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10- Neutron capture and the production of heavy elements (s-process…
10- Neutron capture and the production of heavy elements
Elements beyond iron peak
Thermonuclear fusion beyond iron is endothermic
if thermonuclear made heavier elements abundance would drop rapidly with A due to larger coulomb barrier
new process needed, neutron capture
Free neutrons are unstable particles with a half life around 15 minutes
Nuclei formed by neutron capture are often unstable wrt beta decay
Evidence for neutron capture
Certain stages in a stars evolution large fluxes of neutrons are created in the interiors
Neutron capture xsections for heavy elements very large compared to those for light elements
3% of iron peak elements are needed to synthesise all A>60 heavy elements, enough material exists
technetium is in stellar atmospheres, which has half life 10e6 yrs, less than a stars lifetime, so must have been produced in star
Neutron capture processes
S-process
neutron capture time much larger than beta decay lifetime
energy range of neutron 1-300keV
synthesised elements closely follow line of beta stability
timescales vary, minutes-10e6 years
r-process
capture times 10e-3 to 10e-4 s, shorter than beta decay lifetimes
synthesised elements lie away from the line of beta stability
Chart of nuclides observations
some nuclei can be produce by s and r processes
general those with few neutrons can only be produced by the s-procces
all heavier than lead or bismuth and with largest neutron excess produced by r-process
few cannot be produced by either, bypassed nuclei produced by proton capture maybe. 30 of these, much less abundant
(p,γ) and (p,n) reactions could occur during explosive burning
(γ,n) reaction at temps about 10e9K sufficient energy to eject neutrons from nuclei
inverse beta decay at high T, results in e+ e_ pairs. e+ capture by a nucelus can produce proton rich species
odd-even effect due to nuclear structure, fewer stable nuclides with odd Z and/or odd N
s-process
Neutrons produced during He burning phase on horizontal branch
see equations
inf nuclei are in a neutron flux then trhe (n,γ) capture process produces isotopes of progressively larger A
If an unstable nucleus madee then beta decay happens before neutron capture continuing
nuclides only produce by s-process are expected to have some relationship with neutron capture xsection with larger xsections leading to larger abundance
maths
boundary conditions
Starting material assumed to be Fe-56
Natural termination as Bi-209 is most massive (quasi) stable nucleus
neutron capture by bismuth leads to nucleus which decays to lead
system is self regulating
called local approximation. Does not work where cross section particularly low e.g. magic number region
neutron xsections inversly proportional to neutron velocity v
maths: