We have developed a position-sensitive Parallel Plate Avalanche Counter (PPAC), which has been used as a focal plane detector in the BigRIPS fragment separator and the subsequent RI-beam delivery lines at the RIKEN Nishina Center RI Beam Factory. The PPAC detector plays an important role not only in the tuning of the separator and delivery lines but also in the particle identification of rare isotope (RI) beams. The PPAC detector has a sensitive area of 240 mm × 150 mm, and the position information is obtained by a delay-line readout method. Being called double PPAC, it is composed of two full PPACs, each measuring the particle locus in two dimensions. High detection efficiency has been made possible by the twofold measurement using the double PPAC detector. The sensitivity uniformity is also found to be excellent. The root-mean-square position resolution is measured to be 0.25 mm using an source, while the position linearity is as good as ±0.1 mm for the detector size of 240 mm. Characteristics, operating principles, specifications, performance and issues of the PPAC detector are presented, including its signal transmission system using optical fiber cables.
An exclusive measurement of the Coulomb breakup of 8 B into 7 Be+p at 254 A MeV allowed to study the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates and that E2 multipolarity can be neglected. By using a simple single-particle model for 8 B and treating the breakup in first-order perturbation theory, we extract a zero-energy S factor of S17(0) = 18.6 ± 1.2 ± 1.0 eV b, where the first error is experimental and the second one reflects the theoretical uncertainty in the extrapolation.
An exclusive measurement of the Coulomb breakup of 8 B into 7 Be+p at 254 A MeV was used to infer the low-energy 7 Be(p,γ) 8 B cross section. The radioactive 8 B beam was produced by projectile fragmentation of 350 A MeV 12 C and separated with the fragment separator FRS at GSI in Darmstadt, Germany. The Coulomb-breakup products were momentum-analyzed in the KaoS magnetic spectrometer; particular emphasis was placed on the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates within the angular cuts selected for the analysis. The deduced astrophysical S17 factors exhibit good agreement with the most recent direct 7 Be(p,γ) 8 B measurements. By using the energy dependence of S17 according to the recently refined cluster model for 8 B of Descouvemont, we extract a zero-energy S factor of S17(0) = 20.6 ± 0.8(stat) ± 1.2(syst) eV b. These errors do not include the uncertainty of the theoretical model to extrapolate to zero relative energy, estimated to be about 5% by Descouvemont.
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