G. W. Hitt

Gamow-Teller strengths and electron-capture rates for pf-shell nuclei of relevance for late stellar evolution

Background: Electron-capture reaction rates on medium-heavy nuclei are an important ingredient for modeling the late evolution of stars that become core-collapse or thermonuclear supernovae. The estimation of these rates requires the knowledge of Gamow-Teller strength distributions in the β + direction. Astrophysical models rely on electron-capture rate tables largely based on theoretical models, which must be validated and tested against experimental results. Purpose: This paper presents a systematic evaluation of the ability of theoretical models to reproduce experimental Gamow-Teller transition strength distributions measured via (n,p)-type charge-exchange reactions at intermediate beam energies. The focus is on transitions from stable nuclei in the pf shell (45 A 64). In addition, the impact of deviations between experimental and theoretical Gamow-Teller strength distributions on derived stellar electron-capture rates is investigated. Method: Data on Gamow-Teller transitions from 13 nuclei in the pf shell measured via charge-exchange reactions and supplemented with results from β-decay experiments where available, were compiled and compared with strength distributions calculated in shell models (using the GXPF1a and KB3G effective interactions) and quasiparticle random-phase approximation (QRPA) using ground-state deformation parameters and masses from the finite-range droplet model. Electron-capture rates at relevant stellar temperatures and densities were derived for all distributions and compared. Results: With few exceptions, shell-model calculations in the pf model space with the KB3G and GXPF1a interactions qualitatively reproduce experimental Gamow-Teller strength distributions of 13 stable isotopes with 45 A 64. Results from QRPA calculations exhibit much larger deviations from the data and overestimate the total experimental Gamow-Teller strengths. For stellar densities in excess of 10 7 g/cm 3 , ground-state electroncapture rates derived from the shell-model calculations using the KB3G (GXPF1a) interaction deviate on average less than 47% (31%) from those derived from experimental data for which the location of daughter states at low excitation energies are well established. For electron-capture rates derived from Gamow-Teller strengths calculated in QRPA, the deviations are much larger, especially at low stellar densities. Conclusions: Based on the limited set of test cases available for nuclei in the pf shell, shell-models using the GXPF1a and KB3G interactions can be used to estimate electron-capture rates for astrophysical purposes with relatively good accuracy. Measures of the uncertainties in these rates can serve as input for sensitivity studies in stellar evolution models. Ground-state electron-capture rates based on the QRPA formalism discussed in the paper exhibit much larger deviations than those based on the shell-model calculations and should be used with caution, especially at low stellar densities.

The (t,He-3) and (He-3, t) reactions as probes of Gamow-Teller strength

It is shown via a study on a {sup 26}Mg target that the (t,{sup 3}He) reaction at 115 MeV/nucleon reaction is an accurate probe for extracting Gamow-Teller transition strengths. To do so, the data are complemented by results from the {sup 26}Mg({sup 3}He, t) reaction at 140 MeV/nucleon that allows for a comparison of T=2 analog states excited via the mirror reactions. Extracted Gamow-Teller strengths from {sup 26}Mg(t,{sup 3}He) and {sup 26}Mg({sup 3}He, t) are compared with those from {sup 26}Mg(d,{sup 2}He) and {sup 26}Mg(p,n) studies, respectively. A good correspondence is found, indicating probe independence of the strength extraction. Furthermore, we test shell-model calculations using the new USD-05B interaction in the sd-model space and show that it reproduces the experimental Gamow-Teller strength distributions well. In anticipation of further (t,{sup 3}He) experiments on medium-heavy nuclei aimed at determining weak-interaction rates of relevance for stellar evolution, a second goal of this work is to improve the understanding of the (t,{sup 3}He) and ({sup 3}He, t) reaction mechanisms at intermediate energies because detailed studies are scarce. The distorted-wave Born approximation is employed, taking into account the composite structures of the {sup 3}He and triton particles. The reaction model provides the means to explainmorexa0» systematic uncertainties at the 10%-20% level in the extraction of Gamow-Teller strengths as being because of interference between Gamow-Teller {delta}L=0,{delta}S=1 and {delta}L=2,{delta}S=1 amplitudes that both contribute to transitions from 0{sup +} to 1{sup +} states.«xa0less

Extraction of weak transition strengths via the (He-3, t) reaction at 420 MeV

Differential cross sections for transitions of known weak strength were measured with the (3He, t) reaction at 420 MeV on targets of 12C, 13C, 18O, 26Mg, 58Ni, 60Ni, 90Zr, 118Sn, 120Sn, and 208Pb. Using these data, it is shown that the proportionalities between strengths and cross sections for this probe follow simple trends as a function of mass number. These trends can be used to confidently determine Gamow-Teller strength distributions in nuclei for which the proportionality cannot be calibrated via beta-decay strengths. Although theoretical calculations in the distorted-wave Born approximation overestimate the data, they allow one to understand the main experimental features and to predict deviations from the simple trends observed in some of the transitions.

Collaborative learning in action

In this paper we introduce the Collaborative Learning Environment (CLE), a novel document creation tool, designed at the Etisalat-British Telecomm Innovation Center (EBTIC) and created to enable and enhance team-based negotiation of online, interactive writing tasks. We provide the context in which the tool has been trialed, and discuss a case study where the tool is used to gain a deep understanding into the way students thought about and implemented given tasks.

LENDA: A low energy neutron detector array for experiments with radioactive beams in inverse kinematics

Abstract The Low Energy Neutron Detector Array (LENDA) is a neutron time-of-flight (TOF) spectrometer developed at the National Superconducting Cyclotron Laboratory (NSCL) for use in inverse kinematics experiments with rare isotope beams. Its design has been motivated by the need to study the spin–isospin response of unstable nuclei using (p,n) charge-exchange reactions at intermediate energies ( > 100 MeV / u ). It can be used, however, for any reaction study that involves emission of low energy neutrons (150xa0keV to 10xa0MeV). The array consists of 24 plastic scintillator bars and is capable of registering the recoiling neutron energy and angle with high detection efficiency. The neutron energy is determined by the time-of-flight technique, while the position of interaction is deduced using the timing and energy information from the two photomultipliers of each bar. A simple test setup utilizing radioactive sources has been used to characterize the array. Results of test measurements are compared with simulations. A neutron energy threshold of 150 keV , an intrinsic time (position) resolution of ∼ 400 ps ( ∼ 6xa0cm) and an efficiency > 20 % for neutrons below 4xa0MeV have been obtained.

Gamow-Teller unit cross sections for (t,He3) and (He3,t) reactions

The proportionality between differential cross sections at vanishing linear momentum transfer and Gamow-Teller transition strength, expressed in terms of the unit cross section ({sigma}{sub GT}), was studied as a function of target mass number for (t,{sup 3}He) and ({sup 3}He,t) reactions at 115A MeV and 140A MeV, respectively. Existing ({sup 3}He,t) and (t,{sup 3}He) data on targets with mass number 12{ =}12: {sigma}{sub GT}=109/A{sup 0.65}. The factorization of the unit cross sections in terms of a kinematical factor, a distortion factor, and the strength of the effective spin-isospin transfer nucleus-nucleus interaction was investigated. Simple phenomenological functions depending on mass number A were extracted for the latter two. By comparison with plane and distorted-wave Born approximation calculations, it was found that the use of a short-range approximation for knock-on exchange contributions to the transition amplitude results in overestimated cross sections for reactions involving the composite ({sup 3}He,t) and (t,{supmorexa0» 3}He) probes.«xa0less

Gamow-Teller strength for the analog transitions to the first T=1/2,J(pi)=3/2(-) states in (13)C and (13)N and the implications for type Ia supernovae

The Gamow-Teller strength for the transition from the ground state of (13)C to the T=1/2,J(pi)=3/2(-) excited state at 3.51 MeV in (13)N is extracted via the (13)C((3)He,t) reaction at 420 MeV. In contrast to results from earlier (p,n) studies on (13)C, a good agreement with shell-model calculations and the empirical unit cross-section systematics from other nuclei is found. The results are used to study the analog (13)N(e(-),nu(e))(13)C reaction, which plays a role in the pre-explosion convective phase of type Ia supernovae. Although the differences between the results from the ((3)He,t) and (p,n) data significantly affect the deduced electron-capture rate and the net heat deposition in the star resulting from this transition, the overall effect on the pre-explosive evolution is small.

Engineering design education: Towards design thinking

This work presents an innovative interdisciplinary cornerstone engineering design-and-build course infused at the freshman level. The course promotes systematic design thinking and process using an inductive pedagogy, PBL, as the mode of delivery, in conjunction with technological tools such as robotics kits, a rapid prototyping machine, and C++ interface.

Spectroscopy of B13 via the C13(t,He3) reaction at 115A MeV

Gamow-Teller and dipole transitions to final states in {sup 13}B were studied via the {sup 13}C(t,{sup 3}He) reaction at E{sub t}=115A MeV. In addition to the strong Gamow-Teller transition to the {sup 13}B ground state, a weaker Gamow-Teller transition to a state at 3.6 MeV was found. This state was assigned a spin-parity of 3/2{sup -} by comparison with shell-model calculations using the WBP and WBT interactions which were modified to allow for mixing between n({Dirac_h}/2{pi}){omega} and (n+2)({Dirac_h}/2{pi}){omega} configurations. This assignment agrees with a recent result from a lifetime measurement of excited states in {sup 13}B. The shell-model calculations also explained the relatively large spectroscopic strength measured for a low-lying 1/2{sup +} state at 4.83 MeV in {sup 13}B. The cross sections for dipole transitions up to E{sub x}({sup 13}B)=20 MeV excited via the {sup 13}C(t,{sup 3}He) reaction were also compared with the shell-model calculations. The theoretical cross sections exceeded the data by a factor of about 1.8, which might indicate that the dipole excitations are quenched. Uncertainties in the reaction calculations complicate that interpretation.

Smart grouping tool portal for collaborative learning

Learning theory in education has progressed dramatically with the rapid development of modern technologies. Modern learning theory has found that learning performance can be improved effectively through cooperative/ collaborative learning. How to group students in one class based on their different learning capabilities is a key to success in education. Two grouping types of homogeneous and heterogeneous have been applied into class grouping. Research proves that homogeneous groups are good at specific goal and heterogeneous groups are better in expanded tasks. Learning efficiency is affected in both group level and individual level by different grouping method. However grouping students in an optimal way is a difficult NP-hard research problem with high complexity; and to achieve the best results, such grouping can be very time consuming and tedious for the teachers to perform manually, especially when local cultural context (for instance, gender parameter) has to be taken into account, such as in the Middle East region. So the ability of allowing educators to define rules describing groups is very important for both homogeneous and heterogeneous grouping capabilities. This paper thus describes a smart grouping tool portal which is easy to access and user friendly, and aims to assist teachers to automatically group students based on different criteria. This portal has been trialled at Khalifa University of Science, Technology & Research (KUSTAR). A case study of the trial has also been analyzed to illustrate the derivative benefits of our smart grouping tool portal.