Charge-exchange probes

The (t,3He) charge-exchange reaction, mediated by the strong nuclear force can be used to extract information about reactions mediated by the weak nuclear force (beta decay and electron capture) because of a simple proportionality between the measured reaction cross sections and the weak transition strength [B(GT)]. Such proportionalities also hold for other charge-exchange probes, such as the (p,n) and (7Li,7Be) reactions.

A wide variety of charge-exchange reactions can be used to study the properties of nuclei and the choice of the probe for a particular experiment depends on several factors. Different probes have different sensitivities - for example the (7Li,7Be) reaction, in combination with gamma detection, allows for the isolation of events in which spin is transferred between the projectile and the target. All experiments carried out by the group are performed at about 100 MeV per nucleon, or above. At these relatively high beam energies, the reaction mechanism is relatively simple. An important consequence is that information about weak transition strengths can be extracted because of a proportionality between the measured charge-exchange reaction cross sections (chance of a certain transition taking place) and those strength (see left).

Although most experiments by the group are performed at NSCL, we also take data and participate in experiments at other facilities, such as the Research Center for Nuclear Physics in Osaka, Japan and the RI Beam Factory in Saitama, Japan. In addition, we collaborate with researchers from a wide variety of institutions in and outside of the US on experiments, but also on the development of theoretical tools.

NSCL Coupled Cyclotron Facility

The NSCL Coupled Cyclotron facility consists of 2 cyclotrons (the K500 and K1200) in which beams of stable ions are accelerated to about 40% of the speed of light. Such beams (for example 58Ni as shown in the figure) are impinged on a production target, in which a variety of isotopes are produced. The cocktail of isotopes is purified to one or a few isotopes in the A1900 fragment separator (e.g. 56Ni and 55Co) and then transported to the reaction target (e.g. a hydrogen target). Reaction products are analyzed in the S800 magnetic spectrometer and other detectors, such as the Low-Energy Neutron Detector (LENDA).

Detection systems

The charge-exchange group uses a variety of detection systems in experiments. The S800 magnetic spectrometer (left) is used in all experiments to determine with high precision fast charged reaction fragments. Other detection systems are placed around the target, such as the GRETINA high resolution gamma-ray detector (middle) which is used to detect gamma-rays emitted from excited nuclei with unprecedented precision. To perform charge-exchange experiments in with rare isotope beams, new detectors must be developed. The Low-Energy Neutron Detector Array (LENDA, right) was constructed for use in (p,n) experiments in inverse kinematics and successfully commissioned in a 56Ni(p,n) experiment in inverse kinematics.