The Nobel Prize in Physics 1927
Arthur H. Compton, C.T.R. Wilson
Presentation Speech by Professor K.M.G. Siegbahn, member of the Nobel Committee for Physics of the Royal Swedish Academy of Sciences, December 10, 1927
Your Majesty, Your Royal Highnesses, Ladies
and Gentlemen.
The Royal Academy of Sciences has awarded this year's Nobel Prize
in Physics to Professor Arthur Holly Compton of the University of
Chicago for the discovery of the phenomenon named after him
the Compton effect, and to Professor Charles Thomson Rees Wilson
of the University
of Cambridge for his discovery of the expansion method of
rendering visible the tracks of electrically charged
particles.
Professor Compton has won his prize by work in the field of
X-radiation. Soon after Röntgen's discovery it became known
that matter exposed to X-rays emits radiations of different
character. Besides an emission of electrons, corresponding to the
photoelectric effect known also in the optical region of
radiation, there is also a secondary X-radiation. Even before the
methods of X-ray spectrometry were known, these secondary X-rays
were proved by the investigation of their absorption to be of a
twofold nature. It was Barkla who, through his fundamental
researches, proved that the secondary X-radiation consists partly
in a scattering of X-rays, which he thought to have the same
penetrability as the original radiation, and partly in a specific
X-radiation which was characteristic of the chemical atom and
which was more easily absorbed.
When X-rays fell upon matter with small atomic weight, as for
example graphite, Barkla was not able to detect the mentioned
characteristic X-radiation, but only a scattering; and
consequently the secondary rays ought to have the same properties
as the original X-rays. Barkla, however, in the course of his
investigations of the absorption, had already been able to show
that in this case also the secondary X-rays - at least partly -
are more easily absorbed than the original radiation and
therefore have a greater wavelength. Barkla thought this to be a
new characteristic X-radiation.
This is the point where Compton comes in and affects the
development of science. He made exact spectrometrical
investigations of the secondary X-radiation from matter with
small atomic weight: in other words, he undertook to investigate
exactly the scattered X-radiation. After some preliminary work,
he found an experimental method that gave results which were as
exact as they were astonishing.
Using homogeneous X-rays - corresponding optically to
monochromatic illumination, that is to say, to the use of a
source of light that emits only one single spectral line - he
found that the scattered radiation consists of two lines, one
exactly the same as that of the source of rays, the other with a
somewhat greater wavelength. This is the first evident
manifestation of the Compton effect. Its reality was at first
disputed, but of late years it has been well established and
verified.
The change in wavelength soon proved to be independent of the
nature of the matter used for scattering, while it varies with
the angle between the incident and the scattered rays. Hence the
phenomenon cannot be explained as a new characteristic radiation
of the same nature as that hitherto known; and Compton deduced a
new kind of corpuscular theory, with which all experimental
results showed perfect agreement within the limits of
experimental error.
According to this theory, a quantum of radiation is re-emitted in
a definite direction by a single electron, which in so doing must
recoil in a direction forming an acute angle with that of the
incident radiation. In its mathematical dress this theory leads
to an augmentation of the wavelength that is independent of the
wavelength of the incident radiation and implies a velocity of
the recoil electron that varies between zero and about 80% of the
velocity of light, when the angle between the incident and the
scattered radiation varies between zero and 180°.
Thus this theory predicts recoil electrons with a velocity
generally much smaller than that of the above-mentioned electrons
which correspond to the photoelectric effect. It was a triumph
for both parties when these recoil electrons were discovered by
Wilson's experimental method both by Wilson himself and,
independently, by another investigator. Hereby the second chief
phenomenon of the Compton effect was experimentally verified, and
all observations proved to agree with what had been predicted in
Compton's theory.
Quite apart from the improvements and additions that have been
made to this theory by other investigators, the Compton effect
has, through the latest evolutions of the atomic theory, got rid
of the original explanation based upon a corpuscular theory. The
new wave mechanics, in fact, lead as a logical consequence to the
mathematical basis of Compton's theory. Thus the effect has
gained an acceptable connection with other observations in the
sphere of radiation. It is now so important that, in the future,
no atomic theory can be accepted that does not explain it and
lead to the laws established by its discoverer.
Finally, the fact deserves to be emphasized that the Compton
effect has proved to be of decisive influence upon the absorption
of short-wave electromagnetic - especially radioactive -
radiation and of the newly discovered cosmic rays.
Professor Compton. Your discovery of the
phenomenon known as the Compton effect has already proved so
important that the Royal Academy of Sciences has awarded you a
Nobel Prize, which I now ask you to receive from the hands of His
Majesty.
Professor Wilson has been awarded his prize for the discovery of
a purely experimental method, which dates back from as long ago
as 1911. It is based upon the formation of clouds, which develop
when sufficiently moist air is suddenly expanded. The
refrigeration caused by the expansion brings the temperature to
sink below the dew-point, and the vapour is condensed into small
drops, which form together visible clouds. In the first stage of
condensation a droplet is always formed round a nucleus. The fact
that an electrically charged particle acts as a nucleus in the
formation of drops could, after the discovery of the corpuscular
radiations, be concluded from an experiment that Helmholtz had,
long before, made when he found that a stream of vapour loses its
transparency in the vicinity of electrically charged
objects.
After it had become known that electricity is conducted through
gases by means of ions, and that ions are formed - or, in other
words, gases are ionized - under the influence of X-rays or
radioactive substances, the way lay open for Wilson to follow
photographically the formation of droplets around electrically
charged particles. Alpha and beta particles emitted by
radioactive substances ionize the gases, and their tracks are
marked by a formation of droplets. A suitable photograph of these
droplets then gives a picture of the tracks of the ionizing
particles.
The problem is a little more complicated when the nature and the
details of the ionization caused by X-rays have to be analysed;
and the perfect method for such investigations was not described
until in a paper of 1923. The extremely delicate regulation of
small-time intervals which is necessary in such researches is
attained by the use of three pendulums of adjustable period,
which are all released simultaneously. The pendulum which comes
down first, opens a communication with a vacuum, and the
resulting suction is used, by a mechanical device, to produce a
sudden expansion of the gas that is being examined. The second
pendulum releases an electric spark, which passes through an
X-ray tube, oscillatory sparks being excluded; and thus the
anticathode is brought to send an X-radiation of extremely short
duration through the gas before the lenses of a stereoscopic
camera. The third pendulum releases another electric spark, which
passes through mercury vapour and momentarily illuminates the
clouds. By means of sliding weights on the different pendulums,
just as on an ordinary metronome, Wilson was able to bring it
about that the X-rays were sent through the gas at the moment
when the expansion was complete, and the illuminating spark just
as long afterwards as was needed for a sufficient formation of
droplets round the ions, but before the droplets had time to be
dislocated by currents in the gas, which might have deformed the
tracks visible on the photographic pictures.
Wilson's method attracted attention at first mainly as an elegant
and popular method of demonstration. The formation of droplets by
x-particles is so dense that the resulting cloud
photographs show continuous white lines: and everybody was glad
to recognize on these lines the sharp bendings which correspond
to the sudden change of direction previously known. Along the
beta-rays, on the other hand, are seen isolated droplets, and
their tracks show a multitude of different types according to
differences in initial velocity. For the investigation of such
rays with a comparatively small velocity, the most suitable
method is the excitation by the momentary X-radiation described
above. Here there has been collected a very large photographic
material, from which probably not all possible conclusions have
yet been drawn, and to which Wilson has devoted assiduous
work.
Of late years, new and scientifically important results have been
attained which could not have been gained by other methods. The
consequence of this is that the discovery, although it was made
so long ago, satisfies the provisions for the award of the Nobel
Prize. It would not be of much use to describe these results on
this occasion, as the understanding of them presupposes full
knowledge of the structure of the atom. I will merely call to
mind that in 1923 Wilson gave the experimental proof of the
existence of the recoil electron tracks that had been postulated
by Compton for his explanation of the change in wavelength of
scattered X-rays, and that his method has rendered possible the
closer examination of these tracks.
Professor Wilson. Although a long time has elapsed since you discovered your elegant expansion method, the high value of your discovery has been greatly augmented both through your own assiduous investigations and through results obtained by others. The Academy is happy that an article in the Statutes allows it in such cases to reward even discoveries of comparatively old date; and I now ask you to receive the prize that you have won from the hands of His Majesty.
From Nobel Lectures, Physics 1922-1941, Elsevier Publishing Company, Amsterdam, 1965
Copyright © The Nobel Foundation 1927
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