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A homemade device for testing particle scattering
A homemade device for testing particle scattering; experiments in zero
In 1909 Ernest Rutherford suggested an experiment that led to the first
modern model of the atom. Hans Wilhelm Geiger, a German postdoctoral
researcher working in Rutherford's laboratory, had been studying the passage
of alpha particles through a metal foil. Most of the positively charged
particles passed through the foil directly, striking a detector screen on
the other side. A few particles were deflected slightly by electrical
forces as they went through atoms in the foil. The deflections never
amounted to more than a few degrees because the particles carried
appreciable energy and momentum.
Nevertheless, Rutherford suggested to Geiger that Ernest Marsden, then a
student working with Geiger, look hard for alpha particles deflecting at
greater angles. No one, including Rutherford, expected that any would be
found. Within days, however, Geiger returned to Rutherford with the
startling news that some of the particles were scattered at large angles,
even directly back toward the source. Rutherford later commented: "It was
quite the most incredible event that has ever happened to me. It was almost
as incredible as if you fired a 15-inch shell at a piece of tissue paper and
it came back and hit you.'
The deflection of alpha particles in that experiment is now called
Rutherford scattering. It revealed that the contemporary model of the atom
was wrong. This plum-pudding model, as it was often called, envisioned a
roughly uniform distribution of positively and negatively charged particles.
If an alpha particle penetrated such an atom, it would be attracted by the
negatively charged particles and repelled by the positively charged ones.
The net deflection, if any, would be small.
When Geiger announced that some alpha particles were deflected considerably,
Rutherford realized they must be experiencing a strong electric repulsion
when they entered an atom. That meant the charged components of an atom are
not uniformly distributed. Instead the positively charged ones are
collected in a highly compact core called the nucleus and negatively charged
ones orbit the nucleus at distances that are large compared with the
diameter of the nucleus.
Imagine following an alpha particle as it penetrates an atom made according
to Rutherford's model. As it approaches the atom it experiences essentially
no electric force because the atom as a whole is neutral. Once it passes
through the orbits of the electrons, it begins to be repulsed by the nucleus
because both the particle and the nucleus carry a positive charge. Suppose
it travels directly toward the nucleus. The continuously increasing
repulsion slows the particle to a momentary stop and then propels it back
along the path of entry. The angle of deflection is 180 degrees.
An alpha particle entering an atom along a path offset from the nucleus is
deflected less. The larger the offset, the smaller the angle of deflection.
Since the nucleus is tiny, a small deflection is likelier than a large one.
That is why Geiger and Marsden originally observed small deflections.
The distribution of the particles striking the detection screen is described
in terms of the density of strikes per unit of screen area. The density
depends on the scattering angle. The major strength of Rutherford's model
for the atom was that it correctly predicted how the strikes would vary in
density. Although several earlier workers had proposed similar solar-system
models of the atom, Rutherford is credited with the first modern model
because of his success in explaining the scattering of the alpha particles.
In last year's finals of the International Science and Engineering Fair,
Rudy Timmerman of Wickes, Ark., won first place in the physics division with
his experiments on the scattering of alpha particles from a thin gold foil.
He set out to repeat the basic features of the experiment by Geiger and
Marsden. In particular he wanted to verify Rutherford's mathematical
prediction concerning the angular dependence of the scattering.
Timmerman relied on a detection scheme developed by Charles W. Leming of
Henderson State University in Arkansas. Leming's apparatus, which is now
manufactured commercially by the Daedalon Corporation (35 Congress Street,
Salem, Mass. 01970), incorporates a special type of film that is sensitive
to the impact of alpha particles. When it is developed, it has a small hole
at every point where a particle struck it.
Timmerman constructed his own version of the apparatus. It consisted of
three aluminum plates held by four long bolts. In each plate he drilled a
small central hole. Above the hole in the top plate he positioned a source
of alpha particles. Below the hole in the central plate he mounted a metal
foil. On the upper surface of the bottom plate he placed the detection
He expected that the alpha particles would pass through the holes in the top
two plates, forming a beam. When the particles passed through the foil,
most of them would continue undeflected, but some would scatter into a small
region surrounding the center of the detection film. Timmerman planned to
measure the distribution of holes around the center in order to determine
how the density of particles depended on the scattering angle. Although his
plan was simple, he had to overcome several problems before he finally
obtained the data he submitted at the fair.
Leming supplied Timmerman with the detection film. Leming also worked out a
procedure for developing the film. After exposure the film is kept for 24
hours in a 2.5-molar solution of sodium hydroxide that is maintained at a
temperature of 40 degrees Celsius (104 degrees Fahrenheit). Timmerman built
an apparatus for developing the film. He puts the film and the sodium
hydroxide solution in a canister that is lowered into a water bath inside a
bucket. An aquarium heater warms the water and hence the contents of the
canister. A cover and some insulation around the bucket slow the loss of
heat to the room. A thermometer mounted on the inside of the bucket
monitors the water temperature.
Before beginning his experiments on scattering Timmerman tested the strength
of his source of alpha particles by putting a small piece of film near the
source for a few minutes. He developed the film and put it in a microfilm
machine to magnify it. The film had hundreds of holes, indicating that the
source was strong enough for his plans.
He then positioned the source and a new sheet of film in his apparatus but
still did not insert the metal foil. The apparatus was placed in a bell
jar, which was then evacuated with a pump so that the alpha particles would
not be scattered by air molecules. After five minutes the film was removed,
developed and examined. Again Timmerman found numerous holes spread over
the center of the film. The apparatus seemed to be suitable for the
Timmerman's next move was to put his thin gold foil in the apparatus. (He
had acquired the foil from a painter of window signs.) The apparatus was
evacuated and the film was exposed for two hours. When he looked at the
film later, he discovered that the vacuum had been lost during the test.
After he developed the film he found it was covered with holes, indicating
that the alpha particles had scattered from air molecules and heavily
showered the film.
Timmerman concluded that the bell jar surrounding the scattering apparatus
was leaking air and therefore would have to be evacuated several times
during a test. He tried to rig an automatic pump control by attaching an
electronic switch to an input port of a home computer, which he programmed
to turn on the pump every 30 minutes. The arrangement resulted in a
burned-out switch. Timmerman rigged a switch to control a relay on the
pump, but the relay began to stick. Eventually he discovered that the leak
was not in the bell jar but in the vacuum pump. From then on he closed off
the hose from the pump as soon as the air had been removed from the bell
The next problem was a malfunction of the aquarium heater that resulted in
the ruin of several sheets of film. Unable to find a replacement for the
heater, Timmerman wired it to an interface with the computer. The computer
was to monitor a temperature probe in the water bath and turn on the heater
as needed. This procedure failed because the temperature probe did not
function properly. Timmerman then decided to buy a new aquarium heater.
With a fresh supply of film he set out to determine the proper exposure time
for his experiments. Seven hours seemed to work well. When a sheet of
exposed film was placed in the microfilm machine, he could easily see the
holes created by the alpha particles. Measuring the distribution of the
holes with the microfilm machine proved to be too difficult, and so he
decided to try a more automatic procedure incorporating his computer.
He enlarged his view of the holes in the film by projecting the film onto a
wood screen he built. The holes appeared as bright spots. To chart the
location of the spots Timmerman pivoted a wood arm around one of the lower
corners of the screen. The arm bore a slide with a small aperture. He
rotated the arm and moved the slide until a bright spot fell on the
Wires in the slide and the arm constituted a voltage divider. When the slide
was near the pivot of the arm, the voltage across the wires was low. It
increased as Timmerman moved the slide to the far end of the arm. Hence the
voltage across the wires was proportional to the distance between the
aperture in the slide and the pivot point of the arm.
Timmerman arranged for the center of the film, the point reached by
undeflected alpha particles, to project directly onto the pivot. He next
moved the arm and the slide until the aper aperture in the slide was aligned
with a bright spot. The voltage across the wires was then proportional to
the distance between the center of the film and the point where the particle
responsible for the bright spot struck the film.
Timmerman was interested in the distribution of alpha particles within a
scattering angle of 10 degrees. Therefore he set up his apparatus so that a
bright spot from a particle deflected at 10 degrees fell at the outermost
position of the slide. With the help of James A. Wisman of the University
of Arkansas at Fayetteville, Timmerman constructed an interface between the
voltage divider and his computer. The interface sectioned the maximum
voltage from the divider into 256 steps. Thus each step corresponded to
10/256 degree of scattering.
To measure the scattering distribution on a film Timmerman moved the arm and
slide to each bright spot in the projection. When the aperture in the slide
was aligned with a spot, he triggered a switch so that a signal was sent to
the computer from the interface. The signal indicated which voltage step
(and so which angle of scattering) should be assigned to the spot. The
computer recorded the information.
Timmerman then wrote programs that calculated the density of holes in the
film and printed a histogram of the density versus the angle of scattering.
Not satisfied with the results, he repeated the experiments with longer
exposure times (10 hours and 18 hours). These tests generated such an
abundance of holes in the film that hours were needed to measure their
locations. Timmerman also corrected for a background of holes in the film,
which were presumably due to alpha particles unrelated to his source. He
tried various "best fit' functions to the data to find the angular
dependence of the scattering.
Timmerman concluded that the angular dependence seemed to follow
Rutherford's prediction. Nevertheless, he still had reservations about the
collimation of the particle beam before it reached the foil, and so he
tested his conclusion with two procedures. He weighed the gold foil in
order to determine its thickness. Then with Rutherford's formula for
scattering he calculated the thickness from his measurements of the density
of holes in the film. The two findings agreed to better than an order of
Gravity often modifies the behavior of solids and liquids in subtle ways.
One technique for eliminating its influence is to study materials in free
fall. Donald R. Pettit of the Los Alamos National Laboratory and astronaut
Joseph P. Allen of the Johnson Space Center in Houston have recently done
experiments in free fall in the cargo space of an airplane belonging to the
National Aeronautics and Space Administration. They were assisted by Robert
K. Williams of the Johnson Space Center.
The airplane flew a series of between 40 and 60 vertical parabolic loops.
As the craft neared the top of a loop with a speed of about Mach 5 (half
the speed of sound in the surrounding air) the occupants began free fall.
The floor of the airplane actually fell away from them, but the sensation
was that gravity suddently vanished. This state is referred to as zero g,
where g symbolizes the normal strength of gravity.
About 20 seconds later the free fall ended near the bottom of the loop. For
the next 50 seconds the airplane pushed upward on the occupants, creating
the sensation that gravity was twice its normal strength, a state referred
to as 2 g. The speed of the airplane at the bottom of the dive was about
Mach 88. The brief time of free fall and the subsequent need to protect
oneself and the equipment from the 2-g phase limited the studies to
In addition to their serious research Pettit and Allen had time for a few
recreational experiments. One experiment involved the stability of an egg
spinning about its long axis. Try standing an egg on a table and spinning
it on either end. A hard-boiled egg spins stably for some tens of seconds,
but a fresh egg quickly becomes unstable, falls over and spins for a while
about its short axis until friction from the table drains all its energy.
Pettit investigated the spin of eggs at zero g. On each egg he marked a
line from end to end to enhance the visibility. Then he carefully spun each
egg (in the air) about its long axis with as little initial wobble as
possible. The hard-boiled egg continued to spin stably throughout the
zero-g phase of the loop. The fresh egg completed about two revolutions and
then abruptly began to spin about its short axis. Apparently the fluid in
the egg was set in motion by the initial rotation, even at zero g. The
fluid motion increased the wobble, making the egg spin about its short axis.
Pettit then did a similar experiment with a closed, transparent container
partially filled with water. He released the container at zero g while
giving it a spin about its long axis. The container soon began to wobble
appreciably, but it never stabilized into rotation about its short axis
before the end of zero g.
Pettit and Allen also studied fluid flow in rotating systems. Consider a
cylindrical container of water that is placed at the center of a turntable.
When the turntable begins to rotate, the wall of the container drags the
water in a circle. Eventually the water circulates around the center of the
container at the speed of the turntable. During the transition the water is
said to be in a spin-up state. Suppose the turntable abruptly stops. The
circulation slows and eventually stops. In this phase the water is said to
be in a spin-down state.
During spin-up and spin-down an additional flow arises in the water. This
secondary flow results from unequal pressures created in the water by the
primary flow around the center. In spin-up the secondary flow is downward
along the center line, outward along the bottom, upward along the wall and
then inward along the top surface. In spin-down the secondary flow is
reversed. Evidence for secondary flow is seen in the motion of tea leaves
when the tea is in spin-down. The leaves, which initially are strewn over
the bottom of the cup, are forced to the center and then abandoned in a pile
by the upward flow of water.
What about secondary flow in zero g? Pettit partially filled the
transparent, closed container with water, adding half a teaspoon each of
waterlogged sawdust and aluminum glitter. (The glitter is available in hobby
and art-supply shops.) The sawdust and glitter served as tracers for the
In normal conditions of gravity the secondary flow of spin-down caused the
glitter to collect like tea leaves in a small pile at the center of the
container. The sawdust circulated in a ring just above the bottom center
until near the end of spin-down; then it collapsed onto the pile of glitter.
In spin-up the glitter moved to the wall first, followed by the lighter
sawdust. At zero g Pettit released the container while spinning it. The
water was in spin-up. The sawdust and glitter moved to the wall as before,
but this time they did not collect along the bottom edge. The glitter was
pressed against the wall and the sawdust moved up along the wall.
To generate spin-down Pettit made the container gyrate and then held it
stationary. The sawdust and glitter moved along with the expected secondary
flow, but they failed to pile up on the bottom.
Apparently the secondary flow in spin-up and spin-down is the same at zero g
as it is at normal gravity. In the effective absence of gravity, however,
the sawdust and glitter are no longer confined to the bottom of the
container. The secondary flow can carry them upward at the center of the
container in spin-down and at the wall in spin-up.
Pettit and Allen did another experiment with the container. When water
circulates about a container's long axis at normal gravity, the top surface
is concave. Pettit and Allen wondered how the shape would change as the
effective gravity varied. They found that at 2 g the concave surface was
shallower. At zero g the concavity deepened enough to force all the water
into a layer along the wall.
In a final experiment Pettit and Allen tested a yo-yo at zero g and at 2 g.
They wondered if it could be made to spin at the end of its string, a trick
called sleeping. At normal gravity you must let the yo-yo fall gently to
the end of its string to minimize the usual bounce. Gravity holds the yo-yo
there while it spins loosely in the loop around the spindle.
At 2 g Pettit easily made the yo-yo sleep. At zero g it refused to sleep
even with a gentle toss. It always bounced. The only way Pettit could get
it to sleep was to throw it outward and then pull on the string. That made
the yo-yo circle around his hand. The resulting effective centrifugal force
kept the yo-yo at the end of the string.
Pettit is interested in more experiments that might be done at zero g. If
you have any ideas, write to him at the Los Alamos National Laboratory, MS
P952, Los Alamos, N.M. 87545.
Photo: Rudy Timmerman's particle-scattering apparatus
Photo: How the special film is developed
Photo: Timmerman's device for recording the locations of holes in the film
Photo: The flight path of the airplane for zero-gravity experiments
Photo: The egg experiment
Photo: A container of water rotating in zero gravity
Photo: Secondary flow in normal gravity
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