Zach Meisel, PAN 2016

Nuclear Astrophysics Zach Meisel, PAN 2016

Nuclear Astrophysics is the study of: Energy generation in stars and stellar explosions Extremely dense matter The origin of the elements

If the sun were powered by chemical energy, it would last: 6 10 56 nnnnnnnnnnnn 1eeee/nnnnnnnnnnnnnn 1 10 6 MMMMMM/eeee 1 2 10 39 MMMMMM/ss 3 10 11 seconds a.k.a. 9,500 years.much too short! Nuclear reactions provide the power for stars This was known as of ~1920 due to the sun s age: - Chemical bond energy: ~ev - Nuclear bond energy: ~MeV - Solar Energy release: ~2e39 MeV/s - Nuclei in the sun: ~6e56

What drives fusion in stars? Binding Energy H Fe E=mc 2 Pb C C Fe Pb H

What about stuff heavier than iron? Fe C Pb? neutron capture fusion H

Nuclear reactions in stars transmute elements Reactants (incoming stuff) Products (outgoing stuff) Energy

For instance, our sun makes helium from hydrogen

Everything around you was processed in a star

Everything around you was processed in a star Regular stars made ~1/2 of the stuff around you Exploding stars made most of the rest Most stars eventually become planetary nebulae Some stars explode as supernovae The gas released formed everything around you

Periodic Table by abundance Nuclear astrophysics aims to understand why some elements are more abundant than others lots of oxygen not much gold

Nuclear astrophysics aims to understand why some elements are more abundant than others Big Bang Fusion Magic numbers Neutron-capture Fe-peak N=50 r-process N=82 N=126 s-process

Detour: The valley of beta stability C Fe Pb H

Detour: The valley of beta stability

Unstable nuclei decay back to the valley of stability Z N

Nucleosynthesis can proceed through exotic nuclei s -process r -process

How fast are slow and rapid neutron captures? Slow or rapid neutron capture is with respect to a nucleus s β-decay half-life. Example paths for neutron-capture, starting with 150 Sm (a) ττ nn cccccc 500yyyyyy (b) ττ nn cccccc 1wwwwwwww Z Z N N

Neutron-capture time-scale: s -process Time-scale for slow neutron capture ~10 8 neutrons in a box of 1cm 3, each moving from thermal velocity ~1/6 leave a single box side with a velocity given by: E~kT ~mv 2 v= kkkk mm 25kkkkkk 10 27 kkkk 108 cm/sec 1/6 of the neutrons leave from a cube face, with 1cm 2 area every 10-8 seconds neutron flux ~10 16 neutrons/cm 2 /sec neutron-capture cross-section (at ~25keV): ~100mb = 10-25 cm 2 neutron-capture rate = (flux)x(cross-section) 10 16 neutrons/cm 2 /sec x 10-25 cm 2 = 10-9 /sec neutron-capture time = 1/Rate 10 9 seconds ~ 1 century

Neutron-capture time-scale: r -process Time-scale for rapid neutron capture ~10 22 neutrons in a box of 1cm 3, each moving from thermal velocity ~1/6 leave a single box side with a velocity given by: E~kT ~mv 2 v= kkkk mm 1000kkkkkk 10 27 kkkk 108 cm/sec 1/6 of the neutrons leave from a cube face, with 1cm 2 area every 10-8 seconds neutron flux ~10 30 neutrons/cm 2 /sec neutron-capture cross-section (at ~1000keV): ~1mb = 10-27 cm 2 neutron-capture rate = (flux)x(cross-section) 10 30 neutrons/cm 2 /sec x 10-27 cm 2 = 10-3 /sec neutron-capture time = 1/Rate ~ 1 millisecond

EXERCISE: Map the s-process and r-process paths on the chart of nuclides, starting at 56 Fe

slow (s) and rapid (r) neutron captures make heavy nuclei

slow neutron capture happens in AGB stars

rapid neutron capture may happen in supernovae

or neutron star mergers

So what do nuclear astrophysicists actually do? Astrophysics Nuclear physics theory experiments Nuclear physics theory Astronomical observations Nuclear Astrophysics

Nuclear Physics Experiments Measure properties of nuclei, e.g. masses half-lives reaction cross sections nuclear structure fission products

Nuclear Physics Theory Calculate properties of nuclei, e.g. masses half-lives reaction cross sections nuclear structure fission products

Astrophysics Theory Simulate astrophysical environments, e.g. stellar burning supernovae novae x-ray bursts neutron star mergers

Astronomical Observations Observe astrophysical environments, e.g. old stars supernovae novae x-ray bursts cooling neutron stars

Summary Nuclear astrophysics is the study of the origin of the elements, extremely dense matter, and cosmic nuclear energy generation.

We are star stuff

For a concise summary of nuclear physics with a focus on nuclear astrophysics, see The valley of stability by Matière vidéos on YouTube: https://www.youtube.com/watch?v=utop_2zvzmm