Top of Part 2
Next page
|
Top of Part 2
Next page
|
2-1: The Discovery of Radioactivity |
|
In 1896,
the natural radioactivity
was discovered by
A. H. Becquerel
(France, 1852 - 1908).
He found that
the uranium element
can expose
a photographic
dry plate,
even if it is
separated from the uranium
or covered with
a sheet of black paper.
He also found
that a kind of "ray"
from the uranium
can be sensed
by an
electroscope.
Namely this "ray"
has an electric charge.
Nowadays,
this "ray" is called
a radiation
or radioactivity,
and a substance emitting
the radiation
is called
a radioactive material.
|
|
[The Discovery of
Radium and Polonium]
In 1898,
M. Curie
(Poland, France, 1867 - 1934)
and
P. Curie
(France 1859 - 1906)
confirmed that
the radioactivity
does not depend
on the chemical state
of the relevant atom
but relates
to the atom itself.
They noticed
that the radioactivity
of pitchblende
is stronger
than uranium,
although the former is
the ore of the latter,
and then they discovered
new elements,
radium
and polonium,
out of pitchblende.
By the way,
you should note
that the term
"radioactivity"
was given by M. Curie.
|
|
F. Soddy
(UK, 1877 - 1956)
found that
the radium emits radiation
to decay
and change
into another element,
radon (1903).
This is
the first discovery
of the radioactive decay
of an element.
After the discovery
of the radioactivity or
the radioactive decay
of an element,
people have realized
that the atom is not
an ultimate particle
but it consists
of smaller basic elements.
Namely, an atom has
still an internal structure.
Therefore it became
a very interesting problem
what the structure
of an atom is.
|
|
[Types of Radioactivity]
It has been clarified
that there are
three types of
radioactivity, i.e.
alpha,
beta
and
gamma rays.
By testing how
the radioactive rays curve
in an electric field
or a magnetic field,
their charges
were investigated.
It was seen
that the charge of
alpha rays
is +2e
where e is
the elementary charge,
and its charge-to-mass ratio
is a half of that
of hydrogen ion.
It was also clarified that
beta rays
are nothing other than electrons.
It was made clear that
gamma rays have
no charge and
are very similar
to X rays
which was accidentally
discovered by
W. C. Roentgen
(Germany, 1845 - 1923).
It was later clarified
that both X rays and gamma rays
are electromagnetic waves
of very short wavelength;
usually, the wavelength
of gamma rays
is shorter than
that of X rays.
A schematic drawing
of the orbits of
alpha,
beta
and
gamma rays
in a uniform magnetic field
are shown in
the following figure,
where the magnetic field
is applied
in the direction perpendicular
to this
picture
from front side
to back side.
|
|
Top
|
|
|
Go back to
the top page of Part 2.
Go to
the next page.
|