Study of Reactor-Irradiatedα-Al2O

Hypersonic Absorption in Quartz at Temperatures Below 30°K

“…It is interesting to note that the result just arrived at is essentially the same as calculated for absorption by three-phonon processes, 6,12 in which acoustic phonons are absorbed in collisions with thermal phonons. Such a collision process requires conservation of momentum, such that the momenta of the thermal and acoustic phonon before the collision must be equal to the momentum of the thermal phonon after collision.…”

“…For the measurements on the Z-cut quartz, which is not piezoelectrically active, thin X-cut quartz disks were bonded to the end faces, or magnetic Permalloy films were evaporated on in order to excite sound by the magnetic-film technique. 9 6 The acoustic absorption could then be determined from the heights of the echo pulses. For various reasons, 10 such as misalignment of the two end faces, bulk inhomogeneity of the crystal, and diffraction effects, all of which do not vary with temperature, the echo pattern usually shows a sample-dependent nonexponential decay-even at the lowest temperatures.…”

“…The first, described in Sec. 3A, is the well-known three-phonon process 6,12 which leads to the absorption of slow and, to a lesser degree, fast ultrasonic waves. That it is usually not affected by the dispersion of phonons in crystals is shown in Sec.…”

“…Those fast thermal phonons which are able to a run with the wave," or more precisely, to keep up with the ultrasonic wave fronts, can interact with the sound wave in three-phonon processes, as first described by Landau and Rumer. 6 Consequently, very fast acoustic waves, usually of longitudinal polarization, should not be absorbed by these threephonon processes because few thermal phonons can keep up with them. The manner in which these fast ultrasonic waves are absorbed has seemed to be uncertain and processes of higher order, for example, involving four phonons, have been invoked to explain the absorption.…”

Neutron Scattering at HIFAR—Glimpses of the Past

“…The first scientific publication from a HIFAR neutron beam was from this instrument [7]. That paper in Nature states that the spectrometer was similar to that described by Antal and Goland [8]. The next instrument was the AAEC Crystal Spectrometer on the 4H2 hole, probably installed in late 1960 (It is shown as existing in Drawing AE8526 (8 December 1960)).…”