The dependence on an applied electric field of the ionization current produced by an energetic electron stopped in liquid helium can be used to determine the spatial distribution of secondary electrons with respect to their geminate partners. An analytic expression relating the current and distribution is derived. The distribution is found to be non-Gaussian with a long tail at larger distances.

Results are presented for a simulation carried out to test the precision with which a detector design (HERON) based on a superfluid helium target material should be able to measure the solar pp and 7Be fluxes. It is found that precisions of ±1.68% and ±2.97% for pp and 7Be fluxes, respectively, should be achievable in a 5-year data sample. The physics motivation to aim for these precisions is outlined as are the detector design, the methods used in the simulation and sensitivity to solar orbit eccentricity.

Electrical breakdown of partially filled helium cells below 0.5 K is shown to be the result of Penning ionization of metastable triplet helium excimers bound to the surface of the liquid.

In an experiment to investigate the possibility of using superfluid helium as a detection medium for low energy solar neutrinos, we have studied the currents produced by a radioactive source in a helium cell having a liquid/vacuum interface at 50 mK. A number of phenomena have been observed that appear not to have been described in the literature. These include the following: 1) The current at very low voltages in a cell having a free surface can be 100 times greater than in a filled cell. This additional current is associated with Penning ionization of metastable triplet dimers in surface states. 2) There is a large amplification of current in modest electric fields with a free surface present in the cell. This is the result of charges accelerated across the vacuum having sufficient energy to produce ionization and additional free charges upon hitting a liquid surface. The amplification becomes sufficiently large that breakdown occurs at potential differences across the vacuum of less than 1000 V. The dependence on 3He concentration of these phenomena has been studied.

We have measured the electron-phonon interaction in a 2000 A thick gold film on a quartz substrate in the temperature range of 30 to 200 mK. The magnetization of two small erbium-doped gold sensors was used to determine the temperature of the electrons in the Au and the phonons in the quartz independently. A value of 3.7 x 10(9) W m(-3) K-5 was obtained for the electron-phonon coupling constant. This result provides further evidence that electron-phonon interactions in thin films at low temperatures are dependent upon the properties of the substrate.

Prototype metallic magnetic calorimeters (MMC), in which the magnetization of Au:Er sensors has been measured using a DC SQUID, have shown excellent energy resolution for soft X-rays. However, the results to date have been obtained with devices assembled by hand, placing a small (∼50μm diameter), thin disk of the Au:Er alloy within the loop of the SQUID. This is an unacceptable method for fabricating large focal-plane arrays of detectors required for the next generation of X-ray telescopes. We are therefore investigating the properties of vapor-deposited films produced by DC magnetron sputtering using an Au:Er alloy target. The magnetic properties of 5μm thick Au:Er films have been investigated from room temperature down to 40mK. The measured magnetization matches theoretical values down to 200mK. The films have the same Er concentration as in the target material. At lower temperatures there is indication of enhanced magnetic interactions among the Er ions. We are studying the dependence of this interaction on the parameters used in the deposition process.

The response of a magnetic calorimeter with a sapphire crystal serving as an X-ray absorber has been studied as a function of temperature. Several different Au films were used to connect thermally the magnetic sensor to the absorber. The amplitude and time dependence of the signal resulting from the absorption of an X-ray were fit using an idealized model for the calorimeter. The values of the various parameters resulting from a fit of the data are internally consistent and provide a physical understanding of the processes determining the performance of the calorimeter. The fraction of the energy of the X-ray that is captured by the film without having first been down-converted to thermal phonons in the sapphire is found to depend on both the area and the thickness of the film. The rate at which the energy is transferred between thermal phonons in the sapphire and the electrons in the film is determined by the electron/phonon interaction in the gold. Also, an additional heat capacity was observed to be present in the sapphire, which, for want of a better means of characterization, is ascribed to the tunneling systems. The magnitude of this additional heat capacity and its thermal coupling to the lattice has been studied.

The HERON project is an effort to develop a detector for low-energy solar neutrinos in real time by observing their elastic scattering from electrons using superfluid helium as the target material. By applying appropriate electric fields, the recoil electron can be separated from the positive ion, drifted upward to the liquid–vacuum interface, transmitted through the surface with the aid of a vortex ring, and detected using a calorimeter. By studying the correlation of the 16eV photon signal produced by scintillation and the single-electron signal, we can locate a neutrino event in a large detector and distinguish it from the background events involving multiple Compton scattering.

The transmission of energy across the interface of a gold film with bulk sapphire has been studied using a metallic magnetic calorimeter. The transfer of energy by both high frequency and thermal phonons is found to be dependent upon the thickness of the gold film. For high frequency phonons the thickness dependence occurs when the size of the attenuation length in gold becomes comparable to the thickness. When the wavelength of the thermal phonons is larger than the thickness of the film, the density of modes of such phonons is altered from that of the bulk and the energy transmission is decreased.