18 flashcards · Shared on 19 August 2026 by AtomAI Library
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Which statement about the decay of an individual unstable nucleus is correct?
Its exact decay time is unpredictable
Which statement about the decay of an individual unstable nucleus is correct?
Its exact decay time is unpredictable
Which SI unit is used for radioactive activity?
Becquerel
What happens to a nucleus during beta-minus decay?
A neutron changes into a proton, so atomic number increases by 1
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
A sample passes through three half-lives. What fraction of the original radioactive nuclei remains undecayed?
1/8
A source initially has an activity of 640 Bq. What is its activity after two half-lives?
160 Bq
How does alpha decay change the parent nucleus?
Mass number decreases by 4 and atomic number decreases by 2
What is emitted during alpha decay?
A helium nucleus containing two protons and two neutrons
For most radioactive isotopes, which change leaves the half-life essentially unchanged?
Changing the temperature of the sample
What is the activity of a radioactive source?
The number of nuclear decays per unit time
What does the half-life of a radioactive isotope mean?
The time for half the undecayed nuclei in a sample to decay
What change occurs in a nucleus when it emits gamma radiation?
It loses energy without changing atomic number or mass number
Why does the activity of a radioactive sample decrease over time?
Fewer unstable nuclei remain available 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)
What is radioactive decay?
The spontaneous and random transformation of an unstable nucleus
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
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