New test of modulated electron capture decay of hydrogen-like 142Pm ions: Precision measurement of purely exponential decay

An experiment addressing electron capture (EC) decay of hydrogen-like 142Pm60+ions has been conducted at the experimental storage ring (ESR) at GSI. The decay appears to be purely exponential and no modulations were observed. Decay times for about 9000 individual EC decays have been measured by applying the single-ion decay spectroscopy method. Both visually and automatically analysed data can be described by a single exponential decay with decay constants of 0.0126(7)s−1for automatic analysis and 0.0141(7)s−1for manual analysis. If a modulation superimposed on the exponential decay curve is assumed, the best fit gives a modulation amplitude of merely 0.019(15), which is compatible with zero and by 4.9 standard deviations smaller than in the original observation which had an amplitude of 0.23(4)

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

In-ring velocity measurement for isochronous mass spectrometry

Isochronous mass spectrometry based on heavy-ion storage rings is a powerful tool for direct mass measurements of very short-lived nuclei. Owing to the nature of in-flight separation of high-energy reaction products, many ion species with different mass-to-charge ratios (m/q) can be transmitted to and stored in the ring in one shot. However, high mass resolving power can be achieved only for a limited range of ion species with velocities well matching the isochronous condition of the ring. The knowledge of velocities of each stored ions is required to overcome this restriction. For this purpose, two time-of-flight (TOF) detectors were installed 18 m apart in one straight section of the cooler-storage ring CSRe in Lanzhou. The time sequences measured by the two TOF detectors for each stored ion were used for the precision determination of its velocity. A relative precision of the velocity is achieved to the level of 10-5 for individual ions. The betatron oscillations of the ion motion in the ring were clearly identified in the data and were taken into account in the analysis. The presented technique can be used for measurements of machine tunes and their dependence on particle momenta

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

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