A New Look Inside Ancient Stars


TRIUMF has long been addressing big questions about the origins of matter in our universe by studying the interactions among elementary particles or essential nuclei.  The DRAGON experiment at TRIUMF is an apparatus designed to measure the rates of nuclear reactions that are important in astrophysics and the formation of the chemical elements. The big question we are asking is, "Where do the elements around us come from?" and "What happens inside a supernova and what does it produce?"  One new experiments at TRIUMF, S1227, recently looked at a process that creates lithium and neutrinos within ancient stars.

Researchers working at DRAGON have successfully measured the rate of the 3He + 4He -> 7Be + γ radiative capture reaction, an important reaction in various areas of nuclear astrophysics.  The 7Li observed in ancient stars was created via the radioactive decay of 7Be nuclei formed in the 3He + 4He -> 7Be + γ reaction just minutes after the big bang.  For this reason, measurements of the reaction rate are an important step in resolving the discrepancy between the big bang nucleosynthesis prediction of 7Li abundances and astronomical observations.  In addition, 7Be decay is one process by which stars produce neutrinos.  This process even occurs within our own Sun, so the 3He + 4He ->7Be + γ measurement will lead to a better understanding of the solar neutrinos reaching us on Earth.

TRIUMF experiment S1227 was performed from September 8 to September 14, 2011.  The measurement was done using the DRAGON recoil separator, during which a 4He beam bombarded a 3He gas target.  The 7Be recoils were then separated from the incoming beam particles using DRAGON's world-record beam suppression capabilities and subsequently detected at the focal plane using a silicon detector sensitive to the energy and position of the incident particle.  The 3He + 4He -> 7Be + γ reaction rate was measured at 3 different relative energies (Erel = 1.5 MeV, 2.2 MeV and 2.8 MeV) and a total of approximately 100,000 7Be recoils were collected, a number never reached in previous DRAGON runs.  The completion of the experiment at these energies will add another reaction rate measurement in an energy range where previously only two discrepant measurements existed.

Another first for DRAGON was the use of pure 3He gas in the target chamber, a gas whose use in US homeland security applications has made it both difficult and expensive to obtain.  A complex 3He recycling system was created prior to the September run and successfully operated during the 3He + 4He -> 7Be + γ reaction rate measurement.  It allowed us to retain ~5/6 of our $15 000 3He inventory, leaving a large portion of 3He gas available for future measurements at DRAGON.  In completing this 3He + 4He -> 7Be + γ reaction rate measurement, DRAGON and its collaborators continue to stay on the forefront of measurements important in nuclear astrophysics.


-- by Sarah Reeve, SFU MSc Student