Gamma Strength Functions and the Brink-Axel Hypothesis
157
a counterpart in ground-state absorption experiments on even–even nuclei because
of the pairing gap.
For the low-energy E1 strength in the region of the PDR, the validity of the
BA hypothesis is far from clear when comparing results from the Oslo method with
photoabsorption data. Below particle thresholds most information on the GSF stems
from nuclear resonance fluorescence (NRF) experiments, which suffers from the
problem of unobserved branching ratios to excited states. These can be corrected
in principle by Hauser–Feshbach calculations assuming statistical decay [18]. The
resulting correction factors are sizable and show a strong dependence on the neutron
threshold energy and the g.s. deformation. On the other hand, there are clear
indications of non-statistical decay behavior of the PDR from recent measurements
[19–21]. Violation of the BA hypothesis was also claimed in a simultaneous study
of the (γ , γ ) reaction and average ground-state branching ratios [22] in 142 Nd (see,
however, Ref. [23]). Clearly, information on the GSF in the PDR energy region from
independent experiments is called for.
3 Gamma Strength Function and Level Density from (p,p )
Scattering
A new method for the measurement of complete E1 strength distributions in nuclei
from about 5 to 25 MeV has been developed using relativistic Coulomb excitation
in inelastic proton scattering at beam energies of a few hundred MeV and scattering
angles close to 0 ◦ [24–29]. The experiments also permit extraction of the M1 part
of the GSF due to spinflip excitations [30], which energetically overlaps with the
PDR strength. Furthermore, when performed with good energy resolution, the level
density (LD) can be extracted independently of the GSF in the excitation region of
the IVGDR from the giant resonance fine structure [31]. This allows an important
test of the model-dependent decomposition of LD and GSF in the Oslo method [10].
The case of 208 Pb is used as an example to illustrate the methods [24, 25] and
the comparison to the Oslo data [32]. Details on the experimental techniques can
be found in Ref. [33]. The top part of Fig. 2 shows a spectrum of the 208 Pb(p,p )
reaction in the excitation region E x = 4 − 25 MeV measured with the magnetic
spectrometer placed at 0 ◦ . One observes prominent transitions at low excitation
energies, which can be shown to have E1 character, and a resonance-like structure
around 7 MeV, which contains E1 and M1 parts due to the energetic overlap of the
PDR and the spinflip-M1 resonance. The prominent structure peaking at 13 MeV
represents the IVGDR.
A separation of E1/M1 cross sections and contributions from other multipoles is possible with a multipole decomposition analysis (MDA) of the angular
distributions [25, 26, 29]. The measurement of spin transfer observables with a
polarized beam provides a separation of spinflip and non-spinflip cross sections
[24, 27, 29], which can be related to E1 and M1 components by the different reaction
157
a counterpart in ground-state absorption experiments on even–even nuclei because
of the pairing gap.
For the low-energy E1 strength in the region of the PDR, the validity of the
BA hypothesis is far from clear when comparing results from the Oslo method with
photoabsorption data. Below particle thresholds most information on the GSF stems
from nuclear resonance fluorescence (NRF) experiments, which suffers from the
problem of unobserved branching ratios to excited states. These can be corrected
in principle by Hauser–Feshbach calculations assuming statistical decay [18]. The
resulting correction factors are sizable and show a strong dependence on the neutron
threshold energy and the g.s. deformation. On the other hand, there are clear
indications of non-statistical decay behavior of the PDR from recent measurements
[19–21]. Violation of the BA hypothesis was also claimed in a simultaneous study
of the (γ , γ ) reaction and average ground-state branching ratios [22] in 142 Nd (see,
however, Ref. [23]). Clearly, information on the GSF in the PDR energy region from
independent experiments is called for.
3 Gamma Strength Function and Level Density from (p,p )
Scattering
A new method for the measurement of complete E1 strength distributions in nuclei
from about 5 to 25 MeV has been developed using relativistic Coulomb excitation
in inelastic proton scattering at beam energies of a few hundred MeV and scattering
angles close to 0 ◦ [24–29]. The experiments also permit extraction of the M1 part
of the GSF due to spinflip excitations [30], which energetically overlaps with the
PDR strength. Furthermore, when performed with good energy resolution, the level
density (LD) can be extracted independently of the GSF in the excitation region of
the IVGDR from the giant resonance fine structure [31]. This allows an important
test of the model-dependent decomposition of LD and GSF in the Oslo method [10].
The case of 208 Pb is used as an example to illustrate the methods [24, 25] and
the comparison to the Oslo data [32]. Details on the experimental techniques can
be found in Ref. [33]. The top part of Fig. 2 shows a spectrum of the 208 Pb(p,p )
reaction in the excitation region E x = 4 − 25 MeV measured with the magnetic
spectrometer placed at 0 ◦ . One observes prominent transitions at low excitation
energies, which can be shown to have E1 character, and a resonance-like structure
around 7 MeV, which contains E1 and M1 parts due to the energetic overlap of the
PDR and the spinflip-M1 resonance. The prominent structure peaking at 13 MeV
represents the IVGDR.
A separation of E1/M1 cross sections and contributions from other multipoles is possible with a multipole decomposition analysis (MDA) of the angular
distributions [25, 26, 29]. The measurement of spin transfer observables with a
polarized beam provides a separation of spinflip and non-spinflip cross sections
[24, 27, 29], which can be related to E1 and M1 components by the different reaction