Rapid Gas Analyzer Using Ionization by Alpha Particles PAUL F. DEISLER, JR.~, KEITH W. MCHENRY, JR.*, and RICHARD H. WILHELM Princeton University, Princeton,
N. J.
A method and apparatus are presented for analysis of a flowing or quiescent gaseous mixture, by means of ionization of the mixture using alpha particles from polonium in an aged radium D source. By utilizing a proper combination of applied voltage and electrode spacing in the ionized mixture, ionization currents of the order of 10-8 amp. are obtained. The current depends in its precise value on the composition of the gas (binary, ternary, and possibly more components) at constant temperature and pressure. Primary calibration is necessary with recalibration or compensation at intervals, because of radioactive decay of the source (half life, 22 years). Electrical output may be recorded by a variety of devices or used in control systems. Binary systems: HTN~, HrC2H4, CzHdCzHs and Nt-COz, and the ternary: NT-H-CZH~ have been analyzed satisfactorily. In principle, gases of different molecular, atomic, or electronic structure should be capable of differentiation, examples of exceptions thus being ortho Hz-para H1 and H r D 2 . Theoretical time of response is of the order of second; precision of analysis in the prototype apparatus was about 0.2 to 0.3 mole for binary mixtures. The device is safe and relatively inexpensive.
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N T H E course of an investigation on the application of fre-
quency response techniques to the measurement of diffusional processes in catalyst particles, which is reported elsewhere ($), the need arose for a direct recording, rapid, precise, and relatively inexpensive gas analyzer capable of giving the instantaneous composition of a binary gas mixture. It was also necessary that the instrument be capable of insertion directly into the gases under study, and that it interfere with the flow of these gases as little as possible. Various well known methods of gas analysis were investigated and were found to fulfill only a few of the above requirements. High response speed ruled out thermal conductivity cells, as a response time of 1 to l / b ~second was deemed necessary. Refractometry, while rapid, was found unsatisfactory because of the necessity for a gas depth much greater than would be available, and infrared or ultraviolet absorption techniques were found to be excessively costly for the purpose in hand. Electronic problems, involved in developing methods for measuring the time of traverse of sound for a distance as small as the width of the gas stream, proved to be insurmountable a t any reasonable cost. The search for a new method of gas analysis was initiated] and led to the development of the alpha particle gas analyzer. Further experiments were carried out to determine whether the gas analyzer could be adapted to analysis of ternary mixtures. The device depends on the measurement of the rate of production of ions in a gas by alpha particles. The form of the apparatus is primarily for binary gas mixtures, or for mixtures behaving as binary mixtures; however, one ternary mixture has been analyzed and adaptation to use with more than three components appears possible. For all systems a primary calibration is necessary, with recalibration a t intervals because of the decay of the alpha particle source. The device permits analysis of almost any binary mixture which can be analyzed by means of thermal conductivity cells, with but few exceptions as noted below. In
contrast, the applicability of the analyzer to multicomponent mixtures is more restricted by the relative ionization properties of the pure components in question. Precision, while not equal a t present to commercially available thermal conductivity cells, is satisfactory for many uses, amounting to 0.2 or 0.3 mole %. High response speed and low cost requirements, as well as the other requirements, are all fulfilled by this instrument. The output may be recorded by a variety of recording devices or used in control systems. THEORY AND DESIGN CONSIDERATIONS
If an alpha particle source is placed in a closed gas-filled chamber, the number of ions produced per unit time in the chamber depends, in general, on the nature of the gas for a particular source. These ions render the gas conductive, so that if a pair of electrodes appropriately spaced is placed in the chamber and a voltage is applied to the electrodes] a current flows across the conductive gas and through the circuit external to the chamber. For a binary mixture of gases a t constant temperature and pressure, the rate of production of ions by the alpha particles is a function of gas composition only. If the voltage impressed upon the electrodes is maintained constant, then the current produced is also a function of gas composition only, and measurement and recording of this current make possible the measurement and recording of the gas composition, providing t,hat the device is first calibrated with known gas mixtures. Similar considerations hold for a ternary or more complex mixture of gases, except that for a given spacing of the elctrodes in the chamber and fixed impressed voltage the current is not a unique function of composition. This difficulty can be overcome, in the case of a ternary misture, by submitting the gas mixture to irradiation by alpha particles in a second chamber having a different spacing of electrodes and/or a different applied voltage. After appropriate calibration with known mixtures, the two independent measurements of ionization current yield an analysis. For a C component mixture C-1 independent measurements are necessary. Ionization chambers containing electrodes and a gas are well known devices used for the detection of ionizing radiations. The present gas analyzer chamber differs from such an ionization chamber, in that the gas analyzer chamber contains in addition a fixed known source of ionizing radiations and properly spaced electrodes, and that the gas analyzer is used to determine the 0
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PRESSURE ~
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TEMPERATURE
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DISTANCE FROM SOURCE I C M )
Figure 1. Specific ionization curves for alpha particles from polonium in air and hydrogen
Present address, Shell Development Co., Emeryville. Calif. * Present address, Standard Oil Co. (Ind.), Whiting, Ind.
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V O L U M E 2 7 , NO. 9, S E P T E M B E R 1 9 5 5 c
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D C VOLTAGE APPLIED TO IONIZATION C H A M B E R E L E C T R O D E S
Figure 2.
Typical ionization chamber oiitputapplied voltage curve
composition of the gas within it. Certain operating characteiistics of the gas analyzer are similar to those of ionization chambers; however, some discuqsion of these characteristics, as well as of the characteristics of ionizing radiations in general, are necessary here. Fuller trentment of these subjects is available (1, 6-8). Alpha particles cause ionization in gases by a variety of mechanisms. Whatever the mechanism, for every positive charge created a negative charge is also created, and such pairs of charges are called “ion pairs.” The number of ion pairs, which a particle produces in a gas, depends on the energy of the particle. -1s a given particle loses energy in traversing a gas, the particle’s ionizing power is a function of the distance i t has traveled in the gas, as !vel1 as of its initial energy. Ionizing power of an alpha particle a t a particular distmce from its source is often espressed in terms of the specific ionization i t produces at that point. Specific ionization is the number of ion pails produced per unit of path traversed by an alpha particle. Characteristic curves of specific ionization as a function of distance from the source are shown in Figure 1 for alpha particles from polonium in air and in hydrogen. T h e curve for air \\-as calculated from a curve for alpha particles from radium C in air as given by Lind ( 8 ) ,and the curve for hydrogen was calculated from the data of Hatfield, Lockenvitz, and Young (4). The point at which specific ionization becomes zero is the range of the alpha particles. Ranges of alpha particles from a given source in different gases generally differ R idely. Range is usually expressed in terms of relative stopping power, S , which is the ratio of the range in air a t some standard condition to that in the gas of interest. T h e area under a specific ionization curve is the total number of ion pairs produced by a n alpha particle. This total ionization is expressed frequently as total relative ionization, the ratio of the total ionization in the gas in question to that in air a t standard temperature and pressure. Total relative ionization does not change much from gas to gas, being approximately 1.0 for gases such as hydrogen, nitrogen, oxygen, ammonia, and carbon dioxide. A high total relative ionization is exemplified by t h a t of n-pentane which is 1.38. An analytical chamber designed to measure total ionization is not capable, therefore, of distinguishing very clearly b e b e e n many of the common gases. Furthermore, the analytical cell has to be at least as large as the maximum range encountered in use with any gas mixture. An analytical cell has to be several centimeters in diameter, and holdup in i t is so large for many applications that rapid changes in concentration are averaged out. Such a cell does not give satisfactory performance. T h e analytical cell for binary mixtures is so constructed t h a t its important dimensions are smaller than the minimum range of the alpha particles for all the gas mixtures of interest (Figure 1). If the dectrodes are placed at x1 and x2 in such a manner t h a t
1367 only those ions formed between them are collected, then the ionization current produced when air is in the cell is considerably greater than when hydrogen is in the cell, as can be seen by comparing the areas under the air and hydrogen curves between z1 and r?. The two gases are now easily distinguished from each other. It is this latter design which is used in the present binary apparatus. For a constant gas composition, the ionization current produced is a function both of the rate of production of ions and of the voltage applied to the electrodes. A typical curve of cell output or ionization current against applied voltage is sho\m in Figure 2 . Three regions are readily distinguished. Region A represents that region in which the applied voltage is so low that the ions move slowly enough to permit considerable recombination before they are collected on the electrodes. As the applied voltage is increased, the speed of the ions increases, more ions reach the electrodes before recombining, and cell out,put current increases with increasing voltage. Region B represents a range of voltage over which virtually all of the ions produced reach the electrodes. I n this region the output current is almost constant, regardless of the applied voltage. I n region C, tjhe applied voltage accelerates the ions to the point a t which the ions themselves have sufficient energy to produce additional ions, so that the output current increases rapidly with increasing applied voltage. Region B is the region in which it is most desirable to operate a n analytical cell \%-henanalyzing binary gas mixtures, as no great precautions need be taken to regulate the applied voltage. T h e shape of the output curve varies as the natjure of the gas in the cell is varied, and also as t,he size and configuration of the cell and electrodes are varied. For a given cell, it is necessary t o locate region B for each pure component in the mixture to be used, and then to apply a voltage within this region for the best operation of the cell. For the analysis of ternary mixtures two independent measurements of ionization current (from two different cells, for example) are needed. Some of the considerations pertaining to the operation of a cell in the analysis of binary mixtures can no longer apply to both of t,he cells used in analyzing a ternary mixture, however. This fact can best be demonstrated through a consideration of the theoretical expressions for the output current, for cells operated in the manner recommended for the analysis of binary mixtures, Equations 9 and 11. These equations are derived as follo\vs: Using an empirical expression relating the velocity of a n alpha particle to the distance the particle has traveled from its source, and assuming that the intensity of ionization a t a n y point is proportional to the rate of loss of kinetic energy by the alpha particle, the following expression for the specifir ionization curve has been obtained ( I )
I , = a(L -
z)-1’3
(1)
n-here I , is the specific ionization, L is the range of the alpha particle in the gas in question, P is t,he distance from the source, and a is a proportionality constant. The total ionization, I , is the integral of I , from z = 0 to t = L , so that, from Equation 1,
For an analytical cell whose important dimension, 1, is less than L , and in which the electric field is such t h a t all ions formed are collected, the output current, io,from the cell is given by the expression
(3) where
X. is a ronstant dependent on the rate of emission of alpha
ANALYTICAL CHEMISTRY
1368 particles from the source. From Equations 1, 2, and 3 the following expression is obtained (4) For 1
