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Intruder structures in $^{32}$Si and $^{29}$Al
Authors:
J. Williams,
G. Hackman,
K. Starosta,
R. S. Lubna,
Priyanka Choudhary,
Subhrajit Sahoo,
P. C. Srivastava,
C. Andreoiu,
D. Annen,
H. Asch,
M. D. H. K. G. Badanage,
G. C. Ball,
M. Beuschlein,
H. Bidaman,
V. Bildstein,
R. J. Coleman,
A. B. Garnsworthy,
B. Greaves,
G. Leckenby,
V. Karayonchev,
M. S. Martin,
C. Natzke,
C. M. Petrache,
A. Radich,
E. Raleigh-Smith
, et al. (9 additional authors not shown)
Abstract:
We have studied $^{32}$Si and $^{29}$Al using $^{12}$C($^{22}$Ne,2p) and $^{12}$C($^{22}$Ne,$α$p) fusion-evaporation reactions. In both cases, we observed significant population of high-spin structures distinct from the ground-state yrast bands. In $^{32}$Si, most of the high-energy states feed into a $J^π = 5^-$ nanosecond isomer. In $^{29}$Al, we identified a rotor-like negative-parity band with…
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We have studied $^{32}$Si and $^{29}$Al using $^{12}$C($^{22}$Ne,2p) and $^{12}$C($^{22}$Ne,$α$p) fusion-evaporation reactions. In both cases, we observed significant population of high-spin structures distinct from the ground-state yrast bands. In $^{32}$Si, most of the high-energy states feed into a $J^π = 5^-$ nanosecond isomer. In $^{29}$Al, we identified a rotor-like negative-parity band with a $J^π = 7/2^-$ band-head. Doppler shift lifetime measurements were performed for all observed states. These results were compared to shell model calculations and interpreted in terms of proton and neutron cross-shell excitation.
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Submitted 27 June, 2025;
originally announced June 2025.
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In-beam $γ$-ray spectroscopy towards the proton dripline: The curious case of $^{32}$Ar
Authors:
T. Beck,
A. Gade,
B. A. Brown,
Y. Utsuno,
D. Weisshaar,
D. Bazin,
K. W. Brown,
R. J. Charity,
P. J. Farris,
S. A. Gillespie,
A. M. Hill,
J. Li,
B. Longfellow,
W. Reviol,
D. Rhodes
Abstract:
High-resolution in-beam $γ$-ray spectroscopy was used to study excited states of the neutron-deficient nucleus $^{32}$Ar populated in fast-beam induced four- and six-nucleon removal reactions from $^{36,38}$Ca. One new $γ$-ray transition and indications for an additional two were found, allowing for a glimpse at the level scheme beyond the known $2^+_1$ state. The nature of the new $1900(4)$-keV t…
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High-resolution in-beam $γ$-ray spectroscopy was used to study excited states of the neutron-deficient nucleus $^{32}$Ar populated in fast-beam induced four- and six-nucleon removal reactions from $^{36,38}$Ca. One new $γ$-ray transition and indications for an additional two were found, allowing for a glimpse at the level scheme beyond the known $2^+_1$ state. The nature of the new $1900(4)$-keV transition is discussed in the context of the known energy spectrum of the mirror nucleus $^{32}$Si and shell-model calculations using the FSU and SDPF-M cross-shell effective interactions. Its resulting parent state at $3767(5)$ keV, more than $1.3$ MeV above the proton separation energy, is tentatively assigned to have mixed sd-shell and $2p$-$2h$ character. It might either be the mirror of the $J^π=2^+_2$ state of $^{32}$Si at $4230.8(8)$ keV, but with a decay branch favoring a transition to the $2^+_1$ over the ground state, or the mirror of the $4983.9(11)$-keV state with quantum numbers $0^+$. The resulting mirror-energy differences of $-473(5)$ and $-1218(5)$ keV are both sizable when compared to systematics; in the latter case it would, in fact, be among the largest reported to date in the entire nuclear chart or suggest the potential existence of an additional, hitherto unidentified, low-lying $0^+$ state of $^{32}$Si.
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Submitted 6 December, 2024;
originally announced December 2024.
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In-beam $γ$-ray spectroscopy of negative-parity states of $^{37}$K populated in dissipative reactions
Authors:
T. Beck,
A. Gade,
B. A. Brown,
D. Weisshaar,
D. Bazin,
K. W. Brown,
R. J. Charity,
P. J. Farris,
S. A. Gillespie,
A. M. Hill,
J. Li,
B. Longfellow,
W. Reviol,
D. Rhodes
Abstract:
In-beam $γ$-ray spectroscopy was used to study excited states of the neutron-deficient nucleus $^{37}$K populated in fast-beam inelastic-scattering and proton-removal reactions at high-momentum loss. New $γ$-ray transitions and $γγ$ coincidence relationships were established using the $γ$-ray tracking array GRETINA. The extension of the level scheme up to the first $(13/2^-)$ state highlights the…
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In-beam $γ$-ray spectroscopy was used to study excited states of the neutron-deficient nucleus $^{37}$K populated in fast-beam inelastic-scattering and proton-removal reactions at high-momentum loss. New $γ$-ray transitions and $γγ$ coincidence relationships were established using the $γ$-ray tracking array GRETINA. The extension of the level scheme up to the first $(13/2^-)$ state highlights the potential of this recently demonstrated population pathway for studies of isospin symmetry involving mirror-energy differences. The nature of the newly identified states is discussed in comparison to shell-model calculations with the FSU cross-shell effective interaction. The calculated occupation numbers of individual orbitals are shown to offer a consistent explanation of the measured mirror-energy differences between $^{37}$K and $^{37}$Ar.
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Submitted 23 November, 2024;
originally announced November 2024.
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Coulomb Excitation of $^{80}$Sr and the limits of the $N = Z = 40$ island of deformation
Authors:
R. Russell,
J. Heery,
J. Henderson,
R. Wadsworth,
K. Kaneko,
N. Shimizu,
T. Mizusaki,
Y. Sun,
C. Andreoiu,
D. W. Annen,
A. A. Avaa,
G. C. Ball,
V. Bildstein,
S. Buck,
C. Cousins,
A. B. Garnsworthy,
S. A. Gillespie,
B. Greaves,
A. Grimes,
G. Hackman,
R. O. Hughes,
D. G. Jenkins,
T. M. Kowalewski,
M. S. Martin,
C. Müller-Gatermann
, et al. (11 additional authors not shown)
Abstract:
The region of $N\approx Z\approx 40$ has long been associated with strongly deformed nuclear configurations. The presence of this strong deformation was recently confirmed through lifetime measurements in $N\approx Z$ Sr and Zr nuclei. Theoretically, however, these nuclei present a challenge due to the vast valence space required to incorporate all deformation driving interactions. Recent state-of…
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The region of $N\approx Z\approx 40$ has long been associated with strongly deformed nuclear configurations. The presence of this strong deformation was recently confirmed through lifetime measurements in $N\approx Z$ Sr and Zr nuclei. Theoretically, however, these nuclei present a challenge due to the vast valence space required to incorporate all deformation driving interactions. Recent state-of-the-art predictions indicate a near axial prolate deformation for $N=Z$ and $N=Z+2$ nuclei between $N=Z=36$ and $N=Z=40$. In this work we investigate the shores of this island of deformation through a sub-barrier Coulomb excitation study of the $N=Z+4$ nucleus, \textsuperscript{80}Sr. Extracting a spectroscopic quadrupole moment of $Q_s(2^+_1) = 0.45^{+0.83}_{-0.88}$~eb, we find that \textsuperscript{80}Sr is inconsistent with significant axial prolate deformation. This indicates that the predicted region of strong prolate deformation around $N=Z=40$ is tightly constrained to the quartet of nuclei: \textsuperscript{76,78}Sr and \textsuperscript{78,80}Zr.
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Submitted 15 August, 2024;
originally announced August 2024.
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Probing proton cross-shell excitations through the two-neutron removal from 38Ca
Authors:
T. Beck,
A. Gade,
B. A. Brown,
J. A. Tostevin,
D. Weisshaar,
D. Bazin,
K. W. Brown,
R. J. Charity,
P. J. Farris,
S. A. Gillespie,
A. M. Hill,
J. Li,
B. Longfellow,
W. Reviol,
D. Rhodes
Abstract:
Bound states of the neutron-deficient, near-dripline nucleus $^{36}$Ca were populated in two-neutron removal from the ground state of $^{38}$Ca, a direct reaction sensitive to the single-particle configurations and couplings of the removed neutrons in the projectile wave function. Final-state exclusive cross sections for the formation of $^{36}$Ca and the corresponding longitudinal momentum distri…
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Bound states of the neutron-deficient, near-dripline nucleus $^{36}$Ca were populated in two-neutron removal from the ground state of $^{38}$Ca, a direct reaction sensitive to the single-particle configurations and couplings of the removed neutrons in the projectile wave function. Final-state exclusive cross sections for the formation of $^{36}$Ca and the corresponding longitudinal momentum distributions, both determined through the combination of particle and $γ$-ray spectroscopy, are compared to predictions combining eikonal reaction theory and shell-model two-nucleon amplitudes from the USDB, USDC, and ZBM2 effective interactions. The final-state cross-section ratio $σ(2^+_1)/σ(0^+)$ shows particular sensitivity and is approximately reproduced only with the two-nucleon amplitudes from the ZBM2 effective interaction that includes proton cross-shell excitations into the $pf$ shell. Characterizing the proton $pf$-shell occupancy locally and schematically, an increase of the $sd$-$pf$ shell gap by $250$ keV yields an improved description of this cross-section ratio and simultaneously enables a reproduction of the $B(E2;0^+_1\to2^+_1)$ excitation strength of $^{36}$Ca. This highlights an important aspect if a new shell-model effective interaction for the region was to be developed on the quest to model the neutron-deficient Ca isotopes and surrounding nuclei whose structure is impacted by proton cross-shell excitations.
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Submitted 15 January, 2024;
originally announced January 2024.
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Identifying the spin trapped character of the $^{32}$Si isomeric state
Authors:
J. Williams,
G. Hackman,
K. Starosta,
R. S. Lubna,
Priyanka Choudhary,
P. C. Srivastava,
C. Andreoiu,
D. Annen,
H. Asch,
M. D. H. K. G. Badanage,
G. C. Ball,
M. Beuschlein,
H. Bidaman,
V. Bildstein,
R. Coleman,
A. B. Garnsworthy,
B. Greaves,
G. Leckenby,
V. Karayonchev,
M. S. Martin,
C. Natzke,
C. M. Petrache,
A. Radich,
E. Raleigh-Smith,
D. Rhodes
, et al. (8 additional authors not shown)
Abstract:
The properties of a nanosecond isomer in $^{32}$Si, disputed in previous studies, depend on the evolution of proton and neutron shell gaps near the `island of inversion'. We have placed the isomer at 5505.2(2) keV with $J^π = 5^-$, decaying primarily via an $E3$ transition to the $2^+_1$ state. The $E3$ strength of 0.0841(10) W.u. is unusually small and suggests that this isomer is dominated by th…
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The properties of a nanosecond isomer in $^{32}$Si, disputed in previous studies, depend on the evolution of proton and neutron shell gaps near the `island of inversion'. We have placed the isomer at 5505.2(2) keV with $J^π = 5^-$, decaying primarily via an $E3$ transition to the $2^+_1$ state. The $E3$ strength of 0.0841(10) W.u. is unusually small and suggests that this isomer is dominated by the $(νd_{3/2})^{-1} \otimes (νf_{7/2})^{1}$ configuration, which is sensitive to the $N=20$ shell gap. A newly observed $4^+_1$ state is placed at 5881.4(13) keV; its energy is enhanced by the $Z=14$ subshell closure. This indicates that the isomer is located in a `yrast trap', a feature rarely seen at low mass numbers.
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Submitted 15 November, 2023;
originally announced November 2023.
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Dissipative reactions with intermediate-energy beams -- a novel approach to populate complex-structure states in rare isotopes
Authors:
A. Gade,
B. A. Brown,
D. Weisshaar,
D. Bazin,
K. W. Brown,
R. J. Charity,
P. Farris,
A. M. Hill,
J. Li,
B. Longfellow,
D. Rhodes,
W. Reviol,
J. A. Tostevin
Abstract:
A novel pathway for the formation of multi-particle-multi-hole (np-mh) excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of 38Ca on a Be target. The negative-parity,complex-structure final states in 38Ca were observed following the in-beam gamma-ray spectroscopy of events in the…
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A novel pathway for the formation of multi-particle-multi-hole (np-mh) excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of 38Ca on a Be target. The negative-parity,complex-structure final states in 38Ca were observed following the in-beam gamma-ray spectroscopy of events in the 9Be(38Ca,38Ca+gamma)X reaction in which the scattered projectile lost longitudinal momentum of order p = 700 MeV/c. The characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly-energetic projectile-target collision. Unlike the far-less dissipative, surface-grazing reactions usually exploited for the in-beam gamma-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multi-particle configurations in rare isotopes - final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.
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Submitted 30 November, 2022;
originally announced November 2022.
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Exploiting dissipative reactions to perform in-beam $γ$-ray spectroscopy of the neutron-deficient isotopes \nuc{38,39}{Ca}
Authors:
A. Gade,
D. Weisshaar,
B. A. Brown,
D. Bazin,
K. W. Brown,
R. J. Charity,
P. Farris,
A. M. Hill,
J. Li,
B. Longfellow,
D. Rhodes,
W. Reviol,
J. A. Tostevin
Abstract:
The neutron-deficient Ca isotopes continue to attract attention due to their importance for testing isospin symmetry and their relevance in capture reactions of interest for nova nucleosynthesis and the shape of light curves in Type I X-ray bursts. To date, spectroscopic information on 38,39 Ca is largely limited to data on lower-spin excited states. Here, we report in-beam γ-ray spectroscopy of c…
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The neutron-deficient Ca isotopes continue to attract attention due to their importance for testing isospin symmetry and their relevance in capture reactions of interest for nova nucleosynthesis and the shape of light curves in Type I X-ray bursts. To date, spectroscopic information on 38,39 Ca is largely limited to data on lower-spin excited states. Here, we report in-beam γ-ray spectroscopy of complementary higher-spin, complex-structure states in 39 Ca populated in fast-beam-induced, momentum-dissipative processes leading to neutron pickup onto excited configurations of the projectile, 9 Be(38 Ca , 39 Ca + γ)X. Such a dissipative reaction was recently characterized for the case of inelastic scattering of 38 Ca off 9 Be, 9 Be(38 Ca, 38 Ca + γ)X. Additional data and discussion on the nuclear structure of 38 Ca is also presented. An explanation for the more-complex-structure states, populated with small cross sections in one-nucleon knockout reactions, and observed in the tails of their longitudinal momentum distributions, is also offered.
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Submitted 3 October, 2022;
originally announced October 2022.
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Shape Coexistence at Zero Spin in 64Ni Driven by the Monopole Tensor Interaction
Authors:
N. Mărginean,
D. Little,
Y. Tsunoda,
S. Leoni,
R. V. F. Janssens,
B. Fornal,
T. Otsuka,
C. Michelagnoli,
L. Stan,
F. C. L. Crespi,
C. Costache,
R. Lica,
M. Sferrazza,
A. Turturica,
A. D. Ayangeakaa,
K. Auranen,
M. Barani,
P. C. Bender,
S. Bottoni,
M. Boromiza,
A. Bracco,
S. Călinescu,
C. M. Campbell,
M. P. Carpenter,
P. Chowdhury
, et al. (53 additional authors not shown)
Abstract:
The low-spin structure of the semimagic 64Ni nucleus has been considerably expanded: combining four experiments, several 0+ and 2+ excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0+ excitation is located at a surprisingly high energy (34…
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The low-spin structure of the semimagic 64Ni nucleus has been considerably expanded: combining four experiments, several 0+ and 2+ excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0+ excitation is located at a surprisingly high energy (3463 keV), with a collective 2+ state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N = 40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.
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Submitted 11 August, 2020;
originally announced August 2020.
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In-beam $γ$-ray spectroscopy at the proton dripline: $^{40}$Sc
Authors:
A. Gade,
D. Weisshaar,
B. A. Brown,
J. A. Tostevin,
D. Bazin,
K. Brown,
R. J. Charity,
P. J. Farris,
A. M. Hill,
J. Li,
B. Longfellow,
W. Reviol,
D. Rhodes
Abstract:
We report on the first in-beam $γ$-ray spectroscopy of the proton-dripline nucleus $^{40}$Sc using two-nucleon pickup onto an intermediate-energy rare-isotope beam of $^{38}$Ca. The $^{9}$Be($^{38}$Ca,$^{40}$Sc$+γ$)X reaction at 60.9 MeV/nucleon mid-target energy selectively populates states in $^{40}$Sc for which the transferred proton and neutron couple to high orbital angular momentum. In turn,…
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We report on the first in-beam $γ$-ray spectroscopy of the proton-dripline nucleus $^{40}$Sc using two-nucleon pickup onto an intermediate-energy rare-isotope beam of $^{38}$Ca. The $^{9}$Be($^{38}$Ca,$^{40}$Sc$+γ$)X reaction at 60.9 MeV/nucleon mid-target energy selectively populates states in $^{40}$Sc for which the transferred proton and neutron couple to high orbital angular momentum. In turn, due to angular-momentum selection rules in proton emission and the nuclear structure and energetics of $^{39}$Ca, such states in $^{40}$Sc then exhibit $γ$-decay branches although they are well above the proton separation energy. This work uniquely complements results from particle spectroscopy following charge-exchange reactions on $^{40}$Ca as well as $^{40}$Ti EC/$β^+$ decay which both display very different selectivities. The population and $γ$-ray decay of the previously known first $(5^-)$ state at 892 keV and the observation of a new level at 2744 keV are discussed in comparison to the mirror nucleus and shell-model calculations. On the experimental side, this work shows that high-resolution in-beam $γ$-ray spectroscopy is possible with new generation Ge arrays for reactions induced by rare-isotope beams on the level of a few $μ$b of cross section.
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Submitted 22 July, 2020;
originally announced July 2020.
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Experimental identification of the $T = 1$, $J^π = 6^+$ state of $^{54}$Co and isospin symmetry in $A = 54$ studied via one-nucleon knockout reactions
Authors:
M. Spieker,
D. Weisshaar,
A. Gade,
B. A. Brown,
P. Adrich,
D. Bazin,
M. A. Bentley,
J. R. Brown,
C. M. Campbell,
C. Aa. Diget,
B. Elman,
T. Glasmacher,
M. Hill,
B. Longfellow,
B. Pritychenko,
A. Ratkiewicz,
D. Rhodes,
J. A. Tostevin
Abstract:
New experimental data obtained from $γ$-ray tagged one-neutron and one-proton knockout from $^{55}$Co is presented. A candidate for the sought-after $T=1, T_z = 0, J^π = 6^+$ state in $^{54}$Co is proposed based on a comparison to the new data on $^{54}$Fe, the corresponding observables predicted by large-scale-shell-model (LSSM) calculations in the full $fp$-model space employing charge-dependent…
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New experimental data obtained from $γ$-ray tagged one-neutron and one-proton knockout from $^{55}$Co is presented. A candidate for the sought-after $T=1, T_z = 0, J^π = 6^+$ state in $^{54}$Co is proposed based on a comparison to the new data on $^{54}$Fe, the corresponding observables predicted by large-scale-shell-model (LSSM) calculations in the full $fp$-model space employing charge-dependent contributions, and isospin-symmetry arguments. Furthermore, possible isospin-symmetry breaking in the $A=54$, $T=1$ triplet is studied by calculating the experimental $c$ coefficients of the isobaric mass multiplet equation (IMME) up to the maximum possible spin $J=6$ expected for the $(1f_{7/2})^{-2}$ two-hole configuration relative to the doubly-magic nucleus $^{56}$Ni. The experimental quantities are compared to the theoretically predicted $c$ coefficients from LSSM calculations using two-body matrix elements obtained from a realistic chiral effective field theory potential at next-to-next-to-next-to-leading order (N$^3$LO).
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Submitted 23 March, 2020;
originally announced March 2020.
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Two-neutron knockout as a probe of the composition of states in $^{22}$Mg, $^{23}$Al, and $^{24}$Si
Authors:
B. Longfellow,
A. Gade,
J. A. Tostevin,
E. C. Simpson,
B. A. Brown,
A. Magilligan,
D. Bazin,
P. C. Bender,
M. Bowry,
B. Elman,
E. Lunderberg,
D. Rhodes,
M. Spieker,
D. Weisshaar,
S. J. Williams
Abstract:
Simpson and Tostevin proposed that the width and shape of exclusive parallel momentum distributions of the A-2 residue in direct two-nucleon knockout reactions carry a measurable sensitivity to the nucleon single-particle configurations and their couplings within the wave functions of exotic nuclei. We report here on the first benchmarks and use of this new spectroscopic tool. Exclusive parallel m…
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Simpson and Tostevin proposed that the width and shape of exclusive parallel momentum distributions of the A-2 residue in direct two-nucleon knockout reactions carry a measurable sensitivity to the nucleon single-particle configurations and their couplings within the wave functions of exotic nuclei. We report here on the first benchmarks and use of this new spectroscopic tool. Exclusive parallel momentum distributions for states in the neutron-deficient nuclei $^{22}$Mg, $^{23}$Al, and $^{24}$Si populated in such direct two-neutron removal reactions were extracted and compared to predictions combining eikonal reaction theory and shell-model calculations. For the well-known $^{22}$Mg and $^{23}$Al nuclei, measurements and calculations were found to agree, supporting the dependence of the parallel momentum distribution width on the angular momentum composition of the shell-model two-neutron amplitudes. In $^{24}$Si, a level at 3439(9) keV, of relevance for the important $^{23}$Al(p,$γ$)$^{24}$Si astrophysical reaction rate, was confirmed to be the $2^+_2$ state, while the $4^+_1$ state, expected to be strongly populated in two-neutron knockout, was not observed. This puzzle is resolved by theoretical considerations of the Thomas-Ehrman shift, which also suggest that a previously reported 3471-keV state in $^{24}$Si is in fact the ($0^+_2$) level with one of the largest experimental mirror-energy shifts ever observed.
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Submitted 26 February, 2020;
originally announced February 2020.
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Evidence for rigid triaxial deformation in $^{76}$Ge from a model-independent analysis
Authors:
A. D. Ayangeakaa,
R. V. F. Janssens,
S. Zhu,
D. Little,
J. Henderson,
C. Y. Wu,
D. J. Hartley,
M. Albers,
K. Auranen,
B. Bucher,
M. P. Carpenter,
P. Chowdhury,
D. Cline,
H. L. Crawford,
P. Fallon,
A. M. Forney,
A. Gade,
A. B. Hayes,
F. G. Kondev,
Krishichayan,
T. Lauritsen,
J. Li,
A. O. Macchiavelli,
D. Rhodes,
D. Seweryniak
, et al. (3 additional authors not shown)
Abstract:
An extensive, model-independent analysis of the nature of triaxial deformation in $^{76}$Ge, a candidate for neutrinoless double-beta ($0νββ$) decay, was carried out following multi-step Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and $γ$ bands. Both sequence…
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An extensive, model-independent analysis of the nature of triaxial deformation in $^{76}$Ge, a candidate for neutrinoless double-beta ($0νββ$) decay, was carried out following multi-step Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and $γ$ bands. Both sequences were determined to be characterized by the same $β$ and $γ$ deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in $^{76}$Ge was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured $E2$ matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to $0νββ$.
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Submitted 7 September, 2019;
originally announced September 2019.
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One-proton and one-neutron knockout reactions from $N = Z = 28$ $^{56}$Ni to the $A = 55$ mirror pair $^{55}$Co and $^{55}$Ni
Authors:
M. Spieker,
A. Gade,
D. Weisshaar,
B. A. Brown,
J. A. Tostevin,
B. Longfellow,
P. Adrich,
D. Bazin,
M. A. Bentley,
J. R. Brown,
C. M. Campbell,
C. Aa. Diget,
B. Elman,
T. Glasmacher,
M. Hill,
B. Pritychenko,
A. Ratkiewicz,
D. Rhodes
Abstract:
We present a high-resolution in-beam $γ$-ray spectroscopy study of excited states in the mirror nuclei $^{55}$Co and $^{55}$Ni following one-nucleon knockout from a projectile beam of $^{56}$Ni. The newly determined partial cross sections and the $γ$-decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. A mirror asy…
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We present a high-resolution in-beam $γ$-ray spectroscopy study of excited states in the mirror nuclei $^{55}$Co and $^{55}$Ni following one-nucleon knockout from a projectile beam of $^{56}$Ni. The newly determined partial cross sections and the $γ$-decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. A mirror asymmetry for the partial cross sections leading to the two lowest $3/2^-$ states in the $A = 55$ mirror pair was identified as well as a significant difference in the $E1$ decays from the $1/2^+_1$ state to the same two $3/2^-$ states. The mirror asymmetry in the partial cross sections cannot be reconciled with the present shell-model picture or small mixing introduced in a two-state model. The observed mirror asymmetry in the $E1$ decay pattern, however, points at stronger mixing between the two lowest $3/2^-$ states in $^{55}$Co than in its mirror $^{55}$Ni.
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Submitted 16 August, 2019;
originally announced August 2019.
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Is the structure of 42Si understood?
Authors:
A. Gade,
B. A. Brown,
J. A. Tostevin,
D. Bazin,
P. C. Bender,
C. M. Campbell,
H. L. Crawford,
B. Elman,
K. W. Kemper,
B. Longfellow,
E. Lunderberg,
D. Rhodes,
D. Weisshaar
Abstract:
A more detailed test of the implementation of nuclear forces that drive shell evolution in the pivotal nucleus \nuc{42}{Si} -- going beyond earlier comparisons of excited-state energies -- is important. The two leading shell-model effective interactions, SDPF-MU and SDPF-U-Si, both of which reproduce the low-lying \nuc{42}{Si}($2^+_1$) energy, but whose predictions for other observables differ sig…
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A more detailed test of the implementation of nuclear forces that drive shell evolution in the pivotal nucleus \nuc{42}{Si} -- going beyond earlier comparisons of excited-state energies -- is important. The two leading shell-model effective interactions, SDPF-MU and SDPF-U-Si, both of which reproduce the low-lying \nuc{42}{Si}($2^+_1$) energy, but whose predictions for other observables differ significantly, are interrogated by the population of states in neutron-rich \nuc{42}{Si} with a one-proton removal reaction from \nuc{43}{P} projectiles at 81~MeV/nucleon. The measured cross sections to the individual \nuc{42}{Si} final states are compared to calculations that combine eikonal reaction dynamics with these shell-model nuclear structure overlaps. The differences in the two shell-model descriptions are examined and linked to predicted low-lying excited $0^+$ states and shape coexistence. Based on the present data, which are in better agreement with the SDPF-MU calculations, the state observed at 2150(13)~keV in \nuc{42}{Si} is proposed to be the ($0^+_2$) level.
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Submitted 10 May, 2019;
originally announced May 2019.