A significant reduction of absorption for single gamma photons has been experimentally observed by studying Mössbauer spectra of 57 Fe in a FeCO 3 crystal. The experimental results have been explained in terms of a quantum interference effect involving nuclear level anticrossing due to the presence of a combined magnetic dipole and electric quadrupole interaction. [16] in Mössbauer spectra, gamma-microwave double resonance [13,17,18], or gamma-optical double resonance [19]. Recently, interesting proposals have been discussed to obtain lasing for gamma rays by utilizing coherent effects [20][21][22][23]. In this Letter, we report on experiments demonstrating the EIT effect at the singlephoton level via the level (anti)crossing technique. A theory of the one-photon interaction with a nucleus has been developed to describe the experimental results. The obtained results open an interesting perspective to extend coherent effects to nuclear transitions. Figure 1 represents the main results of the paper. It shows the observed Mössbauer spectrum of a single crystal of FeCO 3 at a temperature of 30.5(5) K which corresponds to a magnetic hyperfine field of B 15:1 3 T. At this field the hyperfine levels jm 1=2i and jm ÿ3=2i anticross. For the transitions connected to the anticrossing levels, a deficit of absorption of 25% is observed at the peak velocity. It means that some transparency is induced by interference, similar to EIT observed in optics.The experiments were performed by using a conventional Mössbauer setup. It includes a source of gamma radiation ( 57 CoRh), an absorber of FeCO 3 cleaved on the f1014g faces (optical thickness is of the order 10), and a detector. The absorber was mounted on a target holder which allows for a precise temperature control at the target position in the interval 4-600 K. Besides the magnetic hyperfine field, the Fe 2 nucleus in the FeCO 3 crystal [24,25] is subjected to a large axially symmetric electric field gradient (EFG) which results in a well-resolved quadrupole doublet. The level structures of the source and the absorber are shown in Fig. 2. In a magnetic field, the levels might shift to the position where their energies coincide; this situation is referred to as level crossing. But due to the presence of additional fields, the energies of levels might never be equal, and it is the case of level anticrossing.If the magnetic field is collinear with the EFG axis, the axial symmetry is preserved and the m states are eigenfunctions of the total nuclear Hamiltonian if the z axis is chosen along the symmetry axis. However, in such a mineral containing impurities and defects, one can expect a small distribution of fields which are responsible for the

We report the first g factor measurement on microsecond isomers of neutronrich nuclei produced in projectile-fragmentation reactions at intermediate energies. The nuclides in the vicinity of 68 Ni were produced and spin oriented following the fragmentation of a 76 Ge, 61.4 MeV u −1 beam at GANIL. The LISE spectrometer was used to select the nuclei of interest. The time-dependent perturbed angular distribution (TDPAD) method was applied in combination with the heavy-ion-gamma correlation technique to measure the g factors of 69m Cu (J π = 13/2 + , T 1/2 = 350 ns) and 67m Ni (J π = 9/2 + , T 1/2 = 13.3 µs). Specific details of the experimental technique and the comparison of the results (|g(69m Cu)| = 0.225(25) and |g(67m Ni)| = 0.125(6)) with theoretical calculations are discussed. These results provide another indication of the importance of proton excitations across the Z = 28 shell gap.

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