Search

Neutron destruction reactions of the cosmic γ-ray emitter Al26 are of importance to determine the amount of Al26 ejected into our galaxy by supernova explosions and for Al26 production in asymptotic giant branch stars. We performed a new measurement of the Al26(n,α) reaction up to 160-keV neutron energy at the neutron time-of-flight facilities n_TOF at CERN and GELINA at EC-JRC. We provide strengths for ten resonances, six of them for the first time. We use our data to calculate astrophysical reactivities for stellar temperatures up to 0.7 GK. Our results resolve a discrepancy between the two previous direct measurements of this reaction, and indicate higher stellar destruction rates than the most recently recommended reactivity., We thank P. Black (University of Edinburgh) for support in design and construction of the detection system and P. Morrall (Daresbury Laboratory) for production of the B and LiF reference samples. This Letter was supported by the Austrian Science Fund (FWF), Project No. J3503, by JRC-IRMM through the EUFRAT Program, by the U.K. Science and Technologies Facilities Council (STFC), Projects No. ST/L005824/1 and No. ST/M006085/1, the European Research Council ERC-2015-STG Grant No. 677497, and the Cost Action “ChETEC” (Grant No. CA16117).

19 pags., 14 figs., 3 tabs., A γ-ray spectroscopic study of Po212 was performed at the Grand Accélérateur National d'Ions Lourds, using the inverse kinematics α-transfer reaction C12(Pb208,Po212)Be8 and the AGATA spectrometer. A careful analysis based on γγ coincidence relations allowed us to establish 14 new excited states in the energy range between 1.9 and 3.3 MeV. None of these states, however, can be considered as candidates for the levels with spins and parities of 1- and 2- and excitation energies below 2.1 MeV, which have been predicted by recent α-cluster model calculations. A systematic comparison of the experimentally established excitation scheme of Po212 with shell-model calculations was performed. This comparison suggests that the six states with excitation energies (spins and parities) of 1744 (4-), 1751 (8-), 1787 (6-), 1946 (4-), 1986 (8-), and 2016 (6-) keV, which previously were interpreted as α-cluster states, may in fact be of positive parity and belong to low-lying shell-model multiplets. This reinterpretation of the structure of Po212 is supported by experimental information with respect to the linear polarization of γ rays, which suggests a magnetic character of the 432-keV γ ray decaying from the state at an excitation energy of 1787 keV to the 61+ yrast state, and exclusive reaction cross sections., This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 654002 and FEDER funds. We furthermore acknowledge support from the Spanish Ministerio de Ciencia, Inovación y Universidades under contracts FPA2014-57196-C5, FPA2017-84756-C4, and SEV-2014-0398, the Generalitat Valenciana under Grant No. PROMETEO/2019/005, the Swedish Research Council (Vetenskapsrådet, VR 2016-3969), and the German BMBF under Contracts No. 05P18PKFN9 and No. 05P19PKFNA.

5 pags., 4 figs., The low-lying excited states in the neutron-deficient N=Z+1 nucleus Tc444387 have been studied via the fusion-evaporation reaction Fe54(Ar36, 2n1p)Tc87 at the Grand Accélérateur National d'Ions Lourds (GANIL), France. The AGATA spectrometer was used in conjunction with the auxiliary NEDA, Neutron Wall, and DIAMANT detector arrays to measure coincident prompt γ rays, neutrons, and charged particles emitted in the reaction. A level scheme of Tc87 from the (9/2g.s.+) state to the (33/21+) state was established based on six mutually coincident γ-ray transitions. The constructed level structure exhibits a rotational behavior with a sharp backbending at ω≈0.50 MeV. A decrease in alignment frequency and increase in alignment sharpness in the odd-mass isotonic chains around N=44 is proposed as an effect of the enhanced isoscalar neutron-proton interactions in odd-mass nuclei when approaching the N=Z line., This work was supported by the Swedish Research Council under Grants No. 621-2014-5558 and No. 2019-04880, the EU 7th Framework Programme, Integrating Activities Transnational Access, Project No. 262010 ENSAR, the UK STFC under Grants No. ST/L005727/1 and No. ST/P003885/1, the Polish National Science Centre, Grants No. 2017/25/B/ST2/01569, No. COPIN-INFN, No. COPIN-IN2P3, and No. COPIGAL projects, the National Research, Development and Innovation Fund of Hungary (Project No. K128947), the European Regional Development Fund (Contract No. GINOP-2.3.3-15-2016-00034), the Hungarian National Research, Development and Innovation Office - NKFIH, Contract No. PD124717, the Ministerio de Ciencia e Innovación and Generalitat Valenciana, Spanish, under Grants No. SEV-2014-0398, No. FPA2017-84756-C4, and No. PROMETEO/2019/005 and by the EU FEDER funds. X.L. gratefully acknowledges support from the China Scholarship Council, Grant No. 201700260183 for his stay in Sweden.

n_TOF Collaboration: et al., The neutron capture cross sections of several unstable nuclides acting as branching points in the s process are crucial for stellar nucleosynthesis studies. The unstable Tm171 (t1/2=1.92 yr) is part of the branching around mass A∼170 but its neutron capture cross section as a function of the neutron energy is not known to date. In this work, following the production for the first time of more than 5 mg of Tm171 at the high-flux reactor Institut Laue-Langevin in France, a sample was produced at the Paul Scherrer Institute in Switzerland. Two complementary experiments were carried out at the neutron time-of-flight facility (n_TOF) at CERN in Switzerland and at the SARAF liquid lithium target facility at Soreq Nuclear Research Center in Israel by time of flight and activation, respectively. The result of the time-of-flight experiment consists of the first ever set of resonance parameters and the corresponding average resonance parameters, allowing us to make an estimation of the Maxwellian-averaged cross sections (MACS) by extrapolation. The activation measurement provides a direct and more precise measurement of the MACS at 30 keV: 384(40) mb, with which the estimation from the n_TOF data agree at the limit of 1 standard deviation. This value is 2.6 times lower than the JEFF-3.3 and ENDF/B-VIII evaluations, 25% lower than that of the Bao et al. compilation, and 1.6 times larger than the value recommended in the KADoNiS (v1) database, based on the only previous experiment. Our result affects the nucleosynthesis at the A∼170 branching, namely, the Yb171 abundance increases in the material lost by asymptotic giant branch stars, providing a better match to the available pre-solar SiC grain measurements compared to the calculations based on the current JEFF-3.3 model-based evaluation., University of Seville via the V PPIT-US programme, the Spanish Ministerio de Economía y Competitividad FPA2013-45083-P, FPA2014-53290-C2-2-P and FPA2016-77689-C2-1-R projects, the EC FP7 projects NeutAndalus (Grant No. 334315) and CHANDA (Grant No. 605203), and the n TOF Collaboration. The SARAFLiLiT experiment was supported by the Pazy Foundation (Israel). M. P. acknowledges support of the Israel Science Foundation (Grant No. 1387/15). The University of Edinburgh acknowledges funding from the Science and Technology Facilities Council UK (ST/M006085/1), and the European Research Council ERC-2015-STG No. 677497.

The low-lying energy spectrum of the extremely neutron-deficient self-conjugate (N=Z) nuclide Ru448844 has been measured using the combination of the Advanced Gamma Tracking Array (AGATA) spectrometer, the NEDA and Neutron Wall neutron detector arrays, and the DIAMANT charged particle detector array. Excited states in Ru88 were populated via the Fe54(Ar36,2nγ)Ru88∗ fusion-evaporation reaction at the Grand Accélérateur National d'Ions Lourds (GANIL) accelerator complex. The observed γ-ray cascade is assigned to Ru88 using clean prompt γ-γ-2-neutron coincidences in anticoincidence with the detection of charged particles, confirming and extending the previously assigned sequence of low-lying excited states. It is consistent with a moderately deformed rotating system exhibiting a band crossing at a rotational frequency that is significantly higher than standard theoretical predictions with isovector pairing, as well as observations in neighboring N>Z nuclides. The direct observation of such a "delayed" rotational alignment in a deformed N=Z nucleus is in agreement with theoretical predictions related to the presence of strong isoscalar neutron-proton pair correlations.

The neutron capture cross sections of several unstable nuclides acting as branching points in the s process are crucial for stellar nucleosynthesis studies. The unstable Tm171 (t1/2=1.92 yr) is part of the branching around mass A∼170 but its neutron capture cross section as a function of the neutron energy is not known to date. In this work, following the production for the first time of more than 5 mg of Tm171 at the high-flux reactor Institut Laue-Langevin in France, a sample was produced at the Paul Scherrer Institute in Switzerland. Two complementary experiments were carried out at the neutron time-of-flight facility (n_TOF) at CERN in Switzerland and at the SARAF liquid lithium target facility at Soreq Nuclear Research Center in Israel by time of flight and activation, respectively. The result of the time-of-flight experiment consists of the first ever set of resonance parameters and the corresponding average resonance parameters, allowing us to make an estimation of the Maxwellian-averaged cross sections (MACS) by extrapolation. The activation measurement provides a direct and more precise measurement of the MACS at 30 keV: 384(40) mb, with which the estimation from the n_TOF data agree at the limit of 1 standard deviation. This value is 2.6 times lower than the JEFF-3.3 and ENDF/B-VIII evaluations, 25% lower than that of the Bao et al. compilation, and 1.6 times larger than the value recommended in the KADoNiS (v1) database, based on the only previous experiment. Our result affects the nucleosynthesis at the A∼170 branching, namely, the Yb171 abundance increases in the material lost by asymptotic giant branch stars, providing a better match to the available pre-solar SiC grain measurements compared to the calculations based on the current JEFF-3.3 model-based evaluation. Ministerio de Economía y Competitividad FPA2013-45083-P, FPA2014-53290-C2-2-P, FPA2016-77689-C2-1-R European Community 334315, 605203 Israel Science Foundation 1387/15 Science and Technology Facilities Council UK ST/M006085/1 European Research Council ERC-2015-STG-677497

AGATA Collaboration, Lifetimes of the excited states in the neutron-rich Ti52,54 nuclei, produced in a multinucleon-transfer reaction, were measured by employing the Cologne plunger device and the recoil-distance Doppler-shift method. The experiment was performed at the Grand Accélérateur National d'Ions Lourds facility by using the Advanced Gamma Tracking Array for the γ-ray detection, coupled to the large-acceptance variable mode spectrometer for an event-by-event particle identification. A comparison between the transition probabilities obtained from the measured lifetimes of the 21+ to 81+ yrast states in Ti52,54 and that from the shell-model calculations based on the well-established GXPF1A, GXPF1B, and KB3G fp shell interactions support the N=32 subshell closure. The B(E2) values for Ti52 determined in this work are in disagreement with the known data, but are consistent with the predictions of the shell-model calculations and reduce the previously observed pronounced staggering across the even-even titanium isotopes.

21 pags., 18 figs., 10 tabs.-- Open Access funded by Creative Commons Atribution Licence 4.0, Background: Levels fulfilling the seniority scheme and relevant isomers are commonly observed features in semimagic nuclei; for example, in Sn isotopes (Z=50). Seniority isomers in Sn, with dominantly pure neutron configurations, directly probe the underlying neutron-neutron (νν) interaction. Furthermore, an addition of a valence proton particle or hole, through neutron-proton (νπ) interaction, affects the neutron seniority as well as the angular momentum. Purpose: Benchmark the reproducibility of the experimental observables, like the excitation energies (EX) and the reduced electric-quadrupole transition probabilities [B(E2)], with the results obtained from shell-model interactions for neutron-rich Sn and Sb isotopes with N, P.B. and A.M. acknowledge support from the Polish National Science Centre (NCN) under Contract No. 2016/22/M/ST2/00269 and the French LEA COPIGAL project. S.Bi., R.B., S.Bh., S.Bh., and R.P. acknowledge support from CEFIPRA project No. 5604-4 and the LIA France– India agreement. H.L.C. and P.F. acknowledge support from the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Contract No. DE-AC02-05CH11231 (LBNL). R.M.P.V. acknowledge partial support by Ministry of Science, Spain, under the grants BES-2012-061407, SEV2014-0398, FPA2017-84756-C4 and by EU FEDER funds. A.J. was supported by the Spanish Ministerio de Economía y Competitividad under contract FPA2017-84756-C4-2-P.

Background: There have been measurements on roughly 230 nuclei that are β-delayed neutron emitters. They range from 8 He up to 150La. Apart from 210Tl, with a branching ratio of only 0.007%, no other neutron emitter has been measured beyond A = 150. Therefore, new data are needed, particularly in the region of heavy nuclei around N = 126, in order to guide theoretical models and help understand the formation of the third r-process peak at A ∼ 195. Purpose: To measure both β-decay half-lives and neutron branching ratios of several neutron-rich Au, Hg, Tl, Pb, and Bi isotopes beyond N = 126. Method: Ions of interest were produced by fragmentation of a 238U beam, selected and identified via the GSI-FRS fragment separator. A stack of segmented silicon detectors (SIMBA) was used to measure ion implants and β decays. An array of 30 3 He tubes embedded in a polyethylene matrix (BELEN) was used to detect neutrons with high efficiency and selectivity. A self-triggered digital system is employed to acquire data and to enable time correlations. The latter were analyzed with an analytical model and results for the half-lives and neutron-branching ratios were derived by using the binned maximum-likelihood method. Results: Twenty new β-decay half-lives are reported for 204−206Au, 208–211Hg, 211–216Tl, 215–218Pb, and 218–220Bi, nine of them for the first time. Neutron emission probabilities are reported for 210,211Hg and 211–216Tl. Conclusions: The new β-decay half-lives are in good agreement with previous measurements on nuclei in this region. The measured neutron emission probabilities are comparable to or smaller than values predicted by global models such as relativistic Hartree Bogoliubov plus the relativistic quasi-particle random phase approximation (RHB + RQRPA). Spanish Ministerio de Economía y Competitividad-FPA2011- 28770-C03-03, FPA2008-04972-C03-3, AIC-D2011-0705, FPA2011-24553, FPA2008-6419, FPA2010-17142, FPA2014-52823-C2-1-P, FPA2014- 52823-C2-2-P, and CPAN CSD-2007-00042 (Ingenio2010) Program Severo Ochoa-SEV-2014-0398 German Helmholtz Association (Young Investigators)-VH-NG 627 (LISA-Lifetime Spectroscopy for Astrophysics) Nuclear Astrophysics Virtual Institute-VH-VI-417 German Bundesministerium für Bildung und Forschung-06MT7178 / 05P12WOFNF Spanish Nuclear Security Council (CSN)-Catedra Argos UK Science & Technology Facilities Council (STFC)-ST/F012012/1

The β-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with β-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb, and Bi in the mass region N≳126. These are the heaviest species where neutron emission has been observed so far. These measurements provide key information to evaluate the performance of nuclear microscopic and phenomenological models in reproducing the high-energy part of the β-decay strength distribution. This provides important constraints on global theoretical models currently used in r-process nucleosynthesis. Ministerio de Economia y Competitividad SEV-2014-0398 FPA2011-24553 FPA2011-28770-C03-03 FPA2014-52823-C2-1-P AIC-D-2011-0705 Catedra Argos VH-NG 627 German Bundesministerium für Bildung und Forschung 06MT7178 / 05P12WOFNF. UK

A measurement of the reduced transition probability for the excitation of the ground state to the first 2+ state in Sn104 has been performed using relativistic Coulomb excitation at GSI. Sn104 is the lightest isotope in the Sn chain for which this quantity has been measured. The result is a key point in the discussion of the evolution of nuclear structure in the proximity of the doubly magic nucleus Sn100. The value B(E2;0+→2 +)=0.10(4) e2b2 is significantly lower than earlier results for Sn106 and heavier isotopes. The result is well reproduced by shell model predictions and therefore indicates a robust N=Z=50 shell closure. © 2013 American Physical Society., A. J. would like to thank the Spanish Ministerio de Ciencia e Innovacin for financial support under Contract No. FPA2011-29854-C04.

The Ni-63(n, gamma) cross section has been measured for the first time at the neutron time-of-flight facility n_TOF at CERN from thermal neutron energies up to 200 keV. In total, capture kernels of 12 (new) resonances were determined. Maxwellian averaged cross sections were calculated for thermal energies from kT = 5-100 keV with uncertainties around 20%. Stellar model calculations for a 25M(circle dot) star show that the new data have a significant effect on the s-process production of Cu-63, Ni-64, and Zn-64 in massive stars, allowing stronger constraints on the Cu yields from explosive nucleosynthesis in the subsequent supernova. DOI: 10.1103/PhysRevLett.110.022501, The authors would like to thank H. Danninger and C. Gierl of the Technical University of Vienna for their help preparing the 62Ni sample. This work was partly supported by the Austrian Science Fund (FWF), Projects No. P20434 and No. I428. M. P. also acknowledges the support from the Ambizione grant of the Swiss NSF, the NSF Grants No. PHY 02-16783 and No. PHY 09-22648 (Joint Institute for Nuclear Astrophysics, JINA), EU Grant No. MIRG-CT-2006-046520, and EuroGenesis. T. R. acknowledges support from EuroGenesis, the FP7 ENSAR/THEXO project, and a "Distinguished Guest Scientist Fellowship" from the Hungarian Academy of Sciences.

The cross section of the reaction 197Au(n,γ) was measured with the time-of-flight technique at the n-TOF (neutron time-of-flight) facility in the unresolved resonance region between 5 and 400 keV using a pair of C 6D6 (where D denotes 2H) liquid scintillators for the detection of prompt capture γ rays. The results with a total uncertainty of 3.9%-6.7% for a resolution of 20 bins per energy decade show fair agreement with the Evaluated Nuclear Data File Version B-VII.0 (ENDF/B-VII.0), which contains the standard evaluation. The Maxwellian-averaged cross section (MACS) at 30 keV is in excellent agreement with the one according to the ENDF/B-VII.0 evaluation and 4.7% higher than the MACS measured independently by activation technique. Structures in the cross section, which had also been reported earlier, have been interpreted as being due to clusters of resonances.

10 pages, 6 figures.-- PACS nrs.: 25.40.Lw; 27.80.+w; 97.10.Cv.-- ISI Article Identifier: 000250620900078.-- ArXiv pre-print available at: http://arxiv.org/abs/0707.3679, The (n,γ) cross section of Pb-206 has been measured at the CERN n_TOF facility with high resolution in the energy range from 1 eV to 620 keV by using two optimized C6D6 detectors. In the investigated energy interval about 130 resonances could be observed, from which 61 had enough statistics to be reliably analyzed via the R-matrix analysis code SAMMY. Experimental uncertainties were minimized, in particular with respect to (i) angular distribution effects of the prompt capture γ-rays, and to (ii) the TOF-dependent background due to sample-scattered neutrons. Other background components were addressed by background measurements with an enriched Pb-208 sample. The effect of the lower energy cutoff in the pulse height spectra of the C6D6 detectors was carefully corrected via Monte Carlo simulations. Compared to previous Pb-206 values, the Maxwellian averaged capture cross sections derived from these data are about 20% and 9% lower at thermal energies of 5 keV and 30 keV, respectively. These new results have a direct impact on the s-process abundance of Pb-206, which represents an important test for the interpretation of the cosmic clock based on the decay of U-238.

9 pages, 3 figures.-- PACS nrs.: 25.40.Lw; 27.80.+w; 97.10.Cv.-- ISI Article Identifier: 000243896000061.-- ArXiv pre-print available at: http://arxiv.org/abs/nucl-ex/0610033, The neutron capture cross section of Pb-204 has been measured at the CERN n_TOF installation with high resolution in the energy range from 1 eV to 440 keV. An R-matrix analysis of the resolved resonance region, between 1 eV and 100 keV, was carried out using the SAMMY code. In the interval between 100 keV and 440 keV we report the average capture cross section. The background in the entire neutron energy range was reliably determined from the measurement of a Pb-208 sample. Other systematic effects in this measurement were investigated and precisely corrected by means of detailed Monte Carlo simulations. We obtain a Maxwellian average capture cross section for Pb-204 at kT=30 keV of 79(3) mb, in agreement with previous experiments. However our cross section at kT=5 keV is about 35% larger than the values reported so far. The implications of the new cross section for the s-process abundance contributions in the Pb/Bi region are discussed., This work was supported by the European Commission (FIKW-CT-2000-00107), by the Spanish Ministry of Science and Technology (FPA2001-0144-C05), and partly by the Italian MIUR-FIRB grant "The astrophysical origin of the heavy elements beyond Fe".

15 pages, 9 figures.-- PACS nrs.: 25.40.Lw; 26.20.+f; 27.60.+j; 97.10.Cv.-- ISI Article Identifier: 000245332200071.-- ArXiv pre-print available at: http://arxiv.org/abs/nucl-ex/0610034, The nuclear resonance parameters and the capture cross section of the neutron magic isotope La-139 have been measured relative to Au-197 in the energy range of 0.6 eV to 9 keV at the neutron time-of-flight (n_TOF) facility at CERN. We describe the experimental apparatus and the data analysis procedures, which concerned mainly the efficiency correction by means of the pulse height weighting function technique and the determination of different background components. We extracted the resonance parameters and the main nuclear quantities such as the resonance integral RI = 10.8 ± 1.0b, the average γ widths for s and p waves (l=0) = 50.7 ± 5.4 meV and (l=1) = 33.6 ± 6.9 meV, the nuclear level densities (l=0) = 252 ± 22 eV and (l=1) < 250 eV, and the neutron strength functions S-0 = (0.82 ± 0.05)x10^(-4) and S-1 = (0.55 ± 0.04)x10(-4). These results represent a significant improvement over previous data. The deduced Maxwellian-averaged capture cross sections are important for the interpretation of the most recent spectroscopic observations in low-metallicity stars., This work has been partly supported by the EC within the framework of the n TOF-ND-ADS project (nuclear data for accelerator systems, Contract FIKW-CT-2000-00107) and by the funding agencies of the participating institutes.

The neutron capture cross section of 209 Bi has been measured at the CERN n TOF facility by employing the pulse-height-weighting technique. Improvements over previous measurements are mainly because of an optimized detection system, which led to a practically negligible neutron sensitivity. Additional experimental sources of systematic error, such as the electronic threshold in the detectors, summing of γ-rays, internal electron conversion, and the isomeric state in bismuth, have been taken into account. γ-Ray absorption effects inside the sample have been corrected by employing a nonpolynomial weighting function. Because 209 Bi is the last stable isotope in the reaction path of the stellar s -process, the Maxwellian averaged capture cross section is important for the recycling of the reaction flow by α decays. In the relevant stellar range of thermal energies between kT=5 and 8 keV our new capture rate is about 16% higher than the presently accepted value used for nucleosynthesis calculations. At this low temperature an important part of the heavy Pb-Bi isotopes are supposed to be synthesized by the s-process in the He shells of low mass, thermally pulsing asymptotic giant branch stars. With the improved set of cross sections we obtain an s-process fraction of 19±3% of the solar bismuth abundance, resulting in an r -process residual of 81±3%. The present (n, γ) cross-section measurement is also of relevance for the design of accelerator driven systems based on a liquid metal Pb/Bi spallation target. Comisión Europea FIKW-CT-2000-00107 Ministerio de Ciencia y Tecnología FPA2001- 0144-C05

The radiative neutron capture cross section of 207 Pb has been measured at the CERN neutron time of flight installation n TOF using the pulse height weighting technique in the resolved energy region. The measurement has been performed with an optimized setup of two C6D6 scintillation detectors, which allowed us to reduce scattered neutron backgrounds down to a negligible level. Resonance parameters and radiative kernels have been determined for 16 resonances by means of an R-matrix analysis in the neutron energy range from 3 keV to 320 keV. Good agreement with previous measurements was found at low neutron energies, whereas substantial discrepancies appear beyond 45 keV. With the present results, we obtain an s -process contribution of 77±8% to the solar abundance of 207 Pb. This corresponds to an r-process component of 23±8%, which is important forderiving the U/Th ages of metal poor halo stars. Ministerio de Ciencia y Tecnología FPA2001-0144-C05 European Community FIKW- CT-2000-00107