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On the Discovery of the Atomic Nucleus
Shoji Nagamiya
JAEA, 2-4 Shirakata Shirane, Tokai-Mura, 319-1195, Japan
KEK, 1-1 Oho, Tsukuba-shi, 305-0801, Japan
1. Two Models for the Atom
About 100 years ago the atomic
nucleus was discovered. In the early
20th century two different models were
proposed to describe an atom. One
was the watermelon model
(alternatively known as the “plum
pudding” model), which was proposed
by J. J. Thomson. This model posited
that small, negatively charged “plums”
(now called electrons) were surrounded
by a positively charged “pudding”. The
other model, which was proposed later by Thompson’s
student, E. Rutherford, is like the solar system: a
positively charged nucleus at the center is surrounded
by orbiting negatively charged particles (electrons).
Rutherford’s model (the right figure
of Figure 1) is unstable (and changes
with time), because electrons
spontaneously emit radiation in the
Coulomb type potential, keep loosing
energy, and, eventually, are absorbed
by the nucleus. In contrast, the motion
of electrons in the watermelon model
(the left figure of Figure 1) is stable,
in particular, when electrons do not
move and do not lose energy.
Therefore, people had thought that the watermelon
model was much more realistic than the other model.
Figure 1: The watermelon model (left) and Rutherford’s model
(right) for the atom.
From the viewpoint of classical electromagnetism, the
motion of electrons in the solar system model or
Shoji Nagamiya is the President of AAPPS from January of
2011. He is the Director of J-PARC Center, a big center
supported by KEK and JAEA in Japan. He also serves as
President of Physical Society of Japan. He is a member of
Science Council of Japan (SCJ) and serves as a Secretary of
Natural Science Section of the SCJ. His original research field
is relativistic heavy-ion collisions started at Berkeley and then
at Brookhaven.
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AAPPS BULLETIN
Figure 2: J. J. Thomson and E. Rutherford
2. Radiation
Radiation was the most attractive subject for many
physicists in the early 20th century, and three different
types of radiation were discovered. At that time, an
α
-ray was defined as being a positively charged particle
which bends to the left in a magnetic field.
On the Discovery of the Atomic Nucleus ● ●
Figure 3: Three types of radiation: a positively charged α-ray, a negatively charged β-ray and a neutral γ-ray.
A β-ray was defined as being a negatively charged
particle which bents to the right in a magnetic field.
Finally, a γ
-ray was defined as being the radiation that
never bends in a magnetic field. Now, in contemporary
times, every physicist knows that an α-ray is a 4He
nucleus, a β
-ray is an electron emitted by the nucleus
when a nucleus decays via beta-decay, and γ
-rays are
radiation from an excited nucleus (Figure 3).
Rutherford was a leading figure in the field of
radiation. He studied the properties of radiation very
carefully and predicted that the strength of radiation,
which would be the decays of nuclei, must be doubled
when the number of parent nuclei is doubled. Namely,
the strength of radiation per unit second is proportional
to the number of parent nuclei, as shown in Figure 4 [1].
This leads to the following equation;
)
N = N0 exp(-αt) = N0 exp(-t/τ
where τis the lifetime of radiation. This work was
immediately recognized in the chemistry society and
he received the Nobel Prize in chemistry in 1908,
before the discovery of the atomic nucleus. In the early
20th century, the Nobel Prize in chemistry was given to
microscopic studies such as the study of nuclear
decays, whereas the Nobel Prize in physics was given
to macroscopic studies such as wireless telegraphy.
Episode of Rutherford
Some years later, Rutherford received the coat of arms
from the Queen. In his case, the design of the coat of
arms (Figure 5) was taken from his famous decay
curves for radiation (not by the discovery of the
nucleus). Note that the design on the shield features the
curves as seen in Figure 4, but they are rotated 90
degrees [1].
Rutherford was born in New Zealand, one of the
AAPPS countries, and lived there until the age of 25.
His mother was the first woman to become an
elementary school teacher in New Zealand. He very
often wrote letters to his mother. It might imply that his
early education was very important for Rutherford (as
it is, indeed, for many people). His face appears on the
$100 bill in New Zealand.
Figure 4: Rutherford’s prediction of radiation strength and his
experiment.
October 2012 Vol. 22 No. 5
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4. Bohr’s Theory
Figure 5: The Rutherford’s coat of arms (left) which is 90 degrees
rotated from his famous decay curves, and the New
Zealand $100 bill (right).
3. Scatterings of α-rays with Au Nuclei
In 1898 Rutherford discovered both α
-ray and β
-ray. In
the same year he moved from Cavendish Laboratory to
McGill University in Canada and studied the properties
of these forms of radiation. After his return to
Manchester in 1907 his research on the nucleus was
fully initiated. In 1909 H. Geiger and E. Marsden did
the famous Geiger-Marsden experiment in which
α
-rays were scattered by gold foil. The result, however,
was a very puzzling one because α
-rays were scattered
backward, as shown in Figure 6 (right) [2].
If the atom was like the watermelon model, then, one
would not expect the backward scattering as illustrated
in Figure 6 (left) [2]. In 1911, two years after the
Geiger-Marsden experiment, Rutherford theorized
this phenomenon, suggesting that an atom has the
features of a high central positive charge concentrated
in a central small volume (nucleus), and thereby
pioneered the Rutherford model of the atom. This is a
famous story on the discovery of the atomic nucleus.
Figure 6: Expected results are: αparticles passing through the
watermelon model of the atom undisturbed (left);
however, a small portion of the particles were deflected,
indicating the presence of a small, concentrated, positively
charged feature (right).
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AAPPS BULLETIN
Niels Bohr arrived in Cambridge in 1911 at the age of
26. He worked in the group of J. J. Thomson. One
weekend, he visited Manchester and met with
Rutherford. Rutherford was then 39. Bohr learned
Rutherford’s idea regarding the atomic nucleus. His
immediate concern was to determine why electrons did
not slow down and fall into the nucleus, as described in
Section 1. He then spent some time to solve this
problem in connection with the Planck’s quantum
theory. In 1913, he published a series of three papers,
which presented what was then known as the Bohr
model for the atom, where an electron orbit was
quantized. He sent these three papers to Rutherford, but
Rutherford was rather critical to the papers, and
recommended the shortening of their length [3].
Evidently, Rutherford was not quite happy with the
way that Niels Bohr interpreted the Rutherford
discovery of the atomic nucleus, but eventually, he
deeply appreciated to it [3]. With Bohr’s quantized”
atomic model, the long-standing dilemma regarding an
electron’s orbit (to slow down according to the
classical electromagnetism) was finally solved.
5. Constituents of the Nucleus
In 1919 Rutherford conducted an experiment to
bombard a particle from polonium in the air (primarily
a nitrogen gas), and observed an ejection of a hydrogen
nucleus together with the formation of an oxygen
nucleus. This hydrogen nucleus was called a proton.
This experiment was the first trial of nuclear
transmutation, and awakened Rutherford to the
importance of accelerators. J. O. Cockcroft and E.
Walton invented an accelerator in 1932 and the first
man-made nuclear reaction with this accelerator was
accomplished.
What does constitute the nucleus? This was the next
major question after the discovery of the atomic nucleus.
Many researchers believed that the electron must be a
constituent of the nucleus, because an electron was
ejected from the nucleus via β
-decay. Also, the newly
discovered “smallest nucleus,” the proton, must be a
constituent of the nucleus. Many physicists believed that
the nucleus was constituted of protons and electrons for
many years. For example, 14N is made of 14 protons and
7 electrons. The above nuclear transmutation can be
understood in this analogy.
On the Discovery of the Atomic Nucleus ● ●
However, we have already seen a serious dilemma
here. For example, 14N consists of an even number of
protons and an odd number of electrons, so that it must
obey the Fermi statistics. On the other hand, the spin of
14
N is 1 and it obeys the Bose statistics. This dilemma
was not solved until the discovery of the neutron by J.
Chadwick in 1932. By this discovery, it turns out that
14
N is made of 7 protons and 7 neutrons, which allows
it to form the spin 1 nucleus.
6. Yukawa Theory
Immediately after the discovery of the neutron,
physicists focused on a single question of why protons
and neutrons were bound to a tiny object of the order of
10 -(12-13) cm, which is the scale of the nucleus. H.
Yukawa was one of the physicists who tried to
understand this problem successfully. He first tried to
calculate many different short-range forces. When the
force is the Yukawa type (V = exp(-αr)/r), then it
obeys the Klein-Gordon equation with intermediate’s
mass of so called the pion The idea came up in his head
at the Tamino bridge near the Department of Physics at
Osaka University where he worked at that time. Prior to
that point H. Yukawa had not published papers and it
was a complaint of the department chair, Professor
Yagi. The paper written in 1935, which is now known
as the famous Yukawa paper, became his first paper.
The paper gave a strong impact on the present-day
concept of “strong interactions”. Also, it provided a
basic new framework regarding interaction, namely,
that any interactions were intermediated by particles
(which are pions, in this case). The concept became a
fundamental guideline to particle physics. Particle
physics has gradually been separated from nuclear
physics since then. Namely, nuclear physicists have
tried to understand properties of the nucleus, whereas
particle physicists have tried to understand the particle
itself and interactions between particles.
References
[1] S. Devons, F. R. S., in “Rutherford and the Science of His
Day”, Notes and Record of the Roy. Soc. London, 45, 221
(1991).
[2] Taken from: http://en.wikipedia.org/wiki/File:Rutherford_
gold_foil_experiment_results.svg
[3] R. Peierls, in “Rutherford and Bohr” Resonance, May, 2010.
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