D. J. Cousins

An Advanced Dilution Refrigerator Designed for the New Lancaster Microkelvin Facility

We have constructed a large new dilution refrigerator for use with the new Lancaster nuclear cooling facility. The machine is housed in a purpose-rebuilt suite of rooms and has been designed to have a very low base temperature, a very low heat leak environment and to run for long periods between refrigerant refills. The machine has been operated in continuous mode down to ∼1.75 mK and can run for 10 days between refills. Preliminary nuclear cooling experiments suggest that even with an experiment attached the mixing chamber can still run below 2 mK.

A Geometry Dependent Thermal Resistance Between a Saturated Dilute 3He - 4He Solution and Sintered Silver Powder

We present direct measurements of the thermal resistance between a saturated dilute3He-4He solution and sintered silver powder between 5 and 150mK. Measurements obtained using different sinter geometries allow us to distinguish the contribution due to the thermal boundary resistance from that of the size-limited thermal resistance of the3He-4He solution within the pores of the sinter. The thermal boundary resistance per inverse unit volume is found to vary as T−3 and is insensitive to the sinter particle size.

A highly sensitive nuclear recoil detector based on superfluid3He-B

The excitations in superfluid3He have a dispersion curve in which the energy minimum does not coincide with the momentum minimum. As a result, when a mechanical resonator moves through a gas of such excitations, normal and Andreev scattering processes introduce a large asymmetry into the momentum exchange and the mechanical resonator experiences a very large drag force. A gas of such excitations is thus very easy to detect even at very low densities. We have exploited this effect to monitor the increase in excitation density in a small volume caused by a particle interaction. The working volume is filled with superfluid3He-B at around 100 μK. A particle undergoing an interaction in the volume releases a shower of quasiparticle excitations which can be detected by the increase in damping on a vibrating wire resonator. A small hole in the container allows the excitations to leak out into the outside colder liquid to reset the working liquid to the resting state. Using an existing experiment we can detect nuclear recoil interactions depositing energies as low as 500 eV. Two simple modifications should allow us to detect interactions in the 10 eV range.

Andreev reflection of a beam of ballistic quasiparticle excitations incident on a static B-A phase interface in superfluid3He

We have attempted to observe the Andreev reflection of a beam of ballistic quasiparticle excitations as it impinges on a static B-A phase interface. The beam is created, and the Andreev reflected component measured, by the quasiparticle equivalent of a black-body radiator. The A-phase is stabilized at low temperature and pressure by a magnetic field which is generated by a miniature superconducting solenoid, situated next to the radiator orifice and which provides the necessary 0.34 T field in a small, localized region. Preliminary results indicate that we can observe Andreev reflection both from the high-field distorted B-phase energy gap and from the larger equatorial A-phase energy gap.

Texture dependence of the persistent NMR signal in superfluid3He-B

We have studied coherent magnetization precession in superfluid3He with pulsed NMR to temperatures of ≈0.1Tc. Persistent signals were observed with lifetimes up to 60s at a frequency of 0.8MHz. We have found that the behaviour is reproducible for small levels of NMR excitation, whilst becoming irreproducible for higher excitations. We infer from this that textures play an essential role in the formation of persistent signals.

Concentration dependence of the thermal boundary resistance between dilute3He−4He solutions and sintered silver powder

We have measured the3He concentration dependence of the thermal boundary resistanceRB between3He−4He dilute mixtures and submicron sintered silver between 10 and 150 mK. For concentrations greater than one percent, the results for the boundary resistance per inverse volume are insensitive to the concentration and have a magnitude similar to that predicted by the acoustic mismatch theory. For concentrations less than one percent, we observe an increase ofRB for decreasing concentrations.

Andreev reflection from a B-A interface in superfluid3He

Superfluid3He-B has an isotropic energy gap. However, when a magnetic field is applied this gap becomes distorted and at a critical field a transition to the A-phase occurs. In the B-phase a quasiparticle entering a region of increasing field may find that it carries too little energy to continue and is Andreev reflected back along its trajectory. We use a miniature superconducting solenoid to generate a field in a small region. We direct a beam from a Quasiparticle Black Body Radiator into the bore of the solenoid. The Andreev-reflected quasiparticle flux increases the temperature within the radiator. From this we can calculate the maximum gap within the solenoid.

Quasiparticle molasses: the giant force on an object moving through a beam of thermal excitations in superfluid3He

Owing to the unusual dispersion curve for excitations in superfluid3He an object moving through the excitation gas experiences a very much higher drag force than expected for a similar ‘conventional’ gas. We have made investigations of the force on a body moving through a unidirectional beam of thermal excitations in3He-B at very low temperatures which shows that owing to the nature of Andreev reflection by the surrounding flow field the moving body (in this case a vibrating wire resonator) experiences a giant drag forceindependently of the relative direction of motion between object and beam.