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Chapter: 12th Physics : UNIT 9 : Atomic and Nuclear Physics

Radioactivity: Beta decay

In beta decay, a radioactive nucleus emits either electron or positron. If electron (e–) is emitted, it is called β- decay and if positron (e+) is emitted, it is called β+ decay.

Beta decay

In beta decay, a radioactive nucleus emits either electron or positron. If electron (e) is emitted, it is called β- decay and if positron (e+) is emitted, it is called β+ decay. The positron is an anti-particle of an electron whose mass is same as that of electron and charge is opposite to that of electron – that is, +e. Both positron and electron are referred to as beta particles.


β− decay:

In β- decay, the atomic number of the nucleus increases by one but mass number remains the same. This decay is represented by


It implies that the element X becomes Y by giving out an electron and antineutrino (  ). In otherwords, in each β- decay, one neutron in the nucleus of X is converted into a proton by emitting an electron (e) and antineutrino. It is given by

n p + e

Where p-proton,  -antineutrino.

Example: Carbon ( 146C ) is converted into nitrogen ( 147N ) through β- decay.

146C 147N + e


β + decay:

In β+ decay, the atomic number is decreased by one and the mass number remains the same. This decay is represented by


It implies that the element X becomes Y by giving out an positron and neutrino ( υ ). In otherwords, for each β+ decay, a proton in the nucleus of X is converted into a neutron by emitting a positron (e+) and a neutrino. It is given by

p n + e+ + υ

However a single proton (not inside any nucleus) cannot have β+ decay due to energy conservation, because neutron mass is larger than proton mass. But a single neutron (not inside any nucleus) can have β- decay.

A very interesting application of alpha decay is in smoke detectors which prevent us from any hazardous fire.


The smoke detector uses around 0.2 mg of man-made weak radioactive isotope called americium ( 24193Am ). This radioactive source is placed between two oppositely charged metal plates and α radiations from 24195Am continuously ionize the nitrogen, oxygen molecules in the air space between the plates. As a result, there will be a continuous flow of small steady current in the circuit. If smoke enters, the radiation is being absorbed by the smoke particles rather than air molecules. As a result, the ionization and along with it the current is reduced. This drop in current is detected by the circuit and alarm starts.

The radiation dosage emitted by americium is very much less than safe level, so it can be considered harmless.

Example: Sodium ( 2211Na ) is converted into neon ( 2210Ne ) through β+ decay.

2211Na 2210Ne + e+ + υ

It is important to note that the electron or positron which comes out from nuclei during beta decay never present inside the nuclei rather they are produced during the conversion of neutron into proton or proton into neutron inside the nucleus.


Neutrino:

Initially, it was thought that during beta decay, a neutron in the parent nucleus is converted to the daughter nuclei by emitting only electron as given by

AzX z+1AY + e-     (8.30)


But the kinetic energy of electron coming out of the nucleus did not match with the experimental results. In alpha decay, the alpha particle takes only certain allowed discrete energies whereas in beta decay, it was found that the beta particle (i.e, electron) have a continuous range of energies. But the conservation of energy and momentum gives specific single values for electron energy and the recoiling nucleus Y. It seems that the conservation of energy, momentum are violated and could not be explained why energy of beta particle have continuous range of values. So beta decay remained as a puzzle for several years.

After a detailed theoretical and experimental study, in 1931 W.Pauli proposed a third particle which must be present in beta decay to carry away missing energy and momentum. Fermi later named this particle the neutrino (little neutral one) since it has no charge, have very little mass. For many years, the neutrino (symbol υ, Greek nu) was hypothetical and could not be verified experimentally. Finally, the neutrino was detected experimentally in 1956 by Fredrick Reines and Clyde Cowan. Later Reines received Nobel prize in physics in the year 1995 for his discovery.

The neutrino has the following properties

• It has zero charge

• It has an antiparticle called anti-neutrino.

• Recent experiments showed that the neutrino has very tiny mass.

• It interacts very weakly with the matter.

Therefore, it is very difficult to detect. In fact, in every second, trillions of neutrinos coming from the sun are passing through our body without any interaction.

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