Nuclear resonance fluorescence experiments

Introduction:

Nuclear resonance fluorescence (NRF) denotes the process of resonant excitation of definite nuclear states of a target nucleus by absorption of electromagnetic radiation (real photons) and subsequent decay of these levels by re-emission of the equivalent radiation. NRF experiments using bremsstrahlung of relativistic electrons represent an outstanding tool for a precise and systematic investigation of the structure of stable nuclei and for a model-independent determination of lifetimes in the femtosecond (fs)-region. In particular, low-multipolarity (electric and magnetic dipole) transitions can be efficiently studied as a conseqence of the low detection limit of the NRF method. The energies of the excited states, their lifetimes (or - what is equivalent - their line or energetic widths) and their angular momenta provide important information about the fundamental forces between the nuclear constituents. The advantage of this method : Both the excitation as well as de-excitation processes proceed via the electromagnetic interaction - the best understood interaction in physics. In order to efficiently perform such experiments, a high beam intensity of unpolarized as well as linearly polarized bremsstrahlung and excellent background conditions are needed in combination with highly efficient, high-resolution Ge detectors. A typical thin(!)-target bremsstrahlung spectrum generated by the GEANT-? code is shown in (#) fig. 1 (#). This spectrum simulates the experimental conditions very well.

The process of resonant absorption and subsequent re-emission of real photons and the quantities that influence the corresponding photon (#/*) scattering cross sections are illustrated in fig.2 (#) [1,2] (*).

Here, Ji, J and Jf are the spins of the initial, intermediate and final states, respectively. The initial state in NRF experiments corresponds to the ground state [J = J]. The multipolarities Ln (with n = 1,2) refer to the included transitions. The cross section for the absorption and subsequent re-emission of a photon from the ground state with spin and parity Jopi to some excited state Jpi and back to the ground state or low-lying state Jfpi has a resonance shape and is of Doppler-broadened (*) Breit-Wigner type {cf. [3] (*)}. Usually, a continuous bremsstrahlung source is used in NRF experiments. Therefore, the energy integrated differential cross section Is is determined in the experiments. It can be expressed in terms of the ground state (gross_gammao) and final state (gross_gammaf) transition strengths as well as the total transition strength (gross_gamma). Is depends also on the energy of the bremsstrahlung photons and on the angular correlation of the scattered photons with respect to the incoming ones. gross_gamma is connected with the lifetime tau of the excited state : gross_gamma = h_quer / tau eq. (1). Half-lives of several fs can easily be measured with the NRF method. The de-excitation of the excited states can be observed with semiconductor (e.g. CLUSTER and HP Ge) detectors under different angles (e.g. 90 deg and 127 deg in the case of even-even nuclei) to determine the multipolarity of the transitions and, thus, the spins of the corresponding levels. This procedure is based on the different angular distributions of the scattered photons in dependence on their (*) multipolarity {cf. [1] (*)}. A typical NRF spectrum measured for the de-excitation of 88Sr with a EUROBALL cluster detector at an endpoint energy of 6.8 MeV (*) bremsstrahlung at the S-DALINAC accelerator {"[4]" (*)} is (#) displayed in fig. 3 (#).