Validating resonance properties using nuclear resonance fluorescence

Measurement of a resonance's integrated cross section using nuclear resonance fluorescence can be a valuable tool for verifying the properties of the resonance because of the clear and unambiguous physical connection to the spin, lifetime, and ground state branching ratio of the level. We demonstrate this idea by measuring the integrated cross section of the 3.004-MeV level in ${}^{27}\mathrm{Al}$ to 4% using the monoenergetic $\gamma$-ray beam at the High Intensity $\gamma$-ray Source. That level was the subject of much debate experimentally in the 1960s, especially its spin, and even now only has a current tentative spin assignment of $J=(9/2)$. The consistency check between this integrated cross section and the known properties of the level indicate that one (or more) of the literature properties is incorrect. Based on the range of extent of each property, a reassignment of spin to a tentative $J=(7/2)$ may be warranted, but this would need to be confirmed with other measurements. This result demonstrates the utility of NRF as a way to verify the properties of states in the literature before undertaking more extensive measurements.

Figure 1

The experimental setup (distances are not to scale). The NRF scattering signal was observed using four detectors. The two detectors out of the plane of the image are not shown. The monitor detector observed Compton scattering off of a Cu plate which was used to correct for variations in the beam profile during measurement.

Figure 2

The $\gamma$-ray spectrum of the de-excitation of the 2.982- and 3.004-MeV levels of ${}^{27}\mathrm{Al}$ observed using NRF with the beam profile overlaid.

Figure 3

The measured $\gamma$-ray beam profile with fit overlaid.