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Angular correlation spectra of ${e}^{+}{e}^{\ensuremath{-}}$ pairs produced in the $^{3}\mathrm{H}(p,{e}^{+}{e}^{\ensuremath{-}})^{4}\mathrm{He}$ nuclear reaction have been studied at ${E}_{p}=510$, 610, and 900 keV proton energies. The main features of the spectra can be understood by taking into account the internal and external pair creations following the proton capture by $^{3}\mathrm{H}$. However, these processes cannot account for an observed peak around ${115}^{\ensuremath{\circ}}$ in the angular correlation spectra. This anomalous excess of ${e}^{+}{e}^{\ensuremath{-}}$ pairs can be described by the creation and subsequent decay of a light particle during the direct capture process. The derived mass of the particle is ${m}_{\mathrm{X}}{c}^{2}=16.94\ifmmode\pm\else\textpm\fi{}0.12(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.21(\mathrm{syst})$ MeV. According to the mass this is likely the same X17 particle, which we recently suggested [Phys. Rev. Lett. 116, 042501 (2016)] for describing the anomaly observed in the decay of $^{8}\mathrm{Be}$.

Electron-positron angular correlations were measured for the isovector magnetic dipole 17.6 MeV (J^{π}=1^{+}, T=1) state→ground state (J^{π}=0^{+}, T=0) and the isoscalar magnetic dipole 18.15 MeV (J^{π}=1^{+}, T=0) state→ground state transitions in ^{8}Be. Significant enhancement relative to the internal pair creation was observed at large angles in the angular correlation for the isoscalar transition with a confidence level of >5σ. This observation could possibly be due to nuclear reaction interference effects or might indicate that, in an intermediate step, a neutral isoscalar particle with a mass of 16.70±0.35(stat)±0.5(syst) MeV/c^{2} and J^{π}=1^{+} was created.

Y. Fujita et al.; 15 pags.; 6 figs.; 7 tabs.; PACS number(s): 21.10.Hw, 25.55.Kr, 27.40.+z, 25.40.Ep

Background: The Gamow-Teller response of $^{40}\mathrm{Ar}$ is important for the use of liquid argon as a medium for neutrino detection. An ambiguity about the Gamow-Teller strength for the excitation of ${1}^{+}$ states at 2290 and 2730 keV in $^{40}\mathrm{K}$ results in a significant uncertainty for neutrino capture rates. This ambiguity is caused by the large discrepancy observed between strengths extracted from $^{40}\mathrm{Ar}$($p$, $n$) charge-exchange data and the transition strengths for the analog transitions studied in the $\ensuremath{\beta}$ decay of $^{40}\mathrm{Ti}$.Purpose: This study was aimed at resolving the ambiguity between the results from the $^{40}\mathrm{Ar}$($p$, $n$) charge-exchange and $^{40}\mathrm{Ti}$ $\ensuremath{\beta}$-decay data.Method: Shell-model calculations in the $sd\text{\ensuremath{-}}pf$ shell with a new interaction (WBMB-C) were used to study differences between the structure of the transitions from $^{40}\mathrm{Ar}$ and $^{40}\mathrm{Ti}$. Distorted-wave Born approximation reaction calculations were used to investigate uncertainties in the extraction of Gamow-Teller strength from the $^{40}\mathrm{Ar}$($p$, $n$) data. New high-resolution data for the $^{40}\mathrm{Ar}$($^{3}\mathrm{He}$,$t$) reaction were used to gain further insight into the charge-exchange reaction mechanism and to provide more information to test the validity of the shell-model calculations.Results: The shell-model calculations showed that interference between amplitudes associated with $pf$ and $sd$ components to the low-lying Gamow-Teller transitions, in combination with a difference in Coulomb energy shifts for $^{40}\mathrm{Ar}$ and $^{40}\mathrm{Ti}$, can produce the differences on the scale of those observed between the $^{40}\mathrm{Ar}$ charge-exchange and $^{40}\mathrm{Ti}$ $\ensuremath{\beta}$-decay data. In combination with the difference in nuclear penetrability of the ($p$, $n$) and ($^{3}\mathrm{He}$,$t$) probes, the different contributions from amplitudes associated with transitions in the $pf$ and $sd$ shells are likely also responsible for the observed discrepancy between the ratio of the cross sections for the low-lying ${1}^{+}$ states in the $^{40}\mathrm{Ar}$($p$, $n$) and $^{40}\mathrm{Ar}$($^{3}\mathrm{He}$,$t$) data.Conclusions: On the basis of this study, it is recommended to use Gamow-Teller strengths extracted from the $^{40}\mathrm{Ar}$($p$, $n$) data, not the $^{40}\mathrm{Ti}$ $\ensuremath{\beta}$-decay data, for the calculation of neutrino capture rates. Further theoretical studies are required to achieve a consistent quantitative description for the energy differences between low-lying ${1}^{+}$ states in $^{40}\mathrm{K}$ and $^{40}\mathrm{Sc}$ and the experimentally observed Gamow-Teller strengths.

Y. Fujita et al. ; 5 pags. ; 3 figs. PACS numbers: 24.30.Cz, 25.55.Kr, 27.40.+z, Gamow-Teller (GT) transitions in atomic nuclei are sensitive to both nuclear shell structure and effective residual interactions. The nuclear GT excitations were studied for the mass number A=42, 46, 50, and 54 >f-shell> nuclei in (He3, t) charge-exchange reactions. In the Ca42→Sc42 reaction, most of the GT strength is concentrated in the lowest excited state at 0.6 MeV, suggesting the existence of a low-energy GT phonon excitation. As A increases, a high-energy GT phonon excitation develops in the 6-11 MeV region. In the Fe54→Co54 reaction, the high-energy GT phonon excitation mainly carries the GT strength. The existence of these two GT phonon excitations are attributed to the 2 fermionic degrees of freedom in nuclei. © 2014 American Physical Society., This work was in part supported by MEXT, Japan (Grants No. 13002001, No. 15540274, and No. 18540270); MICINN, Spain (Grant No. FPA200806419-C02-01). Y. F. and B. R. acknowledge the support of the Japan-Spain collaboration program by JSPS and CSIC.

Excited states of {sup 112}Sb were investigated through the {sup 112}Sn(p,n{gamma}){sup 112}Sb reaction. {gamma}-ray, {gamma}{gamma}-coincidence, and internal conversion electron spectra were measured with Ge(HP) {gamma} and superconducting magnetic lens plus Si(Li) electron spectrometers at 8.5, 8.9, 9.1, and 9.3 MeV bombarding proton energies. A significantly extended level scheme was constructed. Spins and parities have been assigned to the levels from Hauser-Feshbach analysis of reaction cross sections, internal conversion coefficients, angular distribution of the {gamma} rays, and decay properties of the states. The low lying states were assigned to proton-neutron multiplets on the basis of their decay properties. The energy splitting of these multiplets have been calculated using the parabolic rule. {copyright} {ital 1997} {ital The American Physical Society}

The $\ensuremath{\beta}$-feeding probabilities for three important contributors to the decay heat in nuclear reactors, namely ${}^{102,104,105}$Tc, have been measured using the total absorption spectroscopy technique. For the measurements, sources of very high isobaric purity have been obtained using a Penning trap (JYFLTRAP). A detailed description of the data analysis is given and the results are compared with high-resolution measurements and theoretical calculations.

In this work we have studied ${T}_{z}=+2\ensuremath{\rightarrow}+1$, Gamow-Teller (GT) transitions in the $^{48}\mathrm{Ti}(^{3}\mathrm{He},t)^{48}\mathrm{V}$ charge-exchange reaction at 140 MeV/nucleon and ${0}^{\ensuremath{\circ}}$ at the Research Center for Nuclear Physics, Osaka. From the high-resolution facility, consisting of a high-dispersion beamline and the Grand Raiden spectrometer, the spectrum had an energy resolution of 21 keV, among the best achieved. Individual GT transitions were observed and GT strength was derived for each state populated up to an excitation energy of 12 MeV. The total sum of the $B$(GT) strength observed in discrete states was 4.0, which is 33% of the sum-rule-limit value of 12. The results were compared with the results of shell-model calculations carried out with the GXPF1J interaction. The measured $B$(GT) distribution was also compared with that obtained in the $(^{3}\mathrm{He},t)$ charge-exchange reaction on $^{47}\mathrm{Ti}$. On the assumption of isospin symmetry the $\ensuremath{\beta}$ spectrum of the ${T}_{z}=\ensuremath{-}2$ nucleus $^{48}\mathrm{Fe}$ was deduced from the observed spectrum in the $^{48}\mathrm{Ti}(^{3}\mathrm{He},t)^{48}\mathrm{V}$ reaction and this predicted spectrum was compared with the measured one.

The fission probability of 232Pa was measured as a function of the excitation energy in order to search for hyperdeformed (HD) transmission resonances using the (d,pf) transfer reaction on a radioactive 231Pa target. The experiment was performed at the Tandem accelerator of the Maier-Leibnitz Laboratory (MLL) at Garching using the 231Pa(d,pf) reaction at a bombarding energy of E=12 MeV and with an energy resolution of dE=5.5 keV. Two groups of transmission resonances have been observed at excitation energies of E=5.7 and 5.9 MeV. The fine structure of the resonance group at E=5.7 MeV could be interpreted as overlapping rotational bands with a rotational parameter characteristic to a HD nuclear shape. The fission barrier parameters of 232Pa have been determined by fitting TALYS 1.2 nuclear reaction code calculations to the overall structure of the fission probability. From the average level spacing of the J=4 states, the excitation energy of the ground state of the 3rd minimum has been deduced to be E(III)=5.05 MeV., Comment: 6 pages, 8 figures

The {beta} feeding probability of {sup 102,104,105,106,107}Tc, {sup 105}Mo, and {sup 101}Nb nuclei, which are important contributors to the decay heat in nuclear reactors, has been measured using the total absorption technique. We have coupled for the first time a total absorption spectrometer to a Penning trap in order to obtain sources of very high isobaric purity. Our results solve a significant part of a long-standing discrepancy in the {gamma} component of the decay heat for {sup 239}Pu in the 4-3000 s range.

{gamma}, {gamma}{gamma}-coincidence, internal conversion electron, and {gamma}-ray angular distribution spectra of the {sup 118}Sn({ital p},{ital n}{gamma}){sup 118}Sb reaction were measured at different bombarding proton energies between 5.5 and 7.5 MeV. {gamma}, {gamma}{gamma}-coincidence, and internal conversion electron spectra of the {sup 115}In ({alpha},{ital n}{gamma}){sup 118}Sb reaction were also measured at {ital E}{sub {alpha}}=14.5 MeV. Ge(HP), Ge(Li), Ge(LEPS) {gamma}-ray detectors, as well as a superconducting magnetic lens electron spectrometer (with Si(Li) detectors), were used in the experiments. About 210 (including {similar to}130 new) {gamma} rays have been assigned to {sup 118}Sb. The deduced {sup 118}Sb level scheme contains more than 70 new levels. On the basis of the internal conversion coefficients, Hauser-Feshbach analysis of ({ital p},{ital n}) reaction cross sections, {gamma}-ray angular distributions, and other arguments spin and parity values have been determined. The parabolic rule'' prediction of the energy splitting of different proton-neutron multiplets enabled the identification of many proton-neutron multiplet states. The energy spectrum and electromagnetic properties have been calculated in the framework of the interacting boson-fermion-fermion--odd-odd truncated quadrupole phonon model, and reasonably good agreement has been obtained between experimental and theoretical results.

The fission probability of $^{232}\mathrm{U}$ has been measured using the $^{231}\mathrm{Pa}$($^{3}\mathrm{He}$,df) reaction with an energy resolution of 11 keV in the excitation energy region of ${E}^{*}=4.0\text{\ensuremath{-}}6.4$ MeV. A number of sub-barrier fission resonances have been observed for the first time in the excitation energy range below ${E}^{*}=4.8$ MeV and interpreted as rotational bands with a rotational parameter characteristic to a hyperdeformed nuclear shape ($\ensuremath{\hbar}{}^{2}/2\ensuremath{\Theta}=1.96\ifmmode\pm\else\textpm\fi{}0.11$ keV). The angular distribution of the associated fission fragments was measured to deduce the $K$ value of the rotational bands. The fission barrier parameters of $^{232}\mathrm{U}$ have been determined by analyzing the overall features of the fission probability. A deep third minimum with an excitation energy of ${E}_{\mathrm{III}}=3.2(2)$ MeV and rather low inner barrier height of ${E}_{A}=4.0(3)$ MeV could be established.

The direct and statistical neutron decay of the isoscalar giant dipole resonance was studied in Zr-90, Sn-116, and Pb-208 using the (alpha,alpha(')n) reaction at a bombarding energy of 200 MeV. The spectra of fast decay neutrons populating valence hole-states of the Z,N-1 nuclei were analyzed, and estimates for the branching ratios were determined. The results were compared to recent continuum random-phase approximation calculations.

The lowest excited $K=0$ bands in ${}^{238}\mathrm{U}$ have been studied by internal conversion electron spectroscopy following Coulomb excitation with $\ensuremath{\alpha}$ particles at 19 MeV. Strong $E0$ enhancement was found for transitions between the ${2}^{+}$ members of the ${0}_{2}^{+},$ ${0}_{3}^{+}$ bands and of the ground state rotational band, and $E0$ strength values of 0.0099(18) and 0.175(26) were determined. Based on our more accurate energy value of the ${0}_{3}^{+}$ state, 997.23(24) keV, the moments of inertia for the excited $K=0$ bands were studied.