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Uranium's Scientific History 1789 - 1939 [Image]

This paper was presented by Dr. Bertrand Goldschmidt at the Fourteenth
International Symposium held by the Uranium Institute in London, September
1989. This particular symposium had as its theme the bicentenary of the
discovery of Uranium by the German chemist Professor Klaproth in September
1789.

It is a relief and a treat for a Frenchman to be asked in 1989 to speak
about a bicentenary other than that of the French Revolution. The
bicentenary of the discovery of uranium coincides with the fiftieth
anniversary of the discovery of fission, an event of worldwide significance
and the last episode in the uranium-radium saga, which is the main theme of
this presentation.

The story starts at the beginning of the sixteenth century in the unexplored
mountainous region separating Bohemia from Saxony. This area was then
covered by an impenetrable virgin forest, a refuge for wolves and bears.
Human settlements advanced slowly along small rivers, until the discovery of
silver in one of the rivers triggered the first precious metal rush in
history. This led to the foundation of the town of Sankt Joachimsthal (the
valley of Saint Joachim), soon to become the largest mining centre in Europe
with the relatively large population of 20 000 inhabitants, at a time when
Prague, the capital of Bohemia, had around 50 000.

The discovery of silver led to the minting of some two million large silver
coins called Joachimsthaler, later known more simply as Thaler. These became
a currency accepted worldwide, and their name is, by slight distortion, the
origin of the word dollar.

The prosperity of Joachimsthal was short lived, however. By the middle of
the sixteenth century, depletion of the mines and a lack of pumping machines
needed for deeper mining made it difficult to compete with silver from the
new American colonies, which was arriving in increasing quantities on the
European market. After plague and destruction from the Thirty Years War,
Joachimsthal was almost a ghost town in the first half of the seventeenth
century.

However, the mines, which were the property of the Hapsburg crown, never
closed. The exploitation of bismuth and cobalt deposits continued, and
improvements in mining technology allowed silver production to pick up again
during the eighteenth century. But it was a discovery in chemistry which was
to give to the town and to the mines a second wind and a new direction.

The miners had long since detected in the mines a shiny black mineral whose
presence seemed incompatible with that of silver and had nicknamed it
pechblende, from the German words pech, which means either pitch or bad
luck, and blende, meaning mineral. The first complete analysis of
pitchblende was carried out in 1789 by a talented self-educated German
chemist, Martin Klaproth.

Having extracted from pitchblende what he called 'a strange kind of half
metal' (he had only isolated its oxide), he resisted the temptation to give
his own name to the new element, which was quite customary at the time. It
is due to his modesty that we do not today have to use the rather cumbersome
name Klaprothium.

He made his discovery during the summer, while the people of Paris were
storming the Bastille prison, and his paper was presented to the Berlin
Academy of Sciences on 24 September 1789. He proposed, until a better name
was found, to call his element after the last planet to have been
discovered, thus rendering homage to his compatriot William Herschel.
Herschel was a musician who had emigrated to England and had become both the
director of the orchestra at the celebrated spa, Bath, and a first class
astronomer. His fame was crowned by the discovery of a new planet, named
Uranus after Urania, the muse of astronomy and geometry. This muse was thus
doubly honoured when Klaproth called the new element 'uran', which in its
final form became uranium, a name which is today known worldwide while
Klaproth's own fame has faded.

During the following hundred years the history of the new element was not
particularly dramatic. It was found in many places in the world, but always
in deposits less rich than those at Joachimsthal. Fifteen years after
Klaproth's discovery uranium had been found in Cornwall in Britain, in
Morvan in France, and in Austria and Romania. By the end of the nineteenth
century about 1000 papers had been published on geological and mineral
occurences of uranium around the globe.

The metal, as dense as gold, was first prepared, with some difficulty, in
184l by the French chemist Eugène Peligot, using a thermal reaction of
tetrachloride with potassium. Later, in 1870, an important fact was
established: uranium is the last and heaviest element present on earth. This
was demonstrated by the Russian chemist Dimitri Mendeleev in his famous
periodical classification of the elements by chemical properties and
increasing atomic mass.

For about a century and a half after Klaproth's discovery, the main
application for uranium derived from the vivid colours of its oxides and
salts. These were used to produce yellow glass with green fluorescence, and
glazes for ceramics and porcelain in orange, yellow, red, green and black.
Later, uranyl nitrate was used in early photography to give a sepia tint to
prints and films, and to reinforce negative plates.

The colouring processes for glass and ceramics were for a long time the
jealously guarded monopoly of Bohemia. Thus secrecy appeared for the first
time very early in the history of uranium, before returning in full force in
the 1940s.

The commercial use of uranium for glass and porcelain colouring resulted in
a renewal of activity in Joachimsthal, and soon the production of these
coloured uranium compounds became more important than that of silver. In
1855 a plant for the production of large quantities of various yellow and
orange compounds was built by the Austrian chemist Adolf Patera, and the
export from Bohemia of these expensive materials became an important source
of revenue for the local mining industry. It is difficult to estimate with
precision the quantity of uranium produced for this purpose during the
nineteenth century, but it is probably in the region of 300 to 400 tonnes,
of which a small fraction came from Cornwall and later from Portugal and
Colorado, USA.

However, by the end of the nineteenth century the Joachimsthal deposits once
more ceased to be profitable. The mine shafts were becoming deeper and
deeper, while the price of coloured uranium compounds fell on the world
market with the production of new colouring materials. The town's mines were
once again threatened with closure by their owner, the Austro-Hungarian
government. They were unexpectedly saved by a remarkable series of
scientific discoveries.

At dusk on the evening of 8 November 1895, Wilhelm Röntgen, professor of
physics at the University of Würzburg in Germany, noticed while manipulating
a cathode tube that a sheet of paper some distance away impregnated with
barium platinocyanid had started to fluoresce. Furthermore, when he put his
hand between the tube and the paper, he saw an image of the bones in his
hand on the paper.

He was so surprised by this discovery that he decided to study thoroughly
the properties of what he later called X-rays before mentioning it to
anybody. For seven weeks he pursued his experiments alone, without even his
wife knowing what he was doing. She found him more and more preoccupied and
tense, and even became worried about his mental health.

Those were the happy days when scientific discoveries were not announced
through press conferences even before they had been made. On 28 December
Röntgen decided he had untangled the problem, so he prepared a detailed
paper for a scientific journal and sent reprints with an accompanying X-ray
photograph of his hand to the most renowned physicists in Europe. They were
thus able to check for themselves the validity of the discovery which
brought Röntgen the first Nobel prize in physics ever awarded, in 1901.

The French scientist Henri Poincaré, who was one of the physicists to
receive the reprint, took it to the weekly meeting of the Académie des
Sciences in Paris on 24 January 1896 and showed it to his colleague Henri
Becquerel. Becquerel was the third generation of his family to hold the same
chair of physics at the Museum of Natural History in Paris, and worked in
the same laboratory on the same subjects as his father and grandfather, in
particular on the behaviour and properties of phosphorescent and fluorescent
substances, including uranium salts. Poincaré suggested that, since X-rays
could cause fluorescence, Becquerel should investigate whether some
phosphorescent substances would emit these new rays.

During the next few weeks Becquerel experimented with various substances
with no success, until he tried uranyl potassium sulphate, which becomes
phosphorescent after exposure to the sun. He found that after such exposure
this compound would fog a photographic plate covered in black paper. He
announced this result at the Académie's meeting on 24 February, 1986.

For the following few days bad weather prevented the sun's appearance, and
Becquerel left a photographic plate and the uranium compound in a drawer.
When the sun reappeared he developed the plate to check that the fogging was
caused by the exposure of the compound to the sun. He expected to find the
plate blank, but in fact it was considerably fogged, showing that the rays
emitted by the uranium compound were independent of its exposure to the sun.

Becquerel was soon able to prove that the emission of these rays was linked
to the presence of uranium, and that they would ionize air in the same way
as X-rays. In exactly four months two revolutionary discoveries had taken
place, X-rays and the so-called Becquerel or uranic rays, the study of which
would mark the start of the era of atomic science.

While X-rays created immediate worldwide interest among scientists and
physicians with more than a thousand publications in 1896, the year their
discovery was announced, for two years Becquerel rays remained a scientific
curiosity studied calmly by their discoverer as if scientific progress in
this direction was taking a breathing space before making a leap forward.

This leap was made by the French physicists Pierre Curie and his Polish-born
wife Marie Sklodowska. Pierre Curie had previously discovered, with his
brother Jacques, the piezoelectric properties of quartz and had used them to
measure with precision the ionization of air. He suggested to his wife, just
recovering from the birth of their first child Irène, that the subject for
her doctorate thesis should be the accurate measurement of the rays emitted
by uranium.

Her first results confirmed Becquerel's finding that the intensity of the
rays was proportional to the concentration of uranium in the compound
studied. However, she then decided to investigate uranium ores, starting
with a sample of the 'unlucky' ore pitchblende. She was surprised to find
that the emission of rays from the ore was far greater than would be
expected from its uranium content. She came to the conclusion, published in
April 1898, that this activity was due to trace amounts of a new element far
more active than uranium.

She continued her work with her husband, and they were able to identify non
weighable amounts of a new element which they called polonium after her
native country. Assisted by a chemist, Gustave Bémont, they went on to
discover a second new element, which they named radium. To try to obtain
larger quantities of these new elements, the Curies wrote to Eduard Suess,
the president of the Academy of Sciences of Vienna and a respected
geologist, asking for recent residues from the extraction of uranium at
Joachimsthal.

A few weeks later, a heavily laden horse-drawn cart delivered to their
laboratory - a primitive shed at the School of Physics and Chemistry in
Paris - canvas bags filled with brownish residues still mixed with earth and
pine needles. This was the first and most important transport of radioactive
waste. Nobody could then predict that nearly a century later radioactive
wastes would have strong social and political implications.

From this first tonne of residues the Curies isolated, after two years of
exhausting work, one tenth of a gram of radium bromide. After a further two
years, in 1904, they had separated the first gram of radium from a further
eight tonnes of residues from the Joachimsthal uranium refinery.

By this time medical applications of radium had been discovered and were
developing rapidly. Pierre Curie collaborated with some well known
physicians in a successful study which attracted considerable publicity.
'Curie therapy' became, together with surgery, the only means of treating
deep-seated cancers before chemotherapy was introduced. Fine needles
containing radium or its daughter product radon were most often used for
this purpose.

This therapy contributed to the fame of the Curies, which had already built
up around the extraordinary career of Marie, the first woman to obtain a PhD
in science in France, and the first to be awarded a Nobel prize for physics,
which she shared in 1903 with Becquerel and her husband. Later, in 1911, six
years after Pierre's untimely accidental death, she was awarded a second
Nobel prize, this time alone and in chemistry.

For several decades after the discovery of radium it was believed that
radiation in small doses was beneficial to health, having a stimulating
effect. This belief, neither proven nor disproven, was not affected by the
news in the 1920s of the first casualties caused by the ingestion of radium
and of the deep skin burns to the hands of those who, like Marie Curie,
manipulated strong radioactive sources without precaution. Such a belief was
probably linked to the presence of small amounts of radium and radon in the
waters of the main health spas.

It was not the Curies but a British team working in Canada which was the
first to understand that the presence of polonium and radium in pitchblende
was not due to simple geological and mineral reasons, but that these
elements were directly linked to uranium by a process of natural radioactive
transmutation. The theory of radioactive transformation of elements was
brilliantly elaborated in 1901 by the New Zealand physicist Ernest
Rutherford and the English chemist Frederick Soddy at McGill University in
Montreal.

Thus it was found that uranium is weakly radioactive, decaying slowly but
inexorably at the rate of one milligram per tonne per year. It is
transformed into inactive lead through a chain of radioelements or
daughters, each of which has a characteristic disintegration rate, a
constant of nature that man has never been able to alter. The proportion of
each radioelement in the ore is inversely proportional to its rate of
disintegration. Radium is the fifth radioactive descendant in the chain from
uranium to lead, its daughter is the gas radon, and polonium is the last
radioelement before lead.

While the alchemists of the Middle Ages dreamt of transforming worthless
lead into gold, it is the reverse which occurs with Klaproth's element,
although so slowly that uranium is still present in the earth's crust
despite the great age of our planet, about 4.5 billion years. For every
three tonnes of uranium in the ore there is only one quarter of a milligram
of polonium and one gram of radium, half of which, once separated, will
disintegrate in about 1600 years.

Uranium's Scientific History -