Division of Electronics, Detectors and Computing for Physics (DEDIP)

A predicted N=70 subshell closure in ¹¹⁰Zr has long been considered a potential explanation for the disagreement between theory and observed abundances of the heavy elements near mass 130 resulting from the rapid neutron capture process (r-process). The r-process is thought to be responsible for approximately 50% of the production of heavy elements in the universe but is not well understood, due in part to the uncertainties of the neutron-rich structure evolution mechanisms. The subshell closure in ¹¹⁰Zr, though unlikely, could result from either from a tetrahedral or spherical symmetry in this nucleus due to a weakening of the N=82 shell closure. Each of these symmetries leaves distinct fingerprints on the low lying level structure, and thus their presence can be discerned with a simple spectroscopy measurement.

The MINOS system has been used in combination with the high efficiency DALI2 NaI scintillator array at the Radioactive Isotope Beam Factory of RIKEN in Japan, to perform the first spectroscopy of ¹¹⁰Zr.
MINOS, developed at CEA-IRFU, combines a thick proton target with a vertex tracker to allow high resolution spectroscopy of rare exotic nuclei. Low-lying states in ¹¹⁰Zr were populated via proton removal on the MINOS target, and deexcitation gamma rays detected with the DALI2 array. Results are consistent with a well deformed nucleus, beyond even what was predicted from mean-field models. No evidence is found for a subshell closure at N=70 nor tetrahedral symmetry.

A complementary study explores the systematics of inclusive cross sections for (p,2p) and (p,pn) reactions with neutron rich nuclei. These reactions are a tool of choice for populating the low-lying states in exotic nuclei, but are poorly benchmarked far from stability. Here we have measured approximately 60 inclusive cross sections for single nucleon removal on the MINOS target. The (p,2p) systematics reveal an enhanced cross section to odd-Z daughter nuclei, pointing to the importance of pairing correlations.The (p,pn) cross sections are approximately a factor of 10 larger than the (p,2p) cross sections, and are randomly distributed around 60 mb. Comparison with semi-classical INCL and Fragmentation-Evaporation models suggest that both these systematics can be explained by the important role of pairing correlations. This work highlights this effect in direct reaction systematics for exotic nuclei.

Finally, in view of a future application of a MINOS-like system as a pion tracker for antiproton-nucleus collisions, in a new physics program at CERN called PUMA, electrostatic simulations were performed to optimize the electric field cage of the MINOS time projection chamber.