Cosmic Rays: (B.Sc. Physics)

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Cosmic Rays (B.Sc. Physics)

Introduction to Cosmic Rays
It has been found that a well-insulated gold leaf electroscope slowly loses its
charge, even when apparently no ionising agents are present. At first, it was
assume that the loss of charge of the electroscope is due to the ionising radiations
coming from the radioactive substance present in the earth. But experiment
made by Millikan on the loss of
charge of an electroscope in lakes,
about a mile below the water level,
revealed that there was no decrease
in the rate of discharge. On the other
hand, further experiments indicate
that the rate of loss of charge
increases as we go higher above the
earth. From these observation on the
variation of cosmic ray intensity with
altitude and depth, it was concluded
that a penetrating radiation was
incident from interstellar space. This
radiation given the name cosmic
rays.
The cosmic ray consists of high

energy particles. Most of these
particles have energy of the order of
15 GeV [1 GeV = 1 billion electron
volt] but occasionally some particles
are found to posses energies as high
as 1011 GeV which is a billion times as
large as that largest existing particle
accelerator. It is now well established
that the primary cosmic rays very
high energy protons and to a
Lesser extent heavier nuclei which rain upon the earth in all direction from the
outer space. The primary cosmic radiation approaching the earth’s atmosphere
contains about 92 % protons, 7 % - particle and 1% still heavier nuclei of almost
COSMIC RAYS-ELEMENTARY PARTICLES KAMALKRISHNA2000@YAHOO.CO.IN, +919811983627
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all the atoms from Li to Ni( A<60). The relative abundance of the various nuclides
present in the primary cosmic rays is almost the same as the relative abundance of
elements in the earth’s crust as well as in the stars.
The fact that the primary cosmic rays consists of charged particles and not photons
is established by the latitude effect. The effect was discovered by A.H. Compton in
1933 and it shows that as we proceed from magnetic north to the magnetic south,
the cosmic ray intensity remains steady until a magnetic latitude of 42 is reached.
The intensity then begins to drop, reached a minimum value about 11% less than
the value beyond 42 latitude, at the magnetic equator and then rise to
symmetrical values in the southern hemisphere, as shown in the figure.
The earth’s magnetic field is weak but it

extends far out of the earth’s
atmosphere and can effect the path of
incoming charged particles even at large
distance from the earth. The amount of
deflection depends upon the
momentum and charge on the particles.
All particles with energy less than 104
MeV are so strongly deflected by earth’s
magnetic field at the equator that they
are unable to enter the earth’s
atmosphere. In fact, at low energies, the particles which do not travel parallel to
the magnetic lines of force, spiral round these lines. As the particles approach the
earth , the field strength increases, the radii of their spiral paths become smaller
and smaller till ultimately they are so sharply bend that they get reflected back and
spiral round the lines of force again approaching the earth on the other side. Such
charged particles travelling to and fro constitute the Van Allen Belts, which
surround the earth except at the magnetic poles. The outer built is constituted by
the low energy particles and the inner belt is due to more energetic ones.

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The particle approaching the earth in the direction of its magnetic axis are not
deflected by the magnetic field. Hence primary cosmic particles cosmic particle of
any energy can enter the earth’s atmosphere near the magnetic poles.
The fact that the primary cosmic rays carry a positive charge is shown by the east-
west effect if the particles are positively charged, more will arrive from the west
than from the east and if they are negatively charged more will arrive from the
east than from the west. Actually measurement show that at the equator the
intensity of the rays are composed predominantly of the positively charged
particles and their deflection by the Earth's magnetic field accounts for the east
west asymmetry in cosmic ray intensity.
The primary cosmic Ray particles mostly protons some Alpha particles and a few
heavier nuclei interact with the nuclei of atmosphere of atmospheric gases and
give rise to secondary cosmic rays consisting of a hard component and a soft
component.

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Cosmic Rays (B.Sc. Physics)

The cosmic ray consists of high

The earth’s magnetic field is weak but it

COSMIC RAYS-ELEMENTARY PARTICLES KAMALKRISHNA2000@YAHOO.CO.IN, +919811983627

COSMIC RAYS-ELEMENTARY PARTICLES KAMALKRISHNA2000@YAHOO.CO.IN, +919811983627

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