- Email: [email protected]
Electromagnetic Dipole Strength in 124,128,134Xe
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Nuclear Data Sheets 119 (2014) 317–319 www.elsevier.com/locate/nds
Electromagnetic Dipole Strength in
124,128,134
Xe
R. Massarczyk,1, 2, ∗ R. Schwengner,1 and A.R. Junghans1 1
Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany 2 Technische Universit¨ at Dresden, 01062 Dresden, Germany
The electromagnetic dipole strength in several even nuclei in the chain of Xenon isotopes has been investigated at the bremsstrahlung facility of the ELBE accelerator in Dresden, Germany and at the HIγS facility in Durham, USA. The goal of these measurements is to extend the knowledge about the general behavior of the dipole strength in the energy region below the neutron separation energy under the aspect of neutron excess and nuclear deformation. I.
INTRODUCTION
One of the most fundamental properties in theory as well as from an experimental point of view in a nucleus is the transition probability pi→f between an initial nuclear state i and a final state f . This probability can be determined with the transition width Γif . Therefore one can define pi→f =
Γif , Γtot
(1)
where Γtot represents the sum of widths to all possible final states. The transition width can also be explained in a theoretical picture Γif ∼ |ψi |T |ψf |2 ,
(2)
where it connects the wave function ψ of the states i and f . As shown in Ref. [1] it is possible to express the average transition width at a certain energy Ei XL Γif =
XL (Eγ ) · Eγ2L+1 fif , (Ei )
(3)
where Eγ is the transition energy and (Ei ) the level density at the excitation energy Ei . The superscript XL represents the order transition (L) and distinguishes between magnetic and electric type of strength (X). The strength XL function fif depends only on the transition energy Eγ [2, 3]. If calculations or experiments are performed on a single state, Eq. (3) can be expressed as 2
σγ = 3 (πc) Eγ f E1 (Eγ ) ,
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.08.087 0090-3752/© 2014 Elsevier Inc. All rights reserved.
where σγ is the photo-absorption cross section. Therefore, photo excitation experiments are an excellent tool to determine the strength function and open up the possibility to test theoretical nuclear predictions directly. Intensively studied, (γ,n) reactions have shown the characteristic shape of the Giant Dipole Resonance (GDR) in E1-strength at about 15-20 MeV excitation energies. As shown in recent publications [4–6], an experimentally deduced strength, which differs from standard model descriptions, can have an impact on the (n,γ) cross section and the shape of the photon spectrum following neutron capture. Possible descriptions of the E1 strength are given by the compilation in RIPL3 [7] or the triple lorentzian model (TLO) [8]. Especially in the low energy region below the neutron separation energy Sn these models deviate from each other. Also, extra strength has been found which is often interpreted as an additional resonance, the so called pygmy dipole resonance (PDR) [9–11]. A comparison of experimental results with theoretical calculations [12, 13] has shown that is possible to describe the experimentally deduced cross section. However, more detailed measurements will help to get a deeper understanding of the processes and interactions occurring in the nucleus. A series of measurements [12] have been performed on stable isotopes with neutron number N = 50. The present paper will show results for a series of measurement on 124,128,134 Xe isotopes. In this chain of isotopes, the nuclear deformation is decreasing while the neutron excess is increasing, see also Table I. An additional measurement on 132 Xe is planned to close the gap between the values 1.37 and 1.48 in the N/Z ratio.
(4) II.
EXPERIMENTAL DETAILS
The nuclear resonance fluorescence experiments were performed at two experimental facilities, the
Electromagnetic Dipole Strength . . .
NUCLEAR DATA SHEETS
TABLE I. N/Z ratio of the measured Xenon isotopes.
50
isotope 124 Xe 128 Xe 132 Xe 134 Xe N/Z ratio 1.29 1.37 1.41 1.44
40
R. Massarczyk et al.
124
30
Xe
20
cross section (mb)
10 0 4000 50 40 30
6000
8000
TLO RIPL (2Lorentz) ELBE data
128
10000
Xe
20 10 0 4000
6000
8000
10000
50 134
40
FIG. 1. Measured spectra of filled and empty target experiments. The spectrum of the empty target measurement is normalized to the filled target by the absolute photon flux during the beam time.
Xe
30 20 10 0 4000
bremsstrahlung facility [14] at the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf, Germany, and the HIγS facility [15] at the TUNL nuclear laboratory in Durham, USA. At both experimental sides several high-purity germanium detectors are measuring photons, scattered by highly enriched targets. Several techniques are applied to correct the measured spectra for different effects:
6000
8000
Ex (MeV)
10000
FIG. 2. Final photo-absorption cross section σγ in comparison with predictions from RIPL3 and TLO models.
• To calculate the photo absorption cross section, inelastic scattering events are removed from the measured spectrum and the remaining events are divided by the ground state branching ratio. Both quantities, the inelastic scattering events and the branching ratio, are calculated by a statistical code, γDex [5, 6].
• Empty target measurements are performed to subtract the influence of the steel ball which holds the Xe target. These containers are minimized in terms of the amount of additional material which is put in the beam and optimized to hold high pressures, up 80 bar. The diameter of such a steel ball is 20 mm. A detailed description can be found in Ref. [16]. The background produced by the steel ball can be seen in Fig. 1.
III.
RESULTS AND OUTLOOK
The photo-absorption cross section for the isotopes Xe were deduced from measurements in Dresden. The results are presented in Fig. 2. Small enhancements over the introduced parametrization of the GDR [7, 8] were observed but no large extra pygmy resonance, as for example observed in 139 La [18], was found. Nuclear-resonance fluorescence experiments on 124 Xe and 134 Xe were performed at HIγS. The analysis of these experiments are not yet complete but they can help to estimate the ratio of E1 and M1 dipole strength, as shown in previous experiments [12, 13, 19]. If a reasonable M1strength will be found, Eq. (4) has to be changed. In that case the photo-absorption cross section is a combination of f E1 and f M 1 , the electric and magnetic dipole
• The detector efficiency is determined by building up the setup in real geometry in a GEANT4 [17] simulation.
124,128,134
• The same simulation is used to simulate the detector response for γ-rays impinging on the detector with energies from 0.5 MeV up to 12 MeV. The spectrum is unfolded for detector response. • GEANT4 is also used to simulate the atomic background. These are events registered by the detector which have scattered on the electrons of the nuclei and are not of interest for our results. Details for the GEANT4 simulation can be found in Ref. [13]. 318
Electromagnetic Dipole Strength . . .
NUCLEAR DATA SHEETS
photo-absorption cross sections have been transformed into B(E1) values under the assumption of a minor influence of M 1 strength. As one can see, the trend of data differs from the experimental dataset presented in Ref. [20]. The trend of an increasing B(E1) strength with the N/Z ratio is also found in the theoretical calculations. For comparison an additional datapoint for our measurement on 136 Ba [13] is also plotted in the figure. A reason for the deviation of the data might be the different analysis of the data. We took into account the amount of strength hidden in the continuum of unresolved states. This additional strength can be in the range of 60-70% as shown in previous results [12]. A detailed analysis on our data measured at the HIγS facility will give us more information about the influence of possible M1 strength on total strength and an additional measurement on 132 Xe is planned in order to close the gap in the N/Z region between 128 Xe and 134 Xe.
1.5
Σ B(E1)
2
2
( e fm )
2
1 0.5 0 1.2
1.3
1.4 N/Z
1.5
R. Massarczyk et al.
1.6
FIG. 3. Comparison of summed B(E1) strength between 6 and 8 MeV excitation energy. The data of Xenon-isotopes (black circles) and 136 Ba (black asterisk) are measured at ELBE. The experimental data of other isotopes in the N =82 region (open circles) and a quasiparticle phonon model calculation (black line) are taken from Ref. [20].
Acknowledgements: The authors would like to thank the members of the nuclear physics group in Dresden and Durham as well as the staff of the accelerators located there. This work was supported by Deutsche Forschungsgemeinschaft, project no. SCHW883/1-1, the EURATOM FP7 project ERINDA (031207) and the German BMBF project TRAKULA (02NUK13A).
strength function, respectively. An overview of the experimental results and theoretical predictions is presented in Fig. 3. The experimental
[1] G. Bartholomew et al., Advances Nucl. Phys. 7, 229 (1973). [2] D. Brink, PhD thesis, University of Oxford (1955). [3] P. Axel, Phys. Rev. 126, 671 (1962). [4] M. Beard et al., Phys. Rev. C 85, 065808 (2012). [5] G. Schramm et al., Phys. Rev. C 85, 014311 (2012). [6] R. Massarczk et al., Phys. Rev. C 87, 044306 (2013). [7] R. Capote et al., Nucl. Data Sheets 110, 3107 (2009). [8] A.R. Junghans et al., Phys. Lett. B 670, 200 (2008). [9] A.M. Lane, Ann. Phys. (N.Y.) 63, 171 (1971). [10] S. Volz et al., Nucl. Phys. A779, 1 (2006). [11] A. Zilges et al., Phys. Lett. B 542, 43 (2002).
[12] R. Schwengner et al., Phys. Rev. C 87, 024306 (2013). [13] R. Massarczyk et al., Phys. Rev. C 86, 014319 (2013). [14] R. Schwengner et al., Nucl. Instr. Meth. A 555, 211 (2005). [15] H.R. Weller et al., Prog. Part. Nucl. Phys. 62, 257 (2009). [16] G. Rupp et al., Nucl. Instr. Meth. A 608, 152 (2009). [17] S. Agostinelli et al., Nucl. Instr. Meth. A 506, 250 (2003). [18] A. Makinaga et al., Phys. Rev. C 82, 024314 (2010). [19] G. Rusev et al., Phys. Rev. Lett. 110, 022503 (2013). [20] D. Savran et al., Phys. Rev. Lett. 100, 232501 (2008).
319
Nuclear Data Sheets 119 (2014) 317–319 www.elsevier.com/locate/nds
Electromagnetic Dipole Strength in
124,128,134
Xe
R. Massarczyk,1, 2, ∗ R. Schwengner,1 and A.R. Junghans1 1
Helmholtz-Zentrum Dresden-Rossendorf, 01328 Dresden, Germany 2 Technische Universit¨ at Dresden, 01062 Dresden, Germany
The electromagnetic dipole strength in several even nuclei in the chain of Xenon isotopes has been investigated at the bremsstrahlung facility of the ELBE accelerator in Dresden, Germany and at the HIγS facility in Durham, USA. The goal of these measurements is to extend the knowledge about the general behavior of the dipole strength in the energy region below the neutron separation energy under the aspect of neutron excess and nuclear deformation. I.
INTRODUCTION
One of the most fundamental properties in theory as well as from an experimental point of view in a nucleus is the transition probability pi→f between an initial nuclear state i and a final state f . This probability can be determined with the transition width Γif . Therefore one can define pi→f =
Γif , Γtot
(1)
where Γtot represents the sum of widths to all possible final states. The transition width can also be explained in a theoretical picture Γif ∼ |ψi |T |ψf |2 ,
(2)
where it connects the wave function ψ of the states i and f . As shown in Ref. [1] it is possible to express the average transition width at a certain energy Ei XL Γif =
XL (Eγ ) · Eγ2L+1 fif , (Ei )
(3)
where Eγ is the transition energy and (Ei ) the level density at the excitation energy Ei . The superscript XL represents the order transition (L) and distinguishes between magnetic and electric type of strength (X). The strength XL function fif depends only on the transition energy Eγ [2, 3]. If calculations or experiments are performed on a single state, Eq. (3) can be expressed as 2
σγ = 3 (πc) Eγ f E1 (Eγ ) ,
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.08.087 0090-3752/© 2014 Elsevier Inc. All rights reserved.
where σγ is the photo-absorption cross section. Therefore, photo excitation experiments are an excellent tool to determine the strength function and open up the possibility to test theoretical nuclear predictions directly. Intensively studied, (γ,n) reactions have shown the characteristic shape of the Giant Dipole Resonance (GDR) in E1-strength at about 15-20 MeV excitation energies. As shown in recent publications [4–6], an experimentally deduced strength, which differs from standard model descriptions, can have an impact on the (n,γ) cross section and the shape of the photon spectrum following neutron capture. Possible descriptions of the E1 strength are given by the compilation in RIPL3 [7] or the triple lorentzian model (TLO) [8]. Especially in the low energy region below the neutron separation energy Sn these models deviate from each other. Also, extra strength has been found which is often interpreted as an additional resonance, the so called pygmy dipole resonance (PDR) [9–11]. A comparison of experimental results with theoretical calculations [12, 13] has shown that is possible to describe the experimentally deduced cross section. However, more detailed measurements will help to get a deeper understanding of the processes and interactions occurring in the nucleus. A series of measurements [12] have been performed on stable isotopes with neutron number N = 50. The present paper will show results for a series of measurement on 124,128,134 Xe isotopes. In this chain of isotopes, the nuclear deformation is decreasing while the neutron excess is increasing, see also Table I. An additional measurement on 132 Xe is planned to close the gap between the values 1.37 and 1.48 in the N/Z ratio.
(4) II.
EXPERIMENTAL DETAILS
The nuclear resonance fluorescence experiments were performed at two experimental facilities, the
Electromagnetic Dipole Strength . . .
NUCLEAR DATA SHEETS
TABLE I. N/Z ratio of the measured Xenon isotopes.
50
isotope 124 Xe 128 Xe 132 Xe 134 Xe N/Z ratio 1.29 1.37 1.41 1.44
40
R. Massarczyk et al.
124
30
Xe
20
cross section (mb)
10 0 4000 50 40 30
6000
8000
TLO RIPL (2Lorentz) ELBE data
128
10000
Xe
20 10 0 4000
6000
8000
10000
50 134
40
FIG. 1. Measured spectra of filled and empty target experiments. The spectrum of the empty target measurement is normalized to the filled target by the absolute photon flux during the beam time.
Xe
30 20 10 0 4000
bremsstrahlung facility [14] at the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf, Germany, and the HIγS facility [15] at the TUNL nuclear laboratory in Durham, USA. At both experimental sides several high-purity germanium detectors are measuring photons, scattered by highly enriched targets. Several techniques are applied to correct the measured spectra for different effects:
6000
8000
Ex (MeV)
10000
FIG. 2. Final photo-absorption cross section σγ in comparison with predictions from RIPL3 and TLO models.
• To calculate the photo absorption cross section, inelastic scattering events are removed from the measured spectrum and the remaining events are divided by the ground state branching ratio. Both quantities, the inelastic scattering events and the branching ratio, are calculated by a statistical code, γDex [5, 6].
• Empty target measurements are performed to subtract the influence of the steel ball which holds the Xe target. These containers are minimized in terms of the amount of additional material which is put in the beam and optimized to hold high pressures, up 80 bar. The diameter of such a steel ball is 20 mm. A detailed description can be found in Ref. [16]. The background produced by the steel ball can be seen in Fig. 1.
III.
RESULTS AND OUTLOOK
The photo-absorption cross section for the isotopes Xe were deduced from measurements in Dresden. The results are presented in Fig. 2. Small enhancements over the introduced parametrization of the GDR [7, 8] were observed but no large extra pygmy resonance, as for example observed in 139 La [18], was found. Nuclear-resonance fluorescence experiments on 124 Xe and 134 Xe were performed at HIγS. The analysis of these experiments are not yet complete but they can help to estimate the ratio of E1 and M1 dipole strength, as shown in previous experiments [12, 13, 19]. If a reasonable M1strength will be found, Eq. (4) has to be changed. In that case the photo-absorption cross section is a combination of f E1 and f M 1 , the electric and magnetic dipole
• The detector efficiency is determined by building up the setup in real geometry in a GEANT4 [17] simulation.
124,128,134
• The same simulation is used to simulate the detector response for γ-rays impinging on the detector with energies from 0.5 MeV up to 12 MeV. The spectrum is unfolded for detector response. • GEANT4 is also used to simulate the atomic background. These are events registered by the detector which have scattered on the electrons of the nuclei and are not of interest for our results. Details for the GEANT4 simulation can be found in Ref. [13]. 318
Electromagnetic Dipole Strength . . .
NUCLEAR DATA SHEETS
photo-absorption cross sections have been transformed into B(E1) values under the assumption of a minor influence of M 1 strength. As one can see, the trend of data differs from the experimental dataset presented in Ref. [20]. The trend of an increasing B(E1) strength with the N/Z ratio is also found in the theoretical calculations. For comparison an additional datapoint for our measurement on 136 Ba [13] is also plotted in the figure. A reason for the deviation of the data might be the different analysis of the data. We took into account the amount of strength hidden in the continuum of unresolved states. This additional strength can be in the range of 60-70% as shown in previous results [12]. A detailed analysis on our data measured at the HIγS facility will give us more information about the influence of possible M1 strength on total strength and an additional measurement on 132 Xe is planned in order to close the gap in the N/Z region between 128 Xe and 134 Xe.
1.5
Σ B(E1)
2
2
( e fm )
2
1 0.5 0 1.2
1.3
1.4 N/Z
1.5
R. Massarczyk et al.
1.6
FIG. 3. Comparison of summed B(E1) strength between 6 and 8 MeV excitation energy. The data of Xenon-isotopes (black circles) and 136 Ba (black asterisk) are measured at ELBE. The experimental data of other isotopes in the N =82 region (open circles) and a quasiparticle phonon model calculation (black line) are taken from Ref. [20].
Acknowledgements: The authors would like to thank the members of the nuclear physics group in Dresden and Durham as well as the staff of the accelerators located there. This work was supported by Deutsche Forschungsgemeinschaft, project no. SCHW883/1-1, the EURATOM FP7 project ERINDA (031207) and the German BMBF project TRAKULA (02NUK13A).
strength function, respectively. An overview of the experimental results and theoretical predictions is presented in Fig. 3. The experimental
[1] G. Bartholomew et al., Advances Nucl. Phys. 7, 229 (1973). [2] D. Brink, PhD thesis, University of Oxford (1955). [3] P. Axel, Phys. Rev. 126, 671 (1962). [4] M. Beard et al., Phys. Rev. C 85, 065808 (2012). [5] G. Schramm et al., Phys. Rev. C 85, 014311 (2012). [6] R. Massarczk et al., Phys. Rev. C 87, 044306 (2013). [7] R. Capote et al., Nucl. Data Sheets 110, 3107 (2009). [8] A.R. Junghans et al., Phys. Lett. B 670, 200 (2008). [9] A.M. Lane, Ann. Phys. (N.Y.) 63, 171 (1971). [10] S. Volz et al., Nucl. Phys. A779, 1 (2006). [11] A. Zilges et al., Phys. Lett. B 542, 43 (2002).
[12] R. Schwengner et al., Phys. Rev. C 87, 024306 (2013). [13] R. Massarczyk et al., Phys. Rev. C 86, 014319 (2013). [14] R. Schwengner et al., Nucl. Instr. Meth. A 555, 211 (2005). [15] H.R. Weller et al., Prog. Part. Nucl. Phys. 62, 257 (2009). [16] G. Rupp et al., Nucl. Instr. Meth. A 608, 152 (2009). [17] S. Agostinelli et al., Nucl. Instr. Meth. A 506, 250 (2003). [18] A. Makinaga et al., Phys. Rev. C 82, 024314 (2010). [19] G. Rusev et al., Phys. Rev. Lett. 110, 022503 (2013). [20] D. Savran et al., Phys. Rev. Lett. 100, 232501 (2008).
319