Alexandra Gade

33 Pages·2015·5.23 MB·English

by  Thomas Glasmacher

Nuclear spectroscopy with fast exotic beams
Alexandra Gade
Professor of Physics
NSCL and Michigan State University
… or puzzles in the region around N=28
from 48Ca over 46Ar to 42Si
Alexandra Gade
Professor of Physics
NSCL and Michigan State University
Outline
N=28
• A brief history and motivation
doubly 48Ca 49Ca
• What is wrong with 46Ar?
magic
• Direct neighborhood of 46Ar 47K
• Other chains – Ca isotopes
well … 46Ar 47Ar 48Ar 49Ar
• Other chains – Si isotopes
43Cl 44Cl 45Cl 46Cl
• Open questions and outlook
44S
collective
43P
34Si 36Si 38Si 40Si 42Si collective
…
N=20
A. Gade, 5/13/2015, Slide 3
Why is N=28 still interesting in neutron-
rich nuclei
48Ca Doubly-magic
• Region of rapid shell evolution
47K • First shell closure due to the spin-
orbit force
46Ar
Huh?
• Collectivity of 46Ar is not described
by shell model – puzzling
45Cl 46Cl
deviation
Collective, shape
44S
coexistence proven
• For me personally – 46Ar is a key
nucleus on the path from doubly-
43P
magic 48Ca to deformed 42Si that
theory has to get right if the
42Si Collective
details of shell evolution are
claimed to be understood and
implemented in shell model
40Mg Anybody?
A. Gade, 5/13/2015, Slide 4
The breakdown of N=28 from 48Ca to 42Si
• Neutron f fully filled (as
7/2
compared to 34Si) and that
reduces the d -d SO
3/2 5/2
splitting by almost 2 MeV
(consistent with tensor force:
d -f attractive and d -f
3/2 7/2 5/2 7/2
repulsive)
• N=28 shell gap is reduced by 1
MeV as protons are removed
from 48Ca  42Si
• Particle-hole excitation of sd
shell protons and fp shell
neutrons to unfilled orbitals with
ℓ=2 favor quadrupole
collectivity
B. Bastin et al., PRL 99, 022503 (2007)
A. Gade, 5/13/2015, Slide 5
A brief history –Collectivity of 46Ar
• Reported for the first time in 1974 from an
48Ca(6Li,8B) experiment
• 1996: B(E2) =196(39) e2fm4 (NSCL Coulex)
up
• 2003: B(E2) =218(31) e2fm4 (NSCL Coulex)
up
• Unpublished: Lineshape lifetime measurement
(Stuchbery) – agrees with Coulex B(E2)
• Shell model: factor of 2.5 higher than the
above
• DFT (DD-PCI): B(E2) = 200 e2fm4
up
• 2010: B(E2) =570+335 e2fm4 (INFN lifetime)
up -160
• Shell model claims victory … or agreement
H. Scheit et al., PRL 77, 3967 (1996)
A. Gade et al., PRC 68, 014302 (2003)
• 2012: 47Ar (2+ x p ) multiplet B(E2) consistent
3/2 Z. P. Li et al., PRC 84, 054304 (2011)
with low B(E2):
D. Mengoni et al., PRC 82, 024308 (2010)
R. Winkler et al., PRL 108, 182501 (2012)
• 2014: B(E2) =271+22 e2fm4 (GANIL Coulex)
S. Calinescu et al., Acta Phys. Pol. B 454,
up -26
199 (2014)
A. Gade, 5/13/2015, Slide 6
Intermediate-energy Coulomb excitation
Example: 46Ar + 197Au
A. Gade et al., Phys. Rev. C 68, 014302 (2003)
Experiment:
Max. determines
min. b
adopted
A. Gade, 5/13/2015, Slide 7
The 46Ar lifetime measurement
D. Mengoni et al., PRC 82, 024308 (2010)
• Marginal statistics
• Shell model (SDPF-U with
e /e =1.5/0.5) agrees
p n
• Plunger measurement, uses intensity
ratios of peaks stemming from emission
before or after a degrader
Life is good?
A. Gade, 5/13/2015, Slide 8
Not so fast … Coulomb excitation at
GANIL
S. Calinescu et al., Acta Phys. Pol B45, 199 (2014)
• Intermediate-energy Coulomb
excitation of 46Ar measured
relative to 44Ca
• And again consistent with a
lower B(E2) =271+22 e2fm4
up -26
• Three (3) Coulomb excitation
measurements are consistent
with each other and lower than
the lifetime measurement
A. Gade, 5/13/2015, Slide 9
Coulomb excitation of neutron-rich 47,48Ar
R. Winkler et al., PRL 108, 182501 (2012)
• B(E2) values at 46Ar are
controversial
• SM does not reproduce the lower
of the two values
• Beyond N=28, only excitation
energies are known
• Goal: Assess collectivity in 47,48Ar
• Intermediate-energy Coulex
• PID with the S800
2+ -> 0+ spectrograph
• Gamma-ray spectroscopy with
SeGA
• Collectivity in Ar isotopes
beyond N=28
A. Gade, 5/13/2015, Slide 10