GRETA/GRETINA: physical challenges - view
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GRETA/GRETINA: physical challenges – view
of theorist
Anatoli Afanasjev
Mississippi State University, USA
Basic physics questions to GRETINA/GRETA
1. How do protons and neutrons interact to form nuclei?
2. What are the origins of simple patterns in complex nuclei?
3. What are the limits of angular momentum, excitation
energy, charge and mass for nuclei?
4. What is the origin of elements?
Single-particle degrees of freedom
GRETINA will have a factor of about 8 improvement in resolving power
relative to GAMMASPHERE for lower multiplicity processes, such as Coulomb
excitations and transfer reactions … Æ … will greatly expand the opportunities
for advancing nuclear structure studies to higher spin, heavier mass, and to
odd-A nuclei; all cases where the density of gamma-ray transitions exceeds
the energy resolution achieved using present day gamma-ray detectors for
in-beam spectroscopy
From GRETINA proposal
1. Single-particle properties of heaviest actinides Æ
better understanding of physics of superheavy nuclei
2. Single-particle properties of very neutron-rich
nuclei Æ better mass tables, better predictions for
neutron-drip line, better understanding of
physics of neutron-rich nuclei
Quasiparticle spectra in heaviest actinide nuclei
Experimental quasiparticle
states provide
1. Important constraint for the
selection of effective forces for
the description of superheavy
nuclei
2. Provide information about
spherical subshells (high-j)
active in the vicinity of expected
shells gaps in spherical superheavy
A.V.Afanasjev et al,
PRC 67 (2003) 024309
Analysis allowed
to exclude the NLSH and
NLRA1 RMF forces from
further application
to superheavy nuclei
(the only sets which
predict Z=114 as shell gap)
RMF analysis of single-particle energies in spherical Z=120, N=172 nucleus
corrected by the empirical shifts obtained in the detailed study of quasiparticle
spectra in odd-mass nuclei of the deformed A~250 mass region (PRC 67 (2003) 024309)
Self-consistent
solutionFlat density distribution
mass m*/m~0.8-1.0
in the central
Skyrme part of nucleus:
SkP [m*/m=1]
Large effective
doubleZ=126 appears,
shell closure
N=184
at Z=126,becomes
N=184 larger
(SkM*, ????
and Z=120
(N=172) shrinks
Skyrme SkI3 [m*/m=0.57]
gaps at Z=120, N=184
no double shell closure, Which role effective mass plays???
SLy6
Low effective mass
m*/m ~ 0.65
Gogny D1S
Z=120, N=172(?)
Z=126, N=184
Large density depression
in the central part of nucleus:
RMF
shell gaps at Z=120,
double shell closure
N=172
at Z=120,N=172Physics of neutron rich nuclei
S. Goriely, J.M.Pearson and Co
Skyrme functional: fit to masses allowed
to decrease rms deviation (on masses) from
~ 2.5 MeV down to ~ 0.7 MeV. In total ~
20 parameters fitted to several thousands
of nuclei.
Open questions: 1. No unique fit (how
this affects the r-process abundancies???)
2. Should we use single-particle
properties as an additional
constraint on effective force?Summary on single-particle properties
Single-particle information has to be taken into account in order
to improve the quality of effective interactions (and probably, find
missing channels of interactions) in the self-consistent theories.
Example: extrapolability of mass tables to unknown nuclei
FRDM (Moller, Nix) – good, rms error remains the same
SHF (S. Goriely, J.M.Pearson and Co) – deteriorates for older
parametrizations, unknown for newest ones
From J.Rikowska-Stone, J.Phys. G: 31 (2005) R211
Principal difference between FRDM and SHF: careful fit of single-particle
degrees of freedom.
There are > 80 Skyrme and > 40 RMF parametrizations, but they were
fitted with no single-particle information taken into account.
Theory: careful fit of lowest single-particle states in deformed
nuclei within ‘a la table of mass’ strategy
Experiment: s-p states in heaviest deformed nuclei +
s-p states in deformed neutron-rich nucleiHigh-spin laboratory
deformation
# of p-h excitations ‘maximum’ spin
in the configuration
Energy Jacobi transition Fission
Spin limit
Rotating nuclei:Rotational
the best laboratory
damping for study of shape
coexistence Æ starting from spherical ground state
Order-chaos
by means of subsequent particle-hole excitations one can
transition
build any shape (prolate and oblate [collective and
non-collective], triaxial, superdeformed, hyperdeformed
etc.) Superdeformed
(non-terminating)
(Near-)
spherical Normal-deformed Hyperdeformation
(terminating)
SpinTermination and non-termination of rotational
bands
- basic feature of shell model
- important feature of a finite many-fermion quantum mechanical
system which either do not exist or cannot be experimentally
measured in other quantum systems
Q1: Do all rotational bands end up in terminating states? How
the transition from terminating to non-terminating bands
takes place?
Q2. How the terminating states of smooth terminating bands are
fed? (experimental proof of their termination)Do all rotational bands end up in terminating states ???
T.Troudet and R. Arvieu, Ann. Phys. 134, 1 (1981)
Cranked harmonic oscillator
Imax – the maximum spin which can
be built in the pure configuration
Rotational bands do not terminate
in a noncollective state at Imax if
the deformation exceeds a critical
value at low spin.
Origin: due to the coupling of
different N shells leading to a
mixing of different configurations
Æ
Even higher spins than Imax can
Do not be build within the mixed
Terminate
terminate configuration.Theoretical models:
•CNS: Cranked Nilsson-Strutinsky
•CRMF: Cranked Relativistic Mean Field
No
non-collective
state can be
defined for
I>Imax
Imax
Potential
energy
surfaces for
the GSB
configurationSuper- and hyperdeformation in neutron-rich
nuclei.
- Mean field is well justified + pairing correlations are expected
to have negligible impact at high spin Æ = clean probe of
effective interactions
Q1: How effective interactions are modified by neutron excess
and fast rotation?AA, S.Frauendorf, PRC 72 (2005) 031301(R)
SD(1)
SD(2) is built on ph excitation across
the Z=48 SD shell gap. Crossing freq. SD(2)
confirm its existence.
N=60 and 62 SD shell gaps in CRMF.
J(2) of HD configurations = 67-71 MeV-1Hyperdeformation
First hints for Hyperdeformation
-
(“ridges” in γ−γ spectra)
similar to the first observation of
superdeformation ~20 years ago
compound nucleus at the highest spins
Î most neutron-rich stable isotopes
48Ca + 82Se Î126Xe + 4n
Observation of hyperdeformed nuclei
¾ Gamma-ray tracking array
¾ high intensity neutron-rich beamsHyperdeformation in the A~120-130 mass region
B. Herskind et al
Ridge structures in 3-D rotational mapped spectra are identified
with dynamic moments of inertia J(2) ranging from 71 to 111 MeV-1
Experiment:
Wild variations of J(2): example 122Xe J(2)=77 MeV-1
124Xe J(2)=111 MeV-1
Preliminary CRMF: 122Xe J(2) (HD) ~ 75 MeV-1
Superdeformation in nuclei around 68Zn
Due to Z=30 and N=38 SD shell gaps
See M. Devlin et al, PRL 82 (1999) 5217‘SD’ and ‘HD’ in the crust of neutron stars
The presence or absence
of exotic nuclear shapes
(swiss cheese, spaghetti
and lasagna phases) before
transition to uniform npe
matter depends on the
assumed model of effective
nucleon-nucleon interactionRotating systems: the best
laboratory
for time-odd mean fields
Impact of time-odd mean
fields (in %)
Time-odd mean fields
Open questions:
dependence on deformation,
configuration, spin,
isospin etc. ????
Method: to eliminate the
uncertainties related to
pairing use high-spin dataWobbling motion
- unique signal of triaxiality of nuclei
Q1: Can wobbling excitations be observed in other regions
of nuclear chart in which theoretical calculations strongly
suggests the existence of triaxial shapes at high spin?
Q2: What are the basic conditions for the existence of
wobbling excitations?Theoretical calculations suggests that many rotational bands
possess appreciable triaxiality over considerable spin range:
1. Smooth terminating bands in the A~110 and A~60 mass regions
110Sb yrast band configuration
[21,3] = [#g9/2-1 #h11/2, #h11/2]
see A.V.Afanasjev, D.B.Fossan,
G.J. Lane and I.Ragnarsson,
Physics Report 322 (1999) 1
2. Many normal- and highly deformed rotational
bands in the A~60-80 and A~130 mass regions
3. Superdeformed bands in the A~80 mass region
D.G.Sarantites et al,
PRC 57 (1998) R1Wobbling motion
Wobbling excitations were not observed in these regions
of nuclear chart so far !!!
Possible reasons:
1. Do not exist in these nuclei. Why?
2. Wobbling excitations in are highly non-yrast in these
nuclei??? Will GRETA be able to measure such
excitations?Giant Resonances The GDR position has been measured for about 90 stable nuclei all lying on beta stability line. Extend these measurement to 1. Unstable neutron- and proton-rich nuclei 2. To high spin systems
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