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An early warning fire alarm with an adjustable smoke detector An early warning fire alarm with an adjustable smoke detector The astronauts would need to know if a fire had started or if noxious gases were loose in the vehicle. This ionization smoke detector essentially uses a radioactive element called americium–241 to spot smoke or harmful gasses. When clean air particles of oxygen and nitrogen move through smoke detectors, the americium–241 ionizes them, which creates an electrical current. If foreign smoke particles enter the smoke detector, it disrupts that interaction, triggering the alarm. What is the role of Radioactive element in the alarm? Lets see what this topic has got to offer regarding this.

Learning objectives

After completing the topic, the student will be able to:

  • Understand the phenomenon of radioactivity and discuss the types of radiations that can be emitted by a radioactive substance.
  • Discover what happens when a radioactive substance is being placed in electric and magnetic field and discuss the properties of the radiations emitted by the same radioactive element.
  • Explore the process of radioactive decay and the types of radioactive decays to become a stable element.
  • Differentiate between positron emission and electron capture; also determine the rate of disintegration of the decay process.
  • Investigate the concept of half-life period and its importance in determining the stability of the nucleus.
  • Discuss and probe different types of applications of radioactivity as observed in industries, medicine, etc.
Three types of radiations Three types of radiations
  • An Alpha particle consists of 2 protons and 2 neutrons.
  • A Beta Particle is nothing but an electron.
  • A Gamma Particle is nothing but the high energy photon.
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. All heavy elements with atomic number above 82 show the phenomenon of radioactivity and such substances are called radioactive elements e.g. uranium (Z = 92); radium (Z = 86). It is a self–disintegration and irreversible process. The number of radioactive elements known at present is 40.

There are three types of radiations that can be emitted by a radioactive substance viz.

(a) an α–particle which is simply the helium nucleus (2He4).

(b) β-particle which is an electron identical to one that orbits the nucleus but it is created within the nucleus itself.

(c) γ-rays which are very high energy photons.

The emission of radiations transforms the radioactive element into another element which may be radioactive itself. This radioactive element formed will further change into a still new element. This process goes on till a stable element (82Pb208) is arrived at. After this, the radioactivity (i.e. radioactive decay) stops. Since radioactivity is practically unaffected by temperature, pressure and other conditions, we conclude that radioactivity is a nuclear property. Therefore, 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.

Effect of a magnetic field Effect of a magnetic field on alpha, beta and gamma radiation Alpha rays travel faster with higher energy and are less deflected when compared to ‘β’ rays. Gamma rays are not effected by the magnetic field,hence they go undeflected.
Experiment

A small hole is drilled in a Lead block ‘L’ and a piece of radium is placed in it. Radiations from radium escape only through the hole and others are absorbed by the wall. This arrangement is placed in an evacuated enclosure. The nature of radiations emitted by the radioactive source are studied by applying
i) electric field and ii) Magnetic field

Results for application of electric field
The first part of rays are called α–particles and they bends towards the negative plate. The deflection of α–particles towards the negative plate shows that they are positively charged. It has been found that α–particle is actually a helium nucleus (2He4).

The second part of rays bends towards the positive plate. This part constitutes β–particles. The deflection of β–particles towards the positive plate shows that they are negatively charged. It has been found that β–particles are electrons.

The third part of rays remains unaffected and goes straight without bending. This part constitutes γ–rays. This means that γ–rays are uncharged. It has been found that γ–rays are electromagnetic waves (photons) of very short wavelengths. These are emitted from the nucleus that is in the excited state. The α–particles bend towards the negative plate by a small amount showing that they are heavy positively charged particles.

On the other hand, β–particles bend towards the positive plate by a large amount showing that they are light negatively charged particles.

Results for application of magnetic field
When a uniform magnetic field is applied, perpendicular to the plane of the paper and directed into it then the beam splits into three. By applying Fleming's left hand rule, it can be seen that the ray which bends towards left is α–particle, the ray which bend towards right in a semi-circle is the β–particle and the ray which goes un–deflected is the γ–ray.

Using other experiments, it can be shown that the α–particles are ‘He’ atoms, β–particles are electrons and γ–rays are Electromagnetic radiations.