Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session Y7: Mini-Symposium on Nuclear Masses in Astrophysics |
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Sponsoring Units: DNP Chair: Art Champagne, University of North Carolina at Chapel Hill Room: Delaware A |
Tuesday, February 16, 2010 1:30PM - 2:06PM |
Y7.00001: Mass Measurements for Nuclear Astrophysics Invited Speaker: The masses of nuclei are a crucial input to nucleosynthesis models. Masses of nuclei near the valley of stability have been known with sufficient precision for several decades. On the contrary, only after the recent development of radioactive beam facilities have the first mass measurements of exotic nuclei been made. Enormous effort at several places around the world has been devoted to obtain new mass values of exotic nuclei using a variety of techniques: the most precise measurements are typically obtained by the Penning trap technique, on the other hand the complimentary time-of-flight (TOF) technique can access even more exotic nuclides. The TOF-Brho technique has been recently implemented at the National Superconducting Cyclotron Laboratory and the first experiment, focused on neutron rich isotopes in the region of Z$\sim$20--30 has been successfully performed. An overview of mass measurements important for nuclear astrophysics, mass measurement techniques and recent mass measurements will be given. Results and details of the NSCL experiment will also be presented. [Preview Abstract] |
Tuesday, February 16, 2010 2:06PM - 2:18PM |
Y7.00002: Precision Mass Measurements at CARIBU D. Lascar, J. Van Schelt, G. Savard, S. Caldwell, A. Chaudhuri, J.A. Clark, A.F. Levand, G. Li, M. Sternberg, T. Sun, B.J. Zabransky, R. Segel, K. Sharma Neutron separation energies (S$_{n}$) are essential inputs to models of explosive \emph{r}-process nucleosynthesis. However, for nuclei farther from stability, the precision of S$_{n}$ decreases as production decreases and observation of those nuclei become more difficult. Many of the most critical inputs to the models are based on extrapolations from measurements of masses closer to stability than the predicted \emph{r}-process path. Measuring masses that approach and lie on the predicted \emph{r}-process path will further constrain the systematic uncertainties in these extrapolated values. The Canadian Penning Trap Mass Spectrometer (CPT) at Argonne National Laboratory (ANL) has measured the masses of more than 160 nuclei to high precision. A recent move to the CAlifornium Rare Isotope Breeder Upgrade (CARIBU) at ANL has given the CPT unique access to weakly produced nuclei that cannot be easily reached via proton induced fission of $^{238}$U. CARIBU will eventually use a 1 Ci source of $^{252}$Cf to produce these nuclei. Installation of the CPT at CARIBU as well as the first CPT mass measurements of neutron rich nuclei at CARIBU will be discussed. [Preview Abstract] |
Tuesday, February 16, 2010 2:18PM - 2:30PM |
Y7.00003: Masses of Proton-Rich Nuclides from Nb to Rh and their Influence on the $rp$- and $\nu p$-processes J. Fallis, K.S. Sharma, H. Sharma, J.A. Clark, G. Savard, A.F. Levand, T. Sun, D. Lascar, R. Segel, S. Caldwell, J. Van Schelt, C.M. Deibel, C. Wrede, N.D. Scielzo, A.A. Hecht, A. Parikh, F. Buchinger, J.E. Crawford, S. Gulick, J.K.P. Lee, G. Li The reaction paths of two astrophysical processes on the proton-rich side of stability, the $rp$ and $\nu p$~processes, pass through isotopes of Mo, Tc, Ru and Rh whose masses have only recently been measured. These measurements provide the more precise proton-separation energies,$S_p$, needed to model the paths and final abundances of these two processes. These $S_p$ have been of particular interest to the $\nu p$ process as it is a process which could potentially resolve the long-standing underproduction of light $p$-nuclei such as $^{92}$Mo and $^{94}$Mo. Accordingly, mass measurements of 18 proton-rich nuclides of elements from Nb to Rh have been performed with the Canadian Penning trap mass spectrometer, reducing uncertainties in the associated $S_{p}$ values by up to factors of 60 compared to the 2003 Atomic Mass Evaluation. $^{87}$Mo is found to disagree with the evaluated mass by 3.7$\sigma$, which affects recent $\nu p$-process abundance calculations. This work has been supported by grants from NSERC, Canada and by the U.S. DOE (DE-AC02-06CH11357 and DE-FG02-91ER-40609). [Preview Abstract] |
Tuesday, February 16, 2010 2:30PM - 2:42PM |
Y7.00004: Mass Measurements of Heavy $^{252}$Cf Fission Fragments Near the $r$-Process Path with the Canadian Penning Trap J. Van Schelt, D. Lascar, G. Savard, J.A. Clark, J.P. Greene, A.F. Levand, T. Sun, B.J. Zabransky, S. Caldwell, M. Sternberg, J. Fallis, K.S. Sharma, R.E. Segel, G. Li Precision mass measurements of nuclides near the astrophysical $r$-process path are vital to reduce the uncertainties in the relevant neutron separation energies and the consequent abundance predictions. Before moving to CARIBU, the Canadian Penning Trap mass spectrometer at Argonne National Laboratory undertook a series of mass measurements of spontaneous fission products from a $150~\mu Ci~^{252}$Cf source in a previous large-volume gas catcher. Masses of 38 neutron-rich nuclides ranging from $Z=51$ to $64$ were measured, many of which were closer to the $r$-process path than had previously been measured for these elements. Systematic deviations from the AME 2003 are seen over a wide range of elements, and possible effects of these deviations on the $r$ process will be discussed. These measurements are being extended to even higher neutron excess at CARIBU. [Preview Abstract] |
Tuesday, February 16, 2010 2:42PM - 2:54PM |
Y7.00005: Nuclear Masses: Sharing, Visualization, and Analysis Tools at nuclearmasses.org M.S. Smith, E.J. Lingerfelt, C.D. Nesaraja, H. Koura, F.G. Kondev Nuclear masses form an essential ingredient in simulations of a variety of astrophysical environments and events -- such as r-process nucleosynthesis in supernovae. While lab advances have led to a tremendous increase in the number and precision of new mass measurements, the dissemination of this information has many inadequacies. To address this impediment to progress, we have built an online, dedicated suite of codes that enables researchers to quickly and efficiently share, manage, visualize, access, manipulate, compare, and analyze nuclear mass datasets. Our system, freely available at {\bf nuclearmasses.org}, is a platform-independent client-server application that accommodates the latest mass measurements, theoretical mass models, and large tables of evaluated nuclear masses. With our system, researchers can upload and store their mass datasets, share them with colleagues, quickly and easily visualize them in customizable 1D and 2D plots, and calculate and plot RMS differences. Our system provides an easy mechanism to distribute theoretical models, measurements, and review articles on nuclear masses. [Preview Abstract] |
Tuesday, February 16, 2010 2:54PM - 3:06PM |
Y7.00006: On Isotope Shifts Larry Zamick It was noted by Zamick (Ann.Phys66,784(1971) That the same formula used by Talmi and De Shalit for binding energies (Rev.Mod.Phys.34, 704 (1962)) could also be used for nuclear radii i.e. isotope shifts. The argument is simple-both the radius operator and nuclear interaction are scalars under rotation. The formula has the change of square radius relative to a reference as nC+ (n(n-1)/2)A +[n/2]B. There have been many experimental papers which discuss this work as well as associated work by Talmi(NPA 423,189 (1984).The formula was originally used for the Calcium isotopes but most recently for Argon isotopes (K.Blaum et.al. NPA 799,30(2008)).There was also work on the Bismus and Lead Isotopes by M.R. Pearson et.al. (J.Phys G.26,1829(2000)). The formula displays even-odd staggering for both binding energies and nuclear radii.Sheline et.al. discuss ``inverse staggering'' as possibly evidence of Octupole deformation for certain Barium and Cesium isotopes (PRC38,2952(1988)). Other work of note is due to H.D. Wohlfahrt PRC 23,533(1981). It is my feeling that the above and other experimental results are somewhat scattered and it would be useful to collect them all and have a unified discussion of the implications of these very interesting experiments not only with the above formula but with other theoretical formulations. [Preview Abstract] |
Tuesday, February 16, 2010 3:06PM - 3:18PM |
Y7.00007: An Adaptive QSE-reduced Nuclear Reaction Network for Silicon Burning Suzanne Parete-Koon, William Raphael Hix, Friedrich-Karl Thielemann The nuclei of the ``iron peak'' are formed late in the evolution of massive stars and during supernovae. The nucleosynthesis resulting from silicon burning during these events is responsible for the production of nuclei with atomic mass numbers ranging from 28 to 64. The large number of nuclei involved makes accurate modeling of silicon burning computationally expensive. Examination of the physics of silicon burning reveals that the nuclear evolution is dominated by large groups of nuclei in mutual equilibrium. We present a nuclear reaction network scheme that takes advantage of this quasi-equilibrium (QSE) to reduce the number of independent variables needed to calculate the nucleosynthesis. Because the membership and number of these groups vary as the temperature, density and electron faction change, achieving maximal efficiency requires dynamic adjustment of group number and membership. The resultant adaptive QSE-reduced network calculates the nucleosynthesis up to 20 times faster than the full network it replaces without significant loss of accuracy. These reductions in computational cost make QSE-reduced networks well suited for inclusion within multi-dimensional hydrodynamic simulations, such as those of Type 1A Supernovae. [Preview Abstract] |
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