Chapter 5 Results
5.3 The γ -Strength Function Obtained with the Slope- and Oslo Method
CHAPTER 5. RESULTS 5.3. THEγ-STRENGTH FUNCTION OBTAINED WITH THE SLOPE- AND OSLO METHOD
Figure 5.8: The γ-ray strength function for 150Nd obtained with the Oslo method.
5.3 The γ -Strength Function Obtained with the
Figure 5.9: The144Nd slope method by the FT2 function in MaMa [27] compared to the results from the Oslo method.
Figure 5.10: The 144Nd slope method by the SU function in MaMa [27] compared to the results from the Oslo method.
CHAPTER 5. RESULTS 5.3. THEγ-STRENGTH FUNCTION OBTAINED WITH THE SLOPE- AND OSLO METHOD
Figure 5.11: The 144Nd slope method by the SU function in MaMa [27] compared to the results from the Oslo method with an added 10% uncertainty in the intensity as discussed in 5.1.1.
We see that the slopes of theγ-strength functions are very consistent using the Slope -and Oslo method. The validity of the Slope method is shown to be above 4-5 MeV, below this energy the transitions are no longer statistical but instead get influenced to a greater degree by deterministic transitions to certain structural states the lower one goes inγ-ray energy Eγ. The variance formula ?? has a large uncertainty at low γ-ray energy due to the increase in intensity. The partial derivatives will cause this uncertainty to actu-ally decrease with increasing Eγ. The results support the assumption that the total level density at Sn, for the (p, p0)-reaction in backward direction, can be evaluated from the level spacing parameterD0 according to 4.18. Thus it will be possible to extract normal-ized γ strength functions directly from the standard Oslo method. We assume that this conclusion also holds for the 148,150Nd isotopes.
CHAPTER 6. CONCLUSIONS AND FUTURE WORK
Chapter 6
Conclusions and Future Work
The goal of the present thesis is twofold: (i) To test the assumption of a full spin-population in the (p, p0)144,148,150Nd reaction, and (ii) to investigate the scissors mode as function of deformation.
The spin-population was studied by building further on the work of Wiedeking et al. [1, 2]. The present technique is based on the measured intensities of primaryγ-ray transitions to the ground state and first excited state. Since these two final states have dif-ferent spins (0+ and 2+), it was necessary to introduce a specific spin-distributions. We call this extension to the method for theSlope method. The test is carried out by com-paring the functional form of the γ-ray strength obtained by the Slope- and Oslo method.
The experiments were performed at the Oslo Cyclotron Laboratory using the state-of-the-art OSCAR array which offers high energy resolution, good timing and efficiency.
The result of the comparison of the Slope- and Oslo method shows that the as-sumption of a full spin-population in the (p, p0)144Nd reaction in backwards angles is to a large extend fulfilled. For 144Nd the validity of the Slope method is limited to excitation energies above 4-5 MeV. Below these energies, many transitions connect states with spe-cific nuclear structures and complicate the interpretation of the extracted strength. How-ever, the excitation region from 4-5 MeV and up to the neutron binding energy is large enough to draw firm conclusions. The Slope- and Oslo methods work typically forγ-ray energies between 4-5 MeV up to Sn and between 1 MeV and up toSn −1 MeV, respec-tively, and highlights the complementary nature of these two methods.
Future work would be to delve into the comparison between the Slope- and Oslo method for the 148,150Nd isotopes. Such studies would prove more delicate as the level separation decreases with deformation and mass numberA. However, it is believed that the proof-of-principle-result from144Nd gives validity to the assumption of a full spin-population for 148,150Nd as well. In addition, a future work on evaluating the
uncertain-ties of theγ-ray intensities obtained in the first-generation method by Schiller et al. [49]
would give a more robust relation between the experimental data and the uncertainty of the Slope method.
The extracted γ-ray strength function using the Oslo method shows a small to non-existentM1 scissors resonance at the low energy tail of the GEDR for the near spher-ical and weakly deformed 144Nd and148Nd isotopes, respectively. This was expected for the nearly spherical144Nd isotope, but is rather surprising for the more deformed 148Nd. It is well known [21] that the scissors mode is expected to increase with deformation, which also has been found in an extensive search of the various deformed nuclei at the OCL [53].
This warrants a further discussion and is of great interest. For the150Nd isotope a clear increase at Eγ ∼ 3 MeV is interpreted as the scissors resonance. Extra γ-ray strength at the scissors resonance energies can have an impact on the formation of nuclei in the stellar environment as the strength function is an important quantity in Hauser-Feshbach statisti-cal reaction-rate statisti-calculations.
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