The nature of nuclei and nuclear forces

The chart of nuclei near Carbon. The arrows indicate charge-exchange experiments performed by our group. The experiment on unstable 12B using the (7Li,7Be) reaction was aimed at studying the shell evolution in 12Be.

One of the main goals of fundamental nuclear physics research is to understand what forces bind nucleons into nuclei. Many different types of experiments are performed to learn about different aspects of those forces. Experiments on rare isotopes are particularly helpful, as it becomes possible to isolate particular features of the nuclear forces that are not easily studied in stable nuclei.

Charge-exchange reactions are very useful for studying the structure of nuclei because particular components of the response function, that are not easily accessible through other means, can be selectively probed. Of particular interest for such studies are so-called Gamow-Teller transitions, for which information can be extracted from the data in a model-independent manner. Such data are very important for testing state-of-the-art theoretical models of nuclei and nuclear forces. In addition, these data have very important applications, for example in astrophysics and neutrino physics.

Electron-capture rates in supernovae

Comparison of the electron-capture rates on zinc-64 in the stellar environment, based on transition strengths from a 64Zn(t,3He) charge-exchange experiment and from a shell-model calculation with the GXPF1a interaction.

To understand and simulate the late stages of a massive star prior to its demise as a supernovae, electron-capture rates on many stable and unstable nuclei must be known accurately. Although the electron capture reactions are mediated by the weak nuclear force, the relevant nuclear structure information (the above-mentioned Gamow-Teller strengths) can be extracted from charge-exchange experiments. Consequently, charge-exchange reactions have become the preferred tool for testing theoretical nuclear structure models used to estimate the electron-capture rates of relevance for stellar evolution.

At NSCL, the (t,3He) reaction is used to acquire data on stable nuclei - probes used at other facilities include e.g. the (n,p) and (d,2He) reactions. For unstable nuclei, new experimental techniques are developed, such as the (p,n) and (7Li,7Be) reactions in inverse kinematics.

Double beta decay

The two types of double beta decay: the mode in which 2 neutrinos are emitted (2νββ left), and the exotic neutrinoless mode (0νββ right).

Neutrinoless double beta decay is a yet-to-be observed nuclear decay mode. If discovered, it would indicate that the neutrino, unlike all other constituents of matter, is its own anti-particle, a so-called Majorana particle and provide a way to determine the absolute neutrino mass scale. Several large-scale experiments are being prepared to search for this exotic decay mode. Data obtained from charge-exchange experiments are very useful to test theoretical models for the decay probability (and thus impact the design of the detection systems) and critical to extract information on the Majorana neutrino mass, if neutrinoless double beta decay is discovered. Our group has focused on the case of 150Nd, which is the focus of the discovery experiment at SNO+ (Sudbury Neutrino Observatory).

Giant Resonances

Schematic animation of the isovector giant monopole resonance, in which the neutron and proton fluids in the nucleus oscillate isotropically out of phase (figure credit A. Krasznahorkay (ATOMKI))

In a macroscopic picture, giant resonances are oscillations of the nuclear "fluid". A variety of resonance modes are known to exists and their parameters can provide us valuable information about the bulk properties of nuclear matter. Charge-exchange reactions provide an excellent tool to study so-called isovector giant resonances, in which the neutrons and proton fluids oscillate out-of-phase (in contrast to isoscalar modes, where neutrons and protons oscillate in phase). In addition, it is important to study giant resonances in unstable nuclei as properties of nuclear matter are expected to change in neutron-rich or deficient material. Such studies have important applications, for example in astrophysics, and in particular for better understanding the nature of neutron stars. We use a variety of reactions to excite isovector giant resonances.