Nuclear Experiment

The structure of the doubly magic 132 50 Sn 82 has been investigated at the ISOLDE facility at CERN, populated both by the β − decay of 132 In and β − -delayed neutron emission of 133 In. The level scheme of 132 Sn is greatly expanded with the addition of 68 γ -transitions and 17 levels observed for the first time in the β decay. The information on the excited structure is completed by new γ -transitions and states populated in the β -n decay of 133 In. Improved delayed neutron emission probabilities are obtained both for 132 In and 133 In. Level lifetimes are measured via the Advanced Time-Delayed βγγ (t) fast-timing method. An interpretation of the level structure is given based on the experimental findings and the particle-hole configurations arising from core excitations both from the \textit{N} = 82 and \textit{Z} = 50 shells, leading to positive and negative parity particle-hole multiplets. The experimental information provides new data to challenge the theoretical description of 132 Sn.

In an experiment performed at the SPIRAL1 facility of GANIL, the beta decay of 32Ar has been studied by means of the "Silicon Cube" device associated with germanium clover detectors from the EXOGAM array. Beta-delayed protons and gamma rays have been observed and allowed the determination of all relevant decay branches. The Gamow-Teller strength distribution is compared to shell-model calculations and excellent agreement is found. The Fermi strength is inline with expectations. A quasi-complete decay scheme of 32Ar is established.

We investigate the detection prospects for two-neutrino and neutrinoless second order weak decays of 124 Xe -- double electron capture ( 0/2νECEC ), electron capture with positron emission ( 0/2νEC β + ) and double-positron emission ( 0/2ν β + β + ) -- in multi-tonne xenon time projection chambers. We simulate the decays in a liquid xenon medium and develop a reconstruction algorithm which uses the multi-particle coincidence in these decays to separate signal from background. This is used to compute the expected detection efficiencies as a function of position resolution and energy threshold for planned experiments. In addition, we consider an exhaustive list of possible background sources and find that they are either negligible in rate or can be greatly reduced using our topological reconstruction criteria. In particular, we draw two conclusions: First, with a half-life of T 2νEC β + 1/2 =(1.7±0.6)⋅ 10 23 yr , the 2νEC β + decay of 124 Xe will likely be detected in upcoming Dark Matter experiments (e.g. XENONnT or LZ), and their major background will be from gamma rays in the detector construction materials. Second, searches for the 0νEC β + decay mode are likely to be background-free, and new parameter space may be within the reach. To this end we investigate three different scenarios of existing experimental constraints on the effective neutrino mass. The necessary 500 kg-year exposure of 124 Xe could be achieved by the baseline design of the DARWIN observatory, or by extracting and using the 124 Xe from the tailings of the nEXO experiment. We demonstrate how a combination of 124 Xe results with those from 0ν β − β − searches in 136 Xe could help to identify the neutrinoless decay mechanism.

In β -decay studies the determination of the decay probability to the ground state of the daughter nucleus often suffers from large systematic errors. The difficulty of the measurement is related to the absence of associated delayed γ -ray emission. In this work we revisit the 4πγ−β method proposed by Greenwood and collaborators in the 1990s, which has the potential to overcome some of the experimental difficulties. Our interest is driven by the need to determine accurately the β -intensity distributions of fission products that contribute significantly to the reactor decay heat and to the antineutrinos emitted by reactors. A number of such decays have large ground state branches. The method is relevant for nuclear structure studies as well. Pertinent formulae are revised and extended to the special case of β -delayed neutron emitters, and the robustness of the method is demonstrated with synthetic data. We apply it to a number of measured decays that serve as test cases and discuss the features of the method. Finally, we obtain ground state feeding intensities with reduced uncertainty for four relevant decays that will allow future improvements in antineutrino spectrum and decay heat calculations using the summation method.

Luminosity is a measure of the colliding frequency between beam and target and it is a crucial parameter for the measurement of absolute values, such as reaction cross sections. In this paper, we make use of experimental data from the ESR storage ring to demonstrate that the luminosity can be precisely determined by modelling the measured Rutherford scattering distribution. The obtained results are in good agreement with an independent measurement based on the x-ray normalization method. Our new method provides an alternative way to precisely measure the luminosity in low-energy stored-beam configurations. This can be of great value in particular in dedicated low-energy storage rings where established methods are difficult or impossible to apply.

Nuclear reactions of interest for astrophysics and applications often rely on statistical model calculations for nuclear reaction rates, particularly for nuclei far from β -stability. However, statistical model parameters are often poorly constrained, where experimental constraints are particularly sparse for exotic nuclides. For example, our understanding of the breakout from the NiCu cycle in the astrophysical rp-process is currently limited by uncertainties in the statistical properties of the proton-rich nucleus 60 Zn. We have determined the nuclear level density of 60 Zn using neutron evaporation spectra from 58 Ni( 3 He, n) measured at the Edwards Accelerator Laboratory. We compare our results to a number of theoretical predictions, including phenomenological, microscopic, and shell model based approaches. Notably, we find the 60 Zn level density is somewhat lower than expected for excitation energies populated in the 59 Cu(p, γ ) 60 Zn reaction under rp-process conditions. This includes a level density plateau from roughly 5-6 MeV excitation energy, which is counter to the usual expectation of exponential growth and all theoretical predictions that we explore. A determination of the spin-distribution at the relevant excitation energies in 60 Zn is needed to confirm that the Hauser-Feshbach formalism is appropriate for the 59 Cu(p, γ ) 60 Zn reaction rate at X-ray burst temperatures.

A puzzle has long existed for the α -cluster content in the near-threshold 7.54 MeV state of 10 Be. A new measurement was conducted to measure the cluster-decay partial width of this state, using the reaction 9 Be( 9 Be, 10 Be ∗ →α+ 6 He) 8 Be at 45 MeV beam energy. Special measures were taken to reduce the strong near-threshold background. The neutron-decay strength was also obtained based on the three-fold coincident measurement. A cluster-decay branching ratio of (4.04±1.26)× 10 −4 is obtained, resulting in a reasonably large α -cluster spectroscopic factor. The present work confirms the formation of the σ -bond molecular rotational band headed by the 6.18 MeV state in 10 Be.

The scarce data systematics and complexity of deuteron interactions demand the update of both the experimental database and theoretical frame of deuteron activation cross sections. Various reactions induced by neutrons and protons following the deuteron breakup (BU) should be also taken into account. On the other hand, deuteron reaction cross sections recommended recently for high-priority elements are still based on data fit without predictive power. Purpose: Accurate new measurements of low-energy deuteron-induced reaction cross sections for monoisotopic (55Mn) natural manganese target enhance the related database as well as the opportunity of an unitary and consistent account of the related reaction mechanisms. Method: Activation cross sections of 54,56Mn, and 51Cr nuclei by deuterons on 55 Mn were measured at energies <20 MeV by the stacked-foil technique and high resolution gamma spectrometry at the U-120M cyclotron of CANAM, NPI CAS. Then all available data for deuterons on 55Mn up to 50 MeV are analyzed paying particular attention to BU and direct reaction (DR) mechanisms. Results: Newly measured activation cross sections strengthen the deuteron database at low energies, at once with a consistent account for the first time of all available data. Conclusions: Due account of deuteron-induced reactions on 55Mn, including particularly the new experimental data at low energies, is provided by a suitable BU and DR assessment.

Beta-delayed neutron emission is important for nuclear structure and astrophysics as well as for reactor applications. Significant advances in nuclear experimental techniques in the past two decades have led to a wealth of new measurements that remain to be incorporated in the databases. We report on a coordinated effort to compile and evaluate all the available beta-delayed neutron emission data. The different measurement techniques have been assessed and the data have been compared with semi-microscopic and microscopic-macroscopic models. The new microscopic database has been tested against aggregate total delayed neutron yields, time-dependent group parameters in 6-and 8-group re-presentation, and aggregate delayed neutron spectra. New recommendations of macroscopic delayed-neutron data for fissile materials of interest to applications are also presented. The new Reference Database for Beta-Delayed Neutron Emission Data is available online at: this http URL.

The first measurements of dielectron production at midrapidity ( | η c |<0.8 ) in proton-proton and proton-lead collisions at s NN − − − √ = 5.02 TeV at the LHC are presented. The dielectron cross section is measured with the ALICE detector as a function of the invariant mass m T,ee and the pair transverse momentum p T,ee in the ranges m T,ee < 3.5 GeV/ c 2 and m T,ee < 8.0 GeV/ c 2 , in both collision systems. In proton-proton collisions, the charm and beauty cross sections are determined at midrapidity from a fit to the data with two different event generators. This complements the existing dielectron measurements performed at s √ = 7 and 13 TeV. The slope of the s √ dependence of the three measurements is described by FONLL calculations. The dielectron cross section measured in proton-lead collisions is in agreement, within the current precision, with the expected dielectron production without any nuclear matter effects for e + e − pairs from open heavy-flavor hadron decays. For the first time at LHC energies, the dielectron production in proton-lead and proton-proton collisions are directly compared at the same s NN − − − √ via the dielectron nuclear modification factor R pPb . The measurements are compared to model calculations including cold nuclear matter effects, or additional sources of dielectrons from thermal radiation.