C. Bäumer

Measuring the (d,He-2) reaction with the focal-plane detection system of the BBS magnetic spectrometer at AGOR

At intermediate energies, the (d, He-2) charge-exchange reaction can be used to observe Gamow-Teller strength in the direction. He-2 denotes the two-proton system being in the singlet S-1(0) state. In the present experiment the two protons, which in the laboratory frame are emitted into the forward direction, have been momentum analyzed and detected in coincidence by the same spectrometer and detector. Protons from deuteron breakup processes can induce a large accidental coincidence background because of the much larger breakup cross-section as compared to the (d, He-2) cross-section. Nevertheless, background-free He-2 spectra with a resolution of 145 keV at an incident energy of 170 MeV are obtained, allowing the identification of many levels with high precision in the residual nuclei. The essential features of the detection system and the data-acquisition and analysis techniques which make our (d 2 He) experiments possible are described. Two nuclei, C-12 and Mg-24, have been used as a test case

Studies on the double-β decay nucleus Zn64 using the (d,He2) reaction

The (d, 2 He) charge-exchange reaction on the double-β decay (ββ) nucleus 64 Zn has been studied at an incident energy of 183 MeV. The two protons in the 1 S 0 state (indicated as 2 He) were both momentum analyzed and detected simultaneously by the BBS magnetic spectrometer and its position-sensitive detector. 2 He spectra with a resolution of about 115 keV (FWHM) have been obtained allowing identification of many levels in the residual nucleus 64 Cu with high precision. 64 Zn is one of the rare cases undergoing a ββ decay in β + direction. In the experiment presented here, Gamow-Teller (GT + ) transition strengths have been extracted. Together with the GT - transition strengths from 64 Ni( 3 He,t) data to the same intermediate nucleus 64 Cu, the nuclear matrix elements of the ββ decay of 64 Zn have been evaluated. Finally, the GT + distributions are compared with shell-model calculations and a critical assessment is given of the various residual interactions presently employed for the pf shell.

Testing quantum correlations with nuclear probes

We investigated the feasibility of quantum-correlation measurements in nuclear physics experiments. In the first approach, we measured spin correlations of singlet-spin (S-1(0)) proton pairs, which were generated in H-1(d,He-2) and C-12(d, He-2) nuclear charge-exchange reactions. The experiment was optimized for a clean preparation of the He-2 singlet state and offered a 2pi detection geometry for both protons in the exit channel. Our results confirm the effectiveness of the setup for these studies, despite limitations of a small data sample recorded during the feasibility studies.

Polarization correlations of 1S0 proton pairs as tests of hidden-variable theories

We are investigating the feasibility of nuclear physics experiments designed to overcome the loopholes of observer-dependent reality and satisfying the counterfactuality condition. In a first approach, we have measured polarization correlations of S-1(0) proton pairs produced in C-12(d, He-2) and H-1(d, He-2) reactions in one setting. The results of these measurements are used to test the Bell and Wigner-Belinfante inequalities against the predictions of quantum mechanics

High resolution study of the Gamow–Teller strength distribution starting from the ground state of 14N in the β− and β+ directions

The two charge exchange reactions 14N(d, 2He)14C and 14N(3He, t)14O were studied with high resolution in order to identify the Gamow–Teller (GT) strength distribution starting from the ground state (g.s.) of 14N in the β− and β+ directions. The isospin symmetry in the A = 14 mass system is investigated. An overview of the main features of the experimental investigation is summarized here.

Performance of an ASD-8 based wire chamber readout system

For the medium-energy proton polarimeter mounted at the focal-plane of the Big-Bite Spectrometer at Kernfysisch Versneller Instituut Groningen, a new wire-chamber readout has been developed. The charge-sensitive preamplifier is based on the ASD-8 B chip which has an input impedance of 115 /spl Omega/. This low impedance and the short integration time of 6 ns at a usable sensitivity of 5 fC allow high readout rates and low gas amplifications. This front-end circuit is mounted on all four multi-wire proportional chambers and on two vertical-drift chambers with 3872 wires in total. Measurements have been made using sources and intermediate energy protons. Special attention was given to determine the time-over-threshold properties of the circuit. The time-over-threshold capabilities of the readout system extend future applications to particle discrimination, or, in connection with other detector types, to energy-resolving readout. The operational performance of the readout system is presented.

High resolution studies of low lying GT strength using the (d,He-2) reaction and its impact on electron-capture rates in stellar environments

Abstract Gamow-Teller transitions in fp-shell nuclei have been studied using the 58 Ni( d , 2 He) charge exchange reaction at an incident energy of 170 MeV and forward angles. The measurements have been performed at AGOR using the Big-Bite Spectrometer and the EuroSuperNova focal-plane detection system. The unprecedented energy resolution of about 130 keV FWHM for excitations in the residual nucleus 58 Co allowed for the first time the extraction of single transitions in the β + direction up to about 4 MeV excitation. The impact on electron-capture rates in astrophysical environments has been evaluated.

The (t,3He) reaction at 43 MeV/nucleon on 48Ca and 58Ni : Results and microscopic interpretation

We have used the 43 MeV/nucleon primary tritium beam of the AGOR facility with an intensity of 4x10(7) pps and the BBS experimental setup to study the (t,He-3) reaction between 0 degrees and 5 degrees lab angles on C-12, Ca-48, and Ni-58 targets. The standard ray-tracing procedure has allowed us to obtain excitation-energy spectra up to 30 MeV in six angular bins for each residual nucleus, with an average energy resolution of 350 keV. The reaction mechanism has been described in distorted-waves Born approximation (DWBA) using the DWBA98 code. In this approximation, the form factor is treated as a folding of an effective projectile-nucleon interaction with a transition density. The effective projectile-nucleon interaction has been adjusted to reproduce the 0(degrees) cross section of the 1(+) ground state of B-12 populated in the C-12(t,He-3) reaction. We have employed random-phase approximation (RPA) wave functions of excited states to construct the form factor instead of the normal modes wave functions used earlier. This new DWBA+RPA analysis is used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in K-48 and Co-58 nuclei.

Distribution of the GT strength starting from the ground state of 14N

Two charge‐exchange reactions were performed at intermediate energies in order to investigate the Gamow‐Teller strength distribution starting from 14Ng.s. in β+ and β− directions: 14N(d,2He)14C and 14N(3He,t)14O. We describe here a few delicate details of the experimental techniques used in these investigations.

Spin‐isospin excitations from the ground‐state of 64Ni

Spin‐isospin (Gamow‐Teller) excitations in 64Cu and 64Co have been studied using (3He,t) and (d,2He) charge‐exchange reactions on 64Ni. As the isospin of the 64Ni ground‐state is T0=4, states with T=3, 4 and 5 in 64Cu are excited via the (3He,t) reaction and states with T=5 in 64Co via (d,2He). If we assume that the nuclear interaction is charge symmetric, the T=5 states in 64Cu should appear at corresponding excitation energies (if corrected for the Coulomb displacement) and with similar strengths as the T=5 states in 64Co. As in the 64Cu spectrum the T=5 states are very weakly excited, only by combining the results of the two complementary experiments one can estimate the Gamow‐Teller strength starting from 64Ni in a consistent way.