Momentum distribution of Be-7 produced through B-8 fragmentation

Abstract

The momentum distribution of Be-7 produced through fragmentation of B-8, which is a proton-halo candidate, was measured. The momentum distribution is sensitive to the spatial wave function of a removed proton near the nuclear surface as well as the reaction cross section of B-8 at several tens of MeV/nucleon. Both the longitudinal and transverse momentum distributions of Be-7 on a thin Si target were measured at 50 MeV/nucleon. The B-8 beam was produced through the C-12 + Be-9 collision with a 135 MeV/nucleon C-12 beam provided by RIKEN (Institute of Physical and Chemical Research) Ring Cyclotron, and was separated by RIPS (RIKEN Projectile fragment Separator). The width obtained for the longitudinal distribution of Be-7 is preliminarily 75 +/- 8 MeV/c, which is in agreement with the previous data. The widths for the transverse momentum distributions are about 110 MeV/c independent on the energy and targets, about 40 percent larger than that for the longitudinal one. The present values are, however, much smaller than the value expected from Goldharber theory that accounts for the momentum distribution of stable nuclei. The experimental momentum spectra of Be-7 were compared with the calculation within the Glauber theory. The result is consistent with the calculation and it supports the existence of a proton halo in B-8.

陽子ハローの候補であるB-8の破砕で生成されるBe-7の運動量分布を測定した。運動量分布は数10MeV/核子でのB-8の断面積とともに、核表面近傍の剥離陽子の空間波動関数に敏感である。薄いSi標的上のBe-7の縦運動量分布と横運動量分布の両方を50MeV/nucleonで測定した。B-8ビームは理化学研究所リングサイクロトロンから供給される135MeV/nucleonのC-12ビームのC-12+Be-9衝突によって生成し、RIPS(理化学研究所破砕核分離装置)で選別した。得られたBe-7の縦運動量分布の幅は75±8MeV/cで、以前のデータと一致している。横運動量の幅はエネルギーや標的に無関係に約110MeV/cで、縦運動量の幅より40%大きい。しかし、今回の値は安定核の運動量分布を説明するゴールドハーバー理論から期待される値より非常に小さい。実験で測定したBe-7の運動量スペクトルをグラウバー理論の計算と比較した。結果は計算と一致して、これはB-8に陽子ハローが存在することを支持している。