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After disintegration the radioactive material finally converts into lead and still it holds some radioactivity. This is called nuclear waste.

Cathode rays are used for: 

(i) Converting electrical signal into a visual signal, 

(ii) In television.

The emission of free electrons from a metal surface, when heat energy is imparted to it, is called thermionic emission. 

Purpose of covering cathode by thorium and carbon is to reduce its work function, decrease its melting point. 

There are four types of emissions: 

(i) Thermionic emission. 

(ii) Photoelectric emission. 

(iii) Field emission. 

(iv) Secondary emission.

Hot cathode ray tube is used in T.V.

When a cathode ray beam strikes a zinc sulphide screen, a bright spot is seen on the screen. 

In a CRT, the external P.D. refers to the electrical signal. The P.D. controls the deflection of electrons beam which strikes in the desired fashion direction and converts the electrical signal to visual signal. 

(i) Beta rays and cathode rays both are composed of electrons. 

(ii) Beta rays and cathode rays both are deflected by the electric and magnetic fields in the same direction. 

(iii) Beta rays and cathode rays both ionise the gas through which they pass.

As nucleus is positively charged it strongly attracts the negative charged electrons. The electron orbit close to the nucleus are tightly bound by strong attractive force of nucleus. These electrons are known as bound electrons. 

Electrons in outer orbits are weakly bound with the nucleus. In solids these weakly bound electrons leave their individual atom and become a part of it. These electrons are known as free electrons.

Free electrons leave the metal surface only if sufficient energy is supplied to the metal. This emission of electrons from metal surface is called the electron emission.

The number of protons in the nucleus is known as the atomic number of the element and is denoted by Z. 

The atoms of different elements which have the, same mass number but differ in their atomic numbers are called isobars. 

Examples: Sodium, Magnesium.

The total number of neutrons and protons in the nucleus is called the mass number of the element and is denoted by A.

Rate of emission of thermions depends on: 

(i) Material of the surface 

(ii) Temperature of the surface.

(i) ₁₁Na²⁴ → ₁₂Mg²⁴ + _-1e⁰ + energy

(ii) 24 -  mass no.(no. of protons and neutrons)

11 - atomic number(no. of protons)

(iii) ₁₁Na²⁴ is called  parent nuclei. 

₁₂Mg²⁴ is called daughter product.

₈₄X²⁰² → ₈₂Y¹⁹⁸ + ₂He⁴.

The mass number remains unchanged but atomic number increases by 1 of the nuclei undergoing such a change. 

(i) This nuclear decay is allowed since the atomic number and mass number are conserved. 

(ii) This nuclear decay is not allowed since the mass number is not conserved. 

(iii) This nuclear decay is not allowed since the atomic number and mass number both are not conserved.

(i) Alpha and beta radiations are charged. 

(ii) Beta radiation is similar in nature to cathode rays. 

(iii) Gamma radiation is similar in nature to X-rays. 

(iv) Beta radiation is deviated most in a magnetic field. 

(v) Gamma radiation is not affected by an electric field. 

(vi) Gamma radiation has the maximum penetrating power. 

(vii) Alpha radiation has the highest ionising power. 

(viii) Gamma radiation has the highest speed. 

(ix) Alpha radiation when become neutral become the atom of rare (helium) gas.

γ-radiations can cause severe genetical disorder α and β are deflected by an electric field. 

(i) γ-rays. 

(ii) α-particles.

Speed of γ radiation = 3 × 10⁸m/s in air or vacuum.

(i) This reaction is allowed because, the mass number as well as atomic number are conserved here. 

(ii) This reaction is not allowed because even though charge (atomic number) is conserved in it, the mass number [= A on the L H.S. and equal to (A + 4)] on the R.H.S. is not conserved.

When the nucleus of a radioactive element emits gamma radiation, there is no change inside the nucleus except that its excess energy is released in the form of y-radiation. 

Example: ^A_ZZ* → ^A_ZZ + γ

The star indicates the excited state of nucleus. 

Following three radiations are emitted by the radioactive substances: 

(i) Alpha (α), (ii) Beta (β), and (iii) Gamma (γ). 

All the above radiations are not emitted in a single radioactive decay. There will be either an α emission or a β emission, which may be accompained by γ-emission.

γ-rays travel with the speed of light.

(i) Number of neutrons = Mass number – Atomic number 

= 20 – 9 = 11 

(ii) Number of protons = Atomic number = 9.

γ-rays are not affected by electric or magnetic field.

γ -radiations are nearly 100 times more penetrating than β-radiations and 10,000 times more penetrating than α-radiations. 

The ionising power of α-radiation is nearly 100 times that of β radiations and nearly 10,000 times that of γ-radiations. 

Fluorescent screen helps to obtain images on the screen by converting electrical signals to light (or) visual signals by ionizing radiations to light waves.

The three main parts of a cathode ray tube are (i) Electron gun, (ii) Deflecting system, (iii) Flourescent screen.

(i) Decrease work function. 

(ii) Coating the cathode with a layer of alkaline earth metallic oxides (or) hydrides.

The atoms of same element which have same atomic number but different mass number are called isotopes. Examples: Protium, Deuterium, Tritium.

(i) We should put on special lead lined aprons, lead gloves and special film badges. 

(ii) We should not touch the radioactive substances but should handle them with long lead tongs. 

(iii) The radioactive substance should be kept in a thick walled lead container:

The most important one use of radio-isotopes is in medicines which are used mainly to cure cancer.

Becquerel rays are ‘radioactive radiations’, emitted from radioactive substances like uranium, thorium etc. Since these radiations were discovered by Henry Becquerel, so they are named as ‘Becquerel rays’.

(i) Radioactive radiations (gamma rays) are used to cure diseases such as cancer etc. 

(ii) Radio isotopes such as NaCl etc. are used for diagnosis.

The process of self, spontaneous and random emission of a or P and y radiations from the nucleus of elements of atomic number higher than 82, is called radioactivity.

(i) Nucleus: The central core of an atom is called nucleus. The nucleus contains protons and neutrons. 

(ii) Nucleons: Neutrons and protons are collectively called nucleons.

(i) A cathode ray beam while passing through a magnetic field is deflected in a direction normal to both the direction of magnetic field and the direction of its travel, as given by Fleming’s left hand rule. 

(ii) A cathode ray beam while passing through an .electric field is deflected towards the positive plate (i.e. towards higher potential).

By using materials of low work functions, the emission of electrons from such a surface takes place at relatively low temperature (at 1700°C in instead of 2500°C approximately).

(i) The substance must have high melting point. 

(ii) The work function of the substance should be low so that electrons may be emitted even at low temperature. 

 Two properties are: 

(i) High melting point. 

(ii) Low work function.