Angular Distribution of Energy Flux Radiated from a Pulsed Thermal Source in He II below 0.3°K
The transfer of thermal energy from heated solids to He II has been investigated extensively and the basic features of the experimental data can be accounted for by various phonon coupling models.1 Investigations of the energy flux radiated from pulsed point heaters to low-temperature He II (T 0 < 300 m°K) have produced data which are not entirely consistent with the predictions of these models.2 Therefore a helium blackbody source has been proposed to explain the low-temperature data. According to this model, a layer of hot helium is generated at the heater surface. This helium source, at temperature T s , absorbs the phonon radiation from the solid and reradiates excitations into the ambient bath. A calculation of the special distribution of the flux radiated by such a source,2 assuming long lifetimes of the excitations, indicates that for T s less than 600 m°K the source radiates isotropically a flux of phonons which propagate at the first-sound velocity. For T s in excess of 600 m°K the flux contains additional components which propagate at 131 ± 15 m sec−1. Effects due to the finite lifetime and dispersion of the excitations can, of course, modify this spectral distribution. The following experiment was designed to measure the angular distribution of excitations emitted by two heaters with different surfaces over a wide range of heater power densities to provide further information concerning the flow of energy from heated solids to low-temperature He II.
KeywordsAngular Distribution Energy Flux Detector Angle Thermal Time Constant Energy Flux Radiate
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