September 1956 Popular Electronics
Table of Contents
People old and young enjoy waxing nostalgic about and learning some of the history
of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights
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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
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.
Vapor Trails. 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.
Modern research expansion-type cloud chamber, known as the "Pantograph," has 22"
diameter and is 3 1/2" high.
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.
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.
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.
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 Laboratory.
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.
The signature of a high-energy electron as it travels through the cloud chamber under
the influence of a magnetic field.
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.
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
Cloud Chambers. 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
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!
Posted March 27, 2015