0000000000684571
AUTHOR
M. Valkonen
Corrections for positon annihilation in flight in nuclear spectrometry
Abstract Theories of positon single- and two-quanta annihilation in flight, the Fermi beta-decay theory, and theories of positon energy loss are used in calculations of total probabilities of annihilation in flight of positons in continuous spectra. The results are given in a nomogram useful in correcting for positon annihilation in flight in various nuclear-spectrometry experiments. Confirmation of the theoretical basis employed was obtained by comparing total absolute probabilities for annihilation in flight of 62 Cu positons in Perspex, copper, cadmium and lead, using a new differential method. The agreement with the theory was found to be excellent. A method for obtaining “correct” posi…
The role of target geometry in 14 MeV neutron capture cross-section measurements
Abstract Some activation capture cross sections of 14.5 MeV neutrons have been measured by systematically varying the target geometry. In all cases, the results are found to strongly depend on the geometrical conditions, which implies that a large part of the activation capture cross sections reported earlier are in error. An investigation of a number of materials shows that aluminium gives rise to a fairly small fraction of secondary low-energy neutrons and is, therefore, suitable material for target holders. A thin air-cooled target head specially designed for reliable activation cross-section measurements has been constructed and tested.
A simple two-detector method for precision intercomparisons of source strengths
Abstract A two-detector method for comparing source strengths which largely eliminates errors due to uncertainties of geometric character is described. The simplest version of the method is based on the fact that, when a symmetric face-to-face geometry is being used, the change of the counting rate of one detector caused by the change of the position of the source is approximately compensated for by the change of the counting rate of the other detector. In a more versatile geometry, in which the axes of the detectors are slightly misaligned, the sum and the ratio of the two counting rates can be used in a simple manner to virtually eliminate uncertainties due to the shapes and horizontal po…