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Neutron and β/γ radiolysis of water up to supercritical conditions. 1. β/γ yields for H2, H· atom, and hydrated electron

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Neutron and β/γ radiolysis of water up to supercritical conditions. 1. β/γ yields for H2, H· atom, and hydrated electron
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  Neutron and    /  γ  Radiolysis of Water up to Supercritical Conditions. 1.    /  γ  Yields for H 2 ,H • Atom, and Hydrated Electron Dorota Janik, Ireneusz Janik, and David M. Bartels*  Radiation Laboratory, Uni V  ersity of Notre Dame, Notre Dame, Indiana 46556  Recei V  ed: March 3, 2007; In Final Form: June 7, 2007  Yields for H 2 , H • atom, and hydrated electron production in    /  γ  radiolysis of water have been measured fromroom temperature up to 400  ° C on a 250 bar isobar, and also as a function of pressure (density) at 380 and400  ° C. Radiolysis was carried out using a beam of 2 - 3 MeV electrons from a van de Graaff accelerator,and detection was by mass spectrometer analysis of gases sparged from the irradiated water. N 2 O was usedas a specific scavenger for hydrated electrons giving N 2  as product. Ethanol- d  6  was used to scavenge H • atoms, giving HD as a stable product. It is found that the hydrated electron yield decreases and the H • atomyield increases dramatically at lower densities in supercritical water, and the overall escape yield increases.The yield of molecular H 2  increases with temperature and does not tend toward zero at low density, indicatingthat it is formed promptly rather than in spur recombination. A minimum in both the radical and H 2  yieldsis observed around 0.4 kg/dm 3 density in supercritical water. I. Introduction Commercial nuclear reactors essentially provide a source of heat used to drive a “heat engine” (turbine) to create electricity.A fundamental result of Thermodynamics shows that the higherthe temperature at which any heat engine is operated, the greaterits efficiency. Consequently, one obvious way to increase theoperating efficiency and profitability for future nuclear powerplants is to heat the water of the primary cooling loop to highertemperatures. Current pressurized water reactors run at roughly300  ° C and 100 atm pressure. 1 Designs under considerationwould operate at 500  ° C and 250 atm, 2 - 6 i.e., well beyond thecritical point of water. This would improve the thermodynamicefficiency by about 30% and allow considerable reduction incost. A major unanswered question has been, what changesoccur in the radiation-induced chemistry in water as thetemperature and pressure are raised beyond the critical point,and what do these imply for the limiting corrosion processes inthe materials of the primary cooling loop?Direct measurement of the chemistry in reactor cores isextremely difficult. The extreme conditions of high temperature,pressure, and radiation fields are not compatible with normalchemical instrumentation. There are also problems of access tofuel channels in the reactor core. For these reasons, all reactionvendors and many operators have extensively used theoreticalcalculations and chemical models to simulate the detailedradiation chemistry of the water in the core and the consequencesfor materials. 7,8 The results of these model calculations can beno more accurate than the fundamental information fed intothem, and serious discrepancies remain between model calcula-tions and reactor experiments. 8,9 The problem of modeling asupercritical-water-cooled reactor is even more daunting. Anumber of studies have been published in the last several yearswith the aim of providing the necessary fundamental informationneeded to model radiation chemistry in supercritical water. 10 - 23 Both reaction rates and radiation yields ( G -values) for theprimary free radicals  • OH, H • , and e aq - are required, as well asfor the recombination products H 2  and H 2 O 2 . Moreover, inreactor cores radiation is deposited both via  γ  radiation andenergetic neutrons; 24 this paper represents the first in a seriesthat will report  G  values from both    /  γ  radiation and neutronsusing the same detection methodology.To transfer the information to other systems for modelingstudies, it is very important to know precisely the temperatureand pressure of the fluid under irradiation. This is much easierto achieve with a flowing system than with sealed samples. Forneutron experiments a high-temperature flow system wasconstructed for a small nuclear reactor at the University of Wisconsin, as will be described in subsequent papers. As asource of low-LET radiation for high-temperature experiments,we have found it very convenient to use an electron beam froma 3 MeV van de Graaff accelerator. The choice of detectionmethod and scavengers is dictated by the characteristics of thereactor. The simplest method with sufficiently high sensitivity,reliability, and versatility is the detection of stable gas productsproduced by the radiation 25 using a mass spectrometer.In the following section we describe in some detail thedetection technique that is common to both experiments. Thescavenging experiments and results are then described, and inthe Discussion we compare these results with others in theliterature. II. Experimental Section The   -radiolysis experiments were performed at the NotreDame Radiation Laboratory using a custom-made supercriticalwater (SCW) irradiation block and 2.5 MeV electrons from a3.0 MeV van de Graaff (VdG) accelerator. The apparatusconsisted of sample reservoirs and pumps, a high pressure/ temperature irradiation flowtube, and ambient pressure/temper-ature analysis setup with a directly coupled mass spectrometer(Figure 1).Two glass water reservoirs with copper or stainless steelconnection lines under atmospheric pressure were used to supply * To whom correspondence should be addressed. E-mail: bartels@hertz.rad.nd.edu. Phone (574) 631-5561. Fax: (574) 631-8068. 7777  J. Phys. Chem. A  2007,  111,  7777 - 778610.1021/jp071751r CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 07/24/2007  two independent Alltech 301 HPLC pumps. The final composi-tion of the solution was achieved by changing the flow ratio of the two pumps, keeping the total flow at 6 mL/min. Allexperiments used water purified by the Serv-A-Pure Co.cartridge system (resistivity 18 M Ω  cm, total organic carbon < 5 ppb as CO 2 ) and the solutions were bubbled with therequired gases. A mixture of 20% H 2  in N 2  was used for massspectrometer (MS) calibration, a mixture of 10% N 2 O in Arwas used for the yield measurements, and ultrapure Ar was thesparging gas in the MS detection system. All gases were UHPfrom Mittler Supply, Inc. Absolute ethyl alcohol (200 proof,Aaper Alcohol and Chemical Company), deuterated ethanolCD 3 CD 2 OD (Cambridge Isotope Laboratories, Inc., anhydrous,99 + atom % D), and phenol (Aldrich, redistilled, 99 + %) wereused as received from vendors.The high temperature/pressure flow cell consisted of twopartially separated sections. In the lower section the solutionswere pumped through Hastelloy  1  /  16  in. tubing that was wrappedaround a cylindrical electric heater (1000 W, 120 V). Then,after preheating, solution was introduced to an irradiation zonemade from  1  /  8  in. titanium tubing (0.06 mL total volume) inthe upper section. Behind the irradiation zone a second cartridgeheater (250 W, 120 V) was placed to maintain the temperatureand compensate for the electron beam heating. The temperatureof solution before and after the irradiation zone was monitoredwith a pair of type-K thermocouples with readout by OmegaCN77000 temperature controllers whose accuracy is specifiedas  ( 0.4  ° C. The entire assembly was enclosed in a stainlesssteel block (with a window cut for easier electron penetrationthrough the irradiation volume) and placed in an insulating box(Rescor Ceramic Board, Cotronics Corp.). A fiber-optic wasplaced in a slit in the insulation in front of the irradiated tubingto monitor the dose by means of generated Cerenkov light andfluorescence. The optical signal was detected by a siliconphotodiode and was monitored in one of the spare A/D channelsin the MS (Pfeiffer Vacuum Prisma) along with the molecularion signals.To maintain stable temperature and pressure conditions duringmeasurements, the flow rate through the system was keptconstant. To provide a stable pressure drop, a 0.004 in. i.d.  1  /  16 in. capillary was immersed in a temperature-controlled waterbath. Before reaching the capillary, solution was cooled in theheat exchanger, consisting of several coils of   1  /  16  in. Hastelloytubing immersed in the same water bath, and passed through a5  µ m filter. Changing the temperature of the water bath changesthe viscosity of the solution to provide precise back-pressurecontrol at a given flow rate. Pressure in the system, with anoverall stability of roughly  ( 0.5 bar during each single run,was monitored with a calibrated pressure transducer (OMEGAPX01 series) and strain gage meter (OMEGA DP25B-S). Themanufacturer’s specification is for  ( 0.2 bar accuracy of thepressure measurement. After reaching the lower temperature/ pressure in the water bath, the solution flows through 15 m of  1  /  16  in. stainless steel tubing (i.d. 0.01 in.) to the detection setupoutside the radiation vault.To analyze dissolved gases, precisely 12.0 mL (2 mincollecting time at flow 6 mL/min) of irradiated solution wascollected in the sparging vessel, and then bubbled with UHPargon. The argon stripped out any gaseous products from theirradiated solutions and carried them through the water trap (0.25in. o.d., 3 m long coiled column packed with 4A MolecularSieves) toward the inlet of the MS capillary. The capillary, madefrom 3 cm long and 25  µ m i.d. fused silica uncoated tubing(Chrompack), was placed in the side arm of a T-connection toallow sampling from the center of the main gas stream. Thedimensions of the capillary provided optimal 10 - 5 mbar vacuumpressure in the MS chamber. The sampling stream wasintroduced directly into the closed HS W ion source. Molecularion signals of the gaseous products were monitored as a functionof time with a Balzers/Pfeiffer QMS 200 quadrupole massspectrometer.Radiolysis experiments were performed using a dc electronbeam to produce absorbed doses in the range 100 - 700 Gy. Themaximum dose was limited by the concentration of solvatedelectrons generated during the experiment. Final concentrationslarger than 2.5 × 10 - 4 m  would imply over 10% conversion of the available N 2 O. This concentration corresponds roughly to1000 Gy and was never exceeded during the course of the Figure 1.  Experimental setup for the detection of gaseous productsformed during radiolysis of aqueous solutions at high temperature andpressure. Figure 2.  Set of typical molecular ion current signals (right side axis)and associated shutter current and fiber optic (free channel) signals(left axis) for 0.02  m  EtOH- d  6  and  ∼ 2.5  ×  10 - 3 m  nitrous oxidesaturated aqueous solution (at 300  ° C and 250 bar). 7778  J. Phys. Chem. A, Vol. 111, No. 32, 2007   Janik et al.  experiments. Solutions were in the irradiation zone for 0.6 s,which for the applied doses corresponds to a dose rate of 300 - 1200 Gy/s.A typical set of signals registered during a single experimentalrun is presented in Figure 2. The left axis corresponds to thesignals registered in the detector channels monitoring the dose:the solid line represents signal from the fiber optic and thedashed line represents current on the electron beam shutter. Theright axis corresponds to the molecular ion currents for the gasesmonitored during measurement. At the start of a run the shuttercurrent signal is high and the fiber optic signal is low. After 15s of baseline collection the shutter was opened and irradiationstarted. At this point the shutter signal disappears and simul-taneously the fiber optic signal appears (see Figure 2). Irradiationwas continued for the next 4 min and 30 s. Two minutes afterthe beginning of irradiation the process of collecting sample inthe sparging vessel started. After 2 min the vessel containing12 mL of irradiated solution (6 mL/min flow rate) was switchedfrom the supplying line to sparging argon gas with a three wayvalve. After starting the sparging process the shutter was closed,as reflected in the change of corresponding signals in Figure 2.The sparged out gaseous products were carried to the MSthrough the water trap, which acts like a GC column to separatethe products in time. H 2  and HD reached the ionizing chamberin the MS approximately 30 s after the sparging start time.Nitrogen appeared 2 min later.The procedure presented above was typically repeated forthree different doses at a given temperature and pressure (seethe example in Figure 3). A relative “dose area” was obtainedby integrating the fiber optic signal from 105 s up to 225 s,which corresponds to the 2 min of sample collection. Absorbeddose,  D abs  (Gy), was calculated usingwhere [N 2 ] is measured nitrogen concentration (mol/dm 3 ),  G (N 2 )is the known radiation yield of N 2  from solvated electronscavenging at room temperature (mol/J), and  d   is the densityof water (kg/dm 3 ). This measurement was carried out every dayto calibrate the fiber optic as a relative dosimeter for the givenbeam focusing conditions. A precise estimation of the radiationchemical yields of the measured products with the appliedscavenger concentrations was carried out with the stochasticsimulation package described by Pimblott and co-workers. 26 - 29 For 0.01  m  ethanol solution saturated with 2.5 × 10 - 3 m  N 2 O(which was routinely used for calibrating the fiber opticresponse) values of 2.84  ×  10 - 7 and 7.91  ×  10 - 8 mol/J werecalculated at 25  ° C for N 2  and H 2 , respectively. The ratio  G (N 2 )/  G (H 2 ) obtained at room temperature agreed very well with thesimulated values.In the dose range used, the signals were a linear function of the applied dose. In most cases, the intercept of a plot of yieldvs applied dose went through zero (Figure 4a). However, forconditions where thermal background decomposition was pos-sible or suspected, a blank was measured in the absence of radiation (Figure 4b) and was included in the fit. At temperaturesof 380  ° C and above, nitrogen was observed as a result of decomposition of nitrous oxide on the tubing walls. Themeasured amount varied depending on the history of the cellwalls and if it became too high, the tubing walls could bepassivated by flowing O 2  gas overnight at 450  ° C. At 400  ° Cslight decomposition of ethanol- d  6  took place (1 - 2  µ M of HDobserved). Because radiation yields were calculated on the basisof the slopes of the dose dependence, we assume that thebackground thermal chemistry does not affect the yield values.The data in Figure 4 illustrate the very high precision we areable to attain in these measurements. With three or four pointsin each linear fit, we obtain standard deviations of the slopebelow 3% (typically 1%) in all of the constant pressuremeasurements up to 350  ° C. At supercritical temperatures thescatter is slightly larger, with 7% standard deviation as a worstcase, probably due to a slight drift in density during anexperiment which would change the absorbed dose. Thenumbers are also sensitive to the calibration of the MS sensitivitycarried out for each day. These calibration measurements alsohave standard deviations on the order of 1%. All of themeasurements are made relative to the yield of N 2  in N 2 O/ ethanol solution at 25  ° C, as explained above. Given the highprecision, one can hope for accuracy on the order of 5% up to Figure 3.  Typical set of collected traces for one set of conditions (0.01 m  PhOH at 300  ° C and 250 bar), for three different dose rates. Signalaxes as in Figure 2.  D abs ) [N 2 ] × G (N 2 ) - 1 × d  - 1 (1) Figure 4.  Linear mass spectrometer signals vs dose (a) with zerointercept and (b) a signal at 400  ° C with nonzero intercept due to thethermal breakdown of N 2 O. Neutron and    /  γ  Radiolysis of Water  J. Phys. Chem. A, Vol. 111, No. 32, 2007   7779  350  ° C, and perhaps 10% in the supercritical regime. Particularlyin the latter regime, one also assumes accuracy of the temper-ature and pressure measurements to calculate the density tocorrect for absorbed dose. All of the experimental conditionsreported below in Tables 1 - 3 were repeated at least once ondifferent days. We report results from a final set of experimentsin which the measurement technique had reached its ultimateprecision. III. Results During water radiolysis a number of transient and stableproducts are produced. The initial reaction can be summarizedwithIt should be understood that the “yields” of H + , e aq - ,  • OH, andH • in particular are functions of time because of fast recombina-tion. Scavenged yields of these species, typically for the first-order scavenging rate or “scavenging power “(i.e., rate constanttimes scavenger concentration) of 1.0 × 10 7 s - 1 , are very wellestablished at room temperature. 30 N 2 O is well-known as auseful and very efficient hydrated electron scavenger. 31 Itsreduction in reaction 3 leads to the stable gas nitrogen.This reaction has an activation energy of 15.5 kJ/mol up to300  ° C. Above 300  ° C the rate shows non-Arrhenius behaviorand rate constants up to 400  ° C are at least several times higherthan at room temperature. 13 N 2 O appears to be reasonably stablein supercritical water. All these facts suggest that irradiatingaqueous nitrous oxide solutions should allow us to determineescape yields of e aq - by monitoring production of N 2  over awide range of temperatures. However, N 2 O can also react withH • atoms giving the same product in reaction 4. 32 The relatively small rate coefficient (2.5  ×  10 6 M - 1 s - 1 ) forthis reaction at room temperature does not guarantee thesame at higher temperatures. From pulse radiolysis studies wefound that at 350  ° C, the reaction 4 rate constant is ap-proximately 1.5  ×  10 8 M - 1 s - 1 . 33 However, the detailedtemperature dependence in the entire range up to 400  ° C is stillunknown.Among many possible scavengers phenol molecules areknown to react rapidly with both  • OH and H • atoms but quitemoderately with electrons. 16,31 The reaction with e aq - reachesa maximum rate of 6.5  ×  10 7 M - 1 s - 1 at ca. 125  ° C, and by200  ° C becomes slower than at room temperature, so it shouldnot be any issue at still higher temperatures. 34 Phenol is well-known to be quite stable in supercritical water. Addition of  TABLE 1: Temperature Dependence of Radiation Yields of Gaseous Products in Phenol and Ethanol Aqueous Solutionsin the Presence of N 2 O (2.5  ×  10 - 3  m ) Measured at aConstant Pressure of 250 bar 10 - 7 G (X) (mol/J)in 0.01  m  PhOH in 0.02  m  EtOH- d  6  temp ( ° C)density(kg/dm 3 ) H 2  N 2  H 2  HD N 2 22 1.0000 0.45 3.02 0.44 0.18 2.83100 0.9696 0.48 3.37 0.47 0.51 3.23200 0.8813 0.54 3.62 0.51 0.78 3.55225 0.8527 0.55 3.69250 0.8209 0.57 3.74 0.56 0.95 3.65275 0.7848 0.63 3.75300 0.7430 0.67 3.64 0.67 1.30 4.19325 0.6926 0.71 3.57350 0.6271 0.75 3.40 0.82 1.98 4.02380 0.4508 0.45 1.35 0.86 2.48 1.23400 0.1665 1.09 2.03 1.72 2.92 1.93 TABLE 2: Density Dependence of Radiation Yields of Gaseous Products in Phenol and Ethanol Aqueous Solutionsin the Presence of N 2 O (2.5  ×  10 - 3  m ) Measured at aConstant Temperature of 380  ° C 10 - 7 G (X) (mol/J)in 0.01  m  PhOH10 - 7 G (X)(mol/J)in 0.02  m  EtOH- d  6  density(kg/dm 3 ) H 2  N 2 density(kg/dm 3 ) H 2  HD N 2 0.1229 0.75 2.31 0.1218 2.16 5.26 3.740.1542 0.74 2.08 0.1617 1.67 4.54 2.590.2045 0.65 1.73 0.1980 1.39 4.01 1.760.2501 0.51 1.43 0.2567 1.24 3.36 1.520.3116 0.44 1.12 0.2934 1.07 2.83 1.420.3599 0.32 1.16 0.3639 0.89 2.57 0.940.4004 0.34 1.05 0.4018 0.77 2.43 1.250.4524 0.38 1.20 0.4540 1.03 2.92 2.050.4547 0.45 1.25 0.5126 1.06 3.04 4.070.5010 0.73 2.27 0.5430 1.10 2.83 4.680.5501 0.81 3.12 TABLE 3: Density Dependence of Radiation Yields of Gaseous Products in Phenol and Ethanol Aqueous Solutionsin the Presence of N 2 O (2.5  ×  10 - 3  m ) Measured at aConstant Temperature of 400  ° C 10 - 7 G (X)(mol/J)in 0.01  m  PhOH10 - 7 G (X)(mol/J)in 0.02  m  EtOH- d  6 density(kg/dm 3 ) H 2  N 2 density(kg/dm 3 ) H 2  HD N 2 0.1223 1.05 1.84 0.1211 2.06 3.14 2.240.1485 0.96 1.77 0.1518 1.86 3.15 1.950.2124 0.74 1.53 0.2124 1.73 2.81 1.810.2594 0.62 1.43 0.2594 1.54 2.49 1.670.3090 0.65 1.38 0.3090 1.32 2.29 1.610.3574 0.59 1.35 0.3574 1.32 2.22 1.600.4174 0.57 1.48 0.4094 1.33 2.35 1.51 H 2 O 9 8  irr e aq - ,  • OH, H • , H + , OH - , H 2 O 2 , H 2  (2) Figure 5.  Radiation yields of gaseous products during radiolysis of 0.02  m  EtOH- d  6  /2.5  ×  10 - 3 m  N 2 O aqueous solution compared withproducts from 0.01  m  phenol/2.5 × 10 - 3 m  N 2 O solution as a functionof temperature at a constant pressure of 250 bar. e aq - + N 2 O 9 8  H 2 O • OH + OH - + N 2  (3)H • + N 2 O f  • OH + N 2  (4) 7780  J. Phys. Chem. A, Vol. 111, No. 32, 2007   Janik et al.  0.01  m  phenol to 2.5 × 10 - 3 m  N 2 O/water initiates scavengingof   • OH radicals and formation of dihydroxycyclohexadienylradicals via reaction 5 35,36 ( k  5 ) 6.6 × 10 9 M - 1 s  - 1 ). A similaraddition reaction (6) is observed for H • atoms leading tohydroxycyclohexadienyl radical 31 ( k  6  )  1.7  ×  10 9 M - 1 s - 1 ).The temperature dependence of these rate constants has beenmeasured recently in our laboratory up to 400  ° C. 16 Reaction 6occurs much faster than reaction 4, so the N 2  and H 2  radiolysisyields in the N 2 O/phenol system should correspond to thesolvated electron and molecular hydrogen formed in spurs,respectively.Radiation yields of gases produced during   -radiolysis of 0.01  m  phenol aqueous solutions in the temperature range22 - 400  ° C for isobaric conditions (250 bar) are presented inFigure 5. Detailed values are listed in Table 1. From roomtemperature up to 275  ° C a steady increase in yield was observedfor both N 2  and H 2 . At 275  ° C nitrogen yield reached amaximum of 3.55  ×  10 - 7 mol/J, then started decreasing to aminimum of 1.28 × 10 - 7 mol/J at 380  ° C. The hydrogen yieldkept increasing up to 350  ° C but then dropped to a minimumof 4.30 × 10 - 8 mol/J at 380  ° C. For both hydrogen and nitrogenthe sudden yield drop occurs around the water critical temper-ature of 374  ° C, and yields increase again after the temperatureincreases to 400  ° C.It has been shown that the density of water has a major impacton the yields of transient species in water under supercriticalconditions. 10,17 In this range we have chosen two temperaturesto illustrate the effect of density, 380 and 400  ° C. The flowwas kept constant and the density in the sample was adjustedby changing the pressure in the system. The molal ( m )concentration of solutes was the same as in the isobaricexperiments described above. In the range 0.12 - 0.55 kg/dm 3 at 380  ° C, the density dependence of radiation yields inFigure 6 displays a U-like shape for both N 2  and H 2 , withminima near 0.40 kg/dm. 3 In a narrower range of 0.12 - 0.42kg/dm 3 at 400  ° C, we observed similar behavior of both yields,but with a less pronounced decrease at intermediate density asillustrated in Figure 7. Our pumps did not allow us to extendthe measurements to higher densities (pressures) at this tem-perature. Detailed values for density dependence at 380 and 400 ° C are listed in Tables 2 and 3, respectively.The experiments performed with the phenol/N 2 O solution giveus solid information about molecular hydrogen as well assolvated electron radiation yields over a wide range of temper-atures and densities. In fact, these experiments were performedbecause of problems found with the preferred scavenging systemof N 2 O with ethanol- d  6 , as will now be described. Both hydrogenatoms and hydroxyl radicals abstract hydrogen from ethanolcarbons, forming  R  - and   -hydroxyethyl radicals (C 2 H 4 OH) • ,as shown in reactions 7 and 8 with overall room-temperaturerate constants as indicated.The hydrogen abstraction from the alcoholic hydroxyl groupin reaction 9 is of minor importance (2.5%) 39 at room temper-ature and also leads to R  -hydroxyethyl radicals due to a 1,2-Hshift 40 of the initially formed alkoxyethyl radical in reaction10.Reactions of H • and  • OH with ethanol- d  6  (actually ethanol- d  5 when dissolved in light water) are similar to normal ethanol, Figure 6.  Comparison of radiation yields in deuterated ethanol andphenol solutions vs density at 380  ° C. Figure 7.  Comparison of radiation yields in deuterated ethanol andphenol solutions vs density at 400  ° C. • OH + PhOH f  dihydroxycyclohexadienyl radical (5)H • + PhOH f  hydroxycyclohexadienyl radical (6)H • + C 2 H 5 OH f  H 2 + (C 2 H 4 OH) • k  7 ) 2.0 × 10 7 (M - 1 s - 1 ) 31 (7) • OH + C 2 H 5 OH f  H 2 O + (C 2 H 4 OH) • k  8 ) 1.9 × 10 9 (M - 1 s - 1 ) 31,37,38 (8) • OH + C 2 H 5 OH f  H 2 O + C 2 H 5 O • (9)CH 3 CH 2 O • 9 8  H 2 O CH 3 • CHOH (10) Neutron and    /  γ  Radiolysis of Water  J. Phys. Chem. A, Vol. 111, No. 32, 2007   7781
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