T. Murakami

Constraints on the symmetry energy and neutron skins from experiments and theory

The symmetry energy contribution to the nuclear equation of state impacts various phenomena in nuclear astrophysics, nuclear structure, and nuclear reactions. Its determination is a key objective of contemporary nuclear physics, with consequences for the understanding of dense matter within neutron stars. We examine the results of laboratory experiments that have provided initial constraints on the nuclear symmetry energy and on its density dependence at and somewhat below normal nuclear matter density. Even though some of these constraints have been derived from properties of nuclei while others have been derived from the nuclear response to electroweak and hadronic probes, within experimental uncertainties-they are consistent with each other. We also examine the most frequently used theoretical models that predict the symmetry energy and its slope parameter. By comparing existing constraints on the symmetry pressure to theories, we demonstrate how contributions of three-body forces, which are essential ingredients in neutron matter models, can be determined.

Isoscalar giant resonances in the Sn nuclei and implications for the asymmetry term in the nuclear-matter incompressibility

We have investigated the isoscalar giant resonances in the Sn isotopes using inelastic scattering of 386-MeV alpha particles at extremely forward angles, including 0 degrees. We have obtained completely background-free inelastic-scattering spectra for the Sn isotopes over the angular range 0 degrees-9 degrees and up to an excitation energy of 31.5 MeV. The strength distributions for various multipoles were extracted by a multipole decomposition analysis based on the expected angular distributions of the respective multipoles. We find that the centroid energies of the isoscalar giant monopole resonance (ISGMR) in the Sn isotopes are significantly lower than the theoretical predictions. In addition, based on the ISGMR results, a value of K(tau) = -550 +/- 100 MeV is obtained for the asymmetry term in the nuclear incompressibility. Constraints on interactions employed in nuclear structure calculations are discussed on the basis of the experimentally obtained values for K(infinity) and K(tau).

The Giant Monopole Resonance in the Sn Isotopes: Why is Tin so "Fluffy"?

The isoscalar giant monopole resonance (GMR) has been investigated in a series of Sn isotopes (A=112-124) using inelastic scattering of 400-MeV a particles at extremely forward angles (including 0 degrees). The primary aim of the investigation has been to explore the role of the symmetry-energy term in the expression for nuclear incompressibility. It is found that the excitation energies of the GMR in the Sn isotopes are significantly lower than those expected from the nuclear incompressibility previously extracted from the available data on the compressional-mode giant resonances.

Resolution tests of CsI(Tl) scintillators read out by pin diodes

Cylindrical CsI(Tl) scintillators of 38 mm diameter and 100 mm length read out with PIN diodes of 400 mm2 area were tested with respect to their response to medium energy light particles (p, d, t, α). Resolutions of better than 1% were achieved for 50 MeV protons and 90 MeV α-particles. For many crystals the resolution was found to be limited to 2–3% by local crystal nonuniformities which caused variations of the light output efficiency of several percent. A bench test is described which allows the detection of inhomogeneities to better than 0.5% accuracy. The quality of particle identification obtained with ΔE-E and pulse shape discrimination techniques are investigated as a function of count rate.

Superconducting Dipole Magnet for SAMURAI Spectrometer

A superconducting dipole magnet for a large-acceptance spectrometer named SAMURAI has been constructed and installed at the RIKEN RI Beam Factory. The important features of the SAMURAI superconducting dipole magnet are a large pole gap, a wide horizontal opening, and a large momentum bite. The magnet is an H-type dipole, having circular superconducting coils and cylindrical pole pieces with a diameter of 2 m and a pole gap of 880 mm. The coils are orderly wound by the wet winding method developed by Toshiba using a Nb/Ti superconducting wire. The upper and lower coils are installed in two separate cryostats and cooled by the liquid helium bath cooling method. Each cryostat has six cryocoolers: one for a coil vessel at 4 K, four for thermal shields, and one for high- TC superconducting power leads. The size of the iron yoke is 6.7 m wide, 3.5 m deep, 4.64 m tall, and the total weight of the magnet is about 650 tons. The maximum magnetic field is 3.08 T at 563 A (1.922 MA turns/coil), which gives a bending power (field integral) of 7.05 Tm. The maximum stored energy amounts to 27.4 MJ and the inductance varies from 396 H to 150 H as the magnetic field increases. The fringe fields are smaller than 5 mT at 0.5 m from the magnet. The construction of the SAMURAI magnet started in 2008 and was completed in June 2011. The commissioning of the SAMURAI spectrometer was successfully performed using RI beams in March 2012.

Proton elastic scattering from tin isotopes at 295-MeV and systematic change of neutron density distributions

Crosssectionsand analyzing powersforproton elasticscattering from 116;118;120;122;124 Sn at295 M eV havebeen m easured fora m om entum transferofup to about3.5 fm 1 to deducesystem atic changesofthe neutron density distribution. W e tuned the relativistic Love-Franey interaction to explain the proton elastic scattering ofa nucleuswhosedensity distribution iswellknown.Then, weapplied thisinteraction todeducetheneutron density distributionsoftin isotopes.Theresultof ouranalysisshowstheclearsystem aticbehaviorofagradualincreasein theneutron skin thickness oftin isotopeswith m assnum ber.

Compressional-mode giant resonances in deformed nuclei

Abstract Background-free inelastic scattering spectra have been obtained for the Sm isotopes with 400xa0MeV α particles at forward angles (including 0°) to investigate the effect of deformation on the compressional-mode giant resonances. The strength distributions for the isoscalar giant resonances ( L ⩽3) have been extracted for the spherical nucleus 144 Sm and the deformed nucleus 154 Sm. We have observed that the effects of deformation are different for the low- and high-excitation-energy components of the isoscalar giant dipole resonance in 154 Sm. Evidence for the theoretically predicted coupling between the isoscalar dipole resonance and the high-energy octupole resonance is reported.

Experimental signature of in-medium mass modification of vector mesons at normal nuclear density

We measured invariant mass spectra of e+e− pairs produced in 12 GeV p + A interaction to investigate in-medium mass modification of vector mesons. We observed an enhancement over the known hadronic sources on the low-mass side of the ω meson mass peak. The obtained ρ/ω ratio indicates that this enhancement mainly results from ρ mesons whose mass was modified in nuclear matter. We performed a toy model calculation based on the prediction using the QCD sum rule. It reproduced well the tendency of our data.

New projects at SPring-8 with multi-GeV polarized photons

Abstract The GeV photon beam at SPring-8 is produced by backward-Compton scattering of laser photons from 8 GeV electrons. Polarization of the photon beam reaches to 100 % at the maximum energy if laser photons are 100 % circular polarized. The photon energy will be above 3 GeV thanks to the high energy of the circulating electron beam. The photon energy is measured by tagging the recoiled electron. The photon beam well defined in both the polarization and energy becomes a clean and sharp probe to study the baryon and meson structures in terms of quarks and their dynamics. We present the outline of the project. A few selected topics studied in the SPring-8 Laser-Electron Photon Collaboration are also discussed.

Application of Bragg-curve counters to a target multifragmentation measurement

Abstract We have constructed Bragg-curve counters (BCCs) to detect the intermediate-mass fragments (IMFs) emitted during target multifragmentation induced by 12-GeV primary protons on Au, Ag targets. An energy resolution of ∼1.0% and an atomic-number resolution of 2.0% have been achieved. Long-term stability of the output pulse-heights, which was necessary for actual IMF measurements, was realized by employing a continuous gas-flow system with a constant pressure.