Imagine that one day you receive a gift. The
catch is that you have to guess what it is without opening
the wrapped box. How can you know what it is? Perhaps you
will shake the box and guess the content by sensing its weight
and shape. That is of course unreliable. Or you may watch
the box under a lamp, hoping that part of the light will pass
through it to produce a shadow of its content. An idea pops
up: how about going to the airport and asking the security
guards there to use their X-ray machine to examine your gift.
That’s a good idea. However, all you find out is that
there is a smaller box inside, made of thick plates of lead.
You still cannot see what’s inside the smaller box even
with the X-rays. So, what should you do?
Rest assured that you are not alone. Physicists
who study the ultimate structure of matter face the same problem.
With many years of work, they have already learned of many
levels of structures in matter, but the quest to uncover the
ultimate structure of matter is far from over. One technique
that has helped physicists tremendously is really the same
as your idea of shining X-rays through your gift box. This
technique is called the collision experiment.
Through a series of collision experiments conducted between
1909 and 1914, the New Zealand physicist Ernest Rutherford
and his students Hans Geiger and Ernest Marsden have not only
made a breakthrough in atomic physics, but also set up a paradigm
in modern experimental particle physics. Rutherford shot a
beam of a-particles1 at a speed of about 20,000 km/s at a
thin gold foil (Fig. 1). As he expected, most a-particles
passed through the foil, with only slight changes in their
velocities. However, to his surprise, a small fraction of
a-particles suffered large deflections – some even rebounded
backward! While most of the a-particles were passing straight
through the gold foil as if they met no obstacles, a few crashed
on very hard objects, resulting in back scattering. In Rutherford’s
own words, "It was as if you fired a 15-inch shell at
a piece of tissue paper and it came back and hit you."2.
He concluded that most of the mass inside an atom were concentrated
in a tiny space called the atomic nucleus. Furthermore, Rutherford
deduced from the data that the diameter of an atomic nucleus
is approximately  m,
which is merely one a hundred thousandth of an atom’s
diameter. The atomic nuclei carry positive electric charges,
and they therefore repel a-particles. Most of the space inside
an atom is occupied by electrons, which are negatively charged.
This atomic model of Rutherford proves to be generally correct.
It reveals that atoms, which were previously thought to be
the fundamental and indestructible, also have their own structures.
(Fig. 1 Rutherford’s collision
experiment: colliding a-particles with a gold foil.)
Rutherford's experiment tells us not only the
existence of the atomic nucleus, but also its mass. Assuming
the conservation of momentum and energy, Rutherford calculated
from his data that the masses of various atomic nuclei were
all nearly integral multiples of the proton mass. The multiple
is called the mass number and is approximately two times the
atomic number3 of the element. He therefore speculated that
an atomic nucleus was made up of roughly the same number of
protons and another kind of particles called the neutron4,
which was similar to the proton in mass but electrically neutral.
How could one test this conjecture?
James Chadwick, another one of Rutherford’s students,
eventually discovered the neutron in 1932. Chadwick again
did a collision experiment, but this time he used two collisions
instead of one. It is difficult to detect neutrons, because
they have zero electric charge and high penetrability. Chadwick's
method was to modify Rutherford's experiment mentioned above
(Fig. 1) by adding an "air cushion" behind the foil,
as shown in Fig. 2. The a-particles he used were energetic
enough that they could break up the atomic nuclei in the beryllium
foil to eject protons or neutrons (collectively known as nucleons).
Rutherford had earlier discovered that protons could be released
by colliding atoms with a-particles5, which proved that there
were protons inside atomic nuclei. These protons were readily
absorbed by the foil or air. Chadwick's idea was to intercept
the ejected neutrons by the extra "air cushion"
made of nitrogen gas. The neutrons might hit some nitrogen
nuclei and expel them from the "air cushion". Being
positively charged, these nitrogen nuclei were easily detectable.
(Fig. 2 Chadwick’s collision experiment: to prove the
existence of neutrons using Nitrogen nuclei in secondary collisions.
)
Two fundamentally important concepts are involved in Rutherford's
and Chadwick's experiments. First, nuclei are tiny and their
diameters are far smaller than the wavelength of visible light.
It is therefore impossible to 'see' their structures with
optical microscopes. Chadwick's method was to bombard the
atomic nuclei with high energy 'bullets' so as to break them
up, and then he could infer the "contents" of atomic
nuclei from the remnants. Just for fun, let us call this "the
blast-it-up paradigm". Now you probably know how to deal
with your gift box. Secondly, often it is not so easy to detect
some particles such as the neutron directly. Chadwick therefore
applied an indirect method; he introduced an additional element
– the nitrogen nuclei – to 'catch' the neutrons.
He could then not only prove the existence of the neutron,
but also study its properties through its collisions with
the nitrogen nuclei. This technique is frequently applied
in modern physics. For example, two of the 2002 Nobel Physics
Laureates applied this technique to construct neutrino5 telescopes
successfully and made important discoveries in neutrino astrophysics.
Incidentally, Rutherford not only set up the "blast-it-up
paradigm", but also made an unprecedented achievement.
He was the first person ever to succeed in transforming one
element into another6. It was a historic event in the development
of science, technology, and civilization.
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