.Box The bound-state levelstructures of the 16(cid:3) O and 15(cid:3) N hypernucleiwere studied by (cid:13) -ray spectroscopy using a germ anium detector array (H yperball) via the 16 O ( K (cid:0) ;(cid:25) (cid:0) (cid:13) ) reaction. A level schem e for 16(cid:3) O was determ ined from the observation ofthree (cid:13) -ray transitions from the doublet ofstates (2 (cid:0) ,1 (cid:0) ) at (cid:24) 6 : 7 M eV to the ground-state doublet (1 (cid:0) ,0 (cid:0) ). The 15(cid:3) N hypernucleiwere produced via proton em ission from unbound states in 16(cid:3) O .Three (cid:13) -rays were observed and the lifetim e ofthe 1 = 2 + ;1 state in 15(cid:3) N was m easured by the D oppler shift attenuation m ethod. By com paring the experim entalresults with shell-m odelcalculations,the spin-dependence ofthe (cid:3) N interaction isdiscussed.In particular,them easured 16(cid:3) O ground-statedoubletspacing of26.4 (cid:6) 1.6 (cid:6) 0.5 keV determ inesa sm allbutnonzero strength ofthe (cid:3) N tensorinteraction.

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

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.

. We have observed three γ -ray transitions in Λ 16 O from both 7MeV excited spin-flip and non-spin-flip partners ( 2 - , 1 - 2 to the ground-state doublet ( 1 - 1 , 0 - via the 16 O( K - ,π - ) reaction. The 7MeV excitation energies of the spin-doublet members ( 2 - , 1 - 2 were determined to be 6784±4±4 keV and 6562±1±2 keV, respectively, and thus the spacing was obtained to be 222±4±5 keV. This is the first observation of the spin-flip state directly populated by the ( K - ,π - reaction. Moreover, such directly populated spin-flip and non-spin-flip partners were resolved for the first time.

In a γ-ray spectroscopy experiment with the (K−,π−) reaction on a B10 target, we have observed the 7/2+→5/2+ spin-flip M1 transition in Λ7Li via γ-γ coincidence with the 5/2+→1/2+E2 transition. This is the first successful γ−γ coincidence measurement for hypernuclei. The measured M1 γ-ray energy is 470.8±1.9(stat)±0.6(syst) keV. The spacing of the 7/2+,5/2+ doublet is consistently explained using the strengths of the spin-spin, Λ-spin-orbit, and tensor interactions obtained from previous γ-spectroscopy experiments.Received 3 December 2004DOI:https://doi.org/10.1103/PhysRevC.73.012501©2006 American Physical Society

In a gamma-ray spectroscopy experiment with the (K-, pi(-)) reaction on a B-10 target, we have observed the 7/2(+)-> 5/2(+) spin-flip M1 transition in Li-7(Lambda) via gamma-gamma coincidence with the 5/2(+)-> 1/2(+)E2 transition. This is the first successful gamma-gamma coincidence measurement for hypernuclei. The measured M1 gamma-ray energy is 470.8 +/- 1.9(stat)+/- 0.6(syst) keV. The spacing of the 7/2(+), 5/2(+) doublet is consistently explained using the strengths of the spin-spin, Lambda-spin-orbit, and tensor interactions obtained from previous gamma-spectroscopy experiments.

In a _Λ~(16)O γ-ray spectroscopy experiment with HYPERBALL,two γ-ray transitions from the 6.6MeV excited 1_2~- state to both ground-states spin-doublet members _1~- and 0~- have been observed.The measured ground-state doublet spacing is 26.4±1.6(stat)±0.5(syst)keV.Thus a small but nonzero strength of the tensor interaction between a Λ and a nucleon is deduced to be T=+0.03MeV.The excitation energy of the 1_2~- state is also determined to be 6561.7±1.1(stat)±1.7(syst)keV.

We have searched for neutrons from the 3‐body photon‐induced reaction 9Be + γ → α + α + n using bremsstrahlung produced by electrons from a 2‐MV Van de Graaff. The target was located within a block of beryllium surrounded by an array of 3He proportional counters embedded in paraffin. Based on energy and intensity calibrations of the accelerator and detector using the 9Be + γ → 8Be+n reaction, an upper limit of 93 nb (4σ) was placed on the cross section for neutron production between the 3‐body and 2‐body thresholds. This value is substantially below a previously experimental result using photoexcitation by a 142Pr gamma source and also below an earlier theoretical estimate. We suggest that bremsstrahlung due to beta rays between 1665 keV and the 2160‐keV end‐point of the 142Pr beta‐ray spectrum could account for the photoneutron yield in the 3‐body region that had previously been attributed to 142Pr gamma rays.

We have observed two gamma-ray transitions in (16)(Lambda)O from the 6.6 MeV excited 1(-)(2) state to both ground-state spin-doublet members (1(-)(1),0(-)) by the (K-,pi(-)gamma) reaction. We have obtained the ground-state doublet spacing to be 26.4+/-1.6(stat)+/-0.5(syst) keV and the excitation energy of the 1(-)(2) state to be 6561.7+/-1.1(stat)+/-1.7(syst) keV. The ground-state doublet spacing provides a small but nonzero strength of the tensor interaction between a Lambda and a nucleon. This is the first experimental result on the LambdaN tensor interaction.

We have searched for neutrons from the three-body photon-induced reaction Be-9+gamma-->alpha+alpha+n using bremsstrahlung produced by electrons from a 2-MV Van de Graaff on a gold target. The target was located within a block of beryllium surrounded by an array of He-3 proportional counters embedded in paraffin. Based on energy and intensity calibrations of the accelerator and detector using the Be-9+gamma-->Be-8+n reaction, an upper limit of 93 nb (4sigma) was placed on the cross section for neutron production between the three-body and two-body thresholds. This value is substantially below a previous experimental result using photoexcitation by Pr-142 gamma rays and also below an earlier theoretical estimate. Bremsstrahlung spectra from the gold target were also measured in a NaI(Tl) detector at electron energies from 1.7 to 2.0 MeV and angles of 0degrees to 60degrees with respect to the beam axis. An analysis of photons above the 1665-keV two-body threshold shows that bremsstrahlung due to beta rays between 1665 keV and the 2160-keV end point of the Pr-142 beta-ray spectrum could account for the photoneutron yield in the three-body region that had previously been attributed to Pr-142 gamma rays.

In a previous publication [T. Abbott , E802 Collaboration, Phys. Rev. C 63, 064602 (2001); 64, 029901(E) (2001)], measurements of the A dependence and pseudorapidity interval (deltaeta) dependence of midrapidity E-T distributions in a half-azimuth (Deltaphi=pi) electromagnetic calorimeter were presented for p+Be, p+Au, O+Cu, Si+Au, and Au+Au collisions at the BNL-AGS. The validity of the "nuclear geometry" characterization versus deltaeta was illustrated by plots of the E-T(deltaeta) distribution in each deltaeta interval in units of the measured (p+Au) in the same deltaeta interval for p+Au collisions. These plots, with aperture corrected scale in the physically meaningful units of number of average observed p+Au collisions, were nearly universal as a function of deltaeta, confirming that the reaction dynamics for E-T production at midrapidity at AGS energies is governed by the number of projectile participants and can be well characterized by measurements in apertures as small as Deltaphi=pi,deltaeta=0.3. A key ingredient in these analyses is the probability p(0) for no signal to be detected in a given aperture deltaeta for the fundamental p+Au collision. In fact the measured (p+Au) is biased and the true (true)(p+Au) for the detector aperture is the measured value times 1-p(0). The issues and merits of measuring the E-T(deltaeta) distribution in units of (p+Au) or (true)(p+Au) in the same deltaeta interval are presented and discussed. This method has application at RHIC, where p-p data could be used as the reference distribution for two participants. The E-T distributions for B+A collisions, with E-T(deltaeta) scale normalized by (true)(p-p) in the same aperture for p-p collisions, would then be given directly in the popular unit "per participant-pair" [K. Adcox , PHENIX Collaboration, Phys. Rev. Lett. 86, 3500 (2001); I. G. Bearden , BRAHMS Collaboration, Phys. Lett. B523, 227 (2001); B. B. Back , PHOBOS Collaboration, Phys. Rev. C 65, 031901(R) (2002); C. Adler , STAR Collaboration, Phys. Rev. Lett. 89, 202301 (2002)].