RADIOACTIVITY

Radioactivity is the disintegration of atomic nuclei with the spontaneous emission of particles and/or radiations. Investigations by scientists in the early 1900, particularly by Pierre and Marie curie led to the discovery that elements, including uranium, radium and polonium which disintegrate spontaneously and emitting radiation. Elements of this type are said to radioactive.

        Types of radioactive disintegration:

Rutherford investigated the penetrative power of the radiations from the radioactive substances by subjecting them to electric and magnetic fields, and as a results, deduced that there were four different types of radiation. They are:

1. Alpha radiation: These are fast moving helium nuclei with a nuclear charge of +2. The symbol is . The nuclei has a mass number of 4 and atomic number of 2. The emission of an alpha particle by an unstable nucleus results in the decrease of mass number by 4 units and atomic number by 2 units. For example, emission of alpha particles by radon - 226

In general, emission of 'n' alpha particles by a radioactive substance N with mass number M and atomic number A can be represented as:

Where N* is another different composition  to nuclei N.

                 Properties of alpha particles

• They are of high velocity
• They have little peenetrating power, and can only be stopped by paper.
• They readily ionize gases, for example, air produces flashes on a fluorescent screen.
• A few alpha particles are deflected by thin metal foil.
• They are attracted to the negative plate and south pole; hence are positively charged particles.

 2. Beta radiation: These are of two types,      they are the electrons () and positron (). The electrons are formed by a neutron in the nucleus changing to a proton and electron.

I. e

The positron are formed by the conversion of a proton to a neutron.

• B`- emission: The emission of a negative beta particles results in no change of mass number, but an increase by 1 unit in the atomic number. This is an example of isobars transformation  (no change of mass number). For example, emission of negative beta particles by thorium -234 and lead - 214.

In general, emission of 'n' negative beta particles by a radioactive substance N with mass number M and atomic number A can be represented as 

• B+ -emission  (positron emission): The emission of a positive beta particles results in no change in mass number, but a decrease by 1 unit in the atomic number. For example, emission of positive beta particles by radioactive potassium (K-38) and technetium (Tc - 95).

In general, emission of ' n ' B+- particles by a radioactive substance N with mass number M and atomic number A can be represented as


          Properties of beta- particles

• They are of high velocity .
•  They have more penetrative power than alpha particles.
•  They ionize gases to a lesser extent than alpha particles. 
•  A beam of beta particles is diverged by a metal foil and are repelled by the electrons on the metal atoms (like charges repel and unlike charges attract).
• They are attracted to the positive plate and north poke. Hence are negatively charged particles.

    3. Gamma radiations: These are high energy electromagnetic radiation. The rays are comparable to X-rays, although they have a shorter wavelengths. The symbol is   . They are formed by a readjustment of the nucleus after alpha and beta emission. They have no charge. The emission of a gamma particles results in no ch change of mass and atomic numbers. For example, emission of gamma particles by cobalt-60.

They are also emitted in a process called electron capture. For example


                 Properties of Gamma particles

• They have velocity of light
•  They are not deflected by a magnetic field. 
•  They are extremely penetrating, with a range of 15 - 20 cm in lead.
•  They have little ionization power
•  They have the ability to eject high speed electrons from matter.
• They are not attracted as they are uncharged

      4. Electron capture  (EC): This is a decay process whereby an unstable nucleus captures (picks up) an electron from an inner orbital of an atom. In effect, a proton is changed to a neutron, as in positron emission.

An example is given by potassium -40 which can also decay by electron capture;


The decay by electron capture results in no change in mass number but a decrease by 1 unit in the atomic number.

Effect Of Magnetic Field On The Radiation Emitted From Radioactive Source

The effect of a magnetic field on the radiation from a radium source and deflection pattern of the particles emitted (alpha, beta and gamma) are shown in the figure below:

Artificial Radioactivity

radioactive isotopes of many elements can be made by bombarding the element with high energy particles such as alpha, neutrons, deuterons, or gamma ray. for examples,

Measurement Of Radoactivity

The unit of radioactivity has been the curie, defined as the quantity of any radioactive substance which has a decay rate of 3.7x10*10 disintegration per second. This represents the total number of alpha particles emitted by one gram of radium in one second. Becquerel (or Bq) is defined as one disintegration per second.

1 curie = 3.7x10*10 dis/sec. (3.7x10*10 cps)
               = 3.7x10*10 Bq

This means 1 gram if radium disintegrates at a rate of 3.7x10*10 atoms per second. (Or 3.7x10*10 count per second). The sub-units are:

1 militaries (1mci) = 3.7x10*7 dis/sec
1 microcirculation (1uci) = 3.7x10*4 dis/sec.

Radioactivity is measured in terms of either absolute activity (curve) or relative activity