Summary
Key terms
Radioactivity: It is the process by which a nucleus of an unstable atom loses energy by emitting particles of ionizing radiation. A material that spontaneously emits this kind of radiation is considered radioactive.
Alpha decay: It is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby ‘decays’ into an atom with a mass number 4 less and atomic number 2 less.
Beta decay: It is a type of radioactive decay in which a beta particle (an electron or a positron) is emitted from an atomic nucleus. Beta decay is a process which allows the atom to obtain the optimal ratio of protons and neutrons.
Gamma decay: It refers to electromagnetic radiation of high frequency and therefore high energy per photon. These are produced by the decay from high energy states of atomic nuclei.
Half life: It is the time required for a quantity to fall to half of its value as measured at the beginning of the time period. It is typically used to describe a property of radioactive decay.
Activity: The strength of a radioactive source is called its activity, which is defined as the rate at which the isotope decays. Specifically, it is the number of atoms that decay and emit radiation in one second.
Key concepts
- Radioactivity: The process of spontaneous
(i.e. without external means, by itself) disintegration of the nuclei of heavy elements
with the emission of certain types of radiations is called radioactivity. There are three
types of radiation that can be emitted by a radioactive substance:
- α – particle which is simply the helium nucleus (2He4).
- β – particle which is an electron identical to one that orbits the nucleus but it is created within the nucleus itself.
- γ – rays which are very high energy photons.
- α – particles, β – particles and γ – rays are emitted from the nucleus. It may be noted that electrons revolving around the nucleus are not responsible for radioactivity.
- Given below are the important properties
of α, β, γ rays.
Property α Paticles β Particles γ Particles Charge Positive and equal to twice the charge on an electron. Negative and equal to that on an electron. No charge. Penetration range Very small Can hardly pass through paper. Larger than α – particles. Can travel 10 mm through matter. Very large. Can penetrate through several cm of lead. Fluorescence Can cause fluorescence when they fall on certain substances. Cause fluorescence. Cause less fluorescence. Ionizing power Very high Ionizing power is 1 / 100th that of a – particles. Ionizing power is 1 / 100th that of ß – particles. Photographic effect Can blacken a photographic film. Can blacken a photographic film. Can blacken a photographic film. Speed Less than that of light. Less than that of light. Equal to that of light (3 × 108 m/s). Effect of electric and magnetic fields. Are deflected by electric field and magnetic fields. Are deflected by electric and magnetic fields. Un – deflected by electric and magnetic fields. Mass Mass = mass of helium nucleus. Mass = mass of electron. Zero rest mass. Energy 6MeV 1MeV 0.01MeV - Radio Active Decay:
Radioactive decay is a natural process. An atom of a radioactive isotope will
spontaneously decay into another element through one of the following process:
Mode of decay Participating particles Daughter nucleus Alpha decay An alpha particle (A = 4, Z = 2) emitted from nucleus. (A – 4, Z – 2) β− decay A nucleus emits an electron and an electron anti – neutrino (A, Z + 1) Gamma Decay A nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons),of short wavelength. (A,Z) Positron emission (ß+ decay) A nucleus emits a positron and an electron neutrino (A, Z – 1) Electron capture A nucleus captures an orbiting electron and emits a neutrino; the daughter nucleus is left in an excited unstable state. (A, Z – 1) Electron capture with positron emission A nucleus absorbs one orbital electron, emits one positron and two neutrinos. (A, Z – 2) - Decay Rate: According to decay law, the rate
of disintegration is directly proportional to the number of radioactive atoms present
at the instant of disintegration.,
where λ is a constant of proportionality and is called decay constant
and negative sign shows that N is decreasing with time. The slope of the decay curve
gives the rate of disintegration (dN/dt) of the radioactive substance. - Half Life Period: The time taken by half the
number of the atoms of the radioactive elements to disintegrate completely.
Where ? is the disintegration constant. - Average (mean) life ( Tav): All the atoms in a radioactive substance do not
disintegrate at the same time. Some atoms disintegrate first (earlier) whereas others disintegrate last
(after a long time). The mean of the ages of the atoms of the radioactive element is called the "average life"
or "mean life". Given by, Tav = 1/
which implies that the half life T= 0.693 Tav. - Activity: The rate of decay of atoms (i.e. number of disintegrations
per second) of a radioactive substance is known as its activity A is Activity, A = A 0e
– λt. It can also be expressed in terms of its half – life.
We can also express activity in terms of half – life (T) in another
useful way.,
The activity of a radioactive sample is said to be one curie (1Ci) if it
undergoes 3.7 – 1010 disintegrations per second. - Applications of Radioactivity: The principles of radioactivity and
radioactive decay have wide – ranging applications in.,
- Medicine(like NMR spectroscopy, Radiation Therapy, Emission tomography).
- Industry.
- Radio active dating.
- Arts and sciences.
key formulae
= – λN
N = number of nuclie
d = decay constant- N = N0e-λt
- T1/2 = half-life period =
- Tav (mean life) =
- Activity (A) = λN =
