Nucleosynthesis II

Question 1

why can neutron capture build heavier elements under much less extreme conditions than needed for Si burning?

Answer 1

neutrons have no electric charge and are not repulsed by nuclei, therefore less energy needed to add a neutron in than protons/electrons

Question 2

how do neutron densities (fluxes) differ between the r-process and s-process. what are the implications of this?

Answer 2

r-process, neutron densities (fluxes) are high and successive neutron captures happen more rapidly than Beta decay of the newly formed unstable nuclei

s-process, neutron densities (fluxes) are low, so neutron captures occur more slowly (every few weeks to months) , than the rate of beta decay

r-process > beta decay
s-process < beta decay
timing

Question 3

how are the very high neutron densities produced for the r-process?

Answer 3

short, but very high neutron fluxes, are released during neutron star-neutron star or neutron star-black hole mergers.

Question 4

where are r-process nuclides positioned relative to the valley of stability?

Answer 4

to the far right (more neutrons than protons)

Question 5

what are r-only nuclei?

Answer 5

neutron-rich isotopes which can only be produced by the r-process

Question 6

what are s-only nuclei?

Answer 6

isobars on the proton rich side of the nuclide chart, which are shielded from the r-process

Question 7

the r-processes prdouces maxima in elemental abundances that are related, but slightly shifted to lower valyes of neutrons, to which numbers?

Answer 7

the magic numbers

Question 8

what are neutron drip lines? what do nuclides on the drip lines do?

Answer 8

the limit at which any additional neutron will not be bound. nuclei along the drip lines capture no further neutrons (but beta decay) as they have low neutron capture cross sections. this produces a more stable nuclide which is slightly deficient in neutrons compared to the magic numbers.

Question 9

nuclei with magic numbers of 50, 82 and 126 neutrons along the neutron drip lines have (higher or lower) neutron capture cross sections comparef to nuclei with similar masses with non-magic neutron numbers?

Answer 9

magic neutron number nuclides have much (x100) lower neutron capture cross sections, therefore they have low probabilities of capturing a free neutron and therefore decay before absorbing a further neutron.

Question 10

why are elements beyond 209Bi only formed by the r-process?

Answer 10

intermediate nuclides, between Bi and Th are radioactive with relatively short half lives and so decay before another neutron can be added via the s-process.

Question 11

what is the weak r-process? which elements does it produce?

Answer 11

the lighter trans-Fe elements Ga to Rb (slightly heavier than Fe, which has 26 protons). driven by the neutrino winds in the matter which is ejected from ccSN from massive stars

Question 12

below what solar mass does the iron catastrophe not occur?

Answer 12

stars of less than 8 to 10solar masses

Question 13

how are free neutrons produced in AGB stars?

Answer 13

by episodic mixing of material from the H-rich envelope with the He-burning shell. this leads to reactions that convert 12C to 16O and release free neutrons

Question 14

why is the s-process a secondary nucleosynthesis process?

Answer 14

it requires ‘seed’ nuclei of intermediate to heavier elements. the s-process seeds then capture more neutrons and decay via beta decay transforming into heavier, neutron rich isotopes.

Question 15

why does the s-process not occur in first generation stars (H and He)?

Answer 15

first generation stars consist of only H and He. the s-process requires Fe/Ni seed nuclei to capture more neutrons, which are only produced in second/later generations of stars.

Question 16

what are the two s-process components?

Answer 16

the main s-process and the weak s-process

Question 17

what are the differences between the main s-process and the weak s-process?
1. where they take place
2. which elements they form

Answer 17

the main s-process occurs in low to intermediate mass AGB stars. the weak s-process occurs during core He burning in massive stars and is hence dispersed by ccSN.

main s-process -> produces 1/2 element budget of Sr to Bi
weak s-process produces lighter elements Ga - Sr

Question 18

where are most of the unstable nuclides on the s-process situated on the valley of stability

Answer 18

neutron rich side, RHS/BELOW, decay via beta decay

Question 19

why does the s-process produce only isotopes close to the valley of stability?

Answer 19

the rate of neutron flux in the s-process is such that the nuclides cannot capture another neutron before decay. therefore the s-process can only move along a path through the most stable isotopes of heavier elements.

Question 20

what is the last element produced by the s-process? why?

Answer 20

the last element produced is Pb, which then decays to 210Bi. Pb is the last element because there are no stable or long-lived radioactive nuclei between 210Bi and 232Th.

Question 21

do s-process nuclides with magic neutron numbers have low or high neutron capture cross sections? what are the consequences of this?

Answer 21

low neutron capture cross sections. this means magic neutron number s-process nuclides are associated with peaks in nuclide and elemental abundances e.g. 128Ba isotop with an atomic abundance of 72%