Y. H. Huang

Metallic magnetic calorimeters for particle detection

The principles and theory of operation of a magnetic calorimeter, made of a dilute concentration of paramagnetic ions in a metallic host, is discussed in relation to the use of such a device as a detector of x-rays. The response of a calorimeter to the absorption of energy depends upon size, heat capacity, temperature, magnetic field, concentration of spins and interactions among them. The conditions that optimize the performance of a calorimeter are derived. Noise sources, especially that due to thermodynamic fluctuations of the electrons in the metal, are analyzed. Measurements have been made on detectors in which Er serves as the paramagnetic ion and Au as the host metal. The measured resolution of a detector with a heat capacity of 10−12 J/K was 12 eV at 6 keV. In a detector suitable for use with hard x-rays up to 200 keV a resolution of 120 eV was obtained. Calculations indicate that the performance of both detectors can be improved by an order of magnitude. At temperatures below 50 mK, the time response of the Au : Er calorimeters to an energy deposition indicates the presence of an additional heat capacity, which we interpret as arising from the quadruple splitting of the Au nuclei in the electric field gradients introduced by the presence of the Er ions.

Energy deposition by electrons in superfluid helium and design of a detector for solar neutrinos

Abstract We report recent progress in the development of HERON, a detector of low-energy solar neutrinos that will use as a target material 10 tons of liquid helium. The HERON detector is intended to detect p–p neutrinos from the sun in real time with a an energy threshold of approximately 50xa0keV. The recoil energy of electrons resulting from the elastic scattering of neutrinos is measured with calorimetric wafers above the free surface of the liquid at 40xa0mK. Both the EUV scintillation and the helium atoms produced by quantum evaporation, which result from phonos and rotons hitting the free surface, are detected by the wafers. Background rejection is achieved statistically by a determination of event location with respect to the walls of the helium containment. The wafers will be used to form a coded aperture array. If the wafers have an energy threshold such that they can detect single 16xa0eV photons, a neutrino event having a recoil electron with an energy greater than 50xa0keV could be located to within 10xa0cm or better anywhere in the 10xa0tons of helium.

Potential for precision measurement of solar neutrino luminosity by HERON

Abstract 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 7 Be fluxes. It is found that precisions of ±1.68% and ±2.97% for pp and 7 Be 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.

Progress on HERON: A real-time detector for P-P solar neutrinos

The HERON project is an R&D effort to create a detector suitable for real-time, high rate measurement of neutrinos from both the p-p and 7Be reactions in the Sun using superfluid helium as the target medium. Progress on studies of particle detection processes in superfluid, on development of sensors, on backgrounds, and on event energy and position measurement which are related to this goal are discussed.

Measurement of Electron‐Phonon Interactions in a Gold Film on a Quartz Substrate

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 × 109 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.

Charge Transport in Liquid Helium at Low Temperatures

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.

The suitability of sapphire for large-area calorimeters: The transfer of energy to gold films

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.

Magnetic calorimeters for x-ray and gamma-ray detection

Magnetic calorimeters for particle detection are based on the measurement of the change in magnetization of paramagnetic spins upon the deposition of energy. The use SQUID to measure the flux change of ions in a metallic host has been shown to make a fast detector with high energy resolution. Magnetic ions in a metal constitute a well defined thermodynamic system, the properties of which can be calculated with confidence. Such calculations allows one to optimize the parameters of the system, such a size and concentration, to maximize the sensitivity. Magnetic calorimeters appear particularly suited for use in the detection of hard x-rays since the resolution achievable with such devices decreases with increasing heat capacity as only the one third power. Experimental results on magnetic calorimeters are reviewed.