arXiv subjects

S. G. Pickstone

Publications and source records attributed to S. G. Pickstone.

5 recordsLinked to original sources

arXiv2018

Shape coexisistence and collective low-spin states in $^{112,114}$Sn studied with the $(p,p'γ)$ DSA coincidence technique

Proton-scattering experiments followed by the coincident spectroscopy of $γ$ rays have been performed at the Institute for Nuclear Physics of the University of Cologne to excite low-spin states in $^{112}$Sn and $^{114}$Sn, to determine their lifetimes and extract reduced transitions strengths $B(ΠL)$. The combined spectroscopy setup SONIC@HORUS has been used to detect the scattered protons and the emitted $γ$ rays of excited states in coincidence. The novel $(p,p'γ)$ DSA coincidence technique was employed to measure sub-ps nuclear level lifetimes. 74 level lifetimes $τ$ of states with $J = 0 - 6$ were determined. In addition, branching ratios were deduced which allowed the investigation of the intruder configuration in both nuclei. Here, $sd$ IBM-2 mixing calculations were added which support the coexistence of the two configurations. Furthermore, members of the expected QOC quintuplet are proposed in $^{114}$Sn for the first time. The $1^-$ candidate in $^{114}$Sn fits perfectly into the systematics observed for the other stable Sn isotopes. The $E2$ transition strengths observed for the low-spin members of the so-called intruder band support the existence of shape coexistence in $^{112,114}$Sn. The collectivity in this configuration is comparable to the one observed in the Pd nuclei, i.e. the 0p-4h nuclei. Strong mixing between the $0^+$ states of the normal and intruder configuration might be observed in $^{114}$Sn. The general existence of QOC states in $^{112,114}$Sn is supported by the observation of QOC candidates with $J \neq 1$.

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arXiv2017

Combining γ-ray and particle spectroscopy with SONIC@HORUS

The particle spectrometer SONIC for particle-$γ$ coincidence measurements was commissioned at the Institute for Nuclear Physics in Cologne, Germany. SONIC consists of up to 12 silicon $\mathitΔE$-$E$ telescopes with a total solid angle coverage of 9%, and will complement HORUS, a $γ$-ray spectrometer with 14 HPGe detectors. The combined setup SONIC@HORUS is used to investigate the $γ$-decay behaviour of low-spin states up to the neutron separation threshold excited by light-ion inelastic scattering and transfer reactions using beams provided by a 10 MV FN Tandem accelerator. The particle-$γ$ coincidence method will be presented using data from a $^{92}$Mo(p,p'$γ$) experiment. In a $^{119}$Sn(d,X) experiment, excellent particle identification has been achieved because of the good energy resolution of the silicon detectors of approximately 20 keV. Due to the non-negligible momentum transfer in the reaction, a Doppler correction of the detected $γ$-ray energy has to be performed, using the additional information from measuring the ejectile energy and direction. The high sensitivity of the setup is demonstrated by the results from a $^{94}$Mo(p,p'$γ$) experiment, where small $γ$-decay branching ratios have been deduced.

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arXiv2015

Lifetime measurement of excited low-spin states via the $(p,p^{\prime}γ$) reaction

In this article a method for lifetime measurements in the sub-picosecond regime via the Doppler-shift attenuation method (DSAM) following the inelastic proton scattering reaction is presented. In a pioneering experiment we extracted the lifetimes of 30 excited low-spin states of $^{96}$Ru, taking advantage of the coincident detection of scattered protons and de-exciting $γ$-rays as well as the large number of particle and $γ$-ray detectors provided by the SONIC@HORUS setup at the University of Cologne. The large amount of new experimental data shows that this technique is suited for the measurement of lifetimes of excited low-spin states, especially for isotopes with a low isotopic abundance, where $(n,n^{\prime}γ$) or - in case of investigating dipole excitations - ($γ,γ^{\prime}$) experiments are not feasible due to the lack of sufficient isotopically enriched target material.

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arXiv2015

Mixed-symmetry octupole and hexadecapole excitations in the N=52 isotones

Background: Excitations with mixed proton-neutron symmetry have been previously observed in the $N=52$ isotones. Besides the well established quadrupole mixed-symmetry states (MSS), octupole and hexadecapole MSS have been recently proposed for the nuclei $^{92}$Zr and $^{94}$Mo. Purpose: The heaviest stable $N=52$ isotone $^{96}$Ru was investigated to study the evolution of octupole and hexadecapole MSS with increasing proton number. Methods: Two inelastic proton-scattering experiments on $^{96}$Ru were performed to extract branching ratios, multipole mixing ratios, and level lifetimes. From the combined data, absolute transition strengths were calculated. Results: Strong $M1$ transitions between the lowest-lying $3^-$ and $4^+$ states were observed, providing evidence for a one-phonon mixed-symmetry character of the $3^{(-)}_2$ and $4^+_2$ states. Conclusions: $sdg$-IBM-2 calculations were performed for $^{96}$Ru. The results are in excellent agreement with the experimental data, pointing out a one-phonon hexadecapole mixed-symmetry character of the $4^+_2$ state. The $\big< 3^-_1||M1||3^{(-)}_2\big>$ matrix element is found to scale with the $<2^+_{\mathrm{s}}||M1||2^+_{\mathrm{ms}}>$ matrix element.

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arXiv2015

Measurement of the 187Re(α,n)190Ir reaction cross section at sub-Coulomb energies using the Cologne Clover Counting Setup

Uncertainties in adopted models of particle+nucleus optical-model potentials directly influence the accuracy in the theoretical predictions of reaction rates as they are needed for reaction-network calculations in, for instance, γ-process nucleosynthesis. The improvement of the α+nucleus optical-model potential is hampered by the lack of experimental data at astrophysically relevant energies especially for heavier nuclei. Measuring the Re187(α,n)Ir190 reaction cross section at sub-Coulomb energies extends the scarce experimental data available in this mass region and helps understanding the energy dependence of the imaginary part of the α+nucleus optical-model potential at low energies. Applying the activation method, after the irradiation of natural rhenium targets with α-particle energies of 12.4 to 14.1 MeV, the reaction yield and thus the reaction cross section were determined via γ-ray spectroscopy by using the Cologne Clover Counting Setup and the method of γγ coincidences. Cross-section values at five energies close to the astrophysically relevant energy region were measured. Statistical model calculations revealed discrepancies between the experimental values and predictions based on widely used α+nucleus optical-model potentials. However, an excellent reproduction of the measured cross-section values could be achieved from calculations based on the so-called Sauerwein-Rauscher α+nucleus optical-model potential. The results obtained indicate that the energy dependence of the imaginary part of the α+nucleus optical-model potential can be described by an exponential decrease. Successful reproductions of measured cross sections at low energies for α-induced reactions in the mass range 141{\leq}A{\leq}187 confirm the global character of the Sauerwein-Rauscher potential.

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