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X-Ray Photoelectron Spectroscopy (XPS)
Published in Terrance E. Conners, Sujit Banerjee, Surface Analysis of Paper, 2020
As was stated previously, XPS analysis must be performed under conditions of ultra-high vacuum in order to minimize scatter of photoelectrons and contamination of the samples surface. The central part of any XPS spectrometer, therefore, is a large stainless steel vacuum bell containing the various components of the spectrometer and operating at a pressure of 1 x 10-8 to 1 x 10-10 torr. Most systems utilize multiple vacuum pumps in order to achieve and maintain this pressure. Usually a mechanical vacuum pump is used in combination with a turbo-molecular pump to “rough” the vacuum chamber (pump it down to approximately 1 x 10-6 torr) and to provide vacuum for the sample introduction chamber and various instrumental components (such as an ion gun). An ion diffusion pump or a cryogenic sorption pump is then used to achieve and maintain the ultra-high vacuum in the bell. The use of pumps alone is not sufficient to attain these high vacuums. There must also be a way of “baking the system,” that is, heating the system to a temperature in excess of 100 °C in order to desorb water vapor and organic materials from the inner walls of the chamber.
MBE Growth Techniques
Published in John D. Cressler, SiGe and Si Strained-Layer Epitaxy for Silicon Heterostructure Devices, 2017
Figure 6.2 (right side) shows a schematic view ofa growth chamber. This chamber accommodates all the installations necessary for MBE growth. These are substrate holder, substrate heater, and substrate rotation; evaporation sources for matrix materials and dopants; and monitoring and analyzing facilities. Typically, the growth chamber is evacuated with a pumping cascade existing of an oil-free prevacuum pump, a turbomolecular pump, and a high-speed sorption pump, e.g., a sublimation pump cooled with liquid nitrogen. The growth chamber is always under UHV except for maintenance work or source refilling. After venting the chamber, a conditioning of the MBE system requires bakeout up to 300°C over more than 24 h. For this heater elements in a jacket are welded to the chamber walls from outside. For a minimum of preparation time to the start of bakeout, a layer of thermal isolation fiber is permanently sandwiched between the chamber wall and an aluminum cover. Under growth conditions, cooling water pipes welded to the chamber walls effectively reduce outgassing. Also, all effusion cells exhibit cooled walls to reduce the thermal radiation.
Vacuum Pumps
Published in Pramod K. Naik, Vacuum, 2018
Synthetic zeolites possess higher sorption capacity than activated charcoal but the latter has a higher thermal conductivity. Ease of refrigeration is better for charcoal while zeolites offer less dusting. Explosion hazards are relatively less in zeolites but the costs are comparatively higher. 100 gms of molecular sieve type 13X has a surface area of 5.14 × 10 8 cm 2, and when cooled to 77 K, it has a relative coverage of N 2 as = 0.1 at a pressure of 10 –3 Torr. This corresponds to 3.2 × 10 22 N 2 molecules adsorbed, which is approximately the number of molecules in a volume of 1 liter at STP. This illustrates that modest quantities of porous materials can serve as fore-pumps to reduce the pressure from the atmosphere. Design calculations for cryo-sorption pumping have been given by Manes and Grant37. Jepsen et al38 have described the arrangement for which the system was sealed from atmosphere and a finger containing activated charcoal was immersed in liquid nitrogen after which the pressure dropped to 1 Pa. The cryo-sorption pump consists of a stainless steel container of moderate thermal mass. The sorbent bed has a large surface area against the pump body wall so that the cooling of the low thermal conductivity sorbent is rapid. The gas to be pumped should have a free access to a large area of the sorbent bed. Sorption pumps are provided with heater jackets or rods for activation/degassing. Vanes are provided inside the pump for better thermal conductivity. Good thermal contact between the sorbent and the underlying cooled surface is essential to establish rapid thermal equilibrium. A typical cryo-sorption pump is shown in Fig. 6.30. The cryo-sorption pump producing the ultimate pressure of 10 –7 Torr in a 2 liter volume has been described by Nair and Vijendran 39. The cryo-sorption pumps are commonly employed for roughing the vacuum systems. Low-boiling point gases such as Ne, He and H 2 in the atmospheric air are not pumped effectively. This leaves
Thermostructural Analysis of Large Cryopumping Test Facility
Published in Fusion Science and Technology, 2023
Hemang S. Agravat, Samiran S. Mukherjee, Vishal Gupta, Paresh Panchal, Pratik Nayak, Jyoti Shankar Mishra, Ranjana Gangradey
The pump, along with its American Vacuum Society (AVS)–standard dome, is named the Large Cryopumping Test Facility (LCTF), as the pumping speed tests will be performed in this facility for the different concepts of the cryopump for performance evaluation and design optimization. The said cryopump can serve the requirements of the high pumping speed needed for nitrogen and water vapor along with the pumping of other residual gases, including hydrogen and helium gases. The present system will be less costly and will be easy for infrastructure management compared to liquid-helium-based cryopumps. While handling the liquid-helium-based cryopumps requires large infrastructure to handle cold helium gas and its recovery compressor system, a hybrid sorption pump (like the LCTF) has certain advantages, such as compactness and low running cost as compared to the liquid-helium-based-only cryopumps.
Cryogenic buffer gas beams of AlF, CaF, MgF, YbF, Al, Ca, Yb and NO – a comparison
Published in Molecular Physics, 2022
Sidney C. Wright, Maximilian Doppelbauer, Simon Hofsäss, H. Christian Schewe, Boris Sartakov, Gerard Meijer, Stefan Truppe
Figure 1(a) shows a sketch of the molecular beam source. We use a two-stage closed-cycle He cryocooler (Sumitomo RDK-415DP (with helium pot) with F-50H compressor) to cool the source to about 2.5 K. The first stage cools the aluminium radiation shields to about 40 K. The buffer gas cell, shown in Figures 1(c) and 1(d), is attached to the second stage and surrounded by a copper box, whose internal walls are coated with activated charcoal. The charcoal acts as an efficient sorption pump for helium gas at temperatures below 10 K. The operating pressure of the source chamber under cryogenic conditions is about 10 mbar, measured outside the radiation shields, and typically increases to 3×10 mbar when 2 standard cubic centimetres per minute (sccm) of helium is flowed through the cell.
Development and computational fluid dynamics (CFD) simulation of cryostat thermal shielding for a portable high purity germanium (HPGe) gamma spectrometer
Published in Instrumentation Science & Technology, 2020
Vladislav Malgin, Alan H. Tkaczyk, Olegs Jakovlevs, Marti Jeltsov, Priit Priimägi
Typically, in the operating mode of the spectrometer when the cryostat is cooled, the zeolite sorption pump maintains a high vacuum with a level of 10−6–10−7 mbar. In this case, the heat transfer by the conductance of residual gases is negligible and heat transfer by thermal radiation is dominant. The long periods of storage in the OFF-state with warmed cryostat are characteristic for portable spectrometers. In a warmed state, the sorption pump releases adsorbed gases, which leads to vacuum degradation in the cryostat. In this case, the heat transfer by residual gases becomes dominant and subsequent cooling by a cryocooler with lower cooling power becomes impossible. Moreover, due to gas leakage through the seals, the vacuum in the cryostat always tends to degrade with time. Extending the operational life between services is important for the practical use of portable spectrometers.