In 1896 radioactivity was discovered "by accident" by the French physicist Henri Becquerel (1852-1908). Becquerel's discovery came within two months after Rontgen's published announcement of the discovery of x-rays. Rontgen had pointed out that the x-rays came from the spot on the glass tube where the beam of cathode rays (electrons) was striking and that this spot showed strong fluorescence. It occurred to Becquerel and others that x-ray might be related fluorescence and phosphorescence. (At the time fluorescence was generally called phosphorescence, meaning a substance that emitted light. Now fluorescence is distinguish from phosphorescence. A phosphorescent substance differs from a fluorescent substance in that it continues to emit light for a time after the exciting agency is removed; that is, for example, a phosphorescent substance will glow in the dark after having been exposed to the sunlight.) To find out if x-rays are related phosphorescence, Becquerel tested a number phosphorescent substance to find out whether they emitted x-rays while phosphorescing. He had no success until he tested an uranium compound, the double sulfate of uranium and potassium, a compound that had been shown by his father, Edmond Becquerel (1820-1891), to have the property of phosphorescence. The results of his investigation he communicated to the French Academy on February 24, 1896. He reported,
"Let a photographic plate be wrapped in two sheets of very thick black paper, such that the plate is not affected by exposure to the sun for a day. Outside the paper place a quantity of the phosphorescent substance, and expose the whole to the sun for several hours. When the plate is developed, it displays a silhouette of the phosphorescent substance. So the latter must emit radiations which are capable of passing through paper opaque to ordinary light, and of affecting salts of silver."At the time Becquerel supposed the radiation to have been excited by the exposure of the phosphorescent substance to the sun; but a week later he announced that in one experiment the sun had become obscured almost as soon as the exposure was begun, and yet when the photographic plate was developed, the intensity of the silhouette was as strong as in the other cases. And in addition he found that the radiation persisted for an indefinite time after the substance had been removed from the sunlight, and after the luminosity which properly constitutes phosphorescence had stopped; he concluded that the activity was spontaneous and permanent. He soon found that those salts of uranium that do not phosphoresce, that is, the uranium series of salts and the metal itself, all emit the rays. It became evident that what Becquerel had discovered was a radically new property, possessed by the element uranium in all its chemical compounds.
Becquerel also very soon found that the new rays, like the Rontgen and cathode rays, could make gases electrical conductors, as indicated by the rate of discharge of a charged electroscope. The conductivity of x-rays was being at that time investigated at the Cavendish Laboratory, Cambridge, England, by J.J. Thomson, who had been joined in the summer of 1895 by a young research student from New Zealand named Ernest Rutherford (1871-1937). They found that the conductivity was due to ions, that is, electrical charged particles, which are produced in the gas by the radiation, and which are set in motion when an electric field is applied. Rutherford went on to examine the conductivity produced by the rays of uranium and the absorption of these rays by matter; he found that the rays are not all of the same kind, but that there are present at least two distinct kinds. One of these, to which he gave the name alpha rays, is readily absorbed; while another, which he named beta rays, has a penetrating power hundred times greater than the alpha rays.
One of Becquerel's colleagues in Paris was the physicist Pierre Curie, the husband of Marie Sklodowska Curie. Marie Sklodowska, born in Warsaw in 1867 (died 1934), had studied physics in Paris, and in 1895 she married a young French physicist, Pierre Curie (1859-1900). Early in 1898 she decided as her doctoral thesis to search for other substances having the properties that Becquerel had found in uranium, and in April, 1898, she discovered that the compounds of thorium possessed these properties; of the elements known at that time thorium stood next to uranium in the order of atomic weights; at that time uranium was the heaviest element known. Simultaneously the same discovery about thorium was made independently by G. C. Schmidt in Germany. This was an important discovery because until this time it was thought that this phenomena was a unique property of just one element, uranium. Madame Curie went on to show that, since the emission of rays by uranium and thorium is unaffected by chemical changes, it must be essentially an atomic property. Now the mineral pitchblende, from which the uranium is derived, was found to have an activity much greater than could be accounted for by the uranium it contained in it; pitchblende was about 80% uranium oxide (U3O8). From this Madame Curie inferred that pitchblende must contained another "radio-active" element. Pierre Curie had laid aside his researches in other fields and was assisting his wife. Together they made a systematic chemical analysis of the pitchblende and after a laborious and painstaking search in July, 1898, they discovered a new element which she named polonium, in honor of her native country, Poland. And then in December another element was discovered, having an activity many million times greater than uranium; they named it radium, because of its extraordinary radioactivity. In 1900 another French physicist, Andre Debierne (born 1874), discovered in the uranium residues another radioactive element, which he gave the name actinium.
In the three and a half years they determined the specific properties of these new elements, especially their atomic weights. They had not yet been able to isolate these element, let alone obtain a pure sample of one of their compounds. From the residue containing the strongly radioactive material they had separated a fraction consisting of barium chloride mixed with a small quantity of radium chloride. By additional fractionation gave them an increasing proportion of radium chloride. And to make it even more difficult the properties of the radium chloride were nearly the same as the barium chloride, for it is the difference in chemical properties that makes possible the separation. And to obtain usable amounts in successive fractions, they had to start with a very large amount of the original material. With an initial 100 kilogram shipment of pitchblende which was a gift of the Austrian government of what was considered "useless" and from which the uranium salt had been removed for use in manufacture of glass the Curies prepared a "laboratory" at the School of Physics where Pierre Curie taught. Failing to obtain financial support, the Curies began work without technical assistance in an abandoned wooden shed. Madame Curie wrote later,
"I came to treat as many as 20 kilograms of matter at a time, which had the effect of filling the shed with great jars full of precipitates and liquids. It was killing work to carry the receivers, to pour off liquids and to stir, for hours at a stretch, the boiling matter in a smelting basin."Continuing this tedious process for four years, during which she treated several tons of pitchblende residue, Marie Curie was able to report in July, 1902, that 0.1 grams of radium chloride had been obtained, so pure that spectroscopic examination showed no evidence of any remaining barium. The atomic weight of radium could be computed on the reasonable assumption that radium is divalent, chemically similar to barium and calcium. The value that was obtained 225 (the present day value is 226.05). In 1903 the Nobel Prize for Physics was awarded jointly to Becquerel and the Curies for their work on the discovery of radioactivity. In 1910 Marie Curie isolated radium metal by means of electrolysis of molten radium chloride. In 1911 a unique recognition was given Marie Curie when she also was awarded the Nobel Prize for Chemistry for the discovery of the elements polonium and radium. In 1906 Pierre Curie died as result of a traffic accident.
A few months after the discovery of the alpha and beta rays by Rutherford, it was shown by Fiesel, Becquerel and others, that part of the radiation (the beta-rays) could be deflected by a magnetic field, while another part (the alpha-rays) was not appreciably deflected. After this in 1900, Monsieur and Madame Curie found that the beta rays carried negative electric charges, and Becquerel succeeded in deviating them by an electrostatic field. The beta rays are clearly of the same nature as cathode rays (electrons); and when measurements of the electric and magnetic deviations gave for ratio of m/e, a value in the order 10-7, agreeing with the value found by J.J. Thomson for the electron, the identity of the beta particles with cathode rays was fully established. They differed from the cathode rays only in velocity, the beta rays being very much the swifter. Becquerel found their speeds to be about half the speed of light.
The alpha rays at this time were supposed to be not deviated by a magnetic field; the deviation of the alpha rays is very small, even when the electric field is powerful; but in February, 1903, Rutherford announced that he had succeeded in deviating them by both magnetic and electrostatic fields. The deviation of the alpha rays was in the opposite sense to that of the cathode rays, so the alpha radiation must consist of positively charged particles being ejected with great velocity, and the smallness of the deviation suggested that the expelled particles were massive compared to the electron. The best value for q/m of an alpha particle that he found in subsequent work was about (+)4.8 × 107 coul/kgm, about 4000 times smaller than q/m for beta particles. In fact the value of q/m for alpha particles is just one-half the value of q/m for a hydrogen ion. This can be explained in a reasonable way if the alpha particle were like a hydrogen molecule minus one of its two electrons (H2+), or else like a helium atom, but missing both of its electrons (He++). Other possibilities might have been considered, bare nuclei of carbon, nitrogen, or oxygen would have almost the same q/m. Rutherford, who considered all possibilities, considered the most possible was that the alpha particle is a doubly-ionized helium atom (or a helium nucleus, as we now put it). In a series of experiments from 1906 to 1909 he succeeded in proving his hypothesis in several different ways.
In 1903, P. Villard discovered that in addition to the alpha and beta rays, radium emits a third type of radiation, much more penetrating than either of them, in fact 160 times penetrating as the beta rays. Villard's radiation was called gamma rays. Villard found that the gamma radiation, like x-rays, is not deflected by a magnetic field. Later these gamma rays were found in the emissions of other radioactive elements. And when their wavelength was determined they were found to be shorter than those of x-rays.
The following is a method for separating the three components of radioactive emissions. Some radioactive material is placed at the bottom of a long, narrow hole in a block of lead. A fairly parallel beam of emission thus emerges from the hole, while emissions in all other directions are absorbed by the lead. A strong magnetic field, perpendicular to the direction of travel of the rays, causes the charged particles to be deflected sideways. The whole apparatus is placed in an evacuated box to avoid absorption of any part of the radiation by the air. After separation the beams my be detected by a photographic plate to provide a record.
The almost immediate result of the discovery of radioactivity was a reconsideration of the existing conception of matter and its structure. The emission of the alpha and beta particles was not in accord with the accepted atomic-molecular theory of chemistry. It was assumed that
(a) a pure element is a collection of identical atoms, andTwo of the most basic and useful postulates in physics also seemed threatened, namely,
(b) atoms are indestructible and unchangeable (except for the temporary effects of ionization and excitation).
(c) the principle of conservation of mass, andThe contradictions are clear. Consider the relatively massive alpha particles emitted from pure samples of elements such as radium, they must come from the atoms of the radioactive elements; there is no other source. Yet, according to assumption (b) above, these atoms are unchangeable. Thus we have a situation where a radioactive atom expelling an alpha particle of a definite mass, but without the atom losing mass; this contradicts (c). Similarly the emission of tremendous amounts of energy carried off by the radioactive emissions. But by (d), the atoms initially must have at least as much energy as they give off. The energy released in ordinary chemical reactions comes from the reduction of potential energy as the result of rearrangements of the atoms to form different molecules. But radioactivity does not involve chemical changes between different substances, because it occurs in samples of pure elements.
(d) the principle of conservation of energy.
To resolve these difficulties, Rutherford and Frederick Soddy, in November, 1902, published a bold and radical solution, a theory of radioactive transformation. They swept away the above assumptions, and asserted that:
Radon turns out to be radioactive, each atom emitting an alpha
particle and thereby decaying into an atom of an element which
was called Radium A (RaA):
Rn → RaA + He++.
Radium A is a solid, and is also radioactive. It turns out, that
the original radium atoms experience a succession of transformations
into new radioactive "daughter" elements until one is
reached that is stable, or nonradioactive, namely lead.
The chain beginning with radium has 10 members, some of which
emits beta particles rather than alpha particles; gamma rays do
not appear alone but along with the emission of an alpha particle
or beta particle. Rutherford also suggested that since radium
is always found in uranium minerals, it may be merely only a member
of a series starting with uranium as the ancestor of all members
of the series. We now know that this is the case. Each uranium
atom may in time turn into successive daughter atoms, radium being
the sixth generation and stable lead the fifteenth.
The various radioactive elements show great difference with respect
to decay rates. There is no way of predicting when a particular
atom will decay, nor is there any known way to stimulate or inhibit
its decay. As far as we can tell, the decay rate is not affected
by extreme temperature or pressures, by strong electromagnetic
fields, or by its past history. But regardless of how long a
particular parent has survived, there is a definite probability
when it will decay. The experimental fact is that a given pure
sample of a radioactivity element containing initially the number
N0 atoms will by and by change into atoms of a chemically
different element, as, for example, each radium atom giving off
alpha radiation will change into an atom of the element radon.
After a time t seconds, only N atoms from the original
quantity N0 are still unchanged. The original pure
element decays exponentially according to the following radioactive
decay law:
N = N0e-λt,
where N and N0 are defined above,
e = 2.718...,
and λ is called the decay constant.
If the number N of atoms left at the end of time t is
plotted as a function of t, we get a curve that approaches the
time axis asymptotically, which is another way of saying that there is
no finite time at which the value of N falls to zero; hence, the
concept of "total lifetime" is not applicable. But it is possible to
specify the fraction of N/N0 of a sample to decay.
For convenience the fraction ½ has been chosen. To the time T
for the sample to decay ½ of the original amount, Rutherford
appropriately gave the name half-life. The primary advantage of
this concept of half-life lies in the experimental result that no matter
how old the sample with a given life-time T at a given moment, in
an additional time T half of the atoms will still have survived.
Thus the half-life must not be thought of as an abbreviation for "half of
a life." If one-half of the original atoms remain unchanged after the
time T, one-fourth [= ½ × ½] will remain after
two consecutive half-life intervals 2T, one-eigth after 3T,
and so forth. Note also that the probability of survival for atoms is
unchanged by the age they have already reached. And we are not dealing
with the behavior of individual atoms but with the behavior of very large
number of atoms.
It is impossible to predict how long a given parent atom will live before
undergoing spontaneous decay. There are, however, two statistical measures
of the lifetime in common use: the half-life and the average lifetime.
The average lifetime tav is the mean life of the
element, and is the reciprocal of the decay constant, that is,
tav = 1/λ;
and the half-life T is defined as the time required for
one-half of the initial population N0 to disintegrate.
Mathematically, using the radioactive decay law,
N/N0 = 1/2 = e-λT,
or,
λT = loge2.
Therefore, the half-life is
T = loge2/λ = 0.693/λ.
That is, the half-life T can be determined as soon as the
decay constant λ is known for the specific radioactive element.
After the expiration of n half-lives, the population will be
reduced to (1/2)n of its original value. Naturally occurring
radioactive elements have half-lives ranging from 10-14 seconds
to 1011 years, a range of 32 powers of 10! In other words,
for each given kind of radioactive element there is definite amount
of time in which a given fraction of the original sample will
decay into its daughter element. This time is called the half-life
of the element. For example, the half-life of radium is rather long, 1620
years, which explains why the Curies were unable to detect rapid
diminution in its rate of energy release with time. The half-life
of radon, on the other hand, is only 3.82 days, and Rutherford
was able to detect rapid depletion in the radioactive intensities
of radon samples soon after he had discovered this element. In
the uranium decay chain, uranium itself has the longest half-life
of 4.51 × 109 years.
| Radioactive Element | Decay Emission | Half-life T |
|---|---|---|
| UI | α | 4.51 × 109 years |
| UX1 | β, γ | 24.1 days |
| UX2 | β, γ | 1.18 minutes |
| UII | α | 2.48 × 105 years |
| Io | α, γ | 8.0 × 104 years |
| Ra | α, γ | 1620 years |
| Rn | α | 3.82 days |
| RaA | α | 3.05 minutes |
| RaB | β, γ | 26.8 minutes |
| RaC | β, γ | 19.7 minutes |
| RaC′ | α | 1.64 × 10-4 seconds |
| RaD | β, γ | 19.4 years |
| RaE | β | 5.0 days |
| RaF | α, γ | 138.4 days |
| RaG | (Stable Lead) |