Adrian T. Lee

Broadband cryogenic preamplifiers incorporating GaAs MESFETs for use with low-temperature particle detectors

Two voltage‐sensitive preamplifier designs are presented for operation at 1.6 K. Both designs incorporate GaAs MESFETs (Plessy P35‐1101). The first design has two stages including a common‐source gain stage and a source follower stage. The noise performance, particularly with the low‐frequency noise, was found to improve with cooling. The white‐noise level at low temperature is 1 nV/Hz1/2, and the low‐frequency noise corner occurs at approximately 1 MHz. The voltage gain into 50 Ω is 7.0 dB, with the −3‐dB point occuring at 10 MHz. The noise of the first stage was found to dominate the total noise at the output of a low‐noise room‐temperature post amplifier. The output impedance of the preamplifier is 50 Ω. The second design incorporates one cold FET, dissipating 1 mW, in cascode with a room‐temperature bipolar transistor. The noise of this design was found to be approximately equal to that of the two‐cold‐stage design except for a bulge in the voltage noise centered at 13 MHz due to impedance mismatching...

Detection of elementary particles using silicon crystal acoustic detectors with titanium transition edge phonon sensors

We are developing silicon crystal acoustic detectors (SiCADs), which operate at cryogenic temperatures and use thin-films of superconducting titanium (Tc = 435 mK) to sense phonons generated when an incident particle scatters off a nucleus or electron in pure and cold (< 1 K) silicon. Our motivation for developing SiCADs includes their many direct applications to neutrino physics (e.g. to perform neutrino oscillation experiments), particle astrophysics (e.g. to measure the solar neutrino spectrum or search for the hypothetical dark matter in the universe) and solid state physics (e.g. to study phonon dynamics and focusing effects). We have fabricated and characterized multi-channel SiCADs with phonon sensors instrumented on both sides of a Si wafer substrate, and have used these devices to detect radioactive sources of gamma and X-rays, alpha particles and neutrons with incident energies of < 6 keV to 10 MeV. We discuss our results in terms of ballistic and quasi-diffusive phonon propagation, and show evidence for ballistic phonon focusing effects in [100] silicon.

Prompt phonon signals from particle interactions in Si crystals

We discuss the prompt (ballistic and quasidiffuse) phonon physics associated with elementary particle interactions within silicon crystals at temperatures below 1 K, and the differences in the ballistic and quasidiffuse phonon production from primary electron recoils versus primary nuclear recoils within these crystals. We then summarize the results from a growing body of direct experimental evidence on prompt phonon signals from particle detectors bombarded with alphas, x-rays, and neutrons.

Phonon-mediated detection of X-rays in silicon crystals using superconducting transition edge phonon sensors

The authors present data on the operation of thin-film superconducting strips of titanium as phonon sensors on the surface of silicon crystals. The superconducting films are biased at the foot of the resistive transition in temperature and below the critical latching current (the current above which a normal region in the film grows from self-heating). The interaction of an incident X-ray in the Si crystal generates a phonon source which propagates to the surface at the speed of sound. Such an event produces a several-microsecond-long self-terminating voltage pulse which is proportional to the amount of the sensor area driven normal. It is shown that these Ti superconducting transition edge sensors operated at 0.3 K have sufficient resolution for detecting particles with energy deposition above several keV, which makes them good candidates for use in neutrino (and other) experiments. >

Charge-carrier collection by superconducting transition-edge sensors deposited on silicon

Abstract Superconducting transition-edge sensors deposited on high-purity silicon have been found to operate in two distinct “modes”, distinguished by different intrinsic gains. We propose that this gain-shift is due to a change in the prompt collection of energy carried by electrons and holes.

Charge trapping effects in cryogenic particle detectors made using single-crystal semiconductor substrates

We explore charge-trapping effects in cryogenic particle detectors composed of single-crystal silicon substrates with both titanium transition-edge sensors (TES) and charge-collection electrodes deposited upon them. These effects include transients on various time scales which follow the evolution of different kinds of space charge, intrinsic gain and linearity shifts in signals characteristic of changes in the absorption of energy carried by electrons and holes, variations in charge-collection efficiency and ionization resolution, etc., The physics involved, relevant for many other cryogenic, semiconductor-based devices, includes a variety of charge trapping and transport mechanisms.

Advances in Stanford phonon-mediated elementary particle detectors

Abstract We have demonstrated a new phonon sensor design based on aluminum phonon collection pads connected to tungsten transition-edge sensors (TES). The device is patterned onto a silicon crystal and phonons produced by events in the Si are absorbed into the aluminum films where about half of the energy is converted into long-lived quasiparticle excitations. These diffuse until they encounter the lower gap W regions where they deposit their potential energy to the electron system in the W. With the Si crystal at ∼ 40 mK, the W is kept near the center of its resistive transition ( ∼ 90 mK) using voltage bias and Joule self-heating. Current changes induced by particle events are measured with a high-bandwidth SQUID amplifier readout. We have demonstrated an energy resolution of ∼ 360 eV FWHM and a position sensitivity of ∼ 0.2 mm for 55 Fe X-rays. We have also completed a new analysis of the nuclear recoil versus electron recoil ballistic phonon production experiments which used our earlier generation Ti TES on Si crystals. The new results set an upper limit on the distinction between the two phonon spectra and provide a better understanding of the phonon sensor response.

Phonon-mediated detectors for dark matter searches and neutrino experiments

Abstract Our group at Stanford is developing a new class of elementary particles detectors capable of sensing weakly interacting particles such as neutrinos. Our detectors are based on the propagation of phonons in silicon crystals at low temperatures. We have developed superconducting transition edge devices to sense the arrival of phonons at the crystal surfaces. During the past two years, we have characterized titanium transition-edge devices operated around 0.3 K and are beginning tests with tungsten transition-edge devices operated below 100 mK. The titanium devices have been operated in coincidence on both sides of silicon crystals up to 2 mm thick and have been used in calibration experiments with x-rays, alpha particles and neutrons. An underground facility is near completion at Stanford which provides 20 meters water equivalent of cosmic-ray shielding. This facility will house a pilot dark matter search which we will undertake collaboratively through the Center for Particle Astrophysics and the facility also will be used for developing our reactor neutrino detector.

Phonon-mediated particle detection utilizing titanium superconducting transition edge sensors on silicon crystal surfaces

Alpha particle interactions with crystalline silicon at approximately 400 mK, which produce phonons, were observed with titanium transition edge sensors on the crystal surface. A calculation of the expected mean free path suggest that the phonons should arrive diffusively, but a significant component of ballistic phonons which experience few or no scattering are observed. There are two sources of evidence for this. First, timing differences between sensors on opposite faces of the crystal are consistent with ballistic propagation. Second, the phonon pattern on one face of the crystal shows lobing indicative of phonon focusing, anisotropic propagation of phonons due to direction-dependent elasticity in the crystal. This pattern would be unobservable if the propagation was truly diffusive. A comparison is made between Monte Carlo simulations of phonon propagation and experimental results. The sensors consist of a 400-A-thick line of titanium patterned into a meander. The width of the lines is 2 mu m, and the pitch is 5 mu m. The line is held just below the superconducting transition temperature and biased with a current. Phonons from a particle interaction in the crystal drive sections of the line normal, resulting in a resistance. Potential methods of increasing the sensitivity of the sensors include reducing the line width and using a superconductor with a lower T/sub c/.

Observation of alpha-induced ballistic phonon time-of-flight using multi-channel silicon crystal acoustic detectors

Abstract We have developed a technique for producing double-sided multi-channel silicon crystal acoustic detectors (SiCADs). These detectors, which are operated at = 400 mK and use 40 nm thick films of supercond ucting Ti as the phonon sensors, have been used in time-coincidence experiments to study the propagation of alpha-induced ballistic phonons through 1 mm of crystalline Si.