The nature of the mysterious invisible rays emitted by radioactive substances was determined by Ernest Rutherford. He placed a sample of uranium mineral in a lead box (Fig. 13.1 ). The lead absorbed all other railiations except those which pass through the slit to constitute the beam. The radiations emitted by the uranium mineral could be resolved into three types by the application of electrical field.
(a) The rays which deflected slightly towards –ve electrode were named as alpha (a) rays.
(b) The rays which deflected towards +ve electrode were named as beta (~) rays.
(c) The rays which remained undeflected were named as gamma (y) rays.
Later on, it was established that these three types of radiations were given off from all radioactive substances. Let us study the properties of these three types of radiations.
(i) Nature. The study of a-rays in electrical and magnetic field has shown that these rays consist of material particles carrying 2 units of positive charge and a mass equal to 4 u. In other words, a-particles are helium nuclei [He2+].
(ii) Velocity. They are shot out with a velocity about 1/l0th of the velocity of light In fact, the actual speed of a -particles depends upon the nature of substance from which they are ejected. Energy range of a-particles is 4.9 MeV.
(iii) Penetrating power. They do not possess high penetrating power because of their large mass. They can penetrate through a mica sheet, a thin metal foil or through a few centimetres in air. They can penetrate through a sheet of aluminium up to a thickness of 0.02 cm.
(iv) Ionising power. a -particles can ionise the gas through which they pass. This is because of the fact that they knock off electrons from neutral gas molecules on colliding with them resulting in the ionisation of gas.
(v) Effect on zinc sulphide. Due to the high kinetic energy, they produce luminosity when they strike zinc sulphide plate.
(vi) Effect on photographic plate. a-particles also affect the photographic plate.
BETA (B) RAYS
(i) Nature. B-rays have been found to consist of particles having same charge and mass as electron S. Their (elm) ratio is same as that of electrons. These particles are designated as
(ii) Velocity. B-particles move much faster in comparison to a -particles. They travel with the speed almost approaching the speed of light Energy range is 0.5 to2MeV.
(iii) Penetrating power. B-particles have far more penetrating power due to small mass and high
velocity. They can penetrate through a sheet of aluminium having thickness upto 0.2 cm.
(iv) Ionising power B rays have poor ionising power inspite of their high speed. It is because of the fact that they possess low kinetic energy due to their extremely small mass.
(v) Effect on zinc sulphide and photographic plate. B particles have very little effect on zinc sulphide due to their low kinetic energy. However, their photographic activity is greater than that of a -particles.
GAMMAR ( r) RAYS
(i) Nature. These rays do not consist of material particles but they are electromagnetic waves having wavelength of the order of 10-8 to w-u cm. They are electrically neutral.
(ii) Velocity. They travel with the speed of light. Their energy range is 0.1-2 MeV.
(iii) Penetrating power. They have high penetrating power because of their high velocity and small wavelength. These rays can penetrate through a sheet of aluminium having thickness upto 100 cm.
(iv) Ionising power. They have very poor ionising power because of their non-material nature.
(v) Effect on zinc sulphide and photographic plate. The produce very little effect on zinc sulphide or photographic plate.
The characteristics of a, 13 and r-rays have been summed up in Table 13.1
NUCLEAR CHANGES DURING EMISSION OF β,γ RADIATIONS
It has been pointed out earlier that radioactive element emits, a, f3 and y-rays. The emission of a and β-rays bring some changes in the nucleus of the atom and the new element formed as a result of these changes is called daughter element.
The equations representing the nuclear changes are called nuclear chemical equations. In writing the nuclear chemical equation, the following points must be kept in mind:
(i) Sum of the mass numbers of various reactant species must be equal to the sum o/the mass numbers of the product species.
(ii) Sum of the charge numbers (atomic numbers) of reactant species and that of the product species must be equal.
The notations of some of the species that participate in nuclear reactions are given below in tabular form.
Let us now proceed to discuss the changes produced by a, β and y emission. Emission of ci-rays. When a nucleus emits an a-particle its atomic number is reduced by two units and its mass number is reduced by four units. The daughter element, thus, formed occupies a position in the periodic table two places . left of the parent element. For example, emission of a-particle
from uranium-U) produces a thorium nucleus 234Th ( 90 ) . The above nuclear change can be represented as:
Emission of β-rays. β-particle is merely an electron. When a β-particle is emitted, the nucleus of the new element formed possesses the same mass but nuclear charge or atomic number increases by 1 unit. Beta particle emission is because of the result of decay of neutron into proton and electron. The daughter element formed is an isobar of the parent element.
Emission of y-rays. Gamma rays are emitted due to secondary effects. With the emission of an a-or β-particle, the nucleus left behind remains in the excited state due to recoil. The excess of energy is released in the form of y-rays. As y-rays are electromagnetic radiations with no charge and no mass, their emission from a radioactive element does not produce a new element.
Detection of Radiations
Nuclear radiations are very useful to us, but at the same time these can also be very dangerous. The human senses cannot detect ionizing radiations. The detection of these radiations is done by the use of a Geiger-Muller counter (Fig. 13.2), the cloud chamber, spark counters and scintillation counters.
The Geiger-Muller tube
Radiation, from a radioactive source, are passed into a gas-filled tube (usually argon at low pressure) through a thin mica window. The radiation produces positive ions. And electrons in the gas by ionization. The positive ions move
Fig. 13.2. Detection of radiation using Geiger-Muller counter
towards the wa11 of the counter, which is the cathode, and electrons (negative) go to the central anode. Movement of these particles allows a pulse of electric current to pass through a conductor to the counter. The electrical pulse is amplified and counted as light flashes, clicks, or shown as meter readings. The counter does not distinguish between a., β or y radiation.