Development and operation of an electrostatic time-of-flight detector for the Rare RI storage Ring

An electrostatic time-of-flight detector named E-MCP has been developed for quick diagnostics of circulating beam and timing measurement in mass spectrometry at the Rare-RI Ring in RIKEN. The E-MCP detector consists of a conversion foil, potential grids, and a microchannel plate. Secondary electrons are released from the surface of the foil when a heavy ion hits it. The electrons are accelerated and deflected by 90$^\circ$ toward the microchannel plate by electrostatic potentials. A thin carbon foil and a thin aluminum-coated mylar foil were used as conversion foils. We obtained time resolutions of 69(1) ps and 43(1) ps (standard deviation) for a $^{84}$Kr beam at an energy of 170 MeV/u when using the carbon and the aluminum-coated mylar foils, respectively. A detection efficiency of approximately 90% was obtained for both foils. The E-MCP detector equipped with the carbon foil was installed inside the Rare-RI Ring to confirm particle circulation within a demonstration experiment on mass measurements of nuclei around $^{78}$Ge produced by in-flight fission of uranium beam at the RI Beam Factory in RIKEN. Periodic time signals from circulating ions were clearly observed. Revolution times for $^{78}$Ge, $^{77}$Ga, and $^{76}$Zn were obtained. The results confirmed successful circulation of the short-lived nuclei inside the Rare-RI Ring

Charge-changing cross sections for Ca42-51 and effect of charged-particle evaporation induced by neutron-removal reactions

Charge-changing cross sections sigma(CC) for Ca42-51 on a carbon target at around 280 MeV/nucleon have been measured. Though the existing point-proton radii r(p) of Ca isotopes increase as the neutron number increases, the measured sigma(CC) data show a significant decrease, which is against the expectation from a simple Glauber-like model. We found that this observed phenomenon could be attributed to the charged-particle evaporation effect induced by the neutron-removal reaction. By taking the evaporation effect into account, various sigma(CC) data sets for nuclides from C to Fe isotopes on C-12 measured at around 280 MeV/nucleon are reproduced with a standard deviation of 1.6%. It is also clarified that this evaporation effect becomes negligibly small in the neutron-rich region. The evaluated relation between sigma(CC) and r(p) using the current model indicates that sigma(CC) data for neutron-rich Ca isotopes (A >= 51) are highly sensitive to r(p). This high sensitivity potentially allows one to determine the r(p) of very neutron-rich nuclei.11Nsciescopu

Swelling of Doubly Magic 48^{48}Ca Core in Ca Isotopes beyond N = 28

Interaction cross sections for $^{42\textrm{-}51}$Ca on a carbon target at 280 MeV/nucleon have been measured for the first time. The neutron number dependence of derived root-mean-square matter radii shows a significant increase beyond the neutron magic number $N=28$. Furthermore, this enhancement of matter radii is much larger than that of the previously measured charge radii, indicating a novel growth in neutron skin thickness. A simple examination based on the Fermi-type distribution, and the Mean-Field calculations point out that this anomalous enhancement of the nuclear size beyond $N=28$ results from an enlargement of the core by a sudden increase in the surface diffuseness of the neutron density distribution, which implies the swelling of the bare $^{48}$Ca core in Ca isotopes beyond $N=28$.Comment: 6 pages, 3 figure