Detection and measurement of Radioactivity
The radioactive
radiation can be detected and measured by a number of methods. The important ones
used in modern practice are listed below.
(1) Cloud Chamber
** This
technique is used for detecting radioactivity.
** The chamber
contains air saturated with water vapour.
** When the
piston is lowered suddenly, the gas expands and is supercooled.
** As an α- or
β-particle
passes through the gas, ions are created along its path. These ions provide
nuclei upon which droplets of water condense.
** The trail or
cloud thus produced marks the track of the particle. The track can be seen
through the window above and immediately photographed.
** Similarly, α- or
β-particles form
a trail of bubbles as they pass through liquid hydrogen.
** The bubble
chamber method gives better photographs of the particle tracks.
(2) Ionization Chamber
** This is the
simplest device used to measure the strength of radiation.
** An ionization
chamber is fitted with two
metal plates separated by air.
** When radiation passes through this chamber, it
knocks electrons from
gas molecules and positive ions are formed.
** The electrons migrate to the anode
and positive ions
to the cathode. Thus a small
current passes between the plates. This current can be measured with an
ammeter, and gives the
strength of radiation that passes through the ionization chamber.
** In an ionization chamber called Dosimeter, the total amount of electric charge
passing between the plates in a given time is
measured. This is proportional to the total amount of radiation that has gone
through the chamber.
(3) Geiger-Muller Counter
** This device is
used for detecting and measuring the rate of emission of α- or
β particles.
** It consists
of a cylindrical metal tube (cathode) and a central wire (anode).
** The tube is filled
with argon gas at reduced pressure (0.1 atm).
** A potential
difference of about 1000 volts is applied across the electrodes.
** When an α- or
β-particle enters
the tube through the mica window, it ionizes the argon atoms along its path.
** The argon
ions (Ar+) are drawn to the cathode and electrons to anode. Thus for
a fraction of a second, a pulse of electrical current flows between the electrodes
and completes the circuit around.
** Each
electrical pulse marks the entry of one α- or
β-particle into
the tube and is recorded in an automatic counter.
** The number
of such pulses registered by a radioactive material per minute, gives the
intensity of its radioactivity.
(4) Scintillation Counter
** Rutherford
used a spinthariscope for the detection and counting of α-particles.
** The radioactive
substance mounted on the tip of the wire emitted α-particles.
** Each
particle on striking the zinc sulphide screen produced a flash of light.
** These
flashes of light (scintillations) could be seen through the eye-piece.
** With this
device it was possible to count α-particles
from 50 to 200 per second.
Modern scintillation counter
** A modern
scintillation counter also works on the above principle and is widely used for
the measurement of α- or β-particles.
** Instead of
the zinc sulphide screen, a crystal of sodium iodide with a little thallium
iodide is employed.
** The sample
of the radioactive substance contained in a small vial, is placed in a ‘well’
cut into the crystal.
** The
radiation from the sample hit the crystal wall and produce scintillations.
** These fall
on a photoelectric cell which produces a pulse of electric current for each flash
of light. This is recorded in a mechanical counter.
** Such a
scintillation counter can measure radiation upto a million per second.
(5) Film Badges
** A film badge
consists of a photographic film encased in a plastic holder. When exposed to radiation,
they darken the grains of silver in photographic film.
** The film is
developed and viewed under a powerful microscope.
** As α- or
β-particles pass
through the film, they leave a track of black particles. These particles can be
counted.
** In this way
the type of radiation and its intensity can be known. However, γ-radiation
darken the photographic film uniformly. The amount of darkening tells the
quantity of radiation.
** A film badge
is an important device to monitor the extent of exposure of persons working in
the vicinity of radiation.
** The
badge-film is developed periodically to see if any significant dose of radiation
has been absorbed by the wearer.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
nice explanation and easy to understand
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