312 research outputs found
Increase of volume swelling by a temperature gradient
Verstärkung des Porenschwellens durch einen Temperaturgradienten
Der Temperaturgradient in der Hüllwand eines Brennstabs von Schnellen Brutreaktoren führt zu einer Vergrößerung der Hüllaufweitung verglichen mit Materialbestrahlungen.
Untersuchungen an einem dafür speziell gestalteten Brennstab führten zu dem Schluß, daß die Ursache hierzu verstärktes Porenschwellen ist. Dieses wird durch Heliumbläschen herbei geführt, welche im Temperaturgradienten wandern und durch Koaleszenz wachsen können. Somit wird der kritische Porenkeimradius schneller erreicht, als ohne Gradienten. Folglich ergibt sich hier eine größere Hüllaufweitung als man aus Materialbestrahlungen erwarten konnte, beim vorliegenden Material (DIN 1.4981 lg) um etwa 50%
On the Use of Group Theoretical and Graphical Techniques toward the Solution of the General N-body Problem
Group theoretic and graphical techniques are used to derive the N-body wave
function for a system of identical bosons with general interactions through
first-order in a perturbation approach. This method is based on the maximal
symmetry present at lowest order in a perturbation series in inverse spatial
dimensions. The symmetric structure at lowest order has a point group
isomorphic with the S_N group, the symmetric group of N particles, and the
resulting perturbation expansion of the Hamiltonian is order-by-order invariant
under the permutations of the S_N group. This invariance under S_N imposes
severe symmetry requirements on the tensor blocks needed at each order in the
perturbation series. We show here that these blocks can be decomposed into a
basis of binary tensors invariant under S_N. This basis is small (25 terms at
first order in the wave function), independent of N, and is derived using
graphical techniques. This checks the N^6 scaling of these terms at first order
by effectively separating the N scaling problem away from the rest of the
physics. The transformation of each binary tensor to the final normal
coordinate basis requires the derivation of Clebsch-Gordon coefficients of S_N
for arbitrary N. This has been accomplished using the group theory of the
symmetric group. This achievement results in an analytic solution for the wave
function, exact through first order, that scales as N^0, effectively
circumventing intensive numerical work. This solution can be systematically
improved with further analytic work by going to yet higher orders in the
perturbation series.Comment: This paper was submitted to the Journal of Mathematical physics, and
is under revie
Psychological distress in cancer patients assessed with an expert rating scale
The purpose of this study was to investigate psychosocial stress in a large sample of cancer patients using an expert rating scale. Specific aims were to analyse the relevance of setting variables (type of clinic, contact initiative, therapy) and gender. A total of 6365 patients were assessed in 105 institutions. Univariate and multivariate statistical analysis of setting variables indicated that patients treated in palliative care settings showed highest distress scores compared to patients recruited from hospitals and outpatient clinics (P<0.001). Significant differences were also found for contact initiative (P<0.001); lowest distress was found in patients who were recruited by routine contact. Patients who asked for psychosocial support or who were recruited by the medical staff showed the highest distress scores. The analysis of therapy groups showed that patients receiving radiotherapy or surgery were not more distressed than patients without therapy. The most distressing treatment was chemotherapy. Gender had differential effects on clinic type (P<0.001) and contact initiative (P<0.001) but not on treatment and diagnosis. Expert rating scales are an important complement for self-assessment questionnaires to evaluate psychological distress of cancer patients in psychosocial studies as well as in routine medical care
Effect of atomic transfer on the decay of a Bose-Einstein condensate
We present a model describing the decay of a Bose-Einstein condensate, which
assumes the system to remain in thermal equilibrium during the decay. We show
that under this assumption transfer of atoms occurs from the condensate to the
thermal cloud enhancing the condensate decay rate
Pulsed rotating supersonic source used with merged molecular beams
We describe a pulsed rotating supersonic beam source, evolved from an
ancestral device [M. Gupta and D. Herschbach, J. Phys. Chem. A 105, 1626
(2001)]. The beam emerges from a nozzle near the tip of a hollow rotor which
can be spun at high-speed to shift the molecular velocity distribution downward
or upward over a wide range. Here we consider mostly the slowing mode.
Introducing a pulsed gas inlet system, cryocooling, and a shutter gate
eliminated the main handicap of the original device, in which continuous gas
flow imposed high background pressure. The new version provides intense pulses,
of duration 0.1-0.6 ms (depending on rotor speed) and containing ~10^12
molecules at lab speeds as low as 35 m/s and ~ 10^15 molecules at 400 m/s.
Beams of any molecule available as a gas can be slowed (or speeded); e.g., we
have produced slow and fast beams of rare gases, O2, Cl2, NO2, NH3, and SF6.
For collision experiments, the ability to scan the beam speed by merely
adjusting the rotor is especially advantageous when using two merged beams. By
closely matching the beam speeds, very low relative collision energies can be
attained without making either beam very slow.Comment: 26 pages, 10 figure
Slowing and cooling molecules and neutral atoms by time-varying electric field gradients
A method of slowing, accelerating, cooling, and bunching molecules and
neutral atoms using time-varying electric field gradients is demonstrated with
cesium atoms in a fountain. The effects are measured and found to be in
agreement with calculation. Time-varying electric field gradient slowing and
cooling is applicable to atoms that have large dipole polarizabilities,
including atoms that are not amenable to laser slowing and cooling, to Rydberg
atoms, and to molecules, especially polar molecules with large electric dipole
moments. The possible applications of this method include slowing and cooling
thermal beams of atoms and molecules, launching cold atoms from a trap into a
fountain, and measuring atomic dipole polarizabilities.Comment: 13 pages, 10 figures. Scheduled for publication in Nov. 1 Phys. Rev.
Linear Paul trap design for an optical clock with Coulomb crystals
We report on the design of a segmented linear Paul trap for optical clock
applications using trapped ion Coulomb crystals. For an optical clock with an
improved short-term stability and a fractional frequency uncertainty of 10^-18,
we propose 115In+ ions sympathetically cooled by 172Yb+. We discuss the
systematic frequency shifts of such a frequency standard. In particular, we
elaborate on high precision calculations of the electric radiofrequency field
of the ion trap using the finite element method. These calculations are used to
find a scalable design with minimized excess micromotion of the ions at a level
at which the corresponding second- order Doppler shift contributes less than
10^-18 to the relative uncertainty of the frequency standard
Casimir effect for a -dimensional sphere
The Casimir force on a -dimensional sphere due to the confinement of a
massless scalar field is computed as a function of , where is a
continuous variable that ranges from to . The dependence of
the force on the dimension is obtained using a simple and straightforward
Green's function technique. We find that the Casimir force vanishes as ( non-even integer) and also vanishes when is a negative even
integer. The force has simple poles at positive even integer values of .Comment: 22 pages, REVTeX, 4 uuencoded figures, OKHEP-94-0
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