PHYS Lecture 2 Neutrinos, Helium burning, reac=on rates. Dr Rob WiNenmyer. Office: Old Main 130, in Astrophysics area.

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1 PHYS 3160 Lecture 2 Neutrinos, Helium burning, reac=on rates Textbook reference: pp , 90-91, , Dr Rob WiNenmyer Office: Old Main 130, in Astrophysics area rob@unsw.edu.au hnp://phys.unsw.edu.au/~rob

2 They Might Be Giants The Sun is a mass of incandescent gas

3 Last =me in Physics: Nuclear Energy Release Binding energy per nucleon Peaks at 56 Fe Le^: Fusion! Right: Fission! Proton- proton chain CN cycle

4 Neutrinos: how do they work? In the first step of PPI, a proton is converted into a neutron, resul=ng in a deuteron (p+n). This spits out a positron, which gets zapped by an electron to produce photons. And we get a neutrino (ν)! Neutrino ( linle neutral one ) was proposed as a par=cle with no mass, charge, or magne=c moment to conserve energy and momentum in β- decay. Free neutrons are unstable and decay into a proton, electron, and an=neutrino.

5 Neutrinos: how do they work? Since they have no charge, (apparently) no mass, and no electromagne=c proper=es, how do you ever detect a neutrino????? A stream of neutrinos from the Sun is passing through you (and the Earth) right now. Since they preny well do not interact with anything, if we could observe neutrinos from the Sun, we would have a probe of the condi=ons at the centre of the Sun right now.* [This is because photons take ~10 5 years to escape!! More on that when we discuss opacity] * Insufferable Physics Pedant says: False. Eight minutes ago since neutrinos travel at the speed of light.

6 Neutrino detectors Find 10 parsecs of lead. Neutrino detectors rely on the footprints created when neutrinos interact with other things. Example: Get a big tank of ultra pure heavy water (D 2 O). Very rarely, a neutrino will collide with a deuteron, releasing protons and an electron. That electron briefly travels faster than the speed of light in water, resul=ng in a boom called Cerenkov radia=on. This can be detected with millions of sensi=ve photomul=plier tubes.

7 Neutrino detectors Another type of detector uses a big tank of dry- cleaning fluid. Neutrinos hit Chlorine atoms, producing unstable Argon. (Happens rarely! Need a big tank) The Argon then decays back to 37 Cl and a positron. You detect the positrons (when they annihilate an electron). Less than one neutrino detected per day on average!

8 We re gonna need a bigger detector

9 Some are more equal than others Neutrinos come in different energies detec=on probability goes as (energy) 2 Early experiments could detect only the highest- energy neutrinos. These come from the decay of 8 B in the PPIII reac=on. Recall that only happens 0.3% of the =me in PP chain.

10 We ll need a bener detector That Nature paper Michael was so excited about Problem: The most abundant solar neutrinos are the lowest- energy. So previous types of detectors could not see them as they were overrun by background radia=on.

11 A neat paper about neutrinos hnp://mcba11.phys.unsw.edu.au/~mcba/phys3160/n.pdf

12 We re gonna need a bener detector That Nature paper Michael was so excited about Borexino detector: Went to great lengths to block background radia=on. e.g. Radon, Rubidium, Potassium- 40, Carbon- 14 Opera=ng since 2007 Detec=on threshold ~ 50 kev. Max energy of pp chain neutrinos: 264 kev

13 We re gonna need a bener detector That Nature paper Michael was so excited about Model Predic=on: 131±2 counts per day per 100t. Our models of the Sun s energy genera=on are right! I am not lying to you!

14 The solar neutrino problem The high energy neutrinos we could detect at first come from reac=ons which are very sensi=ve to temperature (~T 18 ). So the neutrino count is a good test of our theories on condi=ons in the centre of the Sun. Result: Only 1/3 of the expected number of neutrinos were found! So something is wrong with the experiment, the model, or the Sun. [Some science fic=on stories are based on the lack of solar neutrinos foretelling some disaster]

15 The solar neutrino problem Standard Model: 3 types of neutrinos. Only electron neutrinos (ν e ) are produced in the Sun. Our experiments can only detect ν e. What if the Sun s neutrinos were oscilla=ng on their way here? We would then only detect the 1/3 of them that arrived as ν e!

16 The solar neutrino problem: sorted The Super- Kamiokande experiment in 1998 showed that neutrinos indeed DO oscillate! So there is nothing wrong with the Sun or our understanding of the reac=ons in it. It DOES mean there is something wrong with the Standard Model, as it required neutrinos to be completely massless. Since this is not a par=cle physics course, the details are beyond our scope but a good summary can be found here: hnp://ctp.berkeley.edu/neutrino/neutrino.html Meanwhile, you can sleep at night knowing the Sun will not explode.

17 Helium burning reac=ons One day, some day, hydrogen is no longer available for fusion in a stellar core. What happens next? Recall equa=ons of stellar structure. A star s life is forever a fight against gravity. Once core H- burning stops, gravity wins (for a bit). The core is compressed un=l the temperature rises enough to permit the next important reac=ons. We have all this 4 He running around. GoNa do something with it. The triple- alpha process Note: 4 He nuclei also known as alpha par1cles.

18 Helium burning reac=ons Two 4 He form 8 Be, which is very unstable and usually fissions back to helium. But some=mes, a third 4 He slams into it and we get 12 C. We skip right over Li, Be, B and go straight to carbon. Resonant reac=on if not, we would get a fourth 4 He to make 16 O instead. This is a Good Thing, since carbon is so important for life! The triple- alpha process Note: 4 He nuclei also known as alpha par1cles.

19 Temperature dependence Energy release is propor=onal to density, the abundance of the reactants, and temperature. We can approximate the dependence as a power law (see dashed lines in figure). For triple- alpha process, this dependence goes as density squared since this is a 3- body reac=on. These power- law approxima=ons are good enough to help us solve the equa=ons of stellar structure.

20 Temperature dependence Things to note: T at which each process occurs Reac=on rates (energy produc=on) very sensi=ve to temperature! Processes with heavier reactants are ever more T dependent. This is because ever- higher kine=c energies are needed to overcome Coulomb barrier for larger nuclei (more posi=vely charged).

Solar Energy Production

Solar Energy Production We re now ready to address the very important question: What makes the Sun shine? Why is this such an important topic in astronomy? As humans, we see in the visible part of the