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References:

A listing of relevant literature, including reference links, keywords and abstracts

Diffusion

Modeling the Pump-down of a Reversibly Adsorbed Phase. II. Multilayer Coverage
P.A. Redhead (1995)
In Part I [J. Vac. Sci. Technol. A 13, 467 (1995)] of this article pressure versus time was calculated for the evacuation of a vacuum system with a reversibly adsorbed phase at coverages less than one monolayer; in Part II this analysis is extended to the multilayer region, i.e., from the saturated vapor pressure (ps) to the pressure corresponding to one monolayer (pm) the gas flow is viscous in this range. In this pressure range the ratio of adsorbed to gas phase molecules is small (less than a few percent). As a practical example, the adsorbed phase is taken to be water at about 300 K. A recently proposed empirical adsorption isotherm is used that is a good fit to experiment from ps to pm , unlike existing isotherms that only cover parts of this range. Using this isotherm, the pressure and coverage as a function of time in the pressure range from ps to pm are calculated. It is shown that, for the reference system chosen (vol 17 l , surface area 4.5x103 cm2), as the system is pumped down, the pressure remains constant for a few seconds at a pressure slightly less than ps and then falls to pm in essentially the same way as in the absence of an adsorbed phase; this occurs because the number of molecules in the adsorbed phase is much less than in the gas phase. For systems with the pumping parameter S / A <5x10-3 (S =pumping speed, l s-1, A =area of adsorbing surface, cm2) the decrease in temperature caused by the desorption of multilayers of water is too small to have any significant effect on the desorption rate; for systems with higher values of S/A the temperature drop may be significant.
Keywords: desorption rate

Hydrogen trapping, diffusion and recombination in austentic stainless steel
R.A. Langley (1984)
Trapping, diffusion, and recombination of hydrogen in austentic stainless steel are reviewed. It is suggested that since all of these processes are strongly interdependent and since the measured recombination rates are found to vary four orders of magnitude at any temperature, the data analysis techniques used to date are insufficient. A two-region diffusion model with surface recombination is proposed in which the surface layer is characterized by a smaller diffusion coefficient than the bulk.
Keywords: diffusion coefficient, stainless steel

Ratio of Diffusion Coefficient to Mobility for Electrons in Liquid Argon
E. Shibamura et al. (1979)
The ratio of the diffusion coefficient D to the mobility μ for electrons has been measured in liquid argon by using a modified Townsend apparatus. The measured ratio of eD/ μ monotonically increases from ~0.1 to ~0.4 eV with an increase in the electric field from ~2 to ~10 kV/cm. The ratio eD/ μ is smaller than that in gaseous argon at the same value of E/N, where N is the atomic density. The mean momentum-transfer cross section < σ > is estimated as a function of < ε > on the assumption < ε > = 3eD/ 2μ, where < ε > is the mean electron agitation energy. Thus the < σ > obtained is nearly constant (3~4x10-17 cm2) for < ε > from 0.15 to 0.6 eV. This value of < σ > is almost the same as the estimated from the low-field mobility of electrons in liquid argon. This result is consistent with both Derenzo’s experiment and Lekner’s theory. It is concluded that there is no Ramsauer minimum at about 0.4 eV of < ε >, where the minimum exists in gaseous argon.
Keyword: diffusion coefficient

Electron Drift Lifetime

Improving the Performance of the Liquid Argon TPC by Doping with tetra-methyl-germanium
P. Cennini et al. (1995)
In order to recover the charge lost by electron-ion recombination, we doped pure liquid argon with photosensitive hydrocarbon, tetra-methyl-germanium (TMG), in the 3 ton ICARUS TPC. A charge increase of 25% to 220% was observed for different electric fields and for energy densities ranging from 1.6 to 32 MeV/cm.
Keywords: electron-ion recombination, doped LAr

A Study of Ionization Electrons Drifting Over Large Distances in Liquid Argon
E. Buckley et al. (1989)
We have measured the lifetime of electrons drifting in liquid argon using a gridded ionization chamber with a drift gap of 153 mm. The electrons were produced by the passage of cosmic rays through the chamber. We find a best fit for the electron lifetime τ at 5 V/cm of 9.2 ms (with 13.2 > τ > 7.1 ms at 95% CL). This value is consistent with all the data below 15 V/cm. This corresponds to an impurity concentration ρ = 0.03 ppb oxygen equivalent.The attenuation length calculated from the above lifetime is 18 m for a field of 1 kV/cm. The liquid remained in the chamber for 14 weeks with no recirculation system and suffered no noticeable deterioration with respect to the lifetime or the pulseheight. The low field mobility μ0  has been measured for fields up to 150 V/cm and we find a value of 545 ± 4 cm2V-1s-1 which is consistent with measurements by other authors. The electron yield as a function of the electric field strength has been measured down to 3 V/cm. The results seem to indicate the need for a modification of the geminate recombination theory.
Keywords: ionization, electron drift lifetime, attenuation length

LAr Impurities

A System to Test the Effect of Materials on Electron Drift Lifetime in Liquid Argon and the Effect of Water
R. Andrews et al. (2009)
A materials test system (MTS) has been developed at FNAL to assess the suitability of materials for use in a large liquid argon time projection chamber. During development of the MTS, it was noted that controlling the cryostat pressure with a ‘raining’ condenser reduced the electron drift lifetime in the liquid argon. The effect of condensing has been investigated using a series of passive materials to filter the condensate. We report the results of these studies and of tests on different candidate materials for detector construction. The inferred reduction of electron drift lifetime by water concentrations in the parts per trillion is of particular interest.
Keywords: water, electron drift lifetime, ‘raining’ condenser

Residence Times of Water Molecules on Stainless Steel and Aluminum Surfaces in Vacuum and Atmosphere
R. Dobrozemsky et al. (2007)
The adsorption and desorption phenomena of water molecules on stainless steel and aluminum surfaces have been studied by means of tritium tracer technique (TTT) with special emphasis on potential differences in the residence times in atmosphere and vacuum. Samples with a geometrical surface area of about 2 cm2 have been exposed in an atmospheric environment loaded with a vapor pressure of HTO (tritium-labeled water) of known specific activity, and the adsorbed amount of water has been determined by liquid-scintillation counting technique. In previous studies it was found that, in atmosphere, adsorbed water quantities and their reproducibility depend significantly on the cleanliness of the surfaces. Furthermore, it was observed that desorption into atmosphere (or exchange with atmospheric humidity) is a rather slow process with a few percent of the adsorbed water molecules staying even for several days and with the water coverage versus time t characterized approximately by a 1/tα law with α≈0.4. In this work, TTT measurements of water desorption have been extended into high vacuum. HTO-covered specimens have been transferred into a vacuum chamber soon after HTO exposure and have been kept under high vacuum for a certain vacuum desorption time which was varied from minutes to several days. It was found that water desorption from stainless steel and aluminum surfaces in vacuum is slower than in atmosphere with the residual coverage again described approximately by a 1/tα law but now with α≈0.14 (stainless steel) and α≈0.17 (Al). Water desorption rates calculated from coverage versus time are within the range of outgassing data reported in literature, indicating that the TTT method can be a powerful tool for the characterization of potential new vacuum materials even if just small specimens are available.
Keywords: water, adsorption, desorption

Temperature-programmed desorption (TPD) of water from iron, chromium, nickel and 304L stainless steel#
J.P. Joly et al. (2000)
Water adsorbed at various temperatures in the range 373–873 K on chromium, iron, nickel and stainless-steel plates was studied by temperature-programmed desorption (TPD) under vacuum (10−6 hPa). Three states were distinguished: state I due to weakly chemisorbed water and states II and III probably associated with hydroxyl groups bound to different sites. Water is adsorbed mainly in states I and II at temperatures T0<500 K and in state III at T0>600 K. These states were present in all the metals which were studied except in iron that did not show state I. TPD spectra of water from stainless steel are explained by the presence of Cr and Fe in the form of oxides at the substrate surface. The heat of adsorption in state I and the activation energies of desorption in states II and III were found around 100–120, 170–190 and 220–270 kJ mol−1, respectively.
Keywords: water adsorption, oxides

Water Vapor in Vacuum Systems#
A. Berman (1996)
A major concern for vacuum processing equipment designed to operate in the high and ultrahigh vacuum range, is the outgassing from the walls of the vacuum chamber. Whenever a vacuum system is opened to air, all the internal surfaces will be covered with layer upon layer of water molecules. The thickness of the layer will depend on the relative humidity of the air, the exposure history, etc. After a few hours of pumping down the system, water desorption from the walls will slow down and pressure will remain steady until the last layer of water is depleted. The economical way to reduce the desorption rate and remove large amounts of water vapor, is to use methods for conditioning the construction materials and transfer energy to the adsorbed gases to break their bonds. Traditional vacuum bakeout cannot be used with some applications such as accelerators, or fusion devices. Several other methods such as flushing with cold or hot neutral gas, glow discharge, or either electron, ion, or ultraviolet radiations have been successfully used. The problem of water vapor will be discussed in detail.
Keywords: water

Effect of an Electric Field on Electron Attachment to SF6, N2O, and O2 in Liquid Argon and Xenon
G. Bakale et al. (1976)
The rate constant for electron attachment to SF6, N2O, and O2 in liquid argon and xenon was studied as a function of an external electric field up to 105 V cm-1. While k(e-+S F6) and k(e-+ O2) decreased with increasing field strength, k(e-+ N2O) exhibited a nearly 100 fold increase. The field effect on the rate constant in the liquified rare gases is related to the increase of the mean electron energy by the external field. By means of the Cohen-Lekner theory electron energy distribution functions were obtained and a rough estimate of the energy dependence of the attachment cross section could be deduced.
Keyword: electron attachment

LAr Purity Monitors

Analysis of Liquid Argon Purity in the ICARUS T600 TPC
S. Amoruso et al. (2004)
The results reported in this paper are based on the analysis of the data recorded with the first half-module of the ICARUS T600 liquid argon Time Projection Chamber (LAr TPC), during a technical run that took place on surface in Pavia (Italy). We include results from the linearity, uniformity and calibration of the electronics, measurements on the electron drift velocity in LAr at different electric fields, as well as the LAr purity achievement of the detector. Two complementary techniques were used to measure the drift electron lifetime inside the active volume: the first, from the data of a purity monitor, gives a measurement localized in space; the second, based on the study of the signals produced by long minimum ionizing tracks crossing the detector, provides a LAr volume averaged value. Both methods yield consistent results over the whole data taking period and are compatible with an uniform LAr purity over the whole volume. The maximal drift electron lifetime value was recorded before the run stop and was about 1:8 ms: From an interpretation of the observed drift electron lifetime as a function of time, we conclude that the adopted technology would allow for drift distances exceeding 3 m.
Keywords: purity monitor, drift electron lifetime

A Simple and Effective Purifier for Liquid Xenon
P. Benetti et al. (1993)
We describe a procedure to obtain ultra high electron lifetime in liquid xenon by menas of a commercial Oxisorb cartridge and the distillation of the liquid. An improved ICARUS-type purity monitor chamber is described as well as details of the purification process. No charge attenuation is observed over the drifiting distance (6 cm) at electric fields ranging from 800 down to 5 V/cm. This implies a free electron lifetime of at least several milliseconds. Results on the drift velocity dependence on the temperature are also reported.
Keywords: purity monitor

A Study of the Factors Affecting the Electron Lifetime in Ultra-pure Liquid Argon
A. Bettini et al. (1991)
As part of the development program for the ICARUS experiment, which consists of a very large time projection chamber (TPC) filled with ultra-pure liquid Ar (LAr), we have carried out tests with different purifier systems, in order to evaluate the performance of the various parts and to improve the purification techniques developed so far.
Electron lifetime τ in LAr has been determined with an improved method based on the measurement of the attenuation of a current due to an electron cloud, photoproduced by a laser pulse impinging on a metallic cathode and moving in a small drift chamber filled with the purified LAr. Results of the above-mentioned tests are reported. During these tests, we observed repeatedly and reproducibly an increase of τ that took place of a period of 10 to 20 h after liquification. Several tests performed in an attempt to elucidate this effect, suggest that the increase in τ is due to adsorption of electron-attaching impurities on the walls of the stainless steel container, a process governed by thermal diffusion. The electron lifetime monitoring system reported here was used to measure the electric field dependence of τ in purified LAr doped with O2 and CO2 for fields 100 V/cm < E < 800 V/cm. The results for O2 are consistent with published data.
Keywords: purity monitor, electron drift lifetime

Electron Lifetime Detector for Liquid Argon
C. Carugno et al. (1990)
A method has been developed for a fast and reproducible measurement of the electronegative contamination of liquid argon. The method is based on the transport of electrons extracted by a Nd-Yag UV laser beam from a metallic photocathode. The transit time of the electrons from the emitting surface to a collecting plate depends on the applied electric field and ranges from a few tens of microseconds to milliseconds. The measurement of the purity is based on the ratio of collected to emitted charges. The operation can be performed in a few minutes and gives values of the electron lifetime with a typical error of a few percent. Filling and emptying the vessel can be done in less than an hour.
Keywords: electronegative contamination measurement

Outgassing

Solution of the Outgassing Equation for the Pump Down of an Unbaked Vacuum System
K. Akaishi (1998)
The outgassing equation based on the Temkin isotherm is applied to consider the outgassing behavior in the pump down of an unbaked vacuum system. A method to solve the equation approximately is discussed first, and then solutions of outgassing rates are derived. Solutions to the equation which can be used to predict the dependence of the outgassing rate on pumping time and pumping speed are discussed. The solutions finally obtained are able to explain the outgassing behavior observed experimentally for a 304 stainless steel chamber after exposure to moist air, in particular the dependence of the outgassing rate on time and pumping speed.
Keywords: outgassing equation, pumping speed

Effects of readsorption on outgassing rate measurements
P. A. Redhead (1996)
Outgassing rates from the walls of a vacuum chamber are usually measured by one of two methods, (1) the throughput method, where the outgassing rate is derived from the pressure drop across a known conductance during evacuation, or (2) the pressure rise method (aka the gas accumulation method) where the chamber is sealed off at a low pressure and the rise in pressure with time measured. A model of the throughput method is analyzed to show that readsorption of desorbed gas results in the measured outgassing rate Qm being considerably less than the intrinsic rate Qi (the outgassing rate in the absence of readsorption). The ratio of outgassing rates Qm/Qi varies with the pumping parameter S/A (S is the pumping speed and A the adsorbing area) and the sticking probability s. Qm is found to vary with the pumping parameter and the sticking probability at zero coverage s0, except when a/As0<<kt-1, where a is the effective area of the orifice, A the adsorbing area, k is a constant involving the heats of adsorption, and t the pumping time. Measurements by the pressure-rise method on stainless steel systems show that the gas evolved is hydrogen, and that the pressure rise is linear for as long as 10 h with the pressure increasing by a factor as large as 106, indicating a constant outgassing rate. It is shown that if readsorption occurs then the p(t) curve will be nonlinear. A model is proposed to explain the observed constancy of outgassing rate and the apparent absence of readsorption which assumes that (a) the outgassing rate is desorption limited and (b) the rate of diffusion of H atoms to the surface is much greater than the rate of desorption of H2 or the rate of arrival of molecules from the gas phase at empty sites.
Keywords: outgassing, throughput method, pressure-rise method

Experimental Study on the Scaling Law of the Outgassing Rate With a Pumping Parameter
K. Akaishi et al. (1996)
The outgassing rate of a small test chamber made of type 304 stainless steel has been measured as a function of pumping speed in the range from 0.3 to 200 l /s. The chamber was first exposed to air and then pumped with different pumping speeds for three days at 20 °C without baking. Typical outgassing rates of the chamber are determined from the measurement of steady pressures at the pumping time of 72 h. It is found, as a result, that measured outgassing rates q obey a set of power laws of q1 = C1m1 and q2 = C2m2, where S/A is the ratio of pumping speed S to the surface area A of the chamber, C1 and C2 are constants, the exponents m1 and m2 are 0.8282 for S/A≥10-3 l /s cm2 and 0.4655 for S/A≤10-3 l /s cm-2. It is shown that the observed power law can be derived from theoretical consideration and is useful to estimate the ultimate pressure of an unbaked vacuum system.
Keywords: outgassing rate, pumping speed

Importance of the Surface Oxide Layer in the Reduction of Outgassing from Stainless Steels
Y. Ishikawa and T. Yoshimura (1995)
Temperature dependence of outgassing was measured for differently surface treated type 316L stainless steel chambers in the temperature range of 25–330 °C. Based on the temperature dependence and outgassing characteristics of major outgassing species, the mechanism of outgassing from the stainless steel surface is discussed and the technique to produce an extremely low outgassing surface is indicated. The following are the important findings. (1) At temperatures below 250 °C, the outgassing species is predominantly hydrogen, which is released by diffusive outgassing from the bulk of the stainless steel. (2) At temperatures above 250 °C, the contribution from water and carbon monoxide outgassing becomes significant because of the enhanced surface reaction to generate these gas species. (3) The surface oxide layer formed by the oxidation in air is predominantly iron oxide and appears to serve as a more effective diffusion barrier for hydrogen outgassing compared with the mixed iron and chromium oxide layer formed on the fully degassed surface. (4) The oxidation reduces carbon contaminants on the surface which are a possible source of carbon containing outgassing species.
Keywords: outgassing, pressure-rise method

Recombination limited outgassing of stainless steel
Hydrogen outgassing in stainless steel gun chambers
Outgassing in thin wall stainless steel cells
Outgassing characteristics of an electropolished stainless-steel pipe with an operating extractor ionization gauge
Outgassing characteristics and microstructure of an electropolished stainless steel surface
The Problem of Water in Vacuum Systems

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