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

The decay represented in Figure 7 is B: beta minus.

In beta minus decay, a neutron in the nucleus of an atom is transformed into a proton while emitting an electron (beta particle) and an antineutrino, which is depicted in the figure.

To estimate the activity of the cobalt-60 source in 2020, we first calculate how many half-lives have passed since 2000:

• The time from 2000 to 2020 is 20 years.
• The half-life of cobalt-60 is 5 years. Therefore, the number of half-lives is: $n = \frac{20}{5} = 4$

Since the activity halves with each half-life, we can calculate the remaining activity after 4 half-lives:

• Initial activity in 2000 = 38.5 kBq
• After 1 half-life (5 years): $\frac{38.5}{2} = 19.25$ kBq
• After 2 half-lives (10 years): $\frac{19.25}{2} = 9.625$ kBq
• After 3 half-lives (15 years): $\frac{9.625}{2} = 4.8125$ kBq
• After 4 half-lives (20 years): $\frac{4.8125}{2} = 2.40625$ kBq

Thus, the estimated activity of the cobalt-60 source in 2020 is approximately 2.41 kBq.