The ? particle as a probe of deeply bound single particle states in heavy nuclei

Abstract

Decay rates of excited states in heavy hypernuclei have been calculated using shell model wave functions. The nuclear Auger effect determines the widths of the states in most cases. The exception is the radiative decay for the A particle in the 1 p and in exceptional cases also 1 d shell. In general, the natural widths of the highly excited states below the threshold for the A emission are smaller than the spacing between the A shells. Spectroscopy of A bound states in heavy hypernuclei thus seems to be feasible. PACS : 21.60Cs; 14.20.Jn MotivationExperimental methods of nuclear spectroscopy are very suitable to investigate the properties of the valence nucleons and the collective nuclear behaviour. Less satisfactory are the methods to study the properties of the deeply bound nucleons. The best way to look at them is by use of the (p,2p), (e,e'p) and in some respect also the pickup reactions. They do supply us, however, with quantitative information only on the nucleons of the last minus one nuclear shell. There is no direct experimental evidence that in heavy nuclei like lead a classification of the nucleons in ls, lp, ld2s shells is justified. Related to the question of the deeply bound nucleons, but much more restrictive, is the question of how well the single particle description of a valence nucleon is valid in the region of the central nuclear density. Recently, a set of the measurements of the electric form factors of lead and bismuth isotopes are tried to determine in how far the charge distribution of the valence 3s proton in bismuth does really correspond to the 3s charge distribution of a single particle also at distances smaller than l fm from the nuclear centre. On the other hand, the A particle in the nucleus fulfills the main requirements of an excellent probe of the particle behaviour in the central nuclear region. After being captured in a nucleus the A particle cascades eventually into its ground state. The lifetime "c~10-1~ of the A particle in the nucleus is long enough that its weak decay AN~2N or A~zN takes place in the l s ground state. It is rather natural to assume that the A particle and the nucleon will show similar behaviour in dependence of the nuclear density and that a deviation from the single particle behaviour of the A particle in the central nuclear region can be extrapolated to that of the nucleon. The aim of this paper is to discuss some of the theoretical aspects relevant to the spectroscopy of heavy hypernuclei. If experimental methods to study heavy hypernuclei should be developed, the first question to answer is how the A particle after being captured in a nucleus cascades to its ground state. For the A particle in a highly excited state the most important mechanism of deexciation is the nuclear Auger effect. For deeply bound states for which the A particle excitation is comparable to the binding energy of the valence nucleons the gamma transition takes over. In Chap. 2 we give the expressions for Auger and

Spectroscopy of 28Al(Lambda), 12B(Lambda), 7He(Lambda) by the (e,e'K+) Reaction

“…Because Λ hypernuclear production in direct interaction converts a nucleon into a Λ particle, most states are excited as nucleon-hole Λ-particle states. Spreading widths of these states (Γ Λ ) were calculated to be less than a few 100 keV [1] [2]. This is much narrower than the width of nucleon deep hole states (Γ N ).…”

“…At step (1), protons which dose not decay in the HKS were chosen by TOF between HTOF1X and HTOF2X. At step (2) and 3, WCs behind a fired HTOF2X were selected. ADC information of WCs was also used to identify protons.…”

“…17: χ pos and χ2 wid as a function of V kin in a 50 keV step at the final step of calibration procedure. The error bars represent the error of each χ 2 caused by the fitting error for the hyperon peaks.…”

Single particle properties of Λ hypernuclei in the density dependent relativistic hadron field theory

“…Different to a previous attempt [2] our approach treats the binding mean-field, wave functions and interactions in a self-consistent manner. The process is described by the production of a neutron-particle-neutron-hole state due to the de-excitation of the Λ, where the particle is unbound.…”

“…Such deeply bound Λ states can ideally be found in heavy hypernuclei. For the investigation of heavy hypernuclei there is, besides the standard spectroscopic methods, also the possibility of the spectroscopy of Auger neutrons [2] as planned in a recently proposed experiment at JLab [3]. We present Auger transition rates for 209 Λ Pb calculated within our self-consistent model and discuss the spectroscopic content of the spectra.…”