The gamma-ray-induced Doppler-broadening method, relying on high-resolution gamma-ray spectroscopy, involves direct measurement of the Doppler broadening of gamma rays emitted when nuclei decay in flight following recoil induced by preceding gamma rays. This method is used to study interatomic collisions at low energy in solids and in this way to probe the repulsive interatomic potential. Line shapes of gamma rays, emitted by the recoiling ${}^{59}\mathrm{Ni}$ isotope after thermal neutron capture in Ni single crystals, were measured and compared to results obtained by molecular dynamics simulations of the slowing down. Several potentials (including different embedded-atom method potentials) are investigated using the observed fine structure of the line shape, which depends on the crystal orientations. From a detailed study of the line shapes measured in two different orientations, a new potential is then derived.
We report on the status of the Laue lens development effort led by UC Berkeley, where a dedicated X-ray beamline and a Laue lens assembly station were built. This allowed the realization of a first lens prototype in June 2012. Based on this achievement, and thanks to a new NASA APRA grant, we are moving forward to enable Laue lenses. Several parallel activities are in progress. Firstly, we are refining the method to glue quickly and accurately crystals on a lens substrate. Secondly, we are conducting a study of high-Z crystals to diffract energies up to 900 keV efficiently. And thirdly, we are exploring new concepts of Si-based lenses that could further improve the focusing capabilities, and thus the sensitivity of Laue lenses.
The binding energies of $^{29}\mathrm{Si}$, $^{33}\mathrm{S}$, and $^{36}\mathrm{Cl}$ have been measured with a relative uncertainty of $<0.59\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ using a flat-crystal spectrometer. The unique features of these measurements are (1) nearly perfect crystals whose lattice spacing is known in meters, (2) a highly precise angle scale that is derived from first principles, and (3) a \ensuremath{\gamma}-ray measurement facility that is coupled to a high-flux reactor with near-core source capability. The binding energy is obtained by measuring all \ensuremath{\gamma}-rays in a cascade scheme connecting the capture and ground states. The measurements require the extension of precision flat-crystal diffraction techniques to the 5- to 6-MeV energy region, a significant precision measurement challenge. The binding energies determined from these \ensuremath{\gamma}-ray measurements are consistent with recent highly accurate atomic-mass measurements within a relative uncertainty of $4.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$. The \ensuremath{\gamma}-ray measurement uncertainties are the dominant contributors to the uncertainty of this consistency test. The measured \ensuremath{\gamma}-ray energies are in agreement with earlier precision \ensuremath{\gamma}-ray measurements.
The evolution of the deformation across the Y isotopic chain, in the vicinity of N = 60 boundary, has been studied using gamma spectroscopy methods. The nuclei of interest have been produced by neutron induced fission of 235 U and 241 Pu targets, during two experimental campaigns named EXILL and FIPPS at the Institute Laue-Langevin in Grenoble. The emitted gamma rays have been collected by HPGe and LaBr 3 detectors providing the identification of the high-spin levels in the 94 Y and 96 Y isotopes up to 6 MeV excitation energy, as well as information about the half-lives of the in-band states in the 98 Y nucleus. The obtained results shed new light on the onset of deformation in the neutron-rich Y isotopes and, in general, in the A ≈ 100 region.
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Next-generation gamma beams beams from laser Compton-backscattering facilities like ELI-NP (Bucharest)] or MEGa-Ray (Livermore) will drastically exceed the photon flux presently available at existing facilities, reaching or even exceeding 10^13 gamma/sec. The beam structure as presently foreseen for MEGa-Ray and ELI-NP builds upon a structure of macro-pulses (~120 Hz) for the electron beam, accelerated with X-band technology at 11.5 GHz, resulting in a micro structure of 87 ps distance between the electron pulses acting as mirrors for a counterpropagating intense laser. In total each 8.3 ms a gamma pulse series with a duration of about 100 ns will impinge on the target, resulting in an instantaneous photon flux of about 10^18 gamma/s, thus introducing major challenges in view of pile-up. Novel gamma optics will be applied to monochromatize the gamma beam to ultimately Delta E/E~10^-6. Thus level-selective spectroscopy of higher multipole excitations will become accessible with good contrast for the first time. Fast responding gamma detectors, e.g. based on advanced scintillator technology (e.g. LaBr3(Ce)) allow for measurements with count rates as high as 10^6-10^7 gamma/s without significant drop of performance. Data handling adapted to the beam conditions could be performed by fast digitizing electronics, able to sample data traces during the micro-pulse duration, while the subsequent macro-pulse gap of ca. 8 ms leaves ample time for data readout. A ball of LaBr3 detectors with digital readout appears to best suited for this novel type of nuclear photonics at ultra-high counting rates.
Delineating the evolution, with proton and neutron numbers, of nuclear phase transitions from spherical to deformed shapes is one of the most challenging tests of nuclear models. This paper presents new data---taken by an international collaboration centered at the Institut Laue-Langevin in Grenoble, France, and analyzed by an international team of researchers led by scientists from TU Darmstadt, Germany---on transition rates in ${}^{148}$Ce, a neutron-rich nucleus located near the iconic $N=88$--90 shape-phase-transition region. In standard geometric models the known experimental data would place ${}^{148}$Ce on the spherical side. However, this paper shows that a more sophisticated analysis that takes into account the finite number of valence nucleons and axial asymmetry places ${}^{148}$Ce just on the deformed side of the transition, thus delimiting for the first time its low-$Z$ edge.
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.
