arXiv subjects

K. Riisager

Publications and source records attributed to K. Riisager.

At least 19 recordsLinked to original sources

arXiv2026

Precision $\beta$-delayed charged-particle emission spectroscopy at FRIB: Proof of principle with the $\beta$-decay of $^{25}\mathrm{Si}$

We report on the $\beta$-delayed proton and $\gamma$-ray emission from $^{25}\mathrm{Si}$, measured at the Facility for Rare Isotope Beams (FRIB). Low-energy $^{25}\mathrm{Si}$ ions extracted from the Advanced Cryogenic Gas Stopper were implanted into a thin carbon foil surrounded by a compact, highly segmented array of silicon detector telescopes and two high-purity germanium detectors. This setup provides high-resolution charged-particle spectroscopy, establishing a proof of principle for precision stopped-beam decay studies at FRIB. We reconstruct the $^{25}\mathrm{Si}$ decay scheme, resolving new high-energy proton transitions and determining the feeding to excited states in $^{24}\mathrm{Mg}$. The observation of spectral interference patterns enables firm spin and parity assignments for highly excited states in $^{25}\mathrm{Al}$. The $^{25}\mathrm{Si}$ $\beta$-strength distribution is extracted and compared with large-scale shell-model calculations.

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arXiv2026

A ground state $^{22}$Al halo is unlikely

We report the decisive resolution of the ground state spin and parity of the proton-dripline nucleus $^{22}$Al, a prime candidate for a proton halo. The resolution stems from the first $β$-delayed charged particle emission experiment in the Gas Stopping Area at the Facility for Rare Isotope Beams (FRIB), leveraging high-intensity, low-energy beams extracted from the Advanced Cryogenic Gas Stopper (ACGS). The pristine beam quality from FRIB and the ACGS enabled a sensitive particle identification technique using thin silicon detectors, allowing for the suppression of the dominant proton background and the first observation of the weak $β$-delayed $α$ transition from the Isobaric Analog State in $^{22}$Mg to the $^{18}$Ne ground state. This observation uniquely fixes the $^{22}$Al ground state as $4^+$. The valence proton is confined by a dominant $d$-wave centrifugal barrier which, combined with the Coulomb repulsion, hinders the tunneling required for halo formation despite the exceptionally low proton separation energy of $^{22}$Al.

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arXiv2025

Beta-delayed particle emission and collective rotations

Beta-delayed proton emission in the lower half of the sd-shell will involve deformed nuclei. We derive the normalized matrix element connecting emission of one particle from an initial rotational nuclear state to another final rotating state, and we extract selection rules involving the $K$ quantum number. The initial state is approximated as having a core identical to the final nuclear state. The formalism is then directly applicable to $β^+$-delayed proton decays of even-$Z$, odd-$N$ nuclei or $β^-$-delayed neutron decays of odd-$Z$, even $N$ nuclei. These beta-decay results are compared to the outcomes of possible transfer reactions. As an example the beta-delayed proton emission of $^{21}$Mg is considered, where new quantum numbers can be assigned to several states in $^{21}$Na.

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arXiv2024

Detailed study of the decay of $^{21}$Mg

Beta-delayed proton and gamma emission in the decay of $^{21}$Mg has been measured at ISOLDE, CERN with the ISOLDE Decay Station (IDS) set-up. The existing decay scheme is updated, in particular what concerns proton transitions to excited states in $^{20}$Ne. Signatures of interference in several parts of the spectrum are used to settle spin and parity assignments to highly excited states in $^{21}$Na. The previously reported $β$p$α$ branch is confirmed. A half-life of 120.5(4) ms is extracted for $^{21}$Mg. The revised decay scheme is employed to test mirror symmetry in the decay and to extract the beta strength distribution of $^{21}$Mg that is compared with theory.

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arXiv2024

Decay study of $^{11}$Be with an Optical TPC detector

The $β$ decay of one-neutron halo nucleus $^{11}$Be was investigated using the Warsaw Optical Time Projection Chamber (OTPC) detector to measure $β$-delayed charged particles. The results of two experiments are reported. In the first one, carried out in LNS Catania, the absolute branching ratio for $β$-delayed $α$ emission was measured by counting incoming $^{11}$Be ions stopped in the detector and the observed decays with the emission of $α$ particle. The result of 3.27(46)\% is in good agreement with the literature value. In the second experiment, performed at the HIE-ISOLDE facility at CERN, bunches containing several hundreds of $^{11}$Be ions were implanted into the OTPC detector followed by the detection of decays with the emission of charged particles. The energy spectrum of $β$-delayed $α$ particles was determined in the full energy range. It was analysed in the R-matrix framework and was found to be consistent with the literature. The best description of the spectrum was obtained assuming that the two $3/2^+$ and one $1/2^+$ states in $^{11}$B are involved in the transition. The search for $β$-delayed emission of protons was undertaken. Only the upper limit for the branching ratio for this process of $(2.2 \pm 0.6_{\rm stat} \pm 0.6_{\rm sys}) \times 10^{-6}$ could be determined. This value is in conflict with the result published in [Ayyad et al. Phys. Rev. Lett. 123, 082501 (2019)] but does agree with the limit reported in [Riisager et al., Eur. Phys. J. A (2020) 56:100]

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arXiv2023

Three-body calculations of beta decay applied to $^{11}$Li

A novel practical few-body method is formulated to include isospin symmetry for nuclear halo structures. The method is designed to describe beta decay, where the basic concept of isospin symmetry facilitates a proper understanding. Both isobaric analogue and anti-analogue states are treated. We derive general and explicit formulas for three-body systems using hyperspherical coordinates. The example of the beta decaying $^{11}$Li ($^{9}$Li+$n$+$n$) is chosen as a challenging application for numerical calculations of practical interest. The detailed results are compared to existing experimental data and good agreement is found at high excitation energies, where the isobaric analogue and anti-analogue states are situated in the daughter nucleus. An interpretation of the decay pattern at lower excitation energies is suggested. Decays of the $^{9}$Li-core and the two halo-neutrons are individually treated and combined to the daughter system with almost unique isospin, which we predict to be broken by about $0.4\%$ probability. Properties of decay products are predicted as possible future tests of this model.

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arXiv2022

Mirror beta transitions

Beta decays of mirror nuclei differ in Q-value, but will otherwise proceed with transitions of similar strength. The current status is reviewed: Fermi transitions are all very similar, whereas Gamow-Teller transitions can differ in strength by more than a factor two. The main cause of the asymmetries appears to be binding energy differences between the mirror systems.

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arXiv2021

Unveiling the Two-Proton Halo Character of 17Ne: Exclusive Measurement of Quasi-free Proton-Knockout Reactions

The proton drip-line nucleus 17Ne is investigated experimentally in order to determine its two-proton halo character. A fully exclusive measurement of the 17Ne(p,2p)16F->15O+p quasi-free one-proton knockout reaction has been performed at GSI at around 500 MeV/nucleon beam energy. All particles resulting from the scattering process have been detected. The relevant reconstructed quantities are the angles of the two protons scattered in quasi-elastic kinematics, the decay of 16F into 15O (including gamma decays from excited states) and a proton, as well as the 15O+p relative-energy spectrum and the 16F momentum distributions. The latter two quantities allow an independent and consistent determination of the ratio of l=0 and l=2 motion of the valence protons in 17Ne. With a resulting relatively small l=0 component of only around 35(3)%, it is concluded that 17Ne exhibits a rather modest halo character only. The quantitative agreement of the two values deduced from the energy spectrum and the momentum distributions supports the theoretical treatment of the calculation of momentum distributions after quasi-free knockout reactions at high energies by taking into account distortions based on the Glauber theory. Moreover, the experimental data allow the separation of valence-proton knockout and knockout from the 15O core. The latter process contributes with 11.8(3.1) mb around 40% to the total proton-knockout cross section of 30.3(2.3) mb, which explains previously reported contradicting conclusions derived from inclusive cross sections.

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arXiv2020

Search for beta-delayed proton emission from $^{11}$Be

We report on an attempt to reproduce the observation of $β^-$-delayed proton emission from $^{11}$Be through detection of the final state nucleus $^{10}$Be with accelerator mass spectrometry. Twelve samples were collected at the ISOLDE facility at CERN at different separator settings, allowing tests of different sources of contamination to be carried out. The observed amounts of $^{10}$Be per collected $^{11}$Be rule out several contamination sources, but do not agree internally. Formation of BeH molecular ions in the ion source may explain our data, in which case an upper limit of the $β$p branching ratio of $2.2 \cdot 10^{-6}$ can be derived.

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arXiv2019

Comment on "Direct Observation of Proton Emission in $^{11}$Be"

We argue that conclusions of [PRL 123, 082501 (2019)] are incorrect. The authors present the direct observation of beta-delayed proton emission in the beta decay of 11Be. From the determined branching ratio for this process and from the energy spectrum of emitted protons the existence of a so far unobserved narrow resonance in 11Be was deduced. The given beta strength for the transition to this state is however wrong. In addition, we show that the combination of peak position and branching ratio is in strong disagreement with models considered by the authors. Furthermore, we identify several deficiencies in the analysis, and we provide possible sources of background, that could explain the error.

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arXiv2019

Measurement of the $2^+\rightarrow 0^+$ ground-state transition in the $β$ decay of $^{20}$F

We report the first detection of the second-forbidden, non-unique, $2^+\rightarrow 0^+$, ground-state transition in the $β$ decay of $^{20}$F. A low-energy, mass-separated $^{20}\rm{F}^+$ beam produced at the IGISOL facility in Jyväskylä, Finland, was implanted in a thin carbon foil and the $β$ spectrum measured using a magnetic transporter and a plastic-scintillator detector. The $β$-decay branching ratio inferred from the measurement is $b_β = [ 0.41\pm 0.08\textrm{(stat)}\pm 0.07\textrm{(sys)}] \times 10^{-5}$ corresponding to $\log ft = 10.89(11)$, making this one of the strongest second-forbidden, non-unique $β$ transitions ever measured. The experimental result is supported by shell-model calculations and has significant implications for the final evolution of stars that develop degenerate oxygen-neon cores. Using the new experimental data, we argue that the astrophysical electron-capture rate on $^{20}$Ne is now known to within better than 25% at the relevant temperatures and densities.

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arXiv2019

Discovery of an Exceptionally Strong $β$-Decay Transition of $^{20}$F and Implications for the Fate of Intermediate-Mass Stars

A significant fraction of stars between 7-11 solar masses are thought to become supernovae, but the explosion mechanism is unclear. The answer depends critically on the rate of electron capture on $^{20}$Ne in the degenerate oxygen-neon stellar core. However, due to the unknown strength of the transition between the ground states of $^{20}$Ne and $^{20}$F, it has not previously been possible to fully constrain the rate. By measuring the transition, we have established that its strength is exceptionally large and enhances the capture rate by several orders of magnitude. This has a decisive impact on the evolution of the core, increasing the likelihood that the star is (partially) disrupted by a thermonuclear explosion rather than collapsing to form a neutron star. Importantly, our measurement resolves the last remaining nuclear physics uncertainty in the final evolution of degenerate oxygen-neon stellar cores, allowing future studies to address the critical role of convection, which at present is poorly understood.

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arXiv2019

Study of multi-neutron emission in the $β$-decay of $^{11}$Li

The kinematics of two-neutron emission following the $β$-decay of $^{11}$Li was investigated for the first time by detecting the two neutrons in coincidence and by measuring their angle and energy. An array of liquid-scintillator neutron detectors was used to reject cosmic-ray and $γ$-ray backgrounds by pulse-shape discrimination. Cross-talk events in which two detectors are fired by a single neutron were rejected using a filter tested on the $β$-1n emitter $^9$Li. A large cross-talk rejection rate is obtained ($> 95 \%$) over most of the energy range of interest. Application to $^{11}$Li data leads to a significant number of events interpreted as $β$-2n decay. A discrete neutron line at $\approx$ 2 MeV indicates sequential two-neutron emission, possibly from the unbound state at 10.6 MeV excitation energy in $^{11}$Be.

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arXiv2019

Clarification of large-strength transitions in the beta decay of 11Be

The shape and normalisation of the beta-delayed alpha spectrum from 11Be was measured by implanting 11Be ions in a segmented Si detector. The spectrum is found to be dominated by a well-known transition to the 3/2+ state at Ex = 9.87MeV in 11B. A significant increase in the observed decay strength towards the higher end of the Q window means, however, that the 9.87MeV state cannot alone be responsible for the transition. Using the R-matrix framework we find that the inclusion of an extra 3/2+ state at Ex = 11.49(10)MeV is required in order to obtain a satisfactory description of the spectrum. Both states show large widths towards alpha decay, exhausting significant fractions of the Wigner limit, a typical signature of alpha clusterisation. The observed Gamow-Teller strength indicate large overlaps between the two states and the ground state of 11Be.

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arXiv2018

First Accurate Normalization of the $β$-delayed $α$ Decay of $^{16}$N and Implications for the $^{12}$C$(α,γ)^{16}$O Astrophysical Reaction Rate

The $^{12}\text{C}(α,γ){}^{16}\text{O}$ reaction plays a central role in astrophysics, but its cross section at energies relevant for astrophysical applications is only poorly constrained by laboratory data. The reduced $α$ width, $γ_{11}$, of the bound $1^-$ level in $^{16}$O is particularly important to determine the cross section. The magnitude of $γ_{11}$ is determined via sub-Coulomb $α$-transfer reactions or the $β$-delayed $α$ decay of $^{16}$N, but the latter approach is presently hampered by the lack of sufficiently precise data on the $β$-decay branching ratios. Here we report improved branching ratios for the bound $1^-$ level [$b_{β,11} = (5.02\pm 0.10)\times 10^{-2}$] and for $β$-delayed $α$ emission [$b_{βα} = (1.59\pm 0.06)\times 10^{-5}$]. Our value for $b_{βα}$ is 33% larger than previously held, leading to a substantial increase in $γ_{11}$. Our revised value for $γ_{11}$ is in good agreement with the value obtained in $α$-transfer studies and the weighted average of the two gives a robust and precise determination of $γ_{11}$, which provides significantly improved constraints on the $^{12}$C$(α,γ)$ cross section in the energy range relevant to hydrostatic He burning.

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arXiv2016

Unbound states in $^{12}$C populated by $γ$-decay of the $(J^π,T) = (2^+,1)$ 16.11 MeV state

The reaction $^{11}\textrm{B}+p$ has been used to populate the $(J^π,T) = (2^+,1)$ state at an excitation energy of 16.11 MeV in $^{12}$C. $γ$-decay to unbound states in $^{12}$C are identified from analysis of the decay of the populated daughter states. Due to a new technique, $γ$-decay to the 10.8 MeV 1$^-$ state is observed for the first time, and transitions to the 9.64 MeV (3$^-$) and 12.71 MeV (1$^+$) are confirmed. Unresolved transitions to natural parity strength at 10 MeV and 11.5-13 MeV are also observed. For all transitions partial widths are deduced

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arXiv2016

Determination of the Neutron-Capture Rate of 17C for the R-process Nucleosynthesis

With the R$^{3}$B-LAND setup at GSI we have measured exclusive relative-energy spectra of the Coulomb dissociation of $^{18}$C at a projectile energy around 425~AMeV on a lead target, which are needed to determine the radiative neutron-capture cross sections of $^{17}$C into the ground state of $^{18}$C. Those data have been used to constrain theoretical calculations for transitions populating excited states in $^{18}$C. This allowed to derive the astrophysical cross section $σ^{*}_{\mathrm{n}γ}$ accounting for the thermal population of $^{17}$C target states in astrophysical scenarios. The experimentally verified capture rate is significantly lower than those of previously obtained Hauser-Feshbach estimations at temperatures $T_{9}\leq{}1$~GK. Network simulations with updated neutron-capture rates and hydrodynamics according to the neutrino-driven wind model as well as the neutron-star merger scenario reveal no pronounced influence of neutron capture of $^{17}$C on the production of second- and third-peak elements in contrast to earlier sensitivity studies.

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arXiv2016

Complete kinematical study of the 3$α$ breakup of the 16.11 MeV state in $^{12}$C

The reaction $^{11}B+p$ has been used to populate the $(J^π,T)=(2^+,1)$ state at an excitation energy of 16.11 MeV in $^{12}$C, and the breakup of the state into three $α$ particles has been studied in complete kinematics. A two-step breakup model which includes interference effects is found to provide the most accurate description of the experimental data. The branching ratio to the ground state of $^8$Be is determined to be 5.1(5)% in agreement with previous findings, but more precise by a factor of two, while the decay to the first-excited state in $^8$Be is found to be dominated by $d$-wave emission.

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