S.H. Zhou

γ-ray spectroscopy in Λ hypernuclei

Abstract The present status of hypernuclear γ -ray spectroscopy with Hyperball is summarized. We observed two γ transitions of 16 Λ O( 1 − → 1 − , 0 − ) and obtained the strength of the ΛN tensor force. In 10 B( K − , π − γ ) data, we did not observe the spin-flip M1 transition of 10 Λ B( 2 − → 1 − ), but γ rays from hyperfragments such as 7 Λ Li( 7 / 2 + → 5 / 2 + ) and 9 Λ Be( 3 / 2 + → 1 / 2 + ) were observed. In 11 B( π + , K + γ ) data, we observed six γ transitions of 11 Λ B. We also attempted an inclusive γ -ray measurement with a stopped K − beam.

Σ-nucleus potential studied with the (π-, K+) reaction on medium-to-heavy nuclear targets

In order to study the Sigma-nucleus optical potential, we measured inclusive (pi^-,K^+) spectra on medium-to-heavy nuclear targets: CH_2, Si, Ni, In and Bi. The CH_2 target was used to calibrate the excitation energy scale by using the elementary process p + pi^- -> K^+ + Sigma^-, where the C spectrum was also extracted. The calibration was done with +-0.1 MeV precision. The angular distribution of the elementary cross section was measured, and agreed well with the previous bubble chamber data, but with better statistics, and the magnitudes of the cross sections of the measured inclusive (pi^-,K^+) spectra were also well calibrated. All of the inclusive spectra were found to be similar in shape at a region near to the Sigma^- binding energy threshold, showing a weak mass-number dependence on the magnitude of the cross section. The measured spectra were compared with a theoretical calculation performed within the framework of the Distorted Wave Impulse Approximation (DWIA). It has been demonstrated that a strongly repulsive sig-nucleus potential with a non-zero size of the imaginary part is required to reproduce the shape of the measured spectra.

Next generation hypernuclear γ-ray spectrometer: Hyperball-J

The next generation hypernuclear γ-ray spectrometer, Hyperball-J, is described. Three of the key aspects in the array construction, namely geometrical configurations, cooling mechanism of Ge detectors, and Compton suppressors, are discussed. Results of simulation for planar and spherical arrangements are compared. Progresses in the development of pulse tube cooling system and PWO background suppression counters are reported.

Study of the Sigma-nucleus potential by the (pi-, K+) reaction on medium-to-heavy nuclear targets

In order to study the Sigma-nucleus optical potential, we measured inclusive (pi^-,K^+) spectra on medium-to-heavy nuclear targets: CH_2, Si, Ni, In and Bi. The CH_2 target was used to calibrate the excitation energy scale by using the elementary process p + pi^- -> K^+ + Sigma^-, where the C spectrum was also extracted. The calibration was done with +-0.1 MeV precision. The angular distribution of the elementary cross section was measured, and agreed well with the previous bubble chamber data, but with better statistics, and the magnitudes of the cross sections of the measured inclusive (pi^-,K^+) spectra were also well calibrated. All of the inclusive spectra were found to be similar in shape at a region near to the Sigma^- binding energy threshold, showing a weak mass-number dependence on the magnitude of the cross section. The measured spectra were compared with a theoretical calculation performed within the framework of the Distorted Wave Impulse Approximation (DWIA). It has been demonstrated that a strongly repulsive sig-nucleus potential with a non-zero size of the imaginary part is required to reproduce the shape of the measured spectra.

γ-ray spectroscopy ofΛ16O andΛ15N hypernuclei via theO16(K−,π−γ) reaction

The bound-state level structures of the 16 Λ O and 15 Λ N hypernuclei were studied by γ-ray spectroscopy using a germanium detector array (Hyperball) via the 16 O (K - , π - γ) reaction. A level scheme for 16 Λ O was determined from the observation of three γ-ray transitions from the doublet of states (2 - , 1 - ) at ∼6.7 MeV to the ground-state doublet (l - , 0 - ). The 15 Λ N hypemuclei were produced via proton emission from unbound states in 16 Λ O. Three γ rays were observed, and the lifetime of the 1/2 + ; 1 state in 15 Λ N was measured by the Doppler shift attenuation method. By comparing the experimental results with shell-model calculations, the spin dependence of the AN interaction is discussed. In particular, the measured 16 Λ O ground-state doublet spacing of 26.4 ± 1.6 ± 0.5 keV determines a small but nonzero strength of the AN tensor interaction.

Gamma-Ray Spectroscopy of (Lambda)O-16 and (Lambda)N-15 Hypernuclei via the O-16(K-, pi- gamma) reaction

The bound-state level structures of the 16 Λ O and 15 Λ N hypernuclei were studied by γ-ray spectroscopy using a germanium detector array (Hyperball) via the 16 O (K - , π - γ) reaction. A level scheme for 16 Λ O was determined from the observation of three γ-ray transitions from the doublet of states (2 - , 1 - ) at ∼6.7 MeV to the ground-state doublet (l - , 0 - ). The 15 Λ N hypemuclei were produced via proton emission from unbound states in 16 Λ O. Three γ rays were observed, and the lifetime of the 1/2 + ; 1 state in 15 Λ N was measured by the Doppler shift attenuation method. By comparing the experimental results with shell-model calculations, the spin dependence of the AN interaction is discussed. In particular, the measured 16 Λ O ground-state doublet spacing of 26.4 ± 1.6 ± 0.5 keV determines a small but nonzero strength of the AN tensor interaction.

GAMMA SPECTROSCOPY OF P-SHELL HYPERNUCLEI AND ΛN SPIN-DEPENDENT INTERACTIONS-REPORT OF BNL E930

High resolution γ spectroscopy of hypernuclei using Hyperball has revealed detailed level structure of several p-shell Λ hypernuclei and allowed us to extract the strengths of ΛN spin-dependent interactions. In the experiment BNL E930, we observed E2(3/2+, 5/2+ → 1/2+) transitions in and confirmed a small size of the Λ-spin-dependent spin-orbit force. Recently we took data for and in order to investigate the strength of the ΛN tensor force. In a preliminary analysis, we observed γ rays of and .