Absorbed dose distribution in a pulse radiolysis optical cell

Znt. J. Rudiut. Phys. Chem. 1975, Vol. 7, pp. 661-666. Pergamon

Press. Printed in Great Britain

ABSORBED DOSE DISTRIBUTION IN A PULSE RADIOLYSIS OPTICAL CELL ARNE MILLER Accelerator

Department,

and WILLIAM L. MCLAUGHLIN*

Danish Atomic Energy Commission, DK-4000 Roskilde, Denmark

Research

Establishment

Rise,

(Received 12 February 1975) Abstract-When a liquid solution in an optical cell is irradiated by an intense pulsed electron beam, it may be important in the chemical analysis of the solution to know the distribution of energy deposited throughout the cell. For the present work, absorbed dose distributions were measured by thin radiochromic dye film dosimeters placed at various depths in a quartz glass pulse radiolysis cell. The cell was irradiated with 30 ns pulses from a field-emission electron accelerator having a dose broad spectrum with a maximum energy of 02 MeV. The measured three-dimensional distributions showed sharp gradients in dose at the largest penetration depths in the cell and at the extreme lateral edges of the cell interior near the optical windows. This method of measurement was convenient because of the high spatial resolution capability of the detector and the linearity and absence of dose-rate dependence of its response. INTRODUCTION FIELD-EMISSION electron accelerators (e.g. Febetron Models 705, 705B, 707) are being used for pulse radiolysis studies of radiation chemical kinetics of liquid solutions held in silica glass optical celW4). A typical radiation spectrum for such an electron beam is broad, with a maximum energy of about 2 MeV. The beam is emitted through a thin metal foil window (e.g. 0.025 mm Ti) and may be diffused or collimated. A non-uniform dose distribution occurs in the pulse radiolysis cell(l). The cell must have dimensions large enough to permit the analyzing light beam to be transmitted. Fig. 1 shows a typical glass cell used for such pulse radiolysis studies(4).

FIG. 1. Silica glass optical cell used at Risa with the Model 705 B Febetron accelerator for pulse radiolysis studies of liquid solutions. Dimensions: (a) 30 mm; (b) 5 mm; (c) 20 mm. Wall thickness: 0.2 mm. *Visiting scientist from the Center for Radiation Washington, D.C. 20234, U.S.A. 661

Research,

National

Bureau of Standards,

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ARNE MILLER and WILLIAML. MCLAUGHLIN

Attempts have been made previously to measure the absorbed dose distribution in an optical cell irradiated by a very high-intensity single pulse of electrons from a field-emission pulse generator(l). The method used for these measurements was calorimetry, with an aluminum probe 1.5 mm in diameter and 20 mm in length used as the calorimetric body containing a thermocouple. The main problems with this method are its poor spatial resolution and the disturbance of the radiation field by the presence of the metallic probe. Simpler methods involving the positioning of thin plastic film dosimeters at different depths in the optical ce1V5) have the problem that most dosimeters of this type show a response which varies with changes in temperature and absorbed dose rate@). For users of field-emission accelerators for pulse radiolysis studies of liquid and solid systems, it is important to know the dose distributions throughout the optical cell in order to be able to correct the optical density values for non-uniformities at the position of the analyzing light beam. The purpose of the present investigation with a practical pulse-radiolysis arrangement using a field-emission accelerator was to determine the three-dimensional dose distribution in the silica glass optical cell. The problem was made relatively simple by a solid thin-film radiochromic dye using as a nearly water-equivalent medium* dosimeter”). This dosimeter registers a high-resolution radiographic image and undergoes a change in optical density with dose which is not dose-rate dependent and only slightly temperature dependenP,‘).

EXPERIMENTAL A Field Emission electron pulse generator (Febetron Model 705B) provided single pulses of electrons of about 30 ns halfwidth and about 2 MeV maximum energy. This accelerator is being used at Riser for pulse radiolysis studies of aqueous and organic solutions as well as gas-phase chemical kinetics studies. Normally, the average absorbed dose in a cell placed close to the accelerator exit window would be of the order of 1 Mrad per pulse. For many radiation chemistry studies the dose per pulse had to be smaller by as much as two or three orders of magnitude. Doses from about 1 to 100 krad per pulse were achieved by placing grounded copper plates with a pattern of drilled holes of different sizes over the accelerator window. This had the effect of attenuating and diffusing the electron beam before it traversed the pulse radiolysis optical cell. Figure 2 shows an exploded view of a simulated pulse radiolysis cell containing the arrangement of thin-film radiochromic dye dosimeters used for determining three-dimensional dose distributions. This arrangement was devised in order to simulate the practical optical cell (see Fig. 1) containing an aqueous solution. In a pulse radiolysis experiment, the electron beam enters from the left (z-direction), and the analytical light beam passes at a 90” angle to the electron beam axis through the optical windows (x-direction). The simulated cell was placed at the position normally cocupied by the pulse radiolysis cell. The rectangular brass collimator opening is slightly smaller than the x-y area of the simulated cell, causing the dose close to the side walls of the cell to be greatly diminished. The simulated cell was irradiated with a series of electron pulses in order to provide a dose at a given depth in the cell sufficient to be registered on the radiochromic film dosimeters. The radiochromic film dosimeterst used in these experiments consisted of approximately 10% by weight of hexahydroxyethyl pararosaniline cyanide dissolved in a Nylon film about 50 pm in thickness. The film is clear and colorless before irradiation; after irradiation it is blue, with an absorption band having a maximum at 600 nm wavelength. The response of the film to 6oCo gamma radiation was calibrated by means of Fricke (ferrous sulfate) dosimetry. Figure 3 shows the change in optical density due to y-ray-irradiation as a function of optical absorption wavelength *These dosimeters have a density of 1.06 g crnw3 and an electron stopping power within 1 ‘A of that of water for the electron spectrum used in this work. tThese dosimeters are obtained from Far West Technology, Inc., 330 S. Kellogg, Goleta, California 93017, U.S.A.

Absorbed

dose distribution

in a pulse radiolysis

663

optical cell

brass(4mm) 5Opm thick

1 - 8 Dosimeters brasstfmm)

a -

Quartz

b -

Nylon

plates 0.2mm thick plates

0.5mmthick

FIG. 2. A simulated

pulse radiolysis cell, showing inside the silica glass walls stacks of Nylon radiochromic dye dosimeter films alternated with slabs of Nylon. The direction of the electron beam is the z-axis and the direction of the analyzing light beam is the x-axis for both pulse radiolytic studies and dose distribution measurements. 2

Change

Change in optical density

in optical density 2

c

400

500

600

Wavelength,

700 “m

0.5 Absorbed

1

1.5 dose

2

in water.

2.5 Mrad

FIG. 3. Fifty-micrometer thick radiochromic dye film dosimeter irradiated with Wo y-rays: (a) optical absorption spectrum for an absorbed dose in water of 2.0 Mrad; (b) calibration curve in terms of change in optical density at 600 nm as a function of dose. and a calibration curve (change in optical density at 600 nm wavelength vs absorbed dose in water). Since the response to gamma radiation has been shown to be the same as that to very high-intensity electron beams from the Febetron (up to w 1016 rads-1(7f*)) this calibration curve could be used for all radiation intensities encountered in the present experiment. Lateral variations in dose across the dimensions of the cell perpendicular to the electron beam axis were represented by differences in the optical densities over the area of the dosimeter films. Differences in dose with depth of penetration of the electron beam were determined by optical density readings at a given location on a series of films in the stack shown in Fig. 2. High-resolution traces of optical density laterally over the films were made either with a specially designed recording microspectrophotometeru’) or with a commercially available computer-controlled scanning microdensitometer, equipped with an interference filter passing light at 600 nm wavelength. RESULTS

Optical density values were traced across the radiographic image of the electron beam energy deposition in the film dosimeters. Figure 4 shows these traces across the center of each film in the y-direction as made with the recording microspectrophotometer on films placed at eight different depths in Nylon.

ARNE MILLER and WILLIAM L. MCLAUGHLIN

664

FIG. 4. Recording microspectrophotometer tg) traces of optical density variations at 600 nm across the radiographic images of the electron beam profile at various depths in the central yz,-plane in the simulated pulse radiolysis cell. Zero krad is above the baseline because of the optical density of the unirradiated film.

Similar traces were made on each film in the x-direction. By analyzing these traces in terms of absorbed dose in Nylon according to the calibration of the film response, it was possible to obtain the dose distribution in the pulse radiolysis cell in the central x,z plane, as shown in Fig. 5. The variation of dose along the z-axis

Absorbed d-xc, t

kmd

FIG. 5. Measured absorbed dose distribution in Nylon throughout the simulated pulse radiolysis cell, showing beam profiles along the x-axis (lateral dose distribution) at various depths in the central x,z-plane, and depth doses along the z-axis from the entrance of the electron beam to the back wall.

Absorbed

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in a pulse radiolysis

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optical cell

(1.65 cm from the side edge) is plotted (see inset) as the central axis depth-dose in the optical cell. It is seen that at the greatest depths of electron beam penetration and at the extreme lateral positions close to the optical windows of the cell the dose is greatly diminished. It was possible to determine the dose distribution across each dosimeter film by using the computer-controlled microdensitometer. The instrument was operated with a series of repetitive scans in the x-direction on the film, each successive scan being slightly displaced in the y-direction from the previous scan so that the entire surface of the film was scanned. Figure 6 shows for one-half of one of the dosimeter

Absorbed dose. krad

l

0.2

0.4

0.6

06

10

1.2

1.4

1.6

1.6

20

x,

cm

FIG. 6. A beam profile plot of dose distributions over one-half of a dosimeter film placed at 0.2 cm depth in the simulated cell, as measured by the computercontrolled scanning microdensitometer. The small peaks are due to dust particles or other imperfections on the dosimeter film during the scan.

films a typical lateral dose distribution obtained in this way. Since the change in optical density at 600 nm wavelength is linear with dose in this range, the dose values given in the ordinate were ascertained simply by means of a calibration factor (absorbed dose per change in optical density). SUMMARY

Thin radiochromic dye dosimeters provided high-resolution dose distributions three-dimensionally in a silica glass optical cell. Measurements could be made simply by means of a scanning microspectrophotometer, without need for corrections due to variations in temperature, dose rate or radiation spectrum. This technique afforded an improvement over previous methods of deriving dose distributions, since (1) calculations of depth-dose for a broad spectrum of electrons having diffuse incidence

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ARNE MILLER and WILLIAM L. MCLAUGHLIN

are not available and (2) most conventional dosimetry methods for very high intensity pulsed electron beams (calorimetry, chemical dosimeters) suffer either from poor spatial resolution or from dose-rate dependence of response. REFERENCES 1. R. E. BUHLER and J. M. BOSSY, ht. J. Radiat. Phys. Chem. 1974, 6, 95, 2. Q. G. MULAZZANI, M. D. WARD, G. SEMERANO, S. S. EMMI and P. GIORDANI, ht. J. Radiat. Phys. Chem. 1974,6,187. 3. V. MARKOVIC, D. NIKOLIC and 0. I. MI&, Int. J. Radiat. Phys. Chem. 1974, 6, 227. 4. K. NILSSON, in press. 5. J. P. KEENE, J. xi. Instrum. 1964, 41, 493. 6. A. MILLER, E. BJERGBAKKE and W. L. MCLAUGHLIN, Int. J. appl. Radiat. Isot. in press. 7. W. L. MCLAUGHLIN, P. E. HJORTENBERG and B. B. RADAK, Absorbed Dose Measurements with Thin Films. Dosimetry in Agriculture, Industry, Biology, and Medicine, International Atomic Energy Agency, Vienna, 1973, pp. 577-591. 8. W. A. FRANKHAUSER, Measurement of the radiation exposed to a pulse electron beam source. EG & G Santa Barbara Division, Goleta, CA 93017, 1973. 9. A. MILLER, W. L. MCLAUGHLIN and B. LYNGG~RD, A thin-film dosimeters. Riser Report M-1525, Danish Establishment Rise, Roskilde, Denmark, 1972.

response of a radiochromic dosimeter Report No. 1183-5024, EG &G, Inc., scanning spectrophotometer for reading Atomic Enerffy Commission, Research