Aluminum Foil HighVacuum GasketStanley Ruthberg and John E. Creedon Citation: Review of Scientific Instruments 26, 1208 (1955); doi: 10.1063/1.1715234 View online: http://dx.doi.org/10.1063/1.1715234 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/26/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highvacuum degassing furnace made from aluminum alloy and its thermal characteristics J. Vac. Sci. Technol. A 7, 2435 (1989); 10.1116/1.575915 HighVacuum Gas Valve Rev. Sci. Instrum. 27, 874 (1956); 10.1063/1.1715403 HighVacuum Butterfly Valve Rev. Sci. Instrum. 26, 989 (1955); 10.1063/1.1715176 A Compact HighVacuum Valve Rev. Sci. Instrum. 25, 1027 (1954); 10.1063/1.1770909 ``Rubber Solution Gaskets'' for High Vacuum Work Rev. Sci. Instrum. 23, 646 (1952); 10.1063/1.1746122

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1208 LABORATORY AND SHOP NOTES

Aluminum Foil High-Vacuum Gasket STANLEY RUTH BERG AND JOHN E. CREEDON

National Bureau of Standards, Washington 25, D. C. (Received August 11, 1955; revised version received September 26, 1955)

AN aluminum foil gasket clamped between two monel flanges as indicated in Fig. 1 serves very well as a simple, easily

manipulated, high-vacuum seal which can be baked at 400°C. A thin gasket of this type avoids the problems associated with gasket expansion during bakeout. It also simplifies design and operation by necessitating a ridge on only one of the flanges, where the area of the seal against the flat flange is that of the ridge face, itself.

An annular ridge, -h in. wide, on flange E drives the gasket, D, against the flat surface of the second flange, C. The gasket is cut from the ordinary household variety of aluminum foil, which is about 0.001 in. thick. The flanges are forced together by eight equally spaced 1/4-20 steel bolts, but only moderate torque is needed to tighten the bolts. When glass tubulation is used with the flanges', a support such as F is desirable.

The expansion coefficients are a factor in the choice of the materials used in the flanges and bolts. Here, for the selection made, the compressive forces on the gasket are greater during bakeout where increased protection against leakage is desirable. Over the range from room temperature to 400°C the expansion coefficient of monel metal is approximately 1 X 1O-&/co larger than that for cold rolled steel. This insures increased compression of the gasket without apparent deformation of the rest of the system. No tightening of the bolts has been necessary on the return to room temperature after bake-out.

The flat surface of C and the face of the ridge on E are prepared by abrading on No. 600 wet-or-dry carborundum paper. It has not been found necessary to remove the fine, radial, abrasion lines which can occur. The paper is supported by plate glass and lubri­cated with paraffin wax dissolved in mineral spirits. When the system is disassembled, the aluminum foil adheres strongly to the surfaces and is removed with sodium hydroxide which does not affect the flange surface. Before reassembly, the surfaces may then need a minor quick touchup on the abrasive paper.

This system has been used as part of a semidemountable vacuum tube. It has been leak tested by coupling the vacuum system to a helium mass-spectrograph leak detector and surrounding the demountable tube with a helium atmosphere. No measurable leak was indicated until a bakeout temperature of 230°C was reached. As the temperature was increased to 400°C, the reading increased

JE j1 it~ FIG. 1. Sketch of gasket seal;

A, glass; B, Kovar cylinder; C, flange, "R" Monel; D, gasket, O.OOI-in. aluminum foil; E, flange, uR" Monelj F, support, brass.

nonlinearly to 42X1O-7 cc atmos/sec for a glass envelope 10 cm in length, 1 mm thick, and of 150 cm' area. The rate of helium influx as a function of temperature and time gave a hysteresis curve. With temperature diminution to room value, no leak was detected. The estimated amount of helium permeation through the glass envelope at these temperatures was more than ample to explain the magnitude of the leak rate and the hysteresis effect.! A pressure of 8 X 10-8 mm Hg has been attained after bakeout in air at 400°C with an all glass vacuum system using a glass stopcock lubricated with Apiezon N, an oil diffusion pump, a DPI VG-IA ion gauge tube, and a liquid nitrogen cold trap. On isolation from the pump by the glass stopcock, pressure rose to 1.5 X 10-8 mm Hg after 20 hours. Then the refrigerant was removed and the ion gauge tube turned off. Each day the coolant was reapplied and the ion tube turned on. After equilibrium was attained each day, the pressure and time to reach equilibrium were recorded. At the end of a week, the equilibrium pressure still remained at 1.5 X 10-& mm Hg, whereas the time to establish equilibrium had increased from 20 minutes to 2 hours. This schedule of recording pressure was chosen for convenience and to allow some differentiation between true and virtual leakage. There is an apparent background pressure of 1.5 X 10-8 mm Hg, due to vapor which cannot be removed by 'the pumping action of the VG-IA, and a small amount of outgassing and/or leakage.

The results indicate this gasket-flange system to be at least a~ leak-tight as the vacuum system with which it was used.

1 Rogers, Buritz, and Alpert, J. App\. Phys. 25, 868 (1954).

Coincidence Sorter for Scintillation Spectrometers* LEE GRODZINS

Brookhaven National Laboratory, Upton, Long Island, New York (Received September 15, 1955)

ONE of the standard tools for the study of level schemes of nuclei has been the coincidence scintillation spectrometer.

In general the source of radiation is viewed by two counters, the pulse height accepted from one of the counters is fixed and the pulse height distribution out of the other crystal is explored by multi-channel or single channel analysers. A technique which records all coincidences and associated pulse heights simul­taneously on photographic film is described below.

Figure 1 shows a simplified block diagram of the apparatus. When a coincidence occurs the pulses from the two crystals are stretched at their maxima and applied to the X and Y axes, respectively, of an oscilloscope whose intensifier is activated for a few microseconds after the pulses have been stretched. Thus a spot appears on the oscilloscope face at the X -Y position corre­sponding to the pulse heights from the crystals. A long time ex­posure records the pattern of dots. An examination of this pattern is usually sufficient to determine the coincidence spectra in the source.

Figure 2 illustrates the decayl,2 of Bi207 to Pb207• The first excited

FIG. 1. Block diagram of coincidence sorter.

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LABORATORY AND SHOP NOTES 1209

3-min exposure (a) 570-1063 key

20~min exposure (b) 570-1770 key

40-hr exposure (e) 900-1440 key

FIG. 2. Coincidence sorter photographs of decay of Bi207 •

state at 570 kev is fed by 1063 and 1770 kev gamma rays in the approximate ratio of 10: 1. The upper level also decays by a 1440-900 kev transition approximately 2% of the time. The source strength was about 0.5 microcurie. The detectors were I! X 1 in. NaI(Tl) crystals mounted on RCA 5819 photomultipliers. Figure 2(a) shows the coincidences of the 570-1063 kev photopeaks, the photopeak-Compton and Compton-Compton coincidences.3 A longer exposure, Fig. 2 (h), brings out the 570-1770 kev coinci­dences ,vhile a very long, 40 hour, exposure, Fig. 2(c), was used to bring out the weak 1440-900 coincidences. The intensity of this Jast transition is estimated to be about 0.2% of the total dis­integrations of Pb2J7 by comparison with the 1063-1063 random coincidence intensity.

Successful pictures have been taken of electron-gamma and electron-electron coincidences as well as gamma-gamma coinci­dences in very complex decay schemes. The present instrument with I! X 1 in. crystals is approximately equivalent, for energy determinations, to a 75X75 channel matrix. We are now in the process of determining order of magnitude intensities of transitions by measuring, in standard geometry, the widths of lines and dots as a function of time and number of coincidences.

We are indebted to R. L. Chase for the design of the circuits which were incorporated into a grey wedge analyser4 chassis. We wish to thank him as well as W. A. Higinbotham, M. Goldhaber, and A. W. Sunyar for their encouragement and stimulating discussions.

* Work carried out under contract with the U. S. Atomic Energy Com~ mission.

1 D. E. Alburger and A. W. Sunyar, Phys. Rev. (to be published). 2 N. H. Lazar and E. D. Klema, Phys. Rev. 98,710 (1955). 3 To reduce halation and to bring out the zero dot a discriminator is used

to reject all coincidences the sum of whose pulse heights is less than a given amollnt; this accounts for the missing dots near zero.

'Bernstein, Chase, and Schardt, Rev. Sci. Instr. 24, 437 (1953).

Acoustic Optical Bench for Ultrasonic Lens Measurements*

E. E. SVCKLI:-<G, Siale University of Ne1V York, College of lVIedicine at New York City, Brooklyn, New York

AND

W. R. MACLEAN, Polytechnic institute of Brooklyn, Brooklyn, New York (Received September 15. 1955)

I N testing ultrasonic focusing systems it is usual to use methods whereby the rays are rendered visible along their paths by

light diffraction effects.' In optical lens work on the other hand it is more common to view a plane image falling on a viewing screen and to check focus, distortion, etc., by the appearance of this image. The use of this second method is useful also in ultra­sonics but usually requires tiny probe microphones manipulated from above the fluid in which the ultrasonic field exists to de­lineate the image.

The observation has been made that when many independent small electrodes are placed across the surface of a single quartz crystal located in a liquid these electrodes show piezoelectrically generated voltages that depend on the sonic intensity in the quartz under them. Excitation does not seem to spread across the crystal face.2 This property has been used in the development of an ultra­sonic optical bench using image detection which is found to be convenient and capable of considerable precision.

A plastic box 30 in. long with an inner cross section 6X6 in. and capable of being filled with water is prepared. Lenses, ultrasonic generating source, etc., are fitted to plastic slides along the edges of the box so that they can be moved in the box, parallel to the main axis of the system in the usual manner of an optical bench. One 6 by 6 in. end of the box is formed of brass having a square cutout just less than 2X2 in. Four 1 in. square quartz crystals, X-cut and ground to the frequency of interest (3067 kc in the model made) are cemented edge to edge to form a 2 by 2 in. square and then are cemented over the square cut out in the brass end plate. Epoxy resin is a satisfactory cement. (A single 2 by 2 in. square crystal is being obtained and will doubtless be as satis­factory as the composite unit.) The box is filled with water so that the crystals have water on one side and are exposed to the air on the other. In operation the ultrasonic image occurs as a vibration field across the crystals. The inner side of the crystals is at all times at a substantially uniform (ground) potential because of the water in contact with it but the outer surface exposed to the air reproduces the vibration field as an alternating piezoelectric voltage field. A capacity probe is used to detect the variations in this electric field. The probe is constructed as follows. A # 20 2-in. long hypodermic needle is ground to a flat end and all burrs are taken from the ground edges. A length of # 26 B & S enameled wire is then dipped in Dupont cement and threaded through the needle. The wire is cut off almost flush with the ground end of the needle and is then drawn a short distance (about one­fifth of a millimeter) back into it. The body of the needle is connected to ground and mounted in a manil;ulator constructed

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FIG. 1. Shielded probe for detection of crystal surface voltage pattern.

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