Discovery of the Neutron (English)

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1-3: The Discovery of the Neutron

[Problem of Isotopes]

J.J. Thomson

(UK, 1856-1940) discovered

that the natural neon gas

is a mixture of two kinds of

elements which have different

atomic weights (1912).

Afterwards, F.W. Aston

(UK, 1877-1945) carried out

precise mass-spectroscopic

analyses and

he confirmed that

many various elements

are mixtures of two or more

kinds of isotopes

which differ in mass

(atomic weight).

Namely,

there exist some elements

(or atoms) which have the same atomic

number and,

therefore, essentially

the same chemical properties

but different atomic masses.

This means that there exist

different nuclei having the same

electric charge but

different masses.

These are also called isotopes.

What is

a reasonable explanation

of the existence of isotopes ?

[Does the Nucleus Consist of Protons and Electrons?]

It is known well that

some kinds of nucleus decay

by emitting electrons.

This is called the beta decay.

If we consider that the atomic nucleus

consists of protons and

electrons, then the beta decay could

easily be understood.

However, the idea that protons and

electrons are basic constituents

of a nucleus has the following

serious difficulties.

(1) Problem on the spin and the statistics

As for the details of

the spin and the statistics,

please refer to some other

advanced books concerning

"Quantum Mechanics".

Here, let us give only

a rough outline of the discussion

about the present problem.

(See the page of

"Microscopic World -2-:

[The Pauli Principle, Spin]" ).

For simplicity of

our explanation, let us take

as an example

the case of nitrogen-14 nucleus ()

whose atomic number is 7

and atomic weight is 14.

A single proton and

a single electron

are both

fermion.

A composit system of even number

of fermions is a boson

as a whole.

And it has the property of

Bose-Einstein statistics.

Contrarily, a composit system

of odd number of fermions

behaves as a fermion

as a whole,

and it has the property of

Fermi-Dirac statistics.

If we consider that

nitrogen-14 nucleus ()

consists of 14 protons

and 7 electrons,

the total particle number is 21

that is an odd number.

In contrast to this,

the experimental result

of the spectroscopy

shows that the nitrogen-14 nucleus

must be a boson as a whole.

This is the first problem.

The second problem is as follows:

Each of the proton and the electron

has spin .

While the total spin of

a system of even number of protons

(or electrons)

is an integral multiple of ,

that of odd-proton (or odd-electron) system

must be an odd-integral multiple of

However, the experimental data show

that

the spin of the nitrogen-14 nucleus

is 0 (= even).

This means that total particle number

of the nitrogen-14 nucleus

must be even.

This is also an important problem.

(2) Problem concerning the uncertainty relation

If we consider that

the atomic nucleus consists of

protons and electrons,

an alpha particle

(= helium nucleus)

should be made of 4 protons

and 2 electrons.

The size (diameter) of the

alpha particle is about

If an electron is confined

within this size of space,

the uncertainty of the position

of the electron should be .

Using the uncertainty relation in "Quantum Mechanics",

we can estimate the uncertainty

of the momentum of the electron,

and then the

uncertainty of the speed of

the electron will be

This value largely exceeds

the light speed.

This is quite unreasonable

on the basis of

the Special Theory of Relativity.

Namely, it is completely impossible

to confine such a light particle

as electron within

a narrow space like a nucleus.

This is another serious problem.

[Rutherford's Prediction]

As discussed above,

the problems of the idea

that the nucleus consists of

protons and electrons

could at all once be solved

if we assume that

there exists an electrically

neutral particle with

almost the same mass as a proton;

so to say a "neutral proton".

If it is so, the existence of

isotopes could easily be understood.

This was Rutherford's

prediction and he sometimes

talked this idea to his

students in his lecture.

One of his top students

influenced strongly by him,

J. Chadwick

(UK, 1891-1974),

made an effort to find out

that "neutral proton"

and finally got the honor

of discovering that particle;

Chadwick named it "neutron".

[History of the Discovery of the Neutron]

The discovery of the neutron

was based on the results of efforts

by some research groups

for several years.

(1) Experiment of Bothe and Becker

W. Bothe (Germany, 1891-1975)

and his student H. Becker

bombarded beryllium with

strong alpha-rays from a polonium source

and found a high-energy radiation

of strong penetrability (1928).

They observed similar radiations

not only from beryllium,

but also from lithium and boron.

At first, these radiations

of strong penetrability

were considered to be high-energy

gamma rays.

(2) Experiment of I. Curie and Joliot

Around 1931,

I. Curie (France, 1897-1956)

and F. Joliot

(France, 1900-58)

reported that the above

high-energy radiation found

by Bothe and Becker

was able to eject

protons from a paraffin layer.

I. and F. Joliot-Curie

thought that this phenomenon

is caused by the Compton effect

due to high-energy gamma-rays

emitted from the polonium-beryllium

source.

Namely, they thought that

a gamma particle (photon)

of Bothe-Becker's high-energy

radiation hits a proton

in the paraffin layer

to be scattered

and the observed proton

is nothing but the recoil proton

from this Compton effect.

(As for the Compton effect, refer to

the page of

"Microscopic World -1-:

3-7:The Compton Effect").

(3) The Discovery of the Neutron by Chadwick

Chadwick was much interested

in the report of Joliot-Curie.

He repeated the experiments,

using the polonium-beryllium

(Po-Be) source.

The schematic drawing

of his experimental apparatus

is shown in a figure below.

The high-energy radiation

emitted from the (Po-Be) source

on the left-hand side

is led to the

cloud chamber,

in front of which

a paraffin layer is placed.

The radiation is scattered

by a proton in the parafin

and a photograph of the recoil proton

is taken in the cloud chamber

on the right-hand side.

Chadwick collided the

radiation

emerging from the (Po-Be) source

not only with proton (paraffin),

but also with helium and nitrogen.

Comparing the results of these

experiments with each other,

Chadwick concluded that

this mysterious radiation from the (Po-Be)

source cannot be interpreted

by assuming it to be a gamma ray.

He finally concluded that

all were able to be understood

without any contradiction

by assuming that

the mysterious radiation is

electrically neutral particles

with almost the same

mass as a proton.

This is the confirmation

of the existence of

the "neutral proton" predicted

by Rutherford.

Chadwick named this particle

"neutron" (1932).

[Experimental Apparatus of Chadwick]

This is a schematic drawing

of the experimental apparatus

of Chadwick.

High-energy alpha rays

from polonium

on the left-hand side

collide the beryllium target

and a radiation with

strong penetrability

(purple arrows

= neutrons in fact)

comes out to hit the proton

in the paraffin layer.

The knocked proton is

observed in the cloud chamber

behind.

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