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J. Phys. Chem. 1996, 100, 9014-9020
Temperature Dependence of g Values for Aqueous Solutions Irradiated with 23 MeV 2H+ and 157 MeV 7Li3+ Ion Beams A. John Elliot,*,† Monique P. Chenier,† Denis C. Ouellette,† and Vernon T. Koslowsky‡ Reactor Chemistry Branch and Nuclear Physics Branch, AECL, Chalk RiVer Laboratories, Chalk RiVer, Ontario, Canada K0J 1J0 ReceiVed: NoVember 22, 1995; In Final Form: February 28, 1996X
The temperature (T) dependence of the g values for the radiolysis of water with 23 MeV 2H+ and 157 MeV 7 3+ Li ion beams is reported. The results indicate that as the linear energy transfer characteristics of the radiation increase, (i) d(g(eaq-))/dT decreases, (ii) d(g(H2))/dT increases, (iii) d(g(OH))/dT remains constant, and (iv) d(g(H2O2))/dT is approximately constant. These observations are qualitatively consistent with the kinetic-diffusion model of the spur/track. The g values for 2 MeV fission neutrons have been estimated from these ion beam results for the temperature range 25-300 °C.
Introduction At the Chalk River Laboratories, we have been measuring and gathering data on the temperature dependence of the reaction rate constants and of the g values for primary chemical species formed in the radiolysis of light1,2 (reaction 1) and heavy water.2,3
H2O - radn f eaq-, H , OH , H2, H2O2, HO2
(1)
In this paper, g value refers to the yield of the primary species formed in reaction 1 after they are homogeneously distributed in the solution, i.e., after the spur/track reactions are complete following the ionizing event. The term G value is used for the experimentally determined yield from which g values are obtained; e.g., the measured G value is dependent upon the concentrations of solutes used in the chemical system studied. The terms G value and g value are defined here as the number of radicals, ions, or molecules formed per 100 eV of energy absorbed. Our goal is to model the radiolysis of the water in nuclear power reactors in both the primary heat transport circuit (250325 °C) and the auxiliary systems (20-80 °C). We have recently reviewed the data for light water,1 and the information is essentially complete except for the g values for fast neutron radiolysis. There are two experimental approaches that can be used to estimate the effect of temperature on the g values for the primary species formed from the fast neutron radiolysis of water. The direct approach is to irradiate samples in a fast neutron reactor. This approach has been used in Japan, where the fast neutron reactor YAYOI was used as a radiation source.4,5 However, drawbacks to this approach are the difficulty in obtaining accurate dosimetry for the fast neutron and γ-radiation fields and that the γ-radiolysis and fast neutron radiolysis yields have to be separated. The second approach is to irradiate samples with ion beams that have linear energy transfer (LET, defined as initial stopping power, -dE/dx) characteristics similar to those found for the 1H+ (2H+) ion recoils produced in the moderation of fast neutrons.6 Provided the neutron spectrum for a given reactor * Author to whom correspondence should be addressed. † Reactor Chemistry Branch. ‡ Nuclear Physics Branch. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03593-3 CCC: $12.00
is known, the g values for fast neutrons at room temperature can be calculated from data such as shown in Figures 1 and 2. Then, if the temperature dependence for the g values is known in the LET range of interest, the high-temperature g values can also be estimated. As the research reactors at the Chalk River Laboratories are all well moderated, we have opted to use ion beams from the in-house tandem accelerator superconducting cyclotron (TASCC) facility as a means for estimating the fast neutron g values. This paper reports the temperature dependence (up to 180 °C) of the yields for the primary species formed in reaction 1 using ∼23 MeV 2H+ and 157 MeV 7Li3+ ion beams. This information has been combined with previously measured g values for γ-radiation.2 From the determined dependence of g values with LET and temperature, estimates have been made of the g values for 2 MeV monoenergetic neutrons at 25, 250, and 300 °C. These g values supersede those reported in ref 1. Because we have used a different type of experimental configuration for these experiments, we also report on an intercomparison of techniques using Fe2+ solutions irradiated by a 534 MeV 12C6+ beam, a system that has been well-studied elsewhere.16 Experimental Methods Chemistry Procedures. The g values reported in this paper were estimated from G values of products formed in the irradiation of aqueous solutions containing solutes. These solutions were chosen and characterized for temperature stability in γ-radiolysis studies reported earlier.2 The chemicals in this study were of AR grade or better and were used as supplied. The purification of the water, sample preparation, and analytical techniques that were used in these experiments are all described elsewhere.2 In the analysis for Fe3+, the temperature of the solution was measured and the appropriate extinction coefficient used.13 Irradiation Procedures. The apparatus, designed to permit irradiation of solutions at temperatures greater than 100 °C, is shown in Figure 3. The sample ampules, which were not externally pressurized during the irradiations, were prepared from 10 mm diameter Pyrex NMR tubes (wall thickness 0.46 ( 0.02 mm, Wilmad Glass Corp. tube 513-7PP) capable of withstanding internal water vapor pressures of 1 MPa at 180 °C. The degassed sample was filled through a 4 mm o.d. Pyrex neck and then flame sealed. As shown in Figure 3, the ampule
Published 1996 by the American Chemical Society
Temperature Dependence of g
Figure 1. Room temperature data from the literature for the reducing primary species g(eaq-) (O,7 08), g(H2) (1,9 4,7 310), and g(H) (]7). The results from this study are g(eaq-) ([), g(H2) (2), and g(H) (b). Note that our experimental points for g(eaq-) and g(H2) overlap at an LET of 20 eV nm-1.
Figure 2. Room temperature data from the literature for the oxidizing primary species g(OH) (O,10 0,11 ]7), g(H2O2) (1,9 4,10 37), and g(HO2) (- -12). The results from this study are g(OH) (b) and g(H2O2) (2).
Figure 3. Irradiation apparatus; for scale, the ampule is 10 mm in diameter.
was inverted for the irradiation; the neck was used as a holder and was slipped into a latex rubber sleeve mounted on the shaft of a 280 rpm Bodine motor. (For irradiations of oxygenated Fe2+ solutions, open ampules were used; a holder made from a solid Pyrex rod was glass-blown onto the bottom of the ampule.)
J. Phys. Chem., Vol. 100, No. 21, 1996 9015 Rotation of the sample mixed the solution to minimized depletion in the irradiation zone. The ampule was inserted into a brass block that was heated by two 200 W cartridge heaters; the output of a thermocouple inserted into the brass block was used to control the temperature. This block was insulated from the outside holder using ceramic paper (Cotronics Corp.) and Pyrex glass wool. The samples were irradiated through a beam entrance aperture covered with a thin (1.8 or 3.5 mg cm-2) Ni foil heat shield. Temperature gradient tests were undertaken using open-topped ampules filled with a high-boiling-point silicon oil into which a thin, flexible thermocouple could be inserted to various positions inside the vessel. In the upper two-thirds of the ampule surrounding the position where the ion beam enters, the temperature in the oil was within (3 °C of the set temperature. The ion beam exited the beam line through a thin window (2.3 or 17 mg cm-2 Ni or 5.5 mg cm-2 Ti). Because the majority of the samples were sealed and the furnace electrically grounded, the beam current was monitored using a nitrogenflushed ion chamber located immediately ahead of the Ni heat shield. (In the initial experiments, which were done with 2H+ beams, only one heat shield was used, whereas in later experiments with 7Li3+ beams, an extra Ni heat shield was added about 1 mm ahead of the original one to minimize the effect of the furnace temperature on ion chamber amplification.) The ionization chamber gain was several thousand and was calibrated before and after each irradiation using a Faraday cup mounted about 1 m ahead of the apparatus. Tests using a portable, highly suppressed Faraday cup indicated that the beam line cup was also well suppressed. The output of the ion chamber was fed to an integrating electrometer the output of which was recorded continuously by a chart recorder. The accumulated charge was noted at the end of each irradiation. The beam currents were quite low, typically 0.1 electrical nA or less; individual irradiation times ranged between 10 s and 80 min. The length of an experimental session was typically 48 h. Beam position on the target was very stable, but to achieve a stable beam intensity (variation less than (20%), 1-10 nA beams were strongly defocused and then cropped with 1 mm × 1 mm slits at the exit of the accelerator. These slits became the object of the following beam optical elements, which were set for unit magnification at the sample position. The position and size of the beam were initially set up using a beam profile monitor and confirmed using a scintillator at the sample position. The beam position was further confirmed during the session by periodically irradiating an empty, nonrotating ampule and examining the size and position of the brown “burn” mark on the glass. Three ion beams were used in this study: the 2H+ beam (initial energy of 26 or 27 MeV) was used directly from the Tandem Van de Graaff accelerator; the 7Li3+ beam (175 MeV) and 12C6+ beam (540 MeV) were produced in the Tandem Van de Graaff accelerator and postaccelerated by a superconducting cyclotron. The beam energy was known to better than 1%. The energy of the beam entering the solution was calculated from the attenuation of the beam as it passed from the beam line to the sample through the various foils and the Pyrex ampule wall.14 The 2H+ and 7Li3+ beams penetrated about 4 mm into the solutions, i.e., less than halfway through the solution in the spinning ampule. The 12C6+ beam penetrated about 6 mm into the solution, but this beam was only used with a large-volume irradiation cell from the University of Notre Dame as described later. Evaluation of Irradiation Procedures. The aim of the current experiments was to acquire data above 100 °C. This
9016 J. Phys. Chem., Vol. 100, No. 21, 1996
Elliot et al.
TABLE 1: G(Fe3+) from Irradiated 0.01 mol dm-3 Fe2+ Solutions Using the Cell Supplied by the University of Notre Dame gas aeratedb aerated aerated O2 O2 Heb He He He
beam diameter/ mm current/nA 10 1 1 10 1 10 1 1 1
1 1 0.15-0.20 1 1 1 1 0.3-0.4 0.3-0.4
G(Fe3+)a
current measurement
7.86 ( 0.31 (7) 7.06 ( 0.01 (2) 8.05 ( 0.34 (5) 8.62 ( 0.26 (5) 8.46 ( 0.29 (9) 4.52 ( 0.18 (4) 4.60 ( 0.03 (4) 4.53 ( 0.13 (5)
solution solution solution solution solution solution solution ion chamber, solution grounded 4.61 ( 0.13 (4) ion chamber, solution not grounded
a Numbers in parentheses indicate the number of replicate measurements. The uncertainty quoted is the standard deviation of the mean. b Conditions similar to that used by LaVerne and Schuler.16
required an experimental arrangement different from that normally used in other laboratories. Traditionally, a flowing or stirred solution (at room temperature and pressure) is irradiated by a 10 mm diameter ion beam and the current (∼1 nA) is monitored by directly collecting the charge from the irradiated solution.7,10,15 In the present work, a heated sample in a rotating, sealed ampule was irradiated with a 1 mm diameter beam at much lower currents (
