Timo Dickel

The production cross sections of neutron-rich nuclides near the N = 126 neutron shell closure have been measured for the elements ranging from tungsten ( Z = 74 ) to gold ( Z = 79 ) in the fragmentation of a 1000 MeV / u Pb 208 beam on a Be 9 target. Several cross sections, namely those for W 196 − 199 , Re 199 − 201 , and Os 202 − 203 , were determined for the first time with this projectile-target combination. The results are compared with the predictions of several nuclear reaction models and semiempirical parametrizations; special attention is devoted to multiple proton-removal channels and the formation of nuclides with neutron number N = 127 .

Neutron-deficient nuclei in the N ≈ Z , A ≈ 90 region were produced by fragmentation of a relativistic Ag 107 beam at the GSI Helmholtz Centre for Heavy Ion Research (GSI), separated in the fragment separator FRS, slowed down, and thermalized, and high-precision mass measurements were performed employing the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS Ion Catcher. The masses of Rh 91 , 93 , 95 , Ru 89 , Tc 88 , Mo 87 , 88 , 89 , and Nb 87 , 88 , 89 were determined. The mass of Rh 91 was measured for the first time. For Ru 89 , the measured mass excess deviates by more than ten standard deviations (361 keV) from the previous measurement. A possible misidentification of the measured nuclei, including Ru 89 , with contaminants was excluded using a newly developed identification technique for MR-TOF-MS measurements. For the other nuclides, the measured masses agree well with previous measurements. The impact of the new masses on the one-proton separation energy ( S p ) of the nuclei with constant total isospin projection ( T z ) in the island of inversion around N = Z = 40 is investigated, and signatures of the deformed shell gap at Z = 38 are found. A comparison of the experimental S p values with the predictions of several mass models shows a general agreement of the trends.

The missing mass of the ^{12}C(p,d) reaction has been measured near the η^{'}-meson emission threshold in coincidence with a high-momentum proton to selectively collect η^{'}-^{11}C mesic nucleus formation events at GSI, Germany. A 2.5 GeV proton excites a carbon nucleus in order to form an η^{'}-mesic nucleus emitting a deuteron forward with an energy of ∼1.5 GeV. The deuteron is momentum analyzed by the fragment separator used as a high-resolution spectrometer to deduce the excitation energy of the residual system. The large-acceptance detector WASA surrounding the target identifies high-momentum protons emitted in the decay of the η^{'}-mesic nucleus. The measured semiexclusive spectrum exhibits structures below the threshold though the statistical significance is limited. The spectrum is fitted by theoretically calculated spectra varying optical-potential parameters of the η^{'}-nucleus interaction. The analysis results indicate possible formation of η^{'}-mesic nuclei for the real potential depth of ∼-61 MeV with a local statistical significance of 3.5σ and, taking into account the look-elsewhere effect, a global significance of 2.1σ.

The measurement of the bound-state decay of ²⁰⁵Tl at the Experimental Storage Ring (ESR) at GSI, Darmstadt, has recently been reported, with substantial impact on the use of ²⁰⁵Pb as an early Solar System chronometer and on the low-energy measurement of the solar neutrino spectrum via the LOREX project. Owing to the technical challenges in producing a high-purity ²⁰⁵Tl⁸¹⁺ secondary beam, a robust statistical method was developed to estimate the variation in the contaminant ²⁰⁵Pb⁸¹⁺ produced in the fragmentation reaction, which was subsequently transmitted and stored in the ESR. Here, we show that Bayesian and Monte Carlo methods produce comparable estimates for the contaminant variation, each with unique advantages and challenges given the complex statistical problems for this experiment. We recommend the adoption of such methods in future experiments that exhibit unknown statistical fluctuations.

The first direct mass measurement of Pd 93 , the one-proton-decay daughter of the ( 21 + ) isomer in Ag 94 , has been performed, resulting in a mass excess value of − 59 127 ( 35 ) keV and reducing the mass uncertainty by an order of magnitude. As a consequence, the excitation energies of the presumed parent states of the one-proton (1p) decay and two-proton (2p) decay in Ag 94 are found to differ from each other by ten standard deviations. This shows that there is an incompatibility in the previously reported decay scheme of the 1p and 2p branches. Three scenarios are discussed, which could resolve this apparent contradiction, and elucidated by performing state-of-the-art shell-model and mean-field calculations. The latter confirm that, based on the reported decay information, the 2p emission cannot be fed from the same ( 21 + ) isomer as the 1p emission, but indicate that it could originate from a second, structurally different, high-spin state.