Radioactivity
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Radioactivity is defined as the spontaneous disintegration of certain atomic nuclei. This disintegration is accompanied by the emission of alpha particles, beta particles, and gamma radiation. Disintegration of naturally occurring radioisotopes is called natural radioactivity.
RADIOACTIVE RAYS
An experiment to study the effect of electric and magnetic fields on the radioactive rays was carried out by Rutherford. In his experiment, he put a radioactive substance in a thick walled lead box with a small opening in it. He passed the rays coming out of this opening through an electrostatic field between two plates. He observed that the rays of one type were deflected towards the negative plate, rays of the other type were deflected towards the positive plate, and the rays of the third type were not deflected at all. It was also observed that the rays of the first type deflected less than the rays of the second type. Thus, it was concluded that there are three types of radioactive rays. The first type was named alpha rays (a-rays), the second type were named as beta rays (b-rays) and the third type, which did not deflect at all, were termed as gamma rays (g- rays). The a and b rays are actually streams of particles called the a- particle and the g- particle respectively. From the above observations we can see that the a- particles are positively charged particles and the b-particles are negatively charged particles. The g-rays on the other hand are electromagnetic waves like X-rays. The are also called g-photons and are electrically neutral. Also, we can conclude that the b-particles are much lighter than the a-particles.
On passing the rays through a magnetic field, which were perpendicular to their path, we saw that the a-particles are deflected to the left and the b-particles to the right, and the g- rays continued to move in their path. This was when the magnetic field was directed into the screen perpendicular to the plane of the screen. Thus, the same conclusions can be drawn from the above observations.
NOTE: It is to be kept in mind that no radioactive substance emits both the a and b particles simultaneously. Some substance emit a-particles and some emit b-particles. The g-rays are emitted along with both a and b particles.
Also that at any time a piece of a radioactive substance may have more than one element present in it. This happens as a result of new substances that are being formed due to the radioactive emission (discussed later). This explains the presence of all three rays simultaneously in the above experiment.
The detailed properties of the radioactive rays are given below;
1. PROPERTIES OF a-RAYS
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It consists of a stream of positively charged particles. The charge on it is given by 3.2 X 10-19 coulombs. Each particle is made up of two protons and two neutrons. Thus, an alpha particle is nothing but a helium nucleus, or a doubly ionised helium atom.
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The mass of an a-particle is given by 6.645 X 10-27 Kg.
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Being charged, they can be deflected by electric and magnetic fields. The direction of their deflection shows that they are positively charged.
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The velocity of alpha particles is much less than the velocity of light ( 10-6 m/s ). Velocities of alpha particles emitted by different radioactive substances is different, but alpha particles emitted from the same substance have the same velocities ( same energy).
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The penetrating power of alpha particles is low. Even in air it is not able to penetrate more than a few centimeters.
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The cause fluorescence when they fall on certain fluorescent materials such as barium platinocynide ( television screens are coated with this).
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Alpha rays ionise the gases through which they pass.
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It consists of a stream of electrons coming from the nuclei. The charge on it is given by 1.6 X 10-19 coulombs. It is actually a fast moving electron. These are not the orbital electrons of the atoms, but are emitted from the nucleus.
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Being charged, they can be deflected by electric and magnetic fields. The direction of their deflection shows that they are negatively charged.
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The velocity of beta particles is much varies from 1% to 99% of the velocity of light ). Velocities of beta particles emitted by different radioactive substances is different, but beta particles emitted from the same substance have the same velocities ( same energy).
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The penetrating power of beta particles is higher than that of alpha particles. It is 100 times that of the alpha particle.
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The cause fluorescence when they fall on certain fluorescent materials such as barium platinocynide ( television screens are coated with this).
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Beta rays also ionise the gases through which they pass, but the ionising power is 100 times less than that of alpha particles.
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It is a stream of electronegative waves or photons. It has a shorter wavelength than the X-rays.
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The mass of an a-particle is given by 6.645 X 10-27 Kg.
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Being chargeless, they cannot be deflected by electric and magnetic fields.
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The velocity of gamma rays is equal to the velocity of light ( 10-8 m/s ).
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The penetrating power of alpha particles is the largest among the three radioactive rays. It is a 100 times greater than the penetrating power of beta rays.
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Alpha rays ionise the gases through which they pass. Although, the ionising power is the least (about 100 times less than the beta rays).