sábado, 29 de dezembro de 2012

Strassmann proposed an elegant experiment whereby they would use barium chloride to carry the "parent" radioactive substance out of the irradiated uranium solution. Barium chloride precipitates in perfect crystals which can be relied upon to be clean of any of the numerous transuranic elements also unquestionably produced by the irradiation. The apparatus was simple and inexpensive: a tube containing a uranium compound was exposed to neutrons, emitted by a source comprising one gram of radium mixed with beryllium and slowed down by a block of paraffin wax, a source many times weaker than the cyclotrons at the disposal of foreign countries. The irradiated uranium solution, now containing the mysterious 3,5-hour substance among a host of other elements produced by the neutron bombardment, was mixed with barium chloride; the crystals formed now contained the minute quantities of what were believed to be radium isotopes. The presence of these isotopes was confirmed when the crystals were checked with Geiger-Müller counters whose pulses were amplified by a simple amplifier powered by scores of Pertrix HT batteries stacked under the wooden bench. The amplified pulses triggered clockwork counter-mechanisms, which Hahn, Strassmann and their two assistants read at fixed intervals, to establish the half-lives of the radioactive substances they had produced.

It was a difficult experiment; the minute quantities of these new radioactive substances were choked by masses of non-radioactive barium chloride crystals, and this led to a routine attempt to separate the supposed "radium" isotopes from the barium carrier, so that the radioactivity of the isotopes could be examined more conveniently. To separate the "radium", they would use their familiar process of fractional crystallization, the process which had originally been used by Mme. Marie Curie to isolate radium. Hahn and Strassmann had performed this experiment many times before, and were thoroughly familiar with it.

When they applied the method now, however, they found to their surprise that they could achieve no extraction of the supposed "radium" isotopes at all.

Was there some error in their technique? During the third week in December, Hahn decided upon a control experiment: he repeated the fractional crystallization, substituting this time a known radioactive isotope of radium — thorium-X — in the solution for their own supposed "radium" isotopes, and diluting the solution until it showed as little radioactivity as their "radium" had. This control experiment went just as it should: a few atoms of the genuine radium isotope could be separated from the barium carrier just as theory had predicted, so there was nothing wrong with their technique.

On Saturday, December 17, Hahn and Strassmann were stil thoroughly bewildered by this unexpected turn of events, but the truth was gradually dawning upon them; that day they repeated the two experiments, simultaneously this time, with both their artificial "radium" isotope and the natural radium isotope mesothorium-I in the same solution, the latter isotope acting as an "indicator". These radioactive traces were carried out of the uranium solution by the same barium carrier, precipitated and fractionally crystallized together — an extraordinarily complex experiment, confused and fouled at every stage by the production of families of radioactive decay products from all the ingredients. At each stage of the crystallization, samples of the barium crystal were tested for radioactivity: the Geiger–Müller counter showed beyond doubt that the mesothorium — the genuine radium isotope — was concentrating from one stage to the next as it should, while their own artificial "radium" isotope was not. The latter was uniformly distributed among the barium crystals sampled at each stage — as uniform as the barium itself. The uniformity was strange, but significant. That night, Hahn wrote in his diary: "Exciting fractionation of radium/barium/mesothorium".

He himself was in doubt no longer: what they had believed to be a radioactive isotope of "radium" could not be separated from barium by any chemical means, because it was in fact a radioactive isotope of barium.

The slow-neutron bombardment of uranium — the heaviest naturally occurring element on earth — had yielded barium, an element not much over half its weight. The uranium atom had burst asunder. Despite the costly equipment of the great foreign physical laboratories, it was a German chemist working with the most primitive of equipment who had made the discovery that was to throw the world of physics into pandemonium.

in The Virus House — Germany's Atomic Research and Allied Counter-Measures; David Irving; Focal Point, 1967.