Abstracts of papers

clusion but has assigned the cause of the effect to 0 ÷ arising from CO rather than H ÷.

Reference i j H Singleton, J Vac Sci and Technol 3, 103 (1967).

Classification and dynamic calibration of ionization sensors and detectors

Jack A Alealay and Eldon L Knuth, Department o f Engineering, University o f California, Los Angeles, California 90024, USA

Ionization type devices are being used as the sensing element for measuring pressure (pressure gauges), composition (mass spectrometers) and speed distributions (eg, molecular beam detectors). Frequently, the calibration of such sensors consists in measuring the ion current as a function of the density of a gas either at equilibrium or under steady state flow conditions. Use of calibration constants obtained from equilibrium or steady state flow measurements may result in serious misinter- pretation of measuremer.ts at unsteady state flow conditions (such as measurements from satellites).

The dynamic characteristics of the detection process are defined and analyzed here in terms of operational parameters. The concepts "through-flow mode" and "non-through flow mode" of operation are defined as idealized reference points in a continuous range of possible modes of operation. The general class of linear detectors is defined. A linear detector is one whose instantaneous output is a linear combination of the instantaneous beam flux and beam density.

A novel concept of calibration (consistent with the require- ments imposed by applications under unsteady state flow conditions) is presented and discussed. The dynamic calibration junction relates the time dependent ion current function to the time dependent flow field at the sensor entrance. The con- ventional calibration constant of ionization sensors is shown to be equivalent to the zeroth moment (normalization factor) of this function. In general, higher moments of the function are required in order to uncouple speed distribution functions and density obtained from modulated beam measurements.

The object of molecular beam time-of-flight spectroscopy is to determine the speed distribution and density of a molecular beam from time-domain measurements of propagated (time dependent) beam perturbations. In preceding publications a powerful and convenient procedure for obtaining the density, the mean speed, and the random kinetic energy (hence the speed ratio) of a beam was developed). Time-of-flight spectro- scopy was applied in determining the speed distribution of an arc-heated supersonic molecular beam and in measuring the angular distributions of the flux and speed ratios of molecular beams scattered from solid surfaces.

In the present paper, a procedure for determining the dynamic calibration function of a wide class of linear ionization sensors is presented. The moments of the dynamic calibration functions are obtained from the time-dependent ion-current signals obtair, ed by sensing the TOF (time-of-flight) distribution of a modulated beam of known speed distribution and density.

A new type of molecular-beam detector (called an Orbitron detector, after the pressure gauge which is designed on the same principle) was designed and built to provide the sensitivity required to measure the flux distribution of a beam scattered from a solid surface. The three lower moments of the dynamic

calibration function were obtained for this detector by calibra- tion in a thermal beam of known density. A physically meaning- ful analytical expression for the dynamic calibration function is obtained from these moments.

It is expected that the present method would be valuable in designing and calibrating ionization sensors useful in measure- ments of the density and composition of the upper atmosphere by means of orbiting satellites. In addition, the authors hope that the methods and concepts reported here will be useful in characterizing the performance of ion-vacuum pumps.

Surface component in ionization gauges

Josd L de Segovia, Jos6 M L6pez-Sancho and Crist6bal, S Martin, Centro de Investigaciones Fisicas "L Torres Quevedo", Madrid-6, Spain

An attempt has been made to determine both surface com- ponent and grid contamination in several ionization gauges. This study has included the regular Bayard-Alpert and differential gauges, at very low residual pressures (10 -~° torr and lower) and with CO and 0 2 pressures. Also a curve of the sticking coefficient versus grid coverage is presented.

Invited paper (Title to be announced)

J Groszkowski, Warsaw Polytechnic, Warsaw, Poland

Direct measurements of ion collection in Bayard-Alpert gauges

George Comsa and Andrei Mircea, Institute a f Atomic Physics, Bucharest, Roumania

A quantitative theory of the ion collection in Bayard-Alpert gauges with free-ends grid has been worked out in the last three years. This theory led to conclusions which are important from the viewpoint of the practical measurements and on the other hand it put in evidence the fact that the Bayard-Alpert gauge may be a useful tool for getting more subtle informations concerning ultrahigh vacuum physics (eg, the velocity distribu- tion of atoms), ionizing collisions, charge exchange, etc.

The measurement of the collection factor had to be performed, not only in order to check the theoretical results, but also to obtain clearer experimental evidence of some perturbing phenomena which are not considered in the theoretical calcu- lations, eg ion and electron space charges, ion-molecule collisions, grid end-caps, etc.

From the published experimental data concerning the characteristics of Bayard-Alpert gauges, the value of the collection factor might have been eventually obtained indirectly, as the collection factor is one of the factors making up the expression of gauge sensitivity. Unfortunately, neither the value of the other factors entering the sensitivity expression, nor their dependence on different parameters are known.

We constructed an experimental device which enabled us to measure directly the ratio of the number of collected ions to the number of ions created in the grid space, that is the collection factor. The experimental device reproduces to a great extent the conditions encountered in a usual Bayard-Alpert gauge. In most measurements a 30 ~m collecting wire was used. The theoretically expected value of the collection factor was 26 per cent.

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