Thursday, October 15, 2009, 16:00
WHGA Auditorium
M. Jentschel, ILL Grenoble
Abstract:
Einstein's energy/mass equivalence principle is probably the most popular formula in science.
Although its algebraic simplicity it is rather difficult to be directly experimentally verified.
Thermal neutron capture reactions offer the possibility to access this equation directly. A
neutron with an initial kinetic energy of a few MeV stimulates a nuclear reaction with the release
of several MeV of energy. Due to the low initial energy of the neutron this reaction has a ppb
intrinsic relative energy uncertainty. The energy emission occurs as a sequence of gamma rays and
their energies can be measured and summed up. The difference of masses of the involved particles
and nuclei before and after the reaction describes the mass equivalent to this energy and should
be used for comparison.
The practical realisation of this concept involves on the one hand high precision relative atomic mass measurements. For this one uses Penning traps, which compare the atomic masses of particles and atoms to the mass of 12C. This yields data in atomic mass units. On the other hand the measurement of gamma ray energies is done by crystals diffraction. The obtained wave length data are given in meters. If both experiments are put into relation they need to be linked by a conversion factor. It is given by the expression NAh/c, which contains the molar Planck constant NAh. Values for this constant could be derived from measurements of the fine structure and Rydberg constants and the relative electron mass. In this case the validity of Einsteins E=mc2 would be directly verified.
On the other hand one could assume the validity of the mass/energy equivalence principle. In this case the experiments yields a value of NAh. The determination of NAh via this route is attractive as the gamma ray wave length determination uses heavily experimental techniques that are also used for the determination of the Avogadro constant NA. The measurements could be considered as an extension of the Avogadro project for the determination of h, which is of interest in the context of a new definition of the kilogram mass unit.
The talk will give an overview on the experimental techniques involved in the gamma ray wave length measurements. It will summarize the status of measurements done until now and discuss the limitations of these measurements. An outlook on the efforts necessary for a new series of measurements with the goal of an order of magnitude improvement will be presented.