AQA A2 Physics A Nuclear Physics Section 9 Properties of Alpha, Beta and Gamma Radiation

Ionisation Ability and Penetration Power Type of RadiationWhat is it?Relative MassRelative electronic charge Alpha Particle 2 protons and 2 neutrons 4+2 Beta Particle A high energy electron 1/1800 Positron A high energy anti- electron 1/ Gamma Ray A high frequency electromagnetic wave 00 Below is a summary of the four types of radiation.

Ionisation The term ionisation refers to an atom gaining or losing charge by gaining or losing electrons. Radiation has the ability to remove electrons from atoms and molecules hence the term, “Ionising Radiation”. It takes in the order of tens of eV to ionise atoms and molecules. The kinetic energy of alpha and beta/positron particles is typically J and 2x J respectively. The energy of a gamma ray is typically 5x J. Given that it takes about J to ionise a typical atom estimate the number of particles that can be ionised for alpha, beta/positron and gamma rays. The energy alpha, beta/positron and gamma radiation is reduced with each ionisation. When this energy has run out the particle will be completely absorbed.

Penetration Power The depth over which a certain type of radiation can penetrate depends on: The type of radiation (how likely it is to ionise an atom or a molecule when it passes). The initial energy of the radiation. The energy it takes to produce an ion. The frequency at which collisions occur between radiation and atoms of the material.

An alpha particle is heavy and has a large charge. Its ability to ionise atoms and molecules is therefore large. As alpha particles pass through materials they rapidly start to ionise the atoms or molecules of that substance. They therefore lose their energy very quickly and get absorbed very easily. Alpha particles can be stopped by paper or even a few millimetres of air. Their penetration power is low. paper Ionisation Ability and Penetration Power

A beta particle/positron is less heavy and has less charge than an alpha. Its ability to ionise atoms and molecules is reasonable but not as great as that of the alpha particle. As beta particles/positrons pass through materials they ionise atoms or molecules but not as rapidly as alpha particles. They therefore do not lose their energy as quickly so penetrate deeper, they still do eventually get absorbed. Beta particles/positrons can penetrate paper but get stopped by a thin layer of aluminium or a few tens of centimetres of air. Their penetration power is intermediate. paper aluminium Ionisation Ability and Penetration Power

A gamma ray has no mass and no charge. Its ability to ionise atoms and molecules is weak. As gamma rays pass through materials they seldom ionise atoms or molecules. They therefore do not lose their energy very quickly and so can penetrate very deep and through most materials. Gamma rays can penetrate aluminium and are in fact only weakened in intensity by very thick lead. Their penetration power is very high. paper aluminium thick lead Ionisation Ability and Penetration Power

The Inverse Square Law Like all electromagnetic radiation, gamma rays obey the inverse square law. R Area of window on detector, A Fraction of  -radiation entering the detector =

The Inverse Square Law Like all electromagnetic radiation, gamma rays obey the inverse square law. R Area of window on detector, A Count Rate for Gamma Radiation =

A radiation worker measures background radiation as 60 counts in 3 minutes. Calculate the background count rate in counts per minute. A Geiger-Muller tube is situated 1m away from a radioactive source. In 2 minutes the counter reads 105. Determine the corrected count rate. The Geiger-Muller tube is moved to a distance 50cm away from the source. Predict the number of counts that would occur in 2 minutes (including the background count). The Inverse Square Law

A Geiger-Muller tube is situated 5m away from a radioactive source. The corrected count rate is measured as 6s -1. The Geiger-Muller tube is moved away from the source. The corrected count rate reduces to 2.5s -1. Calculate the distance between the Geiger-Muller tube and the source. The Inverse Square Law