In the manufacture of newsprint paper, heavy rollers are used to adjust the thickness of the moving paper - Leaving Cert Physics - Question d - 2011

As the paper passes through the radiation field created by the Sr-90, the reading on the detector will vary based on the thickness of the paper.

1. With increasing paper thickness: The amount of beta-particles that reaches the detector decreases due to absorption by the paper, resulting in a lower reading.
2. With decreasing paper thickness: More beta-particles will be detected, leading to a higher reading.

The radioisotope Am-241 emits alpha-particles, which have poor penetrating power.

Due to their limited ability to penetrate materials, alpha-particles can be easily blocked by even a few millimeters of paper. This would make Am-241 ineffective for the task of measuring the thickness of the paper, as it would not be able to provide reliable readings.

To find the number of atoms present in the sample, we can use the formula for activity:

$A = \lambda N$

Where:

• $A$ is the activity (in Bq),
• $\lambda$ is the decay constant,
• $N$ is the number of atoms.

First, we need to calculate the decay constant $\lambda$ using the formula: $\lambda = \frac{0.693}{T_{1/2}}$

Substituting the half-life of Sr-90 (28.78 years): $\lambda = \frac{0.693}{28.78 \times 365.25 \times 24 \times 60 \times 60} \approx 7.63 \times 10^{-10} \text{ s}^{-1}$

Now, rearranging the activity formula to find $N$: $N = \frac{A}{\lambda} \\ = \frac{4250}{7.63 \times 10^{-10}} \\ = 5.57 \times 10^{12} \text{ atoms}$

Thus, the number of atoms present in the sample of Sr-90 is approximately $5.57 \times 10^{12}$.