Vibrating-reed Electrometer Research Articles

Vibrating Reed Electrometer With Attocoulomb Charge Resolution and Subattoampere Current Resolution

An extremely sensitive vibrating reed electrometer is built, which has attocoulomb charge resolution and subattoampere current resolution within a 1-min measurement. The input leakage current of the setup is typically less than 1 aA with values as low as 0.12 aA, which is less than 1 electron/s. Allan deviation analysis shows that the lowest standard deviation that can be achieved in current measurements is 0.065 aA with a 3-h integration time. Crucial for this extreme sensitivity and low leakage current was the development of an input measurement stage with only vacuum-gap capacitors. The stability and sensitivity of this new electrometer make it suitable for accurate low-frequency characterization of vacuum-gap capacitors as for example used in the electron counting capacitance standard experiment. As a first step toward such a study, the internal vacuum-gap feedback capacitor of the electrometer was measured as a function of applied charge and for different heat treatments resulting in changing surface layers of the capacitor plates.

Electrostatic charge and field sensors based on micromechanical resonators

We have developed highly sensitive electrometers and electrostatic fieldmeters (EFMs) that make use of micromechanical variable capacitors. Modulation of the input capacitance, a technique used in macroscale instruments such as the vibrating-reed electrometer and the field-mill electrostatic voltmeter (ESV), moves the detection bandwidth away from the 1/f-noise-limited regime, thus improving the signal-to-noise ratio (SNR). The variable capacitors are implemented by electrostatically driven resonators with differential actuation and sensing to reduce drive-signal feedthrough. The resonators in the electrometer utilize a balanced comb structure to implement harmonic sensing. Two fabrication methods were employed - a hybrid technology utilizing fluidically self-assembled JFETs and SOI microstructures, and an integrated process from Analog Devices combining 0.8-μm CMOS and 6-μm-thick polysilicon microstructures. All devices operate in ambient air at room temperature. Measured data from one electrometer with an input capacitance of 0.7 pF indicates a charge resolution of 4.5 aC rms (28 electrons) in a 0.3 Hz bandwidth. The resolution of this electrometer is unequaled by any known ambient-air-operated instrument over a wide range of source capacitances. The EFM has a resolution of 630 V/m, the best reported figure for a MEMS device.

A new technique in contact electrification

A new type of instrument to study contact-charging phenomena is described. The two functions of any contact-charging apparatus, namely touching two materials together, separating them and then measuring the transferred charge, are performed by a single element, a flexing cantilever, bonded to a piezoelectric strip. The displacement of the end of the cantilever is controlled remotely by supplying electrical pulses to the strip. A DC pulse makes a metal-insulator contact and an AC signal transforms the strip into a vibrating-reed electrometer. Using phase-sensitive signal recovery, initial tests showed a sensitivity of about 250 electronic charges, a factor of several orders of magnitude better than can be achieved using DC measurement techniques.

577.電力スペクトル密度によるバースト状放射線の平均強度測定法

The intensive bremsstrahlung emitted from an electron LINAC is pulse modulated with a period of T0. If we detect these γ bursts with a scintillation detector, the output of the detector reflects the pulse trains with the same period T0 but with random pulse heights due to the piling up of γ photons.We have derived the power spectral density of such piled-up pulse trains and have found that the power spectral density in composed of two parts. The one is the line spectra whose interval is 1/T0 with amplitude proportional to the square of the average intensity of the incoming bursts. The other is a continuous spectrum due to variations in the intensity of the incoming bursts.This characteristic of the power spectral density can be used so measure the radiant intensity of the γ burst with period T0 by using a watt-meter connected with a narrow band-pass filter. This method is simpler and less expensive than using the vibrating-reed electrometer, and can be well adapted to use as surveymeter or in a monitoring system. We have measured the average intensity of γ bursts with this method, and experimental results are described.

The measurement of small ion currents with the aid of photomultipliers

A short survey of the various methods for measuring small ion currents (10 −9 to 10 −20 A) is given: 1. Ion current measurement (with d.c. amplifier, vibrating-reed electrometer amplifier or multiplier). 2. Ion counting (with open multiplier or scintillation detector). In particular, technical problems encountered in lowering the background in a Daly-type scintillation counter are discussed. In the arrangement used, the ions separated in a mass spectrometer are accelerated on to an auxiliary electrode which is at a potential of −25 kV. Opposite this auxiliary electrode (converter) is situated a scintillator. The secondary electrons released from the converter by the ions are accelerated on to the scintillator and give rise to photons which generate photoelectrons in the cathode of a photomultiplier. These photoelectrons are finally detected as a voltage pulse at the anode of the photomultiplier with the of well-known pulse counting methods. The influence of the following parameters on the background of the arrangement is discussed: field emission from the converter, the absorption of cosmic radiation and radioactive radiation in the scintillator, the choice of the scintillator material, the optimum dimensions of the scintillator, thermal emission from the photocathode, and spurious effects of the electronic equipment. Methods for lowering these unwanted contributions are discussed. The lowest background obtained was 0.02 counts/sec corresponding to approximately 3 × 10 −21 A. A method is described to determine the average number γ ′ of electrons released at the converter per impinging active ion i.e. an ion actually releasing electrons.

Total Ionization and Range of Low-Energy Recoil Particles in Pure and Binary Gases

The total ionization and range of the 103-keV $^{206}\mathrm{Pb}$ recoil particle that results from the $\ensuremath{\alpha}$ decay of $^{210}\mathrm{Po}$ deposited on a metallic backing have been determined for a number of gases. The method employed a hemispherical ion chamber and a vibrating-reed electrometer. The 5.3-MeV $\ensuremath{\alpha}$ particles were used for calibration. The gases used were Ne, Ar, Xe, ${\mathrm{N}}_{2}$, dry air, C${\mathrm{H}}_{4}$, ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$, 95% Ar+5% ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$, and other binary mixtures. The equivalent range, in microns, of the $^{206}\mathrm{Pb}$ particle in the listed gases at STP is 131, 79, 44, 80, 83, 84, 58, and 82, respectively. The uncertainty in each value is \ifmmode\pm\else\textpm\fi{}2 \ensuremath{\mu}. The average energy loss per ion pair produced, ${W}_{r}$, in eV/(ion pair), for the $^{206}\mathrm{Pb}$ particle in passing through the gases, omitting Ne, is 121\ifmmode\pm\else\textpm\fi{}6, 158\ifmmode\pm\else\textpm\fi{}8, 116\ifmmode\pm\else\textpm\fi{}6, 120\ifmmode\pm\else\textpm\fi{}6, 121\ifmmode\pm\else\textpm\fi{}6, 115\ifmmode\pm\else\textpm\fi{}4, and 81.7\ifmmode\pm\else\textpm\fi{}2, respectively. Theory and experiment for the range of the $^{206}\mathrm{Pb}$ particle in the monatomic gases lie within 5% of each other. A rough calculation is made of the energy the recoil particle loses through molecular, nonionizing processes per ion pair produced. For ${\mathrm{N}}_{2}$ this amounts to approximately 10 eV/(ion pair) of the ${W}_{r}$ value.

An Electrometer for Use in Scientific Space Instruments

A wide-range vibrating-reed electrometer that obtained extensive detailed data on the "solar wind" during the Mariner II flight to Venus is described. This circuit used an inverting carrier-type dc amplifier with a logarithmic diode as its principal feedback element and represented the first successful application of a space-qualified dynamic capacitor electrometer. This system also represents one of the earliest all-semiconductor amplifiers to be used in a dynamic capacitor electrometer. The wide dynamic range of the application (10-13 to 10-6 A) required the stabilization of a closed loop having 7 decades of continuous variation in the dynamic resistance of the primary feedback element. Some details of design, analysis, and performance associated with the electrometer amplifier and its feedback system are presented.

A comparison of ionization-chamber and liquid-scintillation methods for measurement of beta emitters.

At the outset it should be remarked that there is no unequivocal general method of comparing the ionization-chamber and liquid scintillation methods for the measurement of beta emitters. For a useful comparison, one must make arbitrary, but reasonable and specific, assumptions, and then carry out calculations in a reliable statistical manner. This is the approach attempted by the authors in this paper. What has been done is to calculate the minimum sample specific activity that could be determined in a 30-min measurement time to a standard deviation of ± 10% for two widely used commercially available measuring systems: the Applied Physics Corporation Vibrating-Reed Electrometer and the Packard Tri-Carb Liquid Scintillation Spectrometer. Consideration is limited to the two most widely used beta-emitting tracer radioisotopes: tritium and C14.

Mixed radiation dosimetry of a plutoniumberyllium neutron source.

The dose rate delivered to an assumed standard tissue by the fast- neutron and gamma -radiation from a standard 1 c plutonium-beryllium neutron source was determined by measurements with ethylene-filled polyethylene and CO²-filled Teflon ionization chambers. The resulting ionization currents as measured with a calibrated vibrating-reed electrometer were used to obtain a measure of both the gamma - and fast-neutron components of the emitted radiatdon. The tissue dose rate due to the fast neutron extrapolated to 1 cm from the source was found to be 1.74 rads/hr. The gamma -component was found to contribute 0.16 rad/hr. From the neutron dosc rate and the neutron spectrum for Pu- Be, the emission rate of the source was calculated to be 1.65 x 10 neutrons/ sec-c. A discussion of the neutron and gamma spectra of the emitted radiation from this source and some aspects of ionization chamber design are also included in this report. (auth)

Exact Solution of a Time-Varying Capacitance Problem

By means of a new method, a closed-form solution is obtained for the harmonics generated by a sinusoidally varying capacitance in series with a fixed resistor and battery. The solution describes the behavior of the condenser microphone, the vibrating-reed electrometer, a vibrating plate contact potential measuring apparatus, and a special loudspeaker improvement. With only minor modifications the solution can also apply to the case of a sinusoidally varying resistance in series with a fixed inductance and battery; thus, it may, in addition, be usedl to caculate the response of a carbon microphone. The present large-signal solution, which applies for any finite values of the modulation index and frequency, is compared with previous small-signal approximate results, and the dependence on modulation index and frequency is investigated for such quantities as output waveform, total harmonic distortion, harmonic amplitude and phase, and average input and output power. A very distorted waveshape is obtained for low relative frequencies and values of the modulation index near and including unity.

Gas Phase Radioassay of Labelled Organic Compounds. I Measurement of 14C with an Ionization Chamber and a Vibrating-Reed Electrometer

The method of C/sup 14/O/sub 2/ assay by means of a Borkowski ionization chamber and a vibrating-reed electrometer was examined with respect to reliability, accuracy, and measurable lowest limits of the activity. Changes in temperature and humidity in the operating room affected observed drift-rates, which are related to the amount of C/sup 14/ (dpm) in the ionization chamber. When the room was air-conditioned, the drift-rate was stable and independent of the temperature. The ionization chamber, which was filled with C/sup 14/O/sub 2/ and evacuated by repetition of evacuation and refilling with CO/sub 2/ still retained about 1/1000 of the original activity. After repetition of this procedure two to three times the background drift-rate returned to the normal value. The higher the C/sup 14/O/sub 2/ pressure in the ionization chamber, the larger the drift rate was. The ratio of C/sup 14/ to the observed drift-rate (minus background) was constant with one ionization chamber, but with another chamber this ratio changed with the amount of C/sup 14/ contalned in it. Therefore, for significant determination of C/sup 14/ from an observed drift- rate, it is necessary to use a plot of the drift-rate as a function of the ratio of the amount ofmore » C/sup 14/ to the drift-rate. With these precautions, the radioactivity of as low as 10/sup -2/ m mu c/mmol of C/sup 14/O/sub 2/ can be measured with a 100 ml ionization chamber. This lowest measurable limit of specific radioactivity is a few hundred times lower than that of the end-window G- M counting method using BaC/sup 14/O/sub 3/ at infinite thickness. (auth)« less

A simplified beta-ray extrapolation chamber.

The design and calibration of an extrapolation chamber for the measurement of surface dosages emanating from β-sources is described. The unit can be used to measure dosages from β-ray plaques or other sources of various sizes and intensities. It is a versatile low-cost instrument of quite simple construction. It is designed for direct insertion into the multiple resistor turret of a commercial vibrating-reed electrometer.

Precision Integrator for Ion Beams

The integrator consists of a circuit which operates relays to turn off data-storage devices, timers, etc., when the charge conducted by an ion beam reaches a specified quantity. A vibrating-reed electrometer (without feedback) senses when the potential drop across a capacitor into which the beam current flows becomes equal to the potential of several standard cells in series. Errors in sensing the potential are less than 0.001%, and the over-all time delay of the relay operation is about 15 msec. The major sources of error are associated with soakage and changes in capacitance with temperature, less than 0.05% and about −0.01% per °C, respectively. It should be feasible to attain a considerable reduction in all the errors of integration caused by these known sources of error.

Energy Loss Per Ion Pair for Protons in Various Gases

Values of $w$ for protons in A, ${\mathrm{N}}_{2}$, C${\mathrm{O}}_{2}$, dry air, and tissue-equivalent gas were measured as 26.66\ifmmode\pm\else\textpm\fi{}0.26, 36.68\ifmmode\pm\else\textpm\fi{}0.34, 34.37\ifmmode\pm\else\textpm\fi{}0.33, 35.18\ifmmode\pm\else\textpm\fi{}0.42, and 30.03\ifmmode\pm\else\textpm\fi{}0.29 ev/ion pair, respectively. A 2-Mev positive-ion accelerator was used as the source of protons. The energy of the protons was determined with a precision gaussmeter that was calibrated by the $\mathrm{Li}(p, n)$ and $\mathrm{T}(p, n)$ threshold reactions. These protons were scattered from a gold foil into a parallel plate ionization chamber. The fast-electron pulses were collected on one electrode, amplified, and counted. The positive-ion charge was collected on the other electrode and measured by means of a standard capacitor that was connected between the input and feedback terminals of a vibrating-reed electrometer.

The Backscattering of Beta-Particles from Gases

During the course of experimental measurements in an ionization chamber (1, 2) to determine relative values of W, the average energy to produce an ion pair in various gases by (-particles, large discrepancies from the expected ionization current values were often observed when the active source took the form of a thin sheet at the center of a large flat electrode. The diminution of ionization current which occurred especially in certain gases, was at last traced to a phenomenon of scattering of the/ -particles from the gas itself back to the electrode plate. This resulted in a reduced ionization path for such /-particles and hence a reduction in the observed value of the ionization current. Since such an effect may necessitate very large corrections in absolute 3-ray ionization chamber measurements, and since it might under certain experimental conditions be easily overlooked, it seems worth while to give a summary of our experiments here. Relative ionization current measurements in different gases were carried out for thin /-particle sources of H3, Ni6, and C14. The active source was in each case deposited in a thin layer upon the central collecting electrode of an ionization chamber, shown schematically in Fig. 1. The collecting electrode varied greatly in size and shape throughout the course of the experiment. Since the range of the /-particles is much larger for C14 than for H3 and Ni63, it was found advantageous to use two similar ionization chambers of different size. For H3 and Ni63 the chamber was a brass cylinder of internal diameter 9.5 cm and height 7.0 cm, as is shown in Fig. 1. For C14, in order to avoid the use of excessive gas pressures, the chamber was made larger-of diameter 20.5 cm and height 21.0 cm. The general technique of making the ionization current measurements has already been described (1, 2). In brief, the central collecting electrode of the ionization chamber was connected to a vibrating-reed electrometer, which in turn fed into a Brown strip-chart recorder. As the potential of the insulated chamber electrode rose with the collection of ions, the needle of the Brown recorder drifted across the scale. This drift was opposed at frequent intervals by counterpotentials applied to

High-Speed, Direct Recording Fabry-Perot Interferometer

A direct recording Fabry-Perot interferometer is described which has a high optical speed and provides a direct tracing of spectral line shapes on a linear scale. A photomultiplier views the center spot of the interference pattern. The output of the photomultiplier goes to a vibrating-reed electrometer and then to a pen recorder. The wavelength of the light at the center of the interference pattern is made to vary with time by changing the index of refraction between the interferometer plates. This is accomplished by enclosing the plates in a gas-tight housing and admitting an inert gas such as argon or helium to the housing. By the proper choice of gas and of the rate of pressure change, one obtains the desired rate of tracing the line shape. Orders of magnitude reduction in ``exposure'' time are achieved by eliminating the use of photographic plates.

Electrochemistry of Stainless Steel in Sulphuric Acid

IN a study of the electrochemical behaviour of stainless steels, the electrode potential has been continuously recorded during the transformation of an air-stable oxide film to the surface state that exists in dilute, aerated sulphuric acid at temperatures between 25° and 90° C. Several definite inflexions or stages in the process have been observed which are similar in certain respects to the oscillograms obtained by Bonhoeffer and Vetter1 in the activation and re-passivation of carbon steel in nitric acid. With stainless steel, the processes are much slower than those observed with carbon steel, so that the details of the several stages may be more readily studied. Thus, Fig. 1 shows the results obtained at 85° C. when a niobium-stabilized steel (type 347) was heated in air at 445° C. for 2 hr. and then placed in an aerated 0.1 N mixture of sulphuric acid and sodium sulphate which had a pH of 1.46. A vibrating-reed electrometer measured the potential between the steel electrode and a reference electrode kept at 24° C, so that all potentials to be given include a small potential due to the thermal gradient in the bridge. Dissolved iron was determined at intervals spectro-photometrieally by use of thioglycollic acid2.

Precision Measurement of Uniformity of Materials by Gamma-Ray Transmission

The uniformity of the product of thickness and density (i.e., mass per unit area) of materials of constant mass absorption coefficient is determined by measurements of the variation in gamma-ray transmission. The radiation is detected with a scintillation detector and vibrating-reed electrometer. For a given incident intensity, maximum sensitivity is attained when a source is chosen which emits gamma rays whose mean free path in the material under investigation equals the thickness, assuming the detector response is a slowly varying function of quantum energy. This optimum condition for cobalt-60 radiation obtains for thicknesses of 3, 1, and ½ in. of Al, Fe, and U, respectively; using a one-curie source, variations of 0.01 percent in the uniformity of plates of these thicknesses have been measured. The method may be applied also to the scanning of curved surfaces and extended materials where the source and detector cannot permanently be fixed with respect to each other. For this latter case, the problem of compensating for the effect of misalignment of source and detector is considered.

Precision Obtained with Vibrating Reed Electrometer in Radioassaying Meta- and Para-Substituted Benzoic-Alpha-Carbon-14 Acids

Precision Obtained with Vibrating Reed Electrometer in Radioassaying Meta- and Para-Substituted Benzoic-Alpha-Carbon-14 Acids