Question 1:

Let’s break down what "activity" means in the context of radioactive decay.

In physics, the activity A of a radioactive sample is defined as the rate at which decay occurs:

where λ is the decay constant and N is the number of radioactive nuclei present.

What that means in words:
Activity is the number of decays per second (or the number of radioactive atoms that decay per unit time).

Now check each option:

• A says "total number of radioactive atoms that have decayed" — that's just a total count over time, not a rate.

• B says "total number of radioactive atoms remaining undecayed" — that's N, not the activity.

• C says "number of gamma rays emitted per second" — this is only one possible decay mode; activity counts all decay modes, not just gamma.

• D says "number of radioactive atoms that have decayed per second" — this matches the definition of activity exactly.

The correct choice is: D

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 2:

Let’s clarify the setup here:

• A = measured total activity from the rock (including background radiation)

• B = background count (activity from the environment alone, with no rock present)

• R = radiation coming only from the rock itself

Since the total A includes radiation from both the rock and the background, the rock’s own activity is:

Checking the options:

• A+B would be more than total, so that can’t be right.

• A−B matches our reasoning.

• B−A would be negative unless B>A, but A normally includes B plus more, so B<A usually.

• B×A has no physical sense here.

The correct answer is: B

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 3:

Why Graph B is Correct

Radioactive decay follows an exponential decay model. This means that in every fixed time interval (the half-life), the amount of the substance—and therefore the count rate—decreases by half.

Graph B shows this characteristic curve. The count rate drops from 800 to 400 in 10 years, and then from 400 to 200 in the next 10 years (at year 20). This consistent doubling of time for each halving of the rate is the hallmark of exponential decay.

Graph A shows a decay that is too rapid to be a simple exponential decay of a single isotope.

Graph C shows a linear decrease, which implies a constant amount decays per year, regardless of how much is left.

Graph D shows the rate of decay increasing over time, which is the opposite of how unstable nuclei behave.

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 4:

Based on the description of Graph B:

• Initial count rate = 800 at t = 0

• At t = 10 years, count rate = 400 (half) → half-life = 10 years

Half-life = B. 10 years

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 5:

After 2 half-lives = 20 years:
800 → 400 (1st half-life) → 200 (2nd half-life)

Activity after 2 half-lives = C. 200 counts per second

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 6:

After 30 years = 3 half-lives (since 10 years each):
800 → 400 → 200 → 100

Count rate after 30 years = A. 100 counts per second

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 7:

The half-life is 8 hours.

After 24 hours, the number of half-lives that have passed is:

The fraction remaining after n half-lives is:

So the answer is: D

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 8:

Half-life = 8 hours.

After 16 hours, number of half-lives:

Fraction remaining = (½)²=¼

Initial mass = 12 g, so mass remaining after 16 hours:

The correct answer is: B

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 9:

Let’s think carefully:

• In radioactive decay, the original isotope changes into a different element/isotope (because the nucleus loses or changes particles).

• In beta decay, for example, a neutron turns into a proton, emitting an electron and an antineutrino. The atomic number increases by 1, but the mass number stays the same — so it’s a different element chemically.

• The "rest of the mass" (the part that’s no longer the original isotope) has become a new isotope/element, not just converted into pure energy or heat.

Option C matches this: "Converted into a new isotope/element" — correct.
Option A is wrong because only a tiny fraction of mass is converted to energy in radioactive decay, but the rest of the mass remains as matter (new isotope).
Option B is incorrect — heat is an effect of energy release, but that’s not what happens to "the rest of the mass."
Option D is wrong — it hasn’t evaporated, it has transmuted.

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.

Question 10:

Let's examine each suggestion for beta-emitting isotope safety:

• I. Do not handle directly – use tongs.
  For beta emitters, handling with tongs reduces contamination risk and keeps the source away from the skin to avoid beta burns — sensible.

• II. Store in a thick lead or steel box.
  Beta particles are stopped by a few mm of plastic/aluminum. Thick lead is better for gamma, but steel/plastic first stops betas then lead for bremsstrahlung X-rays. For safety, thick shielding is realistic — not necessary for beta alone, but not unreasonable.

• III. Do not look directly at the isotope whilst handling it.
  Beta particles won't hurt the eyes unless very high energy, and beta sources don't produce bright light. This is meaningless for beta radiation hazards — unrealistic and not sensible.

So I and II are realistic and sensible. III is unnecessary and irrelevant for pure beta emitter (unless there’s gamma too, but even then, "looking" at it doesn't matter since radiation is invisible).

The correct choice is: B

*These A.I. responses have been individually checked to ensure they match the accepted answer, but explanations may still be incorrect. Responses may give guidance but the A.I. might not be able to answer the question! This is particularly the case for questions based on diagrams, which the A.I. typically cannot interpret.
Grade Gorilla uses Gemini, Deepseek and a range of other A.I. chatbots to generate the saved responses. Some answers have had human intervention for clarity or where the A.I. has not been able to answer the question.