NI. J. BRONSKILL AND J. W. HUNT
3762
A Pulse-Radiolysis System for the Observation of Short-Lived Transients by M. J. Bronskill’ and J. W. Hunt Ontario Cancer Institute and Department of iMedical Biophysics, Unirersity of Toronto, Toronto, Ontario, Canada (Received M a y 7, 1968)
Several interesting problems in the field of radiation chemistry occur in the time interval from 0.01 to 1 nsec following the passage of an ionizing particle. Existing pulse-radiolysis systems are unable to investigate these problems because their time resolution is limited to about 1 nsec. In this paper a new technique is described which is theoretically capable of observing transient absorption signals as short-lived as 0.02 nsec. This technique will use the Cerenkov light produced by the 30-MeV electron beam from the Toronto linear accelerator t o detect the absorption of transient species produced by the fine-structure pulses (0.01 nsec wide) of the electron beam. -4 stroboscopic effect is created by varying the phase difference between the Cerenkov light flashes and the fine-structure electron pulses. With this technique a conventional slow detection system can be used to achieve a time resolution of 0.02 nsec. Experimental observations indicate that such a detection system is completely practical, although no short-lived transients have yet been observed.
Introduction The absence of a suitable short-radiation pulse and a high-speed optical detector limits present pulse-radiolysis systems to a time resolution of about 1 nsec. Nevertheless, the direct observation of events taking place in the “physiochemical” stage of radiolysis (from 10+ t o lo-’ nsec) may be attainable with a slow system using presently available radiation sources and optical detectors. Such controversial problems as solvated electron f ~ r m a t i o n , ~ion - ~ recombination,6te “spur” reaction^,^,^,^ and triplet excited state f o r m a t i ~ ncan ,~ be readily studied with such a system. The study of the radiolysis of dilute aqueous solutions does not provide a completely satisfactory model for the interaction of ionizing radiation with living cells. I n living cells a high solute concentration is present (a cell is about 75% water) and hence direct radiation effects become important.’O Under cellular conditions the solute-radical reactions start to occur a t 0.01 nsec. Such reactions can only be studied by pulse-radiolysis techniques if a system having a greatly improved time resolution is devised. I n order to attack these problems, a system having a theoretical time resolution of 0.02 nsec has been conceived and constructed. Stroboscopic Pulse Radiolysis System A . Radiation Source. The “new breed” of linear accelerators (linacs) available for pulse-radiolysis studies are capable of producing short, intense bursts of highenergy electrons. The important parameters of the linac recently installed in the Physics Department at the University of Toronto are given in Table I. From this machine, a beam current of over 0.5 A has been obtained in a 30-nsec pulse which is focused by quadrupole lenses into a spot less than 4 mm in diameter. Such a 30-nsec pulse is not actually a continuous stream of electrons but consists of a train of 100 finestructure pulses spaced 0.35 nsec apart, the period of The Journal of Physical Chemistrv
Table I : University of Toronto Linac Parameters (as Used for Stroboscopic Pulse Radiolysis) Manufacturer
Beam energy Beam current Accelerating microwave frequency Time interval between fine-structure pulses Beam diameter (minimum)
Vickers-Armstrong Ltd., Swindson, England >30 MeV 1 A (30-mec pulse) -30 A (fine-structure pulse) 2.86 X 109 Hz 0 . 3 5 nsec 1, where p is the ratio of the speed of the charged particle to the speed of light in vacuo, and n is the index of refraction of the medium. Cerenkov light has a continuous spectrum and is radiated outward from the path of the charged particle along the surface of a cone of half-angle 8 where cos 8 = I/pn." The time sequence of events for a detection system utilizing Cerenkov light is presented in Figure 1. The electron fine-structure pulses (previously described) are shown producing a rapidly decaying concentration of absorbing species. Simultaneously, the electron finestructure pulses produce pulses of Cerenkov light. The Cerenkov light pulses are delayed a variable time with respect to the fine-structure electron pulses and then used as analyzing light flashes. Summation over the train of delayed light flashes produces an absorption signal corresponding to a fixed point in time relative t o the electroll fine-structure pulse. This stroboscopic method makes it unnecessary for the detection system to be particularly fast. The detection system need only add up the total number of photons received from a train of analyzing light flashes to provide a usable signal. By varying the delay of the light flashes, a sweep over the time interval between fine-structure pulses may be obtained.
Figure 2. Components of a stroboscopic pulse-radiolysis system. The electron beam (broken line) emerges from the electron drift tube, passes through 10 cm of air and two thin mirrors, and finally irradiates the sample. The analyzing Cerenkov light (solid lines) is produced in the air path of the electron beam. I t is transmitted over a variable length optical path !nd focused to pass through the irradiated sample. This Cerenkov light then is focused through a monochromator and detected by a photomultiplier. Special integrating circuits give a dc signal which may be displayed on a recorder.
C . System Design Parameters. The detailed design of a feasible stroboscopic pulse-radiolysis system must overcome three main problems : (1) the incorporation of a suitably adjustable time delay for the Cerenkov light flashes, (2) the production of a detectable concentration of absorption species by the fine-structure electron pulse, and (3) the reduction of noise to well below the signal intensity. A system has been designed and constructed which is capable of overcoming these obstacles. 1. Time Delay. For the operation of this system, a variable delay of a t least twice the time interval between fine-structure pulses is desired in order t o show that the absorption signal builds up and decays with the same period as the fine-structure pulses. This variable delay is 0.6 nsec and corresponds to a distance of 18 cm for a light beam in air. The delay is accomplished by reflecting the Cerenkov light beam through 90" onto a pair of movable mirrors which reflect the light back through 180" (see Figure 2) to a fourth mirror in the electron beam. If the pair of mirrors is moved back by 9 cm, the path length is increased by 18 cm, and the desired delay of 0.6 nsec is achieved. The optical system must collect the analyzing Ceren. kov light and focus it through the irradiated volume in the sample cell. The light intensity in the sample cell should not vary as the optical path length is changed. This condition can only be fulfilled by extremely accurate alignment of the optical system. This system must also have constant magnification. The ultimate time resolutions of such a system is not limited by the width of the fine-structure electron pulse but rather by the problem of synchronizing the fastmoving electron pulses (velocity pc) with the somewhat slower Cerenkov light flashes (velocity c/n) as they pass (11) J. V. Jelley, "Cerenkov Radiation a n d Its Applications," P e r g a m o n Press, L o n d o n , 1958.
Volume 7.3, Number 11
October 1968
nlr. J. BRONSKILL AND J. W. HUNT
3764 through the sample cell. From the front to the back of the cell these pulses lose synchrony by a time
nL L -c Pc
t = -
=
-c
0.01L nsec
NKOV FROM S A M P L E
where L is the sample cell length and n is the index of refraction of the sample (1.33 for water). The cell length becomes, therefore, a compromise between short time resolution and high sensitivity (i.e,, large absorption due to long cell path length). In the detailed system described later, L was chosen as 2 em, giving a theoretical time resolution of about 0.02 nsec. Another possible limit on time resolution is caused by differences in the optical path length for individual Cerenkov light rays. In our focusing system, the time spread is negligible, being in the order of loF3nsec.12,13 2. Detectable Concentration of Transient Species. The average dose calculated for each fine-structure pulse in a h i m diameter, 1-A, 30-MeV electron beam is about 250 rads. This corresponds t o a concentration of solvated electrons of about 6 X 10-7 mol/l., or an absorption signal of about 4y0. Such a signal should be easily detectable. 3’. Noise. There are two principal sources of noise in this system: the shot noise caused by the finite number- of photoelectrons being detected, and noise caubed by variations in electron and Cerenkov pulse intensity . ( a ) Shot Noise. An analyzing light beam for pulse radiolysis studies must be sufficiently intense to overcome the statistical fluctuations in the production of photoelectrons a t the photocathode of the detecting device. This “shot” noise level varies as the square root of the number of photoelectrons.2 In order t o detect a 4y0 absorption signal with a 1 O O : l signal-tonoise ratio, we require at least 4 X lo7 photoelectrons during a 30-nsec pulse. Assuming a 10% cathode efficiency and a 10% light collection eficiency, 4 X lo9 photons are required in the Cerenkov light pulse at the wavelength of interest. The calculations summarized in Table I1 show that the necessary light intensity can easily be obtained with a 10-em air path.
Table I1 : Cerenkov Light Production by a aO-nsec, 30-MeV, 0.5-A Electron Pulse ( p = 0.999)”
Medium
Electron path length , om
Water (liquid) Zenon (1 atm) Air (1 atm)
10 10
2
No. of photons produced in a 10-mp band width a t 700 m@ll
4
x
10”
x
10’0
1 . 5 x 10’0 1.0
a eerenkov light intensity per unit wavelength (A) is proportional to 1/A2. Thus the figures given in this table should represent minimum values for a 10-mp band width throughout the visible and near-ultraviolet regions.
The Journal of Physical Chemistry
Signal observed
Three reference light signals from system gAMPLE
L E A D BEAM STOPPER
-
ANALYZING LIGHT LESS ABSORPTION IN C E L L PLUS BACKGROUND L I G H T FROM C E L L
ANALYZING LIGHT ONLY
2 BACKGROUND L I G H T FROM C E L L ONLY
MOVABLE CARRIAGE
ABSORPT16N = A.-(B+CI
Figure 3. The three types of light pulses used to produce an absorption signal. A rotating “chopper” wheel produces a cycle of three different signals (A,B,C) from consecutive electron pulses. For pulse A, all light and electron paths are open. For pulse B, the electron beam produces the analyzing Cerenkov light, but is blocked from irradiating the sample by a lead block. For pulse C, _the electron beam irradiates the sample, but the analyzing Cerenkov light is blocked. Lenses have been omitted from the optical system for simplicity.
(b) Noise f r o m Variation in Pulse Intensity. Variations in the intensity of the fine-structure electron pulses produce variations in the concentration of absorbing species and variations in the intensity of the Cerenkov analyzing light. The detection system, however, averages the consequent fluctuations in the absorption signal by integrating the light received over 100 pulses. Unfortunately, variations in intensity from one 30-nsec pulse t o the next present a more difficult problem. A portion of the light reaching the detection system will be Cerenkov light generated in a cone of about 40” half-angle by the passage of the electron pulse through the sample. This “background” Cerenkov light level must be subtracted before the absorption signal can be obtained. In order t o reduce noise and extract the absorption signal from the background light level, three different types of light pulses can be generated as shown in Figure 3. The “A” light pulse consists of the analyzing Cerenkov light, less the absorption in the sample, plus the background Cerenkov light generated in (and beyond) the sample cell. The “B” light pulse consists (12) D. A. Hill, D. 0. Caldwell, D. H. Frisoh, L. S. Osborne, D. M. Ritson, and R. A. Sohluter, Rev. Sci. Instrum., 32, 112 (1961). (13) E. K. Zavoisky and S. D. Fanchenko, A p p l . Opt., 4, 1165 (1965).
PULSE-RADIOLYSIS SYSTEMFOR OBSERVATION O F SHORT-LIVED TRANSIENTS
3765
Further Details
Figure 4. Block diagram nf the deleelion system and signal-processing electronic circoits. The light pulses paaa through the moimchromntar to the photomiiltiplier where they are converted into electronic pulses. These elertronic pulses are "stretched" and fed seqoentislly to three "hnlding" circuits. The de outpiits of these holding circuits arc proportional to signals A, B, atid C. Thcae outputs are fed to B diflerential amplifier which yields the absnrption signal,
A
- (I3 + C).
of analyzing Cerenkov light alone with the electron beam blocked in front of the sample cell. The "C" light pulse consists only of background Cerenkov light, sinre the anulyzing light is blooked before the sample cell. The actual nbsorpt.ion signal is A - (B C) and can be obtained by electronicully compnring the three separate light pulses. A rhopper mhecl, driven by a synchronous mot.or a t constant speed, is phased to provide the three types of light pulscs, in sequence, a t the rate of 60 Hz. The sequence of signals is ABCABCA., .etc. The detcct.ion system converts these light pulscs into clcctronir pulses which are then proC), cessed to yield the nbsorpt,ion signal, A - (B averaged ovcr a period of severul .seconds, Le., over several hundred individual pulses. A block diugram of the elertronic circuit.s is shown in Figure 4. By averaging over several hundred pulses, the noise rsnsed by variations in t.he electron pulse intensity is great.ly redurcd. D. Suitable Chemical Sys/ems. The choice of a suitable chemical system for this stroboscopic puLse radiolysis technique must be made with great care. The lifetime of t,hc absorbing species under observation must fall within the time runge from 0.02 t.o 0.3 nsec. Absorbing specics 1vit.h lifetimes longer than 0.3 nsec will be observed, but the kinetics of their decay will be obscured by the buildup of the absorption signal from one fine-structure pulse to the next. Observation of the decay of the solvated electron in acidic solntion is an ideal test system for the stroboscopic pulse radiolysis technique. High concentrations of H+ can be used to reduce the lifetime of the solvated electron to less than 0.3 nsec. Since the solvation time for electrons in water is thought to be 0.01 nsec" or less,'.' their buildup and decay should provide an accurate measurement of the t.ime resolution of our apparatus.
+
The analyzing Cerenkov light flashes are generated in 10 em of air immediately beyond the exit window of the linac. The electron beam, focused to a 5-mm diameter spot by quadrupole magnets, traverses this air path, then passes t.hrough two thin (0.01 rm) mirrors to the sample cell. The Cerenkov light rays enter the sample cell a t the same point as the electron beam (Figure 2). Once through the sample cell, the light rays are reflected out of the beam line and through a hole in a shielding wall to a separate room containing the monochromator and detection system. The optical system is designed to keep the position and size of the Cerenkov light image constant as the light path length is varied. The movement of the pair of mirrors is accomplished without vibration by an air cylinder mounted coaxially with a hydraulic checking cylinder (Nodernair Corporation, Nodel 1320). A multi-turn potentiometer is coupled to this linear motion and provides a voltage signal proportional to the mirror position. This signal is fed to the X input of an X-Y recorder while the absorption signal, A (B C), is fed to the Y input. Thus the X-Y recorder displays the absorption signal a3 a function of time within the interval between fine structure pulses, i.e., from 0.02 to 0.3 nsec. The critical alignment of the optical system is performed by a combination of two methods. A laser shining along the line of the electron beam is used to align all mirrors and lenses up to the sample cell. Once the apparatus is positioned in front of the linac, a light
-
+
+
~ r:LO : ~~~si(.illos~.iqii. ph I I . : ~ . s h n v i i q the Figtire Ir. I ' l ~ ~ t ~ ~ c>f ~ three types of m : s I y z i u ~licht puis,*. This s i w d u ' : thserved at. the output of the stretdicr circuit (SW Figwc 4). T h e vertical scale is 1 V per division rind the iiitervd between pulses corresponds to a frequency of GO 11%. IClertron beam riirrent WBB >0.S A in a 30-11secpelae; beam energy was nhout 30 MeV. The unusual pulse sequence B, A, C wan cnlleed by rotating the chopper wheel in the reverse direction tu the normal sequence A, B, C. (14) R. L. Platzman.~"Physienl and Chemical Aspects of Basic Mechanisms in Ilndiobiologv." U. S. Nntionnl I b e n r e h Counc4
Publication 305. 1953, p 22.
Volume 7% Kutnlcr I 1 Oclobcr 1968
E.-G. NIEMANN AMI 3’1. KLENERT
3766
bcam, imaged on thc cxit slit of the monoclhromator, is directed back through thc monochromator and optical system to align the components beyond the samplc cell. Because of the high doses absorbed in the sample, a cooling water jacket surrounds the sample cell. In addition, buildup of interfering radiation products is rcduccd by a flow system which rapidly charigcs the solution in thc sample cell during the run.
Results Tests of this stroboscopic pulse-radiolysis system are currently in progrcss. Beam diameters less than 5 mm havc been measured with polyvinyl chloride and perspex HX sheets. A dosc of over 10 l
