the two-step g cascade method as a tool for studying g -ray strength functions
DESCRIPTION
The two-step g cascade method as a tool for studying g -ray strength functions. Milan Krtička. Outline. The method of two-step γ - cascades following thermal neutron capture (setup at Rez near Prague) Data processing - DICEBOX code Examples. - PowerPoint PPT PresentationTRANSCRIPT
Athens, July 9, 2008
The two-step cascade method as a tool for studying -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:
Data acquisition: - Energy E1
- Energy E2
- Detection-time difference
Three-parametric, list-mode
Athens, July 9, 2008
Athens, July 9, 2008
TSC spectra
Det
ectio
n-tim
e di
ffere
nce
Tim
e r
esp
on
se f
un
ctio
n
Spectrum of energy sums
Bn-Ef
Energy sum Eγ1+ Eγ2
List-mode data
i=-1 i=0 i=+1
j=-1
j=0
j=+1
Accumulation of the TSC spectrum from, say, detector #1:
The contents of the bin, belonging to the energy E1, is incremented by
q = ij,
where ij is given by the position and the size of
the corresponding window in the 2D space “detection time”דenergy sum E1+E 2”.
background-free spectrum
Athens, July 9, 2008
TSC spectra
Det
ectio
n-tim
e di
ffere
nce
Tim
e r
esp
on
se f
un
ctio
n
Spectrum of energy sums
Bn-Ef
Energy sum Eγ1+ Eγ2
List-mode data
i=-1 i=0 i=+1
j=-1
j=0
j=+1
2000 4000 60000
200
T
SC
Inte
nsity
(arb
. uni
ts)
Gamma-Ray Energy (keV)
0
30
2000 4000 60000
200
TS
C I
nte
nsi
ty (
arb
. u
nits
)Gamma-Ray Energy (keV)
0
30
X 5
TSC spectrum - taken from only one of the detectors
Athens, July 9, 2008
Example of sum-energy spectra (57Fe)
Gamma-ray energy sum (keV)
6000 7000 8000
Cou
nts
per
1 k
eV
0
10000
20000
30000
40000
50000
60000
TSC from thermal 56Fe(n,)57Fe reactionGamma-ray energy sums
Athens, July 9, 2008
Example of a TSC spectrum
Dynamic range 1:1000
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Example of a TSC spectrum
Athens, July 9, 2008
Normalization of experimental spectra
Knowing intensity of one -ray cascade TSC intensities to all final levels can be normalized
Corrections to angular correlation and vetoing must be done
5 %
20 %
Athens, July 9, 2008
How to process data from this experiment?
Result of interplay of level density and -ray SF
Comparison with predictions from decay governed by different level density formulas and -ray strength functions
Code DICEBOX is used for making these simulations Simulates gamma decay of a compound nucleus within
extreme statistical model
Athens, July 9, 2008
Simulation of 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 if (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 decay are fully observed
Any pair of partial radiation widths if (XL) is statistically uncorrelated
Athens, July 9, 2008
Modelling within ESM
0
Level Number
Excitation Energy
12
1
2
c
3
n
E1
Ec
EEcrit
0
E
Bn
1
2
3
Precursor
c
1
2
s1
0 1s4
0 s2 1
0 s3 1
Outcomes from modelling are compared with experimental data
Deterministic character of random number generators is exploited
Simulation of the decay: “nuclear realization”
(106 levels Ţ 1012 f) Ţ “precursors” are introduced
fluctuations originating from nuclear realizations cannot be suppressed
Athens, July 9, 2008
Main feature of DICEBOX
There exists infinite number of artificial nuclei (nuclear realizations), obtained with the same set of level density and -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,)96Mo
Athens, July 9, 2008
Example of a TSC spectrum
Wide-bin TSC spectra
E = 2 MeVIntegrated TSC spectra
Athens, July 9, 2008
Examples of spectra (96Mo)Integrated TSC Simulation
Experiment
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Some features of TSC spectra (1)
100 nuclear realizations
Problems with presentation of results
distribution of values from 1400 nuclear realizations
2235 keV
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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
Entire absence of SRs is assumed
DICEBOX Simulations
Řež experimental data
πf = -
πf = +
1/2 +
M1
E1
Athens, July 9, 2008
TSCs in the 162Dy(n,γ)163Dy reaction
A “pygmy E1 resonance” with energy of 3 MeV
assumed to be built on all levels
Athens, July 9, 2008
TSCs in the 162Dy(n,γ)163Dy reaction
SRs assumed to be built only on all levels
below 2.5 MeV
Athens, July 9, 2008
TSCs in the 162Dy(n,γ)163Dy reaction
Scissors resonances assumed to be built on
all 163Dy levels
Athens, July 9, 2008
163Dy: models for photon strength function used
-ray strength functions plotted refer to the transitions to the ground state of 163Dy
KMF+BA
SRSF
EGLO
f(E
,T
=0)
(M
eV-3)
The role of E1 transitionsto or from the ground state
is reduced
Athens, July 9, 2008
TSCs in the 167Er(n,γ)168Er reaction
Entire absence of scissors resonances
is assumed
Athens, July 9, 2008
TSCs in the 167Er(n,γ)168Er reaction
Scissors resonances assumed to be built on
all 168Er levels
Athens, July 9, 2008
Enhanced PSF at low energies - 96Mo
Athens, July 9, 2008
Enhanced PSF at low energies - 96Mo
Athens, July 9, 2008
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 levelKrticka 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
Athens, July 9, 2008
Pygmy resonance in 198Au revisited
No pygmy resonance postulated
Athens, July 9, 2008
Pygmy resonance in 198Au revisited
Pygmy resonance at 5.9 MeV
Athens, July 9, 2008
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
Athens, July 9, 2008
PSF used
Exactly the same fitdescribes perfectly alsodata obtained with DANCE 4detector
Athens, July 9, 2008
Spectra from 4 ball (DANCE, n_TOF, …)NeutroncapturingstatesE1
E2
E4 Ground state
E3
Bn+En
0
200
159Gd
Multiplicity 1 - 15
0
10Multiplicity = 1
0
20
Multiplicity = 2
0 3000 60000
50 Inte
nsity
(ar
b. u
nits
)
Inte
nsity
(ar
b. u
nits
)
Energy sum (keV)
Multiplicity = 3
0 3000 60000
50Multiplicity = 4
Energy sum (keV)
0 3000 60000
50Multiplicity > 4
Energy sum (keV)
0 3000 60000
200
Multiplicity > 4
Inte
nsity
(ar
b. u
nits
)
Energy (keV)
0 3000 60000
100
Energy (keV)
Multiplicity = 4
0
50Multiplicity = 3
0 3000 60000
10
Energy (keV)
Multiplicity = 2
0
10Multiplicity = 1
E1
E2E3
E4
Athens, July 9, 2008
data from the Karlsruhe 4 BaF2 calorimeter
Pygmy resonance at 5.5 MeVNo pygmy resonance postulated Suppression of PSF below 5 MeV
… but postulating a pygmy resonance leads to too large total radiation widthNo difference in fits
Pygmy resonance in 198Au revisited
Athens, July 9, 2008
Athens, July 9, 2008
Conclusions
Measurement of TSC cascades provides valuable information on -ray strength functions
DICEBOX simulations can be used for obtaining information of -ray strength functions from many experiments
Special thanks to:F. Becvar
Charles University, Prague, Czech Republic
I. Tomandl, J. HonzatkoNuclear Physics Institute, Rez, Czech Republic
G. MitchellNCSU
Athens, July 9, 2008
Thank you
for your attention