How radiation can be detected
Detecting radiation using a Geiger-Muller tube
The ionising effect of radiation is used in the Geiger-Muller (GM) tube as a means of detecting the radiation.
The GM tube is a hollow cylinder filled with a gas at low pressure. The tube has a thin window made of mica at one end. There is a central electrode inside the GM tube. A high voltage supply is connected across the casing of the tube and the central electrode as shown in the following diagram.
When alpha, beta or gamma radiation enters the tube it produces ions in the gas. The ions created in the gas enable the tube to conduct. A current is produced in the tube for a short time. The current produces a voltage pulse. Each voltage pulse corresponds to one ionising radiation entering the GM tube. The voltage pulse is amplified and counted.
The greater the level of radiation, the more ionisation in the tube so the greater the number of counts.
The GM tube counting the number of ionisations may not provide a completely accurate reading, as the number of counts will simply keep increasing.
The quantity Activity gives an indication of how radioactive a substance is. Activity is the number of radioactive atoms which disintegrate and emit radioactivity per second.
Activity is measured in units called Becquerels (Bq) named after Henri Becquerel, the French scientist who shared the Nobel Prize for Physics in 1903 with his students - Marie and Pierre Curie.
\(Activity = \frac{{number\,of\,disintegrations}}{{time\,taken\\}}\)
\(A = \frac{N}{t}\)
Where time is in seconds \((s)\) and activity is measured in Units of activity. Usually shortened to Bq. \((Bq)\). The number of disintegrations has no units.
The number of disintegrations cannot be determined easily in practical work, but the count of radioactive particles detected by a Geiger Muller counter is a useful approximation at this level, and can give an indication of the rate of change of activity.
Watch this video to see the procedure for measuring the activity of a radioactive sample, taking into account the background radiation.
How to measure the activity of a radioactive sample.
Question
A pupil measures the background activity in the classroom.
The pupil records 45 disintegrations in 60 seconds.
What is the background activity in the room?
\(Number\,of\,counts = 45\)
\(Time = 60s\)
\(A = \frac{N}{t}\)
\(A = \frac{{45}}{{60}}\)
\(A = 0.75Bq\)
Background radiation
Radiation from natural sources and man made sources is around us all the time. This is called Background Radiation. It is very low level, usually less than \(1 Bq\).
However, if you were to measure the activity of a source in the classroom, you would also be measuring the background activity too.
To ensure that you calculate the correct activity for a source, the background activity must first be measured and taken into account.
Background radiation level can be found as follows:
- With no radioactive sources in the room start a stopwatch and GM tube (with counter) at the same time.
- After 60 seconds, stop the GM tube and counter and record the number of counts in 60 seconds.
- Calculate background activity in the room.
- Repeat the process a number of times and find the average background activity.
