Accelerators pulse radiolysis

Pulse radiolysis requires access to an electron accelerator or similar device. This requirement usually restricts work to specialized laboratories. Thorough descriptions of the experimental apparatus and protocols have been given.23,24... 

Proton inventory technique. 21.9-220 Pseudo-first-order kinetics, 16 Pulse-accelerated-flow method. 255 Pulse radiolysis, 266-268 Pump-probe technique. 266... 

More common in the liquid phase is pulse radiolysis . In this technique, electron accelerators which can deliver intense pulses of electrons lasting a very short time (ns up to fis) are used. Each single pulse can produce concentrations of intermediates which are high enough to be studied by methods such as light absorption spectroscopy or electrical conductivity. 

The electron itself is frequently used as a primary source of radiation, various kinds of accelerators being available for that purpose. Particularly important are pulsed electron sources, such as the nanosecond and picosecond pulse radiolysis machines, which allow very fast radiation-induced reactions to be studied (Tabata et al, 1991). Note that secondary electron radiation always constitutes a significant part of energy transferred by heavy charged particles. For these reasons, the electron occupies a central role in radiation chemistry. 

Application of pulse-radiolysis techniques revealed that the following intramolecular and intermolecular electron-transfer reactions all exhibit a significant acceleration with increasing pressure. The reported volumes of activation are -17.7 0.9, 18.3 0.7, and... 

The intramolecular electron transfer kg, subsequent to the rapid reduction, must occur because the Ru(III)-Fe(II) pairing is the stable one. It is easily monitored using absorbance changes which occur with reduction at the Fe(III) heme center. Both laser-produced Ru(bpy)3 and radicals such as CO (from pulse radiolysis (Prob. 15)) are very effective one-electron reductants for this task (Sec. 3.5).In another approach," the Fe in a heme protein is replaced by Zn. The resultant Zn porphyrin (ZnP) can be electronically excited to a triplet state, ZnP which is relatively long-lived (x = 15 ms) and is a good reducing agent E° = —0.62 V). Its decay via the usual pathways (compare (1.32)) is accelerated by electron transfer to another metal (natural or artificial) site in the protein e. g.. 

Fast pulse radiolysis studies have shown that geminate recombination occurs on the picosecond time scale [12,13]. Bartczak and Hummel [14] predicted that for -dodecane, 82% of the geminate ions still remain at 5 psec for 1 MeV irradiation. Future accelerators, with pulses of a few picoseconds length, may soon provide experimental measurements of Gtot directly. 

Pulse radiolysis was performed using e from a linear accelerator at Osaka University [42 8]. The e has an energy of 28 MeV, single-pulse width of 8 nsec, dose of 0.7 kGy, and a diameter of 0.4 cm. The probe beam for the transient absorption measurement was obtained from a 450-W Xe lamp, sent into the sample solution with a perpendicular intersection of the electron beam, and focused to a monochromator. The output of the monochromator was monitored by a photomultiplier tube (PMT). The signal from the PMT was recorded on a transient digitizer. The temperature of the sample solution was controlled by circulating thermostated aqueous ethanol around the quartz sample cell. Sample solution of M (5 x 10 -10 M) was prepared in a 1 x 1 cm rectangular Suprasil cell. 

In pulse radiolysis experiments these radicals are formed by a short pulse, 10-12-10-6 s depending on the experimental set up, in concentrations enabling their physical observation. The linear electron accelerator of the Hebrew University of Jerusalem, which is used, forms up to... 

Details of the picosecond pulse radiolysis system for emission (7) and absorption (8) spectroscopies with response time of 20 and 60 ps, respectively, including a specially designed linear accelerator (9) and very fast response optical detection system have been reported previously. The typical pulse radiolysis systems are shown in Figures 1 and 2. The detection system for emission spectroscopy is composed of a streak camera (C979, HTV), a SIT... 

As has already been mentioned, picosecond pulsed radiolysis offers great possibilities for studying the short-lived transient processes. In Ref. 326 the solutions of 2,5-diphenyloxazol (DPO) in different solvents were irradiated by picosecond electron pulses obtained from an accelerator. The authors have found two types of excitations of DPO, which they have named the fast and the slow excitations. With fast excitation the luminescence appears during the electron pulse and stops growing at the end of the pulse, after 10 ps. With slow excitation the luminescence is formed within 1 ns. At small DPO concentrations the observed intensity of fast luminescence cannot be explained by direct excitation by electrons (cf. data of Ref. 325). Analyzing the results of experiments with different solvents and different types of additives, Katsumura et al.326 conclude that the main part of the fast luminescence of DPO is due to VCR absorption. 

Tripathi GNR (1998) Electron-transfer component in hydroxyl radical reactions observed by time resolved resonance Raman spectroscopy. J Am Chem Soc 120 4161-4166 TsaiT, Strauss R, Rosen GM (1999) Evaluation of various spin traps for the in vivo in situ detection of hydroxyl radical. J Chem Soc Perkin Trans 2 1759-1763 Tsay L-Y, Lee K-T, Liu T-Z (1998) Evidence for accelerated generation of OH radicals in experimental obstructive jaundice of rats. Free Rad Biol Med 24 732-737 Ulanski P, von Sonntag C (2000) Stability constants and decay of aqua-copper(lll) - a study by pulse radiolysis with conductometric detection. Eur J Inorg Chem 1211-1217 Veltwisch D, Janata E, Asmus K-D (1980) Primary processes in the reactions of OH radicals with sul-phoxides. J Chem Soc Perkin Trans 2 146-153... 

Another aspect of pulse radiolysis which has been improved is the pulse duration. For most experiments of interest to the physical organic chemist the common machines with pulse durations of 10 7-10-5 s are quite satisfactory, though for certain reactions, such as those involving protonation, examination on a shorter time scale can be of value. Several accelerators which supply nanosecond pulses are currently in use, but they are employed mostly with microsecond detection systems. Work in the 10-12-10-1° s region has recently become possible by the stroboscopic technique utilizing the fine structure pulses from a linear accelerator (Bronskill et al., 1970). More recently, a system which produces a single pulse of 40 picoseconds has been constructed (Ramler et al., 1975) and utilized for the observation of hydrated electrons at very short times (Jonah et al., 1973). 

Fig. 1. Block diagram of the nanosecond pulse radiolysis system using the Hokkaido University 45 MeV electron linear accelerator...

So far the microwave electron linear accelerator is the most suitable for this purpose. In this accelerator electrons are injected into an evacuated cylindrical waveguide in which pulsed radiofrequency of several megawatts from a klystron oscillator travels. Electrons enter the radiofrequency field at the correct phase are accelerated to a velocity close to that of light. By means of gun control, electrons are injected only during the radiofrequency pulse, and thus the electron pulses of several nanosecond duration, useful for conventional nanosecond or microsecond pulse radiolysis, are produced. 

Pulse radiolysis systems capable of picosecond time resolution use the fine structure of the output from the electron linear accelerator. Electrons in the accelerating tube respond to positive or negative electric field of the radiofrequency, and they are eventually bunched at the correct phase of the radiofrequency. Thus the electron pulse contains a train of bunches or fine structures with their repetition rate being dependent on the frequency of the radiofrequency (350 ps for the S-band and 770 ps for the L-band). 

Perkin Elmer MPF-3 spectrofluorometer. X- and Q-band measurements of EPR spectra were carried out at liquid nitrogen and liquid helium temperatures. Microcalorimetric measurements were performed on a LKB 10700 batch microcalorimeter. Temperature-jump relaxation kinetics were measured using a double beam instrument (18) with a cell adapted for anaerobic work. The relaxation signals were fed into an H.P. 2100 computer and analyzed as described in Ref. 7. The pulse radiolysis exepriments were carried out on the 5-MeV linear accelerator at the Hebrew University. Details of the system have been published previously (19). 

Fig. 3.8. Possible lay-out for a simple pulse radiolysis instrument. Electrons from the accelerator impinge on the cell in a short pulse which is followed at controlled time intervals by the light pulse which probes the spectra of transients and their time dependence.

Although, in principle, ESR spectroscopy is the most powerful method for detecting radical intermediates, it has not been widely used in conjunction with steady-state or pulse radiolysis, mainly because of technical difficulties. Important early work was done by Fessenden, Schuler and their co-workers (see for example Eiben and Fessenden, 1968 and Fessenden and Schuler, 1971) using steady-state radiation from a Van der Graaf accelerator. In this way, liquid-phase ESR spectra were generated for a range of radicals never previously observed by ESR methods. 

The study of the very fast processes that follow on the absorption of radiation in organic systems is a very active field with pulse radiolysis with picosecond time resolution being one of the major tools. This technique, the latest version of which employs twin linear accelerators(69), has time resolution of about 20 ps. These methods are being used to investigate the fast recombination of charges, the formation of excited states and free radicals, mainly in hydrocarbon media, but have also recently been applied to the study of radiation effects in polymers(70). 

Samples were irradiated by a 10 ps single or 2 ns electron pulse from a 35 MeV linear accelerator for pulse radiolysis studies (17). The fast response optical detection systems of the pulse radiolysis system for absorption spectroscopy (18) is composed of a very fast response photodiode (R1328U, HTV.), a transient digitizer (R7912, Tektronix), a computer (PDP-11/34) and a display unit. The time resolution is about 70 ps which is determined by the rise time of the transient digitizer. 

Fig. 6-3. Magnetic field effects observed in the radiation reaction of a squalane (S) solution of fluorene (M) for pulse radiolysis with a 4-MeV electron accelerator. The reaction temperature is not described in the present papers, but may be room temperature, (a) Time profile of fluorine fluorescence during and after pulse radiolysis of a squalane solution (1) at the minimum field less than 0.05 mT, where the residual field of an electromagnet is cancelled by passing a small reverse current through the magnet s coils (2) at 0.3 T. (b) The time dependence of the magnetic field enhancement of the fluorescence intensity (A) 15-ns pulse ( ) 50-ns pulse, (c) The MFE on the increase in fluorescence intensity at 200 ns after the pulse. (Reproduced from Ref. [18b] by permission from The American Chemical Society)...

Pulse radiolysis was modeled after flash photolysis. The time resolution of laser flash photolysis has always been better than for pulse radiolysis. There are multiple reasons for this effect. (1) Flash photolysis equipment is cheaper than electron accelerators so there have been many more practitioners of the art. (2) Photons do not repel each other so it is possible to focus a larger number of them in a small volume over a short time period than it is possible to do for electrons. (3) The velocity of relativistic electrons in a dense material is much higher than photons in the same material so sample thicknesses must be much thinner for pulse radiolytic experiments than for flash photolytic experiments, thus meaning that signals would be smaller. 

Oulianov DA, Crowell RA, Gosztola DJ, Shkrob lA, Korovyanko OJ, Rey-de-Castro RC. (2007) Ultrafast pulse radiolysis using a terawatt laser wake-field accelerator. /Ap P/tyr 101 053102-053102-9. 

Ogata A, Nakajima K, Kozawa T, Yoshida Y. (1996) Femtosecond single-bunched linac for pulse radiolysis based on laser wakefield acceleration. IEEE Trans Plasma Science 24 453-459. 

Instrumentation has undergone drastic changes since the first successful performance of pulse radiolysis experiments some 50 years ago. In keeping with the purpose of this book, recent trends in the instrumentational realization of detection methods, in accelerators, and in the application of computers are discussed in this chapter. 

Table 1. Comparison of specifications of photocathode electron gun accelerators for picosecond pulse radiolysis.

Fig. 6. Scheme of the laser-driven RF electron accelerator of pulse radiolysis facility ELYSE. IP ion vacuum pump, CPC cathode preparation chamber, W vacuum valve, SOL solenoid, D dipole, TRl and 2 triplets, Q quadrupole, WCM wall current monitor, PC Faraday cup, T translator for Cerenkov light emitter and visualization screen LME laser entrance mirror, LMEx laser exit mirror, VC virtual cathode FIS horizontal slit, VS vertical slit. (Reproduced with permission from Ref 28.)... 

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