Transcript ppt - Institute of Nuclear and Particle Physics

The two-step g cascade method as a tool for
studying g-ray strength functions
Milan Krtička
Athens, July 9, 2008
Outline

The method of two-step γ-cascades following
thermal neutron capture
(setup at Rez near Prague)

Data processing - DICEBOX code

Examples
Athens, July 9, 2008
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
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HPGe #2
Athens, July 9, 2008
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 = ij,
j=0
where ij 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
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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
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2000
4000
6000
Gamma-Ray Energy (keV)
Example of sum-energy spectra (57Fe)
60000
Counts per 1 keV
50000
TSC from thermal 56Fe(n,g)57Fe reaction
Gamma-ray energy sums
40000
30000
20000
10000
0
6000
7000
Gamma-ray energy sum (keV)
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8000
Annih.
Example of a TSC spectrum
7279.3 keV
366.8 keV
4000
0
Gamma-ray energy (keV)
25
Annih.
Annih.
50
0
0
Annih.
Dynamic range 1:1000
5920.7 keV
8000
Counts per 1keV bin
Two-step g cascades
terminating at the 57Fe ground state
1725.4 keV
12000
2000
4000
Gamma-ray energy (keV)
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6000
Example of a TSC spectrum
J = 5/2+, 251 keV terminal level in 163Dy
TSC intensity (relative units)
9000
6000
3000
0
0
2000
4000
Gamma-ray energy (keV)
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6000
Normalization of experimental spectra

Knowing intensity of one
g-ray cascade 
TSC intensities to all final levels
can be normalized

Corrections to angular correlation
and vetoing must be done
5%
20 %
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How to process data from this experiment?



Result of interplay of level density and g-ray SF
Comparison with predictions from decay governed by
different level density formulas and g-ray strength functions
Code DICEBOX is used for making these simulations

Simulates gamma decay of a compound nucleus within
extreme statistical model
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Simulation of g cascades - DICEBOX algorithm
Main assumptions:



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) and ρ are also a priori known


Selection rules governing the g decay are fully observed
Any pair of partial radiation widths igf (XL) is statistically
uncorrelated
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Modelling within ESM
Simulation of the decay:
 “nuclear realization”
(106 levels 1012 lgf)
“precursors” are introduced

fluctuations originating from
nuclear realizations cannot be
suppressed
Deterministic character of random
number generators is exploited
Level
Number
c
1
2
0
1
2
3
Outcomes from modelling are
compared with experimental data
n
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Excitation
Energy
Precursor
s1
1
0
 c E 
Bn
s2 1
0
 1 E 
s3 1
c
1
 2 E 
Ecrit
2
0 s4
1
E
0
3
Main feature of DICEBOX





There exists infinite number of artificial nuclei (nuclear realizations),
obtained with the same set of level density and g-ray SFs models that
differ in exact number of levels and intensities of transitions between
each pair of them
 leads to different predictions from different nuclear realizations
DICEBOX allows us to treat predictions from different nuclear
realizations
The size of fluctuations from different nuclear realizations depend on
the (observable) quantity - in our case intensity of TSC cascades - and
nucleus
Due to fluctuations only “integral” quantities can be compared
Simulation of detector response must be applied
Athens, July 9, 2008
Results of GEANT3 simulations - 95Mo(n,g)96Mo
Backscattering
0.94 %
Annih.
0.96 %
Annih.
Backscattering
102
7656.2 keV
103
719.6 keV
Backscattering
Annih.
104
Bremsstrahlung
101
100
0
2000
4000
6000
8000
102
Bremsstrahlung
101
Bremsstrahlung
100
0
1000
2000
3000
4000
Gamma-Ray Energy (keV)
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5000
6000
Backscattering
0.54 %
Annih.
0.57 %
Annih.
Backscattering
Backscattering
103
Annih.
104
Annih.
105
Doublet 3279.9 leV
106
Backscattering
Number of GEANT Events per 10 keV
105
Backscattering
Annih.
106
Example of a TSC spectrum
Wide-bin TSC spectra
DEg = 2 MeV
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Integrated TSC spectra
Examples of spectra (96Mo)
Integrated TSC
Simulation
Experiment
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Some features of TSC spectra (1)
Problems with presentation of results
100 nuclear realizations
2235 keV
distribution of values from
1400 nuclear realizations
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Some features of TSC spectra (2)
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And some results
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TSCs in the 162Dy(n,γ)163Dy reaction
1/2 +
M1
E1
Řež experimental data
πf = +
πf = -
DICEBOX Simulations
Entire absence
of SRs is assumed
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TSCs in the 162Dy(n,γ)163Dy reaction
A “pygmy E1 resonance”
with energy of 3 MeV
assumed to be built on
all levels
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TSCs in the 162Dy(n,γ)163Dy reaction
SRs assumed to be
built only on all levels
below 2.5 MeV
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TSCs in the 162Dy(n,γ)163Dy reaction
Scissors resonances
assumed to be built on
all 163Dy levels
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163Dy:
models for photon strength function used
f(Eg,T=0) (MeV-3)
g-ray strength functions plotted refer to the
g transitions to the ground state of 163Dy
The role of E1 transitions
to or from the ground state
is reduced
EGLO
SR
SF
KMF+BA
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TSCs in the 167Er(n,γ)168Er reaction
Entire absence of
scissors resonances is
assumed
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TSCs in the 167Er(n,γ)168Er reaction
Scissors resonances
assumed to be built on
all 168Er levels
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Enhanced PSF at low energies - 96Mo
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Enhanced PSF at low energies - 96Mo
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Enhanced PSF at low energies - 96Mo
Athens, July 9, 2008
Enhanced PSF at low energies - 96Mo
Pictures with comparison similar but correct
statistical analysis excludes also this model at
99.8 % confidence level
Krticka et al., PRC 77 054319 (2008)
 the enhancement is very weak if any
analysis of data from DANCE confirm this
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Pygmy resonance in 198Au revisited
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Pygmy resonance in 198Au revisited
No pygmy resonance postulated
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Pygmy resonance in 198Au revisited
Pygmy resonance at 5.9 MeV
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Pygmy resonance in 198Au revisited
Abrupt suppression of PSF below 5 MeV
The best fit obtained – it does not seem that there is a pygmy
resonance in 198Au
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PSF used
Exactly the same fit
describes perfectly also
data obtained with DANCE 4
detector
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Eg1
Neutron
capturing
states
200
Multiplicity
1 - 15
10
Multiplicity = 1
Multiplicity = 2
Intensity (arb. units)
Bn+En
Intensity (arb. units)
Spectra from 4 ball (DANCE, n_TOF, …)
20
159
Gd
0
0
0
50
Multiplicity = 3
Multiplicity = 4
Multiplicity > 4
50
50
Eg2
0
0
Eg3
3000
0
0
6000
Energy sum (keV)
0
0
6000
Energy sum (keV)
3000
6000
Energy sum (keV)
Ground
state
10
Intensity (arb. units)
Eg4
3000
Multiplicity = 1
50
Multiplicity = 3
100
Multiplicity = 4
0
Multiplicity > 4
10
0
0
0
0
0
200
Multiplicity = 2
3000
6000
Energy (keV)
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0
0
3000
6000
Energy (keV)
Eg1
Eg3
3000
Energy (keV)
6000
Eg4
Eg2
Pygmy resonance in 198Au revisited
data from the Karlsruhe 4 BaF2 g calorimeter
No pygmy resonance postulated
Pygmy resonance at 5.5 MeV
Suppression of PSF below 5 MeV
No difference in fits
… but postulating a pygmy resonance leads to too large total radiation width
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Athens, July 9, 2008
Conclusions
 Measurement of TSC cascades provides valuable information on g-ray
strength functions
 DICEBOX simulations can be used for obtaining information of g-ray
strength functions from many experiments
Special thanks to:
F. Becvar
Charles University, Prague, Czech Republic
I. Tomandl, J. Honzatko
Nuclear Physics Institute, Rez, Czech Republic
G. Mitchell
NCSU
Athens, July 9, 2008
Thank you
for your attention
Athens, July 9, 2008