Experiment
Natural Radioactive – A small quantity of the radioactive source was placed in a hole dug in a block of lead (fig. 18.1) so that parallel rays ma come out of the nosal at S. A photographic plate was placed at some distance above the block so that all the radiations fall on this plate. All of these were placed in a lead chamber which was evacuated with the help of a pump, so that the radiations may not be absorbed by air.
A magnetic field was applied on this chamber vertically downwards, i.e., the magnetic field was perpendicular to the plane of the paper and directed inwards. After some time, when the photographic plate was developed, it carried black spots at three different points which showed that these radiations are of three types (fig. 18.1) the spot on the left is due to the arc of a large circle indicating a group which carried positively charged particles, these are known as β rays, the spot in the middle indicates the particles which do not have any charge and are known as Y rays.
Fig. 18.1
Properties of Alpha (A), Beta (Β) and Gamma (Y) Rays Alpha (A) Rays
- The a rays are emitted by radioactive elements. Their speed ranges between
. The speed of these rays depends upon the radioactive element from which these are emitted. Their speed is different for different elements, but the speed of all the a rays emitted by one element is the same. - a rays ionize the gas through which they pass.
- a rays affect the photographic plates.
- They produce fluorescence in zinc sulphide.
- At atmospheric pressure they travel a small distance in air. This distance is different for a rays emitted by different elements. For example the range for a rays emitted by uranium is 2.7 cm while is 8.62 cm for a rays emitted by thorium.
- The atomic and magnetic fields affect the a rays which shows that these are charged particles.
- a rays consist of such positively charged particles whose mass is 4 times that of hydrogen atom and they carry a charge twice that of a proton i.e., the a particles are in fact the nuclei of helium.
Emission of a particle from the nucleus
Beta (β) rays
- B rays are emitted by the radioactive elements with great speed. Their speed ranges from one percent to 98% that of speed of light. The speed of all the B rays emitted by one element is not the same.
- B rays produce ionization in air, but this ionization is nearly 1/100 times less than that produced by a rays. Although the speed of B rays is very high, since their mass is very small, hence, their energy is less as compared to a rays.
- B rays affect photographic plates but the effect produced by B rays is much higher than that produced by a particles.
- They produce fluorescence in barium platinocnide.
- They can pass through a thick layer of matter. For example they can easily pass through one millimeter thick sheet of aluminium.
- B rays are affected by electric and magnetic fields.
- B rays carry negative charge. The mass and charge of B rays is equal to the mass and charge of an electron. So we can say that B rays are electrons which are emitted by the nucleus.
Emission of B particle from the Nucleus
Gamma (Y) rays
- They move with the speed of light.
- The ionization produced in a gas by them, s very small as compared to a or B rays.
- Y rays affect photographic plates but this effect is much higher than that of B rays.
- Y rays also produce fluorescence in bariam platinocynide.
- The penetrating power of Y rays is much higher than the B rays and they can easily pass through 30 cm thick sheet of iron.
- They are not affected by electric and magnetic fields.
- Y rays are such electromagnetic waves which are emitted by the nucleus.
Emission of y particle from the nucleus