1,113 research outputs found
Metastable helium molecules as tracers in superfluid liquid He
Metastable helium molecules generated in a discharge near a sharp tungsten
tip operated in either pulsed mode or continuous field-emission mode in
superfluid liquid He are imaged using a laser-induced-fluorescence
technique. By pulsing the tip, a small cloud of He molecules is
produced. At 2.0 K, the molecules in the liquid follow the motion of the normal
fluid. We can determine the normal-fluid velocity in a heat-induced counterflow
by tracing the position of a single molecule cloud. As we run the tip in
continuous field-emission mode, a normal-fluid jet from the tip is generated
and molecules are entrained in the jet. A focused 910 nm pump laser pulse is
used to drive a small group of molecules to the vibrational state.
Subsequent imaging of the tagged molecules with an expanded 925 nm probe
laser pulse allows us to measure the velocity of the normal fluid. The
techniques we developed demonstrate for the first time the ability to trace the
normal-fluid component in superfluid helium using angstrom-sized particles.Comment: 4 pages, 7 figures. Submitted to Phys. Rev. Let
A Path to the Direct Detection of sub-GeV Dark Matter Using Calorimetric Readout of a Superfluid He Target
A promising technology concept for sub-GeV dark matter detection is
described, in which low-temperature microcalorimeters serve as the sensors and
superfluid He serves as the target material. A superfluid helium target has
several advantageous properties, including a light nuclear mass for better
kinematic matching with light dark matter particles, copious production of
scintillation light, extremely good intrinsic radiopurity, a high impedance to
external vibration noise, and a unique mechanism for observing phonon-like
modes via liberation of He atoms into a vacuum (`quantum evaporation'). In
this concept, both scintillation photons and triplet excimers are detected
using calorimeters, including calorimeters immersed in the superfluid. Kinetic
excitations of the superfluid medium (rotons and phonons) are detected using
quantum evaporation and subsequent atomic adsorption onto a microcalorimeter
suspended in vacuum above the target helium. The energy of adsorption amplifies
the phonon/roton signal before calorimetric sensing, producing a gain mechanism
that can reduce the techonology's recoil energy threshold below the calorimeter
energy threshold. We describe signal production and signal sensing
probabilities, and estimate electron recoil discrimination. We then simulate
radioactive backgrounds from gamma rays and neutrons. Dark matter - nucleon
elastic scattering cross-section sensitivities are projected, demonstrating
that even very small (sub-kg) target masses can probe wide regions of as-yet
untested dark matter parameter space
A 83Krm Source for Use in Low-background Liquid Xenon Time Projection Chambers
We report the testing of a charcoal-based Kr-83m source for use in
calibrating a low background two-phase liquid xenon detector. Kr-83m atoms
produced through the decay of Rb-83 are introduced into a xenon detector by
flowing xenon gas past the Rb-83 source. 9.4 keV and 32.1 keV transitions from
decaying 83Krm nuclei are detected through liquid xenon scintillation and
ionization. The characteristics of the Kr-83m source are analyzed and shown to
be appropriate for a low background liquid xenon detector. Introduction of
Kr-83m allows for quick, periodic calibration of low background noble liquid
detectors at low energy.Comment: Updated to version submitted to JINS
Calibration of liquid argon and neon detectors with
We report results from tests of Kr, as a calibration
source in liquid argon and liquid neon. Kr atoms are
produced in the decay of Rb, and a clear Kr
scintillation peak at 41.5 keV appears in both liquids when filling our
detector through a piece of zeolite coated with Rb. Based on this
scintillation peak, we observe 6.0 photoelectrons/keV in liquid argon with a
resolution of 6% (/E) and 3.0 photoelectrons/keV in liquid neon with a
resolution of 19% (/E). The observed peak intensity subsequently decays
with the Kr half-life after stopping the fill, and we
find evidence that the spatial location of Kr atoms in
the chamber can be resolved. Kr will be a useful
calibration source for liquid argon and neon dark matter and solar neutrino
detectors.Comment: 7 pages, 12 figure
Scintillation and charge extraction from the tracks of energetic electrons in superfluid helium-4
An energetic electron passing through liquid helium causes ionization along
its track. The ionized electrons quickly recombine with the resulting positive
ions, which leads to the production of prompt scintillation light. By applying
appropriate electric fields, some of the ionized electrons can be separated
from their parent ions. The fraction of the ionized electrons extracted in a
given applied field depends on the separation distance between the electrons
and the ions. We report the determination of the mean electron-ion separation
distance for charge pairs produced along the tracks of beta particles in
superfluid helium at 1.5 K by studying the quenching of the scintillation light
under applied electric fields. Knowledge of this mean separation parameter will
aid in the design of particle detectors that use superfluid helium as a target
material.Comment: 10 pages, 8 figure
Calibration of a Liquid Xenon Detector with Kr-83m
We report the preparation of a Kr-83m source and its subsequent use in
calibrating a liquid xenon detector. Kr-83m atoms were produced through the
decay of Rb-83 atoms trapped in zeolite molecular sieve and were then
introduced into liquid xenon. Decaying Kr-83m nuclei were detected through
liquid xenon scintillation. Conversion electrons with energies of 9.4 keV and
32.1 keV from the decay of Kr-83m were both observed. This calibration source
will allow the characterization of the scintillation and ionization response of
noble liquid detectors at low energies, highly valuable for the search for WIMP
dark matter. Kr-83m may also be useful for measuring fluid flow dynamics, both
to understand purification in noble liquid-based particle detectors, as well as
for studies of classical and quantum turbulence in superfluid helium.Comment: Minor edits to match published versio
Decidability of quantified propositional intuitionistic logic and S4 on trees
Quantified propositional intuitionistic logic is obtained from propositional
intuitionistic logic by adding quantifiers \forall p, \exists p over
propositions. In the context of Kripke semantics, a proposition is a subset of
the worlds in a model structure which is upward closed. Kremer (1997) has shown
that the quantified propositional intuitionistic logic H\pi+ based on the class
of all partial orders is recursively isomorphic to full second-order logic. He
raised the question of whether the logic resulting from restriction to trees is
axiomatizable. It is shown that it is, in fact, decidable. The methods used can
also be used to establish the decidability of modal S4 with propositional
quantification on similar types of Kripke structures.Comment: v2, 9 pages, corrections and additions; v1 8 page
Observation of Crossover from Ballistic to Diffusion Regime for Excimer Molecules in Superfluid He
We have measured the temperature dependence of the time of flight of helium
excimer molecules He2* in superfluid 4He and find that the molecules behave
ballistically below 100mK and exhibit Brownian motion above 200 mK. In the
intermediate temperature range the transport cannot be described by either of
the models.Comment: 8 pages, 6 figures, submitted to the Proceedings of the International
Conference on Quantum Fluids and Solids 201
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