18 flashcards · Shared on 19 August 2026 by AtomAI Library
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What is radioactive decay?
The spontaneous and random transformation of an unstable nucleus
What is radioactive decay?
The spontaneous and random transformation of an unstable nucleus
What does the half-life of a radioactive isotope mean?
The time for half the undecayed nuclei in a sample to decay
Which equation gives the number N of undecayed nuclei after time t, where N₀ is the initial number and T is the half-life?
N = N₀(1/2)^(t/T)
A sample passes through three half-lives. What fraction of the original radioactive nuclei remains undecayed?
1/8
A radioactive sample has a half-life of 3 days and an initial mass of 80 g. How much of the original isotope remains after 12 days?
5 g
What is the activity of a radioactive source?
The number of nuclear decays per unit time
Which SI unit is used for radioactive activity?
Becquerel
A source initially has an activity of 640 Bq. What is its activity after two half-lives?
160 Bq
How are activity A, decay constant λ, and the number N of undecayed nuclei related?
A = λN
Which expression relates the decay constant λ to half-life T₁/₂?
λ = ln 2/T₁/₂
Which statement about the decay of an individual unstable nucleus is correct?
Its exact decay time is unpredictable
For most radioactive isotopes, which change leaves the half-life essentially unchanged?
Changing the temperature of the sample
What is emitted during alpha decay?
A helium nucleus containing two protons and two neutrons
How does alpha decay change the parent nucleus?
Mass number decreases by 4 and atomic number decreases by 2
What happens to a nucleus during beta-minus decay?
A neutron changes into a proton, so atomic number increases by 1
What change occurs in a nucleus when it emits gamma radiation?
It loses energy without changing atomic number or mass number
A detector measures a total count rate from a source and its surroundings. How is the source's corrected count rate found?
Subtract the background count rate from the total count rate
Why does the activity of a radioactive sample decrease over time?
Fewer unstable nuclei remain available to decay