Properties of α, β and γ Particles | Radio Isotopes

After reading this article you will learn about the properties of α, β and γ particles.

Properties of Alpha (α) Particles:

The alpha particles are doubly positively charged helium atoms i.e. the alpha rays consist of particles carrying two units of positive charges and having a mass of 4 units. It gives helium spectrum. The energies of α-particles are quite high, being of the order of several million electron volts (MeV).

Thus, these particles during their interaction with matter can strike against the orbital electrons through which they pass, causing ionisation as well as electronic excitation. Ionisation is complete removal of valency electron and the excitation results in the electrons being raised to orbits of higher energy levels. Alpha- particles have relatively little penetrating power and hence the range is relatively small.

Properties of Beta (β) Particles:

Beta particles are electrons emitted by the unstable nuclei of the radioactive atoms at the time of its disintegration or decay. Beta particles are made up of negatively charged particles which are similar to ordinary electrons. Beta particles are shot out with widely varying speeds. Its ionising power is much weaker than that of a-particles.

However, the penetration of the beta particle or the amount of absorbing material requires to prevent the particle depends upon the maximum energy (E_max) and this is expressed as weight per unit area or surface density.

However, the following Celendenin’s equation can be used for calculating the range of beta particles:

It can be applicable where E_max value is 0.8 MeV. Due to small mass, the beta particles are easily deflected and follow irregular paths in traversing matter. When beta particle approaches towards the nucleus of an atom, the speed of the particles becomes slow resulting from the loss of energy and as a result continuous emission of X-rays occurs and of K, L or M orbits of any atom, this is known as “Bremsstrahlung”.

Again, if the beta particle collides with electrons is a production of X-rays of the target element due to falling back of electrons to the ground state. Rutherford’s scattering also takes place due to elastic collision resulting from the interaction between beta particle and the nucleus.

Properties of Gamma (γ) Rays:

The gamma rays differ from alpha and beta rays in that the former rays (gamma rays) are not charged particles, and are not deflected in magnetic and electric fields. It is similar to X-rays but is of shorter wavelength. The source of gamma rays is inside the excited atomic nuclei, usually radioisotopes, while the source of X-rays is extra nuclear where high speed electrons exhibit inelastic collisions with the target atoms.

It is more penetrating than α or β particles and hence it requires several cms thickness of a metal to reduce its intensity which cannot be easily detected. Gamma rays ionise air and possess very high penetrating power. It has short electro-magnetic waves.

Gamma rays are known to interact with matter in six (6) ways of which three (3) are most important and those are shortly described below:

1. Photoelectric Effect:

The gamma photon interacts with orbital electron of the atom in an inelastic collision and the energy not used i.e. excess energy of photon is used in overcoming the binding energy of the electron and some energy is also acted upon the electron as a kinetic energy. However, such photoelectric effect varies with the atomic number or the charge directly, and with the mass number inversely.

2. Compton Effect:

Electron takes energy from the photon when a photon gamma strikes the electron of an atom and recoils take place. However, the direction of recoiling is determined by the direction of the collision. Through such interaction, the gamma photon loses energy and scatters with increased wavelength.

The Compton Effect is of great importance when the absorbing material or substance is of lower atomic weight and energy of the gamma radiation is intermediate.

Due to an increase in atomic weight of the absorbing material the intensity of absorption increases by this effect resulting a series of obstructions or encounters in passing through the absorber which reduces the energy of gamma rays to a level of not detectable.

3. Pair Production:

With the elements of high atomic weights, the photoelectric and Compton effects are not so important for their absorption. However, the pair production involves complete disappearance of the gamma radiation incidence and the production of two particles viz. electron and positron.

This reaction occurs as a result of interaction between the gamma photon and the high energy field in the vicinity of the nucleus. Such pair production is important for absorbers of high atomic weight and high charge.

Neutrons:

Neutrons are being widely used for the methods of determination of soil moisture by using neutron scattering method in the field.

Neutrons are produced by the collision with α-particles and berylium nuclide resulting emission of neutrons as follows:

₄Be⁹ + ₂He⁴ = ₆C¹² + ₀n¹

Neutrons are also produced in atomic reactions and by reactions in the cylocton.

Neutron undergoes spontaneous destruction as:

Neutron → proton + electron + neutrins

It is an unstable particle [half-life: 20 minute (approx.).]. Neutron has no charge but unit mass in the nuclei and is not influenced by magnetic or electric fields.