Radioactivity Explained: The Science Behind Unstable Atoms

What is radioactivity?

Also known as radioactive decay, it is a physical process wherein an unstable nucleus of an atom releases energy and particles in order to achieve a stable configuration. During this process, the unstable nucleus transforms into another isotope or element.

[Isotopes have the same number of protons but different number of neutrons in its nucleus. Because they have the same number of protons, they exhibit the same chemical properties but just differ in atomic masses.]

This natural process occurs because some atoms have an imbalance of protons and neutrons. So to balance it or achieve a stable state, they have to break apart or rearrange themselves.  

When an unstable atom decays, it emits one of the three types of radiations-

1. Alpha particles– they contain 2 protons and 2 neutrons, just like a helium nucleus.
   
2. Beta particles– they are high speed, high energy electrons or positrons [antimatter counterpart/ electron with a positive charge]
   
3. Gamma particles– high energy electromagnetic radiation/ photons.

The rate of decay of a radioactive substance is measured in terms of half-life. It is the amount of time it takes for exactly half of the radioactive nuclei in the sample to decay.

Units of radioactivity

It measures the rate at which a radioactive material decays over time.

SI unit- Bequerel (bq)- 1 decay per second= 2.703×10^-11 Ci

Conventional unit- curie (ci) = 3.7×10¹⁰ radioactive decays per second

Measurement of radioactivity

By measuring radioactivity, we are quantifying the amount of radioactive material present or the dosage of radiation absorbed by a person or object.

Most common measurement devices are

1. Geiger muller counter
2. Scintillation counter

Geiger muller counter

It is an electronic instrument that can measure ionizing radiation like alpha, beta, and gamma particles. It contains a gas filled tube and a display that records the radioactivity.

What gas is present in the gas tube? Inert gases like Argon or Neon with a small amount of quenching gas.

How it works?

1. A voltage (100V- 600V) is applied between the outer wall (cathode) and a central wire (anode).
   
2. When a radioactive particle comes through a tiny window, it strips electrons from the gas. These electrons move toward the anode causing an electron avalanche.
   
3. This avalanche creates a pulse of measurable electric current which the counter translates into a click, visual reading (counts per minute) or a dose rate.

Advantages

1. It is portable
2. Less expensive
3. Highly sensitive

Disadvantages

1. It is not specific
2. It experiences a dead time- a brief period where it needs to reset before detecting another electron avalanche.

Applications

1. Monitoring workplaces and hospitals
2. Checking for radiation or contamination in the environment

Scintillation counter

Principle– when radiation strikes a specific material, it produces a flash of light. The instrument detects this light and converts it into a digital signal for display.

How it works?

1. Scintillator- incoming radiation falls on a luminescent material and interacts with it. The material absorbs the radiation and emits a small flash of light for a few seconds. This flash of light is called the scintillation. The luminiscent material can be solid materials like sodium iodide, plastics, or special liquids. Based on this, it is classified as solid or liquid scintillation counters.
   
2. The light flashes enter a photodetector called the photomultiplyer tube. It converts the light flashes into photoelectrons and multiplies the signal by millions.
   
3. The amplified signal is then sent to an electric circuit which count the individual events and measure the energy.

Advantages

1. They can detect specific type of radiation
2. They can detect radiation in nanoseconds

Applications

1. Used to identify sub atomic particles
2. They are used in medical imaging like PET CT
3. Used in radiation monitoring

Autoradiography

It is a bio-analytical technique which uses radioactive emissions to see how radiolabelled materials are distributed within a biological sample. When you place a sample in close contact with a photographic or X ray film, the ionizing radiation exposes the emulsion to create a visual map of the radiation.

Principle- the photographic plate has silver halide crystals on it. The radioactive particles reduces the silver halide into metallic silver producing black spots.

Steps involved

1. Labelling- the cells or tissues are tagged using a radioisotope. It can be done either by growing the cells in the medium containing tracer elements or by tagging the radioactive element to a specific antibody.
   
2. Sectioning/ sample prep- the tissue is frozen and sliced into thin slices using a cryomicrotome.
   
3. Exposure- the sample is placed in direct contact with an X ray film in total darkness.
   
4. Development- the film is chemically developed to reveal the distribution of radioactivity.

Applications

1. Southern/ Northern blotting
2. Tracing the adsorption, metabolism, and distribution and excretion of drugs in pharmacology.
3. Visualize neurotransmitters and hormones