Gamma Ray Induced Doppler Broadening Research Articles

The negative parity bands in 156Gd

The high flux reactor of the Institut Laue-Langevin is the world most intense neutron source for research. Using the ultra high-resolution crystal spectrometers GAMS installed at the in-pile target position H6/H7 it is possible to measure nuclear state lifetimes using the gamma ray induced Doppler broadening (GRID) technique. In bent crystal mode, the spectrometers allow one to perform spectroscopy with a dynamic range of up to six orders of magnitude. At a very well collimated external neutron beam it is possible to install a highly efficient germanium detector array to obtain coincidences and angular correlations. These techniques were used to study the first two negative parity bands in . These bands are of interest as they seem to show signatures of a tetrahedral symmetry. A surprisingly high B(E2) value of about 1000 W.u. for the transition was discovered. It indicates that the two first negative parity bands cannot be considered to be signature partners.

Application of GRID to Foreign Atom Localization in Single Crystals.

The application of GRID (Gamma Ray Induced Doppler broadening) spectroscopy to the localization of foreign atoms in single crystals is demonstrated on erbium in YAP. By the investigation of the Doppler broadened secondary γ line for two crystalline directions, the Er was determined to be localized on the Y site. Conditions for the nuclear parameters of the impurity atoms used for the application of GRID spectroscopy are discussed.

GRID spectroscopy — a new nuclear method for lattice site localization of foreign atoms

The GRID (Gamma Ray Induced Doppler broadening) spectroscopy has been tested for the first time to investigate the lattice position of Er in yttrium-aluminium-perovskite (YAP). The measured energy distributions of the Doppler-broadened secondary γ-line are clearly different for the two crystalline directions investigated. The best fit of the experimental data is obtained for Er on the Y-site, a life-time of the γ-emitting intermediate level of 32 fs and population of this level from an additional intermediate level (side-feeding). The experimental results confirm the theoretically predicted directional dependence of the Doppler broadening as well as the opportunity to use the method to determine positions of selected foreign atoms in single crystals.

Atomic collision cascades studied with the Crystal-GRID method

Atomic collision cascades within crystals can be produced by recoils induced by γ-emission after thermal neutron capture. Subsequently emitted secondary γ-quanta are Doppler shifted due to the motion of the recoiling atom. We report on the first observation of structure lineshape observed by Gamma-Ray Induced Doppler broadening (GRID) experiments using oriented single crystals. These allow a detailed study of the dynamics of atomic collision cascades and of interatomic potentials in the bulk of materials.

Lifetimes in 15N from gamma-ray line shapes produced in the 2H(14N,p gamma ) and 14N(thermal n, gamma ) reactions.

Mean lifetimes of bound states in [sup 15]N were inferred from the Doppler-shift-attenuation (DSA) of [gamma] rays produced in the inverse reaction [sup 2]H([sup 14]N,[ital p][gamma]) and from the primary gamma-ray-induced Doppler broadening (GRID) of secondary [gamma] rays in the thermal-neutron capture reaction [sup 14]N([ital n],[gamma]). Targets for the DSA measurements were prepared by implanting first neon into a gold backing and then deuterium into the same region such that deuterium was trapped at the neon precipitates. To find out experimentally the initial velocity distributions of the recoiling [sup 15]N nuclei, another target was prepared by implanting deuterium into silicon, which is a slow stopping-power medium. Computer simulations with the Monte Carlo method and experimental stopping powers were used in the DSA analysis of the [gamma]-ray line shapes. This analysis yielded the following lifetimes for eight bound levels in [sup 15]N: 43[plus minus]4 fs (5.30 MeV), [lt]12 fs (6.32 MeV), 11[plus minus]2 fs (7.16 MeV), [lt]3 fs (7.30 MeV), 129[plus minus]6 fs (7.57 MeV), [lt]3 fs (8.31 MeV), [lt]10 fs (8.57 MeV), and [lt]4 fs (9.05 MeV). GRID measurements were made with melamine (C[sub 3]H[sub 6]N[sub 6]) and with silicon nitride (Si[sub 3]N[sub 4]) as both targets and slowing-down media.more » Measurements were also made with air (80% nitrogen). The Doppler-broadened [gamma]-ray line shapes were analyzed by molecular-dynamics simulations of the slowing-down process to obtain the lifetime values of 40[plus minus]3 fs (5.30 MeV), [lt]2 fs (6.32 MeV), [lt]3 fs (7.30 MeV), and [lt]3 fs (8.31 MeV) for levels in [sup 15]N. The extent to which the mirror symmetry of levels in [sup 15]N and [sup 15]O is valid is examined in some detail.« less

Workshop on high resolution gamma ray spectroscopy

Abstract From the October 5–7, 1992 a workshop on “Applications of High Resolution GammaRay Spectroscopy to Atomic Collisions and Nuclear Lifetimes” was held at the Idstitut Laue-Langevin in Grenoble. The workshop was organized in the context of the GRID-method: targets placed in the High Flux Reactor serve as sources of gamma rays following neutron capture reactions. For their analysis they used the instrument GAMS4 at ILL, a double flat crystal spectrometer run as a collaboration between the American NIST, Gaithersburg and the ILL, currently the world's highest resolution gamma ray spectrometer over an energy range 0.5 to 5 MeV. The best resolution obtained to date is close to 3 ppm. Based on this outstanding energy resolution, the so-called GRID-method (Gamma Ray Induced Doppler Broadening) was developed; after capture of a thermal neutron and the subsequent emission of aprimary gamma ray, the nucleus has an initial recoil velocity v where v/c& ≈ 10.5−10−4. The kinetic energy of between 1 and 500 eV is dissipated by interatomic collisions in the solid target within s. If the level populated by the primary transition has a short lifetime of 10-17−10-12 s, then the subsequent secondary gamma ray exhibits a Doppler broadened line profile which, after measurements at high resolution, yields information on the life time of the level and the slowing down process (seealsoNeutronNews2: 1 (1991)20).

The application of the linearized boltzmann transport equation to low energy atomic collisions in solids

A new method has been developed for the solution of the linearized Boltzmann equation far from thermal equilibrium. The solution has been applied to the interpretation of the slowing down of atoms with kinetic energies of a few hundreds of eV as obtained from measured γ-peak profiles, using the gamma ray induced doppler broadening (GRID) technique. From this solution, lifetimes of nuclear excited states can be extracted. These results are compared with the predictions of other models.