Session 1WB: Workshop: Recent Advances in Electromagnetic Decay Properties of Excited Nuclei

Overview of experimental progress on understanding photon strength functions with an emphasis of the Oslo method

The so-called Photon Strength Functions (PSFs) for different multipolarities are, together with the Nuclear Level Density (NLD) the key entities describing the statistical $\gamma$-decay of nucleus. It is well known that PSFs at energies above the threshold for particle emission are well described by the Lorentzian shape of the Giant Electric Dipole Resonance (GEDR). On the other hand, shapes of RSFs at the low-energy tail of GEDR are known rather poorly. Information on the PSFs at the GEDR tail can be obtained from several different experimental techniques. They will be summarized and the most important ones briefly introduced in this contribution. Special emphasis will be put on the so-called Oslo method which allows simultaneous extraction of the NLD and the PSFs from particle-$\gamma$ coincidence measurements. This method has been used for determining the NLD and the PSFs in many nuclei in $A\approx 45-240$ range during past years. Examples of the most interesting results obtained with this method will be shown. The results will be compared to information on the PSFs available from other experimental techniques. The strengths and the weaknesses of the method will be thoroughly discussed.   

New method to study the photon strength function using the beta-decay of unstable nuclei

The photon strength function is a fundamental property of the atomic nucleus that can be linked with many different areas of nuclear science. In particular, a knowledge of the photon strength function can be applied in statistical-model reaction calculations to constrain neutron capture rates useful for nuclear astrophysics and other applications. A new method has been developed which takes advantage of beta-decay to populate high-energy states in a daughter nucleus. This preparation is combined with a total absorption spectrometer to record the subsequent gamma-ray cascade and the overall technique is the so-called beta-Oslo method. The technique is applicable to very low production rates ($\sim$ 1 pps) and, thus, can be used to look at trends across a wide range of neutron and proton numbers. A description of the technique, and preliminary results on neutron-rich nuclei near Z = 28 and N = 40 will be presented.   

Photon strength functions from photon scattering

We present photon-scattering experiments using bremsstrahlung at the $\gamma $ELBE facility of Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and using quasi-monoenergetic, polarized $\gamma $ rays at the HI$\gamma $S facility of the Triangle Universities Nuclear Laboratory (TUNL) in Durham. In the analysis of the spectra measured by using bremsstrahlung at $\gamma $ELBE, we include intensity in the quasi-continuum and perform simulations of statistical $\gamma $-ray cascades using the code $\gamma $DEX to estimate intensities of inelastic transitions to low-lying excited states. Simulated average branching ratios are compared with model-independent branching ratios obtained from spectra measured by using monoenergetic $\gamma $ beams at HI$\gamma $S. Photoabsorption cross sections deduced in this way are presented for selected nuclides. Strength in the energy region of the so-called pygmy dipole resonance (PDR) is considered in nuclei around mass 80 and in xenon isotopes.\\[4pt] In collaboration with Ralph Massarczyk, Los Alamos National Laboratory.   

Low energy magnetic radiation (LEMAR)

A pronounced spike at low energy in the strength function for magnetic radiation (LEMAR) is found by means of Shell Model calculations, which explains the experimentally observed enhancement of the dipole strength at transition energies around 1 MeV. LEMAR originates from statistical low-energy M1-transitions between many excited complex states. Re-coupling of the proton and neutron high-l orbitals generates the strong magnetic radiation. The observation of LEMAR in various nuclei and its absence in other will be reviewed and possible theoretical explanations for its occurrence and absence presented. LEMAR is predicted for nuclides participating in the r-process, and its impact on element synthesis will be discussed. The statistical analysis of the Shell Model B(M1) values reveals unexpected behavior. An exponential decrease of the strength function, close to a Bose-Einstein distribution and a power law for the size distribution of the B(M1) values are found. which strongly deviate from the ones of the GOE of random matrices, commonly used to represent complex compound states.   

Systematics of photon strength functions

The photon strength of high energy E1 transitions is well described by Brink-Axel theory based on the contribution of the Giant Dipole Resonance. No adequate theory is available for M1 and E2 transitions which do not generally compete strongly with high energy E1 transitions. Measurements with the $^{57}$Fe($^{3}$He,$^{3}$He') reaction at the Oslo cyclotron have revealed that the photon strength below 2 MeV greatly exceeds BA predictions. Similar results have been found for numerous other nuclides. In this paper I will discuss my analysis of the $^{56}$Fe(n,$\gamma)^{57}$Fe reaction which we investigated with both cold neutrons from the Budapest Reactor and thermal neutrons from the Rez Reactor (Prague). A \textgreater 99{\%} complete $^{57}$Fe capture $\gamma $-ray decay scheme containing 449 $\gamma $-rays deexciting 100 levels has been constructed on the basis of $\gamma $-ray singles and $\gamma \gamma $-coincidence data. The photon strengths for 90 primary $\gamma $-rays with energies ranging from 92-7646 keV were calculated and compared with the predictions of Brink-Axel (BA) theory. Excellent agreement has been attained for the high energy transitions while the strength below 2 MeV exceeds BA predictions confirming the earlier Oslo ($^{3}$He,$^{3}$He'$\gamma$) results. Photon strengths for another 95 secondary M1, E1, and E2 $\gamma $-rays were also determined to also exceed BA predictions for transitions below 4 MeV. The dependence of photon strength on level energy and the statistical distribution of photon strengths will also be discussed in this talk.   

Radiative neutron capture reactions to study photon strength functions in connection to reaction theories

Statistical Hauser-Feshbach calculations with the width fluctuation correction for the neutron radiative capture process in the fast energy region are still unsatisfactory, in particular to obtain reasonably accurate capture cross sections. This is mainly due to relatively large uncertainties in the model parameters used; the level density, the parity distribution in the continuum, and the photon strength function. It was showed that the calculated cross section for a deformed system strongly depends on the M1 scissors mode, although the amplitude of collective motion is expected to be small. This is indeed consistent with the fact that a global calculation of neutron capture in the fission product region tends to underestimate the experimental data. We study how the neutron capture process is influenced by the nuclear model parameter inputs. Combining nuclear structure theory with the reaction calculation, we estimate the photon strength, which might be distributed at low excitation energies, by comparing the calculation with experimental data.