58 research outputs found
A laser excitation scheme for Th
Direct laser excitation of the lowest known nuclear excited state in
Th has been a longstanding objective. It is generally assumed that
reaching this goal would require a considerably reduced uncertainty of the
isomer's excitation energy compared to the presently adopted value of eV. Here we present a direct laser excitation scheme for
Th, which circumvents this requirement. The proposed excitation
scheme makes use of already existing laser technology and therefore paves the
way for nuclear laser spectroscopy. In this concept, the recently
experimentally observed internal-conversion decay channel of the isomeric state
is used for probing the isomeric population. A signal-to-background ratio of
better than and a total measurement time of less than three days for
laser scanning appear to be achievable
Extending Our Knowledge about the Th-229 Nuclear Isomer
The first nuclear excited state in Th-229 possesses the lowest excitation energy of all currently known nuclear levels. The energy difference between the ground- and first-excited (isomeric) state (denoted with Th-229m) amounts only to approximate to 8.2 eV (approximate to 151.2 nm), which results in several interesting consequences: Since the excitation energy is in the same energy range as the binding energy of valence electrons, the lifetime of Th-229m is strongly influenced by the electronic structure of the Th atom or ion. Furthermore, it is possible to potentially excite the isomeric state in Th-229 with laser radiation, which led to the proposal of a nuclear clock that could be used to search for new physics beyond the standard model. In this article, we will focus on recent technical developments in our group that will help to better understand the decay mechanisms of Th-229m, focusing primarily on measuring the radiative lifetime of the isomeric state
The theory of direct laser excitation of nuclear transitions
A comprehensive theoretical study of direct laser excitation of a nuclear
state based on the density matrix formalism is presented. The nuclear clock
isomer Th is discussed in detail, as it could allow for direct
laser excitation using existing technology and provides the motivation for this
work. The optical Bloch equations are derived for the simplest case of a pure
nuclear two-level system and for the more complex cases taking into account the
presence of magnetic sub-states, hyperfine-structure and Zeeman splitting in
external fields. Nuclear level splitting for free atoms and ions as well as for
nuclei in a solid-state environment is discussed individually. Based on the
obtained equations, nuclear population transfer in the low-saturation limit is
reviewed. Further, nuclear Rabi oscillations, power broadening and nuclear
two-photon excitation are considered. Finally, the theory is applied to the
special cases of Th and U, being the nuclear
excited states of lowest known excitation energies. The paper aims to be a
didactic review with many calculations given explicitly
Sub-3mm spatial resolution from a large monolithic LaBr3 (Ce) scintillator
Abstract
A Compton camera prototype for ion beam range monitoring via prompt (< 1 ns) gamma detection in hadron therapy is being developed and characterized at the Medical Physics Department of LMU Munich. The system consists of a large (50x50x30 mm3) monolithic LaBr3(Ce) scintillation crystal as absorber component to detect the multi-MeV Compton scattered photons, together with a stack of 6 double-sided silicon strip detectors (DSSSD) acting as scatterer component. Key ingredient of the γ-source reconstruction is the determination of the γ-ray interaction position in the scintillator, which is read out by a 256-fold segmented multi-anode photomultiplier tube (PMT). From simulations an angular resolution of about 1.5o for the photon source reconstruction can be expected for the energy range around 3 – 5 MeV, provided that a spatial resolution of 3 mm can be reached in the absorbing scintillator [1]. Therefore, particular effort was dedicated to characterize this latter property as a function of the γ-ray energy. Intense, tightly collimated 137Cs and 60Co photon sources were used for 2D irradiation scans (step size 0.5 mm) as prerequisite for studying the performance of the "k-Nearest-Neighbors" algorithm developed at TU Delft [2] (together with its variant "Categorical Average Pattern", CAP) and extending its applicability into the energy range beyond the original 511 keV. In this paper we present our most recent interaction position analysis in the absorbing scintillator, leading to a considerably improved value for the spatial resolution: systematic studies were performed as a function of the k-NN parameters and the PMT segmentation. A trend of improving spatial resolution with increasing photon energy was confirmed, resulting in the realization of the presently optimum spatial resolution of 2.9(1) mm @1.3 MeV, thus reaching the design specifications of the Compton camera absorber. The specification goal was reached also for a reduced PMT segmentation of 8x8 anode segments (each with 6x6 mm2 active area), thus allowing to reduce the complexity of the signal processing while preserving the performance
Finalization and performance evaluation of a novel PET detector for an in-beam small animal PET scanner
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