4 research outputs found
Analytic Evaluation of Four-Particle Integrals with Complex Parameters
The method for analytic evaluation of four-particle integrals, proposed by
Fromm and Hill, is generalized to include complex exponential parameters. An
original procedure of numerical branch tracking for multiple valued functions
is developed. It allows high precision variational solution of the Coulomb
four-body problem in a basis of exponential-trigonometric functions of
interparticle separations. Numerical results demonstrate high efficiency and
versatility of the new method.Comment: 13 pages, 4 figure
SQUID-based microtesla MRI for in vivo relaxometry of the human brain
SQUID-based MRI (magnetic resonance imaging) at microtesla fields has
developed significantly over the past few years. Here we describe application
of this method for magnetic relaxation measurements in the living human brain.
We report values of the longitudinal relaxation time T1 for brain tissues,
measured in vivo for the first time at microtesla fields. The experiments were
performed at 46 microtesla field using a seven-channel SQUID system designed
for microtesla MRI and MEG. Values of T1, measured for different tissues at
this field, are found to be close (within 5%) to the corresponding values of
the transverse relaxation time T2 at the same field. Implications of this
result for imaging contrast in microtesla MRI are discussed.Comment: To appear in Proceedings of 2008 Applied Superconductivity Conferenc
Variational Approximations in a Path-Integral Description of Potential Scattering
Using a recent path integral representation for the T-matrix in
nonrelativistic potential scattering we investigate new variational
approximations in this framework. By means of the Feynman-Jensen variational
principle and the most general ansatz quadratic in the velocity variables --
over which one has to integrate functionally -- we obtain variational equations
which contain classical elements (trajectories) as well as quantum-mechanical
ones (wave spreading).We analyse these equations and solve them numerically by
iteration, a procedure best suited at high energy. The first correction to the
variational result arising from a cumulant expansion is also evaluated.
Comparison is made with exact partial-wave results for scattering from a
Gaussian potential and better agreement is found at large scattering angles
where the standard eikonal-type approximations fail.Comment: 35 pages, 3 figures, 6 tables, Latex with amsmath, amssymb; v2: 28
pages, EPJ style, misprints corrected, note added about correct treatment of
complex Gaussian integrals with the theory of "pencils", matches published
versio