Wed·Oct 14, 2020Wednesday·October 14, 2020

1. 01*

   Structure of ${}^{30}$Mg explored via in-beam $\gamma$-ray spectroscopy

   N. Kitamura · K. Wimmer · N. Shimizu · V. M. Bader · C. Bancroft · D. Barofsky · T. Baugher · D. Bazin · J. S. Berryman · V. Bildstein · A. Gade · N. Imai T. Kröll C. Langer J. Lloyd E. Lunderberg and 9 other authors

   Background: In the "island of inversion", ground states of neutron-rich $sd$-shell nuclei exhibit strong admixtures of intruder configurations from the $fp$ shell. The nucleus ${}^{30}$Mg, located at the boundary of the island of inversion, serves as a cornerstone to track the structural evolution as one approaches this region. Purpose: Spin-parity assignments for excited states in ${}^{30}$Mg, especially negative-parity levels, have yet to be established. In the present work, the nuclear structure of ${}^{30}$Mg was investigated by in-beam $\gamma$-ray spectroscopy mainly focusing on firm spin-parity determinations. Method: High-intensity rare-isotope beams of ${}^{31}$Mg, ${}^{32}$Mg, ${}^{34}$Si, and ${}^{35}$P bombarded a Be target to induce nucleon removal reactions populating states in ${}^{30}$Mg. $\gamma$ rays were detected by the state-of-the-art $\gamma$-ray tracking array GRETINA. For the direct one-neutron removal reaction, final-state exclusive cross sections and parallel momentum distributions were deduced. Multi-nucleon removal reactions from different projectiles were exploited to gain complementary information. Results: With the aid of the parallel momentum distributions, an updated level scheme with revised spin-parity assignments was constructed. Spectroscopic factors associated with each state were also deduced. Conclusions: Results were confronted with large-scale shell-model calculations using two different effective interactions, showing excellent agreement with the present level scheme. However, a marked difference in the spectroscopic factors indicates that the full delineation of the transition into the island of inversion remains a challenge for theoretical models.

   nucl-exPRC(2020)·8 citations
2. 02*

   Total kinetic energy release in the fast neutron-induced fission of 237Np

   A. Pica · A.T. Chemey · L. Yao · W. Loveland🇺🇸 · H.Y. Lee · S.A. Kuvin

   The total kinetic energy (TKE) in the fast neutron induced fission of 237Np was measured for neutron energies from En = 2.6 - 100 MeV at the LANSCE-WNR facility. The post TKE release decreases non-linearly with increasing incident neutron energy and can be represented as TKE(MeV) = (174.38 +- 0.72) - (5.11 +- 0.5821) log10 En for En > 1 MeV. Analysis of the fragment mass distributions indicates that the decrease in TKE with increasing En is a consequence of two factors; shell effects fade out at high excitation energies, resulting in the increasing occurrence of symmetric fission, and TKEasym decreases rapidly at high En.

   nucl-exPRC(2020)·6 citations
3. 03*

   Quantum entanglement in nuclear Cooper pair tunneling with $\gamma$-rays

   G. Potel🇺🇸 · F. Barranco🇪🇸 · E. Vigezzi🇮🇹 · R. A. Broglia🇩🇰

   While Josephson-like junctions, transiently established in heavy ion collisions (${\tau }_{coll}\approx 10^{-21}$ s) between superfluid nuclei --through which Cooper pair tunneling ($Q$-value $Q_{2n}$) proceeds mainly in terms of successive transfer of entangled nucleons-- is deprived from the macroscopic aspects of a supercurrent, it displays many of the special effects associated with spontaneous symmetry breaking in gauge space (BCS condensation), which can be studied in terms of individual quantum states and of tunneling of single Cooper pairs. From the results of studies of one- and two- neutron transfer reactions carried out at energies below the Coulomb barrier we estimate the value of the mean square radius (correlation length) of the nuclear Cooper pair. A quantity related to the largest distance of closest approach for which the absolute two-nucleon tunneling cross section is of the order of the single-particle one. Furthermore, emission of $\gamma$-rays of (Josephson) frequency ${\nu }_J=Q_{2n}/h$ distributed over an energy range $\hslash /{\tau }_{coll}$ is predicted.

   ↳ nucl-thnucl-exPRC(2021)·21 citations
4. 04*

   Response of a Li-glass/multi-anode photomultiplier detector to collimated thermal-neutron beams

   E. Rofors🇸🇪 · N. Mauritzson🇸🇪 · H. Perrey🇸🇪 · R. Al Jebali🇸🇪 · J.R.M. Annand🇬🇧 · L. Boyd🇬🇧 · M.J. Christensen🇸🇪 · U. Clemens🇩🇪 · S. Desert🇫🇷 · R. Engels🇩🇪 · K.G. Fissum🇸🇪 · H. Frielinghaus🇩🇪 and 13 other authors

   The response of a position-sensitive Li-glass scintillator detector being developed for thermal-neutron detection with 6 mm position resolution has been investigated using collimated beams of thermal neutrons. The detector was moved perpendicularly through the neutron beams in 0.5 to 1.0 mm horizontal and vertical steps. Scintillation was detected in an 8 X 8 pixel multi-anode photomultiplier tube on an event-by-event basis. In general, several pixels registered large signals at each neutron-beam location. The number of pixels registering signal above a set threshold was investigated, with the maximization of the single-hit efficiency over the largest possible area of the detector as the primary goal. At a threshold of ~50% of the mean of the full-deposition peak, ~80% of the events were registered in a single pixel, resulting in an effective position resolution of ~5 mm in X and Y. Lower thresholds generally resulted in events demonstrating higher pixel multiplicities, but these events could also be localized with ~5 mm position resolution.

   ↳ physics.ins-detnucl-exNucl.Instrum.Meth.A(2021)·1 citation
5. 05*

   Evolution of $\beta$-decay half-lives at finite-temperatures

   A. Ravlic🇭🇷 · E. Yuksel🇹🇷 · Y. F. Niu🇨🇳 · N. Paar🇭🇷

   $\beta$-decay properties of nuclei are investigated within the relativistic nuclear energy density functional framework by varying the temperature and density, conditions relevant to the final stages of stellar evolution. Both thermal and nuclear pairing effects are taken into account in the description of nuclear properties and in the finite temperature proton-neutron relativistic quasiparticle random-phase approximation (FT-PNRQRPA) to calculate the relevant allowed and first-forbidden transitions in the $\beta$-decay. The temperature and density effects are studied on the $\beta$-decay half-lives between temperatures $T=0-1.5$ MeV, and at densities $\rho Y_e=10^7$ g/cm${}^3$ and $10^9$ g/cm${}^3$. The relevant Gamow-Teller transitions are also investigated for Ti, Fe, Cd, and Sn isotopic chains at finite temperatures. We find that the $\beta$-decay half-lives increase with increasing density $\rho Y_e$, whereas half-lives generally decrease with increasing temperature. It is shown that the temperature effects decrease the half-lives considerably in nuclei with longer half-lives at zero temperature, while only slight changes for nuclei with short half-lives are obtained. We also show the importance of including the de-excitation transitions in the calculation of the $\beta$-decay half-lives at finite temperatures. Comparing the FT-PNQRPA results with the shell-model calculations for $pf-$shell nuclei, a reasonable agreement is obtained for the temperature dependence of $\beta$-decay rates. Finally, large-scale calculations of $\beta$-decay half-lives are performed at temperatures $T_9(\text{K})=5$ and $T_9(\text{K})=10$ and densities $\rho Y_e=10^7$ g/cm${}^3$ and $10^9$ g/cm${}^3$ for even-even nuclei in the range $8\le Z\le 82$, relevant for astrophysical nucleosynthesis mechanisms.

   ↳ nucl-thastro-ph.SRnucl-ex3 citations
6. 06*

   Measured Nondestructive Assay of ${}^{237}$Np Using Organic Scintillators and Active Neutron Multiplicity Counting

   Michael Y. Hua · Thomas A. Plummer · Jesson D. Hutchinson · George E. McKenzie🇺🇸 · Shaun D. Clarke🇺🇸 · Sara A. Pozzi🇮🇹

   The purpose of nondestructive assay in the context of nuclear safeguards is to precisely verify the declared mass of a sample of nuclear material in a noninhibitive amount of time. 237Np is a proliferation concern, and the capacity to efficiently assay samples of it is a missing piece in the verification and safeguards toolbox. The material is subject to the same safeguards as 235U, is reportable in gram quantities, and is classified as "other nuclear material" according to the United States Department of Energy. Given that 3000 kg of 237Np is annually produced in the US and the bare sphere critical mass is 40-60 kg, it is desirable to augment the safeguards toolbox with a system capable of distinguishing 10 g of 237Np in a 20-minute measurement. One measurement modality is neutron multiplicity counting, which relates the detected multiplicity count rates to the amount of fissionable material. Prior simulation work shows that an organic scintillator-based multiplicity counter can achieve the design criteria, whereas the flagship 3He-based system, the Epithermal Neutron Multiplicity Counter, requires much longer measurement times to achieve the same precision. In this work, simultaneous measurements of a 6-kg sphere of 237Np by organic scintillator- and 3He-based systems are used to confirm the trends in the simulation study; the organic scintillator-based system achieves 1% uncertainty in the neutron double multiplicity rate on the order of minutes, while the 3He-based system requires days to reach the same precision. In conclusion, the International Atomic Energy Agency should consider the development and deployment of an organic scintillator-based multiplicity counter

   ↳ physics.ins-detnucl-ex0 citations
7. 07*

   Rossi-alpha Uncertainty Quantification by Analytic, Bootstrap, and Sample Methods to Inform Fitting Best Practices

   M.Y. Hua · C.A. Bravo · R.M. Marchie · J.D. Hutchinson · G.E. McKenzie🇺🇸 · S. A. Pozzi🇮🇹

   The prompt neutron period (the negative reciprocal of the prompt neutron decay constant) can be estimated using the Rossi-alpha technique that is predicated on fitting Rossi-alpha histograms and of interest in nuclear criticality safety and nonproliferation [1, 2, 3]. The histograms are traditionally fit with a one-exponential model; however, recent work has proposed a two-exponential model to account for reflector-induced phenomenon [4, 5, 6]. Until recently, the uncertainty quantification for either model was inadequate (inaccurate and demanded large measurement times). Measurement uncertainty quantification by sample and analytic methods was developed and validated in Ref. [7]. The purpose of this transaction is to (i) validate a new bootstrap method by comparing bin-by-bin error bar estimates and (ii) demonstrate how to choose bin widths and reset times to optimize precision and accuracy.

   ↳ physics.data-annucl-ex0 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.