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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