Distribution of the absorbed doze and energy of the high-current pulsed electron beam formed by accelerator TEU-500 (350...500 kV, 60 ns, current density 0,3...0,4 kA/sm2) in water sheet depth has been measured. The high-resolution measurement technique of doze and energy distribution with application of dosimetric film based on lavsan with phenazine covering was used. Spatial resolution at registration of the absorbed doze in the range of 5...100 kGr amounts to 20...30 mkm. It was shown that at absorption of electron beam with high current density (in conditions of track overlapping on surface of the absorbing layer) distribution of the absorbed doze in thedepth within the limits of ±10 % coincides with distribution obtained for low-current bea

Pushkarev, A. I.

Sazonov, R. V.

Electronic archive of Tomsk Polytechnic University

Introduction Electron energy distribution in the depth at absorption in condensed media is well studied and approximated by various empirical relations [1, 2]. Distributionof absorbed doze typical for a substance in a condensedphase is conditioned by multiple scattering of primaryelectrons and formation of secondary electrons. By electron delay in the medium ionization loss increase and atthe same time a number of secondary electrons as well asa number of backscattered electrons from deeper layersgrows; density of medium atom ionization increases andas a result energy absorbed in a substance grows. On theother hand, multiple scattering of primary electrons results in significant spread of their range in a substance andthereby, decreases not only a number of electrons moving in original direction but also a number of backscattered electrons. Presence of these two processes results information of wide superimposed maximum in absorbedenergy distribution in substance depth. Owing to scattering processes the distribution maximum (in comparisonwith Bragg curves for heavy particles) is not at the end ofextreme range but it is considerably shifted to the surface layers of the substance [2].This mechanism describes the absorption of electron beams with low current density (oneelectron approximation). At radiation water interaction the mainpart of secondary electrons formed as a result of medium ionization has a low range owing to its low energyand therefore, generates ionization and excitation inimmediate proximity to the place of its formation. Pulse radiolysis of liquidphase compounds in conditionsof influence of electron beam with high current density(more than 100 А/cm2) when tracks are overlapped onthe surface of irradiated liquid is of great interest. In thiscase conditions favorable for multiple collisions of particles with excess energy storage at internal degrees offreedom are formed. In these conditions deviation ofabsorbed doze distribution from known dependences ispossible. The given conditions of carrying out radiolysisof liquid compounds are not studied and they are of scientific and practical interest for studying mechanism ofelectron beam dissipation.In work [3] electron energy absorption was estimated by filter method according to the indices of two detectors arranged behind the filters of different thicknessand calculation by empirical formulas. In works [4, 5]film plastic detectors CPD2F2 were used for measuring absorbed doze [6]. Field of electron absorbed doze(0,5...1 MeV) was measured in water by the depth by filter method in narrow beam geometry. Measuring electron energy absorption in liquid layer by filter method itis difficult to support homogeneous thickness of absorption layer and possibility of varying thickness with apitch less than 0,1 mm that results in low accuracy ofmeasurements.The aim of the work is to study distribution of electron beam with high current density in water sheetdepth and development of highresolution technique ofmeasuring electron energy distribution at absorption inliquid.1. Experimental deviceThe experiments were carried out at pulse electronaccelerator ТEU500 (350...500 kV, 60 ns, current density 0,3...0,4 kA/cm2) [7]. Capacitance divider arrangedin oilfilled chamber was used for measuring voltage.Total current of electron beam was measured by Faradaycup. Oscillograms of accelerating voltage at planar diode cathode at irradiation of water and total current ofelectron beam are given in Fig. 1.Fig. 1. Oscillograms of: 1) voltage applied to planar diode,2) electron beam currentMathematics and mechanics. Physics47UDC 539.12.04; 621.039.587RESEARCH OF HIGHCURRENT PULSED ELECTRON BEAM ENERGY DISTRIBUTION IN DEPTH OF SHEET OF WATERA.I. Pushkarev, R.V. SazonovHigh voltages Research Institute of Tomsk Polytechnic UniversityEmail: aipush@mail.ruDistribution of the absorbed doze and energy of the highcurrent pulsed electron beam formed by accelerator TEU500 (350...500 kV,60 ns, current density 0,3...0,4 kA/sm2) in water sheet depth has been measured. The highresolution measurement technique of dozeand energy distribution with application of dosimetric film based on lavsan with phenazine covering was used. Spatial resolution at registration of the absorbed doze in the range of 5...100 kGr amounts to 20...30 mkm. It was shown that at absorption of electron beamwith high current density (in conditions of track overlapping on surface of the absorbing layer) distribution of the absorbed doze in thedepth within the limits of ±10 % coincides with distribution obtained for lowcurrent beam.To measure absorbed doze at irradiation by continuous and pulse electron beams the dosimetric radiationsensitive film (copolymer with phenazine dye) of the typePOR are widely used [6]. Film thickness is 0,1 mm (sensitive layer thickness is 15 mkm) that allows recording thevalue of absorbed doze with high spatial resolution. Atelectron energy more than 200 keV losses in dosimetricfilm are insignificant. To measure distribution of absorbed doze of pulsed electron beam in liquid sheet depth(water) a special reactor was made, Fig. 2. There is a dimple on the merit of 9° in reactor metal case – 1. Disk ofdosimetric film – 2 is placed on a surface. Lower part ofreactor case is closed with thin aluminum foil. There is adevice for pulling aluminum foil in reactor construction.Fig. 2. Diagram of measuring the absorbed doze of pulsed electron beam in liquid: 1) chamber case, 2) dosimetric film,3) liquid, 4) 10 mkm aluminum foilThe volume between the foil and dosimetric film isfilled up with the examined liquid – 3. Narrow strip ofdosimetric film is arranged below the aluminum foil.Distribution of density of electron beam energy at reactor input was determined by optical density of this strip.2. Calculation of the absorbed doze of electron beam in water Measurement of distribution of the absorbed doze ofelectrons having energy less than 500 keV in liquid depthis technically complicated as the depth of their total absorption does not exceed 1,5 mm. Measuring by filtermethod it is necessary to support uniform thickness ofliquid layer and change it with a pitch less than 0,1 mm.Therefore, experimental data of distribution of the absorbed doze of electrons with energy less than 1 MeV inwater existing in literature differ significantly, Fig. 3.Fig. 3. Distribution of the absorbed doze of electrons with energy 0,5 MeV in water. Data: 1) calculation, 2) works [2],3) works [8]Distribution of the absorbed doze of electrons inaluminum is measured with high accuracy. To determine the depth of electron absorption in a substance theirdistribution in aluminum is recalculated by known values of extrapolated range for the examined substance[1, 2]. The experimental data of distribution of the absorbed doze of monoenergetic electron beam (with different energy) in aluminum coincide well at normalization of proper values of depth t per extrapolated range ofelectrons R with this energy.Distribution of the absorbed doze of monoenergetic electron beam with different energy in the normalized depth of aluminum d=t/R is given in Fig. 4.Magnitude R was calculated by the formula of Flammersfield [1].Fig. 4. Distribution of monoenergetic electron beam in aluminum: 1) experimental data, 2) approximating dependenceof the absorbed doze, 3) density of electron beam energyExperimental values of the absorbed doze given inFig. 4 were approximated by polynomial of the third degree (curve 2, Fig. 4) which was further used for calculating distribution of the absorbed doze of pulsed electronbeam at different accelerating voltage. Electron range inthe examined liquid was calculated by the ratio [1]:(1)where Z is the charge in positron unit charge, A is theatomic mass (in aem) of target (of aluminum or the studied substance), ρAl is the aluminum density, ρх is thedensity of the studied substance.Distribution of density of electron beam energy inthe depth of absorbing layer equals to:(2)where E0 is the density of energy on a surface (J/cm2), ρis the substance density.The obtained normalized dependence of energydensity distribution in aluminum depth is shown inFig. 4 (Curve 3).Distributions of absorbed doze in water for monoenergetic electron beam with energy 0,5 MeV given in00( ) ,tE t E D dtρ= − ∫Al Al( / ) ,( / )x xZ At d R Z Aρρ=Bulletin of the Томsк Pоlytеchnic University. 2007. V. 311. № 248works [2, 8] and dependence calculated by the above given technique are shown in Fig. 3 for comparison. It isseen that distribution of the absorbed doze in water obtained on the basis of the range in aluminum coincideswell with the experimental data given in paper [2]. Ourexperimental data were further compared with dependence 1 Fig. 3.3. Measurement of distribution of density of electron beam energy in water Dosimetric film POR represents a thin layer of radiationsensitive cover coated on lavsan substrate. Measuring electron beam absorption in water two variants ofdosimetric film arrangement are possible – by phenazine cover to water or to reactor vessel.In the first series of experiments dosimetric film wasarranged by phenazine cover to reactor vessel. Beamelectrons passed water sheet, lavsan film and then occurred in phenazine cover of dosimetric film causing thechange of its color. According to the technique [9]change of energy density in cross section of pulsed electron beam at reactor input and at passage of water sheetwas determined by the change of optical density of dosimetric film. Experimental data, Fig. 5, correspond tothree different sectors of disc of dosimetric film irradiated by one pulse. Value r=0 corresponds to outer edge ofthe disc of dosimetric film.Fig. 5. Change of energy density by section of pulsed electronbeam: 1) after passing water sheet, 2) at reactor inputMeasuring absorption in water sheet the change ofdensity of electron beam energy in cross section at thereactor input should be taken into account (Curve 2Fig. 5). It is well described by linear dependence 3(Fig. 5) Е(d)=Е0·(1+0,04.d). Adjusted dependence ofdensity of electron beam energy subject to its inhomogeneity by the section is given in Fig. 6 (Curve 1). Transition from distribution of beam energy density in crosssection Е(r) to distribution of energy density E(t) afterpassing water sheet with the thickness t is fulfilled by theformula t=d·tg(9°).Distribution E(t) of monoenergetic beam in watersheet depth was calculated by the ratio (2) in this caseelectron range in water was determined by the (1). Theobtained dependences are shown in Fig. 6 (Curves2–4). The calculated dependence of distribution of theabsorbed doze of electrons with energy 350 keV isshown as well in Fig. 6. Dependences (3) and (5) are interconnected. Integral of distribution of the absorbeddoze of monoenergetic electron beam in the depth (dimension J·cm/kg) multiplied by water density equalsenergy density on water sheet surface (1,4 J/cm2).Fig. 6. Distribution of density of pulsed electron beam energy inwater depth: 1) experimental data, 2–4) calculation forelectrons with energy 300, 350, 400 keV, 5) calculationfor the absorbed doze of electron beamIt is seen that experimental points correspond to distribution of electrons with energy from 300 to 500 keVin water depth that coincides with average acceleratingvoltage on diode (Fig. 1).4. Measurement of distribution of the absorbed doze of electron beam in water In the next series of experiments the disk of dosimetric film was arranged by phenazine cover to water.Calculated dependences of distribution of energy andabsorbed doze of electrons with energy 350 keV as wellas the experimental data are shown in Fig. 7. Integral ofdistribution of the absorbed doze in the depth multiplied by water density equals to density of electron beamenergy on water sheet surface. It is seen that experimental points corresponds to a large extent to distribution ofelectron absorbed doze in water sheet depth but not toenergy distribution.When radiationsensitive phenazine cover is arranged in water where electron beam is absorbed change ofcoating optical transparency are caused not only by beam electrons but also by secondary electrons. Section ofinteraction of electrons with low energy with the material of phenazine cover is higher than for electrons withhigh energy. Therefore, we record mainly concentrationof secondary electrons which is proportional to the density of energy release in water (absorbed doze). If dosimetric film is arranged by lavsan film to water then only primary electrons of beam get into radiationsensitivecoating and we record decrease of beam energy by increase of thickness of water absorption layer.Mathematics and mechanics. Physics49Fig. 7. Distribution of pulsed electron beam with energy350 keV at absorption in water: 1) experimental data(for spectrum), 2) for the absorbed doze D, 3) for energy density EConclusion At absorption of electron beam with high currentdensity (in conditions of track overlapping on absorbinglayer surface) distribution of the absorbed doze in thedepth coincides satisfactorily with distribution obtainedfor lowcurrent beam. The advanced technique recording distribution by one pulse was used for measuring.The developed technique allows measuring independently distribution of energy and absorbed doze of electron beam in absorptive liquid layer. Spatial resolution isnot lower than 30 mkm that supports high accuracy ofrecording distribution in water sheet of the absorbed doze of electrons with energy less than 500 keV. Error ofmeasuring average energy of beam electrons does notexceed 20 % therefore, the developed technique may beused as express method of calibration of acceleratingvoltage sensors on electron diode cathode.The work was fulfilled with support of RFBR, grant 060800147.Bulletin of the Томsк Pоlytеchnic University. 2007. V. 311. № 250REFERENCES1. Tables of physical magnitudes. Reference book. / Ed. by I.K. Kikoin. – Moscow: Atomizdat, 1976. – 1006 p.2. Chepel L.V. Application of electron accelerators in radiation chemistry. – Moscow: Atomizdat, 1975. – 152 p.3. Fedorov V.M. Measuring electron energy of highcurrent megavoltbeams by hardness of Xray radiation from thick target. – Novosibirsk: Nauka, 1986. – 128 p.4. Kotov Yu.A., Sokovnin S.Yu. Xray diagnostics of highcurrentEbeam // Proc. of 8th IEEE Pulsed Power Conf. – San Diego, California, 1991. – P. 629–632.5. Sokovnin S.Yu. 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Research of high-current pulsed electron beam energy distribution in depth of sheet of water

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