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H. Scheit

Publications and source records attributed to H. Scheit.

At least 37 records · Page 2Linked to original sources
arXiv2018

Weakly Bound Neutron-Rich Nuclei and Cosmic Phenomena

The single particle and bulk properties of the neutron-rich nuclei constrain fundamental issues in nuclear physics and nuclear astrophysics like the limits of existence of quantum many body systems (atomic nuclei), the equation of state of neutron-rich matter, neutron star, nucleosynthesis, evolution of stars, neutron star merging etc.. The state of the art of Coulomb breakup of the neutron-rich nuclei has been used to explore those properties. Unambiguous information on detailed components of the ground-state wave-function along with quantum numbers of the valence neutron of the nuclei have been obtained from the measurement of threshold strength along with the $γ$-rays spectra of the core following Coulomb breakup. The shape of this threshold strength is a finger-print of the quantum numbers of the nucleon. We investigated the ground-state properties of the neutron-rich Na, Mg, Al nuclei around N $\sim$ 20 using this method at GSI, Darmstadt. Very clear evidence has been observed for melting and merging of long cherished magic shell gaps at N = 20, 28. The evanescent neutron-rich nuclei imprint their existence in stellar explosive scenarios (r-process etc.). Coulomb dissociation (CD) is one of the important indirect measurements of the capture cross-section which may provide valuable input to the model for star evolution process, particularly the r-process. Some valuable bulk properties of the neutron-rich nuclei like the density dependent symmetry energy,neutron skin etc. play a key role in understanding cosmic phenomena and these properties have been studied via electromagnetic excitation. Preliminary results of electromagnetic excitation of the neutron-rich nucleus, $^{32}$Mg are presented.

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arXiv2018

Strong neutron pairing in core+4n nuclei

The emission of neutron pairs from the neutron-rich $N\!=\!12$ isotones $^{18}$C and $^{20}$O has been studied by high-energy nucleon knockout from $^{19}$N and $^{21}$O secondary beams, populating unbound states of the two isotones up to 15~MeV above their two-neutron emission thresholds. The analysis of triple fragment-$n$-$n$ correlations shows that the decay $^{19}$N$(-1p)^{18}$C$^*\!\rightarrow^{16}$C+$n$+$n$ is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a $^{14}$C core surrounded by four valence neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay $^{21}$O$(-1n)^{20}$O$^*\!\rightarrow^{18}$O+$n$+$n$, attributed to its formation through the knockout of a deeply-bound neutron that breaks the $^{16}$O core and reduces the number of pairs.

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arXiv2017

Effective proton-neutron interaction near the drip line from unbound states in $^{25,26}$F

Background: Odd-odd nuclei, around doubly closed shells, have been extensively used to study proton-neutron interactions. However, the evolution of these interactions as a function of the binding energy, ultimately when nuclei become unbound, is poorly known. The $^{26}$F nucleus, composed of a deeply bound $\pi0d\_{5/2}$ proton and an unbound $\nu0d\_{3/2}$ neutron on top of an $^{24}$O core, is particularly adapted for this purpose. The coupling of this proton and neutron results in a $J^π = 1^{+}\_1 - 4^{+}\_1$ multiplet, whose energies must be determined to study the influence of the proximity of the continuum on the corresponding proton-neutron interaction. The $J^π = 1^{+}\_1, 2^{+}\_1,4^{+}\_1$ bound states have been determined, and only a clear identification of the $J^π =3^{+}\_1$ is missing.Purpose: We wish to complete the study of the $J^π = 1^{+}\_1 - 4^{+}\_1$ multiplet in $^{26}$F, by studying the energy and width of the $J^π =3^{+}\_1$ unbound state. The method was firstly validated by the study of unbound states in $^{25}$F, for which resonances were already observed in a previous experiment.Method: Radioactive beams of $^{26}$Ne and $^{27}$Ne, produced at about $440A$\,MeV by the FRagment Separator at the GSI facility, were used to populate unbound states in $^{25}$F and $^{26}$F via one-proton knockout reactions on a CH$\_2$ target, located at the object focal point of the R$^3$B/LAND setup. The detection of emitted $γ$-rays and neutrons, added to the reconstruction of the momentum vector of the $A-1$ nuclei, allowed the determination of the energy of three unbound states in $^{25}$F and two in $^{26}$F. Results: Based on its width and decay properties, the first unbound state in $^{25}$F is proposed to be a $J^π = 1/2^-$ arising from a $p\_{1/2}$ proton-hole state. In $^{26}$F, the first resonance at 323(33)~keV is proposed to be the $J^π =3^{+}\_1$ member of the $J^π = 1^{+}\_1 - 4^{+}\_1$ multiplet. Energies of observed states in $^{25,26}$F have been compared to calculations using the independent-particle shell model, a phenomenological shell-model, and the ab initio valence-space in-medium similarity renormalization group method.Conclusions: The deduced effective proton-neutron interaction is weakened by about 30-40\% in comparison to the models, pointing to the need of implementing the role of the continuum in theoretical descriptions, or to a wrong determination of the atomic mass of $^{26}$F.

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arXiv2017

Coulomb breakup of neutron-rich $^{29,30}$Na isotopes near the island of inversion

First results are reported on the ground state configurations of the neutron-rich $^{29,30}$Na isotopes, obtained via Coulomb dissociation (CD) measurements as a method of the direct probe. The invariant mass spectra of those nuclei have been obtained through measurement of the four-momentum of all decay products after Coulomb excitation on a $^{208}Pb$ target at energies of 400-430 MeV/nucleon using FRS-ALADIN-LAND setup at GSI, Darmstadt. Integrated Coulomb-dissociation cross-sections (CD) of 89 $(7)$ mb and 167 $(13)$ mb up to excitation energy of 10 MeV for one neutron removal from $^{29}$Na and $^{30}$Na respectively, have been extracted. The major part of one neutron removal, CD cross-sections of those nuclei populate core, in its' ground state. A comparison with the direct breakup model, suggests the predominant occupation of the valence neutron in the ground state of $^{29}$Na${(3/2^+)}$ and $^{30}$Na${(2^+)}$ is the $d$ orbital with small contribution in the $s$-orbital which are coupled with ground state of the core. The ground state configurations of these nuclei are as $^{28}$Na$_{gs (1^+)\otimesν_{s,d}$ and $^{29}$Na$_{gs}(3/2^+)\otimesν_{ s,d}$, respectively. The ground state spin and parity of these nuclei, obtained from this experiment are in agreement with earlier reported values. The spectroscopic factors for the valence neutron occupying the $s$ and $d$ orbitals for these nuclei in the ground state have been extracted and reported for the first time. A comparison of the experimental findings with the shell model calculation using MCSM suggests a lower limit of around 4.3 MeV of the sd-pf shell gap in $^{30}$Na.

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arXiv2016

Systematic investigation of projectile fragmentation using beams of unstable B and C isotopes

Background: Models describing nuclear fragmentation and fragmentation-fission deliver important input for planning nuclear physics experiments and future radioactive ion beam facilities. These models are usually benchmarked against data from stable beam experiments. In the future, two-step fragmentation reactions with exotic nuclei as stepping stones are a promising tool to reach the most neutron-rich nuclei, creating a need for models to describe also these reactions. Purpose: We want to extend the presently available data on fragmentation reactions towards the light exotic region on the nuclear chart. Furthermore, we want to improve the understanding of projectile fragmentation especially for unstable isotopes. Method: We have measured projectile fragments from 10,12-18C and 10-15B isotopes colliding with a carbon target. These measurements were all performed within one experiment, which gives rise to a very consistent dataset. We compare our data to model calculations. Results: One-proton removal cross sections with different final neutron numbers (1pxn) for relativistic 10,12-18C and 10-15B isotopes impinging on a carbon target. Comparing model calculations to the data, we find that EPAX is not able to describe the data satisfactorily. Using ABRABLA07 on the other hand, we find that the average excitation energy per abraded nucleon needs to be decreased from 27 MeV to 8.1 MeV. With that decrease ABRABLA07 describes the data surprisingly well. Conclusions: Extending the available data towards light unstable nuclei with a consistent set of new data have allowed for a systematic investigation of the role of the excitation energy induced in projectile fragmentation. Most striking is the apparent mass dependence of the average excitation energy per abraded nucleon. Nevertheless, this parameter, which has been related to final-state interactions, requires further study.

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arXiv2013

Beyond the Neutron Drip-Line: The Unbound Oxygen Isotopes 25O and 26O

The very neutron-rich oxygen isotopes 25O and 26O are investigated experimentally and theoret- ically. In this first R3B-LAND experiment, the unbound states are populated at GSI via proton- knockout reactions from 26F and 27F at relativistic energies around 450 MeV/nucleon. From the kinematically complete measurement of the decay into 24O plus one or two neutrons, the 25O ground- state energy and lifetime are determined, and upper limits for the 26O ground state are extracted. In addition, the results provide evidence for an excited state in 26O at around 4 MeV. The ex- perimental findings are compared to theoretical shell-model calculations based on chiral two- and three-nucleon (3N) forces, including for the first time residual 3N forces, which are shown to be amplified as valence neutrons are added.

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arXiv2013

Collectivity evolution in the neutron-rich Pd isotopes towards the N=82 shell closure

The neutron-rich, even-even 122,124,126Pd isotopes has been studied via in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Excited states at 499(9), 590(11), and 686(17) keV were found in the three isotopes, which we assign to the respective 2+ -> 0+ decays. In addition, a candidate for the 4+ state at 1164(20) keV was observed in 122Pd. The resulting Ex(2+) systematics are essentially similar to those of the Xe (Z=54) isotopic chain and theoretical prediction by IBM-2, suggesting no serious shell quenching in the Pd isotopes in the vicinity of N=82.

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arXiv2012

Well-developed deformation in 42Si

Excited states in 38,40,42Si nuclei have been studied via in-beam gamma-ray spectroscopy with multi-nucleon removal reactions. Intense radioactive beams of 40S and 44S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled gamma-gamma coincidence measurements. A prominent gamma line observed with an energy of 742(8) keV in 42Si confirms the 2+ state reported in an earlier study. Among the gamma lines observed in coincidence with the 2+ -> 0+ transition, the most probable candidate for the transition from the yrast 4+ state was identified, leading to a 4+_1 energy of 2173(14) keV. The energy ratio of 2.93(5) between the 2+_1 and 4+_1 states indicates well-developed deformation in 42Si at N=28 and Z=14. Also for 38,40Si energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for 42Si, the results show a rapid deformation development of Si isotopes from N=24 to N=28.

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arXiv2011

Structural evolution in the neutron-rich nuclei 106Zr and 108Zr

The low-lying states in 106Zr and 108Zr have been investigated by means of β-γ and isomer spectroscopy at the RI beam factory, respectively. A new isomer with a half-life of 620\pm150 ns has been identified in 108Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2+ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed sub-shell closure at N = 64. The deformed ground state of 108Zr indicates that a spherical sub-shell gap predicted at N = 70 is not large enough to change the ground state of 108Zr to the spherical shape. The possibility of a tetrahedral shape isomer in 108Zr is also discussed.

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arXiv2010

Exploring the "Island of Inversion" by in-beam gamma-ray spectroscopy of the neutron-rich sodium isotopes 31,32,33Na

The structure of the neutron rich sodium isotopes 31,32,33Na was investigated by means of in-beam gammaray spectroscopy following one-neutron knockout and inelastic scattering of radioactive beams provided by the RIKEN Radioactive Ion Beam Factory. The secondary beams were selected and separated by the fragment separator BigRIPS and incident at ~240 meV/u on a natural carbon (secondary) target, which was surrounded by the DALI2 array to detect coincident de-excitation gammarays. Scattered particles were identified by the spectrometer ZeroDegree. In 31Na, a new decay gammaray was observed in coincidence with the known (5/2+)-> 3/2(+) transition, while for 32,33Na excited states are reported for the first time. From a comparison to state-of-the-art shell model calculations it is concluded that the newly observed excited state in 31Na belongs to a rotational band formed by a 2p2h intruder configuration within the "Island of Inversion".

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arXiv2009

Spectroscopy of 32Ne and the Island of Inversion

We report on the first spectroscopic study of the N=22 nucleus 32Ne at the newly completed RIKEN Radioactive Ion Beam Factory. A single gamma-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a 226 MeV/u 32Ne beam on a Carbon target and proton removal from 33Na at 245 MeV/u. This transition is assigned to the de-excitation of the first J^pi = 2+ state in 32Ne to the 0+ ground state. Interpreted through comparison with state-of-the-art shell model calculations, the low excitation energy demonstrates that the Island of Inversion extends to at least N=22 for the Ne isotopes.

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arXiv2008

Coulomb excitation of $^{68}$Ni at safe energies

The $B(E2;0^+\to2^+)$ value in $^{68}$Ni has been measured using Coulomb excitation at safe energies. The $^{68}$Ni radioactive beam was post-accelerated at the ISOLDE facility (CERN) to 2.9 MeV/u. The emitted $γ$ rays were detected by the MINIBALL detector array. A kinematic particle reconstruction was performed in order to increase the measured c.m. angular range of the excitation cross section. The obtained value of 2.8$^{+1.2}_{-1.0}$ 10$^2$ e$^2$fm$^4$ is in good agreement with the value measured at intermediate energy Coulomb excitation, confirming the low $0^+\to2^+$ transition probability.

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arXiv2008

Quasiclassical description of bremsstrahlung accompanying alpha decay including quadrupole radiation

We present a quasiclassical theory of alpha decay accompanied by bremsstrahlung with a special emphasis on the case of 210Po, with the aim of finding a unified description that incorporates both the radiation during the tunneling through the Coulomb wall and the finite energy E_gamma of the radiated photon up to E_gamma on the order of Q_alpha/sqrt(eta), where Q_alpha is the alpha-decay Q-value and eta is the Sommerfeld parameter. The corrections with respect to previous quasiclassical investigations are found to be substantial, and excellent agreement with a full quantum mechanical treatment is achieved. Furthermore, we find that a dipole-quadrupole interference significantly changes the alpha-gamma angular correlation. We obtain good agreement between our theoretical predictions and experimental results.

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arXiv2007

Observation of the First Excited State in 23O

The first excited state in neutron-rich 23O was observed in a (2p1n) knock-out reaction from 26Ne on a beryllium target at a beam energy of 86 MeV/A. The state is unbound with respect to neutron emission and was reconstructed from the invariant mass from the 22O fragment and the neutron. It is unbound by 45(2) keV corresponding to an excitation energy of 2.8(1) MeV. The non-observation of further resonances implies a predominantly direct reaction mechanism of the employed three-nucleon-removal reaction which suggests the assignment of the observed resonance to be the 5/2+ hole state.

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arXiv2007

Exploring Neutron-Rich Oxygen Isotopes with MoNA

The Modular Neutron Array (MoNA) was used in conjunction with a large-gap dipole magnet (Sweeper) to measure neutron-unbound states in oxygen isotopes close to the neutron dripline. While no excited states were observed in 24O, a resonance at 45(2) keV above the neutron separation energy was observed in 23O.

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arXiv2007

Bremsstrahlung in alpha-Decay Reexamined

A high-statistics measurement of bremsstrahlung emitted in the alpha decay of 210Po has been performed, which allows to follow the photon spectra up to energies of ~ 500 keV. The measured differential emission probability is in good agreement with our theoretical results obtained within the quasi classical approximation as well as with the exact quantum mechanical calculation. It is shown that due to the small effective electric dipole charge of the radiating system a significant interference between the electric dipole and quadrupole contributions occurs, which is altering substantially the angular correlation between the alpha particle and the emitted photon.

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arXiv2007

Energy distributions from three-body decaying many-body resonances

We compute energy distributions of three particles emerging from decaying many-body resonances. We reproduce the measured energy distributions from decays of two archetypal states chosen as the lowest $0^{+}$ and $1^{+}$-resonances in $^{12}$C populated in $β$-decays. These states are dominated by sequential, through the $^{8}$Be ground state, and direct decays, respectively. These decay mechanisms are reflected in the ``dynamic'' evolution from small, cluster or shell-model states, to large distances, where the coordinate or momentum space continuum wavefunctions are accurately computed.

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