Review of PHYSICAL REVIEW C 70, 024301 (2004)

Stability of the N=50 shell gap in the neutron-rich Rb, Br, Se and Ge isotones

Y. H. Zhang, LNL, Italy

David Scraggs

Overview

Motivation Background Experimental Details Experimental Results Shell-Model Calculations Summary

Motivation Populate low and medium spin states

in N=50 neutron rich isotones Compare experimentally observed

excited states in N=50 shell gap region with shell-model calculations

Investigation of the neutron-core breaking excitations and therefore the N=50 shell gap

Explore the energy level structure

Background Precise analytical form of the effective

interaction between nucleons due to their substructure is not known

This fundamental goal of nuclear structure can be achieved by probing nuclei under extreme conditions

Phys. Rev. C70 explores exotic nuclei that have been produced far from stability

Background Nuclei can be represented on nuclear chart Isotones at N=50

126

82

50

28

28

50

82

2082

28

20

neutron number N

prot

on n

umbe

r Z

N=Z nucle

i

Background

Adding neutrons in succession creates valence neutrons

Eventually, neutron is no longer bound and neutron emission occurs

This defines the neutron drip line Radically new features may occur in

these highly exotic nuclei

Background Wave functions of valence neutrons

extend a remarkable distance from nucleus centre

Halos and neutron skins can be formed

Energy levels shift and re-order as the limits of stability are reached and possibly the breakdown of shell gaps

Cornerstone of NS for 50 years

Background Shell gaps can be understood from the shell

model Considers a potential well with a series of

energy levels Neutrons and protons accommodated

according to the Pauli exclusion principle This leads to completely filled shells (closed

shells) and magic numbers! N=50 is a magic number

Background

Large energy gap at N=50, hence stable nucleus

N=50

Background

N=20 and N=28 have exhibited properties inconsistent with shell closure

N=20 shell gap disappearance also predicted by Hartree-Fock calculations

Predictions for N=50 come to differing conclusions! Hence experiment

Experimental Details 87Rb, 85Br, 84Se and 82Ge excited states

populated using (450MeV) heavy-ion multi-nucleon transfer reactions

Expected distribution of neutron rich products Products stopped in target Emitted photons detected with GASP

spectrometer

XGeSeBrRbOsSe ),,,( 8232

508434

508535

508737

11619276

488234

Experimental Details GASP –

4spectrometer consisting of 40 Compton-suppressed, large-volume Ge detectors and an inner BGO ball

Experiment ran for six days

Experimental Details Minimum of 3 Ge and 2 BGO fired in

coincidence Both products detected! rays assigned to nuclide by gating on

previously known rays in conicidence Spatial distribution of photons determine

the parity of emitting level (use ADO) Spectroscopic data summarises nuclei

Experimental Results

Analysis of single- and double-gated spectra identified new -rays from the isotones

The energy levels were populated and compared with the shell model

Results for isotones follow

Rubidium - 87

Previously – Highest excited state was I=9/2+ at 1578keV

Only two rays (1175.3)(402.6) Level scheme extended by

coincidence relationship between these rays

Extended up to 6.8MeV

Rubidium - 87

Previously known

Rubidium - 87

Too weakly populated

Bromine - 85 Level scheme

extended up to 4.343MeV

Seven -rays added

Ordered according to relative intensities

Selenium - 84 Excited states in

band other than the yrast band

Notice the spin and parity of the ground state

This is an even-even nuclei

Low intensities for 704 and 1249keV

Germanium - 82Low intensity and noADO.

Shell-Model Calculations

Two shell-model calculations using RITSSCHIL

SM2 Allows particle-hole excitations across the N=50 neutron core

SM1 Does not Results are similar for all isotones

Shell-Model Calculations – 87Rb Up to 17/2+

(4.1MeV) there is good agreement with SM1 and SM2

Then SM2 has good agreement indicating importance of particle-excitations across N=50 neutron core

Summary For all isotones explored it is necessary to

introduced particle-hole excitations across the N=50 gap

The size of the gap has been kept constant in calculations

Shell-model predictions reproduce observed spectra

Therefore, moving away from stability down to Z=32 the N=50 shell gap remains stable

Persistence of closed shells (or not?)