Hysteretic Response of Torsionally Oscillated Solid 4He and Its Dependence on 3He Impurity Concentration

Abstract

Measurements on hysteretic response of compound torsional oscillator containing annular-shaped solid 4 He samples were carried out by varying the oscillator drive amplitude starting from high to low and then back up to the initial high value. Hysteresis in the oscillator frequency and amplitudes were observed only below an onset temperature. The hysteresis onset temperature (T H ) did not depend on the oscillator frequency, width of the sample annulus, annealing and refreezing after melting. A systematic increase in T H was observed as the 3 He impurity concentration in solid 4 He samples was increased. The dependence of T H on 3 He impurity concentration followed approximately that of the dissipation peak temperatures. Possible relationships of the observed hysteresis phenomena with models of solid 4 He dynamics based on freezing of a vortex liquid and dislocation motion are discussed.

High-sensitivity measurement ofHe3−He4isotopic ratios for ultracold neutron experiments

“…The helium current is viewed as a surrogate to helium partial pressure in the source plasma. The asymptotic value of 3 He/ 4 He ≈ 3 × 10 −13 observed at high 4 He source current (for which source background becomes less significant) is consistent with that obtained using the current source configuration as described in Section IV B. The highest ratio data points were obtained with only oxygen flowing into the source.…”

“…That tune configuration was then scaled to the molecular species H 1+ 3 as an additional check and to make the final tune to the beam cup located before the SPS (see Fig. 2 The AMS 3 He/ 4 He ratio is obtained by comparing the rate of detection of 3 He 2+ ions in the spectrograph detector, corrected for detector efficiency and accelerator transmission, to the beam current of 4 He out of the source. The source operation was monitored by measuring the 4 He 1+ current at the ion source Faraday cup shown in Figure 2.…”

“…In addition, such sources are often operated with helium as a support gas. The typical concentration of 3 He in atmospheric 4 He of 1.4 × 10 −6 [19] is to be compared to a sensitivity goal of one part 3 He in 10 15 parts 4 He. This comparison provides a general perspective on controlling helium backgrounds.…”

“…Attempts were made to further reduce the source of helium background in this geometry by replacing the He ratio observed with an ultrapure sample as a function of the 4 He beam current from the source. The helium current is viewed as a surrogate to helium partial pressure in the source plasma.…”

“…The common feature of these experiments is that each requires accurate measurements of the 3 He- 4 He ratio (R 34 ) at levels below that which can be measured using standard mass spectrometry techniques (the typical abundance sensitivity of a commercial mass spectrometer is ≈ 10 −9 ).…”