Alpha Particle (English)

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2-3: Scattering of Alpha Particle by an Atom

Let us summarize

the properties of atom

that were made clear

up to the preceding section.

	(1)	The radius of atom is approximately
	(2)	An atom includes electrons, each of which has a negative charge -e, where e is the elementary charge. The total mass of the electrons in an atom is much smaller than that of the whole atom. The number of electrons is about a half of the atomic weight.
	(3)	An atom is electrically neutral. So that it includes "something" whose charge may cancel the charge of all the electrons. This "something" must carry the whole atomic mass.

What is the internal

structure of such an atom?

In order to study

the structure of

an invisible atom,

it is better to collide

an appropriate projectile

against the atom

and to look

at the reaction.

The alpha rays

were considered

to be suitable

for this purpose.

[The True Nature of the Alpha Rays]

Before colliding

the alpha rays

against an atom,

we have to know

what the true nature

of the alpha rays.

The alpha rays

are beams emitted

at very high speeds

from uranium and radium.

They were considered

to be collections

of the alpha particles

charged positively.

First,

the charge-to-mass ratio

of the alpha particle,

Q/M,

was measured

by curving its orbit

in electric and

magnetic fields,

where Q is the charge

of the alpha particle

and M the mass.

Thereby it was seen

that the charge-to-mass

ratio of the alpha particle

was 1/2 of that

of the hydrogen ion.

The measurement method

is almost the same as

the case of electron shown

on the page

1-7-A: Measurement of the Charge-to-Mass Ratio of Cathode Rays .

But you should note

that,

while the charge of electron

is negative,

that of the

alpha particle

is positive.

Next, the amount

of the charge of

the alpha particle was

measured.

On the first step

for this purpose,

the number of

the alpha particles

emitted in a unit solid-angle

within a unit time

was measured

by using the system

shown in

Fig. (A)

The scintillation

was used for the detector:

Namely,

the number of the

emission of light,

i.e. luminescence,

which was emitted

from the fluorescent material put

on the window

of the detector

when being hit

by a charged particle,

was counted.

On the second step,

the total charge

of the alpha rays

radiated in a unit time

from the same source

is measured

by using an apparatus

shown in the above figure,

Fig. (B).

Dividing the resultant

total charge by

the number of

the alpha particles,

they obtained

the value of electric charge

per particle.

Thereby

Charge of an alpha particle = +2e

(e = the elementary charge)

was clarified.

An alpha particle

is therefore

speculated to be

a helium ion,

i.e. the helium atom

having lost

two electrons.

It was later confirmed

by E. Rutherford

(UK, 1871 - 1937)

and

T. Royds

that this speculation

was correct (1908).

The schematic drawing

of the apparatus

used for this purpose

is shown in the

following figure,

Fig. (C).

In the above apparatus,

a radiative material

(gas) is enclosed

in a very thin

glass tube A

under a pressure

applied by the mercury

M₂

at the center

of a vacuum glass

container B.

The alpha particles

emitted from the radiative gas

penetrate the thin glass

to be accumulated

in the outside glass

container B.

After leaving

this apparatus

for several days,

the mercury M₁

was lifted up

so that the gas accumulated

in the container

B was led up

into an upper narrow tube

They made a discharge

by applying

a high voltage

between the electrodes,

and observed the spectrum

which was exactly

the same as

that of the helium atom.

Thus they confirmed

that the gas accumulated

in the container

B was helium gas.

It was confirmed

that an alpha particle

came from A to B

captured two units

of elementary charge

from environment

to become a helium atom.

This means

that the alpha particle is a doubly ionized helium atom.

For reference,

they put helium gas

in the thin tube A

and leave it

for several days,

but they could find

no helium gas

in the outer container

at all;

namely,

an alpha particle

can penetrate

the thin glass wall

of the tube A,

but a helium atom cannot.

Let's think why it is so.

[Scattering of the Alpha Rays by an Atom]

The experiment of

the scattering of alpha rays

by an atom were carried out

H. W. Geiger

(Germany, 1882 -1945) and

E. Marsden

(UK, 1889 - 1970)

under Rutherford's

leadership (1909).

They made alpha rays

emitted from radium collide

against a thin metal

(gold or silver) foil.

A schematic graph

of the experiment

is shown in

Fig. (D)

and the details

of the apparatus

are in

Fig. (E).

In the vacuum scattering

chamber in

Fig. (D),

the alpha particles

coming out of the source R

hit the metal foil F

at the center and

they are scattered

in various directions.

These scattered particles

are observed through

their scintillations

with the microscope M.

Thereby

it is measured

what rate of particle number is

scattered in a given

solid angle.

The results obtained

by Geiger and Marsden

showed that

the almost all alpha particles

go straight

in the forward angle,

but

a few number of particles

are occasionally

scattered in a

very large angle

like 90 degrees

or larger.

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