Resonant laser ionization and spectroscopy are widely used techniques at radioactive ion beam facilities to produce pure beams of exotic nuclei and measure the shape, size, spin and electromagnetic multipole moments of these nuclei. However, in such measurements it is difficult to combine a high efficiency with a high spectral resolution. Here we demonstrate the on-line application of atomic laser ionization spectroscopy in a supersonic gas jet, a technique suited for high-precision studies of the ground- and isomeric-state properties of nuclei located at the extremes of stability. The technique is characterized in a measurement on actinium isotopes around the N=126 neutron shell closure. A significant improvement in the spectral resolution by more than one order of magnitude is achieved in these experiments without loss in efficiency.

No abstract

Atomic masses of the neutron-rich isotopes [121][122][123][124][125][126][127][128] 129,131 In, 130−135 Sn, 131−136 Sb, and 132−140 Te have been measured with high precision (10 ppb) using the Penning trap mass spectrometer JYFLTRAP. Among these, the masses of four r-process nuclei 135 Sn, 136 Sb, and 139,140 Te were measured for the first time. An empirical neutron pairing gap expressed as the odd-even staggering of isotopic masses shows a strong quenching across N=82 for Sn, with the Z-dependence that is unexplainable by the current theoretical models.PACS numbers: 21.10. Dr, 27.60.+j The doubly magic 132 Sn nucleus has been probed intensively by nuclear spectroscopy over the last two decades. It has been found to exhibit features of exceptional purity for its single particle structure [1,2]. This provides an ideal starting point for exploring detailed evolution of nuclear structure of more neutron-rich nuclei beyond the N=82 closed shell in the vicinity of Sn. Therefore, it would be necessary to probe the evolution of odd-even staggering of masses [6] around the N =82 neutron shell to learn about the magnitude of pairing and its variation as a function of Z and N beyond 132 Sn.High precision of present-day ion-trap mass spectrometry combined with high sensitivity [7] can provide the needed information on mass differences such as one-and two-nucleon separation energies, shell gaps and empirical pairing energies. For example, the masses of neutronrich Sn and Xe isotopes were recently measured up to 134Sn and 146 Xe with a Penning trap mass spectrometer ISOLTRAP at the CERN ISOLDE facility [8,9]. In this Letter we wish to present new data of high-precision mass measurements of neutron-rich Cd, In, Sn, Sb, and Te isotopes across the N =82 neutron shell by using the JYFLTRAP Penning trap. These nuclides are also of interest for nuclear astrophysics models of element synthesis, in particular, to explain the large r-process abundance peak at A=130 [10], see Fig. 1. In more general context, a vast body of nuclear data on neutron-rich isotopes is needed for r-process nucleosynthesis predictions. Such data include masses, single particle spectra, pairing characteristics as well as decay properties and reaction rates. In all of these the binding energies or masses of ground, isomeric and excited states play key roles [10]. The measurements were performed using the JYFLTRAP Penning trap mass spectrometer [11] which is connected to the Ion Guide Isotope Separator On-Line (IGISOL) mass separator [12]. The ions of interest were produced in proton-induced fission reactions by bombarding a natural uranium target with a proton beam of 25 MeV energy. A thorium target was used in the case of 129 In and isotopes of Sb.Fission products stopped in a helium-filled gas cell at a pressure of about 200 mbar as singly-charged ions were transported out of the gas cell, accelerated to 30 keV energy, and mass separated. A gas-filled radio frequency quadrupole cooler and buncher prepared the ions for the

Total absorption γ-ray spectroscopy of the β-delayed neutron emitters ^{87}Br, ^{88}Br, and ^{94}Rb We investigate the decay of 87,88 Br and 94 Rb using total absorption γ-ray spectroscopy. These important fission products are β-delayed neutron emitters. Our data show considerable βγ-intensity, so far unobserved in high-resolution γ-ray spectroscopy, from states at high excitation energy. We also find significant differences with the β intensity that can be deduced from existing measurements of the β spectrum. We evaluate the impact of the present data on reactor decay heat using summation calculations. Although the effect is relatively small it helps to reduce the discrepancy between calculations and integral measurements of the photon component for 235 U fission at cooling times in the range 1 − 100 s. We also use summation calculations to evaluate the impact of present data on reactor antineutrino spectra. We find a significant effect at antineutrino energies in the range of 5 to 9 MeV. In addition, we observe an unexpected strong probability for γ emission from neutron unbound states populated in the daughter nucleus. The γ branching is compared to Hauser-Feshbach calculations which allow one to explain the large value for bromine isotopes as due to nuclear structure. However the branching for 94 Rb, although much smaller, hints of the need to increase the radiative width Γγ by one order-of-magnitude. This increase in Γγ would lead to a similar increase in the calculated (n, γ) cross section for this very neutron-rich nucleus with a potential impact on r process abundance calculations.

Cs [5], 95 Rb [6], 94 Rb [7], 77 Cu [8], and 75 Cu [9]. The paucity of information is related to the difficulty of detecting weak high-energy γ-ray cascades with the germanium detectors that are usually employed in β-decay studies. This problem has become known as the Pandemonium effect [10] and it also affects the accuracy of the data. There is an analogy [11] between this decay process and neutron capture reactions which populate states in the compound nucleus that re-emit a neutron (elastic channel) or de-excite by γ rays (radiative capture). Indeed the reaction cross section is parametrized in terms of neutron 40 and γ widths, Γ n and Γ γ respectively, which also deter-41 mines the fraction of β intensity above S n that proceeds 42 by neutron or γ emission. Radiative capture (n, γ) cross 43 sections for very neutron-rich nuclei are a key ingredient 44 in reaction network calculations used to obtain the yield 45 of elements heavier than iron in the rapid (r) neutron 46 capture process occurring in explosive-like stellar events. 47 It has been shown [12-14] that the abundance distribu-48 tions in different astrophysical scenarios are sensitive to 49 (n, γ) cross sections. In the classical "hot" r process late 50 captures during freeze-out modify the final element abun-51 dance. In the "cold" r process the competition between 52 neutron captures and β decays determines the forma-53 tion path. Cross section values for these exotic nuclei 54 are taken from Hauser-Feshbach model calculations [15], 55 which are based on a few quantities describing average 56 nuclear properties: nuclear level densities (NLD), pho-57 ton strength functions (PSF) and neutron transmission 58 coefficients (NTC). Since these quantities are adjusted to experiment close to β stability it is crucial to find means 60 to verify the predictions for very neutron-rich nuclei. 61 The Total Absorption Gamma-ray Spectroscopy (TAGS) technique aims at detecting cascades rather than 63 individual γ rays using large 4π scintillation detectors. The superiority of this method over high-resolution ger-65 manium spectroscopy to locate missing β intensity has 66 been demonstrated before [16, 17]. However its appli-67 cation in the present case is very challenging, since the 68 expected γ-branching is very small and located at rather 69 high excitation energies. As a matter of fact previous 70 attempts at LNPI [7] with a similar aim did not lead to 71 clear conclusions. In this Letter we propose and demon-72 strate for the first time the use of the TAGS technique 73 to study γ-ray emission above S n in β-delayed neutron 74 emitters and extract accurate information that can be 75 used to improve (n, γ) cross section estimates far from β 76 stability. 77 Neutron capture and transmission reactions have been 78 extensively used [18] to determine neutron and γ widths 79 (or related strength functions). An inspection of Ref. [18] 80 shows that in general Γ n is orders-of-magnitude larger 81 than Γ γ. In the decay of 87 Br, which is the best stud-82 ied case [1, 19-...