162
P. von Neumann-Cosel
4 Concluding Remarks
The generalized BA hypothesis is a crucial assumption for the application of
statistical nuclear reaction theory with photons in the entrance or exit channel. Of
particular importance is the question whether data from g.s. absorption experiments
represent the GSF in the (quasi)continuum region. While its validity is fairly well
established above neutron threshold in medium-mass and heavy nuclei, the situation
is less clear at lower γ energies when comparing decay and absorption experiments.
There are clear violations like the low-energy enhancement [17] and the larger
scissors mode strength [15] in the decay. For the (PDR + spinflip M1) energy region
there are conflicting results.
The present contribution discusses a new approach to extract the GSF (including
the spin-M1 part) from (p,p ) scattering at energies of a few hundred MeV and at
very forward angles. This method directly measures the g.s. decay width and avoids
the problems of NRF data, where one needs to correct for unknown branching
ratios to excited states. When performed with high energy resolution, such data
not only provide the GSF but also the LD, thus permitting an important test of the
assumptions made in Oslo-type experiments for their decomposition. So far, three
cases have been analyzed. The study of 208 Pb remains inconclusive because the
anomalously low LD leads to large intensity fluctuations [32]. For 96 Mo consistency
within the experimental uncertainties is found [29]. The results in 120 Sn point to a
violation of the BA hypothesis [53]. Clearly, a more systematic study is needed—
e.g. on the role of deformation—and emphasis should be put to establish more cases,
where GSF and LD from Oslo-type and (p,p ) experiments (as well as the LD from
neutron capture) can be compared.
Acknowledgments I thank S. Bassauer for his contribution to the analysis of the present results
and A. Tamii and the collaborators at RCNP for the excellent experiments. This work was funded
by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project number
279384907–SFB 1245.
References
1. M. Arnould, S. Goriely, K. Takahashi, Phys. Rep. 450, 97 (2007)
2. M.B. Chadwick et al., Nucl. Data Sheets 112, 2887 (2011)
3. M. Salvatore, G. Palmiotti, Prog. Part. Nucl. Phys. 66, 144 (2011)
4. M. Wiescher, F. Käppeler, K. Langanke, Annu. Rev. Astron. Astrophys. 50, 165 (2012)
5. D.M. Brink, Ph.D. Thesis, Oxford University (1955)
6. P. Axel, Phys. Rev. 126, 671 (1962)
7. P.F. Bortignon, A.Bracco, R.A. Broglia, Giant Resonances: Nuclear Structure at Finite
Temperature (Harwood Academic, Amsterdam, 1998)
8. C.W. Johnson, Phys. Lett. B 750, 72 (2015)
9. N. Quang Hung, N. Dinh Dang, L.T. Quynh Huong, Phys. Rev. Lett. 118, 022502 (2017)
10. A. Schiller et al., Nucl. Instrum. Methods A 447, 498 (2000)
11. M. Guttormsen et al., Phys. Rev. Lett. 116, 012502 (2016)
P. von Neumann-Cosel
4 Concluding Remarks
The generalized BA hypothesis is a crucial assumption for the application of
statistical nuclear reaction theory with photons in the entrance or exit channel. Of
particular importance is the question whether data from g.s. absorption experiments
represent the GSF in the (quasi)continuum region. While its validity is fairly well
established above neutron threshold in medium-mass and heavy nuclei, the situation
is less clear at lower γ energies when comparing decay and absorption experiments.
There are clear violations like the low-energy enhancement [17] and the larger
scissors mode strength [15] in the decay. For the (PDR + spinflip M1) energy region
there are conflicting results.
The present contribution discusses a new approach to extract the GSF (including
the spin-M1 part) from (p,p ) scattering at energies of a few hundred MeV and at
very forward angles. This method directly measures the g.s. decay width and avoids
the problems of NRF data, where one needs to correct for unknown branching
ratios to excited states. When performed with high energy resolution, such data
not only provide the GSF but also the LD, thus permitting an important test of the
assumptions made in Oslo-type experiments for their decomposition. So far, three
cases have been analyzed. The study of 208 Pb remains inconclusive because the
anomalously low LD leads to large intensity fluctuations [32]. For 96 Mo consistency
within the experimental uncertainties is found [29]. The results in 120 Sn point to a
violation of the BA hypothesis [53]. Clearly, a more systematic study is needed—
e.g. on the role of deformation—and emphasis should be put to establish more cases,
where GSF and LD from Oslo-type and (p,p ) experiments (as well as the LD from
neutron capture) can be compared.
Acknowledgments I thank S. Bassauer for his contribution to the analysis of the present results
and A. Tamii and the collaborators at RCNP for the excellent experiments. This work was funded
by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project number
279384907–SFB 1245.
References
1. M. Arnould, S. Goriely, K. Takahashi, Phys. Rep. 450, 97 (2007)
2. M.B. Chadwick et al., Nucl. Data Sheets 112, 2887 (2011)
3. M. Salvatore, G. Palmiotti, Prog. Part. Nucl. Phys. 66, 144 (2011)
4. M. Wiescher, F. Käppeler, K. Langanke, Annu. Rev. Astron. Astrophys. 50, 165 (2012)
5. D.M. Brink, Ph.D. Thesis, Oxford University (1955)
6. P. Axel, Phys. Rev. 126, 671 (1962)
7. P.F. Bortignon, A.Bracco, R.A. Broglia, Giant Resonances: Nuclear Structure at Finite
Temperature (Harwood Academic, Amsterdam, 1998)
8. C.W. Johnson, Phys. Lett. B 750, 72 (2015)
9. N. Quang Hung, N. Dinh Dang, L.T. Quynh Huong, Phys. Rev. Lett. 118, 022502 (2017)
10. A. Schiller et al., Nucl. Instrum. Methods A 447, 498 (2000)
11. M. Guttormsen et al., Phys. Rev. Lett. 116, 012502 (2016)