arXiv subjects

K. W. Kemper

Publications and source records attributed to K. W. Kemper.

At least 19 recordsLinked to original sources

arXiv2026

Detailed experimental study of excited states in $^{50}$Ti via the $(d,p)$ and $(d,pγ)$ reactions

Excited states of semi-magic $^{50}$Ti were studied up to the neutron-separation energy via the $(d,p)$ and $(d,pγ)$ reactions. In total, 82 excited states were identified based on the measurement of angular distributions with the Super-Enge Split-Pole Spectrograph (SE-SPS) at Florida State University. From the experimental data, sum rules related to vacancies were calculated for the $2p_{3/2}$, $2p_{1/2}$, $1f_{5/2}$, $1g_{9/2}$, and $2d_{5/2}$ neutron single-particle orbitals and compared to predictions obtained with the time-dependent continuum shell model (TDCSM), the quasiparticle-phonon model (QPM), and the relativistic equation of motion theory (REOM$^3$). A comparison for the $(d,p)$ data obtained for $^{51}$Ti and $^{50}$Ti is also presented, focusing on differences of the single-particle strength fragmentation in even-$A$ and odd-$A$ nuclei.

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arXiv2026

Observing the effects of numbers of valence nucleons on $0_{gs}^+ \rightarrow 2_1^+$ transitions in deformed nuclei by comparing proton and neutron transition matrix elements

We examined the ratios of neutron and proton transition matrix elements, $M_n/M_p$, for the $0_{gs}^+ \rightarrow 2_1^+$ transitions in 48 even-even stable nuclei with $N>20$ for which electromagnetic matrix elements were compiled by Pritychenko \textit{et al.} and for which high-quality inelastic proton scattering data were available. Several deformed rare earth nuclei have $(M_n/M_p)/(N/Z)$ values significantly below 1.0, which is not consistent with a simple liquid drop picture. However, this phenomenon can be explained using a schematic picture in which $M_p$ reaches a maximum at proton mid-shell ($Z=66$) and $M_n$ reaches its maximum at neutron mid-shell ($N=104$). Several mid-mass vibrational nuclei have $M_n/M_p$ values significantly below $N/Z$, which is not consistent with the expectation that $M_n/M_p = N/Z$ in such nuclei. A shell model investigation of these observations might yield insights about this behavior.

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arXiv2026

Experimental study of $^{53}$Cr via the $(d,pγ)$ reaction

Excited states in $^{53}$Cr were studied via the $^{52}$Cr$(d,pγ)$ reaction up to the neutron-separation threshold. Proton-$γ$ angular correlations and $γ$ decay branching ratios were measured in particle-$γ$ coincidences between the Super-Enge Split-Pole Spectrograph (SE-SPS) and CeBr$_3$ Array (CeBrA) demonstrator of the John D. Fox Accelerator Laboratory at Florida State University. Previous spin-parity assignments from a $(d,p)$ singles experiment at the SE-SPS are supported and $γ$-ray transitions in $^{53}$Cr reported. We firmly assign higher-lying excited states to $^{53}$Cr because overlapping excited states and contaminants could be identified better due to the complementary $γ$-decay information. We also correct some of the previously reported excitation energies and present a reanalysis of previously measured $^{52}$Cr$(d,p){}^{53}${Cr} angular distributions guided by the complementary $γ$-ray information. Based on this reanalysis, the fragmentation of the neutron $2p_{3/2}$, $2p_{1/2}$, $1f_{5/2}$, $1g_{9/2}$, and $2d_{5/2}$ single-particle strengths is reassessed for $^{53}$Cr. A comparison to the corresponding strengths in $^{55}$Fe is presented.

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arXiv2025

Determination of proton and neutron contributions to the $0_{g.s.}^+ \rightarrow 2_1^+$ excitations in $^{42}$Si and $^{44}$S using inelastic proton scattering in inverse kinematics and intermediate energy Coulomb excitation

We have measured the $0_{g.s.}^+ \rightarrow 2_1^+$ transition in the neutron rich $N=28$ isotope $^{42}$Si using the probes of intermediate energy Coulomb excitation and inelastic proton scattering in inverse kinematics at the Facility for Rare Isotope Beams with beam particle rates of $\approx 5$ particles/s. The results of these two measurements allowed us to determine $M_n/M_p$, the ratio of the neutron and proton transition matrix elements for the $0_{g.s.}^+ \rightarrow 2_1^+$ transition. In addition, we have measured the $0_{\mathrm{g.s.}}^+ \rightarrow 2_1^+$ transition in the isotone $^{44}$S using inverse kinematics inelastic proton scattering. By comparing the $^{44}$S proton scattering result with a recent intermediate energy Coulomb excitation result on the same transition, we were able to determine $M_n/M_p$ for the $0_{g.s.}^+ \rightarrow 2_1^+$ transition in this nucleus as well. This work strengthens the evidence that $^{42}$Si has a stable quadrupole deformation in its ground state and that $^{44}$S does not. Both conclusions are further supported by shell model calculations carried out with the FSU interaction.

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arXiv2024

$^{11}$B states above the $α$-decay threshold studied via $^{10}$B$(d,p){}^{11}$B

The resonance region of $^{11}$B covering excitation energies from 8.4 MeV to 13.6 MeV was investigated with the $(d,p)$ reaction performed on an enriched $^{10}$B target at the Florida State University Super-Enge Split-Pole Spectrograph of the John D. Fox Superconducting Linear Accelerator Laboratory. Complementary measurements were performed with a target enriched in $^{11}$B to identify possible $^{12}$B contaminants in the $(d,p)$ reaction. Four strongly populated $^{11}$B states were observed above the $α$-decay threshold. Angular distributions were measured and compared to DWBA calculations to extract angular momentum transfers and $^{10}\mathrm{B}\left(3^+\right)+n$ spectroscopic factors. The recently observed and heavily discussed resonance at 11.4 MeV in $^{11}$B was not observed in this work. This result is consistent with the interpretation that it is predominantly a $^{10}\mathrm{Be}\left(0^+\right)+p$ resonance with a possible additional $^{7}\mathrm{Li}+α$ contribution. The predicted $^{10}\mathrm{B}\left(3^+\right)+n$ resonance at 11.6 MeV, analogous to the 11.4-MeV proton resonance, was not observed either. Upper limits for the $^{10}\mathrm{B}\left(3^+\right)+n$ spectroscopic factors of the 11.4-MeV and 11.6-MeV states were determined. In addition, supporting configuration interaction shell model calculations with the effective WBP interaction are presented.

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arXiv2024

Proton removal from $^{73,75}$Br to $^{72,74}$Se at intermediate energies

We report new experimental data for excited states of $^{72,74}$Se obtained from proton removal from $^{73,75}$Br secondary beams on a proton target. The experiments were performed with the Ursinus-NSCL Liquid Hydrogen Target and the combined GRETINA+S800 setup at the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory at Michigan State University. Within uncertainties, the inclusive cross sections for proton removal from $^{73,75}$Br on a proton target are identical suggesting that the same single-particle orbitals contribute to the proton-removal reaction. In addition, details of the partial cross section fragmentation are discussed. The data might suggest that $l = 1, 2, 3$, and 4 angular momentum transfers are important to understand the population of excited states of $^{72,74}$Se in proton removal. Available data for excited states of $^{74}$Ge populated through the $^{75}$As$(d,{}^{3}{\mathrm{He}}){}^{74}$Ge proton-removal reaction in normal kinematics suggest indeed that the $fp$ and $sd$ shell as well as the $1g_{9/2}$ orbital contribute. A comparison to data available for odd-$A$ nuclei supports that the bulk of the spectroscopic strengths could be found at lower energies in the even-even Se isotopes than in, for instance, the even-even Ge isotopes. In addition, the population of high-$J$ states seems to indicate that multi-step processes contribute to proton-removal reactions at intermediate energies in these collective nuclei.

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arXiv2023

$g_{9/2}$ neutron strength in the $N=29$ isotones and the $^{52}$Cr($d,p$)$^{53}$Cr reaction

We performed a measurement of the $^{52}$Cr$(d,p)^{53}$Cr reaction at 16 MeV using the Florida State University Super-Enge Split-Pole Spectrograph (SE-SPS) and observed 26 states. While all of the states observed here had been seen in previous $(d,p)$ experiments, we changed five $L$ assignments from those reported previously and determined $L$ values for nine states that had not had such assignments made previously. The $g_{9/2}$ neutron strength observed in $^{53}$Cr in the present work and in the $N=29$ isotones $^{49}$Ca, $^{51}$Ti, and $^{55}$Fe via $(d,p)$ reactions is much smaller than the sum rule for this strength. Most of the observed $L=4$ strength in these nuclei is located in states near 4 MeV excitation energy. The remaining $g_{9/2}$ strength may be located in the continuum or may be fragmented among many bound states. A covariant density functional theory calculation provides support for the hypothesis that the $g_{9/2}$ neutron orbit is unbound in $^{53}$Cr. The ($α,^3$He) reaction may provide a more sensitive probe for the missing $g_{9/2}$ neutron strength. In addition, particle-$γ$ coincidence experiments may help resolve some remaining questions in this nucleus.

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arXiv2023

Hexadecapole strength in the rare isotopes $^{74,76}$Kr

In the Ge-Sr mass region, isotopes with neutron number $N \leq 40$ are known to feature rapid shape changes with both nucleon number and angular momentum. To gain new insights into their structure, inelastic proton scattering experiments in inverse kinematics were performed on the rare isotopes $^{74,76}$Kr. This work focuses on observables related to the $J^π = 4^+_1$ states of the Kr isotopes and, in particular, on the hexadecapole degree of freedom. By performing coupled-channels calculations, hexadecapole deformation parameters $β_4$ were determined for the $J^π = 4^+_1$ states of $^{74,76}$Kr from inelastic proton scattering cross sections. Two possible coupled-channels solutions were found. A comparison to predictions from nuclear energy density functional theory, employing both non-relativistic and relativistic functionals, clearly favors the large, positive $β_4$ solutions. These $β_4$ values are unambiguously linked to the well deformed prolate configuration. Given the $β_2 - β_4$ trend, established in this work, it appears that $β_4$ values could provide a sensitive measure of the nuclear shell structure.

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arXiv2022

$^{54}$Fe($d$,$p$)$^{55}$Fe and the evolution of single neutron energies in the $N=29$ isotones

A measurement of the $^{54}$Fe($d$,$p$)$^{55}$Fe reaction at 16 MeV was performed using the Florida State University Super-Enge Split-Pole Spectrograph to determine single-neutron energies for the $2p_{3/2}$, $2p_{1/2}$, $1f_{5/2}$, $1g_{9/2}$ and $2d_{5/2}$ orbits. Two states were observed that had not been observed in previous (d, p) measurements. In addition, we made angular momentum transfer, \textit{L}, assignments to four states and changed \textit{L} assignments from previous ($d$, $p$) measurements for nine more states. The spin-orbit splitting between the $2p_{3/2}$ and $2p_{1/2}$ orbits is similar to that in the other $N=29$ isotones and not close to zero as a previous measurement suggested. While the $1f_{5/2}$ single neutron energy is significantly lower in $^{55}$Fe than in $^{51}$Ti, as predicted by a covariant density functional theory calculation, the single-neutron energy for this orbit in $^{55}$Fe is more than 1 MeV higher than the calculation suggests, although it is only 400 keV above the $2p_{1/2}$ orbit. The summed spectroscopic strength we observed for the $1g_{9/2}$ orbit up to the single-neutron separation energy of 9.3 MeV is only 0.3. This is surprising because the $1g_{9/2}$ orbit is predicted by Togashi \textit{et al.} to be located only 5.5 MeV above the $2p_{3/2}$ orbit.

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arXiv2022

Investigation of octupole collectivity near the $A =72$ shape-transitional point

Enhanced octupole collectivity is expected in the neutron-deficient Ge, Se and Kr isotopes with neutron number $N \approx 40$ and has indeed been observed for $^{70,72}$Ge. Shape coexistence and configuration mixing are, however, a notorious challenge for theoretical models trying to reliably predict octupole collectivity in this mass region, which is known to feature rapid shape changes with changing nucleon number and spin of the system. To further investigate the microscopic configurations causing the prolate-oblate-triaxial shape transition at $A \approx 72$ and their influence on octupole collectivity, the rare isotopes $^{72}$Se and $^{74,76}$Kr were studied via inelastic proton scattering in inverse kinematics. While significantly enhanced octupole strength of $\sim 32$ Weisskopf units (W.u.) was observed for $^{72}$Se, only strengths of $\sim 15$ W.u. were observed for $^{74,76}$Kr. In combination with existing data, the new data clearly question a simple origin of enhanced octupole strengths around $N = 40$. The present work establishes two regions of distinct octupole strengths with a sudden strength increase around the $A=72$ shape transitional point.

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arXiv2021

$^{50}$Ti($d$,$p$)$^{51}$Ti, single neutron energies in the $N=29$ isotones and the $N=32$ subshell closure

A measurement of the $^{50}$Ti($d$,$p$)$^{51}$Ti reaction at 16 MeV was performed using a Super Enge Split Pole Spectrograph to measure the magnitude of the $N=32$ subshell gap in Ti. Seven states were observed that had not been observed in previous ($d$,$p$) measurements, and the \textit{L} transfer values for six previously measured states were either changed or measured for the first time. The results were used to determine single neutron energies for the $p_{3/2}$, $p_{1/2}$ and $f_{5/2}$ orbitals. The resulting single neutron energies in $^{51}$Ti confirm the existence of the $N=32$ gap in Ti. These single neutron energies and those from previous measurements in $^{49}$Ca, $^{53}$Cr and $^{55}$Fe are compared to values from a covariant density functional theory calculation.

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arXiv2021

Experimental study of the $^{17}$F+ $^{12}$C fusion reaction and its implications for fusion of proton-halo systems

The halo nature of the low-lying $1/2$+ first excited state of the exotic weakly-bound proton drip-line nucleus $^{17}$F has long been hypothesized. The structure of such a halo nucleus would imply special nuclear properties including, possibly, an enhancement in its fusion cross section above the barrier. The total fusion cross section of $^{17}$F + $^{12}$C near the Coulomb barrier was studied using the newly developed "Encore" active-target detector at Florida State University. Total fusion cross sections for the stable counterpart systems $^{16}$O + $^{12}$C and $^{19}$F + $^{12}$C were also measured to enable a systematic comparison. No influence of the halo nature of the $^{17}$F $1/2$+ first excited state on its fusion excitation function was observed when compared with the stable counterpart systems.

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arXiv2020

Important Influence of Entrance Channel Reorientation Coupling on Proton Stripping

While it is well established that the ground state reorientation coupling can have a significant influence on the elastic scattering of deformed nuclei, the effect of such couplings on transfer channels has been much less well investigated. In this letter we demonstrate that the 208Pb(7Li,6He)209Bi proton stripping reaction at an incident energy of 52 MeV can be well described by the inclusion of the 7Li ground state reorientation coupling within the coupled channels Born approximation formalism. Full finite-range distorted wave Born approximation calculations were previously found to be unable to describe these data. Addition of coupling to the 0.478-MeV 1/2- excited state of 7Li, together with the associated two-step transfer path, has little or no influence on the shape of the angular distributions (except that for stripping leading to the 1.61-MeV 13/2+ level of 209Bi which is significantly improved) but does affect appreciably the values of the 209Bi -> 208Pb + p spectroscopic factors. Implications for experiments with weakly-bound light radioactive beams are discussed.

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arXiv2020

Multi-neutron transfer in $^{8}$He induced reactions near the Coulomb barrier

The measured inclusive $^6$He and $^4$He production cross sections of G. Marqu{í}nez-Dur{á}n {\em et al.}, Phys.\ Rev.\ C {\bf 98}, 034615 (2018) are reexamined and the conclusions concerning the relative importance of 1n and 2n transfer to the production of $^6$He arising from the interaction of a 22 MeV $^8$He beam with a $^{208}$Pb target revised. A consideration of the kinematics of the 2n-stripping reaction when compared with the measured $^6$He total energy versus angle spectrum places strict limits on the allowed excitation energy of the $^{210}$Pb residual, so constraining distorted wave Born approximation calculations that the contribution of the 2n stripping process to the inclusive $^6$He production can only be relatively small. It is therefore concluded that the dominant $^6$He production mechanism must be 1n stripping followed by decay of the $^7$He ejectile. Based on this result we present strong arguments in favor of direct, one step four-neutron (4n) stripping as the main mechanism for $^4$He production.

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arXiv2020

Shell structure of $^{43}$S and collapse of the $N=28$ shell closure

The single-particle structure of the $N=27$ isotones provides insights into the shell evolution of neutron-rich nuclei from the doubly-magic $^{48}$Ca toward the drip line. $^{43}$S was studied employing the one-neutron knockout reaction from a radioactive $^{44}$S beam. Using a combination of prompt and delayed $γ$-ray spectroscopy the level structure of $^{43}$S was clarified. Momentum distributions were analyzed and allowed for spin and parity assignments. The deduced spectroscopic factors show that the $^{44}$S ground-state configuration has a strong intruder component. The results were confronted with shell model calculations using two effective interactions. General agreement was found between the calculations, but strong population of states originating from the removal of neutrons from the $2p_{3/2}$ orbital in the experiment indicates that the breakdown of the $N=28$ magic number is more rapid than the theoretical calculations suggest.

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arXiv2020

Multi-step processes in heavy-ion induced single-nucleon transfer reactions

It was first noted during the 1970s that finite-range distorted wave Born approximation (FR-DWBA) calculations were unable satisfactorily to describe the shape of the angular distributions of many single-proton (and some single-neutron) transfer reactions induced by heavy ions, with calculations shifted to larger angles by up to ~ 4 degrees compared with the data. These reactions exhibited a significant mismatch, either of the reaction Q value or the grazing angular momentum of the entrance and exit channels, and it was speculated that the inclusion of multi-step transfer paths via excited state(s) of the projectile and/or ejectile could compensate for the effect of this mismatch and yield good descriptions of the data by shifting the calculated peaks to smaller angles. However, to date this has not been explicitly demonstrated for many reactions. In this work we show that inclusion of the two-step transfer path via the 4.44-MeV 2+ excited state of the 12C projectile in coupled channel Born approximation calculations enables a good description of the 208Pb(12C,11B)209Bi single-proton stripping data at four incident energies which could not be described by the FR-DWBA. We also show that inclusion of a similar reaction path for the 208Pb(12C,13C)207Pb single-neutron pickup reaction has a relatively minor influence, slightly improving the already good description obtained with the FR-DWBA.

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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

Inverse-kinematics proton scattering from $^{42,44}$S, $^{41,43}$P and the collapse of the $N=28$ major shell closure

Excited states of the neutron-rich isotopes $^{42,44}$S and $^{41,43}$P have been studied via inverse-kinematics proton scattering from a liquid hydrogen target, using the GRETINA $γ$-ray tracking array to extract inelastic scattering cross sections. Deformation lengths of the $2^+_1$ excitations in $^{42,44}$S have been determined and, when combined with deformation lengths determined with electromagnetic probes, yield the ratio of neutron-to-proton matrix elements $M_n/M_p$ for the $2^+_1$ excitations in these nuclei. The present results for $^{41,43}$P$(p,p')$ are used to compare two shell model interactions, SDPF-U and SDPF-MU. As in a recent study of $^{42}$Si, the present results on $^{41,43}$P favor the SDPF-MU interaction.

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