SuNTAN – Removing unwanted radioactivity

The majority of radioactive nuclei undergo beta-decay, a process in which a neutron is transformed into a proton, or vice versa. The properties that govern this process, e.g, the decay half-life and probability, depend on the internal structure of the parent and daughter isotopes involved. Thus, measurements of beta-decaying nuclei provide valuable information about the structure of a nucleus, for example its shape. Radioactive nuclei far from stability not only exhibit novel structures and significant deformations, but are also critical in stellar reactions. These nuclei are challenging to study, often because of high levels of background from radioactive beams. At the NSCL, isotopically pure radioactive beams can be produced, and measurements of such isotopes can be performed by simply implanting the beam on a catcher foil. Then, beta or gamma spectroscopy measurements can be made on these primary ions, such as with the Summing Na(I) (SuN) detector. However, often the beta-decay of the implanted ions is followed by subsequent decays that are unwanted. The gamma-rays from the beta-decaying daughters and granddaughters are a significant source of background radiation, and need to be removed from the implantation point. For this reason, the SuN Tape transport of Active Nuclei (SuNTAN) has been developed at the NSCL.

SuNTAN consists of a movable metallic tape and a beta-sensitive, 1 mm thick, annular plastic fiber detector. Low-energy beams will be implanted directly on the tape, at the center of SuN. After a period of time determined by the beta-decay half-life of the primary ion, the tape is quickly moved away from the target, removing the daughter isotopes, and preventing contaminations from being recorded in SuN. SuNTAN was designed and fabricated at MSU in a collaboration with Argonne National Lab and Louisiana State University, while the fiber detector was developed at Hope College. The system is currently being tested and optimized offline. SuNTAN will be used in upcoming experiments, including two approved experiments in the mass = 90 region