Becquerel's early research was almost exclusively in optics.His
first extensive investigations (1875-1882) dealt with the rotation
of plane-polarised light by magnetic fields. He next turned to infra-red
spectra, making visual observations by means of the light released
from certain phosphorescent crystals under infra-red illumination.
He then studied the absorption of light in crystals.With these researches,
Becquerel obtained his doctorate from the Faculty of Sciences of
Paris (1888) and election to the Academy of Sciences (1889). In
1890, he married his second wife. Following the death of his father
in 1891,he succeeded in the following year to his father's two chairs
of Physics at the Conservatoire National des Arts et Métiers
and at the Museum. Thus, in the beginning of 1896, at the age of
forty three, Becquerely was established in the rank and responsibility,
his years of active research behind him and all that for which he
is now remembered still undone!
There are few scientific discoveries whose circumstances
are known as minutely as those around the almost accidental finding
of radioactivity. On January 7, 1896, the great French mathematician
Jules-Henri Poincare (1854-1912) received a letter containing several
astonishing photographs of the bones in someone's hand. The bones
belonged to Wilhelm Conrad Rööntgen (1845-1923), a scientist
Poincare had never visited. The letter explained that the pictures
had been taken with the aid of a new discovery, X-rays that Ròöntgen
had turned up the previous month, and that he was publicizing his
findings by mailing off prints all over Europe. Publicized they
were: The photographs created a sensation across the globe(fig.3).
Within three weeks, little Eddie McCarthy of Dartmouth, New Hampshire,
became a local celebrity when his broken arm was set by physicians
armed with X-rays images of the fracture(fig.4). It is easy to imagine
Poincare's amazement-photographs of the inside of a human being!
-and he quickly asked two local doctors if they could duplicate
Röntgen's work. On January 20, they showed their own X-ray
photographs to the assembled members of the French Academie des
Sciences. The reaction was immediate and extreme. In the next fortnight,
five members of the Academie presented papers on the new phenomenon.
Antoine-Henri Becquerel (1852-1908), too, was
sitting in the audience when the X-ray photographs were shown. He
was fascinated by the strange ghostly images and the mysterious
emanations that produced them.Both he and his father had studied
the phenomenon of phosphorescence-the museum laboratory was filled
with lumps of stone and wood that shone in the dark. The glow of
X-ray emission put Becquerel in mind of the light in his study;
although he had not done much active research in the last few years,
he thought immediately of putting some phosphorescent rock on photographic
paper to see if it would darken it in the same way as one of Röntgen's
X-ray sources. It would not be all that much work. Born on December
15, 1852, Becquerel was the third in the line of Becquerel who held
the chair of applied physics at what is today called the National
Museum of Natural History in Paris. Like his grand father Antoine-Cesar
Becquerel, and father Alexandre-Edmond Becquerel, before him, he
was a member of the French Academy of Sciences and attended its
weekly meetings. During the meeting of January 20, 1896, he felt
that the X-rays appeared to emanate from the area of a glass vacuum
tube made fluorescent when struck by a beam of cathode rays. Poincare
wondered aloud if such radiation was emitted by other luminescent
Becquerel was immediately challenged by this question.
In fact, he was ideally suited to answer it. Not only was he expert
in the investigation of various luminescent effects, a common activity
in physical laboratories of the 19th century, but he had studied
the phosphorescence of some uranium compounds in particular. He
also was skilled in laboratory applications of photography. And,
like most physicists, he sought a better understanding of the nature
of matter, so perhaps the mechanics of phosphorescence would bring
him closer to reaching this final, philosophical goal.
What is Radioactivity?
Imagine that you are holding a water melon
in your hands. All of a sudden, for no apparent reason, one
of its seeds comes flying out through the thick skin. At the
same time, you find that the water melon has turned into a
musk melon. Before you realise what happened, the musk melon
throws out a seed and turns into an apple.As you are looking
at the apple wondering to bite into it or not, a seed shoots
out of it, and now what you have in your hand is an orange.
By the time you try comprehend this unbelievable chain of
event, the orange throws out a seed and becomes a emon. Surely,
you would not like to eat a lemon, so you wait for it to turn
into a berry or into a grape. You keep waiting, but nothing
happens. The lemon remains a lemon. You may think that may
be the a magician is trying to keep you awaiting from eating,
or that there is a hitherto unknown power which is responsible
for the entire chain of events. Fortunately, we never find
one fruit changing into another kind of fruit the way it is
described here.However, you may like to note that a very similar
process is called Radioactivity.
Atoms are the smallest constituents that
make up of elements, and hence all matter. At the centre of
each atom, there is a much smaller nucleus that contains even
tinier particles called Protons and Neutrons. The nucleus
of a "Radioactive"atom - throw out one or more of
these tinier articles, sometime particles other than proton
or neutron thereby changing into a different element, or sometimes
electromagnetic radiation. Such a nucleus is said to decay,
or break apart when the decay of a nucleus occur. One type
of atom is changed into a different type., and hence one element
and will consider his watch to be slow!
An expert on uranium phosphorescence:
In the second half of the 19th century, Henri's
father, Alexandre-Edmond Becquerel (1820-1891), was the leading
authority in Europe on the subject of the phosphorescence of solids.
It was an important field, made prominent by Robert Wilhelm Bunsen's
(1811-1899) and Gustav Robert Kirchhoff's (1824-1887) recent spectroscopic
analyses. Incidentally, "Fluorescence" is defined as the
emission of light only during stimulation by external radiation.
"Phosphorescence" persists after the external radiation
ceases. "Luminescence" is the umbrella term. Edmond was
drawn to the investigation of uranium salts because of their exceptionally
bright phosphorescence and their interesting spectra. One of his
contributions was to show that the uranic series of salts is phosphorescent
and that the uranous series is not.His son, Henri, began publishing
on phosphorescence in 1883, and wrote twenty papers on this and
related areas of study over the next 13 years, being attracted especially
to the effects of infrared radiations. Like his father, Henri was
fascinated by uranium salts, and he examined their absorption bands
in both infrared and visible regions.
Although uranium and its compounds interested
the Becquerels, the study of these substances remained in something
of a scientific backwater throughout the 19th century. Uranium had
been discovered in 1789 by a German analytical chemist, Martin Klaproth,
while he had been examining pitchblende from Saxony. Its name was
chosen in honor of William Herschel's discovery of the planet Uranus
in 1781 (a practice continued in the 20th century with the naming
of neptunium and plutonium). Not until 1841, however, was it recognized
that Klaproth had obtained only the oxide. Eugene Peligot, a noted
French chemist,then succeeded in separating the metal.
Attention was again directed to uranium when Dmitri
Ivanovich Mendeleev (1834-1907) formulated his periodic table in
1869 and showed it to be the heaviest element. But in an age of
burgeoning chemical production, few applications for it were found.
Compounds were tried as toning agents in photography, as dyes or
stains for leather and wool and as mordants for silk and wool, and
attempts were made with the metal to form an alloy with steel. The
greatest use was in the ceramic and glass industries, in which uranium
was valued for making coloured glazes and coloured clear glass.
By varying the percentage of the salt used, one could get yellow,
orange, brown, green or black.
Photography entered the laboratory around the
middle of the 19th century, being used to complement the microscope,
telescope and balloon (for aerial photography),and to capture events
such as sound waves, flying bullets, drop splashes, the motion of
animals and lightning. Röntgen's encounter with X-rays, which
evoked tremendous public interest, relied heavily on photography
for its fame. By far the greatest scientific use of this tool came
in the century's last two decades,which suggests the impact of dry,
gelatin emulsion plates. By 1896 Becquerel would probably have had
at his disposal dry photographic plates of relatively good quality,
uniform emulsion and long shelf life. Luminescence, uranium, photography
Becquerel was in the right place at the right time. But he still
might have failed to recognizes radioactivity as a phenomenon separate
from phosphorescence if he had not been an accomplished physicist
an apple remain an apple?
Most atoms are not radio active, fortunately.
Their neuclei are "stable",i.e. they do not decay.
This is why an ordinary object, such as an apple or a watermelon
with millions and millions of atoms with stable neuclei, always
remains the same. Incidentally, we call a radioactive nucleus
unstable because it can decay. When it does decay, both the
number of protons and neutrons can change. Will the resulting
nucleus be stable? Well, it may be stable or it too may be
radioactive. If it is radio active, it may be further decay
to form yet another new nucleus, which could be radioactive
as well. This is how a "decay series" could occur.
Each kind of chemical element in the series changes into the
next kind. Ultimately, the series may end with an element
such as lead, i.e. not radio active. The atoms in this element
are stable, they do not decay. Thus after a sufficiently long
time one may find that radium or uranium has completely changed
A model of scientific method:
Becquerel's working hypothesis was that a body
had to luminesce to emit penetrating radiation such as Röö;ntgen
had found. His technique was to wrap a photographic plate in light
tight black paper, position the mineral on the plate, and leave
the experiment on his window sill where sunlight would stimulate
the mineral to glow. At a meeting of the Academy of Sciences on
24 February 1896, he claimed success, reporting that several materials
in particular, phosphorescent crystals of potassium uranyl sulfate
emitted rays that penetrated thick black paper and exposed the photographic
plate. This exposure was little more than a smudge. To refine the
results and to make them more attractive to others Becquerel also
placed coins and other thin, metallic objects under the crystals,
producing interesting silhouettes and showing their penetrating
power. It must, however,be stated that X-rays, which produced far
sharper photographic images in less time were overwhelmingly more
On Wednesday and Thursday, 26 and 27 February,
1896, Becquerel prepared several arrays of crystals and photographic
plates. The Parisian winter, however, brought half a week of overcast
skies, forcing Becquerel to postpone the experiments;he felt that
he needed strong sunlight. The plates rested in a dark drawer until
Sunday, 1 March, when Becquerel developed them, "expecting
to find very weak images. To the contrary", he wrote in his
memoirs, "the silhouettes appeared with great intensity".
The following day, on 2 March 1896, Becquerel
reported to the Academy of Sciences that the potassium uranium sulfate
crystals could be stimulated to emit the new rays by diffuse daylight
hrough a thin cloud cover, as well as by reflected and refracted
direct sunlight. He also described using different thicknesses of
copper foil to examine the absorption of the rays. But the most
astounding result that Becquerel offered was that stimulation of
the crystals by sunlight immediately before or during the experiment
was apparently not necessarry.
The Uranium, it seemed, was spitting out X-rays
all by itself. This, too, was not entirely correct. In fact, the
lump of potassium uranyl sulfate was emitting a whole spectrum of
radiation, of which only a small portion was X-rays. Nonetheless,
the discovery caused a sensation, in part because it was so easy
to duplicate. Almost every laboratory in the world had construction
paper,photographic plates, and chunks of uranium ore. Within weeks,
scientists across the Continent were looking in astonishment at
the blurred black patches on their photographs making Becquerel
a celebrity (Figure).
Within weeks, news of Becquerel's findings had
spread to Germany, Great Britain, Italy, and the United States,
further exciting researchers already stirred by the discovery of
X-rays. Tests of the two phenomena were often conducted on the same
workbench. The consequences of each discovery, however, were far
different. X-rays were found to be simply pulses of light light
of an intensity and power never before seen, but light nonetheless.
Radioactivity, on the other hand, was something entirely new, something
that did not fit anywhere. The existence of radioactivity metal
that somehow shot out energy! was a direct attack on the most ardent
beliefs of Becquerel and his colleagues. When the strange behaviour
of uranium was first noted, Becquerel wrote in his memoirs, "There
was no reason to presume that the phenomenon was [anything but]
a new example of a known type of energy transformation. Contrary
to every expectation, the first experiments demonstrated the existence
of an apparently spontaneous production of energy" . They had
spent many years, those nineteenth century scientists, establishing
the law of conservation of energy: Energy was neither created not
destroyed. But every single piece of uranium seemed of its own accord
to produce radiation that fogged photographic plates, electrified
gases, and sometimes even burned physicists and the energy needed
to do these things evidently came from no place at all. The metal
just sat there, its atoms quietly working away, continuously beaming
out penetrating rays in seeming disregard for the conservation of
A page from the doctoral
thesis of Marie Curie (1903)
Marie Curie's representation of alpha, beta
and gamma rays in a magnetic field from a radioactive material
placed in a narrow but deep cavity in a block of lead. The
magnetic field is applied in a direction perpendicular to
and out of the plane of the paper. In the absence of electric
and magnetic fields, the rays would emerge as a thin vertical
beam. The alpha particles being positively charged and relatively
heavy, would be slightly deflected to the right. The beta
particles, being negatively charged and light, would be deviated
to a greater extent to the left, whereas the gamma rays, carrying
no electric charge, would not be deflected at all.
What prompted Becquerel to develop the plates?
But why had Becquerel bothered to develop those
plates, which he thought were faintly exposed at best? His behavior
has been explained as thoroughness: Jean Becquerel has suggested
that his father planned to resume his experiments and wished to
use fresh plates, so why to develop the old ones anyway? The explanation
(proffered by G.E.M. Jauncey in a 1946 paper in the American Journal
of Physics) is "impatience after awaiting four days for the
sun to shine". Yet other reasons, suggested, are "simple
thrift or an overriding curiosity". We can dismiss the belief
that Becquerel planned to resume his experiments on that Sunday:
Meteorological records indicate that the day was less sunny than
the average of the ceding four days
A better explanation for Becquerel's activity
is that he wanted to have sufficient material to report at the next
day's session of the academy. In previous experiments he had already
found, or so he believed, that weak illumination triggered his crystals
somewhat. Perhaps he thought that these newly prepared plates had
been exposed to some diffuse daylight, if not a short period of
sunlight, before he placed them in the dark drawer. Thus, even if
he could not describe many additional experiments, he might furnish
evidence of the connection between the intensity of the photographic
image and the intensity and duration of phosphorescence.
Another page from Marie Curie's doctoral thesis
describing the set-up for measuring the ionisation power of "uranium
rays" The method employed consists in measuring the conductivity
acquired by air under the action of radioactive bodies; this method
possesses the advantage of being rapid and of furnishing figures
which are comparable. The apparatus employed by me for the purpose
consists essentially of a plate condenser, AB (Figure 1). The active
body, finely powdered, is spread over the plate B, making the air
between the plates a conductor. In order to measure the conductivity,
the plate B is raised to a high potential by connecting it with
one pole of a battery of small accumulators. P, of which the other
pole is connected to earth. The plate A being maintained at the
potential of the earth by the connection CD,an electric current
is set up between the two plates. The potential of plate A is recorded
by an electrometer, E. If the earth connection be broken at C, the
plate A becomes charged, and this charge causes a deflection of
the electrometer. The velocity of the deflection is proportional
to the intensity of the current, and serves to measure the latter.
But a preferable method of measurement is that of complensating
the charge of plate A, so as to cause no deflection of the electrometer.
The charges in question are extremely weak; they may be compensated
by means of a quartz electric balance, Q, one sheath of which is
connected to plate A and other to the earth. The quartz lamina is
subjected to known tension, produced by placing weights in a plate,T;
the tension is produced progressively, and has the effect of generating
progressively a known quantity of electricity during the time observed.
The operation can be so regulated that, at each instant, there is
compensation between the quantity of electricity that traverses
the condenser and that of the opposite kind furnished by the quartz.
In this way, the quantity of electricity passing through the condenser
for a given time, i.e., the intesity of the current, can be measured
in absolute units. The measurement is independent of the sensitiveness
of the electrometer. (Source Resonance, March 2001) (fig.6). That
he found the plates as blackened as they would have been had the
crystals phosphoresced continuously, and that he recognized the
significance of his surprising observation, shows that the discovery
of radioactivity was not simply a happy accident but also a product
of genuine scientific talent. Becquerel's example is comforting
to us: His genius emerged because he mistakenly believed in a connection
between the penetrating rays and phosphorescence, and because he
felt compelled to speak at the academy's meeting.
Though a major step, this event does not deserve
to be called the discovery of radioactivity. The discovery was a
process, not an instantaneous occurrence, for even at this point
Becquerel had not sufficiently localized the phenomenon. No doubt
Becquerel was a skilled and ingenious experimenter. However, in
this early research he was not sufficiently meticulous to exclude
extraneous influences and to see that some of his experimental results
could bear more than one explanation. Thus, he often concluded that
his experiments proved uranium rays to posses a certain physical
property, only to have it shown later that the effect was due to
another cause. Indeed, his investigations are particularly interesting
for their many false trails, unreproducible results and misinterpreted
Yet,his erroneous conclusions inexorably led him
to further experiments, which often revealed the true nature of
the phenomenon. This uneven progress is perhaps the most striking
facet in the story of the discovery of radioactivity. But it must
be understood that few scientists are able to avoid false trails.
He recognized that the next step must be to determine
if any light at all was necessary to stimulate the crystals. Working
in a dark room, he placed different minerals atop photographic plates
in an opaque cardboard box. When developed five hours later, the
plates showed strong images in samples in which the crystals lay
directly on the emulsion and less intense images in those in which
the crystals were separated from the emulsion by sheets of aluminum
and glass. Besides showing attenuation, the samples involving aluminum
and glass also indicated that chemical action was not the explanation
for the photographic smudges. Nor could the smudges result from
the luminous radiation, because the phosphorescence of uranium salts
is perceptible only for about 0.01 second, too short a time to expose
a plate.Becqurel therefore suggested that phosphorescent bodies
might give off an invisible emission that lasts much longer than
the visible radiation.
Even before Rö;ntgen's discovery of X-ray,
it had become almost a standard procedure for scientists exploring
various types of radiation to perform some of the experiments that
Rö;ntgen conducted to determine the properties of X-rays. Becquerel
followed suit, as was only logical, because he believed that his
own rays were similar to X-rays. He only had to substitute a layer
of uranium salts for a cathode-ray tube, for example, to show that
the separate gold leaves of an electroscope were made to fall. Having
established this electrical property, he next examined whether the
rays were reflected and refracted and he claimed they were. This
conclusion, however, would be corrected by Rutherford some three
Through March and the succeeding months of 1896,
Becquerel found that those crystals kept in darkness retained their
ability to expose a photographic plate. Surely, he felt, this was
a remarkable example of long-lived phosphorescence. But he was at
a loss to explain the equally intense images produced by non-phosphorescent
uranous sulfate. This discovery led him on a new path of investigation.
Since uranium nitrate ceases to luminesce when dissolved or melted
in its water of crystalization, Becquerel, in darkness, heated a
crystal in a sealed glass tube, protecting it even from the light
of the alcohol flame. He then allowed it to recrystallize in darkness.
All phosphorescence had been destroyed in this process, yet the
salt still produced results on a photographic plate as strong as
crystals exposed to light. Indeed, Becquerel admitted the anomalous
behaviour of his samples: All salts of uranium emitted the invisible
radiation, while other phosphorescent bodies did not. Finally, he
tried a disk of pure uranium metal and found that it produced penetrating
radiation three to four times as intense as that he had first seen
with potassium uranyl sulfate. With this last announcement, on May
18, 1986, Becquerel's discovery of radioactivity was complete, although
he continued with ionization studies of his penetrating radiation
until the following spring. The new rays emerged from the element
uranium, and with the implicit consequence that this was an atomic
phenomenon, it may be said that the process of the discovery of
radioactivity was essentially over. It was a process that took several
months, notable for a number of conclusions that were later overturned!
Enter Marie and Pierre Curie
Marie Curie (1867-1934) leaped into this exciting new field. She
soon discovered at roughly the same time that Becquerel and Ernest
Rutherford (1871-1937)(fig8) did that the radiations given off by
uranium were composed of more than one type. Some rays were bent
one way by a magnetic field; others were bent another way. Rutherford
named the positively charged rays alpha rays and the negatively
charged ones beta rays (also known as alpha particles and beta particles).
Exactly what these rays or particles were composed of, no one knew,
but by 1898 Marie Curie suggested a name for these radiations radioactivity
and that is the name that stuck. And in 1900, Paul Ulrich Villard
discovered a third, unusually penetrating type of ray in radioactive
radiation, one that did not bend at all n a magnetic field, which
he named the gamma ray. The use of Greek letters to name these rays
simply meant that their identity was unknown, as with the X in X-ray.
The Law of Exponential Decay Rutherfod and Soddy observed in 1902
that the activity of a radioactive element was diminishing in an
exponential or (logarithmic) manner. This implied that the rate
of decay of an active species, that is, the number of atoms that
disintegrate in a unit interval of time, is proportional to the
total number of atoms of that species present at that time. If we
suppose that at a given instant, there are N atoms present of a
particular radioelement, the rate of disintegration is represented
by dN/dt. Since the rate of disintegration is proportional to the
total number of atoms N, the relationship between the two, following
methods of simple calculus, can be written as
- dN/dt = N
where l is is a constant which Rutherford and
Soddy called the "radioactive constant". It is now referred
to as the disintegration constant or the decay constant of the element
under consideration. The negative sign is due to the fact that the
number of atoms of the radioactive element decreases with time,
and hence the rate dN/dt is a negative quantity. The value of l
depends on the property of a given radioelement and is independent
of the physical condition or state of chemical combination. In an
equivalent exponential form, the above equation yields the result,
N t = N 0 e - lt
where N 0 is the number of atoms present at any
arbitrary zero time and N t is the number remaining after the lapse
of a further time t. Another constant introduced by Rutherfod in
1904, called the "half-life" is the time required for
the radioactivity of a given amount of the element to decay to half
its initial value, that is, when half of the N0 atoms present at
the zero time have decayed.
Marie Sklodowska, a Polish girl came to Paris
at the Sorbonne University to study physics and mathematics and
qualified with honours and distinction. She married Pierre Curie
(1859-1906) of the same university in 1895. Pierre was already famous
for his discovery of piezo electricity - a property shown by some
crystals such as quartz of developing an electrical voltage between
opposite ends when subjected to pressure. Marie Curie used the discovery
of her husband (see Box) to measure radioactivity. Radioactive rays,
like X rays, ionized any gas they passed through (including air)
making it capable of conducting electricity. She found that she
could measure the current so conducted with a galvanometer and offset
it with the potential of a crystal under pressure. By measuring
the amount of pressure it took to balance the current, she could
obtain the reading of the intensity of the radioactivity. She systematically
tested radioactive salts and succeeded in showing that the degree
of radioactivity was in proportion to the amount of uranium in the
radioactive material thereby narrowing the source of the radioactivity
in her samples down to uranium. Then in 1898 she made yet another
find: the heavy element thorium was also radioactive. It was already
known that natural pitchblende is three or four times more active
than uranium. Even more interesting is the fact that as Marie was
working to separate uranium out of pitchblende, she found that the
residues she produced had a much higher measurement of radioactivity
than the uranium content alone could account for. Since the other
minerals present in the ore were not radioactive, that could mean
only one thing. Some other radioactive element, in amounts too small
to detect, must also be present! By this time, Marie's work had
developed so much potential that her husband Pierre joined her to
help with the backbreaking, tedious work of crystallizing the elements
from the ores. Though himself a fine scientist with a successful
career, he set his own work aside and spent the remaining seven
years of his life assisting her, recognizing both her extraordinary
gifts as a scientist and the importance of the path she was following.
By July 1898 the two had succeeded. Working together, they had isolated
a tiny amount of powder from the uranium ore from the fraction that
contained bismuth. It was a new element, never before detected,
with a level of radioactivity 400 times higher than uranium. They
named the new element polonium, after Marie's home country. But
something still seemed strange. The ore still gave off more radioactivity
even than the uranium and polonium combined could account for. There
must still be something else. In December 1898 they found the answer:
another, even more radioactive element obtained from the fraction
that contained barium which was 900 times more active than uranium.
This one they named radium (from Latin radius meaning ray). Marie
and Pierre could not really offer a good description of new element
radium because the amount they were able to derive from the ore
they had was so minuscule. They could measure its radiations, and
Eugene Demarcay, a specialist in elemental line spectra, was able
to provide the spectral characteristics. (Different elements give
off different wavelengths of electromagnetic radiation or light,
and these can be observed as discrete lines.) The next project was
to produce a large enough quantity of radium that they could weigh
it and measure it and see it. For this, they required a much bigger
laboratory and financial resources which Sorbonne University could
not provide. Undaunted by the circumstances, they set to work in
a make-shift laboratory housed in a neighbouring abandoned court-yard.
Through the courtesy of the Academy of Sciences, Vienna, they managed
at a reasonable cost, stacks of the required ore pitchblende. The
new laboratory was damp with a leaky glass roof, walls made of card-boards,
a few tables knocked together as the work tables, a gas stove and
no exhaust to remove noxious fumes arising from the work upto 20
kg of the ore every day. It was a back breaking, hazardous and almost
suicidal adventure with no help coming from any quarters. They spent
their life savings to obtain large masses of waste ore from a nearby
mine, and they began the monumental task. They spent four years,
during which Marie lost 5 pounds, purifying and repurifying the
ore into small amounts of radium, say, about 0.1 gram. Marie Curie
wrote her doctoral dissertation on the subject in 1903, for which
she, Pierre and Henri Becquerel shared the Nobel Prize in physics
that year. In 1906, two years after receiving an appointment as
professor of physics at the Sorbonne, Pierre Curie was run over
by a horse-drawn truck at the age of 47. Marie was appointed in
his place and she became the first woman to teach physics at the
Sorbonne. Eight years after Pierre's death (1914), she received
another Nobel for her discovery of two new elements, viz. Polonium
and Radium, this time in chemistry and this time alone, Pierre -
her partner and collaborator - no longer at her side. Years later,
in 1935 to be precise, their daughter Irene and her husband Frederic
Joliot-Curie - the second husband and wife team - were awarded the
Nobel Prize in chemistry for their discovery of artificial radioactivity.
Nobel Prizes awarded for work with radioactivity
The discovery of radioactivity brought about a revolution
in our conceptiual understanding of the matter and found applications
various fields of human activity. Here is a list of Nobel
Prizes awarded for work with radioactivity.
Physics for his discovery of spontaneous radioactivity
in Physics for extraordinary
services rendered by research on the radiation phenomena
discovered by Professor Henri Becquerel
Chemistry for his services in the discovery of the
inert gaseous elements in air, and his determination
of their place in the periodic system
Chemistry for his investigations into the disintegration
of the elements, and the chemistry of radioactive
Chemistry for her services to the advancement of chemistry
by the discovery of the elements radium and polonium,
by the isolation of radium and the study of the nature
and compounds of this remarkable element
Chemistry for his contributions to our knowledge of
the chemistry of radioactive substances, and his investigations
into the origin and nature of isotopes
Chemistry for his discovery of heavy hydrogen
Chemistry in recognition of their synthesis of new
Physics for his demonstrations of the existence of
new radioactive elements produced by neutron irradiation,
and for his related discovery of nuclear reactions
brought about by slow neutrons
Physics for the invention and development of the cyclotron
and for results obtained with it, especially with
regard to artificial radioactive elements
Chemistry for his work on the use of isotopes as tracers
in the study of chemical processes
Chemistry for his discovery of the fission of heavy
| Sir John Douglas CockcroftErnest
Thomas Sinton Walton
Physics for their pioneer work on the transmutation
of atomic nuclei by artificially accelerated atomic
| Edwin Mattison McMillanGlenn
| in Chemistry for their
discoveries in the chemistry of the transuranium
| Willard Frank Libby
|| in Chemistry for his
method to use Carbon-14 for age determination in archaeology,
geology, geophysics, and other branches of science
| Rosalyn S. Yalow
|| in Physiology or Medicine
for the development of radioimmunoassays
of peptide hormones"
Parent Transforms Into a Daughter
When Ernest Rutherford (1871-1937) arrived on
scholarship at the Cavendish Laboratory from his native in New Zealand,
he was 24 years old, a large dark haired man with strong opinion,
plenty of ambition and no money. He was tireless in every quest
and loved his role as one who put endless questions to nature. A
consummate experimentalist, Rutherford had a talent for designing
experiments and an uncanny ability to pick out one significant fact
from a mass of confusing detail. Rutherford began by examining the
Becquerel rays from uranium. Indeed, until about 1904, the emission
from radioactive elements received far more attention than the emitters.
Passage of the radiation through foils revealed one type that was
easily absorbed and another with greater penetrating ability, which
Rutherford named alpha and beta. In 1898, he took up the professorship
of physics at the McGill University, Montreal, the authorities of
which were convinced by Sir J.J. Thomson's testimonial that said,
"I have never had a student with more enthusiasm or ability
for original research than Mr. Rutherford". While in Cambridge,
Rutherford's work in radioactivity was solely with uranium salts;
in Montreal his first inclination was to examine thorium substances,
since the activity of these substances had been noticed only a few
months earlier. With passage of time, the number of radio elements
had increased. Rutherford added several more to the list. Rutherford
was joined by Frederick Soddy (1877-1956)(fig.10) in May 1900. Soddy
was the youngest son of a London merchant and was raised in the
Calvinist tradition by his dominant half-sister. Before coming to
McGill, he was engaged in independent chemical research at Oxford.
Soddy joined with Rutherford in a series of investigations which
produced the theoretical explanation of radioactivity. Rutherford
and Soddy proposed in 1902 the theory of "radioactive disintegration".
They suggested that the atoms of radio elements, unlike those of
inactive elements, undergo spontaneous disintegration with the emission
of alpha or beta particles and the formation of atoms of new elements.
In their words: "The disintegration of the atom and the expulsion
of a charged particle leaves behind a new system lighter than before
and possessing physical and chemical properties quite different
from those of the original parent element. The disintegration process
once started, proceeds from stage to stage with measurable velocities
in each case". It appeared to Rutherford and Soddy that the
activity was diminishing in what the mathematicians call an exponential
(or logarithmic) manner. This would mean that the rate of the decay
of an active species, i.e. the number of atoms which disintegrate
in a unit interval of time, is proportional to the total number
of atoms of that species present at that time. Given the disintegration
is taking place continuously, the number of atoms present is changing,
and so also is the rate of disintegration (see Box: How the radioactive
In 1903, Soddy joined Sir William Ramsay (1852-1916)(fig.9),
the great chemist best known for the discovery of the inert gaseous
elements. They experimentally confirmed the prediction of Rutherford
and Soddy that disintegration of radium would continously produce
helium (or what was termed alpha particle earlier). It may be of
interest to note that helium was first detected in the Sun way back
in 1868 by the French Astronomer Pierre J. Janssen (1824-1907) while
studying a total solar eclipse in the tobacco fields of Guntur,
Andhra Pradesh. Yet another important contribution by Soddy was
that of the existence of "isotopes". It turned out that
decay of several species of radioactive elements disintegrated to
products which had identical chemical properties but they differed
in their atomic weights. Soddy called them isotopes (from the Greek,
iso-same and topos-place). Today we know that such atoms have the
same number of protons (i.e. the atomic number) but may have different
number of neutrons. Further studies established that the phenomenon
of radioactivity involved emission of alpha particles (helium nuclei),
beta-rays (electrons) or gamma rays (electromagnetic radiation like
X-rays, but of very short wave-length) transforming the "parent"
nucleus into a "daughter" nucleus.
Alpha particles help Unravel the Structure
of the Atom
The discovery of radioactivity shattered the age
old ideas about the indivisibility of atoms. It also helped develop
our understanding about their structure. In 1908, Rutherford had
returned to England, to the University of Manchester, where a young
German physicist named Hans Geiger teamed up with him. Together,
they bombarded thin pieces of gold foil with alpha particles from
radium. Based on his discovery of the electron, Sir J.J. Thomson
(1856-1940) had suggested in 1898 that atoms were spheres of positively
charged matter with negatively charged electrons embedded in them,
something like "raisin in poundcake". However, the results
of the scattering of alpha particles from the gold atoms (1911)
suggested that atoms consisted of a tiny positive nucleus with electrons
circling outside it - somewhat similar to our solar system. Most
of the atom's mass would be contained in the nucleus, and an equal
number of negatively charged electrons would be found in motion
around it. Rutherford's idea about the atomic nucleus was indeed
extra-ordinary, for which he earned the title "the Newton of
Making Stable Atoms Unstable
In 1934, Irene Joliot Curie (1897-1956) and Frederic
Joliot (1900-1958) discovered "artificial" radioactivity.
In the course of a study of the effect of alpha particles from the
naturally occuring radioelement polonium (discovered) by Irene's
parents Marie and Pierre Curie), on the nuclei of aluminium in particular,
Irene and Frederic found that after the source of alpha particles
was removed, the aluminium foil on which alpha particles were irradiated
became radioactive and followed the exponential law of radioactive
decay described earlier. The same phenomenon was observed with boron
and magnesium as well. The "transmuation" of boron, magnesium
and aluminium by alpha particles had given birth to new radioelements!
These new elements not found in nature, would then be the radioactive
species which decays with the emission of a positron (i.e. a positive
electron) or what is also called beta decay. An alpha particle bombarded
on an aluminium atom would transform it into a phosphorus atom,
which being unstable, would in turn emit a positron to transform
into a stable, naturally occuring isotope of silicon. The discovery
of artificial radioactivity opened up altogether a new field. Man-made
Radioisotopes found applications in industry, biology, health, agriculture,
archaeology and many other areas of human activity.
Legacy of radioactivity
The discovery of radioactivity evolved into the
study of nuclear physics almost a century ago. The applications
that have flowed from the work of Becquerel and others are primarily
nuclear medicine, nuclear reactors, and nuclear weapons. In the
field of medicine, diagnostic procedures such as tracer techniques,
therapeutic applications like the treatment of cancer by radiation,
have proved to be highly valuable. For example, gamma rays from
an isotope of cobalt are used in the treatment of cancer. The radioactivity
of isotopes of elements used as tracers enables the scientists to
locate them, even inside a living body. One isotope of carbon, known
as carbon-14 (i.e. with 6 protons and neutrons) has a sufficiently
long half-life (that is, the time in which half of the initial number
of carbon-14 atoms would decay in a given sample containing this
isotope). It is present in the bones and woods. When an archaeologist
digs them up, a comparison of radioactivity of carbon-14 present
in them with that in the living specimens allows him to estimate
But, the future of the nuclear reactors and weapons
appears to be problematic. This is so, because the nuclear reactors
are always fraught with possible meltdowns (remember the Three Mile
Island and the Chernobyl disasters?), and the skyrocketing costs.
Nuclear weapons, too, seem to share similar concern. No doubt, after
a hundred years of the discovery of radioactivity, we now face even
newer challenges through its all pervading influence.
- The discovery of radioactivity Lawrence Badash Physics Today,
February 1996 .A well researched article written in a lucid style.
The present article draws heavily on it.
- The Second Creation Robert P. Crease and Charles C. Mann Affiliated
East-West Press Pvt. Ltd. 1986 A wonderful resource book. A must
for anyone interested in knowing about the makers of the twentieth
- The History of Science from 1895-1945 Ray Spangenburg and Diane
K. Moser Universities Press (India) Ltd. 1999 Highly readable.
A set of five volumes on history of science from the ancient Greeks
- Uranium Glenn Seaborg Encyclopaedia Britannica (1957) Vol.
- Sourcebook on Atomic Energy Samuel Glasstone Affiliated East-West
Press Pvt. Ltd. 1969 A superb and invaluable resource on history,
atomic and nuclear phenomena, and applications of atomic energy.
- Resonance (March 2001), page 97 A monthly journal of science
education published by Indian Academy of Sciences, Bangalore.
Important terms used in connection with radioactivity are
given below. The terms given do not necessarily appear in the present
Alpha particle : Charged particles
emitted from a radioactive atom. Each charged particle consists
of two protons and two neutrons. Atom : This is the smallest unit
of an element. It contains a nucleus with neutrons and protons,
surrounded by orbiting electrons. Atomic mass : The mass of an atom
usually expressed as atomic mass unit (amu).
Beta particle : (often designated
beta rays) Charged particles emitted from a radioactive atom. These
particles are identical except for their charge. The charge is classified
as positive (positron) or negative (electrons or negatron).
Carbon-14 : A naturally occuring
radio isotope of carbon having a mass number of 14 and half-life
5780 years. Used in Radio carbon dating for determination of age
of ancient objects.
Cathode rays : Electrons originating
at the cathodes of gaseous discharge devices. These electrons are
often focused in a small area such as a tube and intensified on
a surface. The most familiar form of a cathode-ray tube is the television
Conductivity : The ratio of electric
current to the electric field in a material. Passage of electric
charge which can occur a variety of ways such as passage of electrons
or ionized atoms.
Curie : A unit of radioactivity,
defined as that quantity of any radioactive nuclide which has 3700
X 1010 disintegrations per second.
Deuterium : The isotope of element
hydrogen with one neutron and one proton in the nucleus.
Electrons : A negative charged
particle that orbits the nucleus of an atom. It is lighter in weight
than a proton or neutron.
Elements : An element is a substance
made up of atoms with the same atomic number. 75% of the elements
are metals and the others are nonmetals. A few examples are oxygen,
iron, gold, chlorine, and uranium.
Fluorescence : Electrons absorb
energetic radiation (for example ultraviolet light) raising an electron
to a higher "Bohr" orbit. The energized electron soon
drops down in a series of steps through lower energy states and
in the process releases photons at lower energy states corresponding
to visible light. The bright color occurs because the photons are
concentrated in a narrow range of wavelengths.
Geiger counter : A radiation
counter that uses a Geiger-Mésarller tube in appropriate
circuits to detect and count ionizing particles, each particle crossing
the tube produces ionization of gas in the tube which is roughly
independent of the particle's nature and energy resulting in a uniform
discharge across the tube. Also known as Geiger-Mésarller
: A radiation-counter tube usually consisting of a gas-filled cylindrical
metal chamber containing a fine-wire anode at its axis. Also known
as Geiger-Mésarller Counter tube.
Half-life : The period of time
it takes for half the nuclei of a radioactive element to undergo
decay to another nuclear form.
Heavy water : A compound of hydrogen
and oxygen containing a higher proportion of the hydrogen isotope
deuterium than does naturally occuring water.
Ionization chamber : A particle
detector which measures the ionization produced in the gas filling
the chamber by the fast-moving charged particles as they pass through.
Isotope : An atom having the
same number of protons in its nucleus as other varieties of the
element but has a different number of neutrons.
Magnetic field : All magnetic
fields are created by moving electric charge. The single moving
electron around a nucleus is a tiny electric current. These orbiting
electrons create magnetic fields and their net effect is to provide
the atom with a magnetic field.
Neutron : A particle with no
charge that is located in the nucleus of an atom.
Nuclear physics : A branch of
physics that includes the study of the nuclei of atoms, their interactions
with each other, and with constituent particles.
Nucleus : The central part of
every atom that contains protons and neutrons.
Nuclide : A species of atom characterized
by the number of protons, number of neutrons, and energy content
in the nucleus, or alternatively by the atomic number, mass number,
and atomic mass. To be regarded as a distinct nuclide, the atom
must be capable of existing for a measurable life time. Also known
as nuclear species.
Pitchblende : A brown to black
fine grained, amorphous, variety of uraninite which has a dull luster
and contains small quantities of uranium. Also called pitch ore
Phosphorescence : Luminescence
that persists after a light source has been removed. Materials such
as phosphors or phosphorogens are activated from a light source
to emit the light in the form of photons of light.
Polonium : A naturally radio
active chemical element, Po, atomic number 84. It is used in photographic
film to reduce the static charge.
Proton : A positively charged
particle that is located in the nucleus of an atom.
Radiation effects : The harmful
effects of ionizing radiation on humans and other animals, such
as production of cancer, cataracts, and radiation ulcers, loss of
hair, reddening of skin, sterilization, nausea, etc.
Radioactive contaminant : A radioactive
material which has spread to places where it may harm persons, spoil
experiments, or make products or equipment unsuitable or unsafe
for consumption by living beings, or for some specific purpose.
Radioactive decay : The spontaneous
transformation of a nuclide into one or more different nuclides,
accompanied by either the emission of particles from the nucleus,
nuclear capture or ejection of electrons. Also known as radioactive
transformation, radioactive disintegration or radioactivity. Radioactive
element : An element all of whose isotopes spontaneously transforms
into one or more different nuclides, giving off various types of
radiations, examples include uranium, radium and thorium.
Radioactive emanation : A radiactive
gas given off by certain radioactive elements, all of these gases
are isotopes of the element radon. Also known as emanation.
Radioactive waste : Liquid, solid,
or gaseous waster resulting from mining of radioactive ore, production
of reactor fuel materials, reactor operations, processing of irradiated
reactor fuels, and from use of radioactive materials in research,
industry, and medicine.
Radioactive waste disposal :
The disposal of waste radioactive materials and equipment contaminated
by radiation: the two basic disposal methods are concentration for
burial underground or in the sea, and dilution for controlled dispension:
reprocessing of reactor fuel is a major source of radioactive waste.
Radioactivity : A behaviour of
an element in which nuclei are undergoing change and emitting particles.
This occurs naturally in approximately fifty elements. It can be
Radioactivity equilibrium : A
condition which may arise in the decay of a radioactive parent with
short-lived descendants, in which the ratio of the activity of a
parent to that of a descendant remains constant.
Radiochemistry : Area of chemistry
concerned with the study of radioactive substances.
Radio isotope : An isotope which
exhibits radioactivity. Also known as radioactive isotope.
Radio isotope assay : An analytical
technique including procedures for separating and reproducibly measuring
a radioactive tracer.
Radiometric dating : A technique
for measuring the age of an object or sample of material by determining
the ratio of the concentration of a radio isotope to that of a stable
isotope in it: for example, the ratio of carbon-14 to carbon-12
reveals the approximate age of bones, pieces of wood, and other
archaeological specimens. Also known as radioactive age determination
or radiogenic dating.
Radium : A naturally radio active
chemical element, Ra, that has an atomic number 88. It is used as
a source of neutrons alpha particles.
Thorium : A naturally radio active
chemical element, Th, that has an atomic number 90. It is used as
fuel in nuclear reactors called the "breeder" reactors.
Thorium is available in India on the Kerala coast in the form of
Tracer : A foreign substance,
usually radioactive, that is mixed with or attached to a given substance
so the distribution or location of the latter can later be determined;
used to trace chemical behaviour of a natural element in an organism.
Also known as tracer element.
Transmutation : A nucleus process
in which one nuclide is transformed into the nuclide of a different
Tritium : The hydrogen isotope
having mass number 3. It is one form of heavy hydrogen.
Uranium : A chemical element,
U, that has an atomic number 92. It reacts with nearly all nonmetals
and is used as fuel for nuclear reactors, available in the mineral
form at Jeduguda in Bihar.
X rays : Invisible electromagnetic
radiation with wavelengths shorter than visible light. X rays are
produced when high energy charged particles collide with other charged
particles or with atoms.