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( 4 ) A. P. Reuvers, J. D. Chapman. and J. Borsa, Nature (London), 237,
402 (1972). J . D. Chapman, A. P. Reuvers, J. Eorsa, A. Petkau, and D. R. McCalia, Cancer Res., 32, 2616 (1972). (6)C. L. Greenstock, J. A. Raleigh, W. Kremers, and E. McDonald, lnt. J. Radiat. Biol., 22,401 (1972). 17) C. L. Greenstock and I. Dunlop, Int. J. Radiat. Biol., 23, 197 (1972). (8) J. D. Chapman, C. L. Greenstock, A. P. Reuvers, E. M. McDonald, and I. Dunlop, Radiat. Res.. 53, 190 (1973). (9) J. W. Hunt, C. L. Greenstock, and M. J. Eronskill, Int. J. Radiat. Phys. Chem., 4, 87 (1972). (10)G. E. Adams, J. W. Boag, J. Currant, and E. D. Michael, "Pulse Radiolysis," M. Ebert, et a/., Ed., Academic Press, London, 1965, p
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77. (11) G. Czapski and 9. H. J. Bieiski, J. Phys. Chem., 57, 2180 (1963). (12) C. L. Greenstock, I. Dunlop, and P. Neta, J. Phys. Chem., in press. (13) Haif-wave potential measured against a saturated calomel reference electrode at pH 7 in 0.05 M tris buffer. (14) J. Lilie, Z. Naturforsch., 266, 197 (1971). (15) P. Neta and C. L. Greenstock, Radiat. Res., in press. (16) R. L. Willson, Int. J. Radiat. Biol., 17, 349 (1970). (17) G. E. Adams, J. W. Boag, and B. D. Michael, Trans. Faraday Soc., 61, 1417 (1965). (18) R. L. Willson, C. L. Greenstock, G. E. Adams, L. M . Dorfman, and R. Wagemann, lnt. J. Radiat. Phys. Chem., 3, 211 (1971). (19) G . E. Adams, C. L. Greenstock, J. J. vanHemmen, and R. L. Wiilson, Radiat. Res., 49, 85 (1972).
ields and Decay of the Hydrated Electron at Times Greater Than 200 Picoseconds' C. B. Jonah," E. d. Hart, and M. S. Matheson Chemistry Division, Argonne National Laboratory, Argonne, lllinois 60439 (Received February 20, 1973) Publication costs assisted by Argonne National Laboratory
We have measured the yield of the hydrated electron a t times greater than 200 psec after irradiation with 19-MeV electrons as 4.1 i 0.1 e,,-/100 eV of energy deposited. We have also measured the yield and the decay of ea,- in the presence of (a) 1 M CH30H, (b) 1 M NaOH, and (c) 1M CH30H plus 1M NaOH. The results confirm the concept of high local concentrations in the initial stages after energy deposition (the spur theory), but the time dependence of [e,,-] differs from that predicted by published spur diffusion model calculations.
Introduction It is a fundamental concept of the radiation chemistry of condensed phases that the energy of a charged particle is deposited inhomogeneously along its track in the absorbing medium, producing high local concentrations (spurs) of reactive intermediates. Samuel and Magee28 developed the first theoretical model for the radiolysis of water, in which the transient species, initially at high local concentrations, participate in the competing processes of reaction with each other and diffusion into the bulk of the medium to form a homogeneous solution. This model with various refinements has been used quite successfully2b to correlate yields of products obtained as a function of concentration of added scavenger (10-6-1 M ) and as a function of linear energy transfer (LET). (LET = energy deposited per unit path length.) For fast electrons with low LET the spurs are about 500 nm apart, while for protons or a particles of a few MeV the LET is high and the spurs overlap into a continuous track. In the past 20 years the following has been established: (1) the identity of the transient species escaping the spurs (eaq-, H, OH, H30+, and OH-); (2) the G values for these species that escape the spur (although there is disagreement for G(H&+) and G(OH-)); (3) the rate constants for reaction among the intermediates; and (4)the diffusion constant of eaq-. This accumulation of data has aided in the formulation of the model but restricted the number of variable parameters. The Journal of Physical Chemistry. Vol. 77. No. 75. 7973
Among the predictions of the spur-diffusion model are sec, and the the initial yields of species at about time dependence of these yields as the spur expands. In the present work, we have measured the value of G(e,,-) after 200 psec and followed the time dependence of [eaq-1 to 40 nsec in H20, 1 M CHzOH, 1 M NaOH, and in 1M NaOH + 1 M CH30H. A preliminary report of the measurement of the early G(e,,-) has been published3 jointly with Hunt and his collaborators a t the University of Toronto, who have independently measured G(e,,-) at 30 psec. Our results confirm that eaq- initially is distributed inhomogeneously, but the time dependence of eaq- concentration is quite a t variance with spur model predictions.*+S Work such as that reported here can eventually restrict the choice of spur model parameters to (1)the radius of the spur, (2) the number of particles per spur, and (3) the percentage of spurs of a given radius and particle content, and thus define the inhomogeneity of the early stages of the radiation chemistry of water. In this paper in addition to our results we describe some of the details of our apparatus, so that the reliability of these critical data can be judged, and so that an adequate experimental background for future papers in this series is provided. Experimental Section Linac. Irradiations were carried out using an Applied Radiation Corporation linear electron accelerator.'j
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Yields and Decay of the Hydrated Electron Upgrading at Argonne includes the addition of a subharmonic prebuncher which enables the production of very short pulses of estimated duration 1 5 0 psec and with 7 nC of charge per pulse at 19 MeV.7 For experiments discussed in this paper the electron beam was pulsed 30 times a second and focussed to a cross sectional area -5 mm2 to give a dose per pulse of 2 krads in the irradiated volume. Optical System. A schematic diagram of the experimental system is shown in Figure 1. Either an argon ion Model 71BR or krypton ion Model 71KR TRW pulsed gas ion laser with peak powers around 0.5 W was used for the analyzing light source. Since the laser and electron beam pulses were synchronized so that the electron pulse entered the cell a t the peak of the much broader (5 or 50 p e c ) light pulse, the light pulse was flat on the reaction time scale. A Baird Atomic interference filter in front of the photodiode isolated either the 514.5- or 647.1-nm line and rejected essentially all Cerenkov radiation. The pulsed lasers produced an easily measurable output from the photodiode and simplified alignment. On the other hand, the number of wavelengths available from these lasers is limited, and their output includes much “high-frequency’’ noise which we attribute to mode beating in the laser. A thin mirror of 0.5-mm fused silica front coated with aluminum reflected the light beam 90” to pass through the cell collinear with and in the same direction as the electron beam, and a similar mirror behind the cell reflected the light again perpendicular to the electron beam. Other mirrors directed the light beam out of the linac room through a hole in the wall and onto the active surface of the photodiode. The best signal-to-noise ratio was obtained by focussing the laser beam into the cell, making it parallel again after passage through the cell, and refocussing the light to fill the active surface of the photodiode. Three ITT F 4014 photodiodes were used, two with S-20 photocathodes for detection at 647.1 nm, and the other with a n S-5 active surface for measurement at 514.5 nm. Our two diodes with S-20 photocathodes lost all sensitivity after short usage, but, fortunately, no such problem was encountered with the S-5 photocathode. The diodes were operated a t 1600 V, with risetimes which were measured to be less than 100 psec. There is, however, some overshoot and ringing lasts 400 to 500 psec. Signal Sampling System. The photodiode output was sampled by a Tektronix S-4 sampling head with a rated risetime of 25 psec. As in all sampling oscilloscopes, the sampled signal voltage is held until the next trigger pulse. Since the sampling trigger is capacitively coupled to the pulse that fires the injector of the linear accelerator, the timing jitter between the sampling system and the electron pulse is less than 50 psec. The signal from the sampling oscilloscope (Tektronix 7704) is biased so that the signal voltage is always positive (between 0 and 2 V for a signal on the scope face) and then is fed to a Model 241 Vidar voltage to frequency converter. The Vidar gives a pulsed output whose frequency is proportional to the input voltage. These pulses were counted by a multichannel analyzer used in a multiscaling mode. The analyzer was stepped from channel to channel a t a fixed rate of 4 to 10 channels per second. An output signal which is linearly proportional to the number of the channel being counted in the analyzer was used to sweep the sampling oscilloscope. Thus each channel in the analyzer has a unique time (relative to the triggering
VOLTS
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RECORDER
SUBNANOSECOND PULSE RADIOLYSIS APPARATUS
Figure 1. Schematic experimental set u p for the picosecond
pulse radiolysis experiments.
pulse) associated with it. A shutter in front of the photodiode automatically interrupts the analyzing light as the last 20 analyzer channels are swept to give a measurement of zero light. After each sweep of the analyzer through all channels, the data from the analyzer channels are transferred to the Chemistry Division’s Sigma 5 computer.8 The number of sweeps desired for a run are input by means of a teletype to the computer, which, at the end of a run, automatically transfers the data back into the analyzer one sweep at a time. Each sweep can be displayed on an oscilloscope and examined for experimental artifacts. Any sweep or sweeps showing experimental artifacts such as those caused by accelerator faulting can be rejected, without loss of the data of the whole run. The accepted sweeps of a run are summed and stored in disk storage. Ten sweeps were generally made to enhance signal-to-noise. The linearity of response of the overall system was confirmed by measuring a Cerenkov pulse with and without the analyzing laser light. The measured intensity of the Cerenkov was the same in both cases within 5%, which was the signal-tonoise limitation of that experiment. The peak Cerenkov light intensity was about one-sixth that of the analyzing light. Three points should be made about the operation of the system: (1) the frequency of stepping channel-to-channel in the analyzer is not tied to the linac pulsing frequency; (2) the counting in the analyzer does the averaging and the analog to digital conversion; and (3) the system must spend the same length of time in each channel. Irradiation Cell and Flow System. Since the experimental technique requires many electron pulses per run (20,000 pulses or 40 Mrads typically), a flow system was necessary to avoid overheating and buildup of radiolytic products. In each run 2 1. or more of fresh solution were circulated through a closed system at -200 ml/min by a CRC vibrostaltic pump. The solution enters the bottom of the 2-cm long fused silica cell (windows of high-purity silica), is spread by a perforated plate for more uniform flow through the main body of the cell, and exits a t the top of the cell. Total volume of the cell is about 3 cc. The solution can be purged continuously during a run by bubbling helium or other gas through the reservoir. At the end of a run in which such purging by helium had been done, analysis showed average concentrations of 2 K M 0 2 and
