“We are made of star stuff.” – Carl Sagan. The quote isn’t some fantastic claim made by a science fiction writer. A lot of the “stuff” found on earth is created in stars. Two astronomical phenomena that are thought to be responsible for the creation of elements such as iron and heavier elements are core-collapse and thermonuclear supernovae. In the former type, a massive star collapses after it has burned its nuclear fuel through fusion and can no longer withstand the gravitational forces. Eventually, it ejects material into space after the density has increased so much that a powerful shockwave destroys the star. The latter kind is thought to involve a white dwarf star that slowly accretes matter from a nearby companion star, which can ignite and explode due the increased pressure, density and temperature in the star.
Many questions remain about how exactly these gigantic explosions take place, but they have one thing in common – a particular type of nuclear reaction called “electron-capture” plays an important part in both.
An experiment recently conducted by an international team of researchers led by the charge-exchange group at NSCL is helping to answer some of the questions about electron capture. The experiment uses a new technique to extract information about nuclear reactions involving the unstable isotopes that are critical for performing accurate simulations of the supernovae. The technique involves impinging a beam of unstable isotopes of the kind that are thought to exist in pre-supernova stars on a target of liquid hydrogen and measuring recoiling neutrons.
In the first experiment, a beam of unstable Nickel-56 isotopes was used and the researchers determined the probability of the Gamow-Teller transitions occurring from Nickel-56 to Copper-56. The information about these Gamow-Teller transitions can be used to estimate the chance that electrons in the stellar plasma are captured by Nickel-56 ions. The new data for Nickel-56 are not only important because this isotope is abundant in supernovae stars, but also because it is key to unlocking better methods to use theory to estimate the electron-capture rates on many more unstable nuclei in the iron region.
The Nickel-56 (p,n) experiment ran in October of 2010 at NSCL. It involved the use of the S800 spectrograph and two new devices—the Low-Energy Neutron Detector Array (LENDA) and a Liquid Hydrogen Target. LENDA was developed at NSCL specifically for measuring low-energy neutrons (150 keV-10 MeV) that recoil from the Hydrogen target in a (p,n) charge-exchange reaction. The construction of the Liquid Hydrogen target, led by Ursinus College under an NSF MRI grant, was important for the experiment as its use increased the counting rate and strongly reduced the background compared to running the experiment with a CH2 (plastic) target. Additionally, a thin, in-beam diamond detector was used to provide a high-resolution timing reference for neutron time-of-flight timing. The Nickel-56 beam (at 110 MeV/u) was produced by fast-fragmentation of a 160 MeV/u Nickel-58 primary beam. The A1900 fragment separator was used to purify the beam. The S800 served to detect heavy fragments produced in the Nickel-56 (p,n) reaction and also provided the trigger for LENDA.
With the development of the new technique to measure charge-exchange reactions on unstable isotopes, the road is now open to extend such studies to regions of the nuclear chart previously unreachable and up to high excitation energies. Since a large fraction of the isotopes prevalent in stars prior to their explosion are unstable and many of the signals that nuclear physicists are keen to study are enhanced in unstable nuclei, the successful experiment at NSCL leads the way for many new discoveries.
The results were published in Physical Review Letters (http://prl.aps.org/abstract/PRL/v107/i20/e202501) as an Editors’ Suggestion. It was also chosen for its importance to the field for a Viewpoint by one of the referees (http://physics.aps.org/articles/pdf/10.1103/Physics.4.91). This work was supported by the US NSF (Grants No. PHY-0822648 (JINA), No. PHY-0606007, No. PHY-0758099, PHY-0922615 and No. PHY-1068217), the US DOE (Grant No. DE-FG02-94ER40848), and the Research Corporation of Science Advancement.