ASTON'S EXPERIMENT
In the year 1896
John Thomson investigated
cathode rays and discovered electron - the first elementary
particle. In 1911, in the similar way, he tried to study
atomic nucleus. He obtained
a beam of positive ions inserting gas into a vessel. There were two
electrodes in the vessel. Between them there occurred magnetic
field dislocating electrons (picture no. 1).
The electrons were
moving to the positive charged anode, and the positive ions were
moving to the cathode. In the cathode there was a channel leading
ions out of the instrument. The ions ran then between the charged
electrodes D and E. Between them there was the electric field with
intensity E. And the ions were running also between magnet's poles
S and N. The intensity of those poles was equal B. The ions beam
was abberated both by the electric field (vertically) and by the
magnetic field (horizontally). The force from the first field
equals:
(1)
From the second Newton's
law:
(2)
where m - electron mass, a - molecule acceleration The particle in
the field moved along the path trajectory which was the circular
arc; the circle radius was equal r. So the occurring acceleration
was the acceleration of the circular motion:
(3)
After connecting formulas (1), (2) and (3):
(4)
Iv of every ion is the
same and there is only the electric field on then the beam will hit
the screen at the point 1.
The force of the
magnetic field, directed perpendicular to the field's direction and
to ions' velocity, abberating the path trajectory of the beam
(which becomes the circular arc with the circle radius
r1) is equal:
(5)
After connecting that formula, the second Newton's law and the
formula of the centripetal acceleration of the circular
motion:
(6)
If v of all ions are the
same and there is only the magnetic field switched on then the beam
will hit the point 2.
And if the both fields
are switched on then every ion will be influenced by both forces
and by the constant v the beam will hit the point 3. The beam
leaves the track on the screen which is the photographic plate. But
the v of different ions is different so, on the screen there is not
a single point but the sign of parabola. If all ions have the same
mass and the charge then there is only one sign of parabola on the
screen.
But investigating neon,
Thomson got two parabolas because
of two atomic masses of the element (equal 20 and 22). Isotopes were discovered.
In 1919 Francis William Aston found different, more
exact way to measure the content of isotopes in the total amount of
the element. He constructed an instrument called mass
spectrograp. The instrument consisted from ions source (working
similar to the Thomson' one), two
narrow slice (Sz1 and Sz2), two parallel plates P1 and P2 (between
them, by impressing voltage, there creates the electric field),
beam stop, permanent magnet, screen and photographic plate (picture
no. 2).
Ions from the source,
after going through slice, create a narrow beam which next is
diffused by the electric force (the yaw angle of ions is inversely
proportional to their kinetic energy thanks to what the separation
of the ions of different velocities is possible). Than, the beam of
ions goes through the magnetic field so, ions trajectories are
abberated to the other side. The magnetic field abberates the ions
trajectory by the angle which is directly proportional to particles
momentum. Ions go through that two fields with the same q/m (the
charge to mass proportion). Thanks to it they can agglomerate in
the one point of the photographic plate. By analysing the dark
points of the plate (where ions hit) its possible to investigate
isotopes. Different in mass hit different in places on the plate.
The blackening of the points is proportional to the number of
hitting points. The spectrograph, made in 1919 and working until
1925, let evaluate the atomic mass of isotopes with the accuracy to
1 percent. That accuracy was not enough because the differences of
many isotopes masses to the total masses were also of order of 1
percent.
Not a long time later
another scientist - Arthur Jeffrey Dempster constructed more
developed instrument. The ions source was the incandescent spiral
with metal evaporating. Leaving the spiral ions had very small
velocity. Then they were accelerated by the electric field. It let
to get a beam of ions of almost identical energies. To separate the
ions with different masses the magnetic field was enough. The
instrument let the precision of measurements be 100 times
multiplied.
I n the 30-ties K. T.
Bainbridge constructed the spectrograph with double focal distance
of the ions beam. The measurements were even more exact. The beam
went through the space where the magnetic and the electric field
were crossed. According to the formula (1) there is a upward force
influencing in the electric field. And according to the formula (5)
there is a downward force influencing the ions in the magnetic
field. He constructed the instrument this way not to let other ions
then the ones with the not curved trajectory move out the space of
fields. That were the ones on which the magnetic and the electric
force equilibrate:
(7)
So the velocity of ions moving farther equals:
(8)
There is a force
influencing the charged molecules moving with the similar velocity
v. The velocity is directed perpendicularly to the line of magnetic
field (the vector of induction B) created by the permanent magnet.
The force is perpendicularly to the vector of velocity v and to the
vector B. The particle moves by the circle. The force given by
formula (5) is the centripetal force: :
(9)
where m - particle's mass, so:
(10)
Until 1937 Aston evaluated the quantity of atomic mass
of different isotopes. The accuracy of atomic mass of different
isotopes. The accuracy of his measurements were of the order of the
5-th place after the decimal point.
The discovery of
isotopes gave the answer for the question of not total masses of
atoms. Almost a hundred years before William Prout brought forward a hypothesis
that all atoms consisted of basal elements which mass is equal to
hydrogen mass. But chlorine ruined the hypothesis because hi mass
was equal about 35,5 masses of hydrogen. Now it was proved that
there is no chlorine with mass equal 35,5 but two isotopes of
chlorine with masses equal 35 and 37 in the proportions making the
mean mass equal 35,46.
All the beginning of the
20th century scientist were of the opinion that atomic nucleus
consisted of protons but also of some other component. That thesis
was proved by the discovery of isotopes - atom of the same atomic
number (the number of protons and electrons) but of different mass
number.
In 1932 James Chadwick
gave final, experimental proofs for the subsistence of the
neutrons - neutral particles of
nucleus, their mass is similar to proton's mass. The subsistence of
neutron in the nucleus explained the fact of isotope's
subsistence.
- Computer programm - experiment
simulation
