Speaker
Description
The Brink-Axel (BA) hypothesis states that the transition probability between two groups of states, described by the photon strength function (PSF) for a given multipolarity, only depends on the energy difference between the states and not on their intrinsic properties. As a consequence, the upward (absorption/excitation) and downward (emission/deexcitation) PSF are expected to be the same. For $^{96}Mo$, significant discrepancies were found [1] with various experimental probes between upward (bremsstrahlung/$\left(\gamma,\gamma′\right)$) and downward (Oslo-method/$\left(^3He,^3He'\gamma\right), \left(p, p'\right)$) PSF in the energy region of the Pygmy Dipole Resonance.
A new method [2] allows for the simultaneous model-independent measurement of upward and downward PSF in a single $\left(\overrightarrow{\gamma},\:\gamma'\gamma''\right)$ nuclear resonance fluorescence (NRF) experiment using quasi-monochromatic linearly-polarized $\gamma$-ray beams. To study the discrepancies and BA hypothesis, NRF experiments on $^{96}Mo$ were performed at the High Intensity $\gamma$-ray Source $\left( HI\gamma S\right)$ at energies of 3.9 to 9.25 MeV.
In this contribution, first results will be shown and discussed.
Supported by the State of Hesse, grant “Nuclear Photonics” (LOEWE program), the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 279384907 – SFB 1245, and the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under grant nos. DE-FG02-97ER41041 and DE-FG02-97ER41033.
[1] A. Bracco, E. G. Lanza and A. Tamii, Prog. Part. Nucl. Phys. 106, 360 (2019)
[2] J. Isaak et al., Phys. Lett. B 788, 225 (2019)
