5.2.1 Detection of Radioactivity

Objective: Use count rate measured in counts/s or counts/minute

Radioactive decay involves the spontaneous breakdown of unstable atomic nuclei, resulting in the emission of particles or energy. The rate at which these radioactive emissions occur is quantified by the count rate. This section focuses on understanding and using count rates measured in counts per second (counts/s) or counts per minute (counts/min).

What is a Geiger-Müller Tube?

A Geiger-Müller (GM) tube is a common instrument used to detect ionizing radiation. It consists of a metal tube with a gas filling. When ionizing radiation enters the tube, it ionizes the gas, creating an electrical pulse that can be counted.

How is Count Rate Measured?

The count rate is the number of radioactive events detected by the instrument per unit of time. For a Geiger-Müller tube, the count rate is typically measured in counts per second (counts/s) or counts per minute (counts/min).

The instrument records each detection as a 'count'. A higher count rate indicates a higher rate of radioactive decay in the sample.

Factors Affecting Count Rate

Several factors can influence the count rate of a radioactive sample:

• Type of radioactive isotope: Different isotopes have different decay rates.
• Amount of radioactive substance: A larger amount of radioactive material will generally produce a higher count rate.
• Distance from the source: The intensity of radiation decreases with distance.
• Shielding: Materials like lead can absorb radiation and reduce the count rate.

Using a Geiger-Müller Tube to Determine Activity

The activity of a radioactive substance is the rate at which it decays. It is defined as the number of radioactive decay events per unit time.

The relationship between activity ($A$), count rate ($C$), and time ($t$) is:

$$A = \frac{C}{t}$$

Where:

• $A$ is the activity (usually in Becquerels (Bq) or Curies (Ci)).
• $C$ is the count rate (counts/s or counts/min).
• $t$ is the time interval (in seconds or minutes).

Example Calculation

Suppose a Geiger-Müller tube has a count rate of 50 counts/s. Calculate the activity of the radioactive source after 30 seconds.

1. Identify the given values: $C = 50$ counts/s, $t = 30$ s.
2. Apply the formula: $A = \frac{C}{t}$
3. Substitute the values: $A = \frac{50 \text{ counts/s}}{30 \text{ s}}$
4. Calculate the activity: $A = 1.67 \text{ counts/s}$

Therefore, the activity of the radioactive source is 1.67 counts/s.

Table Summarizing Count Rate and Activity

Quantity	Unit	Definition
Count Rate	counts/s or counts/min	Number of radioactive events detected per unit time.
Activity	Becquerel (Bq) or Curie (Ci)	Rate of radioactive decay (number of decay events per unit time).

Practical Considerations

When using a Geiger-Müller tube, it's important to consider:

• Background radiation: Radiation from natural sources in the environment. This needs to be subtracted from the sample's count rate to get the actual activity.
• Dead time: The time it takes for the instrument to recover after detecting a radiation event. This can affect the accuracy of count rate measurements at very high activities.