Storage System in Measuring ProblemsMario De Rossi Citation: Review of Scientific Instruments 35, 764 (1964); doi: 10.1063/1.1746765 View online: http://dx.doi.org/10.1063/1.1746765 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/35/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dielectric Breakdown Problems in Electric Energy Transmission and Storage AIP Conf. Proc. 19, 153 (1974); 10.1063/1.2948455 Problem of Measuring Hydrogen Pressure in Ultrahigh-Vacuum Systems J. Vac. Sci. Technol. 2, 257 (1965); 10.1116/1.1492437 Vacuum Problems of Electron and Positron Storage Rings J. Vac. Sci. Technol. 2, 130 (1965); 10.1116/1.1492415 Environmental Problems with Acoustic and VibrationMeasurement System Components J. Acoust. Soc. Am. 34, 1989 (1962); 10.1121/1.1937054 The energy storage problem Phys. Today

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764 NOTES

+300V

01

1/2 12AT7

100 330

~.I

'--------''--+---i TO MODULATION DIODES

FIG. 1. A part of the original bias circuit with the addition of a cathode bypass capacitor.

setting of the amplitude control (360 -kn potentiometer). The potentiometer was checked and was found not at fault. The circuit was analyzed. The voltage gain is given

0.4

0.2

0 Q2

by Co

Ci

0.4 0.6

{3

O.S

1

FIG. 2. Plot of volt­age gain vs (3, (3 =RdRo. Solid curve is a plot of Eq. (1). Circles are experi­mental points.

1 +[Rg(r p+ Ro)/ R O(iJ.(3R g+r p) J' (1)

where iJ. is the amplification factor of the tube, r p is the plate resistance, and (3 is the ratio R 1/ Rg. Figure 2 shows

.1

-jr-r-+

(bl

FIG. 3. Two recommended cathode follower circuits with amplitude control.

the calculated gain (solid curve) and the experimental data (circles), and the agreement is excellent. The non- . linearity is, therefore, a property of the circuit itself. If a linear control of the output voltage is desirable in a cathode follower, it is recommended that one should follow either circuit (a) or circuit (b) of Fig. 3, where the potentiometer is no longer in series with the output resistor Ro. An experimental check in each case did show the ex­pected linear relationship between Co and (3.

1 Richard Conley La Force, Rev. Sci. Instr. 32, 1387 (1961).

Storage System in Measuring Problems MARIO DE ROSSI

lileclrical "Yf easurements Department, Science Faculty, University of Rome, Rome, Italy

(Received 26 December 1963; and in final form, 17 February 1964)

A MONG the different methods employed to measure the ionic beam intensity in a mass spectrometric

analysis that of storing the electric charges by means of a capacitance has the advantage of high sensitivity and a reduction of noise, since such a method carries out an inte­gralmeasurement of the ionic current over the time during which the capacitance of the storage system is charged. This method was first used by Thomson,! Dempster,2 and more recently by Chupka and Inghram.3

However, a problem in connection with such a method is that of switching the charge on the capacitor to the measuring instrument. To overcome this problem, an elec­tronic switching system was tried. This system possesses the advantage of eliminating any mechanical type of switch and, in addition, of making the measurements repeatable and representable on an oscilloscope. Such a system may also be used for measurements other than mass spectro­metric ones, as the advantages mentioned above remain valid whenever very weak currents (as with the output of G. M. tubes, photomultipliers, etc.) are measured by col­lecting the charge over a fairly long period of time and then measuring the charge that has accumulated. Consequently it seems worthwhile to describe here the principle of the new system.

Referring to Fig. 1, T is the switch, in this case a high vacuum photoelectric cell, that switches the charge col­lected by the capacitor C to the input impedance of the oscilloscope. At the beginning of the measurement T is open, i.e., it has a very high resistance (in fact the resist­ance is that of the photoelectric cell when neither anodic potential nor light are applied). As the electric charge is gradually collected by the pickup electrode P, the potential of the capacitor C increases. When this potential is to be measured, a flash of light is directed onto the cathode of

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NOTES 765

FIG. 1. Schematic circuit of the storage system. C=l00 pF Araldit capacitor, C1

=40 pF, and RI = 10 Mn oscilloscope input impedance. c,

p II GHT

~

90 AV PHILIPS

PHOTOELECTRIC CELL

TO SYN C.

the photoelectric cell, whereupon the capacitor discharges across the input of the oscilloscope.

Knowing the capacitance C of the capacitor and the charging time te, the unknown current Ie can be obtained from the relation

Ie=CVclte,

where Ve is the potential across the capacitor at the instant te when the charging process finishes and the discharge begins. However, the oscilloscope measures not Ve, but rather the potential V m, the peak value of the transient signal, at a later moment tm• In order to obtain the value of Ve from V m, a function relating the two must be derived from consideration of the circuit.

It has been shown that in this case

where V m is the peak value of the signal read on the screen of the CRT at the instant tm , Ve is the value of the potential actually reached across the capacitor C at the instant to of the beginning of the discharge, and K is an exponential transfer function which is related to the time constant of the two circuits; that of the input of the measuring instru­ment and that of the photoelectric cell.

A restriction with respect to the greatest duration of the charging time (that is proportional to the weakest value of the detectable current) is represented by the discharge time constant of the storage circuit when open. However, it has to be noted that the potential applied to the photocell at the end of the charging process, for fair values of the electric constants of the circuit, does not usually reach more than 1 mY.

Consequently, considering that the "dark current" of the photocell is about 10-8 A, when an anode potential of 100 V is applied, in the region below saturation, it is reasonable to assume that the dark current is lowered to a value of 10-1a A at room temperature when the anode potential is 10-a V.

If the weakest currents are to be recorded with such a method, the technique of cooling the photocell has to be used4 : a further reduction of the dark current is achieved by lowering the thermionic emission of the cathode; at a temperature of -120°C, the dark current is about 10-a

times its value at room temperature.

In this case, employing the typical circuit values as indi­cated in Fig. 1, intensity currents as low as 10-15 A were easily recorded with a time constant of the storage circuit of about HF sec. Obviously, care must be taken in the insulation of the remaining components of the circuit.

The cooling system is a Dewar filled with liquid air; the terminals of the photocell are brought out from the Dewar while a conventional tungsten filament dc lamp of 30 W sends the flash of light inside the Dewar through a mirror.

The capacitance of the charging circuit is 100 pF and the interelectrode capacitance of the 90 A V Philips high vacuum photocell 0.7 pF. The impedance of the measuring instrument is represented by the input of the "low level type E" Tektronix plug-in unit with a sensitivity of 50,!LV/cm.

It may be noted that if there is more than one unknown electric current, it is possible to make use of the system by employing a number of capacitors and photoelectric cells equal to the number of unknown currents. Such a multiple system was applied to a multicollector mass spectrometer; the experimental results are in good agreement with pre­diction and they are published elsewhere.'

I J. J. Thomson, Rays of Positiz'e Electricity and their Application to Chemical Analysis (Longmans Green & Company, New York, 1921), 2nd ed., p. 4.

2 A. G. Dempster, Phys. Rev. 20, 631 (1922). 3 W. A. Chupka and M. G. Inghram, J. Phys. Chern. 59, 100

(1955). 4 R. W. Engstrom, J. Opt. Soc. Am. 37, 420 (1947). 5 M. De Rossi, Ric. Sci. Suppl. 33,497 (1963).

Epoxy Resin Seals to Copper and Nylon for Cryogenic Applications*

J. C. WHEATLEY

Department of Physics and Materials Research Laboratory, University of Illinois, Urbana Illinois

(Received 30 December 1963; and in final form, 7 February 1964)

T HE purpose of this note is both to point out the use­fulness of the epoxy resin Epibond 100-AI in making

vacuum and pressure seals to copper and nylon2 for low temperature work and to give some quantitative indica­tions of how the seals are made.

The present work was initially motivated by the need for an electrically nonconducting, nonmagnetic vacuum case for adiabatic demagnetization apparatus where small susceptibilities must be measured. The need for such a vacuum case was pointed out by Horwitz and Bohm,a but they suggest glass as material, which is usually quite mag­netic.4 Strongin and Maxwells have suggested reducing the problem of the electrically conducting case by slitting a brass case longitudinally and filling the gap with Emerson and Cumming's Stycast 2850 GT. This is not a complete

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