5. (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 7 represents a decay that can happen inside the nucleus of an atom - Edexcel - GCSE Physics Combined Science - Question 5 - 2020 - Paper 1

Figure 7 indicates a beta minus decay, where a neutron in the nucleus decays into a proton and an electron (beta particle) is emitted. Thus, the answer is 'beta minus'.

To find the activity of the cobalt source in the year 2020, we use the half-life of cobalt-60, which is 5 years. Since the year 2000 has a starting activity of 38.5 kBq, and from 2000 to 2020 is 20 years, that means there are 4 half-lives:

1. Calculate the activity after each half-life:
   • After 1 half-life (2005): $38.5 \text{ kBq} \div 2 = 19.25 \text{ kBq}$
   • After 2 half-lives (2010): $19.25 \text{ kBq} \div 2 = 9.625 \text{ kBq}$
   • After 3 half-lives (2015): $9.625 \text{ kBq} \div 2 = 4.8125 \text{ kBq}$
   • After 4 half-lives (2020): $4.8125 \text{ kBq} \div 2 = 2.40625 \text{ kBq}$

Thus, the estimated activity in the year 2020 is approximately 2.41 kBq.