Nuclear Force (English)

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3-2: The Nuclear Force

On the preceding page,

we learned that

the origin of the huge amount

of nuclear energy

is the binding energy

of nuclei.

Then, what is the

binding energy of nuclei?

It has been explained

in detail

on the page,

2-3: "Mass of Nuclei, Binding Energy".

We can review it

as follows:

The atomic nucleus consists

of many nucleons

(protons and neutrons)

which bind to each other

through the interaction

called nuclear force.

The mass of a nucleus

is slightly less

than the sum of masses

of its constituent nucleons.

This mass difference

called the mass defect.

According to

Einstein's Mass-Energy Equivalence,

the mass defect

is converted into an energy.

This is

the binding energy

of the nucleus.

The strength of its binding

is therefore represented by

the magnitude of

the binding energy.

Hence, we can say

that the final origin of the origin of the nuclear energy

is nothing but

the nuclear force.

Then, what is the nuclear force?

[The Origin of the Coulomb force]

The Coulomb force

works between two electric charges,

and

q'.

If the two charges are of

the same sign,

it is repulsive.

If they are of opposite sign,

then they attract each other.

In the Quantum Mechanics of Electromagnetic Fields

(or Quantum Electrodynamics),

the exchange of photons

between two charges

causes

the Coulomb force

between them.

(See the following figure.)

[The Origin of the Coulomb Force]

If two charges,

and

, exchange photons,

the Coulomb force

occurs between them.

This figure is its schematic

drawing.

The solid lines represent

particles running upward.

[Yukawa's Meson Theory]

In 1935, H. Yukawa

(Japan, 1907 - 81)

proposed a theory for the

origin of the nuclear force

on an analogy with the above-mentioned

interpretation of the Coulomb force.

He thought that

the nuclear force

might be caused by

the exchange of some massive unknown particles

between nucleons.

Now this particle

is known as

the pi-meson

or pion.

(See the following figure.)

The pion was found

in cosmic rays

by C. F. Powell

(UK, 1903 - 69) et al.

in 1947

and produced artificially

by an accelerator in 1948.

If pions are exchanged

between two nucleons,

the nuclear force

occurs.

(See the following figure.)

There exist

three types of pions,

with charge 0,

with charge +e, and

with charge -e.

Their masses are

nowadays known well as

[Properties of the Nuclear Force]

The nuclear force is

one of the strong interactions.

It is about ten times

as strong as the Coulomb force.

However it is of very short-range

compared with the Coulomb force;

namley, the nuclear force is

very strong but it does not work

unless two nucleons approach

to each other

in a very short distance.

Moreover, it is known that,

when they approach

very close to each other, ( ),

an extremely strong repulsive force

works.

The detailed properties

of the nuclear force

at the distance smaller than

are not sufficiently clarified yet.

In the following figure,

the potential of the nuclear

force between two nucleons

is roughly compared

with the Coulomb potential

between two charges,

and

-e.

[Comparison between the Nuclear Force and the Coulomb Force]

A rough comparison

between the nuclear force

(between two nucleons)

and the Coulomb force

(between charges

and

-e

) is shown.

The red solid

curve indicates the

nuclear force

and

the blue dashed

curve the

Coulomb force.

The abscisa denotes the distance

between particles, r.

In the region of

r < 3 fm,

the nuclear force is

overwhelmingly stronger than

the Coulomb force.

You should notice that,

in the very inner region (

< ),

an extremely strong

repulsive force works.

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