Nuclear Astrophysics tries to answer one simple question: what is the origin of the elements in the universe?
Alan Chen gave us a general introduction to the topic and then went on to tell us about the distribution of 26Al sources in the milky way.
26Al emits a beta-delayed gamma transition at 1.8 MeV that has been used to characterize stellar evolution in our galaxy (as we heard in a couple other talks as well). 26Al has a short-lived isomer of ~6sec but the decay of its ground state goes to the 1st excited state of 26Mg, (therefore, the gamma lines comes from here).Some important reactions to study in the laboratory are:
26Al(p,g)27Si
25Al(p,g)26Si
What one wants is to determine reaction rates from cross section measurements. In a lot of cases, the reaction rate is dominated by resonant reactions in the compound nucleus, the rate goes as T3/2 (ωγ) exp (-ER/T). Alan described an experiment performed at TRIUMF-ISAC (located in Vancouver, Canada). The goal: determine (p,g) strength of the 188 keV resonance. The technique: inverse kinematics. Experimental details: Beam energy ~ 200 keV/u. 10^9 ions/ sec, Target: silicon carbide (since mass should be comparable to that of the beam). Challenge: build a gas target of H2 without windows, then measure yield of 27Si recoils with DRAGON recoil separator in coincidence with prompt gammas.
The DRAGON recoil separator at TRIMF was designed to perform (p,γ) and (α,γ) reactions relevant for nuclear astrophysics. From the experiment described they measured ωγ= 35 ± 7 μeV!!! Supersensitive measurement!
Future experiment: Direct 25Al(p,g)26Si in order to determine energy levels of 26Si – this could be done indirectly with in-beam gamma ray spectrometry at NSCL p(27Si, 26S)d.
In summary: a sample of completed as well as planned experiments aimed at improving our understanding of stellar explosions were presented. This field benefits from interactions with astronomers and theorists/modelers. Future rare isotope facilities promise a bright future: FAIR, FRIB, RIKEN, TRIUMF, etc.
the speaker, here.
ReplyDeletewow, i'm really impressed - the blogger even got the reaction rate formula down! either (1) the speaker was extraordinarily clear, or (2) the blogger is a bright student who was paying very close attention to the lecture. most likely (2)...