We have directly observed the effects of roton-roton scattering by crossing two beams of ballistic rotons in liquid "^He at a temperature of -0.1 K. One beam is weak and is used as the probe. Its attenuation is measured as it passes through the stronger scattering beam which is predominantly perpendicular to the probe beam. The beams are pulsed and the dispersion of the roton velocities enables the momenta of the rotons to be resolved by time-of-flight measurements. It appears that the scattering is strongly wave-vector dependent.PACS numbers: 67.40.-w, 63.2aHpThe excitation model of liquid "^He envisages a ground state and excitations from this ground state. The excitations, phonons and rotons, are well defined with an inverse lifetime T<^(o, where (o is the energy of the excitation. The excitation picture is well established and the dispersion curve of the excitations has been measured by neutron scattering (see Fig. 1). The lifetime of the excitations is limited by scattering between the excitations and so the inverse lifetime is directly proportional to the prevailing density of excitations. Therefore T increases with temperature as the ambient excitation density increases. Various methods of measuring the lifetimes confirm this. ^"^ However, these measurements can only be made at high temperatures where the lifetime is short enough to be detectable. At low temperatures the lifetimes are orders of magnitude longer and this can only be seen by time-of-flight measurements.^The scattering measurements to date rely on a thermal distribution of excitations, so the excitation lifetimes which are obtained are necessarily averages over the energies of the excitations and the scattering angles. As it is likely that scattering cross sections are dependent on energy and angle, a more refined type of experiment is desirable. In principle one would like to have two well-15 10 v \--
Faecal egg counts (FECs) are commonly promulgated as a means of determining the status and impact of parasitic gastroenteritis (PGE) in cattle, yet there is little scientific evidence for consistent, quantitative associations between FECs, worm burdens or the effects of PGE on the hosts. Examples are provided of applications of FEC in cattle that do have an evidence base and also of alternative aids to anthelmintic treatment decisions, such as growth rates in young cattle. The potential value of FECs in veterinary/farmer interactions is also addressed as is the need to justify diagnostics through demonstration of favourable cost–benefit ratios.
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