Yasuo Nagashima

Systematic measurements of pulse height defects for heavy ions in surface-barrier detectors

The pulse height defect (PHD) in several Si-surface-barrier detectors for 12C, 16O, 28Si, 63Cu, 79Br and 127I ions was measured. The electric field strength dependence as well as the energy dependence were investigated. The results are compared with the recombination model. An empirical formula that describes well the total PHD is suggested.

Fusion and breakup at the barrier with 11Be

Abstract The cross-section excitation functions for various neutron-evaporation channels have been measured in the 11,10,9 Be+ 209 Bi systems around the Coulomb barrier, for energies ranging from 35 to 70 MeV, in order to see any eventual effect from the expected breakup of the 11 Be unstable halo nucleus. Within the simplified reaction process considered, the data are consistent with a breakup of 11 Be at the top of the Coulomb barrier contributing to less than 25% to the total fusion cross-section; however contributions from other type of breakup mechanisms cannot be excluded.

Charged-particle calorimetry of 40Ar + 27Al reactions from 36 to 65 MeV/u☆

Abstract Temperatures and excitation energies have been independently determined for hot nuclei formed in the 40Ar + 27Al reaction at energies ranging from 36 to 65 MeV/u. Charged products have been measured in a geometry close to 4π in the center of mass with the multidetectors MUR and TONNEAU. Events were sorted as a function of the impact parameter and the products emitted from equilibrated nuclei were separated from both pre-equilibrium and target-like products. Temperature (slope parameter) and excitation energy values were deduced from the kinetic energies of particles in the frame of the reconstructed equilibrated nucleus. Both have been found to increase with decreasing impact parameter. For the most central collisions impact parameters — less than 2 fm — the temperature was seen to increase with incident energy (slowly above 45 MeV/u) and reach a value of 7 MeV at 65 MeV/u. The excitation energy per nucleon exhibited a similar behaviour. The correlation between excitation energy and temperature was found to remain compatible with the statistical theory.

Global variables and impact parameter determination in nucleus-nucleus collisions below 100 MeV/u

Abstract Several global variables are tested for determining the impact parameter in 4π measurements of charged products. For the system Ar + Al from 25 to 85 MeV/u, simulated reactions are used to study the correlation between the real impact parameter value and the experimentally determined one. Even with a perfect detector, the total multiplicity and the mid-rapidity charge give a poor correlation. Filtering through the acceptance of an actual detector (Mur + Tonneau at GANIL) provides a poor correlation with the total charge, and an acceptable one with the total perpendicular momentum. The average (mass weighted) parallel velocity V av gives the best correlation and is less sensitive to the incident energy and to the mechanisms used in the simulation. Measurements extending to very forward angles are needed to accurately sort events by impact parameter. Real data are used to study the variation of other global variables versus V av : they exhibit the expected average trend, with broad fluctuations. The eccentricity and flow angle allow the evolution of central reactions with the incident energy to be followed. The proportion of relative motion transformed into energy perpendicular to the beam direction decreases when the incident energy increases; its absolute value, however, doubles from 25 to 65 MeV/u. In experimental studies where good separation between participant (pre-equilibrium) and spectator (equilibrium) particles is wanted in addition to good impact parameter sorting, the projectile mass should be in the range 1.2–2.5 times the target mass.

Components of collective flow and azimuthal distributions in 40Ar + 27Al and 40Ar + 58Ni collisions below 85 MeV/u☆

The transverse and longitudinal components of collective flow have been measured for the nuclear reactions 27Al(40Ar, X), E = 25–85 MeV/nucleon and Ni(40Ar, X), E = 36 and 65 MeV/u with the 4π array Mur+Tonneau. Monte Carlo simulations show that, even for peripheral reactions, the transverse-momentum analysis method gives a better determination of the reaction plane than the method which uses only the projectile-like fragments. The measured in-plane transverse-flow values reach −30 MeV/c at low energies for Z = 2 particles and tends to zero around 85 MeV/u. Flow-angle values are in the range −65° to −35°. The azimuthal distributions of the measured charged products show that there is no squeeze-out of nuclear matter, but the reaction system has a rotation-like behaviour which increases with the impact-parameter value.

Strong impact parameter dependence of pre-equilibrium particle emission in nucleus-nucleus reactions at intermediate energies☆

Abstract Charged particles and fragments emitted in reactions between 40 Ar at 45 and 65 MeV/u and an 27 Al target have been detected in a geometry close to 4π in the center of mass. A new global variable, the average parallel velocity, has been used to sort the events as a function of the impact parameter value. For particles with Z = 1 and 2, a pre-equilibrium component is present. Its multiplicity increases strongly when the impact parameter value decreases, and reaches 7 in head-on reactions.

State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling

Global cooling in intermediate glacial climate with northern ice sheets preconditions climatic instability with bipolar seesaw. Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear. Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instability with bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets.

Estimation of Groundwater Residence Time Using the 36Cl Bomb Pulse

We propose a methodology for estimating the residence time of groundwater based on bomb-produced (36)Cl. Water samples were collected from 28 springs and 2 flowing wells located around Mt. Fuji, Central Japan. (36)Cl/Cl ratios in the water samples, determined by accelerator mass spectrometry (AMS), were between 43 × 10(-15) and 412 × 10(-15). A reference time series of the above-background (i.e., bomb-derived) (36)Cl concentration was constructed by linearly scaling the background-corrected Dye-3 data according to the estimated total bomb-produced (36)Cl fallout in the Mt. Fuji area. Assuming piston flow transport, estimates of residence time were obtained by comparing the measured bomb-derived (36)Cl concentrations in spring water with the reference curve. The distribution of (36)Cl-based residence times is basically consistent with that of tritium-based estimates calculated from data presented in previous studies, although the estimated residence times differ between the two tracers. This discrepancy may reflect chlorine recycling via vegetation or the relatively small change in fallout rate, approximately since 1975, which would give rise to large uncertainties in (36)Cl-based estimates of recharge for the period, approximately since 1975. Given the estimated ages for groundwater from flowing wells, dating based on a (36)Cl bomb pulse may be more reliable and sensitive for groundwater recharged before 1975, back as far as the mid-1950s.

Status of the 36Cl AMS system at the University of Tsukuba

Abstract The current status of the 36Cl AMS system at the University of Tsukuba is reported. A “tri-molecular pilot beam” method is used to stabilize the terminal voltage of a 12UD tandem instead of an ordinary GVM stabilizer. The terminal voltage is kept stable within 0.1%. The capabilities of the ion source have been tremendously improved. Both Cl− negative ions and a 12 C 3 − tri-molecular pilot beam can be produced simultaneously with reasonable stability and intensity. The 36Cl/35Cl ratio can be measured with a precision of ±2% for a 15 μA Cl− beam current. A high sulfur reduction is achieved by introducing two independent chemical procedures, so that the background reaches up to 2×10−14. This sensitivity is still insufficient. Therefore, our current effort is focused on improving it.

An accelerator mass spectrometry system with the 12UD Pelletron at the University of Tsukuba

Abstract Construction of a new facility for AMS has commenced at the tandem accelerator center based on a 12UD Pelletron tandem. A “di-molecular pilot beam” method is used for AMS measurements without any modification to the existing tandem system. The absolute 26Al/27Al ratio is measured with two sets of standard 26Al samples to confirm the di-molecular pilot beam concept.