Radioactivity

I particularly insist on the following fact, which appears to me exceedingly important and not in accord with the phenomena which one might expect to observe: the same encrusted crystals placed with respect to the photographic plates in the same conditions and acting through the same screens, but protected from the incident rays and kept in the dark, still produce the same photographic effects. I may relate how I was led to make this observation: among the preceding experiments some had been made ready on Wednesday the twenty-sixth and on Thursday the twenty-seventh of February and as on those days the sun had only shown itself intermittently, I kept my arrangements all prepared and put back the holders in the dark in the drawer of the case, and left in place the crusts of the uranium salt. Since the sun did not show itself for several days I developed the plates on the first of March, expecting to find the images very feeble. The silhouettes appeared on the contrary with great intensity. I at once thought that the action might be able to go on in the dark …
— Antoine Henri Becquerel (1852 – 1908), Comptes rendus 122, 420 (1896), translated by Magie in A Source Book in Physics, McGraw-Hill, New York, 1935.

These rays which are emitted spontaneously from some substances are called Becquerel rays, and we call the substances emitting these rays radio-active.

Mme Curie and I have discovered new radio-active substances which are present as traces in certain minerals, but from which the radio-activity is very intense. We have separated polonium, a substance with similar chemical properties to bismuth, and radium, a close chemical neighbour to barium. Since then, M. Debierne has separated actinium, a radioactive substance similar to the rare earths.

Polonium, radium and actinium produce radiation a million times more intense than that of uranium and thorium. With these substances, the phenomena of radio-activity may be studied in detail, and many researches have been carried out by various physicists in recent years. Tonight I wish to speak of radium, because we have recently proved that it is an element which we have isolated in the pure state.
— Pierre Curie (1859 – 1906), lecture to the Royal Institution, June 19, 1903.

In a previous paper I have given an account of the indirect experimental evidence in favour of the view that alpha rays consist of projected charged particles. Preliminary experiments undertaken to settle this question during the past two years gave negative results. The magnetic deviation, even in a strong magnetic field, is so small that very special methods are necessary to detect and measure it. The smallness of the deviation of the alpha rays, compared with that of the cathode rays in a vacuum-tube, may be judged from the fact that the alpha rays, projected at right angles to a magnetic field of strength 10,000 C. G. S. units, describe the arc of a circle of radius about 39 cm, while under the same conditions the cathode rays would describe a circle of radius about 0.1 cm.

In the early experiments, radium of strength 1000 was used, but this did not give out strong enough rays to push the experiment to the necessary limit. The general method employed was to pass the rays through narrow slits and to observe whether the rate of discharge, due to the issuing rays, was altered by the application of a magnetic field. When, however, the rays were sent through sufficiently narrow slits to detect a small deviation of the rays, the rate of discharge of the issuing rays became too small to measure, even with a sensitive electrometer.

I have recently obtained a sample of radium of strength 19,000, and using an electroscope instead of an electrometer, I have been able to extend the experiments, and to show that the alpha rays are all deviated by a strong magnetic field.
— Ernest Rutherford, 1st Baron Rutherford of Nelson, Philosophical Magazine, February 1903.

Soon thereafter, when the discovery of artificial radioactivity by Frederic Joliot and Irene Curie was announced, I suddenly saw that the tools were at hand to explore the possibility of such a chain reaction. I talked to a number of people about this. I remember that I mentioned it to George Thomson and to M. S. Blackett, but I couldn’t evoke any enthusiasm…

In the spring of 1934 I had applied for a patent which described the laws governing such a chain reaction. This was the first time, I think, that the concept of critical mass was developed and that a chain reaction was seriously discussed. Knowing what this would mean — and I knew it because I had read H. G. Wells — I did not want this patent to become public. The only way of keeping it from becoming public was to assign it to the government. So I assigned the patent to the British Admiralty.
— Leo Szilard (1898 – 1964), Leo Szilard: His Version of the Facts, MIT Press.

The detailed investigations of the fission products formed by uranium fission resulted in the chemical identification of about 100 different radioactive isotopes belonging to 25 different elements. In no case, however, was it possible to furnish convincing evidence of the presence of two complementary primary fission fragments.

Four krypton isotopes, for instance, were demonstrated to be primary fragments, and one would have expected to find four corresponding primary barium isotopes. But not one of the detected barium isotopes were primary products; all of them were found to be disintegration products of xenon. If, however, these xenon isotopes represented primary products one would have expected the complementary fission fragments to be strontium isotopes. All strontium isotopes so far detected, however, were disintegration products of krypton.

Two reasons may account for this fact. Firstly, when using chemical methods of separation it will not be possible to demonstrate the presence of very short lived products. Secondly, the large number of the products detected as well as the theory advanced by Bohr and Wheeler, support the view that the manner in which the excited uranium nucleus divides, varies greatly. The curves constructed by Bohr and Wheeler clearly indicate that the greater the energy evolved the higher is the charge of a fragment, the mass of which is given. In other words a radioactive isotope created by the uranium fission can be formed as primary fragment as well as disintegration product of an isobar* of lower charge depending on the amount of energy released in the respective fission process. When attempting its chemical identification it may depend on the method of separation used, whether it will appear as primary or secondary product.
— Lise Meitner (1878 – 1968), An Attempt to Single Out Some Fission Processes of Uranium by Using the Differences in Their Energy Release, Reviews of Modern Physics, 17 (2 & 3), 1945, 287-291.

* [Isobars in this context are nuclides with the same mass number, or total of neutrons and protons.]

Sir:

Some recent work by E. Fermi and L. Szilard, which has been communicated to me in manuscript, leads me to expect that the element uranium may be turned into a new and important source of energy in the immediate future. Certain aspects of the situation which has arisen seem to call for watchfulness and, if necessary, quick action on the part of the Administration. I believe therefore that it is my duty to bring to your attention the following facts and recommendations:

In the course of the last four months it has been made probable – through the work of Joliot in France as well as Fermi and Szilard in America – that it may become possible to set up a nuclear chain reaction in a large mass of uranium, by which vast amounts of power and vast quantities of new radium-like elements would be generated. Now it appears almost certain that this could be achieved in the immediate future.

This new phenomenon would also lead to the construction of bombs, and it is conceivable – though much less certain – that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port together with some of the surrounding territory. However, such bombs might very well prove too heavy for transportation by air.

The United States has only very poor ores of uranium in moderate quantities. There is some good ore in Canada and the former Czechoslovakia, while the most important source of uranium is Belgian Congo.

In view of this situation you may think it desirable to have some permanent contact maintained between the Administration and the group of physicists working on chain reactions in America. One possible way of achieving this might be for you to entrust with this task a person who has your confidence and who could perhaps serve in an inofficial [sic] capacity. His task might comprise the following:

a) to approach Government Departments, keep them informed of the further development, and put forward recommendations for Government action, giving particular attention to securing a supply of uranium for the United States;

b) to speed up the experimental work, which is at present being carried on within the limits of the budgets of University laboratories, by providing funds, if such funds be required, through his contacts with private persons who are willing to make contributions for this cause, and perhaps also by obtaining the co-operation of industrial laboratories which have the necessary equipment.

I understand that Germany has actually stopped the sale of uranium from the Czechoslovakian mines which she has taken over. That she should have taken such early action might perhaps be understood on the ground that the son of the German Under-Secretary of State, von Weizsacker, is attached to the Kaiser-Wilhelm-Institut in Berlin where some of the American work on uranium is now being repeated.

Yours very truly,

(Albert Einstein)
— Albert Einstein (1879 – 1955), letter to President F. D. Roosevelt, August 2, 1939.

When you see something that is technically sweet, you go ahead and do it and you argue about what to do about it only after you have had your technical success.
— J. Robert Oppenheimer (1904 – 1967), quoted in R. W. Reid, Tongues of Conscience: Weapons Research and the Scientist’s Dilemma, 1969.

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