RADIOLYSIS OF WATERBY PARTICLES OF HIGH LINEARENERGY TRANSFER
June, 1959
833
RADIOLYSIS OF WATER BY PARTICLES OF HIGH LINEAR ENERGY TRANSFER. THE PRIMARY CHEMICAL YIELDS I N AQUEOUS ACID SOLUTIONS OF FERROUS SULFATE, AND I N MIXTURES OF THALLOUS AND CERIC IONS BY MARCLEFORTAND
&VIER
TARRAGO
Ddpartement de radiochintie, Laboratoire de Physique Nuclbaire de la Facult6 des Sciences de Paris, Or.sau, Prance Received J a n u a r y 23,1969
I n order to measure the primary chemical yields of water radiolysis, mixtures of thallous and ceric ions in aqueous 0.8 N HzSOasolutions were irradiated by polonium a-particles. Absolute dosimetry measurements were carried out. A G-value of 5.5 & 0.1 was found for ferrous sulfate oxidation. From this value and from results obtained on the ceric system, the following values were determined: GOA = 0.50, &os = 0.11, Ga,o, = 1.45, GH = 0.60, G H ~= 1.57 and GH*O= 3.62. However these yields depend on the concentration of thallous ions which inhibit the intratrsck reaction H z 0 ~ OH -+ HzO HOa. An explanation also is given for the low yield (3.6) obtained for the oxidation of ferrous sulfate in deaerated solutions.
+
+
Although a large part of the early work in radia- tion of oxygen from pure water irradiated with ation chemistry was done on mixtures of radon with rays, which shows a linear increase with the abother gases, studies of r a d i a t i ~ n l - induced ~ reac- sorbed dose. Hart13 confirmed the primary formations in water were carried out by a number of pio- tion of HOz with light particles in his study of yneers among them S. C. Lind5 who investigated the ray induced reactions in aqueous ferrous sulfateradiolysis of aqueous potassium iodide solutions by cupric sulfate solutions. With similar solutions a-particles. He also wrote the first monograph on irradiated by polonium a-rays, Donaldson and radiation chemistry,6 a book which is of interest Miller14 measured the oxygen yield (Go, = 0.23) even today. In the period between the wars, few and suggested that HOzis a primary product of wa, ~ decomposition. Pucheault15 has assumed that papers appeared on the chemical effects of a - r a y ~ ~ter ~ while much work was reported on reactions induced the values of primary yields GOH, GH, G H O and by X-rays, probably because the latter are easier GH,o could vary in the presence of a solute which and less dangerous to use than the radon techniques. might scavenge more or less OH radicals or H atMore recently, however, interest has been aroused in oms and therefore inhibit the local reaction of the chemical effects of particles of high linear energy these species with HzOz or Hz. Intratrack reactransfer (LET) by studiesg-10 on the formation and tions are extensively discussed in references 16 and interaction of radicals in the neighborhood of the 17. The purpose of our research was first to make an tracks produced by the particles. Furthermore, new sources of such particles have become available. accurate determination of the oxidation yield of Attempts have been made to determine the molec- ferrous sulfate in aqueous sulfuric acid solutions irular and radical yields along tracks of high LET radiated with polonium a-rays, and second to obparticles. It has been shown recently, mainly with tain more information on the magnitude of intrapolonium a-particles that in the radiolysis of water track reactions and on their dependence on different the radical HOz is produced inside each spur in solutes. We have called “primary products’’ all those nearly the same time as molecular hydrogen and hydrogen peroxide; it is believed that the local which are formed in dilute solutions from water, without direct interaction of the radiation on solfollowing reactions occur utes. These products may be formed by inany OH HzOz +HOz HzO (1) mechanisms such as ion neutralization, Stern-VolH HzOz +OH + Hz0 (2) mer reactions, ion-molecule reactions or radical Reaction 1 which was proposed in 1951l’ was later combinations a t a very early stage. For the yield of shown t o be an intratrack reaction because of the primary products we have used the notation GH, high local concentration of OH radicals and H202 GOH, while the fneasured yields of stable products molecules.12 It was assumed to explain the produc- are written as G(H202),G(H2).. . . A. Oxidation of Ferrous Sulfate in Aerated ( I ) A. T. Cameron and W. Rainsay, J . Chem. Soc., 91, 931 (1907); 92, 966 (1908). Aqueous Solutions, 0.8 N H2S04 (2) J. Duane and 0. Scheuer, Le R a d i u m , 10, 33 (1913). (3) $1. Kernbautn, ibid., 6, 225 (1909). Experimental
+ +
+
A. Debierne, Ann. Phy8., 2, 115, 126 (1914). ( 5 ) 8. C. Lind, Le R a d i u m , 8, 289 (1911). ( 6 ) 9. C. Lind, “The Chemical Effects of a-Particles and Eleo(4)
trona,” Interscience Publishers, New York, N. Y., 1921. (7) %. C. Lanning and S. C. Lind, THISJOURNAL, 42, 1229 (1938). (8) C . Nurnberger, ibid., 38, 47 (1934); 41, 431 (1937). (9) D. E. Lea, Brit. J . Rad., sup. 1 (1947). (10) L. H. Gray, J . c h i m . p h y s . , 48, 172 (1951): M . Lefort, ibid., 47, 624 (1050) L. Monchick, J. L. Magee and A. H. Samuel, J . C h e m . Phga., 26, 935 (1957); H. Fricke, Ann. N . Y.A c a d . Sci., 59, 567 (1955); D. A. Flanders and H. Fricke, J . C h e m . P k y s . , 28, 1126 (1958). (11) h1. Lefort, J . e h i m . phyc., 48, 339 (1951). (12) M. Lefort, “Actions chimiques et biologiques des radiations,” s6rie 1, Masson e t Cie., Paris, 1955, p. 118. ~
Several drops of carefully purified polonium in 0.3 N nitric acid (50 microcuries per drop) was evaporated on a steam-bath in a silica crucible which had been irradiated with ?-rays. Following Dr. Hart’s advice, we used polonium chloride in several runs instead of a nitric acid solu(13) E. J. Halt, R a d i a t i o n Rea., 2, 33 (1955). (14) D. hf. Donaldson and N. Miller, T r a n s . F a r a d a y Soe., 62, 652 (1956). (15) J. Pucheault, C o m p t . retid. ad. SCP., 246, 409 (1958). (16) A. 0. Allen and H. A. Schwarz, Proceedings of the second international conference on the peaceful uses of atomic energy-Geneva, 1958. (17) A I . Lefort, -4nn. Rea. Phus. Clrem., 9, 123 (1958).
834
A4.4RC
n
I
3.103
1 ~
2.103
P
i
a‘
LEFORTAND %VIER TARRAGO / I
4 7
Vol. 63
TABLE I OXIDATIONOF FERROUS IONI N SOLUTION IRRADIAT~D WITH CY-PARTICLES Mean
G-valur
I
Polonium, rc./co.
103
30 40 50 Channel. Fig. 1.-The counting of a-particles for absolute dosimetry: 9, background measurements. The straight line indicates the energy calibration of pulse amplitude. 20
tion; however, we obtained the same yield in the presence or absence of chloride ion. The M ferrous sulfate solution then was added. The solution was stirred and transferred to spectrophotometer cells. Formation of ferric ions was followed by measurement of the optical density at 3040 A. The cells were thermostated at 25 f 0.02’ and the extinction coefficient was found to be 2205 f 20 mole-’ cm.-’. The usefulness of the Fricke dosimeter has been discussed frequently in connection with X- and r-rays. The polonium activity in the kradiated solution was measured with a pulse ionization chamber collecting free electrons, and a multichannel system for pulse height recording and measurement. A very small aliquot of the solution was diluted to serve as a-emitter source. The a-ray width was measured and compared with that of a pure polonium emitter. When the width was larger because of self absorption, the preparation was not used for dosimetry (Fig. 1). The pulse ionization chamber was filled with a mixture of COz and argon and worked at 4.500 volts. It had been calibrated with standards and the maximum error of the counting measurements was lower than 1%. The dilution factor was of the order of 2:10,000. We made several determinations of activity for the same initial solution and checked that the mean square deviation was about 2%.
Schuler and Barr,18working with lo-’ M ferrous sulfate solutions, report a yield of Fe+++ of 4.22 i 0.08 for B(n,a)Li recoils, and 5.69 f 0.12 for ‘jLi(n, a) 3He. The first published yields for polonium a-rays (5.3 Mev.) ranged from 5.9 to 6.2,19but recently values of 5.6*O and 5.221 have been reported. Our results are recorded in Table I from which a value of G F ~ + + + = 5.5 f 0.1 was obtained. The ferric ion yield was independent of the polonium concentration, ie., of the dose rate; between 20 and 1000 microcuries per cc.; (1.4 X 10l6 e.v. cc.-I hr.-l to 1 X 10’8e.v. cc.-l hr.-l), and remained almost constant throughout the oxidation. G F e + + + was also independent of the ferrous sulfate concenM , but tration between 2 x lob4 and 2 x with a concentration of low2M , we obtained a yield of 5.65 which seems t o be definitely higher than that of more dilute solutions. This is in agreement with Schuler and Barr’s18 results on the oxidation rate induced by B(n, a)Li recoils. (18) R . H. Scliuler and N. F. Barr, J. A m . Cham. Soc., 78, 5756 (1956). (19) (a) M. Haissinsky and M.C. Anta, Compt. rend. acad. a m . , 236, 11631 (1953); (b) N. Bliller, Trane. Faraday Soc., 60, 690 (1954); (c) R . XfoDonelI and E. J. Hart, J. A m . Chenz. Soc., 7 6 , 2121 (1954). (20) RI. Lefort, Compt. rend. acad. s c i . , 248, 1023 (1957). (21) C . N. Trumbore, J . A m . Chem. SOC.,80, 1772 (1958).
20 50 100 100 200 250 250 1000
Measd. alonq-the dose Time of Oxidation straight rate X 1017, observation, rate X 1016, line of e.v./cc. hr. ions C C . - ~ hr.-1 oxidn.
0.11 .30 .65 .76 1.18 1.75 1.82 6.80
162 80 20 20 12 19 19 8
0.60 1.55 3.59 4.25 6.50 9.50 10.20 37.40
5.45 5.50 5.52 5.58 5.50 5.40 5.60 5.50
B. Reduction of Ceric Ion Induced by a-Particles Experimental Several authors22-24 have studied the reduction of ceric ions in dilute aqueous solutions of H2S04 by polonium aparticles. We determined this yield in 0.8 N H2S04 by the technique used for fefious sulfate. Very pure solutions were prepared as described previously.24 The disappearance of ceric ion? was measured directly in spectrophotometer cells a t 3200 A. for the most dilute solutions. We have taken an extinction coefficient of 5580 f 50 mole-’ cm.-l. For irradiation in the absence of oxygen, 20 to 30 cc. of solution was degassed under vacuum at the same time as blanks, by trapping and warming up several times. We found that the ion yield was independent of both the dose rate and the ceric ion concentration. The G-value was constant up to high doses and dropped off slightly when nearly all the ceric ions had been reduced. As shown in Fig. 2, there was a difference between the reduction yields determined in the presence and in the absence of air. G,,, = 1.05 f 0.01 (Fig. 2) GV,,. The ratio G(Fe+++)/G(Ce+++)also was determined on the same solution by the following procedure: the reduction of ceric ions was followed for some time, and then 1 cc. of 2 X molar ferrous sulfate solution was added to 2 cc. of the initial ceric and polonium mixture. After stirring, all the remaining ceric ions were reduced. Ferric ions were measured, and the value obtined was used to verify the direct determination made with the spectrophotometer a t 320 mM. Then the oxidation of the excess ferrous ions in the new solutipn was followed; the dose rate was 2/3 of the initial rate. The ratio G(Fe+++)/C(Ce+++)could be determined with great precision; we had found that cerous ions have no effect on the oxidation yield of ferric ions. I n presence of air G(Fe+++)/G(Ce+++) = 1.72 i 0.02 (Fig. 3). Gas analysis was carried out with an apparatus built in the laboratory several years ago,26 by measuring the total pressure with a McLeod gauge and the ionization current produced in an alphatron. In this apparatus, the ionization source is a polonium deposit in equilibrium with RaD (Pbz’o). The ionization current produced through the mixture of gases is proportional to energy loss per unit of track length of a-particles, dE/dx. This value is proportional to the pressure, and the ionization current is therefore proportional to the pressure. dE/dx is very dependent on the nature of gases, particularly on the oxygen and hydrogen content of the mixture. After careful calibration we were able to analyze in one minute mixtures of hydrogen and oxygen with a precision of I%, when the total amount was of the order of 10 mm.3 at atmospheric pressure. Table I1 gives our results. TABLEI1
g(E5) 5.5*00.1
C(Ce”1 aer.) 3.20*0.06
G(Ce1” in vacuo) 3.10fO.OF
G(W G(02) 1.57=t00.05 1 . 5 7 & 0 . 0 5
(22) M . Lefort and M. Haissinsky, J. chim. phgs., 48, 368 (1051). (23) M . C. Anta and RI. Haissinsky, ibid., 61, 33 (1954). (24) M.Lefort, ibid., 64, 782 (1957). (25) M. Lefort, J. phus. radium, 178, 105 (1950).
June, 1959
C.
RADIOLYSIS OF WATERBY PARTICLEB OF HIGHLINEARENERGY TRANSFER
835
Aqueous Acid Solutions of Ceric and Thallous Ions
The previous data are of interest for the following experiments from which primary yields of water decomposition could be calculated. Sworskian has shown that the reduction of ceric ions by yradiation follows the rate: G(CeII1) = 2GH20r GH - GOB; he also found that the addition of thallous ions a t a concentration high enough to scavenge every OH radical increases the reduction yield to ~ G H ~ O ~ , GH GOH,according to reactions 3 and 4 T1I OH +TI11 OH(3) TI11 CeIV +Tl'II Ce"1 (4) If H02 is locally produced through reaction 1, the yield of 0 1 2 3 4 5 6 7 8 cerous ions in the absence of thallium is not changed since Time, hours. one ceric ion is reduced by one HO2; this is strictly equivalent t o the reduction of two ions by H2Oz and the reoxidaFig. 2.-The rate of reduction of ceric sulfate: CeIV, 1.4 tion of one cerous ion by OH. This is true also for ferric X lo4M ; dose rate 2.2 X lOl7e. v. c c - 1 hr.-l; X, reduction ion formation in the presence of oxygen, since 3Fe+++ions in evacuated solution; 0, reduction in aerated solution. are oxidized by H 0 2 as well as by HIOz OH. Moreover, the addition of thallous ions to ceric sulfate solutions permits the determination of CEO,,as can be seen from the equations G(CeII1) = ~GHZO~GH G H O ~ GOH ( 5 ) O ~BGA DOH aGHoz G(FelI1) = Z G H ~-t (6) G(Celll)(Tll added) = Z G H ~ O ~GH G H O ~ Gon ( 7 ) G(Hd = CHI = G H ~ 4o ~'/~GoH- '/~GH a / & ~ ~ (8) l From Sworski's experiments,a equation 7 is obtained with M for thallous ions and initial concentrations of 2 X to 10-8 M for ceric ions. Such mixtures were irradiated with dissolved polonium in the absence and presence of av. The disappearance of ceric ions was followed spectrophotometrically until half of the ceric ions had disM ferrous sulfate solution then was appeared. A added; the ferric ions formed are a measure of the amount of ceric and thallic ions remaining in the solution. Therefore G(T1111) is equal to the difference between Gpo+++and Go,* determined above. From the above scheme GOH = G(T~III),but the difference is small and the precision is not good. The results were very reproducible, much more so than for solutions containing only ceric ions (Fig. 4). The yield was constant as long as the ratio of the concentrations of ceric 10IBe.v. cc.-l. to thallic ions was large. However, the initial concentraFig. 3.-The measurement of the reduction of ceric sulfate tion of thallous ions has a slight effect on the initial yield of ceric ion reduction. Table I11 gives the values which were and the oxidation of ferrous sulfate; -, dose rate: 0.26 obtained in evacuated solution for several thallous concen- lo1*e.v. cc-1 hr.-1 in ceric ions solution IO-' M; 0.175 10l8e.v. cc.-1 hr.-l in ferrous ions solution; X, the measuretrations, togebher with the yields Goa, GaoI, G H ~ owhich ~, ment of cerous ions; 0 , measurement of ferric ions. The can be calculated from the equations dotted line indicates the addition of ferrous solution to the (7) - (6) = G(Fe) - G(Ce TI) = 2(GH GEO~) ceric sulfate solution and the first point corresponds to the oxidation of ferrous ions by remaining ceric ions. T1) - G(Ce) = ~ G O H = 2G(TlIII) (7) ( 5 ) = G(Ce (6) 2 (8) =: G(Fe) - 2G(H2) = ~ G H 5 .1 3(7) - (6) = 3G(Ce TI) - G(Fe) = 4&30a WOE I I The values given in Table I11 are in accord with the equa4 tion Gtor, = G H ~ o ~ GHO~,. a finding which confirms our I results. Thus, the radiation induced decom osition of water may be calculated by -GH~o = GH +2&, GHO~ = GOH+ ~ G H O Z ~ G H Z O Z . In presence of aw the reduction yields were slightly higher, and the ratio GBeI/GV&. is the same as that found for pure ceric solutions, 1.05. Although values for the absolute yields are given within ?yo,differences between yields, corresponding to differences in thallous ion concentration, have a much better precision. Discussion of Table 111.-(1) The increase of 0 1 2 3 4 5 6 7 G(Ce+++) with initial thallous concentration inTime, hours. volves an increase of GOH and a decrease of GHO~. Fig. 4.-The effect of presence of thallous ions in irradiaIntratrack reaction 1 is therefore inhibited by thal- tion of deaerated solutions of ceric ions: I, T1 10-8, Ce 2 X lo-' M dose rate 1.81 X 10'7 e.v. cc. -1 hr.-l; 11, Ce 2 X lous ions. An upper limit of 0.22 f 0.2 could be lo-' dose rate 1.44 X lo1' e.v. m-1 hr.-l.
+
+
+ +
+ +
+
+ + + + + + +
+
-
+ + + +
+
'
+
_.
-
extrapolated for GHO~, a value which is in agreement with Donaldson and Miller's re~u1ts.l~ (2) It is therefore believable that when solutions containing only ceric ions are irradiated, the initial yield of reduction is equal to Gee+++ = ~GH,o, G H O ~ GH - GOH with GH,o, = 1.35; GHO, =
+
(20)
T.J. Sworski, Radiation Research, 4, 483 (1956).
+
1
I
4
0.22, GOH = 0.40 and GH = 0.60. This is in disagreement with Pucheault's calculations16 in which GHO, was assumed to be negligible for dilute solutions of ceric ion. Ai (3) At concentrations greater than 2 X thallous ion, the oxygen yield decreases, G(Ce+++)
MARCLEFORTAND XAVIER TARRAGO
836
Vol. 63
TABLEI11 IRRADIATION OF DEAERATED SOLUTION CONTAINING CERIC AND THALLOUS IONS Initial
TI1 conon.,
M
3 X 10-6 10-4 2 X lo-' 10-3 2 X IOd3 7 X 10-
f0.1
G(Hd
G(0d
G (T P ) 2t0.2
3.90 4.0 4.05 4.1 4.18 4.25
1.57 i O . 0 5 1.57 zk 0 05 1.57 1.57 1.58 1.58
1.56 f 0.05 1.55
0.6 .5
1.55 1.58 f 0.05 1.30 f 0 . 1
.6 .8
G(Ce"1)
is unaffected, and apparently G(Tl+++) increases. As GHO~ cannot be decreased any further, and since G@z) = GHOI -k GHIOWit follows that GHZOZ decreases from about 1.48 to 1.25. The concentration of thallous ions is SO high that within each spur, thallous ions compete for OH radicals with the combination of OH radicals and thus inhibit the primary formation of HzOz. No change is found for G(Ce+++) since ceric ions which are not reduced by HzOz are destroyed by: TIr1 according to reaction 4. Thls effect seems similar to Miller and Donaldson's observation on the decrease of G(Hz) when the cupric ion concentration is increased to molar. (4) It is possible t o compare the mean concentration of hydrogen peroxide when it goes into reaction with OH radicals along the a-track with the HzOz concentration in the bulk of a solution irradiated by y-rays. From our results we can assume that a concentration of 2 or 3 X lou3molar thallous ion is necessary to suppress GHO~,while a concentration of 3 X 10-6 M is high enough, from Sworski's experiments, to protect OH radicals against back reactions in the bulk of the solution. This means that the concentration of hydrogen peroxide is about 100 times higher in local cylindrical columns where intratrack reactions occur during expansion of the track, than in the bulk of the solution. (5) The slightly larger reduction yield in aerated solutions is difficult to understand since KOz reduces ceric ions as well H atoms. Positively OXYgen is acting as a strong scavenger for H atoms or free electrons, and therefore the primary yield GH is enhanced while GH, decreases.
D. Comments on the Yield of Oxidation of Ferr o w Sulfate in Deaerated Solutions
If we use the primary yields obtained above for a calculation of ferrous sulfate oxidation in the ab+ ) 4.36. sence of oxygen, G ( F ~ + + =
GOK
GHOZ
GHZOZ
GH
- GHZO
0.40 .43 .45 .50 .54 .8
0.20 .16 .15 .11 .08 .02
1.36 1.41 1.42 1.45 1.50 1.25
0.59 .59 .59 .59 .58 .58
3.52 3.57 3.58 3.62 3.70 3.70
+ +
G(F~+++) = ZGH~OZ GH
+ GOH = 2.7 + 0.60 + 0.66 + 0.40 = 4.36
*
3GHOZ
This is much higher than the value of 3.6 observed by several authors14.20 together with G(Hz)= 1.8. There is no reason to reduce GHOz= 0.23 since Donaldson and Miller14 found a yield of 0.23 when cupric ion is added to ferrous sulfate, under conditions where reaction 1 can occur in the same manner as for pure ferrous ions. On the other hand, Ga may be calculated from G(H2) - G H ~since ferric ions are formed through reaction 9 Fe++(H,O),
+
-
FeOH++ + HZ + (H20)n-l
(9)
GH* has been found t o be 1.6 and G(H2) = 1.8. Then with GH = 0.2, GHO, = 0.23 and G(Fe+++) = 3.6 ~GHSOZ+ Goa = 3.6 - 0.2 - 0.69 = 2.71 This value is much lower than in the cupric-ferrous solutions (3.12). But cupric ions, as well as oxygen molecules in aerated solutions of ferrous ions, scavenge atomic hydrogen and therefore inhibit intratrack reaction 2. However, when only ferrous ions are present, reaction 2 occurs and could explain a drop in GH,O~ from 1.35 to 0.95 because of a yield of 0.4 for reaction 2. GOH would increase for the same reason from 0.4 t o 0.8 and ~GH,o, GOHwould decrease to 2.7. As it has been suggested several time^'^^'^^^^ it is difficult to assign the same Primary Yields for all aqueous solutions; for example, aerated and deaerated aqueous solutions of ferrous sulfate support the Yields 8s in Table Iv.
+
TABLEI V
+
Fe+++ 0 2 Fe+++in vacuo
GR
G H ~ GOH
GHOZ GHZOZ - O H Z U
0.59 1.57 0 4 0 . 2 2 1.35 0 . 2 1 . 6 0 . 8 0 23 0.95
3.55 3.2
The yield of decomposition of water itself varieP according to the inhibition of intratrack reactions 1 and 2 by solutes. The lower and upper limits seem t o be 3.2 and 3.8 for 5.3 mev. a-particles.
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