When a small radioactive source is placed in a cloud chamber, straight tracks about 4 cm long are observed - AQA - A-Level Physics - Question 18 - 2019 - Paper 2

To determine the types of radiation emitted by the source, we need to analyze the behavior of the radiation when it encounters a material.

The provided information states that when the aluminum sheet (5 mm thick) is placed between the source and the Geiger tube, the count rate falls to the background count rate. This indicates that the radiation is being absorbed by the aluminum.

Types of Radiation:

1. Alpha (α) Radiation:

   • Alpha particles are not very penetrating and can be stopped by a sheet of paper or a few centimeters of air.
   • However, they are typically absorbed by materials such as aluminum.

2. Beta (β) Radiation:

   • Beta particles are more penetrating than alpha particles but can still be stopped by thin sheets of plastic or aluminum.
   • Their significant absorption by the aluminum sheet indicates their presence.

3. Gamma (γ) Radiation:

   • Gamma rays are much more penetrating and require denser materials like lead or several centimeters of concrete to significantly reduce their intensity.
   • The fact that the count rate reduces to background suggests that gamma radiation was not significantly produced by this source when the aluminum barrier was introduced.

Given this analysis, the correct answer must include alpha and beta radiation, which corresponds to option D (α and β).