RADIOACTIVITY Discovery of radioactivity In the mid 1800s, the invention of the photographic plate gave scientists to a sensitive detector that responded to radiations not visible to the human eye. Henri Becquerel (1852–1908), while attempting to show that fluorescent materials could be the source of the X-rays, discovered that rays invisible to the human eye were emitted from a uranium ore. These rays made patches on the photographic film.
Types of radiations There are three types of radiations: alpha (), beta () and gamma (). Alpha particles The alpha particles are helium nuclei, that is, they consist of two neutrons and two protons. Their mass is about four times that of a hydrogen atom, and each alpha particle carries a double positive charge. Alpha particles are emitted at velocities of about 1–20% of the speed of light. They are good ionizers, but do not have high penetrating power. Their range in air is only a few centimeters, and they can be absorbed and stopped by a sheet of thin paper. They are deflected by both magnetic and electric fields.
Beta particles Beta particles have the same mass and charge as an electron. They are seven thousand times lighter than alpha particles. Their velocities are extremely high, approaching that of light. Beta particles are about 100 times more penetrating than alpha particles and can travel a metre in air. They are absorbed and stropped by a thin sheet of aluminium. They are deflected by both magnetic and electric fields.
Gamma rays Gamma rays are not particles; they are electromagnetic radiation of extremely short wavelength. Since they have no charge, they do not interact with atoms electrostatically and produce very little ionisation. They have tremendous penetrating power, and are stopped by a few centimeters of lead. They are not deflected by both magnetic and electric fields. NOTE: Radioactive sources may give out any or all three of these radiations.
The effect of magnetic and electric fields In a typical experiment, a small amount of radioactive material is placed in a small hole drilled in a lead cylinder, placed in an evacuated chamber. A magnetic field is applied in a direction perpendicular to the plane of the paper. A photographic plate placed above the hole when developed shows three dark spots.
• The spot on the left is due to positively charged alpha particles. The small curvature of their track from the source to the photographic plate indicates that they are comparatively massive particles. • The central spot is not affected by the magnetic field and must be due to uncharged particles. The penetrating properties of these emissions show them to be gamma rays. • The spot on the right is due to negatively charged beta particles. These are lighter than the alpha particles as indicated by the larger curvature of their tracks. NOTE: The alpha and beta rays are deflected by the magnetic field. Therefore they must contain charged particles. The gamma rays are not deflected by the magnetic field and therefore they must be uncharged. In electric fields alpha behaves like a positive charge and is attracted towards the cathode whereas the beta particles behave like an electron and are attracted towards the anode. The gamma rays are not affected by an electric field either because they are uncharged.
Detection of Radioactivity Geiger – Muller (GM) Tube. The GM tube contains argon gas. When radiation enters a GM tube through the mica window it ionizes the argon gas producing electrons and argon ions. The electrons are accelerated towards the anode whereas the positive argon ions are attracted to the cathode. A current pulse is created which is amplified and fed into the scaler or the ratemeter. The ratemeter counts the pulses and shows the counts per second.
Diffusion Cloud Chamber In a diffusion cloud chamber vapour from alcohol in the felt ring diffuses downwards and is cooled by the dry ice. When radiation passes through the air in the chamber it ionizes the air molecules and the vapour condenses on the air ions. This results in a white line of tiny droplets of liquid which shows up as a track when illuminated.
Tracks of alpha
Tracks of beta
Tracks of gamma
Photographic Plate The photographic plate shows dark patches when exposed to radiations.
Radioactive decay Radioactive atoms have unstable nuclei and decay into atoms of different elements with more stable nuclei by emitting alpha or beta particles. The decay is random and is unaffected by temperature.
Alpha decay This occurs when an unstable nuclei becomes stable by emitting an alpha particle. X
Y + He
For example:
Ra
Rn + He
Beta decay This occurs when an unstable nuclei emits a beta particle to become stable. X
Y+ e
For example:
C
N+ e
Half–life. This is the time for half the atoms of a radioactive material to decay. Example: The half life of a radioactive material is 12 years. If there are 2000 particles to start with, after how many years will there be 500 particles left. Answer: 2000 / 2 = 1000 first half life (12 years) 1000 / 2 = 500 second half life (12 years) So it takes two half lives to drop to 500 particles i.e. 12 + 12 = 24 years.
Decay curve Activity counts/s
Time /mins
Uses of radioactivity Radioactive substances made in the nuclear reactors called radioisotopes are used as following: 1. Thickness gauge: Used to measure the thickness of a paper, plastic and metal sheets during manufacture. A radioactive source is placed at one side and the GM tube is placed at the other end of a paper. If the thickness of paper increases the count rate drops and the thickness is adjusted automatically. 2. Tracers: A small amount of weak radioisotope is injected in the system and then traced with the GM tube. This method is used in medicine to detect brain tumors and in agriculture to study the uptake of fertilizers by plants. 3. Radiotherapy: Gamma rays are used to treat cancer. 4. Sterilization: Gamma rays are used to sterilize medical instruments. 5. Archaeology: A radioisotope of carbon , present in air , is taken in by plants and trees and its rate of decay is used to date archaeological remains.