Gamma (γ) Radioactivity


Gamma radiation is one of the three types of natural radioactivity. Gamma rays are electromagnetic radiation, like X-rays.  The other two types of natural radioactivity are alpha and beta radiation, which are in the form of particles.   Gamma rays are the most energetic form of electromagnetic radiation, with a very short wavelength of less than one-tenth of a nanometer.

A nucleus which is in an excited state may emit one or more photons (packets of electromagnetic radiation) of discrete energies. The emission of gamma rays does not alter the number of protons or neutrons in the nucleus but instead has the effect of moving the nucleus from a higher to a lower energy state (unstable to stable). Gamma ray emission frequently follows beta decay, alpha decay, and other nuclear decay processes.

Experimental Setup

To detect gamma radiation the following geiger tube has been used :


Sensor : Geiger tube LND-712
Sens,: 18 CPS/mR/h
Bkg: 0.10 CPS
Voltage: 500 volt
Resistence: 5.6 megaohm
Type of Radiation Alfa+Beta+Gamma

Setup image with sensor and source

Gamma Radiation Interaction with Matter

When a gamma ray passes through matter, the probability of absorption is proportional to the thickness of the layer, the density of the material and the cross-section of the absorption material. The total absorption shows an exponential decrease of intensity with increasing distance from the incident surface :


where x is the distance from the incident surface, μ = nσ is the absorption coefficient, measured in cm-1, n the number of atoms per cm3 of the material (atomic density) and σ the absorption cross-section in cm2. As it passes through matter, gamma radiation loses energy through three processes : the photoelectric effect, Compton scattering and pair production.

In the image above it is shown the trend of the total absorption coefficient for aluminum (atomic number 13 ) for gamma rays: is plotted the total absorption curve and the contribution by the three effects. As usual, the photoelectric effect is predominant at low energies, the Compton diffusion dominates at intermediate energies, and pair production dominates at high energies.

Lead gamma attenuation measurement for increasing thickness
Cs 137 gamma spectrum without shield and with a 20 mm steel shield

Inverse Square Root Law

To the attenuation with a screen is added the solid angle effect, which is purely geometric and states that the number of photons that strike a target decreases in proportion to the square of the distance from the source.


The inverse square law generally applies when a force, energy, or other conserved quantity is evenly radiated outward from a point source in a three dimensional space. Because the surface of a sphere (4πr2) is proportional to the square of the radius, as the radiation is emitted away from the source, it extends on a surface that is increasing in proportion to the square of the distance from the source. Therefore, the intensity of the radiation that passes through any unit of surface (directly opposite the point source) is inversely proportional to the square of the distance from the point source.

Experimental measurement of the decrease in intensity with increasing distance

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