RADIOACTIVITY (Radiochemistry)

RADIOACTIVITY (Radiochemistry) 

 

An atom of an element consists of a small, central nucleus of positive charge surrounded by orbital electrons, each carrying a negative charge. The nucleus is composed of protons (with single positive charge) and neutrons (with no charge). 

• The atomic number of an element is the number of protons in the nucleus, where as the mass number is the total number of protons and neutrons. A nucleide is an atomic species (element) with given atomic number and mass number.

 

RADIOACTIVITY (Radiochemistry)

Isotopes are nuclides with the same atomic number but different mass numbers. Most elements occurring in nature are mixtures of isotopes. Isotopes of lighter elements are stable, but isotopes of some heavy elements are unstable. 

• These unstable nuclides undergo spontaneous decay to produce stable nucleides by a process called radioactive decay. Decay is accompanied by emission of energy in the form of radiation. The half-life of a radioactive substance is the time required for the isotope to decay to half its activity.\

Radiation

Radiation from an atom or its nucleus may either be particulate or electromagnetic.

Particulate radiation (decay) 

consists of small particles of nuclear fragments. These are of two types, a-decay and B-decay. The a-decay emits & particles made of two neutrons and two protons. In the B-decay, the nuclide loses one neutron and gains one proton.

 Electromagnetic radiations 

These can be divided into two groups. Those with very low energy and long wavelength are called radio waves while those of high energy and short wavelength are called light waves. The radiations with the highest energy are called x-rays and gamma rays. The ganuma rays can penetrate material which block both &- and B- radiations.

Detection and Measurement of Radioactivity

There are various methods by which radioactivity can be detected. For example, darkening of photographic emulsion is used for X-rays. In clinical chemistry, scintillation counting systems are in common use. The isotopes used for radioactivity measurements include 1251 and 'H (tritium). 1251 emits gamma rays where as PH emits B-particles.

• Gamma rays are detected by a scintillation counter, which is a large sodium iodide crystal with thallium as an activator. The crystal is in close contact with a photomultiplier tube. When a gamma ray strikes a molecule of sodium iodide, a photon of light energy is produced. 

• This light is amplified by the photomultiplierr tube and converted to a pulse of electrical energy. The number of pulses is thus proportional to the quantity of radioactive material in the sample.

• B particles are detected by a liquid scintillation system. These substances called fluors are dissolved in special grade solvents such as toluene. The light signals are picked up by two photomultiplier tubes and converted to pulses. 

Caution All individuals working in the restricted area where radioactivity is present must be aware of the health problems associated with radiation exposure.

Applications of Radioactivity

In clinical chemistry, radioactivity is used for the estimation of hormones, drugs and other substances present in minute quantities in body fluids. Radioimmunoassay (RIA) is the method of choice (refer to Basic Immunology section for the principle of RIA).