arXiv subjects

V. Z. Goldberg

Publications and source records attributed to V. Z. Goldberg.

At least 19 recordsLinked to original sources

arXiv2024

Radiative decay branching ratio of the Hoyle state

Background: The triple-alpha process is a vital reaction in nuclear astrophysics, characterized by two consecutive reactions [$2α\leftrightarrows{^{8}\rm{Be}}(α,γ){^{12}\rm{C}}$] that drive carbon formation. The second reaction occurs through the Hoyle state, a 7.65 MeV excited state in ${^{12}\rm{C}}$ with $J^π=0^{+}$. The rate of the process depends on the radiative width, which can be determined by measuring the branching ratio for electromagnetic decay. Recent measurements by Kibédi et al. conflicted with the adopted value and resulted in a significant increase of nearly 50\% in this branching ratio, directly affecting the triple-alpha reaction. Purpose: This work aims to utilize charged-particle spectroscopy with magnetic selection as a means to accurately measure the total radiative branching ratio ($Γ_{\rm{rad}}/Γ$) of the Hoyle state in $^{12}{\rm C}$. Methods: The Hoyle state in $^{12}{\rm C}$ was populated via $^{12}\rm{C}(α, α')^{12}\rm{C^{*}}$ inelastic scattering. The scattered $α$ particles were detected using a $Δ$E-E telescope, while the recoiled $^{12}{\rm C}$ ions were identified in a magnetic spectrometer. Results: A radiative branching ratio value of $Γ_{\rm{rad}}/Γ\times10^{4}=4.0\pm0.3({\rm stat.})\pm0.16({\rm syst.})$ was obtained. Conclusions: The radiative branching ratio for the Hoyle state obtained in this work is in agreement with the original adopted value. Our result suggests that the proton-$γ$-$γ$ spectroscopy result reported by Kibédi et al. may be excluded.

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arXiv2022

alpha-cluster structure of 18Ne

In this work we study alpha-clustering in 18Ne and compare it with what is known about clustering in the mirror nucleus 18O. The excitation function of 18Ne was measured in inverse kinematics from the resonant elastic scattering reaction of 14O on 4He in the excitation energy range from 8 to 17 MeV, using the active target TexAT. The analysis was performed using a multi-channel R-matrix approach. Detailed spectroscopic information is obtained from the R-matrix analysis: excitation energy of the states, spin and parity as well as partial alpha and total widths. This information is compared with theoretical models and previous data. Clustering structures appear to be robust and mostly isospin symmetric. A good correspondence was found between the levels in 18O and 18Ne. We carried out an extensive shell model analysis of the experimental data. This comparison suggests that strongly clustered states remain organized in relation to the corresponding reaction channel identified by the number of nodes in the relative alpha plus core wave function. The agreement between theory and experiment is very good and especially useful when it comes to understanding the clustering strength distribution. The comparison of the experimental data with theory shows that certain states, especially at high excitation energies, are significantly more clustered than predicted. This indicates that the structure of these states is collective and is aligned towards the corresponding alpha reaction channel.

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arXiv2021

The Lowest Broad Alpha Cluster Resonances in $^{19}$F

There is a deep astrophysical interest in the structure of $^{19}$F states close to the alpha decay threshold. The nuclear structure of these states is important for understanding of the development of $α$ clustering in the $^{20}$Ne region. Emergence of clustered states and generally states that favor coupling to reaction channels near the corresponding decay thresholds is currently of special interest in theoretical physics. Excitation function for $^{15}$N($α$,$α$) elastic scattering was measured by the TTIK method. These new data together with old, high energy resolution data, were analyzed using the R matrix approach. $^{19}$F nuclear structure was calculated using configuration interaction methods with the recently developed effective interaction Hamiltonian. The parameters of broad low spin $\ell = 0$ and 1 relative partial wave resonances close to the $α$ decay threshold in $^{19}$F were identified. Detailed theoretical analysis was carried out identifying all states coupled to the $\ell = 0$ and 1 alpha cluster channels. Considering hierarchy of states with different harmonic oscillator shell excitations allows to evaluate coupling to the alpha channels with different number of nodes in the relative wave function and helps to explain the distribution of the clustering strength and emergence of broad clustering resonances. Comparison of clustering in $^{20}$Ne into $^{16}$O+$α$ and consideration of spin-orbit splitting of the $^{15}$N+$α$ channel provides additional evidence. Detailed analysis of new and old experimental data allows to identify a series of $α$ clustering resonances in $^{19}$F and to assess the distribution of the clustering strength which is of importance to questions of astrophysics and for theoretical understanding of many-body physics and emergence of clustering in loosely bound or unstable nuclei.

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arXiv2020

Observation of T=3/2 Isobaric Analog States in 9Be using p+8Li resonance scattering

Background: Resonance scattering has been extensively used to study the structure of exotic, neutron-deficient nuclei. Extension of the resonance scattering technique to neutron-rich nuclei was suggested more than 20 years ago. This development is based on the isospin conservation law. In spite of broad field of the application, it has never gained a wide-spread acceptance. Purpose: To benchmark the experimental approach to study the structure of exotic neutron-rich nuclei through resonance scattering on a proton target. Method: The excitation function for p+8Li resonance scattering is measured using a thick target by recording coincidence between light and heavy recoils, populating T=3/2 isobaric analog states (IAS) in 9Be. Results: A good fit of the 8Li(p,p)8Li resonance elastic scattering excitation function was obtained using previously tentatively known 5/2- T=3/2 state at 18.65 MeV in 9Be and a new broad T=3/2 s-wave state - the 5/2+ at 18.5 MeV. These results fit the expected iso-mirror properties for the T=3/2 A=9 iso-quartet. Conclusions: Our analysis confirmed isospin as a good quantum number for the investigated highly excited T=3/2 states and demonstrated that studying the structure of neutron-rich exotic nuclei through IAS is a promising approach.

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arXiv2020

The TTIK approach for neutrons

We applied Thick Target Inverse Kinematics Method for a study of resonances decaying through neutron emission. As a test we used a well-studied, because of its role in s-process in stars, $^{13}$C($α$; n) reaction. The observed energy resolution for the $^{13}$C($α$; n) excitation function was $\sim$60 keV, the largest contributions coming from the time structure of the beam and the thickness of the neutron detector. These measurements demonstrated the high efficiency of the approach and revealed a disagreement with R-matrix calculations based on parameters of the most recent previous analysis.

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arXiv2020

Texas Active Target (TexAT) detector for experiments with rare isotope beams

The TexAT (Texas Active Target) detector is a new active-target time projection chamber (TPC) that was built at the Cyclotron Institute Texas A$\&$M University. The detector is designed to be of general use for nuclear structure and nuclear astrophysics experiments with rare isotope beams. TexAT combines a highly segmented Time Projection Chamber (TPC) with two layers of solid state detectors. It provides high efficiency and flexibility for experiments with low intensity exotic beams, allowing for the 3D track reconstruction of the incoming and outgoing particles involved in nuclear reactions and decays.

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arXiv2020

Constraining the $^{22}$Ne($α$,$γ$)$^{26}$Mg and $^{22}$Ne($α$,n)$^{25}$Mg reaction rates using sub-Coulomb $α$-transfer reactions

The $^{22}$Ne($α$,$γ$)$^{26}$Mg and $^{22}$Ne($α$,n)$^{25}$Mg reactions play an important role in astrophysics because they have significant influence on the neutron flux during the weak branch of the s-process. We constrain the astrophysical rates for these reactions by measuring partial $α$-widths of resonances in $^{26}$Mg located in the Gamow window for the $^{22}$Ne+$α$ capture. These resonances were populated using $^{22}$Ne($^6$Li,d)$^{26}$Mg and $^{22}$Ne($^7$Li,t)$^{26}$Mg reactions at energies near the Coulomb barrier. At these low energies $α$-transfer reactions favor population of low spin states and the extracted partial $α$-widths for the observed resonances exhibit only minor dependence on the model parameters. The astrophysical rates for both the $^{22}$Ne($α$,$γ$)$^{26}$Mg and the $^{22}$Ne($α$,n)$^{25}$Mg reactions are shown to be significantly different than the previously suggested values.

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arXiv2019

Structure of 9C through proton resonance scattering with Texas Active Target detector

Background: Level structure of the most neutron deficient nucleon-bound carbon isotope, 9C, is not well known. Definitive spin-parity assignments are only available for two excited states. No positive parity states have been conclusively identified so far and the location of the sd-shell in A=9 T=3/2 isospin quadruplet is not known. Purpose: We have studied the level structure of exotic nucleus 9C at excitation energies below 6.4 MeV. Methods: Excited states in 9C were populated in 8B+p resonance elastic scattering and excitation functions were measured using active target approach. Results: Two excited states in 9C were conclusively observed, and R-matrix analysis of the excitation functions was performed to make the spin-parity assignments. The first positive parity state in A=9 T=3/2 nuclear system, the 5/2+ resonance at 4.3 MeV, has been identified. Conclusions: The new 5/2+ state at 4.3 MeV in 9C is a single-particle L=0 broad resonance and it determines the energy of the 2s shell. The 2s shell in this exotic nucleus appears well within the region dominated by the p-shell states.

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arXiv2019

Strong resonances at high excitation energy in 17O+alpha resonance scattering

The Thick Target Inverse Kinematic (TTIK) approach was used to measure excitation functions for the elastic 17O (α, α) scattering at the initial 17O beam energy of 54.4 MeV. We observed strong peaks corresponding to highly excited α-cluster states in the 21Ne excitation energy region of 8-16 MeV, which have never been investigated before. Additional tests were done at a 17O beam energy of 56.4 MeV to estimate a possible contribution of resonance inelastic scattering.

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arXiv2019

$^{21}$Ne level structure in the resonance $^{17}$O+$α$ elastic scattering

The first study of resonances in $^{17}$O+$α$ elastic scattering was carried out using the Thick Target Inverse Kinematics (TTIK) method. The data were analyzed in the framework of an $\textit{R}$-matrix approach. Many $α$-cluster states were found in the $^{21}$Ne excitation region of the 9-13 MeV excitation energy including the first observation of a broad $\textit{l}$=0 state in an odd-even nucleus, which is likely the analog of the broad 0$^+$ at 8 MeV in $^{20}$Ne. The observed structure in $^{21}$Ne appeared to be strikingly similar to that in $^{20}$Ne populated in the resonance $^{16}$O+$α$ scattering. The results are also useful for refinement of data on an $^{17}$O($α$,$\textit{n}$) reaction important for astrophysics.

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arXiv2018

Searching for states analogous to the 12C Hoyle state in heavier nuclei using the thick target inverse kinematics technique

Identification of alpha cluster states analogous to the 12C Hoyle state in heavier alpha- conjugate nuclei can provide tests of the existence of alpha condensates in nuclei. Such states are predicted for 16O, 20Ne, 24Mg, 28Si etc. at excitation energies slightly above the multi-alpha particle decay threshold, but have not yet been experimentally identified. The Thick Target Inverse Kinematics (TTIK) technique can be used to study the breakup of excited self-conjugate nuclei into many alpha particles. The reaction 20Ne+α was studied using a 20Ne beam at 12 MeV/nucleon from the K150 cyclotron at Texas A&M University. The TTIK method was used to study both single α-particle emission and multiple α-particle decays. Events with alpha multiplicity up to four were analyzed. The analysis of the three α - particle emission data allowed the identification of the Hoyle state and other 12C excited states decaying into three alpha particles. The results are shown and compared with other data available in the literature. Although the statistics for events with alpha multiplicity four is low, the data show a structure at about 15.2 MeV that could indicate the existence in 16O of a state analogous to the 12C Hoyle state. This structure is confirmed by the re-analysis of alpha multiplicity four events from a previous experiment performed at 9.7 MeV/nucleon with a similar setup but lower granularity. Moreover, the reconstructed excitation energy of 24Mg for these events peaks at around 34 MeV, very close to the predicted excitation energy for an excited state analogous to the 12C Hoyle state in 24Mg.

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arXiv2017

Structure of $^{20}$Ne states in the resonance $^{16}$O+$α$ elastic scattering

Background The nuclear structure of the cluster bands in $^{20}$Ne presents a challenge for different theoretical approaches. It is especially difficult to explain the broad 0$^+$, 2$^+$ states at 9 MeV excitation energy. Simultaneously, it is important to obtain more reliable experimental data for these levels in order to quantitatively assess the theoretical framework. Purpose To obtain new data on $^{20}$Ne $α$ cluster structure. Method Thick target inverse kinematics technique was used to study the $^{16}$O+$α$ resonance elastic scattering and the data were analyzed using an \textit{R} matrix approach. The $^{20}$Ne spectrum, the cluster and nucleon spectroscopic factors were calculated using cluster-nucleon configuration interaction model (CNCIM). Results We determined the parameters of the broad resonances in \textsuperscript{20}Ne: 0$^+$ level at 8.77 $\pm$ 0.150 MeV with a width of 750 (+500/-220) keV; 2$^+$ level at 8.75 $\pm$ 0.100 MeV with the width of 695 $\pm$ 120 keV; the width of 9.48 MeV level of 65 $\pm$ 20 keV and showed that 9.19 MeV, 2$^+$ level (if exists) should have width $\leq$ 10 keV. The detailed comparison of the theoretical CNCIM predictions with the experimental data on cluster states was made. Conclusions Our experimental results by the TTIK method generally confirm the adopted data on $α$ cluster levels in $^{20}$Ne. The CNCIM gives a good description of the $^{20}$Ne positive parity states up to an excitation energy of $\sim$ 7 MeV, predicting reasonably well the excitation energy of the states and their cluster and single particle properties. At higher excitations, the qualitative disagreement with the experimentally observed structure is evident, especially for broad resonances.

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arXiv2017

Structure of 10N in 9C+p resonance scattering

The structure of exotic nucleus 10N was studied using 9C+p resonance scattering. Two L=0 resonances were found to be the lowest states in 10N. The ground state of 10N is unbound with respect to proton decay by 2.2(2) or 1.9(2) MeV depending on the 2- or 1- spin-parity assignment, and the first excited state is unbound by 2.8(2) MeV.

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arXiv2015

Nuclear structure beyond the neutron drip line: the lowest energy states in $^9$He via their T=5/2 isobaric analogs in $^9$Li

The level structure of the very neutron rich and unbound $^9$He nucleus has been the subject of significant experimental and theoretical study. Many recent works have claimed that the two lowest energy $^9$He states exist with spins $J^π=1/2^+$ and $J^π=1/2^-$ and widths on the order of hundreds of keV. These findings cannot be reconciled with our contemporary understanding of nuclear structure. The present work is the first high-resolution study with low statistical uncertainty of the relevant excitation energy range in the $^8$He$+n$ system, performed via a search for the T=5/2 isobaric analog states in $^9$Li populated through $^8$He+p elastic scattering. The present data show no indication of any narrow structures. Instead, we find evidence for a broad $J^π=1/2^+$ state in $^9$He located approximately 3 MeV above the neutron decay threshold.

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arXiv2014

$α$-cluster structure of $^{18}$O

Background: Clustering phenomena in $N \neq Z$ nuclei provide an opportunity to understand the interplay between cluster and nucleon degrees of freedom. Purpose:To study resonances in the $^{18}$O spectrum, populated in $^{14}$C+$α$ elastic scattering. Method: The Thick Target Inverse Kinematics (TTIK) technique was used to measure the excitation function for the $^{14}$C+$α$ elastic scattering. A 42 MeV $^{14}$C beam was used to populate states of excitation energy up to 14.9 MeV in $^{18}$O. The analysis was performed using a multi-level, multi-channel R-Matrix approach. Results: Detailed spectroscopic information, including spin-parities, partial $α$- and neutron- decay widths and dimensionless reduced widths, was obtained for excited states in $^{18}$O between 8 and 14.9 MeV in excitation energy. Cluster-Nucleon Configuration Interaction Model calculations of the same quantities are performed and compared to the experimental results. Conclusions: Strong fragmentation of large $α$-cluster strengths is observed in the spectrum of $^{18}$O making the $α$-cluster structure of $^{18}$O quite different from the pattern of known quasi-rotational bands of alternating parity that are characteristic of $N=Z$, even-even nuclei like $^{16}$O and $^{20}$Ne.

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arXiv2012

Suppression of the centrifugal barrier effects in the off-energy-shell neutron+$^{17}$O interaction

The reaction $^{17}$O($n,α$)$^{14}$C was studied at energies from $E_{cm}=0$ to $E_{cm}=350$ keV using the quasi-free deuteron break-up in the three body reaction $^{17}$O$+d \rightarrow α+ ^{14}$C$+p$, extending the Trojan Horse indirect method (THM) to neutron-induced reactions. It is found that the $^{18}$O excited state at $E^*=8.125 \pm 0.002$ MeV observed in THM experiments is absent in the direct measurement because of its high centrifugal barrier. The angular distributions of the populated resonances have been measured for the first time. The results unambiguously indicate the ability of the THM to overcome the centrifugal barrier suppression effect and to pick out the contribution of the bare nuclear interaction.

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arXiv2011

Molecular Structures in T=1 states of 10B

Multi-center (molecular) structures can play an important role in light nuclei. The highly deformed rotational band in 10Be with band head at 6.179 MeV has been observed recently and suggested to have an exotic alpha:2n:alpha configuration. A search for states with alpha:pn:alpha two-center molecular configurations in 10B that are analogous to the states with alpha:2n:alpha structure in 10Be has been performed. The T=1 isobaric analog states in 10B were studied in the excitation energy range of E=8.7-12.1 MeV using the reaction 1H(9Be,alpha)6Li*(T=1, 0+, 3.56 MeV). An R-matrix analysis was used to extract parameters for the states observed in the (p,alpha) excitation function. Five T=1 states in 10B have been identified. The known 2+ and 3- states at 8.9 MeV have been observed and their partial widths have been measured. The spin-parities and partial widths for three higher lying states were determined. Our data support theoretical predictions that the 2+ state at 8.9 MeV (isobaric analog of the 7.54 MeV state in 10Be) is a highly clustered state and can be identified as a member of the alpha:np:alpha rotational band. The next member of this band, the 4+ state, has not been found. A very broad 0+ state at 11 MeV that corresponds to pure alpha+6Li(0+,T=1) configuration is suggested and it might be related to similar structures found in 12C, 18O and 20Ne.

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arXiv2010

High-spin states in $^{22}$Ne

The structure of high-lying states in $^{22}$Ne has been studied using the $^{14}$C($^{12}$C,$α$)$^{22}$Ne reaction at E($^{12}$C)= 44 MeV. The spins were determined by measuring double ($α$,$α$) angular correlations. Selective population of the 9$^-$ and 11$^-$ states at E$_x$=20.1 and 20.7 MeV, respectively, identifies those states as the 9$^-$ and 11$^-$ members of the first $K^π$ = 0$^-$ band, whose lower members were investigated by a method using inverse kinematics and a thick gas target. The spin and parity of four other new levels were determined to be 9$^-$ (21.5 MeV),12$^+$ (22.1 MeV),9$^-$ (25.0 MeV) and 8$^+$ (22.9 MeV). The two levels 9$^-$ and 12$^+$ may belong to the rotational doublets.

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