Vacuum Lifetime Research Articles

Positronium reemission yield from mesostructured silica films

The reemission yield of ortho-positronium (o-Ps) into vacuum outside mesoporous silica films on glass is measured in reflection mode with a specially designed lifetime (LT) spectrometer. Values as high as 40% are found. The intensity of the 142ns vacuum LT is recorded as a function of reemission depth. The LT depth profiling is correlated to the 2γ and 3γ energy ones to determine the annihilation characteristics inside the films. Positron lifetime in capped films is used to determine the pore size. For the first time, a set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size, is directly determined in surfactant-templated mesoporous silica films.

Verifying the RTE model: ortho‐positronium lifetime measurement on controlled pore glasses

AbstractIn porous media, the vacuum lifetime of ortho‐positronium (o‐Ps) of τ = 142 ns can be reduced markedly by pick‐off annihilation (interaction with electrons of the host material). So the o‐Ps lifetime is determined by the pore size which can be extracted by utilising approved models like the Tao‐Eldrup model for pore sizes smaller than 1 nm and the Tokyo model or RTE model for larger pore sizes. The RTE model contains an explicit temperature dependence of the o‐Ps lifetime. Experiments on controlled pore glasses (CPG) with different pore sizes (2‐70 nm) at different temperatures (50‐500 K) were performed to verify the RTE model. A general agreement for T = 300 K could be found. The temperature dependence of the lifetime, especially for low temperatures, could not be approved sufficiently. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

EMPIRICAL CONSTRAINTS ON VACUUM DECAY IN THE STRINGY LANDSCAPE

It is generally considered as self evident that the lifetime of our vacuum in the landscape of string theory cannot be much shorter than the current age of the universe. Here I show why this lower limit is invalid. A certain type of "parallel universes" is a necessary consequence of the string-landscape dynamics and might well allow us to "survive" vacuum decay. As a consequence our stringy vacuum's lifetime is empirically unconstrained and could be very short. Based on this counterintuitive insight I propose a novel type of laboratory experiment that searches for an apparent violation of the quantum-mechanical Born rule by gravitational effects on vacuum decay. If the lifetime of our vacuum should turn out to be shorter than 6 ×10-13 seconds such an experiment is sufficiently sensitive to determine its value with state-of-the-art equipment.

What can the quantum liquid say on the brane black hole, the entropy of extremal black hole and the vacuum energy?

Using quantum liquids one can simulate the behavior of the quantum vacuum in the presence of the event horizon. The condensed matter analogs demonstrate that in most cases the quantum vacuum resists to formation of the horizon, and even if the horizon is formed different types of the vacuum instability develop, which are faster than the process of Hawking radiation. Nevertheless, it is possible to create the horizon on the quantum-liquid analog of the brane, where the vacuum life-time is long enough to consider the horizon as the quasistationary object. Using this analogy we calculate the Bekenstein entropy of the nearly extremal and extremal black holes, which comes from the fermionic microstates in the region of the horizon -- the fermion zero modes. We also discuss how the cancellation of the large cosmological constant follows from the thermodynamics of the vacuum.

Correcting lifetime measurements for temperature

Typical pressure sensitive paint (PSP) methods use intensity measurements to predict the pressure. Unfortunately, there are a number of difficulties that arise when using intensity measurements, such as: uneven illumination conditions, photodegradation, inhomogeneous paint coatings, and thickness effects. On the other hand, lifetime measurements are free of these problems. In addition, lifetime methods may be able to eliminate the troublesome `wind-off' condition by using the internal reference τ vac (the vacuum lifetime of the sensor). However, both methods require correction due to the temperature dependence of the paint. We propose a method of correcting lifetime measurements for temperature. A non-oxygen quenched, temperature-dependent phosphor is added to the standard UW paint. The temperature is determined from the lifetime of the phosphor. Then, using the lifetime of PtTFPP and the temperature-selected pressure calibration curve, the pressure can be calculated more accurately.

Seventy years of the Klein paradox

The Klein paradox is examined. Its explanation in terms of electron–positron production is reassessed. It is shown that a potential well or barrier in the Dirac equation can produce positron or electron emission spontaneously if the potential is strong enough. The vacuum charge and lifetime of the well/barrier are calculated. If the well is wide enough, a seemingly constant current is emitted. These phenomena are transient whereas the tunnelling first calculated by Klein is time-independent. Furthermore, tunnelling without exponential suppression occurs when an electron is incident on a high barrier, even when it is not high enough to radiate. Klein tunnelling is therefore a property of relativistic wave equations and not necessarily connected to particle emission. The Coulomb potential is investigated in this context: it is shown that a heavy nucleus of sufficiently large Z will bind positrons. Correspondingly, it is expected that as Z increases the Coulomb barrier will become increasingly transparent to positrons. This is an example of Klein tunnelling.

KLEIN TUNNELLING AND THE KLEIN PARADOX

The Klein paradox is reassessed by considering the properties of a finite square well or barrier in the Dirac equation. It is shown that spontaneous positron emission occurs for a well if the potential is strong enough. The vacuum charge and lifetime of the well are estimated. If the well is wide enough, a seemingly constant current is emitted. These phenomena are transient whereas the tunnelling first calculated by Klein is time-independent. Klein tunnelling is a property of relativistic wave equations, not necessarily connected with particle emission. The Coulomb potential is investigated in this context: it is shown that a heavy nucleus of sufficiently large Z will bind positrons. Correspondingly, it is expected that as Z increases the Coulomb barrier will become increasingly transparent to positrons. This is an example of Klein tunnelling.

The vacuum charge, Klein tunnelling and the Klein paradox

The Klein paradox is reassessed by considering the properties of a finite square well or barrier in the Dirac equation. It is shown that spontaneous positron emission occurs for a well if the potential is strong enough. The vacuum charge and lifetime of the well are estimated. If the well is wide enough, a seemingly constant current is emitted. These phenomena are transient whereas the tunnelling first calculated by Klein is time independent. Klein tunnelling is an intrinsic property of relativistic wave equations. These ideas could in principle be tested in positron-heavy nucleus scattering. It is also possible that they have analogues in superfluid helium.

Vacuum system and lifetime for the LSB storage ring

A detailed design for a synchrotron light source optimised for the necessities of the Spanish scientific community is in progress. An overview of the general features, requirements and problems associated with the vacuum system for this future light source is presented here. For a good performance of the storage ring, a beam-on operating pressure of ca 10 −9 mbar or better is needed. An analysis of the effects of gas desorption due to the emision of synchrotron radiation on the vacuum system as well as its effect on the lifetime of the machine will be described.

Lifetime of metastable magnesium atoms in superfluid helium

The metastable 3s3p3P0, 1, 2 states of the magnesium atom immersed into superfluid helium have been investigated. Absorption-fluorescence measurements were carried out to monitor the population of the3P0,3P1 and3P2 level as a function of time. The population of these levels was found to decrease exponentially with a constant of τ=15±2 ms. This is about three times as long as the vacuum lifetime of the3P1 level. In the resonant excitation band of the 3s3p3P states to the 3s4s3S state a blueshift of 70 nm compared to the emission and a large broadening were detected. The3P2 and3P0 states are not at all metastable any more. Additionally the weak intercombination transition of the3P1 state to the1S0 ground state was investigated by monitoring this emission line as a function of time and of wavelength. The experiments resulted in the same exponential decay time as the excitation measurement. This outcome indicates a rather effective fine structure mixing of the considered Mg states in superfluid helium. Moreover, this raises the question whether common atomic quantum numbers are conserved and the selection rules are still valid.

Plasma diagnostics in a metal vapor vacuum arc ion source (abstract)

We have developed a metal vapor vacuum arc ion source, which was used for film deposition. Based on this source, some experimental diagnostics of plasma parameters are presented in this paper. Characters of vacuum arc cathode spots, including number, size, motion, and lifetime, are studied using a high-speed photograph technique. Substrate bias voltage dependence of plasma microparameters, such as electron density and electron energy, are measured by a Langmuir probe measurement system. In conclusion, some discussion of experimental results are also presented.

Angular correlation studies of positron annihilation in the noble gases

Two-dimensional angular correlation distributions have been measured for the first time for positron and positronium annihilation in low-density helium, neon, argon, krypton and xenon, and in a range of helium-xenon mixtures. The full widths of the (cylindrically averaged) free positron components are in reasonable agreement with theoretical values. The intensity of the mixed-state positronium (Ps) component (mean vacuum lifetime 9.7 ns in a magnetic field of 0.8 T) in helium is consistent with the Ps fraction previously determined in lifetime measurements; the widths of the Ps component in helium, neon and argon are consistent with a Ps-atom scattering cross section below 6.8 eV in all three gases of approximately (9-0.5 E) pi a02, where E is the Ps energy in eV.

The mean lifetime of orthopositronium in vacuum

Various measurements and calculations of the vacuum lifetime of ortho-positronium have given widely divergent values. There are indications that these differences are now gradually being resolved.