The SπRIT time projection chamber.

The Superconducting Analyzer for MUlti-particles from RAdioIsotope (SAMURAI) Pion-Reconstruction and Ion-Tracker Time Projection Chamber (SπRIT TPC) was designed to enable measurements of heavy ion collisions with the SAMURAI spectrometer at the RIKEN radioactive isotope beam factory and provides constraints on the equation of state of neutron-rich nuclear matter. The SπRIT TPC has a 50.5 cm drift length and an 86.4 × 134.4 cm2 pad plane with 12 096 pads that are equipped with the generic electronics for TPCs. The SπRIT TPC allows for an excellent reconstruction of particles and provides isotopic resolution for pions and other light charged particles across a wide range of energy losses and momenta. The details of the SπRIT TPC are presented, along with discussion of the TPC performance based on cosmic rays and charged particles emitted in heavy ion collisions.

Probing the Symmetry Energy with the Spectral Pion Ratio.

Many neutron star properties, such as the proton fraction, reflect the symmetry energy contributions to the equation of state that dominate when neutron and proton densities differ strongly. To constrain these contributions at suprasaturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be 42

Extraction of the Landau-Migdal Parameter from the Gamow-Teller Giant Resonance in

The key parameter to discuss the possibility of the pion condensation in nuclear matter, i.e., the so-called Landau-Migdal parameter g^{'}, was extracted by measuring the double-differential cross sections for the (p,n) reaction at 216 MeV/u on a neutron-rich doubly magic unstable nucleus, ^{132}Sn with the quality comparable to data taken with stable nuclei. The extracted strengths for Gamow-Teller (GT) transitions from ^{132}Sn leading to ^{132}Sb exhibit the GT giant resonance (GTR) at the excitation energy of 16.3±0.4(stat)±0.4(syst) MeV with the width of Γ=4.7±0.8 MeV. The integrated GT strength up to E_{x}=25 MeV is S_{GT}^{-}=53±5(stat)_{-10}^{+11}(syst), corresponding to 56% of Ikeda's sum rule of 3(N-Z)=96. The present result accurately constrains the Landau-Migdal parameter as g^{'}=0.68±0.07, thanks to the high sensitivity of the GTR energy to g^{'}. In combination with previous studies on the GTR for ^{90}Zr and ^{208}Pb, the result of this work shows the constancy of this parameter in the nuclear chart region with (N-Z)/A=0.11 to 0.24 and A=90 to 208.

Candidate Resonant Tetraneutron State Populated by the

A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186 MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst) MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

Structural evolution in the neutron-rich nuclei ¹°6Zr and ¹°8Zr.

The low-lying states in ¹°6Zr and ¹°8Zr have been investigated by means of ß-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹°8Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64. The deformed ground state of ¹°8Zr indicates that a spherical subshell gap predicted at N = 70 is not large enough to change the ground state of ¹°8Zr to the spherical shape. The possibility of a tetrahedral shape isomer in ¹°8Zr is also discussed.

ß-decay half-lives of very neutron-rich Kr to Tc isotopes on the boundary of the r-process path: an indication of fast r-matter flow.

The ß-decay half-lives of 38 neutron-rich isotopes from (36)Kr to (43)Tc have been measured; the half-lives of (100)Kr, (103-105)Sr, (106-108)Y, (108-110)Zr, (111,112)Nb, (112-115)Mo, and (116,117)Tc are reported here. The results when compared with previous standard models indicate an overestimation in the predicted half-lives by a factor of 2 or more in the A≈110 region. A revised model based on the second generation gross theory of ß decay better predicts the measured half-lives and suggests a more rapid flow of the rapid neutron-capture process (r-matter flow) through this region than previously predicted.

Wavelength-dependent fragmentation and clustering observed after femtosecond laser ablation of solid C60.

We report here the resonance effect in femtosecond laser ablation of solid C60 by investigating wavelength and fluence dependence of product ion species. When the ablation laser wavelength is far from the molecular absorption band of C60, we observe both C60-2n+ fragment ions and C60+2n+ cluster ions as well as C60+ parent ion. Delayed ionization of C60 is not significant. When the ablation laser wavelength is near resonant with the molecular absorption, we observe C60+ and some amount of C60-2n+ fragment ions depending on the laser fluence. Delayed ionization of C60 is significant in this case, which indicates high internal energy of C60 molecule. From the observations, we confirm the strong coupling of femtosecond laser energy with C60 molecule when the molecular absorption is high at the ablation laser wavelength.

Ionization and fragmentation of solid C60 by femtosecond laser ablation.

Ionization and fragmentation of solid C(60) dispersed on a silicon plate are investigated by femtosecond laser ablation. Bimodal mass distribution with large fragment ions C(60-2n) (+) (0< or =n< or =11) and small fragment ions C(n) (+) (13< or =n< or =28), formation of dimer ion (C(60))(2) (+), and delayed ionization of C(60) have been observed as reported in gas phase experiments with nanosecond laser excitation. Metastable dissociation of small fragment ions C(n) (+) has been observed for the first time, which suggests different structures of fragment ions compared with those of well-studied carbon cluster ions. From these observations, strong coupling of laser energy to electronic degrees of freedom of solid C(60) has been revealed for femtosecond laser ablation as compared with excitation in the gas phase.