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Dipole-strength distributions up to the particle-separation energies and photodissociation of Mo isotopes
Nuclear Physics A 788 (2007) 331c–336c
Dipole-strength distributions up to the particle-separation energies and photodissociation of Mo isotopes R. Schwengnera , N. Benouareta∗ , R. Beyera , F. D¨onaua , M. Erharda , S. Frauendorfa† , E. Grossea‡ , A.R. Junghansa , J. Kluga , K. Koseva , C. Naira , N. Nankova§ , G. Ruseva , K.D. Schillinga , A. Wagnera a
Institut f¨ ur Strahlenphysik, Forschungszentrum Rossendorf, PF 510119, 01314 Dresden, Germany
Dipole-strength distributions in the nuclides 92 Mo, 98 Mo and 100 Mo have been investigated in photon-scattering experiments with bremsstrahlung at the superconducting electron accelerator ELBE of the Forschungszentrum Rossendorf. A simulation of γ cascades was performed in order to estimate the distribution of inelastic transitions to low-lying states and thus to deduce the primary dipole-strength distribution up to the neutronseparation energies. The absorption cross sections obtained connect smoothly to (γ, n) cross sections and give novel information about the low-energy tail of the Giant Dipole Resonance below the neutron-separation energies. The experimental cross sections are compared with predictions of a Quasiparticle-Random-Phase Approximation (QRPA) in a deformed basis. Photoactivation experiments were performed at various electron energies to study the 92 Mo(γ, n), 92 Mo(γ, p), 92 Mo(γ, α) and 100 Mo(γ, n) reactions. The deduced activation yields are compared with theoretical predictions. 1. Introduction The accurate knowledge of the photoabsorption cross section σγ at excitation energies close to the threshold of the (γ, n) reaction is important for the understanding of astrophysical processes [1] as e.g. for the modelling of the p-process in which 35 neutrondeficient nuclides (p-nuclei) are created that cannot be produced in neutron-capture reactions [2]. The energy region around the neutron threshold belongs to the low-energy tail of the Giant Dipole Resonance (GDR) which contains only a few percent of the total cross section of the GDR. Closely below the neutron threshold σγ can be measured via γ rays emitted after photoexcitation. However, the increasing density of nuclear states towards the threshold leads to a complex deexcitation pattern which includes not only the deexcitations to the ground state but also deexcitations to many intermediate states. Since the assignment of ∗
On leave from Facult´e de Physique, Universit´e des Sciences et de la Technologie d’Alger, 16111 BabEzzouar-Alger, Algerie † Also Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA ‡ Also Institut f¨ ur Kern- und Teilchenphysik, Technische Universit¨ at Dresden, 01062 Dresden, Germany § On leave from Institute for Nuclear Research and Nuclear Energy, BAS, 1784 Sofia, Bulgaria 0375-9474/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2007.01.062
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R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
Figure 1. Spectra of γ-rays scattered from 92 Mo, 98 Mo and 100 Mo, respectively, at an electron energy of Ekin = 13.2 MeV and an angle of 127◦ relative to the beam.
these several hundred transitions to particular levels is not possible, the determination of the absolute dipole-strength distribution in the region of several MeV below the neutron emission threshold requires new techniques of the analysis. The p-nuclei are thought to be produced during supernova explosions through chains of photodissociation reactions like (γ, n), (γ, p) and (γ, α) reactions on heavy seed isotopes. In many network calculations of the p-process nucleosynthesis Mo and Ru isotopes are produced with too small abundances [2]. Therefore, the photodissociation rates in the region of 92 Mo, which are part of the nuclear physics input to the network calculations, are to be deduced precisely. 2. Photon-scattering experiments Photon-scattering experiments on 92 Mo, 98 Mo and 100 Mo were carried out using the bremsstrahlung facility installed at the electron accelerator ELBE of the Forschungszentrum Rossendorf [3]. Bremsstrahlung was produced by irradiating a 6 mg/cm2 thick niobium radiator with a continuous-wave electron beam of a kinetic energy of 13.2 MeV and an average current of 600 μA. Gamma rays scattered from the target were measured with four high-purity germanium detectors of 100% efficiency relative to a 3 × 3 in2 NaI detector, two of them placed at 90◦ and the other two at 127◦ relative to the incident photon beam. All detectors are equipped with escape-suppression shields made of bismuth-germanate scintillation detectors. The three nuclides were studied under identical experimental conditions. Spectra measured during the irradiation of 92 Mo, 98 Mo, and 100 Mo for 57, 64 and 64 hours, respectively, are shown in Fig. 1. These spectra show that (i) in average the intensities of the transitions decrease with increasing neutron number N while the number of transitions increases and (ii) the γ-ray intensities drop at the neutron threshold because the deexcitation via emission of neutrons dominates above the threshold. The high level density and PorterThomas fluctuations of the decay widths have the consequence that many transitions will not be observed as isolated peaks but instead form a continuum. In order to estimate the strength in the continuum we performed GEANT3 simulations of the non-resonant background caused by atomic processes. The simulated background is compared with
R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
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Figure 2. Left panel: Comparison of efficiency- and time-corrected spectra of 98 Mo and 100 Mo with the simulated non-resonant background. Right panel: Averaged reduced decay widths f1 (E) = 1/Δ · Δ Γ0 /E 3 deduced from the isolated peaks and from the spectra after subtraction of the non-resonant background using energy bins of Δ = 200 keV.
the experimental spectra in Fig. 2. The relevant intensity of the scattered photons is then obtained from a subtraction of the non-resonant background from the experimental spectrum. The subtracted spectra contain the ground-state (elastic) transitions and in addition, transitions to lower-lying excited states (inelastic transitions) as well as transitions from these states to the ground state (cascade transitions). The different types of transitions cannot be clearly distinguished. However, for the determination of the absorption cross section only the intensities of the ground-state transitions are needed. This means that contributions of inelastic and cascade transitions have to be removed from the spectra. We corrected the intensity distributions by simulating γ-ray cascades from the levels in the whole energy range analogous to the strategy of the statistical DICEBOX code [4]. Spectra of γ-ray cascades were generated for groups of levels in 100 keV bins. Examples are given in the left panel of Fig. 3. These spectra resemble strongly the ones measured in experiments with tagged photons [5] indicating that the simulated quasimonoenergetic spectra can describe experimental ones. Starting from the high-energy end of the experimental spectrum, which contains ground-state transitions only, the simulated intensities of the ground-state transitions were normalised to the experimental ones in the considered bin and the intensity distribution of the branching transitions was subtracted from the experimental spectrum. Applying this procedure step-by-step for each energy bin moving towards the low-energy end of the spectrum one obtains the intensity distribution of the ground-state transitions. This intensity distribution is compared with the uncorrected continuum in the middle panel of Fig. 3. The right panel of Fig. 3 shows the distribution of the branching ratios B0 deduced simultaneously from the simulations of the γ-ray cascades as the ratios of the intensities of the ground-state transitions to the total intensities of all transitions depopulating particular levels. Dividing the intensities of the ground-state transitions, which are proportional to the elastic scattering cross sections σγγ , by the corresponding branching ratios B0 = Γ0 /Γ we calculated the absorption cross sections σγ = σγγ /B0 . The absorption cross sections obtained for 92 Mo, 98 Mo and 100 Mo before correction and
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R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
Figure 3. Left panel: Simulated intensity distributions of transitions depopulating levels at 4, 6 and 8 MeV, respectively. Middle panel: Continuum spectrum of 98 Mo before (upper spectrum) and after (lower spectrum) correction for branching transitions. Right panel: Distribution of the branching ratios B0 = Γ0 /Γ for ground-state transitions.
after correction with the described procedure [6] are shown in Fig. 4. They are compared with the results of (γ, n) experiments [7]. In the case of 92 Mo the calculated cross sections for the (γ, p) reaction are added [8]. It can be seen that the absorption cross sections obtained from our photon-scattering experiments connect smoothly to the (γ, n) data. Thus, we have identified in our experiments for the first time how the tail of the GDR extends across the threshold region towards low energy. The cumulative integrated cross x sections Σ(Ex ) = E σγ (Ei ) ΔE for the three isotopes are also shown in Fig. 4. One i observes an increase of Σ when going from 92 Mo to 98,100 Mo. In order to investigate the origin of this increase of Σ with the neutron number we compare the experimental results with predictions of a Quasiparticle-Random-Phase-Approximation (QRPA) in a deformed basis.
3. QRPA calculations in a deformed basis The QRPA calculations used a deformed modified oscillator basis, a separable dipoleplus-octupole interaction, and the nuclear selfconsistency approach [9]. The possible contamination of the calculated E1 strength with spurious motion of the centre-of-mass is completely removed by means of the suppression method described in Ref. [10]. The relative strength of the isovector part of the dipole-plus-octupole interaction was adjusted such that it reproduces the position of the maximum of the GDR. Similar QRPA calculations including the deformation parameters used for the Mo isotopes are described in our recent study of the magnetic-dipole (M1) strength in Mo isotopes [11]. The calculated Σ for 92 Mo, 98 Mo, and 100 Mo are compared with the experimental ones in Fig. 4. They reproduce the relative increase of the experimental Σ with increasing N . In the calculations, the enlargement of the Σ with increasing N is caused by the increasing deformation. This has been concluded from a comparison with QRPA calculations in which a spherical shape was taken for the heavy Mo isotopes.
R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
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Figure 4. Left panel: Comparison of the absorption cross sections determined from our photon-scattering experiments [6] and from the measurements of the (γ, n) reaction [7]. Right panel: Cumulative sums of the integrated photoabsorption cross sections for 92 Mo, 98 Mo and 100 Mo obtained from our experiments and the (γ, n) measurements (top) and obtained from QRPA calculations in a deformed basis (bottom). The discrete values calculated in QRPA were folded with a Lorentzian of a width Δ = 0.5 MeV.
4. Photoactivation experiments Reaction rates were determined in the present photoactivation experiments from the intensities of γ transitions following the β decay or electron capture of the nuclei produced in the photodissociation reactions. The photoactivation of 92 Mo and 100 Mo was performed in the following way. A sample of natural Mo was irradiated together with a gold sample in the electron beam dump where the highest photon flux is available. At the same time another gold sample was irradiated together with a 11 B sample at the position of the photon-scattering target. By means of the known integrated cross sections of transitions in 11 B the photon flux at the photon-scattering site was determined. Using the known cross section for the 197 Au(γ, n) reaction and the activities of the gold samples irradiated at the photon-scattering site and in the electron beam dump, respectively, the activation yields for the Mo isotopes were deduced. This procedure is described in detail in Ref. [12]. Fig. 5 shows experimental activation yields for the reactions 100 Mo(γ, n), 92 Mo[(γ, n) + (γ, p)] and 92 Mo(γ, α) relative to the yield of the 197 Au(γ, n) reaction at various electron energies [13]. The experimental values agree roughly with values based on calculated photodissociation cross sections taken from Refs. [8,14].
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R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
Figure 5. Experimental and calculated activation yields for the 100 Mo(γ, n) reaction (triangles), the 92 Mo[(γ, n) + (γ, p)] reactions (boxes) and the 92 Mo(γ, α) reaction (diamonds) relative to the yield of the 197 Au(γ, n) reaction [13]. The solid and dashed lines were obtained by using photodissociation cross sections given in Refs. [8] and [14], respectively.
5. Summary Dipole-strength distributions of even-even Mo isotopes have been studied at the photonscattering facility of the ELBE accelerator. Photoabsorption cross sections were deduced by means of simulations of γ-ray cascades which allowed us to estimate the intensities of branching and feeding transitions in the spectra. The obtained absorption cross sections connect smoothly with the low-energy tails of the Giant Dipole Resonances. QRPA calculations in a deformed basis ascribe the observed increase of the dipole-strength with the neutron number to the increasing deformation. Photoactivation experiments have been carried out to investigate reaction rates of the photodissociation of the p-nucleus 92 Mo. The activation yields obtained for the reactions 92 Mo[(γ, n) + (γ, p)] and 92 Mo(γ, α) are in rough agreement with the values predicted on the basis of calculated photodissociation cross sections. The 92 Mo(γ, α) reaction has been observed for the first time at energies of astrophysical interest. This work was supported by Deutsche Forschungsgemeinschaft under contract DO 466/1-2. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
S. Goriely and E. Khan, Nucl. Phys. A 706 (2002) 217. M. Arnould and S. Goriely, Phys. Rep. 384 (2003) 1. R. Schwengner et al., Nucl. Instr. Meth. A 555 (2005) 211. F. Beˇcv´aˇr, Nucl. Instr. Meth. A 417 (1998) 434. R. Alarcon et al., Phys. Rev. C 36 (1987) 954. G. Rusev et al., Eur. Phys. J. A 27 s01 (2006) 171. H. Beil et al., Nucl. Phys. A 227 (1974) 427. T. Rauscher and F.-K. Thielemann, At. Data Nucl. Data Tables 88 (2004) 1. H. Sakamoto and T. Kishimoto, Nucl. Phys. A 501 (1989) 205. F. D¨onau, Phys. Rev. Lett. 94 (2005) 092503. G. Rusev et al., Phys. Rev. C 73 (2006) 044308. M. Erhard et al., Eur. Phys. J. A 27 s01 (2006) 135. M. Erhard et al., Proceedings of Nuclei in the Cosmos IX, PoS(NIC-IX) 056. A. Koning et al., AIP Conf. Proc. 769 (2005) 177.
Dipole-strength distributions up to the particle-separation energies and photodissociation of Mo isotopes R. Schwengnera , N. Benouareta∗ , R. Beyera , F. D¨onaua , M. Erharda , S. Frauendorfa† , E. Grossea‡ , A.R. Junghansa , J. Kluga , K. Koseva , C. Naira , N. Nankova§ , G. Ruseva , K.D. Schillinga , A. Wagnera a
Institut f¨ ur Strahlenphysik, Forschungszentrum Rossendorf, PF 510119, 01314 Dresden, Germany
Dipole-strength distributions in the nuclides 92 Mo, 98 Mo and 100 Mo have been investigated in photon-scattering experiments with bremsstrahlung at the superconducting electron accelerator ELBE of the Forschungszentrum Rossendorf. A simulation of γ cascades was performed in order to estimate the distribution of inelastic transitions to low-lying states and thus to deduce the primary dipole-strength distribution up to the neutronseparation energies. The absorption cross sections obtained connect smoothly to (γ, n) cross sections and give novel information about the low-energy tail of the Giant Dipole Resonance below the neutron-separation energies. The experimental cross sections are compared with predictions of a Quasiparticle-Random-Phase Approximation (QRPA) in a deformed basis. Photoactivation experiments were performed at various electron energies to study the 92 Mo(γ, n), 92 Mo(γ, p), 92 Mo(γ, α) and 100 Mo(γ, n) reactions. The deduced activation yields are compared with theoretical predictions. 1. Introduction The accurate knowledge of the photoabsorption cross section σγ at excitation energies close to the threshold of the (γ, n) reaction is important for the understanding of astrophysical processes [1] as e.g. for the modelling of the p-process in which 35 neutrondeficient nuclides (p-nuclei) are created that cannot be produced in neutron-capture reactions [2]. The energy region around the neutron threshold belongs to the low-energy tail of the Giant Dipole Resonance (GDR) which contains only a few percent of the total cross section of the GDR. Closely below the neutron threshold σγ can be measured via γ rays emitted after photoexcitation. However, the increasing density of nuclear states towards the threshold leads to a complex deexcitation pattern which includes not only the deexcitations to the ground state but also deexcitations to many intermediate states. Since the assignment of ∗
On leave from Facult´e de Physique, Universit´e des Sciences et de la Technologie d’Alger, 16111 BabEzzouar-Alger, Algerie † Also Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA ‡ Also Institut f¨ ur Kern- und Teilchenphysik, Technische Universit¨ at Dresden, 01062 Dresden, Germany § On leave from Institute for Nuclear Research and Nuclear Energy, BAS, 1784 Sofia, Bulgaria 0375-9474/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2007.01.062
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R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
Figure 1. Spectra of γ-rays scattered from 92 Mo, 98 Mo and 100 Mo, respectively, at an electron energy of Ekin = 13.2 MeV and an angle of 127◦ relative to the beam.
these several hundred transitions to particular levels is not possible, the determination of the absolute dipole-strength distribution in the region of several MeV below the neutron emission threshold requires new techniques of the analysis. The p-nuclei are thought to be produced during supernova explosions through chains of photodissociation reactions like (γ, n), (γ, p) and (γ, α) reactions on heavy seed isotopes. In many network calculations of the p-process nucleosynthesis Mo and Ru isotopes are produced with too small abundances [2]. Therefore, the photodissociation rates in the region of 92 Mo, which are part of the nuclear physics input to the network calculations, are to be deduced precisely. 2. Photon-scattering experiments Photon-scattering experiments on 92 Mo, 98 Mo and 100 Mo were carried out using the bremsstrahlung facility installed at the electron accelerator ELBE of the Forschungszentrum Rossendorf [3]. Bremsstrahlung was produced by irradiating a 6 mg/cm2 thick niobium radiator with a continuous-wave electron beam of a kinetic energy of 13.2 MeV and an average current of 600 μA. Gamma rays scattered from the target were measured with four high-purity germanium detectors of 100% efficiency relative to a 3 × 3 in2 NaI detector, two of them placed at 90◦ and the other two at 127◦ relative to the incident photon beam. All detectors are equipped with escape-suppression shields made of bismuth-germanate scintillation detectors. The three nuclides were studied under identical experimental conditions. Spectra measured during the irradiation of 92 Mo, 98 Mo, and 100 Mo for 57, 64 and 64 hours, respectively, are shown in Fig. 1. These spectra show that (i) in average the intensities of the transitions decrease with increasing neutron number N while the number of transitions increases and (ii) the γ-ray intensities drop at the neutron threshold because the deexcitation via emission of neutrons dominates above the threshold. The high level density and PorterThomas fluctuations of the decay widths have the consequence that many transitions will not be observed as isolated peaks but instead form a continuum. In order to estimate the strength in the continuum we performed GEANT3 simulations of the non-resonant background caused by atomic processes. The simulated background is compared with
R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
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Figure 2. Left panel: Comparison of efficiency- and time-corrected spectra of 98 Mo and 100 Mo with the simulated non-resonant background. Right panel: Averaged reduced decay widths f1 (E) = 1/Δ · Δ Γ0 /E 3 deduced from the isolated peaks and from the spectra after subtraction of the non-resonant background using energy bins of Δ = 200 keV.
the experimental spectra in Fig. 2. The relevant intensity of the scattered photons is then obtained from a subtraction of the non-resonant background from the experimental spectrum. The subtracted spectra contain the ground-state (elastic) transitions and in addition, transitions to lower-lying excited states (inelastic transitions) as well as transitions from these states to the ground state (cascade transitions). The different types of transitions cannot be clearly distinguished. However, for the determination of the absorption cross section only the intensities of the ground-state transitions are needed. This means that contributions of inelastic and cascade transitions have to be removed from the spectra. We corrected the intensity distributions by simulating γ-ray cascades from the levels in the whole energy range analogous to the strategy of the statistical DICEBOX code [4]. Spectra of γ-ray cascades were generated for groups of levels in 100 keV bins. Examples are given in the left panel of Fig. 3. These spectra resemble strongly the ones measured in experiments with tagged photons [5] indicating that the simulated quasimonoenergetic spectra can describe experimental ones. Starting from the high-energy end of the experimental spectrum, which contains ground-state transitions only, the simulated intensities of the ground-state transitions were normalised to the experimental ones in the considered bin and the intensity distribution of the branching transitions was subtracted from the experimental spectrum. Applying this procedure step-by-step for each energy bin moving towards the low-energy end of the spectrum one obtains the intensity distribution of the ground-state transitions. This intensity distribution is compared with the uncorrected continuum in the middle panel of Fig. 3. The right panel of Fig. 3 shows the distribution of the branching ratios B0 deduced simultaneously from the simulations of the γ-ray cascades as the ratios of the intensities of the ground-state transitions to the total intensities of all transitions depopulating particular levels. Dividing the intensities of the ground-state transitions, which are proportional to the elastic scattering cross sections σγγ , by the corresponding branching ratios B0 = Γ0 /Γ we calculated the absorption cross sections σγ = σγγ /B0 . The absorption cross sections obtained for 92 Mo, 98 Mo and 100 Mo before correction and
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R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
Figure 3. Left panel: Simulated intensity distributions of transitions depopulating levels at 4, 6 and 8 MeV, respectively. Middle panel: Continuum spectrum of 98 Mo before (upper spectrum) and after (lower spectrum) correction for branching transitions. Right panel: Distribution of the branching ratios B0 = Γ0 /Γ for ground-state transitions.
after correction with the described procedure [6] are shown in Fig. 4. They are compared with the results of (γ, n) experiments [7]. In the case of 92 Mo the calculated cross sections for the (γ, p) reaction are added [8]. It can be seen that the absorption cross sections obtained from our photon-scattering experiments connect smoothly to the (γ, n) data. Thus, we have identified in our experiments for the first time how the tail of the GDR extends across the threshold region towards low energy. The cumulative integrated cross x sections Σ(Ex ) = E σγ (Ei ) ΔE for the three isotopes are also shown in Fig. 4. One i observes an increase of Σ when going from 92 Mo to 98,100 Mo. In order to investigate the origin of this increase of Σ with the neutron number we compare the experimental results with predictions of a Quasiparticle-Random-Phase-Approximation (QRPA) in a deformed basis.
3. QRPA calculations in a deformed basis The QRPA calculations used a deformed modified oscillator basis, a separable dipoleplus-octupole interaction, and the nuclear selfconsistency approach [9]. The possible contamination of the calculated E1 strength with spurious motion of the centre-of-mass is completely removed by means of the suppression method described in Ref. [10]. The relative strength of the isovector part of the dipole-plus-octupole interaction was adjusted such that it reproduces the position of the maximum of the GDR. Similar QRPA calculations including the deformation parameters used for the Mo isotopes are described in our recent study of the magnetic-dipole (M1) strength in Mo isotopes [11]. The calculated Σ for 92 Mo, 98 Mo, and 100 Mo are compared with the experimental ones in Fig. 4. They reproduce the relative increase of the experimental Σ with increasing N . In the calculations, the enlargement of the Σ with increasing N is caused by the increasing deformation. This has been concluded from a comparison with QRPA calculations in which a spherical shape was taken for the heavy Mo isotopes.
R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
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Figure 4. Left panel: Comparison of the absorption cross sections determined from our photon-scattering experiments [6] and from the measurements of the (γ, n) reaction [7]. Right panel: Cumulative sums of the integrated photoabsorption cross sections for 92 Mo, 98 Mo and 100 Mo obtained from our experiments and the (γ, n) measurements (top) and obtained from QRPA calculations in a deformed basis (bottom). The discrete values calculated in QRPA were folded with a Lorentzian of a width Δ = 0.5 MeV.
4. Photoactivation experiments Reaction rates were determined in the present photoactivation experiments from the intensities of γ transitions following the β decay or electron capture of the nuclei produced in the photodissociation reactions. The photoactivation of 92 Mo and 100 Mo was performed in the following way. A sample of natural Mo was irradiated together with a gold sample in the electron beam dump where the highest photon flux is available. At the same time another gold sample was irradiated together with a 11 B sample at the position of the photon-scattering target. By means of the known integrated cross sections of transitions in 11 B the photon flux at the photon-scattering site was determined. Using the known cross section for the 197 Au(γ, n) reaction and the activities of the gold samples irradiated at the photon-scattering site and in the electron beam dump, respectively, the activation yields for the Mo isotopes were deduced. This procedure is described in detail in Ref. [12]. Fig. 5 shows experimental activation yields for the reactions 100 Mo(γ, n), 92 Mo[(γ, n) + (γ, p)] and 92 Mo(γ, α) relative to the yield of the 197 Au(γ, n) reaction at various electron energies [13]. The experimental values agree roughly with values based on calculated photodissociation cross sections taken from Refs. [8,14].
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R. Schwengner et al. / Nuclear Physics A 788 (2007) 331c–336c
Figure 5. Experimental and calculated activation yields for the 100 Mo(γ, n) reaction (triangles), the 92 Mo[(γ, n) + (γ, p)] reactions (boxes) and the 92 Mo(γ, α) reaction (diamonds) relative to the yield of the 197 Au(γ, n) reaction [13]. The solid and dashed lines were obtained by using photodissociation cross sections given in Refs. [8] and [14], respectively.
5. Summary Dipole-strength distributions of even-even Mo isotopes have been studied at the photonscattering facility of the ELBE accelerator. Photoabsorption cross sections were deduced by means of simulations of γ-ray cascades which allowed us to estimate the intensities of branching and feeding transitions in the spectra. The obtained absorption cross sections connect smoothly with the low-energy tails of the Giant Dipole Resonances. QRPA calculations in a deformed basis ascribe the observed increase of the dipole-strength with the neutron number to the increasing deformation. Photoactivation experiments have been carried out to investigate reaction rates of the photodissociation of the p-nucleus 92 Mo. The activation yields obtained for the reactions 92 Mo[(γ, n) + (γ, p)] and 92 Mo(γ, α) are in rough agreement with the values predicted on the basis of calculated photodissociation cross sections. The 92 Mo(γ, α) reaction has been observed for the first time at energies of astrophysical interest. This work was supported by Deutsche Forschungsgemeinschaft under contract DO 466/1-2. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
S. Goriely and E. Khan, Nucl. Phys. A 706 (2002) 217. M. Arnould and S. Goriely, Phys. Rep. 384 (2003) 1. R. Schwengner et al., Nucl. Instr. Meth. A 555 (2005) 211. F. Beˇcv´aˇr, Nucl. Instr. Meth. A 417 (1998) 434. R. Alarcon et al., Phys. Rev. C 36 (1987) 954. G. Rusev et al., Eur. Phys. J. A 27 s01 (2006) 171. H. Beil et al., Nucl. Phys. A 227 (1974) 427. T. Rauscher and F.-K. Thielemann, At. Data Nucl. Data Tables 88 (2004) 1. H. Sakamoto and T. Kishimoto, Nucl. Phys. A 501 (1989) 205. F. D¨onau, Phys. Rev. Lett. 94 (2005) 092503. G. Rusev et al., Phys. Rev. C 73 (2006) 044308. M. Erhard et al., Eur. Phys. J. A 27 s01 (2006) 135. M. Erhard et al., Proceedings of Nuclei in the Cosmos IX, PoS(NIC-IX) 056. A. Koning et al., AIP Conf. Proc. 769 (2005) 177.