The short answer
Alpha, beta and gamma decay, and the fixed-rate maths of half-life.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Unstable nuclei decay at a fixed, predictable rate. The key number is the half-life — the time for half of the nuclei present to decay. It's constant: each half-life, half of whatever remains decays, regardless of how much you started with.
The three decays
Alpha (α) & Beta (β)
- α: emits a helium nucleus (2p + 2n) → mass −4, atomic number −2
- β⁻: a neutron becomes a proton + emitted electron → atomic number +1
- Both change the element (the proton number changes)
Gamma (γ)
- Emits high-energy electromagnetic radiation
- No change to mass or atomic number
- Just sheds excess energy from the nucleus
The half-life rule
After half-lives, the fraction of the original sample remaining is . So after 1 half-life, remains; after 2, ; after 3, — and so on.
Worked example
A radioactive isotope has a half-life of 5 years. You start with 80 g. How much remains after 15 years?
Check yourself
After how many half-lives has a radioactive sample decayed to 1/16 of its original amount?
Key takeaways
- Half-life = time for half the nuclei to decay; it's constant.
- After n half-lives, the fraction left is (1/2)ⁿ.
- Alpha: emits a He nucleus (mass −4, atomic number −2).
- Beta⁻: a neutron → proton + emitted electron (atomic number +1).
- Gamma: energy only — no change to mass or atomic number.
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