My research focuses on the field of Experimental Nuclear Astrophysics. I study the structure of exotic nuclei that participate in different astrophysical processes in an attempt to better understand the synthesis of the elements we see around us. In addition, I study nuclear reactions that take place in stellar environments. Some of these reactions can be measured directly with the facilities and equipment we have available today. Others, however, are impossible to measure directly. For these "elusive" reactions, I try to find indirect ways to extract the necessary information by studying basic nuclear properties.
The elements that we observe today on earth (and the Universe) were all created inside stars through different types of nuclear reactions. Starting with hydrogen and helium, the light elements are produced by reaction cycles that burn the existing fuel and slowly build the heavier nuclei up to the region of iron. Beyond iron, most elements are created through two main processes (the s- and r-process), which involve neutron-induced reactions along with β-decays. There is also a small group of proton-rich nuclei, called “p-nuclei”, which cannot be created by these neutron-processes, but rather via a different process called “p process”.
There are several open questions concerning the synthesis of the heavy elements. My focus as an experimentalist is to study the nuclear reactions involved in these astrophysical processes and also the structure of the participating nuclei. For this purpose, my group developed the SuN detector - a total absorption gamma-ray spectrometer that is used for very efficiently detecting the gamma rays emitted from nuclear reactions or from β-decay processes.
My group performs experiments in all three of the experimental areas of the Laboratory:
1) We measure β decays with fast beams, where the beam is implanted in a charge-particle detector at the center of SuN.
2) For nuclei with longer half-lives we can study beta-decays in the “stopped” beam area, where we use a tape transport system (SuNTAN) for removing any radioactive decay products.
3) We use re-accelerated rare isotope beams from the ReA3 facility to measure nuclear reactions at energies that correspond to the temperature of the astrophysical environment.
SuN: Summing NaI
SuN is a γ-ray detector. It is made of NaI(Tl) scintillator material which has a very high efficiency in detecting γ-rays. The source of radiation can be placed inside the borehole of SuN, at its geometric center, and in this case the solid angle coverage is about 98%. SuN is the main instrument my group uses for experiments at NSCL. We use it to detect γ rays coming from our studies of astrophysical reactions or from the β decay of exotic radioactive nuclei.
One of the main sources of background in some experiments with SuN is cosmic ray radiation. Shielding against cosmic rays is not very practical (some experiments go deep underground for this exact reason). We can, however, remove some of this background by using an "active shield", namely a cosmic ray veto detector. This is the purpose of SuNSCREEN. It is made of nine long plastic scintillator bars that cover SuN in a roof-like structure. We use the signal from SuNSCREEN to reject any events where both detectors recorded a signal and this way remove 50-70% of the cosmic-ray background. Sunscreen was built by our collaborators at Hope College.
SuNTAN - Tape transport system
In β-decay experiments a radioactive beam is implanted at the center of SuN. Often, the daughter nucleus is also unstable and can be a source of background in the measurement. For this reason, a tape transport system is needed. The initial (mother) nucleus is implanted on a tape, and after waiting for a reasonable amount of time (depending on the nucleus half-life) the tape is rotated so that the daughter decays will take place away from SuN. The SuN tape transport system was funded by my NSF-CAREER award.