Bulletin of the American Physical Society
5th Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 63, Number 12
Tuesday–Saturday, October 23–27, 2018; Waikoloa, Hawaii
Session DP: The r-process |
Hide Abstracts |
Chair: Anna Simon, University of Notre Dame Room: Hilton Kona 1 |
Thursday, October 25, 2018 9:00AM - 9:15AM |
DP.00001: ABSTRACT WITHDRAWN
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Thursday, October 25, 2018 9:15AM - 9:30AM |
DP.00002: Benchmarking the extraction of statistical neutron capture cross sections on short-lived nuclei Sean Liddick, Artemis Spyrou, Ann-Cecilie Larsen, Jørgen Midtbø, Benjamin Crider, Farheen Naqvi, Magne S. Guttormsen, Georgios Perdikakis, Darren L Bleuel, Lucia Crespo Campo, Aaron J Couture, Alexander C Dombos, Rebecca Lewis, Katherine L Childers, Shea Mosby, Christopher J Prokop, Sunniva Siem, Therese Renstrom Neutron-capture rates on short-lived rare isotopes are required input into a number of physical processes; one such example is the astrophysical r-process. The recent LIGO and Virgo gravitational-wave detection of a direct signal from two colliding neutron stars combined with the wealth of follow-up measurements across the electromagnetic spectrum demonstrated that an r-process had occurred during the collision. Despite knowing at least one location for the r-process, many questions remain. The uncertainties in the nuclear physics inputs, such as neutron-capture rates, present a large barrier to accurately model the r-process abundances in large-scale nucleosynthesis calculations. Neutron-capture rates are difficult to measure and theoretical predictions range over orders of magnitude far from stability. The β-Oslo method is an indirect technique to determine neutron-capture rates and the present work will present the results of the 50Ti neutron-capture cross section extracted following the β decay of 51Sc. The inferred cross section will be compared to the directly measured 50Ti neutron capture cross section. |
Thursday, October 25, 2018 9:30AM - 9:45AM |
DP.00003: β-decay strengths of Co isotopes from total absorption spectroscopy Stephanie Lyons, Artemis Spyrou, Sean N. Liddick, Farheen Naqvi, Benjamin Patrick Crider, Alex C Dombos, Darren L Bleuel, B Alex Brown, Aaron J Couture, Lucia Crespo-Campo, Magne S. Guttormsen, Ann-Cecilie Larsen, Rebecca Lewis, Peter Moller, Shea Mosby, Matthew R Mumpower, George Perdikakis, Christopher J Prokop, Therese Renstrom, Sunniva Siem, Mallory K Smith, Stephen Quinn Recent multi-messenger observations of GW170817 have provided evidence of r-process nucleosynthesis in neutron-star mergers. While progress has been made in understanding a site of this critical process, sensitivity studies show that the final abundance distributions of r-process nuclei are greatly impacted by β-decay properties. More specifically, β-delayed neutron-emission can affect the flow back to stability and enrich the environment with neutrons for further neutron capture. While it is assumed that neutron emission dominates above the neutron threshold, recent measurements of β-decay on r-process nuclei observed γ-emission above the neutron threshold. This competition may effect the final abundances of r-process nuclei. For this reason, β-decay intensities for 69,71Co were measured using the technique of total absorption spectroscopy at the NSCL. The resultant β-decay intensities and deduced Gamow-Teller strengths are compared to QRPA calculations, which are commonly used in r-process calculations. The experimental results of these odd-mass isotopes contain some feeding above the neutron separation energy, though not as strong as observed in 70Co, which has then motivated further investigation into the mechanism that is driving the observed n-γ competition. |
Thursday, October 25, 2018 9:45AM - 10:00AM |
DP.00004: Search for a low-energy enhancement in the $\gamma$-decay strength of neutron-rich $^{64}$Fe Mallory Smith, Artemis Spyrou, Wei Jia Ong, Sunghoon Ahn, Alex C Dombos, Sean N. Liddick, Fernando Montes, Farheen Naqvi, Debra Richman, Hendrik Schatz, Justin E Browne, Katherine L Childers, Benjamin Patrick Crider, Christopher J Prokop, Eric Deleeuw, Paul A Deyoung, Christoph Langer, Rebecca Lewis, Zachary P Meisel, Jorge Pereira, Stephen Quinn, Konrad Schmidt, Ann-Cecilie Larsen, Magne Guttormsen Far from stability, little is known about $\gamma$ strength functions ($\gamma$SFs) and nuclear level densities (NLDs). In certain isotopes, e.g., in the Fe-Cd region, an unexpected increase in the low energy $\gamma$-decay probability has been observed. The presence of this enhancement, or upbend, can have a significant influence on neutron capture rates. These rates are crucial for nucleosynthesis models. Recently, the upbend was found in $^{70}$Ni, the first time in a neutron-rich nucleus. It is unknown how this feature evolves throughout the nuclear landscape. An indirect method known as the $\beta$-Oslo method has been developed to constrain neutron capture rates for radioactive nuclei. With the $\beta$-Oslo method, the reaction product is populated in $\beta$-decay, and the NLD and $\gamma$SF are extracted simultaneously. At the NSCL, excited states were populated via $\beta$-decay for neutron-rich $^{64}$ Fe. $\gamma$-rays are recorded with the 4$\pi$ Summing Na(I) (SuN) segmented total absorption spectrometer, which allows a simultaneous extraction of the NLD and $\gamma$SF. These results will be presented for $^{64}$Fe, a nucleus expected to exhibit an upbend in the $\gamma$SF. |
Thursday, October 25, 2018 10:00AM - 10:15AM |
DP.00005: Experimentally constrained 70Ni(n,γ)71Ni cross section Rebecca Lewis, Sean N. Liddick, Stephanie M Lyons, Artemis Spyrou, Darren L Bleuel, Katherine L Childers, Benjamin Patrick Crider, Alexander C Dombos, Caley Harris, Ann-Cecilie Larsen, Alicia Palmisano, Debra Richman, Nicholas David Scielzo, Anna Simon, Mallory K Smith, Antonius W Torode, Adriana Ureche, Remco G.T. Zegers The merger of two neutron stars was recently observed in a combination of gravitational and electromagnetic radiation. The time profile of the electromagnetic signature of the merger event confirmed that an r-process event had taken place, identifying neutron-star mergers as at least one site for the astrophysical r-process. The neutron-capture cross sections of the neutron-rich nuclei needed to inform r-process abundance predictions in neutron-star merger scenarios are poorly known and must be constrained through indirect techniques. One indirect technique, the β-Oslo method, extracts the nuclear level density (NLD) and γ-ray strength function (γSF) following beta decay and uses these two quantities in a Hauser-Feshbach calculation to constrain the neutron capture cross section. The β-Oslo method has been used previously to obtain experimentally-constrained neutron-capture cross sections of 68,69Ni. An experiment at the National Superconducting Cyclotron Laboratory measured the β-delayed γ rays of nuclei in the A~70 region to extract NLD and γSFs of neutron-rich nuclei. Results for 70Ni(n,γ)71Ni will be presented and the statistical properties and neutron-capture cross section will be compared to those of the lighter nickel isotopes. |
Thursday, October 25, 2018 10:15AM - 10:30AM |
DP.00006: Constraining the cross section of 82Se(n, γ)83Se to validate the β-Oslo method Katherine L Childers, Sean N. Liddick, Artemis Spyrou, Ann-Cecilie Larsen, Magne S. Guttormsen, Darren L Bleuel, Lucia Crespo-Campo, Benjamin Crider, Aaron J Couture, Alex C Dombos, Rebecca Lewis, Shea Mosby, Farheen Naqvi, Georgios Perdikakis, Christopher J Prokop, Sunniva Siem, Therese Renstrom, Stephen Quinn Neutron star mergers have recently been confirmed as one of the sites of the rapid neutron-capture process (r-process). In order to better understand the r-process, nuclear physics properties of the nuclei involved are needed, including neutron-capture cross sections. However, many r-process nuclei are not viable for direct measurements of neutron-capture rates, so their neutron-capture cross sections are poorly known. This has led to the development of indirect measurement techniques such as the β-Oslo method. This method will be validated with the 82Se(n,γ)83Se reaction, where the neutron-capture product, 83Se, can be accessed through the β-decay of 83As, which has been studied at the National Superconducting Cyclotron Laboratory with the Summing NaI (SuN) total absorption spectrometer. The nuclear level density (NLD) and γ-ray strength function (γSF) of 83Se have been extracted using the β-Oslo method and fed into a statistical Hauser-Feshbach model to obtain a neutron-capture cross section. The comparison of this constrained neutron-capture cross section to a direct measurement of the neutron capture on 82Se using the Detector for Advanced Neutron Capture Experiments (DANCE) at Los Alamos National Laboratory will be presented. |
Thursday, October 25, 2018 10:30AM - 10:45AM |
DP.00007: Nuclear Level Density and Gamma Strength Functions for i-process nuclei, 103,104Mo Andrea L. Richard, Sean N. Liddick, Alex C. Dombos, Artemis Spyrou, Farheen Naqvi, Stephen J. Quinn, Alejandro Algora, Thomas Baumann, Jaclyn Brett, Benjamin P. Crider, Paul A DeYoung, Thomas Ginter, Jason P. Gombas, Elaine Kwan, Stephanie Lyons, Wei Jia Ong, Alicia Palmisano, Jorge Pereira, Christopher J. Prokop, Dustin P. Scriven, Anna Simon, Mallory K. Smith, C. S. Sumithrarachchi Neutron-capture nucleosynthesis occurs via a variety of processes depending on the astrophysical sites and conditions. Recent observations and stellar evolution models suggest that an intermediate process, known as the i-process, exists between the slow (s-process) and rapid (r-process) neutron-capture processes. The abundance patterns of i-process nuclei are affected by various nuclear inputs such as masses, β-decay probabilities, and neutron-capture cross sections. Direct neutron-capture measurements can only be done on long-lived nuclei, while for short-lived, exotic nuclei, indirect techniques are required. One such technique is the β-Oslo method in which the nuclear level density (NLD) and γ-strength function (γSF) are extracted following the β-decay of a neutron-rich nucleus and are used in a statistical reaction model to constrain the neutron-capture cross section. The neutron-rich region around the Se-Nb isotopes has been shown to impact i-process abundance patterns. In this work, 103,104Mo were studied at the NSCL via the β-decay of 103,104Nb and detected using the Summing NaI (SuN) total absorption spectrometer. Preliminary results on the NLD, γSF, and neutron-capture cross sections of 102Mo(n,γ)103Mo and 103Mo(n,γ)104Mo using the β-Oslo method will be presented. |
Thursday, October 25, 2018 10:45AM - 11:00AM |
DP.00008: Measuring 84Se(d,p) at 45 MeV/A to reduce uncertainties in spectroscopic factors of states in 85Se Harrison E Sims, David G Walter, Steven D. Pain, Sunghoon Ahn, Sean P Burcher, Jolie Antonia Cizewski, Francesca G Corrado, Michael A Famiano, Heather I Garland, Thomas Nelson Ginter, Alexandre Alban Lepailleur, Filomena Nunes, Jorge Pereira, Andrew Ratkiewicz, Karl Smith, Pei-Luan Tai, Cory R Thornsberry, Rebecca Toomey, Chad C Ummel, Robert L Varner, Marija Vostinar Neutron-transfer reactions with radioactive ion beams (RIBs) enable the structure of neutron-rich nuclei to be studied. With (d,p) reactions, spectroscopic factors can be extracted through a normalization of the observed angular distribution of reaction protons to that modelled using theory. They are, therefore, heavily dependent on the optical-model parameters chosen to model the bound state. A combined method using high and low energy RIBs allows for both a peripheral and more central probe of the nucleus, thereby constraining the appropriate bound state parameters and reducing uncertainties in the extracted spectroscopic factors. Having previously been demonstrated with the 86Kr(d,p) reaction, this method is now being used to determine spectroscopic factors of states in 85Se through 84Se(d,p). A measurement at 4.5 MeV/A has already been performed [1], providing the low-energy analysis. The high-energy measurement at 45 MeV/A was performed at the NSCL using ORRUBA and SIDAR to measure reaction protons in coincidence with the heavy ion recoil using the S800 spectrograph. Preliminary results will be presented. [1] J.S. Thomas et al., Phys. Rev. C 76, 044302 (2007)
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Thursday, October 25, 2018 11:00AM - 11:15AM |
DP.00009: Validating the Surrogate Reactions Method for Determining $(n,\gamma)$ Cross Sections with the $(d,p\gamma)$ Reaction Andrew Ratkiewicz, Jolie Cizewski, Jutta E Escher, Gregory Potel Aguilar The neutron-capture reaction plays an important role in determining the final abundance pattern of nuclei produced in the r process. However, the $(n,\gamma)$ reaction on the exotic nuclei that participate in the r process cannot be directly measured. It is thus necessary to develop indirect methods for determining these important cross sections. One such technique is the Surrogate Reactions Method (SRM). Recent theoretical advances in modeling the $(d,p)$ reaction [1] and in constraining Hauser-Feshbach calculations of the $(n,\gamma)$ cross section through fits to experimental data [2] have enabled the determination of $(n,\gamma)$ cross sections from $(d,p\gamma)$ reaction data. We will discuss the recent successful effort to benchmark the performance of the SRM with the $^{95}Mo(d,p\gamma)$ reaction. [1] G. Potel, F. M. Nunes, and I. J. Thompson, Phys. Rev. C 92, 034611 (2015) [2] J. E. Escher et al, EPJ Web of Conf. 122, 12001 (2016) |
Thursday, October 25, 2018 11:15AM - 11:30AM |
DP.00010: Using GODDESS instrumentation to explore (n,γ) surrogate reactions through 95Mo(d,pγ) Heather I. Garland, Jolie A. Cizewski, Alexandre A. Lepailleur, Steven D. Pain, Andrew Ratkiewicz, Travis R. Baugher, Harrison E. Sims, David G. Walter, Michael T. Febbraro, Karl Smith The rapid neutron capture process is responsible for the synthesis of more than half of the elements heavier than iron. Unfortunately, the nuclei near the r-process path are too short-lived for the (n,γ) reaction to be studied in the laboratory setting. A possible surrogate for the (n,γ) reaction is the (d,pγ) reaction. The 95Mo(d,pγ) reaction in inverse kinematics was investigated with GODDESS (Gammasphere ORRUBA: Dual Detectors for Experimental Structure Studies) with ATLAS at Argonne National Laboratory to benchmark the (d,pγ) reaction in inverse kinematics as a surrogate for the (n,γ) reaction. GODDESS pairs ORRUBA (Oak Ridge Rutgers University Barrel Array), which consists of twenty-four position-sensitive silicon strip detectors supplemented with annular silicon strip detectors at forward and backward angles, and Gammasphere, an array of 110 Compton-suppressed HPGe detectors with nearly full angular coverage, to measure particle-gamma ray coincidences. This talk will focus on the design of GODDESS instrumentation and will present preliminary results from the 95Mo(d,pγ) measurement. |
Thursday, October 25, 2018 11:30AM - 11:45AM |
DP.00011: Structure of 135Xe from the (d,pγ) reaction with 134Xe beams and GODDESS Alexandre Lepailleur, Jolie Antonia Cizewski, Heather I Garland, Harrison E Sims, David G Walter, Andrew Ratkiewicz, Travis Ray Baugher, Michael T Febbraro, Steven D. Pain, Karl Smith About half of the elements heavier than iron are synthesized during the r-process that proceeds via (n,γ) reactions on neutron-rich nuclei. Direct-semidirect neutron captures are important, if not dominant, near the N=82 closed shell and are expected to have a significant impact during the late stages of the r-process, for example through 130Sn(n,γ)131Sn. Therefore, the evolution of the shell gap at N=82 and the single neutron 3p3/2 and 3p1/2 states are keys to a better understanding of the r-process and for constraining model parameters. To probe the 2f-3p single-neutron configurations in the N=81 isotone 135Xe, we have measured the 134Xe(d,pγ)135Xe reaction in inverse kinematics using GODDESS (Gammasphere ORRUBA: Dual Detectors for Experimental Structure Studies). The 134Xe beam was produced at ATLAS (Argonne National Laboratory) and impinged on a C2D4 target. Reaction protons were measured (ORRUBA) in coincidence with gamma rays (Gammasphere). Several single-neutron states, corresponding to the transfer of a neutron to the p-f configurations above the N=82 shell gap, have been observed for the first time. In collaboration with the GODDESS team.
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