Mon·Apr 9, 2018Monday·April 9, 2018

1. 01*

   First Accurate Normalization of the $\beta$-delayed $\alpha$ Decay of ${}^{16}$N and Implications for the ${}^{12}$C$(\alpha ,\gamma {)}^{16}$O Astrophysical Reaction Rate

   O. S. Kirsebom🇩🇰 · O. Tengblad🇪🇸 · R. Lica🇨🇭 · M. Munch🇩🇰 · K. Riisager🇩🇰 · H. O. U. Fynbo🇩🇰 · M. J. G. Borge🇪🇸 · M. Madurga🇨🇭 · I. Marroquin🇪🇸 · A. N. Andreyev🇬🇧 · T. A. Berry🇬🇧 · E. R. Christensen🇩🇰 and 32 other authors

   The ${}^{12}\text{C}(\alpha ,\gamma ){}^{16}\text{O}$ reaction plays a central role in astrophysics, but its cross section at energies relevant for astrophysical applications is only poorly constrained by laboratory data. The reduced $\alpha$ width, ${\gamma }_{11}$, of the bound $1^{-}$ level in ${}^{16}$O is particularly important to determine the cross section. The magnitude of ${\gamma }_{11}$ is determined via sub-Coulomb $\alpha$-transfer reactions or the $\beta$-delayed $\alpha$ decay of ${}^{16}$N, but the latter approach is presently hampered by the lack of sufficiently precise data on the $\beta$-decay branching ratios. Here we report improved branching ratios for the bound $1^{-}$ level [$b_{\beta ,11}=(5.02\pm 0.10)\times 10^{-2}$] and for $\beta$-delayed $\alpha$ emission [$b_{\beta \alpha }=(1.59\pm 0.06)\times 10^{-5}$]. Our value for $b_{\beta \alpha }$ is 33% larger than previously held, leading to a substantial increase in ${\gamma }_{11}$. Our revised value for ${\gamma }_{11}$ is in good agreement with the value obtained in $\alpha$-transfer studies and the weighted average of the two gives a robust and precise determination of ${\gamma }_{11}$, which provides significantly improved constraints on the ${}^{12}$C$(\alpha ,\gamma )$ cross section in the energy range relevant to hydrostatic He burning.

   nucl-exPRL(2018)·8 citations
2. 02*

   Observation of Pendellösung Fringes by Using Pulsed Neutrons

   Shigeyasu Itoh · Masaya Nakaji · Yuya Uchida · Masaaki Kitaguchi🇯🇵 · Hirohiko M. Shimizu🇯🇵

   Pendellösung interference fringes of a single silicon crystal were observed by using pulsed cold neutrons. The nuclear scattering length of silicon was obtained as (4.159\pm0.003(stat.)\pm0.028(syst.)) fm using the observed Pendellösung fringes. This indicates the applicability of pulsed neutron beam to observe the Pendellösung fringes by using the time-of-flight analysis.

   nucl-exphysics.ins-det0 citations
3. 03*

   Mass Measurements of Neutron-Deficient Y, Zr, and Nb Isotopes and Their Impact on $rp$ and $\nu p$ Nucleosynthesis Processes

   Y. M. Xing🇨🇳 · K. A. Li🇨🇳 · Y. H. Zhang🇨🇳 · X. H. Zhou🇨🇳 · M. Wang🇨🇳 · Yu. A. Litvinov🇩🇪 · K. Blaum🇩🇪 · S. Wanajo🇯🇵 · S. Kubono🇨🇳 · G. Martínez-Pinedo🇩🇪 · A. Sieverding🇩🇪 · R. J. Chen🇨🇳 and 39 other authors

   Using isochronous mass spectrometry at the experimental storage ring CSRe in Lanzhou, the masses of ${}^{82}$Zr and ${}^{84}$Nb were measured for the first time with an uncertainty of $\sim 10$ keV, and the masses of ${}^{79}$Y, ${}^{81}$Zr, and ${}^{83}$Nb were re-determined with a higher precision. %The latter differ significantly from their literature values. The latter are significantly less bound than their literature values. Our new and accurate masses remove the irregularities of the mass surface in this region of the nuclear chart. Our results do not support the predicted island of pronounced low $\alpha$ separation energies for neutron-deficient Mo and Tc isotopes, making the formation of Zr-Nb cycle in the $rp$-process unlikely. The new proton separation energy of ${}^{83}$Nb was determined to be 490(400)~keV smaller than that in the Atomic Mass Evaluation 2012. This partly removes the overproduction of the $p$-nucleus ${}^{84}$Sr relative to the neutron-deficient molybdenum isotopes in the previous $\nu p$-process simulations.

   nucl-exastro-ph.SRPLB(2018)·36 citations
4. 04*

   Towards Reliable Nuclear Matrix Elements for Neutrinoless $\beta \beta$ Decay

   Javier Menéndez🇯🇵

   The calculated nuclear matrix elements for the neutrinoless double-beta ($0\nu \beta \beta$) decay suffer from several limitations. Predicted matrix-element values depend on the many-body method used to calculate them and, in addition, they may need to be "quenched", albeit by an unknown amount due to the relatively high momentum transferred in the decay. These uncertainties are hard to figure out from theoretical calculations alone because of the unique character of $0\nu \beta \beta$ decay, not clearly related to other nuclear structure observables. We present recent progress in the determination of the nuclear matrix elements. First, we report improved shell model calculations in an extended configuration space of two harmonic oscillator shells. Second, we note the relation between the $0\nu \beta \beta$ decay and double Gamow-Teller transitions that can in principle be measured in double charge-exchange reactions. These new insights pave the way for a more reliable estimation of the value of the nuclear matrix elements.

   ↳ nucl-thhep-exhep-phnucl-exJPS Conf.Proc.(2018)·2 citations
5. 05*

   Neutrinoless $\beta \beta$ decay mediated by the exchange of light and heavy neutrinos: The role of nuclear structure correlations

   Javier Menéndez🇯🇵

   Neutrinoless $\beta \beta$ decay nuclear matrix elements calculated with the shell model and energy-density functional theory typically disagree by more than a factor of two in the standard scenario of light-neutrino exchange. In contrast, for a decay mediated by sterile heavy neutrinos the deviations are reduced to about 50\%, an uncertainty similar to the one due to short-range effects. We compare matrix elements in the light- and heavy-neutrino-exchange channels, exploring the radial, momentum transfer and angular momentum-parity matrix element distributions, and considering transitions that involve correlated and uncorrelated nuclear states. We argue that the shorter-range heavy-neutrino exchange is less sensitive to collective nuclear correlations, and that discrepancies in matrix elements are mostly due to the treatment of long-range correlations in many-body calculations. Our analysis supports previous studies suggesting that isoscalar pairing correlations, which affect mostly the longer-range part of the neutrinoless $\beta \beta$ decay operator, are partially responsible for the differences between nuclear matrix elements in the standard light-neutrino-exchange mechanism.

   ↳ nucl-thhep-exhep-phnucl-exJ.Phys.G(2018)·125 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.