7. (a) Which of these describes isotopes of an element? A same atomic number different number of neutrons B same atomic number different number of protons C same mass number different number of neutrons D same mass number different number of protons (b) Figure 9 represents a decay that can happen inside the nucleus of an atom - Edexcel - GCSE Physics - Question 7 - 2020 - Paper 1

The decay represented in Figure 9 is C: beta plus. In this decay process, a proton is transformed into a neutron and a positron, which is consistent with the illustration given.

To estimate the activity in the year 2020, we recognize that 20 years have passed since 2000. Since the half-life of cobalt-60 is 5 years, this corresponds to 4 half-lives (20/5 = 4).

Using the formula for activity after n half-lives:

$A = A_0 \left(\frac{1}{2}\right)^n$

where:

• $A_0 = 38.5$ kBq (initial activity)
• $n = 4$ (number of half-lives)

Thus:

$A = 38.5 \left(\frac{1}{2}\right)^4 = 38.5 \times \frac{1}{16} = 2.40625 \text{ kBq}$

Rounded to three significant figures, the activity in the year 2020 is approximately 2.4 kBq.