W. Nazarewicz. Limit of stability for heavy nuclei Meitner & Frisch (1939): Nucleus is like liquid drop For Z>100: repulsive Coulomb force stronger than.

W. Nazarewicz

Limit of stability for heavy nucleiLimit of stability for heavy nuclei

• Meitner & Frisch (1939): Nucleus is like liquid dropFor Z>100: repulsive Coulomb force stronger than

attractive nuclear force

• Shell Model explains stability of MAGIC NUCLEI due to large binding of closed shells

• Strutinsky combined liquid drop model and shell model

• Superheavy nuclei are stabilized only by shell effects

Exact position of magic proton shell gap is in question

• N=184 neutron shell gap is predicted by all theoretical models• Position of proton shell gap very sensitive on details of the theory

(Z=114, 120, 126 ?) structure of superheavy elements provides sensitive test for models

114

120

120

126

Z

N

Heavy-ion fusion reactions to produce new elementsHeavy-ion fusion reactions to produce new elements

Hot fusion light beam on actinide target (traditional approach)

(also due to limits of accelerators) large asymmetry predicted to lead to larger cross section relatively high excitation energy of Compound Nucleus

Cold fusionheavy beam on double magic 208Pb or 209Bi (most successful for Z > 108)

(possible with modern accelerators) low excitation energy of Compound Nucleus higher survival probability

The challenge of detecting and identifying superheavy elementsThe challenge of detecting and identifying superheavy elements

• expected count rateN = Nt Np

production cross section = 1 pbarn ( 10-35 cm2)

projectiles per second Np = 5 ·1012 s-1

target nuclei Nt = 1018 cm-2

efficiency of detection system = 50 %

detection rate : N = 2.5 ·10-6 s-1 ( 1 atom per 5 days)

• competition by fission = 100 mbarn ( > 1011 times stronger)

• scattered beam particles or transfer products can have the same kinematics

• need good separation of produced elements• up to Z=104 : standard chemical separation• up to Z=106 : fast chemistry, atom by atom

• Z > 106: separation in flight

• unique identification necessary• alpha - alpha parent-daughter correlation

Separation in flightSeparation in flight

• Filters (Vacuum)• SHIP Velocity filter GSI Darmstadt, Germany• VASILISSA Energy filter JINR Dubna, Russia

• Gas-filled separators• GNS JINR Dubna, Russia• GARIS RIKEN Tokyo, Japan• HECK GSI Darmstadt, Germany• BGS LBNL Berkeley, USA

• Filters (Vacuum)• SHIP Velocity filter GSI Darmstadt, Germany• VASILISSA Energy filter JINR Dubna, Russia

• Gas-filled separators• GNS JINR Dubna, Russia• GARIS RIKEN Tokyo, Japan• HECK GSI Darmstadt, Germany• BGS LBNL Berkeley, USA

Separation between beam particles

superheavy element transfer products fission fragments

target

Separatorbeam detector

beam stop

new element

The SHIP Velocity Filter at GSI, Darmstadt, GermanyThe SHIP Velocity Filter at GSI, Darmstadt, Germany

Electric dipole

Magnetic dipole

Beam stop

Magneticquadrupole

Targetwheel

Position sensitivefocal plane

detector

Time of flightdetectors

eq

mv Eρ

eq

mv Bρ

2

rigidity Electric

rigidity Magnetic

Gas-filled separatorGas-filled separator

• magnet filled with ~ 1 Torr He gas

• heavy ions leave target with charge distribution

• scattering of heavy ions with gas combination of charge states into narrow distribution larger acceptance than vacuum system since vacuum system can only accept few charge states • magnetic rigidity B is velocity independent since average charge state depends on velocity

B = 0.0227 A v/v0 q-1

q = v/v0 Z1/3

- effective radius of trajectoryq - average charge state

• large acceptance BUT reduced resolution reduced suppression

The Berkeley Gas-filled Separator

V. Ninov, K. Gregorich, et al.Phys. Rev. Lett.

- mother daughter correlation technique- mother daughter correlation technique

• detection of -decay chain at one position• energies• time correlation

• correlation with known daughter decays uniquely identifies mother nucleus

Discovery of Z=114 in DubnaDiscovery of Z=114 in Dubna

The problem:

• no daughter product known• no link to known nuclei• short decay chain• correlation not very strong• identification not clear

Discovery of Z=118 at the BGS in BerkeleyDiscovery of Z=118 at the BGS in Berkeley

The problem:

• no daughter product known no link to known nuclei

BUT:• strong correlation

• No clear identification• not yet confirmed by GSI, RIKEN

• confirmation experiment at BGS in March

The problem:

• no daughter product known no link to known nuclei

BUT:• strong correlation

• No clear identification• not yet confirmed by GSI, RIKEN

• confirmation experiment at BGS in March

Three consistent chains observed!

Three consistent chains observed!

Perspectives with radioactive beams

92Sr would allow Z=120production with ~1nb

Yields predicted for Munich accelerator for fission fragments (MAFF)

Structure study of heavy nuclei (254No)Structure study of heavy nuclei (254No)

Gamma-rays at target position in coincidence withrecoils detected at the focal plane of the separator

Unique identification by use of - correlations

Test of deformation, fission barrier

Odd-A Nuclei will reveal single-particle strucutre

Gamma-rays at target position in coincidence withrecoils detected at the focal plane of the separator

Unique identification by use of - correlations

Test of deformation, fission barrier

Odd-A Nuclei will reveal single-particle strucutre

RITU + Jurosphere & Gammasphere + FMA P. Reiter et al.R Julin et al.

SASSYER (Small Angle Separator System at Yale for Evaporation Residues)

Combine SASSY 2 with YRAST Ball powerful system for channel selection, fission suppression

recoil decay tagging (RDT) capabilities

Combine SASSY 2 with YRAST Ball powerful system for channel selection, fission suppression

recoil decay tagging (RDT) capabilities

SASSY 2 from LBNL comes to Yale in March

• gas-filled separator• large acceptance• high transmission efficiency

SASSY 2 from LBNL comes to Yale in March

• gas-filled separator• large acceptance• high transmission efficiency

Only two other labs worldwide: Berkeley (BGS) and Jyvaskyla (RITU)

Physics program:- exotic nuclei

proton emittersheavy elementsneutron-rich nuclei

- reactions studies relevant to production of superheavy elements

Only two other labs worldwide: Berkeley (BGS) and Jyvaskyla (RITU)

Physics program:- exotic nuclei

proton emittersheavy elementsneutron-rich nuclei

- reactions studies relevant to production of superheavy elements

SASSYER - Physics Projects

• Light actinide nuclei near A = 204• Magnetic Rotation• Superdeformation• High-spin structure

• Actinides around A = 224• Octupole deformation• Collective excitations

• Structure of heavy nuclei• Spectroscopy of transactinides• Alpha spectroscopy• -spectroscopy at focal plane

• Reaction studies relevant for production of superheavy elements

• Structure of nuclei near the proton dripline• Shape coexistence• Onset of deformation• High-K isomers

• N=Z nuclei• Mass measurements of r-process nuclei• -decay to T=0 and T=1 states

• Study of fission fragments

• Nuclear Astrophysics