and the type of radiation emitted. If the product emits Il, under the experimental conditions, the ionization produced by a rays, the range of ionization of the a particle in air is given. an a particle from the second product is K times that from the first product, the activity observed is proportional to AP+KAQ. In
TABLE OF RADIOACTIVE PRODUCTS this way, it is possible to compare the theoretical activity curves of a mixture of products with those deduced experimentally.
Range Analysis of Radioactive Changes. The analysis of the suc
Transformacessive changes occurring in uranium, thorium, radium and
Cms. 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 dis
5 X 109 years
Uranium X tinctive physical or chemical properties; (b) that the product
lonium . under consideration arises directly from the product preceding it in the scheme of changes, and is transformed into the product RADIUM
3.5 succeeding it.
3.86 days In general, it has been found that each product shows some
4.83 Radium B
26 mins. slow B1 distinctive chemical or physical behaviour which allows of its
19 mins. a +8+ partial or complete separation from a mixture of other products.
17 years slow B It must be remembered that in most cases the amount of radio
Radium F active matter under examination is too small to detect by
Radium G=lead? weight, but its presence is inferred from its characteristic radiations and rate of change. In some cases, a separation may be THORIUM
about 1010 yrs. effected by ordinary chemical methods, for example thorium X
(Th. 1) 5.5 years rayless is separated from thorium by precipitation of thorium with
Mesothorium (Th. 2)
737 days ammonia. The Th X remains in the filtrate and is practically
3.6 days free from thorium. In other cases, a separation is effected by a
54 secs separation of a metal in the solution of active matter. For
10-6 hours slow B
Thorium B example, polonium (radium F) always comes down with bismuth
55 mins. Thorium C
very short? and may be separated by placing a bismuth plate in a solution.
3 mins. By Radium C is separated from radium B by adding nickel filings to a solution of the two. Radium C is deposited on the nickel.
ACTINIUM In other cases, a partial separation may be effected by electrolysis
19-5 days atp or by differences in volatility when heated. For example,
11.8 days when radium A, B and C are deposited on a platinum plate, on Act Emanation
5.8 heating the plate, radium B is volatilized and is deposited on any
36 mins. slow B cold surface in the neighbourhood. A very striking method of
5.50 Actinium C
5.1 mins. Bty separating certain products has been recently observed depending upon the recoil of an atom which breaks up with the expulsion
In each of the groups under the heading uranium, thorium and of an a particle. The residual atom acquires sufficient velocity |
actinium, each product is derived from the direct transformation of in consequence of the ejection of an a particle to escape and be the product above it. deposited on bodies in the neighbourhood. This is especially marked in a low vacuum. This property was independently Products of Radium.-Radium is transformed directly into the investigated by Russ and Makower (21) and by Hahn (22). emanation which in turn goes through a rapid series of transThe latter has shown that by means of the recoil, actinium C formations called radium A, B and C. The complete analysis may be obtained pure from the active deposit containing of these changes has involved a large amount of work. The actinium A, B and C, for B emits a rays, and actinium C is emanation changes first into radium A, a substance of period driven from the plate by the recoil. In a similar way a new 3 minutes emitting only a rays. Radium A changes into radium product, thorium D, has been isolated. By the recoil method, | B, a product of period 26 minutes emitting Brays of penetrating radium B may be separated from radium A and C. The recoil power small compared with those emitted from the next product method is one of the most definite and certain methods of radium C. The product radium C has proved of considerable settling whether an a ray product is simple or complex.
importance, for it not only emits very penetrating a rays and While in the majority of cases the products break up either B rays, but is the origin of the y rays arising from radium in with the emission of a or B particles, some products have been equilibrium. When a wire charged negatively has been exposed observed which do not emit any characteristic radiation and have for some time in the presence of the radium emanation, it becomes been called "rayless products.” For example, radium D and coated with an invisible film of radium A, B ad C. After