| |
Marie
Curie - The Science of Radioactivity -
Xrays and Uranium Rays
MARIE
SKLODOWSKA CURIE opened up the science of radioactivity. She
is best known as the discoverer of the radioactive elements
polonium and radium and as the first person to win two Nobel
prizes. For scientists and the public, her radium was a key
to a basic change in our understanding of matter and energy.
Her work not only influenced the development of fundamental
science but also ushered in a new era in medical research
and treatment. This exhibit brought to you by The Center for
History of Physics A Division of The American Institute of
Physics Further Reading and Links Material on this site is
copyright © 2000 American Institute of Physics and Naomi
Pasachoff and is based on the book Marie Curie and the Science
of Radioactivity by Naomi Pasachoff, Oxford University Press,
copyright © 1996 by Naomi Pasachoff.
Radium
and Radioactivity
By Mme. Sklodowska Curie, Discoverer of Radium
from Century Magazine (January 1904), pp. 461-466
The discovery
of the phenomena of radioactivity adds a new group to the
great number of invisible radiations now known, and once more
we are forced to recognize how limited is our direct perception
of the world which surrounds us, and how numerous and varied
may be the phenomena which we pass without a suspicion of
their existence until the day when a fortunate hazard reveals
them.
The radiations
longest known to us are those capable of acting directly upon
our senses; such are the rays of sound and light. But it has
also long been recognized that, besides light itself, warm
bodies emit rays in every respect analogous to luminous rays,
though they do not possess the power of directly impressing
our retina. Among such radiations, some, the infra-red, announce
themselves to us by producing a measurable rise of temperature
in the bodies which receive them, while others, the ultra-violet,
act with specially great intensity upon photographic plates.
We have here a first example of rays only indirectly accessible
to us.
Yet further
surprises in this domain of invisible radiations were reserved
for us. The researches of two great physicists, Maxwell and
Hertz, showed that electric and magnetic effects are propagated
in the same manner as light, and that there exist “electromagnetic
radiations,” similar to luminous radiations, which are
to the infra-red rays what these latter are to light. These
are the electromagnetic radiations which are used for the
transmission of messages in wireless telegraphy. They are
present in the space around us whenever an electric phenomenon
is produced, especially a lightning discharge. Their presence
may be established by the use of special apparatus, and here
again the testimony of our senses appears only in an indirect
manner. If we consider these radiations in their entirety
- the ultra-violet, the luminous, the infra-red, and the electromagnetic
- we find that the radiations we see constitute but an insignificant
fraction of those that exist in space. But it is human nature
to believe that the phenomena we know are the only ones that
exist, and whenever some chance discovery extends the limits
of our knowledge we are filled with amazement. We cannot become
accustomed to the idea that we live in a world that is revealed
to us only in a restricted portion of its manifestations.
Among
recent scientific achievements which have attracted most attention
must be placed the discovery of cathode rays, and in even
greater measure that of Roentgen rays. These rays are produced
in vacuum-tubes when an electric discharge is passed through
the rarefied gas. The prevalent opinion among physicists is
that cathode rays are formed by extremely small material particles,
charged with negative electricity, and thrown off with great
velocity from the cathode, or negative electrode, of the tube.
When the cathode rays meet the glass wall of the tube they
render it vividly fluorescent. These rays can be deflected
from their straight path by the action of a magnet. Whenever
they encounter a solid obstacle, the emission of Roentgen
rays is the result. These latter can traverse the glass and
propagate themselves through the outside air. They differ
from cathode rays in that they carry no electric charge and
are not deflected from their course by the action of a magnet.
Everyone knows the effect of Roentgen rays upon photographic
plates and upon fluorescent screens, the radiographs obtainable
from them, and their application in surgery.
The discovery
of Becquerel rays dates from a few years after that of Roentgen
rays. At first they were much less noticed. The world, attracted
by the sensational discovery of Roentgen rays, was less inclined
to astonishment. On all sides a search was instituted by similar
processes for new rays, and announcements of phenomena were
made that have not always been confirmed. It has been only
gradually that the positive existence of a new radiation has
been established. The merit of this discovery belongs to M.
Becquerel, who succeeded in demonstrating that uranium and
its compounds spontaneously emit rays that are able to traverse
opaque bodies and to affect photographic plates.
It was
at the close of the year 1897 that I began to study the compounds
of uranium, the properties of which had greatly attracted
my interest. Here was a substance emitting spontaneously and
continuously radiations similar to Roentgen rays, whereas
ordinarily Roentgen rays can be produced only in a vacuum-tube
with the expenditure of energy. By what process can uranium
furnish the same rays without expenditure of energy and without
undergoing apparent modification? Is uranium the only body
whose compounds emit similar rays? Such were the questions
I asked myself, and it was while seeking to answer them that
I entered into the researches which have led to the discovery
of radium.
First
of all, I studied the radiation of the compounds of uranium.
Instead of making these bodies act upon photographic plates,
I preferred to determine the intensity of their radiation
by measuring the conductivity of the air exposed to the action
of the rays. To make this measurement, one can determine the
speed with which the rays discharge an electroscope, and thus
obtain data for a comparison. I found in this way that the
radiation of uranium is very constant, varying neither with
the temperature nor with the illumination. I likewise observed
that all the compounds of uranium are active, and that they
are more active the greater the proportion of this metal which
they contain. Thus I reached the conviction that the emission
of rays by the compounds of uranium is a property of the metal
itself—that it is an atomic property of the element
uranium independent of its chemical or physical state. I then
began to investigate the different known chemical elements,
to determine whether there exist others, besides uranium,
that are endowed with atomic radioactivity—that is to
say, all the compounds of which emit Becquerel rays. It was
easy for me to procure samples of all the ordinary substances—the
common metals and metalloids, oxides and salts. But as I desired
to make a very thorough investigation, I had recourse to different
chemists, who put at my disposal specimens—in some cases
the only ones in existence—containing very rare elements.
I thus was enabled to pass in review all the chemical elements
and to examine them in the state of one or more of their compounds.
I found but one element undoubtedly possessing atomic radioactivity
in measurable degree. This element is thorium. All the compounds
of thorium are radioactive, and with about the same intensity
as the similar compounds of uranium. As to the other substances,
they showed no appreciable radioactivity under the conditions
of the test.
I likewise
examined certain minerals. I found, as I expected, that the
minerals of uranium and thorium are radioactive; but to my
great astonishment I discovered that some are much more active
than the oxides of uranium and of thorium which they contain.
Thus a specimen of pitch-blende (oxide of uranium ore) was
found to be four times more active than oxide of uranium itself.
This observation astonished me greatly. What explanation could
there be for it? How could an ore, containing many substances
which I had proved inactive, be more active than the active
substances of which it was formed? The answer came to me immediately:
The ore must contain a substance more radioactive than uranium
and thorium, and this substance must necessarily be a chemical
element as yet unknown; moreover, it can exist in the pitch-blende
only in small quantities, else it would not have escaped the
many analyses of this ore; but, on the other hand, it must
possess intense radioactivity, since, although present in
small amount, it produces such remarkable effects. I tried
to verify my hypothesis by treating pitch-blende by the ordinary
processes of chemical analysis, thinking it probable that
the new substance would be concentrated in passing through
certain stages of the process. I performed several experiments
of this nature, and found that the ore could in fact be separated
into portions some of which were much more radioactive than
others.
To try
to isolate the supposed new element was a great temptation.
I did not know whether this undertaking would be difficult.
Of the new element I knew nothing except that it was radioactive.
What were its chemical properties? In what quantity did it
appear in pitch-blende? I began the analysis of pitch-blende
by separating it into its constituent elements, which are
very numerous. This task I undertook in conjunction with M.
Curie. We expected that perhaps a few weeks would suffice
to solve the problem. We did not suspect that we had begun
a work which was to occupy years and which was brought to
a successful issue only after considerable expenditure.
We readily
proved that pitch-blende contains very radioactive substances,
and that there were at least three. That which accompanies
the bismuth extracted from pitch-blende we named Polonium;
that which accompanies barium from the same source we named
Radium; finally, M. Debierne gave the name of Actinium to
a substance which is found in the rare earths obtained from
the same ore.
Radium
was to us from the beginning of our work a source of much
satisfaction. Demarçay, who examined the spectrum of
our radioactive barium, found in it new rays and confirmed
us in our belief that we had indeed discovered a new element.
The question
now was to separate the polonium from the bismuth, the radium
from the barium. This is the task that has occupied us for
years, and as yet we have succeeded only in the case of radium.
The research has been a most difficult one. We found that
by crystallizing out the chloride of radioactive barium from
a solution we obtained crystals that were more radioactive,
and consequently richer in radium, than the chloride that
remained dissolved. It was only necessary to make repeated
crystallizations to obtain finally a pure chloride of radium.
But although
we treated as much as fifty kilograms of primary substance,
and crystallized the chloride of radiferous barium thus obtained
until the activity was concentrated in a few minute crystals,
these crystals still contained chiefly chloride of barium;
as yet radium was present only in traces, and we saw that
we could not finish our experiments with the means at hand
in our laboratory. At the same time the desire to succeed
grew stronger; for it became evident that radium must possess
most intense radioactivity, and that the isolation of this
body was therefore an object of the highest interest.
Fortunately
for us, the curious properties of these radium-bearing compounds
had already attracted general attention and we were assisted
in our search.
A chemical
factory in Paris consented to undertake the extraction of
radium on a large scale. We also received certain pecuniary
assistance, which allowed us to treat a large quantity of
ore. The most important of these grants was one of twenty
thousand francs, for which we are indebted to the Institute
of France.
We were
thus enabled to treat successively about seven tons of a primary
substance which was the residue of pitch-blende after the
extraction of uranium. Today we know that a ton of this residue
contains from two to three decigrams (from four to seven ten-thousandths
of a pound) of radium. During this treatment, and as soon
as I had in my possession a decigram of chloride of radium
recognized as pure by the spectroscope, I determined the atomic
weight of this new element, finding it to be 225, while that
of barium is 137.
The properties
of radium are extremely curious. This body emits with great
intensity all of the different rays that are produced in a
vacuum-tube. The radiation, measured by means of an electroscope,
is at least a million times more powerful than that from an
equal quantity of uranium. A charged electroscope placed at
a distance of several meters can be discharged by a few centigrams
of a radium salt. One can also discharge an electroscope through
a screen of glass or lead five or six centimeters thick. Photographic
plates placed in the vicinity of radium are also instantly
affected if no screen intercepts the rays; with screens, the
action is slower, but it still takes place through very thick
ones if the exposure is sufficiently long. Radium can therefore
be used in the production of radiographs.
The compounds
of radium are spontaneously luminous. The chloride and bromide,
freshly prepared and free from water, emit a light which resembles
that of a glow-worm. This light diminishes rapidly in moist
air; if the salt is in a sealed tube, it diminishes slowly
by reason of the transformation of the white salt, which becomes
colored, but the light never completely disappears. By redissolving
the salt and drying it anew, its original luminosity is restored.
A glass
vessel containing radium spontaneously charges itself with
electricity. If the glass has a weak spot,—for example,
if it is scratched by a file,—an electric spark is produced
at that point, the vessel crumbles like a Leiden jar when
overcharged, and the electric shock of the rupture is felt
by the fingers holding the glass.
Radium
possesses the remarkable property of liberating heat spontaneously
and continuously. A solid salt of radium develops a quantity
of heat such that for each gram of radium contained in the
salt there is an emission of one hundred calories per hour.
Expressed differently, radium can melt in an hour its weight
in ice. When we reflect that radium acts in this manner continuously,
we are amazed at the amount of heat produced, for it can be
explained by no known chemical reaction.The radium remains
apparently unchanged. If, then, we assume that it undergoes
a transformation, we must therefore conclude that the change
is extremely slow; in an hour it is impossible to detect a
change by any known methods.
As a result
of its emission of heat, radium always possesses a higher
temperature than its surroundings. This fact may be established
by means of a thermometer, if care is taken to prevent the
radium from losing heat.
Radium
has the power of communicating its radioactivity to surrounding
bodies. This is a property possessed by solutions of radium
salts even more than by the solid salts. When a solution of
a radium salt is placed in a closed vessel, the radioactivity
in part leaves the solution and distributes itself through
the vessel, the walls of which become radioactive and luminous.
The radiation is therefore in part exteriorized. We may assume,
with Mr. Rutherford, that radium emits a radioactive gas and
that this spreads through the surrounding air and over the
surface of neighboring objects. This gas has received the
name emanation. It differs from ordinary gas in the fact that
it gradually disappears. [The modern name for this element
is radon.]
Certain
bodies—bismuth, for instance—may also be rendered
active by keeping them in solution with the salts of radium.
These bodies then become atomically active, and keep this
radioactivity even after chemical transformations. Little
by little, however, they lose it, while the activity of radium
persists.
The nature
of radium radiations is very complex. They may be divided
into three distinct groups, according to their properties.
One group is composed of radiations absolutely analogous to
cathode rays, composed of material particles called electrons,
much smaller than atoms, negatively charged, and projected
from the radium with great velocity—a velocity which
for some of these rays is very little inferior to that of
light.
The second
group is composed of radiations which are believed to be formed
by material particles the mass of which is comparable to that
of atoms, charged with positive electricity, and set in motion
by radium with a great velocity, but one that is inferior
to that of the electrons. Being larger than electrons and
possessing at the same time a smaller velocity, these particles
have more difficulty in traversing obstacles and form rays
that are less penetrating.
Finally,
the radiations of the third group are analogous to Roentgen
rays and do not behave like projectiles.
The radiations
of the first group are easily deflected by a magnet; those
of the second group, less easily and in the opposite direction;
those of the third group are not deflected. From its power
of emitting these three kinds of rays, radium may be likened
to a complete little Crookes tube acting spontaneously.
Radium
is a body which gives out energy continuously and spontaneously.
This liberation of energy is manifested in the different effects
of its radiation and emanation, and especially in the development
of heat. Now, according to the most fundamental principles
of modern science, the universe contains a certain definite
provision of energy, which can appear under various forms,
but cannot be increased.
Without
renouncing this conception, we cannot believe that radium
creates the energy which it emits; but it can either absorb
energy continuously from without, or possess in itself a reserve
of energy sufficient to act during a period of years without
visible modification. The first theory we may develop by supposing
that space is traversed by radiations that are as yet unknown
to us, and that radium is able to absorb these radiations
and transform their energy into the energy of radioactivity.
Thus in a vacuum-tube the electric energy is utilized to produce
cathode rays, and the energy of the latter is partly transformed,
by the bodies which absorb them into the energy of Roentgen
rays. It is true that we have no proof of the existence of
radiations which produce radioactivity; but, as indicated
at the beginning of this article, there is nothing improbable
in supposing that such radiations exist about us without our
suspecting it.
If we
assume that radium contains a supply of energy which it gives
out little by little, we are led to believe that this body
does not remain unchanged, as it appears to, but that it undergoes
an extremely slow change. Several reasons speak in favor of
this view. First, the emission of heat, which makes it seem
probable that a chemical reaction is taking place in the radium.
But this can be no ordinary chemical reaction, affecting the
combination of atoms in the molecule. No chemical reaction
can explain the emission of heat due to radium. Furthermore,
radioactivity is a property of the atom of radium; if, then,
it is due to a transformation this transformation must take
place in the atom itself. Consequently, from this point of
view, the atom of radium would be in a process of evolution,
and we should be forced to abandon the theory of the invariability
of atoms, which is at the foundation of modern chemistry.
Moreover,
we have seen that radium acts as though it shot out into space
a shower of projectiles, some of which have the dimensions
of atoms, while others can only be very small fractions of
atoms. If this image corresponds to a reality, it follows
necessarily that the atom of radium breaks up into subatoms
of different sizes, unless these projectiles come from the
atoms of the surrounding gas, disintegrated by the action
of radium; but this view would likewise lead us to believe
that the stability of atoms is not absolute.
Radium
emits continuously a radioactive emanation which, from many
points of view, possesses the properties of a gas. Mr. Rutherford
considers the emanation as one of the results of the disintegration
of the atom of radium, and believes it to be an unstable gas
which is itself slowly decomposed.
Professor
Ramsay has announced that radium emits helium gas continuously.
If this very important fact is confirmed, it will show that
a transformation is occurring either in the atom of radium
or in the neighboring atoms, and a proof will exist that the
transmutation of the elements is possible. [In fact radium
does emit helium, as alpha particles.]
When a
body that has remained in solution with radium becomes radioactive,
the chemical properties of this body are modified, and here
again it seems as though we have to deal with a modification
of the atom. It would be very interesting to see whether,
by thus giving radioactivity to bodies, we can succeed in
causing an appreciable change in their atoms. We should thus
have a means of producing certain transformations of elements
at will. [These observations were misleading. True artificial
radioactivity was not produced until the work of Irène
and Frédéric Joliot-Curie in 1934.]
It is
seen that the study of the properties of radium is of great
interest. This is true also of the other strongly radioactive
substances, polonium and actinium, which are less known because
their preparation is still more difficult. All are found in
the ores of uranium and thorium, and this fact is certainly
not the result of chance, but must have some connection with
the manner of formation of these elements. Polonium, when
it has just been extracted from pitch-blende, is as active
as radium, but its radioactivity slowly disappears; actinium
has a persistent activity. These two bodies differ from radium
in many ways; their study should therefore be fertile in new
results. Actinium lends itself readily to the study of the
emanation and of the radioactivity produced in inactive bodies,
since it gives out emanation in great quantity. It would also
be interesting, from the chemical point of view, to prove
that polonium and actinium contain new elements. Finally,
one might seek out still other strongly radioactive substances
and study them.
But all
these investigations are exceedingly difficult because of
the obstacles encountered in the preparation of strongly radioactive
substances. At the present time we possess only about a gram
of pure salts of radium. Research in all branches of experimental
science—physics, chemistry, physiology, medicine—is
impeded, and a whole evolution in science is retarded, by
the lack of this precious and unique material, which can now
be obtained only at great expense. We must now look to individual
initiative to come to the aid of science, as it has so often
done in the past, and to facilitate and expedite by generous
gifts the success of researches the influence of which may
be far-reaching
Research
Breakthroughs (1897-1904)
X-rays
and Uranium Rays
MARIE
CURIE'S CHOICE of a thesis topic was influenced by two recent
discoveries by other scientists. In December 1895, about six
months after the Curies married, German physicist Wilhelm
Roentgen discovered a kind of ray that could travel through
solid wood or flesh and yield photographs of living people's
bones. Roentgen dubbed these mysterious rays X-rays, with
X standing for unknown. In recognition of his discovery, Roentgen
in 1901 became the first Nobel laureate in physics.
In early
1896, only a few of months after Roentgen's discovery, French
physicist Henri Becquerel reported to the French Academy of
Sciences that uranium compounds, even if they were kept in
the dark, emitted rays that would fog a photographic plate.
He had come upon this discovery accidentally. Despite Becquerel's
intriguing finding, the scientific community continued to
focus its attention on Roentgen's X-rays, neglecting the much
weaker Becquerel rays or uranium rays.
THE IGNORED
URANIUM RAYS appealed to Marie Curie. Since she would not
have a long bibliography of published papers to read, she
could begin experimental work on them immediately. The director
of the Paris Municipal School of Industrial Physics and Chemistry,
where Pierre was professor of physics, permitted her to use
a crowded, damp storeroom there as a lab.
A clever
technique was her key to success. About 15 years earlier,
Pierre and his older brother, Jacques, had invented a new
kind of electrometer, a device for measuring extremely low
electrical currents. Marie now put the Curie electrometer
to use in measuring the faint currents that can pass through
air that has been bombarded with uranium rays. The moist air
in the storeroom tended to dissipate the electric charge,
but she managed to make reproducible measurements.
“Instead
of making these bodies act upon photographic plates, I preferred
to determine the intensity of their radiation by measuring
the conductivity of the air exposed to the action of the rays.”
You can
exit this site to an exhibit on the discovery of the electron
With numerous
experiments Marie confirmed Becquerel's observations that
the electrical effects of uranium rays are constant, regardless
of whether the uranium was solid or pulverized, pure or in
a compound, wet or dry, or whether exposed to light or heat.
Likewise, her study of the rays emitted by different uranium
compounds validated Becquerel's conclusion that the minerals
with a higher proportion of uranium emitted the most intense
rays. She went beyond Becquerel's work, however, in forming
a crucial hypothesis: the emission of rays by uranium compounds
could be an atomic property of the element uranium--something
built into the very structure of its atoms.
MARIE'S
SIMPLE HYPOTHESIS would prove revolutionary. It would ultimately
contribute to a fundamental shift in scientific understanding.
At the time scientists regarded the atom--a word meaning undivided
or indivisible -- as the most elementary particle. A hint
that this ancient idea was false came from the discovery of
the electron by other scientists around this same time. But
nobody grasped the complex inner structure or the immense
energy stored in atoms. Marie and Pierre Curie themselves
were not convinced that radioactive energy came from within
atoms--maybe, for example, the earth was bathed in cosmic
rays, whose energy certain atoms somehow caught and radiated?
Marie's real achievement was to cut through the complicated
and obscure observations with a crystal-clear analysis of
the set of conclusions that, however unexpected, were logically
possible.
Marie
tested all the known elements in order to determine if other
elements or minerals would make air conduct electricity better,
or if uranium alone could do this. In this task she was assisted
by a number of chemists who donated a variety of mineral samples,
including some containing very rare elements. In April 1898
her research revealed that thorium compounds, like those of
uranium, emit Becquerel rays. Again the emission appeared
to be an atomic property. To describe the behavior of uranium
and thorium she invented the word “radioactivity”
--based on the Latin word for ray.
|
|
|
Cosmos
of Happiness
