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
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
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
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?)