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Dating Fundamentals (Radioactive decay (Nature of particle emitted (= f (…
Dating Fundamentals
Most techniques
Depend on
radioisotope decay
(unstable nuclides)
Element Stability
Stable N/Z = 1.5
stick around a long time
Unstable NZ > 1.5
Elements w atomic no. > 89
decay via mass loss instantly
= f ( mass / atomic number ) ∴ position on periodic table
Radioactive decay
Unstable nuclides with high energy
Decay via emission of heavy particles e.g. α-particles
∴ reducing nuclide mass & release energy
Nature of particle emitted
= f ( location of unstable nuclide wrt 'valley' )
Decay types
Isobaric decay
mass of nuclide
does not change
isobars = nuclides of equal mass
Heavy particle emission
α - decay
mass of nuclide
does change
Unstable isotopes
Spontaneous decay / fission occurs
occurs in nuclides that are unstable
(have atomic number > 89)
Decay by mass loss
as lose mass & emit energy
U & Th important
= Highly unstable
in periodic table
Decay by spontaneous fission
-->
to release mass & energy
Significance radioactive decay
One parent isotope
--> Decays to 2 fission fragments/daughters
similar size & different mass
Mass loss & energy emission
Energy emission
distorts lattice
Produces
damage tracks
238 U & 235 U
Undergo natural fission
But insignif track contribution
Why?
Less abundant isotopes
Longer fission half lives
Energy emitted by fission
Distorts lattice structure
i.e. surrounding decaying atom
within a finite distance
of location of spontaneous fission event
Fission track is produced
by this distortion
i.e. damage track of fission reaction
Fission track
Defines areal extent around atom damaged/distorted by energy emission
Fission may be induced
by bombarding mineral with nuclear radiation
With time
Radioactive decay increase
Damage increase
Tracks = increasingly abundant