Inelastic proton scattering at energies of a few hundred MeV and very-forward scattering angles including 0° has been established as a tool for the study of electric-dipole strength distributions in ...nuclei. The present work reports a systematic investigation of the chain of stable even-mass Nd isotopes representing a transition from spherical to quadrupole-deformed nuclei. Extraction of the equivalent photo-absorption cross sections and analysis of their fine structure in the energy region of the isovector giant dipole resonance (IVGDR). Method: Proton inelastic scattering reactions of 200 MeV protons were measured at the iThemba Laboratory for Accelerator Based Sciences in Cape Town, South Africa. The scattering products were momentum-analyzed by the K600 magnetic spectrometer positioned at θLab = 0°. Using dispersion-matching techniques, energy resolutions of ΔE ≈ 40–50 keV (full width at half maximum) were obtained. After subtraction of background and contributions from other multipoles, the spectra were converted to photoabsorption cross sections using the equivalent virtual-photon method. Wavelet-analysis techniques are used to extract characteristic energy scales of the fine structure of the IVGDR from the experimental data. Fine structure of the IVGDR is observed even for the most deformed nuclei studied. Comparisons between the extracted experimental energy scales and those energy scales obtained from the quasiparticle-phonon model (QPM) and Skyrme separable random phase approximation (SSRPA) predictions provide insight into the role of different giant-resonance damping mechanisms. It can be seen that the scales in the spherical and most likely also in the deformed nuclei mainly result from the fragmentation of the one-particle-one-hole (1p1h) strength into several dominant transitions serving as doorway states. In cases where calculations beyond the 1p1h level are available, some impact of the spreading due to coupling of the two-particle-two-hole (2p2h) states to the 1p1h doorway states is observed. New virtual-photon absorption data for the chain of stable Nd isotopes and 152Sm are presented, with a focus on the phenomenon of nonstatistical cross-section fluctuations, referred to as fine structure, in the energy region of the IVGDR. The wavelet-analysis techniques used allowed for the features of the fine structure to be quantified in the form of characteristic scales. Finally, comparisons between experimental results and model predictions indicate that Landau damping seems to be the main source of the fine structure in both the spherical and deformed nuclei, but calculations including 2p2h degrees of freedom would be beneficial to confirm this for the deformed cases.
Abstract
The iThemba Laboratory for Accelerator Based Sciences (iThemba LABS) is a centre of expertise and innovation in the field of nuclear-structure physics and is a leader in several high-impact ...studies. One of the highlights of these nuclear-structure experiments is the study of the broad structure of the IsoVector Giant Dipole Resonance (IVGDR) in the rare-earth region. Proton inelastic scattering experiments with
E
p
= 200 MeV were performed on the even-even Nd isotope chain and
152
Sm at very forward scattering angles including zero degrees with the K600 magnetic spectrometer. The evolution of the shape of the IVGDR in the transition from spherical to deformed nuclei was investigated. One of the goals of this highlighted study was to confirm the
K
-splitting observed in previous photo-absorption measurements from Saclay. Significant discrepancies were found between the direct (γ, xn) data obtained at Saclay and the equivalent photo-absorption cross sections obtained using (p, p′) data from the K600. Furthermore, discrepancies exist for several nuclei between photo-absorption data taken at the Saclay and Livermore laboratories. These discrepancies, possible reasons for them and future investigations will be presented and discussed.
Electric dipole polarizability of ^{40}Ca R. W. Fearick; P. von Neumann-Cosel; S. Bacca ...
Physical review research,
05/2023, Letnik:
5, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The electric dipole strength distribution in ^{40}Ca between 5 and 25 MeV has been determined at RCNP, Osaka, from proton inelastic scattering experiments at very forward angles. Combined with total ...photoabsorption data at higher excitation energy, this enables an extraction of the electric dipole polarizability α_{D}(^{40}Ca)=1.92(17) fm^{3}. Together with the measured α_{D} in ^{48}Ca, it provides a stringent test of modern theoretical approaches, including coupled-cluster calculations with chiral effective field theory interactions and state-of-the art energy density functionals. The emerging picture is that for this medium-mass region dipole polarizabilities are well described theoretically, with important constraints for the neutron skin in ^{48}Ca and related equation of state quantities.
Electric dipole polarizability of Ca 40 Fearick, R. W.; von Neumann-Cosel, P.; Bacca, S. ...
Physical review research,
05/2023, Letnik:
5, Številka:
2
Journal Article
The 34Ar(α,p) 37K reaction is believed to be one of the last in a sequence of (α,p) and (p,γ) reactions within the Tz = -1 , sd-shell nuclei, known as the αp-process. This process is expected to ...influence the shape and rise times of luminosity curves coming from type I x-ray bursts (XRBs). With very little experimental information known on many of the reactions within the αp-process, stellar rates are calculated using a statistical model, such as Hauser-Feshbach. Questions on the applicability of a Hauser-Feshbach model for the 34Ar (α,p) 37K reaction arise due to level density considerations in the compound nucleus, 38Ca . We have performed high energy-resolution forward-angle 40Ca (p,t) 38Ca measurements with the K = 600 spectrograph at iThemba LABS in order to identify levels above the α-threshold in 38Ca . States identified in this work were then used to determine the 34Ar (α,p) 37K reaction rate based on a narrow-resonance formalism. Comparisons are made to two standard Hauser-Feshbach model predicted rates at XRB temperatures.
The Langevin model provides a description of axial channeling that is intermediate between a full Monte Carlo treatment and that of a statistical equilibrium treatment. Quantities are readily ...interpreted in terms of useful variables such as the transverse energy. We show that with appropriate averaging, the usual diffusion model of dechanneling is recovered from the Langevin description. This places the Langevin model more firmly in context.
The ALICE High Level Trigger combines and processes the full information from all major detectors in a large computer cluster. Data rate reduction is achieved by reducing the event rate by selecting ...interesting events (software trigger) and by reducing the event size by selecting sub-events and by advanced data compression. Reconstruction chains for the barrel detectors and the forward muon spectrometer have been benchmarked. The HLT receives a replica of the raw data via the standard ALICE DDL link into a custom PCI receiver card (HLT-RORC). These boards also provide a FPGA co-processor for data-intensive tasks of pattern recognition. Some of the pattern recognition algorithms (cluster finder, Hough transformation) have been re-designed in VHDL to be executed in the Virtex-4 FPGA on the HLT-RORC. HLT prototypes were operated during the beam tests of the TPC and TRD detectors. The input and output interfaces to DAQ and the data flow inside of HLT were successfully tested. A full-scale prototype of the dimuon-HLT achieved the expected data flow performance. This system was finally embedded in a GRID-like system of several distributed clusters demonstrating the scalability and fault-tolerance of the HLT.
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