Measurements of half lives and neutron emission probabilities of the neutron drip line nuclei B-19, C-22, and N-23

Abstract

The neutron drip line nuclei B-19, C-22, N-23 were produced by projectile fragmentation, using Ar-40 primary beam and Ta target. Each fragment was identified on-line by measuring its time of flight and its energy loss. The typical intensities of B-19, C-22, and N-23 were 0.04, 0.003, and 0.09 cps, respectively. The unstable nuclei were separately implanted into a 12 mm thick plastic scintillator. After the implantation of a nucleus with Z which is great then or equal to 5, the primary beam was turned off for 100 ms, a much larger period than the expected half lives, during which the decay of nuclei was measured. The plastic scintillator was used for the detection of beta rays emitted from the implanted unstable nuclei. A neutron detector array was placed around the scintillator to measure beta-delayed neutrons in coincidence with beta rays. Neutrons and gamma rays were separated by a pulse shape analysis. The values of half life were deduced from the least-squares fits to the data, assuming a functional form given by an exponential plus and constant background. The values of the neutron emission probabilities were determined from the ratio of the number of detected neutrons to that of beta rays. The tentatively deduced neutron emission probabilities of B-19, C-22, and N-23 are 125+/-32 percent, 99+/-39 percent, and 80+/-21 percent, respectively. Here, both statistical and systematic errors of the n-gamma separation are taken into account.

Ar-40の1次ビームとTa標的を使った入射体破砕によって中性子ドリップ線核B-19、C-22、N-23を生成した。各破砕片の飛行時間とエネルギー損失をオンラインで測定して同定した。B-19,C-22,N-23の典型的な強度はそれぞれ、0.04、0.003、0.09cpsであった。不安定核は別々に12mm厚さのプラスチックシンチレータへ打ち込んだ。Z≧5の核を打ち込んだ後、1次ビームは半減期の期待値よりも長い100msの間止められ、核崩壊を測定した。プラスチックシンチレータを使って、打ち込んだ不安定核から放出されるβ線を検出した。中性子検出器アレイをこのシンチレータ周囲に設置し、β線と同時にβ-遅延型中性子を測定した。中性子とγ線はパルス形状解析によって分離した。崩壊データが指数関数と一定のバックグラウンドとからなると仮定して最小自乗法で半減期を求めた。中性子放出確率は検出した中性子数とβ線の数の比から決定した。試験的に求めたB-19、C-22、N-23の放出確率は、それぞれ125±32%、99±39%、80±21%であった。ここで、誤差にはn-γ分離の統計誤差と系統誤差が含まれている。