A new approach to positron angular correlation studies

Three-Dimensional Image Reconstruction with Application in Positron Annihilation

“…Of course, if one performs the reconstruction on the plane, all the curves must be normalised to the same area. However, the "experimental" values of N,(t) ( N r P ( t ) ) are equal to the values of N,(t) (see (8)) only in the interpolation points p = cosy. So, in the case of a bad experimental statistics the area under N6(t) can differ from that for NpP(t).…”

“…Recently, on the base of the idea of Majumdar [7] as well as Howells and Osmon [8] that the expansion of densities into spherical harmonics series allows to determine more terms of the expansion than there are linear projections data, a new method was proposed by Pecora [9] and applied to the experimental data for vanadium [lo, 111. I n this method the reconstruction of e ( p ) in 3D space is performed at the same time (i.e.…”

“…According to our experience, in order to calculate the coefficients ii:, A y should be equal to lo, i.e. 90 values of N,,(t) for each n ought to be used in (8). Then, in the eveluation of en@) (see (6)) the optimal number of coefficients a; becomes equal to 60 (for n = 0 ) and 30 for n + 0.…”

Reconstruction techniques in angular correlation of positron annihilation experiments

“…With this approach one can get more p I m ( p ) than there are data sets. Mijnarends (1967) Hansen (1980 2D-ACPAR Majumdar (1971) Pecora (1987) (Howells and Osmon 1972) On a practical level, one would want to use more than one data set, since information on particular p,,(p) coefficients may not be present or may be less than the noise or error level. This can occur, for example, in data planes that are also high-symmetry planes in the sample, so that the spherical harmonics have nodes, or zeros, in these planes and therefore do not contribute to the data.…”

International Union of Crystallography Commission on Charge, Spin and Momentum Densities. Comparison of experimental methods for the measuring electron momentum distributions

“…To regain some of the time lost in the crossed-slit geometry due to decreased counting rates, several schemes have been proposed which increase, or effectively increase, the number of counters. have set up an apparatus which uses 11 pairs of counters and Howells & Osmon (1972) have attempted to use a spark chamber to obtain a two-dimensional array of detectors. Berko's method is currently producing data whereas Howells & Osmon's method has not proved practical due to the low efficiency of the spark chamber for 7-rays.…”