A nucleus of lanthanum-140 emits gamma radiation - AQA - GCSE Physics Combined Science - Question 6 - 2019 - Paper 1

Gamma radiation is only weakly ionizing, which means it can pass through materials without interacting significantly. Most gamma radiation is very penetrating, allowing it to go undetected by standard detection devices.

The count rate from a Geiger-Muller tube represents only a portion of the gamma radiation emitted by the source. Some of the radiation spreads out in all directions, while only a fraction enters the GM tube, resulting in a lower count rate compared to the actual activity of the sample.

The teacher stood far away from the apparatus to reduce exposure to gamma radiation, which can pose health risks. Standing further away helps to minimize any potential radiation exposure while still being able to observe the experiment.

The count rate did not decrease proportionally as the thickness of lead increased. For instance, from 0.5 cm to 1.0 cm thickness, the count rate dropped from 110 to 60 counts per second, but this does not represent a direct inverse relationship. The decrease in count rate becomes less significant with added thickness, indicating that other factors may influence the radiation absorption.

The completed equation is: $140_{57}La \to e^{-} + 140_{58}Ce$.

From Figure 7, the number of cerium atoms when 100 seconds old is 5.0 x 10^23 and when 350 seconds old is approximately 1.5 x 10^23. The ratio can be calculated as: $\frac{5.0 \times 10^{23}}{1.5 \times 10^{23}} = \frac{5}{1.5} = \frac{10}{3} = 3.33 \text{ or simplified as } 32:1.$

From the graph in Figure 7, the number of atoms of cerium-148 when 20 seconds old is approximately 6.5 x 10^23. Since the half-life is 50 seconds, we can determine activity using the formula:
$A = \lambda N$, where $N$ is the number of atoms. The activity is approximately 1.3 x 10^6 Bq.