Dose buildup effects in irradiation of food products

Radiat. Phys. Chem. Vol. 25. Nos. 1-3, pp. 135-139, 1985

0146-5724/85 $3.00 + .00 Pergamon Press Ltd

Printed in Great Britain.

DOSE BUILDUP EFFECTS IN IRRADIATION OF FOOD PRODUCTS E. T. O'Sullivan,

A. L. Gunby,

and K. M. O'Sullivan

International Nutronics, Inc. 1237 N. San Antonio Road, Palo Alto, CA

94303

ABSTRACT Dose buildup in food products under gamma irradiation was demonstrated in small-scale and large-scale tests with citrus. An analytical approach for including buildup in food irradiator design was verified against test results.

KEYWORDS

Food irradiation; dose buildup; food dose distribution.

gamma radiation buildup; measured buildup;

INTRODUCTION Interest in commercializing food irradiation has increased with two recent developments: (a) the recognition of no toxic effects within desired exposures, a result of decades of research, and (b) increased concern with using chemicals to treat post-harvest food production. A strong base for commercial food irradiators exists in the radiation services industry, which has developed sound designs for medical device irradiators and proven them with many years of efficient operation. At first glance, however, the greater density of food products (s.g. = 0.3 to 0.9) compared to that for medical products (s.g. = 0.05 to 0.2) seems to dictate food irradiator designs with thin targets for food products, in order to hold the maximum/minimum exposure ratio within limits. However, the increased absorbance of dense target material is partially offset by the phenomenon of dose buildup. Although buildup was recognized in early irradiator designs (1,2), the mean free path of photons in medical products (greater than 150 cm for s.g. = 0.i) is sufficiently greater than the target dimensions so that buildup has little impact on current medical irradiator designs. Food targets, however, will have mean free paths from 15 cm to 50 cm, less than practical target dimensions. Maintenance of desired max/min exposure ratios will depend on the presence of buildup, and good design will depend upon the ability to predict it. Determination of the max/min exposure ratio depends on the food to be irradiated and the goal of the irradiation. Generally, a max/min ratio of 2.0 or less will be required because of (a) phytotoxicity in fruits and vegetables vs quarantine release, (b) regulatory limits vs desired effect, or (c) economy of the process. For example, an exposure of 0.26 kGy (26 kRad) is sufficient to disinfest papaya for quarantine release, while slightly higher exposures (up to about 0.75 kGy) can provide significant extension of shelf life; however, exposure of papaya above 1.0 kGy will produce some surface scalding (3). Thus, for quarantine release, the max/min exposure can be 1.0/0.26 = 3.85, while for shel~ life extension, it is limited to 1.0/0.75 = 1.33. To achieve a tight max/min ratio of 1.33 with a target density for papaya (s.g. = 0.3), designers of irradiators must be able to predict the effect of buildup. The need for more studies in this area has recently been stressed (4) and preliminary examinations by International Nutronics, Inc. (INI) have indicated the strong influence of buildup on food irradiation (5). Today's designer needs easily-applied, experimentally-verified techniques for applying dose buildup to irradiator design. Facilities for experimentally verifying dose distributions in large targets require a large irradiation cell and a large plaque-type source. Such facilities are not generally available to researchers. To this end, INI has performed several tests in its commercial facilities. A relatively small-scale test was performed in its Palo Alto, CA, facility and a large-scale test was performed in its Irvine, CA, plant. Results of these experiments and related analyses are presented herein.

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STUDY

As a first step, b u i l d u p was e x a m i n e d in a o n e - d i m e n s i o n a l a r r a n g e m e n t in INi's Palo Alto facility. Four boxes of l o o s e - p a c k e d citrus w e r e a r r a n g e d to a l l o w an i n s t r u m e n t e d t r a v e r s e of 86 cm (34 in.) through the heart of the target, as shown in Fig. I. The Co-60 source was e f f e c t i v e l y a r e c t a n g l e of 41 cm by 64 cm (16 in. by 25 in.), located in a w a t e r - f i l l e d , s t e e l - s i d e d "finger" e x t e n d i n g from the side of the s t o r a g e tank. The target was p l a c e d in air 91.5 cm (3 feet) from the source in o r d e r to a p p r o x i m a t e a p o i n t - s o u r c e arrangement. D o s i m e t e r s (Far West-70 system) w e r e p l a c e d on a m e t e r stick at intervals of 5.1 cm (2 in.). The stick was then p l a c e d b e t w e e n the boxes as shown in Fig. i, with care taken to e l i m i n a t e gaps b e t w e e n the boxes by filling the b o t t o m boxes c a r e f u l l y w i t h fruit (oranges and lemons). D e n s i t y of the b o x e d fruit was m e a s u r e d at 0.46 g/cc, ±0.02. F o l l o w i n g p r e - r e a d i n g of the dosimeters, the i n s t r u m e n t e d target was e x p o s e d for s u f f i c i e n t time to a c h i e v e good d o s i m e t e r readings, the d o s i m e t e r s were read, and the d o s e r a t e in the target for each l o c a t i o n was d e t e r m i n e d . The r e s u l t i n g data, n o r m a l i z e d to the d o s e r a t e at the l e a d i n g edge of the target, are d i s p l a y e d in Fig. 2, along w i t h t h e o r e t i c a l values for the doserate, with and w i t h o u t buildup. The c o m p u t e d curves of Fig. 2 are b a s e d on first principles, a s s u m i n g the p o i n t - s o u r c e a p p r o x i m a t i o n (6). G e o m e t r i c effects and target a b s o r p t i o n are i n c l u d e d with w a t e r p r o p e r t i e s for a b s o r p t i o n and buildup, as m o d i f i e d by the t a r g e t s p e c i f i c gravity. P o i n t - s o u r c e dose b u i l d u p values were used (6), w h i c h are v e r i f i e d by r e c e n t i n f o r m a t i o n w i t h i n 2% (7). As shown, the data c l e a r l y s u p p o r t the i n c l u s i o n of b u i l d u p in the s u p p o r t i n g analysis. In p a r t i c u l a r , the s u r f a c e - t o - c e n t e r dose ratio, c o m p u t e d by a s s u m i n g a 1 8 0 - d e g r e e turn of the target for e q u a l time on the o p p o s i t e face, is m a r k e d l y i m p r o v e d by buildup. This result gives great c o n f i d e n c e for h o l d i n g the m a x / m i n ratio w i t h i n limits for large targets. The e x c e l l e n t a g r e e m e n t b e t w e e n theory lowing for the o n e - d i m e n s i o n a l case: buildup

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To v e r i f y the f o r e g o i n g a p p r o a c h to a c c o u n t i n g for buildup, a s i m i l a r irrad i a t i o n test was p e r f o r m e d in the INI Irvine facility. A p p r o x i m a t e l y 5700 kg (2600 ib) of g r a p e f r u i t were loaded into ten s t a n d a r d f a c i l i t y totes in front of the source as s h o w n in Fig. 3. In the seven totes nearest the source, a 15 cm s p a c e r was i n s t a l l e d so that the d i s t a n c e from source to target surface was 30.5 cm (12 in.). This air gap a p p r o x i m a t e s the gap e x p e c t e d for a t y p i c a l food i r r a d i a t o r w i t h n o m i n a l l y - s i z e d packages. A n o t h e r air gap of 15 cm was p r e s e n t b e t w e e n the two rows of totes. Thus, in the d i r e c t i o n of p h o t o n travel from the source, the e x p e r i m e n t a l d i m e n s i o n s were as follows: 0 - 30.5 cm air space; 30.5 - 76 cm target; 76 - 91.5 cm air space; 91.5 152.5 cm target. T a r g e t d e n s i t y was m e a s u r e d at 0.44 g/cc ±0.02. The target was i n s t r u m e n t e d by two c o n c e n t r i c p l a s t i c pipes p a s s i n g t h r o u g h two totes of the u p p e r level and p o s i t i o n e d in the center of the two totes (Fig. 3). D o s i m e t e r s w e r e p l a c e d at r e g u l a r i n t e r v a l s in the inner p l a s t i c pipe (which was sliced l e n g t h w i s e into a s e m i - c i r c u l a r section), with spacers of carrot p l a c e d b e t w e e n the d o s i m e t e r s to p r e v e n t streaming. H a r w e l l Red P e r s p e x and Far W e s t - 7 0 d o s i m e t e r s w e r e used. The inner h a l f - t u b e could be r e m o v e d from the fixed o u t e r tube for access to the d o s i m e t e r s and then be repositioned. Totes w i t h g r a p e f r u i t w e r e p o s i t i o n e d to p r o v i d e a b s o r b e r b e t w e e n all source l o c a t i o n s and d o s i m e t e r locations.

Dose Buildup Effects in Irradiation of Food Products

137

The source plaque was raised for the required time to achieve desired dosimeter exposure, dosimeters were removed in sequence (front to rear) as the desired dose was achieved, dosimeters were read, and doserate for each location was determined. Dosimetry data for both Far West and Perspex were consistent and reproducible. The doserate distribution in air at the same location as the instrument traverse was also measured. Data for the relative doserate through the instrument traverse are shown in Fig. 4. Two dosimeters were exposed at each location; average values are plotted. Dosimeters at each location agreed within 5%. The flattening of the photon field in the air gap between the rows of totes is clearly shown. The analytical model for the large-scale test was based on the results of the small-scale test. As before, point source assumptions were used. However, the source plaque was divided into 216 small segments (15 by 23 cm). The doserate along the instrument traverse was then computed by the method of superposition, summing the contributions of all source segments. As before, absorption and buildup data for water were used, modified by the target density. The large number of computations required computerization, resulting in the IRVINE-2B code. However, computations were well within the capabilities of a desk-top computer. The expected relative doserates along the instrument traverse as computed by IRVINE-2B are shown in Fig. 5, with and without buildup. As demonstrated earlier in the small-scale test, buildup is crucial in matching analysis to experiment, and must be included in any design analysis for predicting the behavior of food irradiation systems. CONCLUSIONS Experiment and supporting analysis of food targets in a commercial irradiator configuration have clearly demonstrated that buildup is a key element in maintaining desired dose distribution. Analysis of the buildup contribution can use several simplifying assumptions: (a) homogeneous model for a heterogeneous array, (b) point source approximation with the method of s u p e r p o s i t i o n and (c) absorption and buildup properties for water, modified by the target density. REFERENCES i.

Frankfort, J. A., S. Haram, and S. Wallach (1960). An Industrial Gamma Irradiator for Medical Supplies, NYO-9433. U. S. Atomic Energy Commission, Washington, D. C.

2.

Manowitz, B., R. H. Bretton, L. Galanter, and F. X. Rizzo (1964). Computational Methods of Gamma Irradiator Design, BNL 889(T-361). Brookhaven National Laboratory, Upton, NY.

3.

Akamine, E. K., and J. M. Moy (1983). Delay in postharvest ripening and senescence of fruits. In E. S. Josephson, and M. S. Peterson (Ed.), Preservation of Food by Ionizing Radiation, Vol. III, Chapter 5. CRC Press, Boca Raton, FL.

4.

McMullen, W. H., and J. G. Yeager (1982). Workshop on Low-Dose Radiation Treatment of Agricultural Commodities. U. S. Department of Energy, Albuquerque, NM.

5.

O'Sullivan, E. T., and A. L. Gunby (1983). Food irradiation experience in a large medical products irradiator. Proceedings of the International Conference on Radiation Disinfestation of Food and Agricultural Produgts, Honolulu, HI.

6.

Blizard, E. P. (1958). Nuclear radiation shielding. In H. Etherington (Ed.), Nuclear Engineering Handbook, Section 7.3. McGraw-Hill, New York.

7.

Chilton, A. B., J. K. Shultis, and R. E. Faw (1984). Principles of Radiation Shielding. Prentice-Hall, Englewood Cliffs, NJ.

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