24 research outputs found
Hydrogen-assisted laser-induced resonant transitions between metastable states of antiprotonic helium atoms
Laser resonance transitions between normally metastable states of antiprotonic helium atoms were observed making use of state dependent quenching effects caused by small admixtures of \htwo\ molecules. By selectively shortening the lifetimes of states with higher principal quantum number as compared to those of lower , this method for the first time provides access to all initially populated metastable states of \pbar\hep\ atoms. This was demonstrated by observing the transitions and
Quenching of metastable states of antiprotonic helium atoms by collisions with H molecules
Laser resonance transitions between normally metastable states of antiprotonic helium atoms were induced making use of state dependent quenching effects caused by trace admixtures of \mbox{H}\ to the target helium gas. With this method of ``\mbox{H}-assisted inverse resonances'' the decay rates of the states and of \mbox{}\mbox{He}\ were determined as a function of the \mbox{H}\ admixture. The quenching cross sections at 30~K deduced therefrom for the states with were found to be of the order of the geometrical cross section for \mbox{}\mbox{He}-\mbox{H}\ collisions (~cm), with a moderate decrease with increasing . Within a given cascade with constant , the quenching cross sections for states with are smaller by a factor of 4--6 than those for states with
Accuracy of conventional radiography and computed tomography in predicting implant position in relation to the vertebral canal in dogs.
OBJECTIVE
To compare the accuracy of radiography and computed tomography (CT) in predicting implant position in relation to the vertebral canal in the cervical and thoracolumbar vertebral column.
STUDY DESIGN
In vitro imaging and anatomic study.
ANIMALS
Medium-sized canine cadaver vertebral columns (n=12).
METHODS
Steinmann pins were inserted into cervical and thoracolumbar vertebrae based on established landmarks but without predetermination of vertebral canal violation. Radiographs and CT images were obtained and evaluated by 6 individuals. A random subset of pins was evaluated for ability to distinguish left from right pins on radiographs. The ability to correctly identify vertebral canal penetration for all pins was assessed both on radiographs and CT. Spines were then anatomically prepared and visual examination of pin penetration into the canal served as the gold standard.
RESULTS
Left/right accuracy was 93.1%. Overall sensitivity of radiographs and CT to detect vertebral canal penetration by an implant were significantly different and estimated as 50.7% and 93.4%, respectively (P<.0001). Sensitivity was significantly higher for complete versus partial penetration and for radiologists compared with nonradiologists for both imaging modalities. Overall specificity of radiographs and CT to detect vertebral canal penetration was 82.9% and 86.4%, respectively (P=.049).
CONCLUSIONS
CT was superior to radiographic assessment and is the recommended imaging modality to assess penetration into the vertebral canal.
CLINICAL RELEVANCE
CT is significantly more accurate in identifying vertebral canal violation by Steinmann pins and should be performed postoperatively to assess implant position
Populations and lifetimes in the and 3 metastable cascades of measured by pulsed and continuous antiproton beams
Using the laser spectroscopy, the time evolution of the state population in the v equivalent n-l=2 and 3 metastable cascades of antiprotonic helium atoms were studied. The effects of the collision between antiprotonic helium and the ordinary helium atoms on the atomic cascade were also analyzed. The measurements were done using the pulsed and continuous types of antiproton beams supplied by the Low Energy Antiproton Ring. The studies revealed five phases in the life history of the metastable antiprotonic helium. (Edited abstract) 71 Refs
Collisional quenching of metastable states of antiprotonic helium by hydrogen and deuterium molecules
The quenching of metastable states of antiprotonic helium by collisions with
hydrogen and deuterium molecules was studied. A systematic investigation of the
delayed annihilation time spectra at various and admixture
ratios at the ppm level revealed characteristic changes of their shape, which
indicated a
strong
principal and orbital quantum number dependent
quenching of levels in both cases. Applying a laser
spectroscopy technique to measure the lifetimes of individual states and
cascades we deduced and quenching cross-sections for the states
(n,l)=(39,35) and (37,34). These
cross-sections
establish for molecules
the strong
increase of the quenching efficiency with increasing principal quantum number
n of the state under investigation
previously reported
for the case of
admixtures.
Our experiments indicate that
the low-n state (37,34)
is somewhat less affected by than by , while the high-n state
(39,35) is equally quenched by both isotopes