E. Jacobs

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

Polarised bremsstrahlung nuclear-resonance fluorescence set-up at the 15~MeV linac in Gent.

Abstract Nuclear resonance fluorescence experiments using unpolarised as well as off-axis linearly polarised bremsstrahlung represent an outstanding tool to determine in a completely model independent way transition probabilities, multipole orders and parities of electromagnetic transitions to bound states in nuclei. A new polarised bremsstrahlung facility has been constructed at the 15 MeV linac in Gent. The experimental arrangement is discussed and first results are presented.

Nuclear data sheets update for A = 110

Abstract The 1983 evaluation on mass A=110 has been revised, taking into account all data available before November 1991. Detailed experimental information is presented from the neutron rich nucleus 110 Mo to the neutron deficient 110 Xe nucleus.

A DSP-BASED READOUT AND ONLINE PROCESSING SYSTEM FOR A NEW FOCAL-PLANE POLARIMETER AT AGOR

Abstract A Focal-Plane Polarimeter (FPP) for the large acceptance Big-Bite Spectrometer (BBS) at AGOR using a novel readout architecture has been commissioned at the KVI Groningen. The instrument is optimized for medium-energy polarized proton scattering near or at 0°. For the handling of the high counting rates at extreme forward angles and for the suppression of small-angle scattering in the graphite analyzer, a high-performance data processing DSP system connecting to the LeCroy FERA and PCOS ECL bus architecture has been made operational and tested successfully. Details of the system and the functions of the various electronic components are described.

On the determination of polarization observables in proton-induced reactions using large-acceptance magnetic spectrometers

Abstract For proton-induced reactions, a formalism is presented to determine polarization observables using a focal-plane polarimeter in combination with a large-acceptance spectrometer, where also the effect of the scattering in the non-dispersive plane of the spectrometer is taken into account. The inclusive proton-carbon analyzing power needed for the determination of spin observables was studied over a wide angular range and a new parameterization for this quantity was deduced.

Strong fragmentation of low-energy electromagnetic excitation strength in Sn-117

Results of nuclear resonance fluorescence experiments on Sn-117 are reported. More than 50 gamma transitions with E gamma < 4 MeV were detected. For the first time microscopic calculations making use of a complete configuration space for low-lying states are performed for a heavy odd-mass spherical nucleus. The theoretical predictions are in good agreement with the data. It is concluded that although the El transitions are the strongest ones also M1 and E2 decays contribute substantially to the observed spectra. In contrast to the neighboring even Sn116-124 nuclei, in Sn-117 the 1(-) component of the two-phonon [2(1)(+) x 3(1)(-)] multiplet built on top of the 1/2(+) ground state is proved to be fragmented.

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.

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.

Gamow-Teller transitions in the Ni-64(He-3, t)Cu-64 reaction

In order to study the Gamow–Teller (GT) transitions in the fp-shell nucleus 64Cu, the 64Ni(3He, t)64Cu charge-exchange reaction was investigated at MeV/nucleon [1]. The outgoing tritons were momentum analysed by the Grand Raiden spectrometer at 0°. The very high energy resolution of 35 keV (FWHM) allowed the separation of individual levels in the excitation energy region from 0 to 3.5 MeV. An angular distribution analysis was performed for the observed transitions to these states. In addition to the ground state (g.s.), known to be a Jπ = 1+ GT state, several excited states showed L = 0 nature, making them candidates of GT states. At higher excitation energies, the level density becomes very high and a bump-like structure, the so-called GT Giant Resonance, dominates the spectrum.