ApriI, 1963
IONIZATION O F a - P A R T I C L E S I S
are somewhat greater than those estimated from 20” acidity data; however, this is the direction expected, and of the approximate magnitude predicted, from studies of the temperature coefficient of U(V1) hydrolysis in nitrate medium.*a The question of the importance of other species which may be present in minor amounts is no more resolved by the present results than by the earlier study. For example, it was apparent that about as satisfactory an interpretation could be obtained with a scheme including UOzOH+, although there would be considerable uncertainty in the values of el,l. Similarly, our conclusions for the perchlorate solutions agree in general with those of SuttonJ6though he includes a small contribution of the (3,4) species. The most important point of disagreement with the interpretation of acidity measurements on a core-link
BINARY GAS3 h X T U R E S
825
modelll seems to be resolved, since the St>ockholm group now postulate an important contribution by the (3,5) species, which is not of the core-link type.l2 They still postulate the presence of several higher corelink species, for which we do not find any evidence in our results, but since the constants quoted for these species are rather low, the remaining disagreement is perhaps more in a conceptual model of hydrolysis rather than in a practical description of U(V1) solution chemistry. From a recent publication,13 it appears that even this disagreement may no longer exist. Acknowledgment.-We wish to express indebtedness to Kurt A. Kraus for many helpful and stimulating discussions and to Neva Harrison for technical assistance. (11) S. Ahrland, S. Hietenan, and L. G. SillBn, Acta Chem. Scand., 8 , 1907 (1954). (12) L. G. Sillh, private communication. (13) L. G. SillBn, Acta Chem. Scand., 16, 1051 (1962).
IONIZATIOK BY ALPHA PARTICLES IS BINARY GAS MIXTURES BY T. D. STRICKLER~ Health Physics Division, Oak Ridge National Laboratory,, Oak Ridge, Tennessee, and Department of Physics, Berea College, Berea, Kentucky Receiiied September 31, 196W The W value (average energy loss per ion pair) for a-particles has been measured in a number of binary gas mixtures of molecular gases as a function of the fract’ionalpressures. The W of the mixture ( Wij) can be represented in terms of the 14”s of the pure constituent gases Wi, Wj) and the fractional pressures ( P i , Pj) by the relation: Wij = (Wi -- Wj)Zij” Wj, where Zij” = Pi/(Pi f i j Pj),in whichfij is a constant determined empirically for each pair of gases. These constants verynearly satisfy the relationshipfij = fj/fi = (fj/fk)/(fi/jk) = jkj/jki,where i, j , and k refer to any three gases. Thus, if the followingj-values are assigned to the gases in this study (Nz, 1; COz,1.8; H,, 0.5; 02, 1.3; CH,, 1.8; CeH4, 3.4; GH6, 3.5; CJ&, 4.5; C,Hs, 4.5; C9H8,6.3) then the constant f i j determined from the ratio of any two of these will serve t o predict the W of any mixture of these two gases with an accuracy better than 1%. Slight departures from the W predicted by the above equation have been noted in the case of nitrogen mixtures, indicative of an effect similar to t’hat observed in the noble gases when small amounts of impurities are added.
+
+
Introduction
If an a-particle of kinetic energy Eo is completely stopped in a gas, a,nd in becoming stopped produces Ni ion pairs, then the mean energy lost per ion pair E,/Ni is commonly called the W of the gas for aparticles. The W of most gases lies in the range 20 to 46 e.v. per ion pair and in many cases is found to be practically independent of the energy of the initial ionizing particle. The practical importance of W (in the measurement of radiation dose, in the calculation of energies of particles in nuclear reactions, and in the interpretation of radiation-induced chemical reactions) and its theoretical significance have been pointed out in many recent publications on the ~ u b j e c t . ~ I n the case of binary mixtures of gases, two distinct phenomena have been observed. One is the marked increase in ionization, and consequent decrease in W, when small amounts of some gases are mixed with the noble gases. This has been studied by Jesse and Sadauski^^-^ using helium and neon, and by Melton, Hurst, (1) Dent. of Physics, Berea College, Berea, Kentucky. (2) Operated by Union Carbide Corporation for the U. S. Atomic Energy Commission. (3) (a) S. C. Curran and J. M. Valentine, Rept. Progr. Phys., 21, 1 (1958); (b) W. Binks, Acta Radiol., Splppl., 117, 85 (1954); (c) R. L. Platzman, NAB-National Researoh Council Publication 752, 109 (1960). (4) W. P. Jesse and J. Sadauskis, Phys. Rev., 88, 417 (1952). (5) W. P. Jesse and J. Sadauskis, kbid., 90, 1120 (1953). (6) W. P. Jesse and J. Saudauskis, ibid., 100, 1755 (1955).
and Bortner’ using argon. The effect is attributed, in part, to the excitation of the metastable level in the noble gases and the subsequent ionization of the impurity (by interaction with the excited atom), provided the ionization potential of the impurity is lower than that of the metastable state. This has been referred to as the “Jesse effect.”8 The fact that increased ionization occurs in argon, even when the impurity has an ionization potential greater than that of the metastable level, has been demonstrated by Melton, Hurst, and Bortner,’ but the explanation of the effect is not entirely clear. On the other hand, in the molecular gases, the W of the mixture is found to lie between the extreme values for the pure gases and to change smoothly from one limit to the other as the composition of the mixture is changed. This has been studied by Huber, et U Z . , ~ ~ ~ ~ and by Hurst, et uZ.11--13 It has been shown that the W of any mixture of two of these gases can be expressed (7) C. E. Melton, G. S. Hurst, and T. E. Bortner, ibid.,96, 643 (1954). ( 8 ) R. L. Platzrnan, “The Physical and Chemical Basis of Mechanisnis in Radiation Biology,” “Radiation Biology and Medicine,” W. D. Claus, Ed., Addison-Wesley Publishing Co., Inc., Reading, Mass., 1958, pp. 15-72. (9) P. Huber, E. Baldinger, and W. Haeberli, Helu. Phyg. Acta, 23, Suppl. 111 (1949). (10) W. Haeberli, P. Huber, and E. Baldinger, ibid., 26, 145 (1963). (11) T. E. Bortner and G. S. Hurst, Phys. Rev.,93, 1236 (1954). (12) H. J. Moe, T. E. Bortner, and G. S. Hurst, J. Phya. Chem., 61, 422 (1957). (13) G. S. Hurst and T. D. Strickler, NAS-National Researoh Council Publication 752, 134 (1960).
T. D. STRICKLER
826
0
1
2
3
6
t , ] -1CURVES
5 6 7 8 W I T H P O S I T I V E SLOPE1
9
1
Val. 67
on the particular measurements shown here, it was assumed that these remaining impurities would not greatly affect the results described. Only relative measurements of W were made by comparing the rate of charge collection in the mixture with that in pure nitrogen.’6 Nitrogen was assumed to have a W of 36.3 e.v./ion pair and all measurements reported are relative to this number.Ie The gases used in this study are listed in Table I along with the measured W values. These results are comparable to those measured a t other laboratories, with the possible exception of hydrogen, for which the value quoted in Table I appears to be somewhat 10w.l~ The ionization potential also has been included in the table18 as well as the ratio of W to the ionization potential, since the constancy of this ratio is of some theoretical interest. The constant f shown in column 6 is an empirical constant used in predicting the W of mixtures of the gases listed and is described in the section below.
0
Fig. l.-Plot of W , , vs. Z,,” for mixtures of NZwith COz, 0 2 , CH,, CZHZ,CdL, C Z H ~C3He , (cyclopropane), CT”, C ~ H S(1butene), and CAHS(2-butene) using Z,,” calculated from measured constants shown in column 3 of Table TI.
e.v./ion pair
Gas -
--
__
1
I
Fig. 2.-Plot of W,, vs. Z,,” for mixtures of Hz with COS, CH,, CzH2,C2H,, C3H6(cyclopropane), and C4Hs (1-butene) using Z,,” calculated from measured constants shown in column 3 of Table 11.
in terms of the W’s of the pure gases, the partial pressures, and an empirical constant characteristic of the two gases. This report coiisists mainly of an extension of the work on binary gas mixtures in a n attempt to determine more of these empirical constants and to study possible relationships among them.
Experimental Technique and Results with Single Gases The measurements reported here were made with the same ionization chamber and associated equipment described and used by Bortner and Hurst.I1 This consisted of a large parallel-plate ionization chamber enclosed in a vacuum tank capable of holding gas a t pressures up to 2 atm. Alpha particles from the uncollimated source of PuZ3O,located a t the center of the bottom plate, were completely stopped in the chamber and the total ionization was measured by the time required to charge a capacitor to a fixed potential. Electric fields up to 103 volts/cm. could be applied to thechamber. Since W is practically independent of the CY-particleenergy, a rather strong Puzsgsource (approximately 3 X lo6 c.p.m.) was used in the chamber even though the self-absorption in this source was quite high. A spectral analysis of this source showed that over 90y0 of the ionization was from a-particles of energy above 3.0 MeV. No purification techniques were employed and commercially available gases were used throughout. These were reported by the manufacturers t o have purities of 99% or more in all cases except acetylene.14 Since small impurities have no great effect
Sitrogen Carbon dioxide Hydrogen Oxygen Methane Acetylene Ethylene Ethane Cyclopropane Propylene Propane IbObutylene 1-Butene 2-Butene Toluene a Assumed value. By from NTC& mixture.
pot.,
Ionization pot.
f 36.3“ 15.6 2.32 1.0” 34.1 13.85 2.46 1.8 35.8 l5,43 2.32 0.5 32.1 12.1 2.65 1.3 28.9 13.12 2.20 1.8 27.4 11.42 2.40 3.3 27.85 10.56 2.64 3.4 26.4 11.65 2.27 3.8 25.8 10.23 2.52 4.5 27.0 9.80 2.76 ... 26.1 11.21 2.33 4.5 26.6 9.35 2.85 ... 26 5 9.72 2.73 6.3 9 3 2.81 6.3 26.1 2gc 8.7 3.22 15 definition. Value extrapolated V.
Summary of Results Using Mixtures of “Regular G.ases”--The &Ratio This report is concerned mainly with the determination of the W of mixtures of two gases. It has been shown1l that the W for a mixture of two “regular gases” (in which the Jesse effect is not pronounced) can be given by
where zij
Pi
= Pi
+
UijPj
Here W i j refers to the W of the mixture, W i and TVj to that of the pure gases, each of which is present with fractional pressures P i and Pi, and aij is a constant determined empirically for any particular pair of gases. (14) Mass spectrographio analysis of a similar tank of acetylene showed: Cnf-12 (97.8%), NZ CO (1.40/0),acetone (0.5%), HzO (0.3%). (16) Mas- spectrographic analysis of actual tank of nitrogen showed: Kz (QQ.997%), HnO (0.0008%), and Ar (0.002%). (16) Measurements by Jesse, substantiated recently by the group at Oak Ridge National Laboratory, indicate that a more accurate value for this FT’ of pure nitrogen for a-particles in this energy range would he 36.6 e.v./ion pair, in which case all measurements reported here would be increased by about 1%. (17) Mass spectrographio analysis of the tank of hydrogen showed: He (99.3%), HnO (0.3%), Nz CO (0.26%), with tracesof CHI, 0 2 , HC, Ai-, and
+
+
COZ. (18) F. J. Field and L.
J. Franklin, “Electron Impact Phenomena.” Academic Press,New York, N. Y., 1957, pp. 105-127.
April, 1963
IONIZBTION O F CY-PARTICLESI N
Z’fj-( 1.0
CURVES W I T H NEGATIVE .6 .5 4
.I
0
I
I
I
I
I
I
I
.3
4
.5
.6
.7
.8
I
I
.9
1.0
of Wij vs. Zij” for mixtures of COz with 0 2 , CHa, C~HZ,C2H4, CzHs, C3H6 (cyclopropane), C3H& C ~ H S(1-butene), and C4Hs (2-butene), using Zii” calculated from measured constants shown in column 3 of Table 11.
where
Cyclopropane 1-Butene O?
Zij” =
Pi
Pi
+
fijpj
If the measured value of Wij is plotted against Zij”, a linear result is obtained (as predicted by eq. 2), provided the proper value of the empirical constant fij is used for any pair of gases. This has been done for many pairs of gases listed in Table I. The results are shown in Fig. 1 through 5 and the empirical constant f i j associated with each pair of gases can be found in column 3 of Table 11. These constants have each been TABLErI MEASURED AND CALCULATED RATIOSf j / f i FOR VARIOUS COMBINATIONS OF MOLECULAR GASES fj/’fi
