The Nobel Prize in Physics 1935
James Chadwick
Presentation Speech by Professor H. Pleijel, Chairman of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences, on December 10, 1935
Your Majesty, Your Royal Highnesses, Ladies
and Gentlemen.
This year like two years ago the Academy of Sciences awards the
Nobel Prize for Physics as a reward for discoveries in the world
of atoms and molecules. However, a fundamental difference is to
be observed between the prizewinners of this year and the prizes
that were awarded last time. The latter formed the reward for
investigations of more theoretical nature, viz. the discovery of
laws regulating the great many phenomena having been brought into
light by experimental research. This year the Nobel Prize for
Physics is awarded as a reward for a discovery, confirmed in an
experimental way, of a new fundamental building-stone of atoms
and molecules, viz. the discovery of the so-called
neutron. By a combination of intuition, logical thought,
and experimental research Professor J. Chadwick, the laureate of
this year, has succeeded in proving the existence of the neutron
and establishing its properties.
One of the Nobel Prize winners for the year 1933, Professor Heisenberg, had concluded by
his researches that, owing to reasons of principle as well as to
the roughness of our senses and our instruments, it would be
impossible for us to arrive at an exact knowledge of what takes
place within the atoms. However, experimental research has made
undaunted progress, and by the aid of refined methods and new
instruments today's Nobel Prize winners in Physics and Chemistry
have succeeded in presenting science with a new and deeper
knowledge of the structure and qualities of matter.
The Nobel Prize for Physics for this year is awarded as a reward
for the discovery of the neutron.
The neutron is a heavy particle without any electric charge and
of the same weight as the nucleus of an atom of hydrogen.
At the decomposition of the radioactive substances and at the
disintegration of atoms and molecules two kinds of particles were
always found. One of them that has been called electron,
has an extremely small weight, amounting to about 1/2000 of the
weight of an atom of hydrogen. The electron is charged with
negative electricity, the quantity of the charge being always the
same, in whatever way the electron may have appeared. The other
kind of particles proved to have a weight of the same size as
that of the atom of hydrogen, or a multiple of the same. This
heavy particle is always combined with a charge of positive
electricity, whose quantity turned out to be equal to or a
multiple of the charge of the electron. The smallest particle
with positive charge, found in this way, consists of the nucleus
of the atom of hydrogen, and its positive charge equals the
negative charge of the electron. This smallest, heavy particle
with positive charge has received the name of proton.
Owing to the disintegration of atoms always resulting in protons
and electrons, the theory was established that the atoms were
composed of protons and electrons. The atom was thought of as
having the form of a planetary system where the central body
consists of protons, combined to a nucleus; outside this nucleus
the negative light electrons circle like the planets round the
sun. The number of electrons is different with different
substances. The lightest element, hydrogen, has only one
electron, helium has two, etc.
That the atom may be in a neutral state of electricity, the
positive charge of the nucleus must be the same as the total
charge of the exterior electrons. The simplest relation would
here have been that the number of protons in the nucleus had been
the same as that of the electrons circling about the nucleus.
This proved, however, not to be the case. In the atoms belonging
to different elements it was found that, apart from hydrogen, the
nucleus had about twice as many protons as the number of exterior
electrons. Thus e.g. helium has the weight four in relation to
the nucleus of hydrogen but only two exterior electrons. That the
atom may be neutral in electric respect, the supposition is
necessary that the surplus of positive electricity that the
nucleus thus receives owing to the greater number of protons, was
compensated by negative electrons also entering the nucleus. The
nucleus of helium was thus supposed to consist of four protons
and two electrons, and about this nucleus there circle two
electrons.
At first this idea of the atom could be made to agree fairly well
with experience. The nucleus-charge resulting determines the
character of the atom and its place among the elements. The
number of exterior electrons and the distribution of their paths
at different distances from the nucleus are determinative of the
physical and chemical qualities of the element; if one electron
suddenly passes from one path to another, light is emitted, and
if electrons from the paths closer to the nucleus are flung from
the atom, X-rays are emitted, and so on. If the number of protons
is increased or diminished in a nucleus, but the charge of the
nucleus is still kept unaltered by the addition or the loss of
negative electrons, the same element is still obtained but with
different atomic weight; a so-called isotope is obtained. Thus
e.g. lead is found in several different forms with different
weight; and heavy hydrogen, the object of last year's Nobel Prize
for Chemistry, is a similar modification of normal
hydrogen.
A continued study of the conditions of energy at the
disintegration of the nuclei of atoms showed, however, that the
theory of the nuclei being composed of protons and electrons
could scarcely be brought to agree with theoretical and
experimental facts. As often happens in these spheres, it was the
discovery of new phenomena, difficult to explain, that gave rise
to the solution of the problem about the structure of the nuclei
of atoms. In 1930 the scientists Bothe and Becker had
found a new strange radiation that appeared, when the substance
beryllium was bombarded with nuclei of helium. This new
radiation, which was called the radiation of beryllium
proved extremely penetrating. The rays could pierce a brass
plate, several centimeters thick, without any noteworthy loss of
velocity. When hitting nuclei of atoms, this new radiation caused
a disintegration of them, similar to an explosion.
As a matter of course the new rays became at once the object of
intensive experimental research, in which today's Nobel Prize
winners in Chemistry, the couple Joliot, have taken an
active and important part. At that time it was generally supposed
that the radiation of beryllium was of the same nature as the
electromagnetic waves of extremely short wavelength arising at
the disintegration of radioactive substances. This radiation has
received the name of g-radiation and
has the same qualities as the well-known X-rays. However, it was
found that the new radiation possessed a power considerably
superior to that of the strongest radioactive g-rays; a correspondent radiation from another
element, boron, proved, however, still stronger.
During their investigations of the radiation of beryllium, the
couple Joliot made the important observation that a block of
paraffin or another substance containing hydrogen being bombarded
with the new rays, will emit an intensive stream of protons. With
the assistance of the expansion chamber, constructed by the Nobel
Prize winner Wilson, in which the
paths of particles with electric charge - protons or electrons -
could be made visible, it was possible to calculate the energy of
the protons emitted from paraffin and thus also that of the
radiation of beryllium causing the stream of protons. Then it
turned out that the values of energy obtained, if the radiation
of beryllium was supposed to be a g-radiation, became absurdly high. Nor could
these values of energy be brought to agree with the energy to be
reckoned with in the radiation giving rise to the radiation of
beryllium. Chadwick, who had undertaken investigations of the
radiation of beryllium, found a similar radiation from quite a
number of other elements, e.g. helium, lithium, carbon, nitrogen,
and argon. By his extensive studies and calculations on
conditions of energy at collisions, he was soon convinced that
the radiation of beryllium could not be a g-radiation.
Already in 1920 Lord Rutherford had
suggested that, apart from protons and electrons, there also
existed particles of the same weight as a proton but without any
electric charge. To this particle was given in advance the name
of neutron. This neutron had long been searched for but
without any result. It is also easily understood how difficult it
would be to discover this particle without electric charge. The
neutron and the proton are certainly, like the electron, both
particles of extremely small dimensions. But owing to their
charges, the proton as well as the electron are accompanied by
electric fields, which make them act as bodies of considerably
larger dimensions, and their charges are influenced by the
charges of the atoms they pass; these charged particles are
therefore strongly checked when passing through substantial
bodies. The neutron, on the contrary, having no electric charge
is not affected and is not checked in its way, until it directly
hits another particle, which happens extremely seldom owing to
the small dimensions of the particles in relation to the distance
between them. This explains why a neutron may pass through
several kilometers of air, before losing its energy of motion.
The motion of a proton or an electron may be observed in the
above-mentioned Wilson chamber, and these particles being charged
with electricity, their courses will be curved, if they are
exposed to electric or magnetic fields. This curve may be studied
in the Wilson chamber. The neutron, on the other hand, being
without any charge, is not affected by such fields and may be
discovered only in the case of a direct collision with the
nucleus of an atom.
Chadwick now studied how, at a collision between radiation of
beryllium and nuclei of atoms, the exchange of energy would be,
supposing that the radiation of beryllium consisted of neutrons
flung out from beryllium, and he then found that the experimental
results attained agreed well with his own calculations. The same
was the case also with radiation from other substances. By these
facts the existence of the neutron was beyond all doubt. Chadwick
then examined the exchange of mass taking place when by collision
the nuclei of different substances are changed into new nuclei,
belonging to other substances, and into neutrons. As an example
may be mentioned that the nucleus of helium, when meeting that of
beryllium, gives rise to a nucleus of carbon plus a neutron.
Knowing the masses of different nuclei, it is possible directly
to calculate the mass of the neutron. By examining the exchange
of mass at a great number of collisions between the nuclei of
different elements Chadwick succeeded in determining exactly the
mass of the neutron, and as was to be expected, he found it
almost the same as that of the proton or that of the nucleus of
hydrogen.
On the other hand these researches have given a new method for
the exact calculation of the size of masses in the nuclei of
different elements. As characteristic for the usefulness of this
new method may be mentioned that in this way Chadwick obtained
another value for hydrogen than the earlier one observed by Aston
with his spectrograph of mass. Aston, having improved his
spectrograph, has obtained new values for the mass of hydrogen
agreeing with those obtained by Chadwick.
The existence of the neutron having thus been proved, it was no
more necessary to suppose compensatory charges of electron in the
nuclei. The nucleus of atoms is nowadays considered to be
composed of a number of protons and neutrons. Thus the nucleus of
helium consists of two protons and two neutrons; about the
nucleus there circle in the atom two electrons. Isotopes are
formed by surplus or lack of the number of neutrons in the solid
atom.
Owing to its weight and its great penetrating power, the neutron
has become a powerful resource to bring about the disintegration
of atoms and of nuclei of atoms, and during the last few years
this power of the neutron to split up atoms and molecules has
been largely made use of.
The existence of the neutron having been fully established,
scientists have, as has just been mentioned, come to a new
conception of the structure of atoms which agrees better with the
distribution of energy within the nuclei of atoms. It has proved
obvious that the neutron forms one of the buildingstones of atoms
and molecules and thus also of material universe.
However, there are still many questions to be answered, among
others the one about the relations of protons and neutrons to
each other. There are certain signs indicating that these two
particles are modifications of one and the same primitive
particle. The existence of the positive electron, found by
Dirac by
theoretical research, having now been experimentally proved, the
task of physical science will be to examine, more closely, the
relations existing between this electron and the parts of the
nuclei of atoms - the proton and the neutron; the neutron
discovered by Chadwick has here given a powerful instrument for
future researches on the structure of atoms and molecules. If the
qualities of the neutron are made use of, this will certainly in
the immediate future give us a new and deeper knowledge of matter
and its transformations.
Professor Chadwick. The Royal Academy of
Sciences has awarded you the Nobel Prize for Physics for your
discovery of the neutron.
We congratulate you to this most important result by which has
been revealed a new building-stone of matter playing the same
fundamental part as the proton and the electron.
By means of a new method, created by you, you have been able to
determine the mass of the neutron, and by the same method you
have found new, more exact values of the atomic weights of a
number of elements.
In the neutron Science has obtained a powerful means of splitting
up atoms and molecules which has already given important
results.
I now ask you, Mr. Chadwick, to receive the prize from the hands
of His Majesty.
From Nobel Lectures, Physics 1922-1941, Elsevier Publishing Company, Amsterdam, 1965
Copyright © The Nobel Foundation 1935
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