September 1956 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
By now, most people
involved in science and engineering have seen the iconic photos of cosmic rays and
other subatomic particles leaving a signature of their presence as streaks in a
cloud chamber. Invented by Scottish physicist Charles Wilson, the cloud chamber
is a sealed volume containing super-saturated water vapor that can be ionized by
energetic particles passing through it. The result is a tell-tale whitish line that
can be straight arced, or even a spiral, depending on the nature of the particle.
First developed in the early part of the 20th century, many particles predicted
by researchers were detected and identified. Many unexpected particles were also
encountered that gave physicists reasons to sharpen their pencils and develop new
theories to explain. Similar research and discoveries occur today using super-sensitive
electronic detectors instead of cloud chambers. CERN's
Large Hadron Collider
(LHC) is currently the world's grandest particle collider for performing atomic
and subatomic particle research.
After Class: Subatomic Footprints
At this very moment - as you read this - you are being riddled by
sub-microscopic bullets, sprayed and permeated by potential death rays, and assailed
from every side by wild energies of which you are not even aware. Were it not for
our sensitive scientific instruments, we should still be unconcernedly going through
life, blithely unconscious of the invisible "energy-world" around us. Cosmic rays,
radio waves, high-energy electrons, and a host of other energy packages buzz into
us and through us as if we didn't exist. Our radios, television, and radar receivers
tell us about the variety of waves that surround us, but what of the dozen or so
subatomic particles which do not advertise themselves quite as loudly?
Modern research expansion-type cloud chamber, known as the "Pantograph,"
has 22" diameter and is 3 1/2" high.
World's largest cloud chamber, originally designed for study
of cosmic ray air showers, is now in use at the "Bevatron" at U. C. Radiation Laboratory.
The signature of a high-energy electron as it travels through
the cloud chamber under the influence of a magnetic field.
Experimental setup for use with cloud chamber. In the background,
you can see the concrete shielding of the 184" cyclotron at the U. C. Radiation
A five-billion-volt meson from the world-famous Bevatron is shown
above striking a hydrogen atom in a diffusion cloud chamber, thus producing six
charged particles, This is an outstanding example of the phenomenon known as multiple
meson production. (The meson enters the picture from the left, as indicated by the
arrow, and the collision occurs at the photo's exact center.) All Photos Courtesy
of University of California Radiation Laboratory
The existence of these tiny elemental bits of matter had been suspected for many
years before the first "viewing" device was even conceived. As a matter of fact,
they announced their presence to Henri Becquerel in 1896 by leaving their "footprints"
behind them as they flashed through the emulsion of a piece of photographic film
inadvertently left in a drawer near some uranium. Becquerel simply remarked in his
notes that radioactive emanations from the uranium had fogged the film, not realizing
that he had "invented" the first subatomic particle detector.
Although we cannot see these particles, we can at least see where they have been.
One of the methods used to study them involves special photographic emulsions; another
is the cloud chamber, invented by the English physicist C. T. R. Wilson in 1911.
In the cloud chamber, the space within a hollow chamber is super-saturated with
water vapor. Electrically charged particles have the ability to serve as condensation
nuclei, i.e., they encourage water vapor in their immediate vicinity to change to
liquid water droplets. So - as a subatomic particle of the charged variety strays
into the chamber, it causes condensation of vapor all along its path - leaving a
trail behind it much like the vapor trails that appear behind a speeding jet plane.
The trail is substantially thicker than the particle itself; hence it becomes visible
and may be studied.
There are two basic types of cloud chambers. The Wilson cloud chamber is an expansion
type in which saturated water vapor is brought to the super-saturated state by a
sudden expansion of the volume of the chamber. The subatomic footprints which form
in this instrument are very fleeting, lasting for about 1/30 second. Later, in 1939,
Langsdorf at the University of California invented a cloud chamber in which the
conditions required for track formation are maintained continuously; after many
years of research, the Langsdorf chamber was perfected to the point where it could
be used as a research instrument.
Cosmic Rays. When no source of radiation is placed in or near the cloud chamber,
almost all the tracks seen will be caused by those mysterious vagrants from outer
space, the cosmic rays. Although cosmic rays in themselves are incapable of leaving
tracks, their energy is so high, particularly in the upper atmosphere, that they
smash into and disrupt atoms of any elements that happen to be present, releasing
a shower of all kinds of charged (and uncharged) particles.
These particles include the now familiar electrons, protons, and neutrons as
well as numerous other recently discovered atomic debris - -such as nine different
kinds of mesons, positrons, antiprotons, lambda particles, sigma particles, and
cascade particles. Most of them, with the notable exception of the neutron, two
types of mesons, and the neutrino, leave characteristic vapor trails which make
it possible to identify the causative agents. For instance, electrons make much
fainter and thinner tracks than the heavier positive bodies.
An alpha particle is a helium atom which has been stripped of its orbital electrons
so that only the nucleus remains; the nucleus contains two protons and two neutrons,
is quite massive, and is heavily ionizing. Alpha particles radiate from the gas
radon which is always present in the air in small quantities. At first an alpha
track is sharply defined, but then suddenly billows out as if it were exploding
into a puff of smoke.
Neutrons and other neutral particles do not leave their signatures behind them
because they have no electric charge and cannot act as nuclei for condensation.
There is, however, ample evidence of their very real existence. Neutrons smash into
atoms and liberate other charged particles that can be identified in the cloud chamber;
for instance, a fast neutron charging through the vapor may collide head-on with
a hydrogen atom, tear away the latter's lone electron, and cause the proton in the
hydrogen nucleus to make a track that tells the story of the atomic catastrophe.
Of all the scientific devices used for research in the first half of the 20th
century, the cloud chamber is probably responsible for the making of more Nobel
prize winners in physics than any other.
C. T. R. Wilson was awarded the Nobel prize in 1927 for the invention of the
chamber itself and some of the discoveries he made with it. The positron - a particle
which is the counterpart of the familiar negative electron in all respects except
for the fact that it carries a positive charge - was discovered by the American,
C. D. Anderson, during an investigation of cosmic rays by the use of a cloud chamber.
It was also in a cloud chamber that the elusive dream of the ancient alchemists
was first observed actually occurring: transmutation of one element into another.
P. M. S. Blackett, Nobel prize winner in 1948, filled a cloud chamber with nitrogen
and bombarded this gas with high-energy alpha particles. The alpha particles smashed
into nitrogen nuclei, set up a complex series of nuclear changes from which a proton
and an oxygen atom then emerged; thus, nitrogen was transmuted to oxygen.
As recently as September, 1953, a new discovery of the first rank was made with
a cloud chamber. At Brookhaven National Laboratory, research workers bombarded hydrogen
gas in a cloud chamber with very high energy pi mesons coming from the now-famous
Cosmotron accelerator. They observed the first artificially produced V-particles.
Naturally occurring V-particles from cosmic rays had been studied previously with
the aid of cloud chambers.
The life history of a cosmic particle from space that generates its track in
a cloud chamber and then comes to rest is something to capture the imagination.
It may be the nucleus of a helium, iron, or nickel atom torn from its electrons
millions of years ago in the very heart of an immense star thousands of times more
massive than our own sun. Blown out of the star by the unimaginable energy of billions
of exploding atoms ... accelerated through the vastness of interstellar space by
ever-present magnetic and electric fields ... escaping collision with cosmic dust
for millions of years ... it finally stumbles into our atmosphere and - in a single
split second - loses all the energy stored in it since birth!
"After Class" Topics
Posted May 18, 2020(original