Mica windows

In the microwave region tunable monochromatic radiation is produced by klystrons, each one being tunable over a relatively small frequency range, or a backward wave oscillator, tunable over a much larger range. Both are electronic devices. Absorption experiments are usually carried out in the gas phase, and mica windows, which transmit in this region, are placed on either end of the absorption cell, which may be several metres in length. Stark... 

Mica windows can be fused to glass using a special powder of a lead boroshicate glass softening at 450°C (J. S. Donal, 1942). A suitable glass for solder seals of this kind is BTH No. 93. 

In order to measure the absorption of the beam of rays coming from the mercury vapor an ionization chamber with a thin mica window in it and containing methyliodide was set up opposite the window, F, and lead plates with holes in them were placed in the line of the beam so that only the radiation coming from the impacts of the electrons against the mercury entered the chamber with sufficient intensity to be detected. That this was the case in the actual experiments is indicated by the fact that no perceptible ionization current could be observed when the mercury pump was not running. A quadrant electrometer measured the ionization current. 

Samples were made either by adding lipid to known amounts of salt solution or by adding lipid to dextran-plus-salt solutions. Mixtures were allowed to equilibrate for 48 hr and then were mounted between mica windows. Mixing and waiting at 50°C for up to 48 hr provided no advantage in reacning equilibria nor did the addition of a Ca2+ ionophore, A23187. All measurements were at room temperature (25°C). 

Table I contains our new values for the L2 and Ls absorption limits of Pt, Au and Bi. These measurements were taken with almost the same arrangement of apparatus as before. The X-ray tube was provided with an arm which reached nearly to the first slit of the spectrometer, thus reducing the absorption of X-rays by air. At the end of this arm and also on the front of the ionization chamber thin mica windows were employed which still further reduced the absorption of X-radiation. Finally the sensitivity of our electrometer had been increased many times. We were thus supplied with a much more sensitive detector of X-ray spectra than in our earlier measurements. This enabled us to use narrower slits and to obtain at the same time greater drops in our ionization currents for the absorption limits.

Exposures, for both the gamma-irradiated samples at the Argonne facility and the x-irradiated samples, were based on the standard ferrous sulfate dosimeter solution (using a G value of 15.6) (12, 15). In the latter case, the mica window sample holder used for sample irradiations was filled with the dosimetric solution for dosage estimations. 

This group has further developed multitechnique cells, and designed a cell for SAXS/WAXS and XAFS, and SAXS/WAXS/XAFS and UV-vis spectroscopy (Beale et al., 2006 Grandjean et al., 2005). They reported use of this cell to investigate hydrothermal crystallization processes of inorganic catalysts such as CoAPO-5. The synthesis cell design was simple in concept essentially a mini-autoclave, with a usable volume of 2 mm3, and mica windows, heated by an insulated aluminum block. 

In order to calculate the theoretical amount of radiation that should pass through the test sheet of aluminum, if the distribution in the spectrum follows the above inverse square law, it is necessary to determine accurately the absorption of x-rays of different wave-lengths by the mica windows and the sheets of aluminum. This has been done as described in the previous note by measuring the absorption of a few mono-... 

Various types of Geiger-Muller counters are shown in Fig. 7.12. The end-window counter equipped with mica windows of about 1.5 to 3 mg/cm is a very simple... 

Although has not been extensively used as yet, it promises to become one of the most useful of the isotopes. First made in 1940 by Ruben and Kamen, it has the advantage of a very long half-life and can be obtained having an appreciable specific radioactivity (3-6%). It emits electrons with essentially no 7-ray emission, and because of its weak radiation, must be measured with a thin mica-window counter or a suitable ionization chamber. 

Very thin mica windows (= lOpm) were later designed. Since these strips of mica are sensitive to electron bombardment, they are protected with sheets of beiyllium. The tubes used today are equipped with beryllium windows. Figure 3.5 shows the absorption curves according to the wavelength for these different windows. 

The irradiation techniques, gas purification methods, and analytical procedures have been described (2, 3). Borosilicate glass cells with mica windows and all-silica cells were used at a gas pressure of about 300 mm. at ambient temperatures. The energy of the incident protons was about 1.5 Mev. which was reduced to about 1.2 Mev. on passage through the cell window (3-4 mg./sq. cm.). Dose rates were varied from 2 X 1016 to 2 X 1018 e.v. cc.-1 sec."1 at STP. 

A GM counter with a mica window is to be used for measurement of C activity. What should the thickness of the window be if it is required that at least 90 percent of the C betas enter the counter ... 

Figure 3-3. Typical targeting system (top)[38, reprinted with permission, 1987 American Chemical Society] and irradiation cell for pulse radiolysis system (bottom)[39, reprinted with permission, 1977 Radiation Research Society] (Q quartz cell body B BNC connector, P paraffin gasket M mica window E electrical tape gasket. A aluminum frame S standard taper joint).

Isotopic. Iron-55 is an electron-capture isotope with a half-life of 2.94 years. Iron-59 is a mixed /3-7 emitter the main radiations of which areO.27 M.e.v. 8 (46%), 0.46M.e.v. /8 (54%), 1.10 M.e.v. 7 (57%), and 1.29 M.e.v. 7 (43%). The half-life of Fe is 45.1 days. The radioactive samples are most readily assayed with a standard thin-mica-window Geiger counter which will detect mainly the Fe /3-particles. Decomposition of ferrocene and ferricinium salts in a boiling mixture of four parts by volume of concentrated nitric acid and one part of 75% perchloric acid and the subsequent electrodeposition of iron on copper disks gives samples which exhibit excellent counting reproducibility. 

In most techniques for studying adsorption on metals, xmiform, clean, and reproducible metal surfaces are difficult to prepare and the adsorption process cannot be followed continuously [2, 3,4,7,10,11,16, 18,2l]. Clean and reproducible metal surfaces are also difficult to prepare and maintain in methods that measure adsorption continuously and directly on a metal-coated window of a Geiger tube [l, 6,7,13]. A recently developed apparatus and technique provide controlled conditions for the production and maintenance of relatively clean metal films and the precise measurement of adsorption [20j. Metal is evaporated onto a mica window supported within a high-vacuum apparatus adsorption onto the metal film is measured directly and continuously by a counter tube below the window. 

Apparatus and Procedure. The metal films were deposited and adsorption thereon was carried out in a high-vacuum apparatus, as shown in Figure 1. The mica window was supported between two chambers that were evacuated simultaneously to prevent breakage the window was coated by evaporating metal from a heated filament in the upper chamber. The gas was then admitted to both chambers until the pressure was 1 atm. Valve 3 was closed to isolate the upper chamber, the bottom plate was removed, and the coxmter tube was positioned mder the window. After the film had been exposed 15 minutes to the gas, the solution was admitted and counting was started with an automatic coxmter-printer system. 

The n-hexadecane was distilled twice and passed through a silica gel column under nitrogen to remove polar impurities. A 0.0005 molal solution of radiostearic acid in n-hexadecane was stored imder nitrogen in Teflon bottles. Five milliliters of solution was used—enough to cover the mica window to a depth of about 1.5 mm. A depth of 0.015 mm. contained enough radiostearic acid for a continuous monolayer. 

For each experiment, a new mica window was prepared from a thin sheet (5 to 7 microns) of cleaved Indian ruby mica. The window was moimted in the adsorption chamber and thoroughly washed with twice-distilled benzene. 

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