Transcript PHOTON STRENGTH FUNCTIONS IN RARE

Scissors Mode of Excited Nuclei
from (n,γ) reaction
Milan Krtička
Kazimierz, September 25, 2010
Outline
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Properties of the “ground-state” scissors mode
Description of (n,g) experiments
(that allow us to study decays of excited nuclei) –
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Two-Step Cascade Measurement
(using Ge detectors)
Multistep Cascade Measurements
(using 4p BaF2 detector arrays)
Data processing – statistical model simulations
Results/Conclusions – Scissors mode on excited states
Kazimierz, September 25, 2010
Scissors mode build on the ground state
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Resonance–like structure in magnetic orbital strength
Discovered in (e,e’) experiment in 156Gd
D. Bohle et al., Phys. Lett. B137 (1984) 27
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Rich data on even-even nuclei measured in NRF experiments
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Strength concentrated near 3 MeV
in several transitions (e-e nuclei)
Total strength depends on the square
of deformation
For well-deformed e-e nuclei
SB(M1)  3 mN2
Problems with the strength in odd
nuclei due to very high
fragmentation
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How is it on excited states?
Kazimierz, September 25, 2010
U. Kneissl, H.H. Pitz, A. Zilges,
Prog. Part. Nucl. Phys. 37 (1996) 349
Scissors mode build on excited states
There exists two experiments/reactions which were used to study the
scissors mode on excited states
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Coincident measurement of products from (3He, 3Heg) or (3He, ag)
reactions – the method is known as “Oslo method”
A. Schiller et al., Phys. Lett. B 633 225 (2006),
A. Schiller et al., Nucl. Instrum. Methods Phys. Res. A 447, 498 (2000), …
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Measurement of spectra g-ray spectra from (n,g) reaction
Kazimierz, September 25, 2010
The method of two-step γ-cascades following TNC
Geometry:
HPGe #1
6 m long neutron guide
γ1
Target
n
γ2
4 mm
Data acquisition:
Three-parametric, list-mode
- Energy Eg1
- Energy Eg2
- Detection-time difference
Kazimierz, September 25, 2010
HPGe #2
TSC spectra
n
Bn
Spectrum of energy sums
Eg
A
Quasi-continuum
Eg
Bn-Ef
Ef
A+1
G.S.
Accumulation of the TSC spectrum from, say,
detector #1:
Energy sum Eγ1+ Eγ2
Detection-time difference
Time response function
i=-1 i=0 i=+1
The contents of the bin, belonging to the energy
Eg1, is incremented by
j=-1
q = aij,
j=0
where aij is given by the position and the size of
the corresponding window in the 2D space
“detection time”דenergy sum Eg1+Eg 2”.
j=+1
List-mode data
 background-free spectrum
Kazimierz, September 25, 2010
TSC spectra
n
Bn
Spectrum of energy sums
Eg
A
Quasi-continuum
Eg
Bn-Ef
Ef
A+1
TSC Intensity (arb. units)
Detection-time difference
i=-1 i=0 i=+1
j=-1
j=0
j=+1
200
TSC Intensity (arb. units)
TSC spectrum - taken from only one of the detectors
Energy sum Eγ1+ Eγ2
Time response function
G.S.
200
30
30
X5
0
0
0
2000
4000
6000
Gamma-Ray Energy (keV)
0
List-mode data
Kazimierz, September 25, 2010
2000
4000
6000
Gamma-Ray Energy (keV)
The method of multistep-step γ-cascades (MSCs)
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Measured with (a 4p) array of BaF2 detectors
Installed at neutron TOF facilities
DANCE @ LANSCE (160 crystals), n_TOF @ CERN (40 crystals),
formerly FzK Karlsruhe (40 crystals)
Kazimierz, September 25, 2010
Eg1
Neutron
capturing
states
Multiplicity
1 - 15
200
10
Multiplicity = 1
Multiplicity = 2
Intensity (arb. units)
Bn+En
Intensity (arb. units)
MSC spectra
20
159
Gd
0
0
0
Multiplicity = 3
50
Multiplicity = 4
Multiplicity > 4
50
50
Eg2
0
0
Eg3
3000
0
0
6000
Energy sum (keV)
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0
0
6000
Energy sum (keV)
3000
6000
Energy sum (keV)
Ground
state
We can gate on strong
resonances possibly with
different spins (DANCE,
n_TOF)
unresolved resonance
region (Karlsruhe)
10
Intensity (arb. units)
Eg4
3000
Multiplicity = 1
50
Multiplicity = 3
100
Multiplicity = 4
0
Multiplicity = 2
Multiplicity > 4
10
0
0
0
0
0
200
3000
Energy (keV)
Kazimierz, September 25, 2010
6000
0
0
3000
6000
Energy (keV)
Eg1
Eg3
3000
Energy (keV)
6000
Eg4
Eg2
Data processing
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TSC and MSC spectra are compared with predictions based on
simulations within Extreme statistical model
The statistical model was incorporated in the Monte-Carlo code
(DICEBOX) – all the fluctuations are taken into account during
simulation of g decay
Detector response must be applied to simulated cascades
– GEANT simulations of 4p BaF2 arrays
– simple in the case of TSC spectra – knowledge of detector
efficiencies is sufficient
Kazimierz, September 25, 2010
Simulation of g cascades
Main assumptions:
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For nuclear levels below certain “critical energy” spin, parity and decay
properties are known from experiments
Energies, spins and parities of the remaining levels are assumed to be a
random discretization of an a priori known level-density formula
A partial radiation width igf (XL), characterizing a decay of a level i to
a level f, is a random realization of a chi-square-distributed quantity the
expectation value of which is equal to
f (XL)(Eγ) Eγ2L+1/(Ei),
where f (XL) – g-ray strength functions – and ρ – level density – are
also a priori known
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Selection rules governing the g decay are fully observed
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Any pair of partial radiation widths igf (XL) is statistically uncorrelated
Kazimierz, September 25, 2010
Models used - examples
g-ray strength functions are needed not
only for transitions to the ground state
but also for transitions between
excited states –
Brink hypothesis (suggested/works for
GDER) is usually adopted
f(Eg,T=0) (MeV-3)
g-ray strength functions
EGLO
SR
SP
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SF
KMF+BA
Is the Brink hypothesis a good
approximation for scissors mode?
Are the parameters of the scissors
mode the same in even-even, odd,
and odd-odd nuclei?
Kazimierz, September 25, 2010
Level density
TSCs in the 162Dy(n,γ)163Dy reaction
M. Krticka et. al, PRL 92 (2004) 172501
Probability
6271 keV ½+
n
162Dy
SR Probability
3/2-
Eγ1
SR
251 keV 5/2+
G.s. 5/2163Dy
Eγ1+Eγ2 = 2 ESR
Eγ2
Sharpening the peak at the midpoint
of the TSC spectrum
for the 251 keV final level
A unique possibility of a sensitive test for presence of SRs
built on the levels in the quasicontinuum
Kazimierz, September 25, 2010
TSCs in the 162Dy(n,γ)163Dy reaction
1/2 +
M1
-+
E1
-
E1-M1 & M1-E1
+
E1-E1 & M1-M1
Kazimierz, September 25, 2010
TSCs in the 162Dy(n,γ)163Dy reaction
πf = +
πf = -
DICEBOX Simulations
Corridors predicted by
simulations reflect
fluctuations involved
in the decay (mainly
Porter-Thomas fluctuations)
Entire absence
of SRs is assumed
Kazimierz, September 25, 2010
TSCs in the 162Dy(n,γ)163Dy reaction
A “pygmy E1
resonance” with energy
of 3 MeV assumed to be
built on all levels
Kazimierz, September 25, 2010
TSCs in the 162Dy(n,γ)163Dy reaction
SRs assumed to be built
only on all levels below
2.5 MeV
Kazimierz, September 25, 2010
TSCs in the 162Dy(n,γ)163Dy reaction
Sharpening the 3 MeV peak
well reproduced
plus
a quantitative agreement
between the predicted and
simulated TSC spectra
Scissors resonances
assumed to be built on
all 163Dy levels
SB(M1)  6 mN2
Kazimierz, September 25, 2010
MSC spectra from the capture of keV n’s in 162Dy
SRs built on all 163Dy levels
…only on levels with Ef <2.5 MeV
En = 90-100 keV
Kazimierz, September 25, 2010
MSC spectra from the capture of keV n’s in 162Dy
SRs built on all 163Dy levels
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The same parameters of f (XL)
(i.e. also parameters of the
scissors mode) needed for nice
reproduction of TSC spectra
are used:
Lorentzian with E = 3 MeV,
 = 0.6 MeV, SB(M1)  6 mN2
En = 90-100 keV
Kazimierz, September 25, 2010
Additional results – rare-earth nuclei
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Scissors mode is build upon all (or at least majority of) level up to neutron
separation energy and its parameters (energy, width, total strength) seem to
be very stable with excitation energy
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Preliminary analysis of well-deformed odd Gd isotopes from DANCE
measurement (spectra from isolated resonances) indicates the total strength
similar to 163Dy, i.e. about 5-7 mN2, is required
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Similar strength seems to be needed for description of TSC spectra for oddodd nucleus 160Tb - poster of Jiri Kroll
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For even-even Gd isotopes the scissors mode needed for reproduction of
DANCE spectra is much weaker – at maximum SB(M1)  3 mN2, but we
need additional “smooth” M1 strength
Analysis of TSC spectra for even-even Gd isotopes should be available soon
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Kazimierz, September 25, 2010
Additional results – actinides
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Several actinides (235U, 237U, 239U, 242Am, 244Am) has been measured
during past years at DANCE and n_TOF experiments;
more measurements are planned
Experimental data from DANCE look very good and the analysis of
the MSC spectra has just started
The analysis of n_TOF data is much more complicated due to the
pile-up of detected events
Kazimierz, September 25, 2010
Thank …
you for your attention!
and my collaborators
Prague – F. Bečvář, J. Kroll
Karlsruhe – F. Käppeler, F. Voss, K. Wisshak, R. Reifarth
DANCE – G.E. Mitchell, M. Jandel, U. Agvaanluvsan,
T.A. Bredeweg, R. C. Haight, J. M. O’Donnell,
R. S. Rundberg, J.L. Ullmann, D.J. Vieira,
J.B. Wilhelmy, J.M. Wouters
Kazimierz, September 25, 2010
MSC spectra from the capture of keV n’s in 162Dy
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Spectra from unresolved region are
very similar for different neutron
energies in the range
En = 10 – 100 keV
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Almost no contribution of p-wave
resonances at the lowest neutron
energies while significant
contribution (more than 50%) for
the highest energies
Kazimierz, September 25, 2010
Even-even rare-earth nuclei
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For even-even Gd isotopes the scissors mode needed for reproduction of
MSC spectra (from DANCE) seems to be much weaker – at maximum
SB(M1)  3 mN2 , but we need additional “smooth” M1 strength
Analysis of TSC spectra should be available soon
Kazimierz, September 25, 2010