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INTRODUCTION

Radioactivity or Radioactive decay  is  simply defined as a phenomenon where an element emits radiation due to the unstable nucleus. It is the spontaneous disintegration of unstable nuclides to form stable ones with the emission of radiation.  These radiations are in the form of high energy photons or particles, which are caused by nuclear reactions.

Radioactivity was discovered by Henri Becquerel in 1869, this was followed  by a successful isolation of the first two radioactive elements,Polonium (z=84) and Radium (z= 88)  by  Marie and Pierre Curie in 1898.

Radioactivity occurs in those elements that have an unstable atomic structure, i.e., unstable proton and neutron bonds. The SI unit of radioactivity is Becquerel. During the emission of these radioactive radiations, there are three variations of particles that are emitted, alpha particle, beta particle and gamma particle.

Substances that undergo radioactivity  have  unstable nuclides which  continue to disintegrate until a stable atom is formed. In the process Alpha (α) and beta (ϐ) particles are emitted and the gamma rays (ϒ) accompany the ejection of both alpha and beta particles.

Classification of Radiations

These radiations are categorised into three groups on the basis of their penetration capability. The three types of radioactive decay are :-

1. Alpha Decay: Alpha decay radiates from the particles, which are basically made up of two protons or two neutrons. Ideally, these are identical to the helium atom. These particles are heavy in nature, and they radiate at a slow pace. Due to having a slow speed, the alpha particles have a weak penetration power. However, these particles are highly ionising.
2. Beta Decay: Beta decay emits beta particles that are fast-moving electrons compared to alpha particles. These are lightweight and are negatively charged. It has great penetration power. As these particles are negatively charged, they are attracted towards positively charged particles.
3. Gamma Decay: Gamma rays are the highest energy influenced particles. These are emitted from the radioactive elements, and these have the most energetic photons in comparison to Alpha and Beta particles. It has high penetration power as well as the wavelength is the shortest among all the particles.

  Properties of Radioactive Emissions

1. a) Alpha (α) particles

They are represented as⁴H₂, hence with a nucleus number 4 and a charge of +2.

When a radioactive atom emits an alpha particle, the original atom’s atomic number decreases by two (because of the loss of two protons), and its mass number decreases by four (because of the loss of four nuclear particles).

We can represent the emission of an alpha particle with a chemical equation—for example, the alpha-particle emission of uranium-235 is as follows:

 Properties

1. Their speeds are 1.67 × 10⁷m/s, which is 10% the speed of light.
2. They are positively charged with a magnitude of a charge double that of an electron.
3. They cause intense ionization hence loosing energy rapidly hence they have a very short range of about 8 cm in air.
4. They can be stopped by a thin sheet of paper, when stopped they capture two electrons and become helium gas atoms.
5. They can be affected by photographic plates and produce flashes when incident on a fluorescent screen and produce heating effect in matter.
6. They are slightly deflected by a magnetic field indicating that they have comparatively large masses.

b) Beta (ϐ) particles

 They are represented by e⁰₁ meaning that they have no mass but a charge of -1.

The net effect of beta particle emission on a nucleus is that a neutron is converted to a proton. The overall mass number stays the same, but because the number of protons increases by one, the atomic number goes up by one. Carbon-14 decays by emitting a beta particle:

Properties

1. Their speeds are as high as 99.9% or more than the speed of light
2. They are deflected by electric and magnetic fields but in a direction opposite to that of alpha particles.
3. Due to their high speed they have a higher penetrative rate than alpha particles (about 100 times more)
4. They can be stopped by a thin sheet of aluminium
5. Their ionization power is much less intense about 1/100th that of alpha particles.
6. c) Gamma (ϒ) particles

 They have very short wavelengths in the order of 10^-10 m and below.

 Properties

1. They travel at the speed of light.
2. They have less ionization power than that of both alpha and beta particles
3. They accompany the emission of alpha and beta particles
4. They carry no electric charge hence they are not deflected by both electric and magnetic fields.
5. They have more penetrating power than X-rays.

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Alpha, beta, and gamma emissions have different abilities to penetrate matter. The relatively large alpha particle is easily stopped by matter (although it may impart a significant amount of energy to the matter it contacts). Beta particles penetrate slightly into matter, perhaps a few centimeters at most. Gamma rays can penetrate deeply into matter and can impart a large amount of energy into the surrounding matter. Table below summarizes the properties of the three main types of radioactive emissions and Figure thereafter summarizes the ability of each radioactive type to penetrate matter.

Table 3.1 The Three Main Forms of Radioactive Emissions

Illustration of the relative abilities of three different types of ionizing radiation to penetrate solid matter. Typical alpha particles (α) are stopped by a sheet of paper, while beta particles (β) are stopped by an aluminum plate. Gamma radiation (γ) is damped when it penetrates lead.