Procedimientos de dosimetria en procesado electrónico.pdf

7/28/2019 Procedimientos de dosimetria en procesado electrnico.pdf

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A.I.I . I . Newsletter, No. 25,Vol. 1 &2, pp. 242-251, Hay 1992.

DOSIMETRY PROCEDURES IR ELECTROR PROCESSIRGAndras KovacsInstitute of Isotopes of the Hungarian Academy ofSciences, H-1525, Budapest, P.O.B. 77, HungaryKishor MehtaVhiteshell Laboratories, ABCL ResearchPinawa, Manitoba, Canada ROE 1LOArne MillerRiso National LaboratoryDK-4000, Roskilde, Denmark

ABSTRACTDosimetry is of basic significance in the various stages of electronprocessing: during characterization of the irradiation facil i ty , in processvalidation, and in the routin e control of the irradiation process. Variousreference and routine dosimetry systems of solid and liquid state are in regularuse for these procedures. The paper describes the basic role of dosimetry inthe course of these processes inc luding the applicability of various dosimeters.

IRTRODUCTIORElectron accelera tors are widely used in radiation processing, and themain applications include:* Cross-linking of wires and cables (100 to 200 kGy),* Curing (10 to 200 kGy),* Sterilization (25 to 50 kGy), and* Food processing (0.5 to 10 kGy).Dosimetry plays an important role in controlling these processes byproviding documentation that the radiation treatment has been carried outcorrectly. I t is essential that the uncertainty of the dosimetry system isknown and the system is traceable to a national standard. There are t hree bas icstages i n e lect ron processing where dosimetry performs a vita l function throughthe use of various types of dosimeters. These three stages are: thecharacterization of an irradiation facility, the validation of the irradiationprocess, and the process control during production (1,2,3J.

DOSIMETRY SYSTEMS ABD SELECTIOR CRITERIAFrom the variety of dosimetry systems available fo r electronprocessing, i t is important that an appropriate one be selected, careful lytaking into account the actual purpose of irradiation and the expectedirradiation conditions, such as dose, dose-rate and environmental effects.


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Energy dependence of the dosimeter response is another factor to beconsidered; since the energy spectrum of the radiation field is usually notknown, the dosimeter and the product should have very similar energyabsorption/scattering characteristics. The size of the dosimeter should besuch that i t yields the requi red spatial resolut ion for detailed dosedistribution measurements. The most commonly used systems are thefollowing:1. Calorimeters: Vater and graphite calorimeters, used asreference systems, are applicable in the 4 to 10 MeV energy range fo rpractical processing applications. The most commonly used systems havebeen designed, built and used at Riso National Laboratory, Denmark (4),National Institute of Standards and Technology, USA (5), and VhiteshellLaboratories, AECL Research, Canada (6). The schematic view of the Risotype water calorimeter is shown in Fig. 1. These instruments contain athermistor. placed in the water or graphite absorber for measurement oftemperature rise due to irradiation. Absorbed dose can generally becalculated as specific heat multiplied by the temperature rise.2. Liquid-state dosimeters: These types of dosimeters are not aswidely used for electron processing as fo r gammas because of theirrelatively large size. For example, spatial resolution considerationsexclude their application for dose distribution measurements. According torecent studies, however, potassium dichromate (10 to 40 kGy), ceric-cerous(1 to 100 kGy) and ethanol-monochlorobenzene (1 to 400 kGy) solutions sealed in glass ampoules - are suitable for measurement of nominal dose andfor reference dosimetry (7). The dosimeters must be placed in phantoms forthese purposes.3. Solid-state dosimeters: The following solid-state dosimetersare the most commonly used systems in e lect ron processing (8):

DOSIMETERS DOSE RANGE(kGy)Radiochromic Dye Film:Gafchromic 0.1 - 40FVT-60 0.5 - 100B3 (Riso) 5 - 100Cellulose Triacetate Film 5 - 300Alanine (rod and film) 0.01 - 100PHHA:

Gammachrome 0.1 - 3Amber Perspex 1 - 30Red Perspex 5 - 50Radix 5 - 50Among a ll these systems, the thin radiochromic films are the mostwidely used both during dose mapping and fo r routine process control.However, because of their dependence on environmental conditions, their useneeds careful consideration.


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DOSlllETRY PROCEDURES1. Characterization of the Irradiation Facili tyThe purpose of this procedure is to determine the principal

characteristics of the facility. For example, the relationships betweenthe operating parameters and the absorbed dose in a reference material mustbe established. In an electron irradiation facility, the dose depends on anuaber of operating parameters. For example, variation in the electronbeam current affects the absorbed dose, whereas variation in the. beamenergy affects electron penetration and depth dose distribution. Thus thecharacterization of the facility consi sts o f the measurement of thefollowing parameters:* nominal dose,* electron energy,* beam spot size,* scan width and dose uniformity, and* dose distributions in reference material(s).The characterization of the facility must be carried out after its

in i t ia l commissioning and whenever changes that can influence the dose areintroduced. I t is also suggested that this procedure be executed atregular intervals, for example, yearly.1.1 Measurement of nominal dose

The aim of this procedure is to determine the relationshipsbetween the operating parameters and the absorbed dose in a specificgeometry [9]. Vhen the beam current, the beam energy and the scan widthare kept constan t, the re is an inverse relationship between the dose andthe conveyor speed. Vhen, however, the conveyor speed is kept constant,the dose and the beam current are directly proportional to each other.In electron irradiation faci l i t ies, the nominal dose can bemeasured by either a calorimeter (water or graphite) , or a liquid or soliddosimeter placed in a phantom (Fig. 2).

1.2 Measurement of e lect ron energyA change in the electron energy affects the depth dosedistribution and the absorbed dose in a product; hence, the determinationof the electron energy is of basic significance. The most probable energy(Ep ) and the average energy (E.) of an electron beam at the surface of theproduct can be determined by the following formulas [II]:Ep (MeV) - C + C z ' ~ , andE. (MeV) - C,.Rso

where, ~ and Rso are, respectively, the practical range and the half-valuedepth in a homogeneous reference material. The values of the threeconstants Cl ' Cz and C, for water and aluminum have been determinedempirically and may be found in Ref 11. Relationships between ~ in waterand in other low-atomic-number materials are also given in this reference.


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The range parameters can be de te rmined by using the wedgetechnique along with a thin film dosimeter strip , such as FVT-60, 83, orGafchromic (p 97, 8). The details of the experimental setup and theresulting depth dose profile are shown in Fig. 3.1.3 Measurement of beam spot size

Yhen using a scanned and a pulsed electron beaa, i t is importantto know the dose distribution within a single pulse. This information willassist in ensuring the proper overlap between the pulses and scans, thusavoiding non-uniform lateral dose distribution.Again, the dose distribution with in the beam spot is measured witha film dosimetry system.

1.4 Measurement of scan widthThe aim of measuring scan width is to determine the lateral extent

of the radiation zone in the scanned direction with respect to doseuniformity. Film dosimeters, including radiochromic and cellulosetr iacetate, as well as radiation-sensitive indicators, such as PVC, aresuitable for the purpose. This information can be of practical importancein determining the allowable height of a product box when irradiating fromabove.1.5 Determination of dose distribution in reference product

During the characterization of an irradiation facili ty, thedetermination of the dose distribution, and especially the minimua andmaximum dose regions in a reference product box, is of basic significance.The measurement is carried out by placing individual dosimeters (mainlyfilm dosimeters) in the product box containing reference material, which isthen irradiated under production conditions. A new product validation isneeded i f the dose distribution is signif icantly al tered after amodif icat ion to the facili ty.

2. Validation of the ProcessProcess validation includes several components such as:* assessment of materials compatibility,* determination of the process dose, and* process qualification.

In a ll of these activities, accurate dosimetry is of great importance.During a materials assessment program, samples of products must beirradiated with known doses, and the propert ies of the product must betested. Such an evaluation ensures not only that the product and i tspackaging can withstand the radiation process employed, but also that thefinal product will meet the manufacturer's specifications and labellingclaims. The test irradiation must present a challenge to the materials atleast as severe as the process. For determination of the process dose,taking steril ization of medical devices as an example, samples of productsare irradiated to relatively small doses and, based on the extrapolation of


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the microbiological data from these samples, a dose is chosen that issuitable for the process. A pro tocol for this procedure has beenestablished by the Association for the Advancement of MedicalInstrumentation [12) and has also been proposed by the InternationalOrganization for Standardization [1) and the European Committee forStandardization [2).

The last activity, namely the process qualification, is of morerelevance to the facility operator. Here, a ll the processing parametersfor a specific product box are established, so as t o ensure that theproduct receives the required dose within the specified limits and underthe specified conditions. The main steps of this procedure are as follows:2.1 Dose mapping process

a) Identification of product (weight, size of product box, type,density, manufacturer of product);b) Selec tion of dosimeters (mainly film), irradiation conditionsand packaging geometry;c) Location of dosimeters in the product box;d) Irradiation of the product box at a nominal dose (setoptionally);e) Determination of the minimum and maximum dose values and theirlocations in the product box, estimation of dose uncertainties,calculation of the dose uniformity ratio; andf) Adjustment of the processing parameters to achieve the requiredminimum dose and the required dose uniformity ratio.

2.2 Verification processa) Irradiation of 8 to 10 product boxes with dosimeters located at

the minimum and maximum dose locations;b) Analysis of results, determination of the variance of D. in andD values; andc) Establishment of the process ing parameters based on thisanalysis, and selection of a reference position for routinedosimetry.For the dose mapping procedure, extreme care should be exercisedin loca ting the dosimeters in product units (boxes). As a guide forplacing dosimeters, the information obtained during facil i tycharacterization may be useful. Special attention should be given,especially for electron processing, to those characteristics of the productunit that may influence the dose distribution. These include inhomogeneous

product distribution within the unit, orientation of the products, voidspresent, local differences in densi ty , and interfaces such as betweenproduct and air. I t is also important to note that the results of theverification process is valid only for a specific packaging and loadingarrangement, and any change in this respect may result in a dosedistribution that is different than that determined earlier.Validation must be repeated whenever the irradiation conditions,including the product , i ts packaging pattern or the operating parameters,are changed.


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3. Routine Process ControlI t is essential to demonstrate that the entire process is undercontrol within a specified confidence level. Routine process control isapplied through dosimetry and is achieved by measuring the absorbed dose at

regular intervals dur ing the production run. For this purpose, dosimetersare usually placed at pre-determined reference positions (see Section2.2(c) above). These reference positions, as well as the relationshipsbetween the dose values at these positions and the minimum dose values inthe product, must be established and documented during the validationexercise. These reference positions can be on the product box or betweenthe boxes. For example, calorimeters, or liquid or solid-state dosimeters,placed between product boxes can be passed under the electron beam duringthe production run. This in fact may be the preferred method because dosegradients on the outside of the product box prevent reproducible referencedosimetry.In the case of electron facil i t ies, the key operating parameters(conveyor speed, beam curre.lt, beam energy and scan width) must also be

monitored, controlled and recorded. This information, along with theroutine dosimetry data, is used to demonstrate that the process was undercontrol throughout the production run.CONCLUSION

An electron processing facility must have available all thedosimetry systems that are needed fo r the three s tages of the irradiationprocess described here. These systems must be calibrated and thecalibration must be traceable to a national standards laboratory.


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REFEREllCES1. ISO Standard, Sterilization of health care products - Validation androutine control - Gamma and electron beam radiation sterilization,(under development). International Organization fo r Standardization.2. CRN Standard, Sterilization of medical devices - Method for validationand routine process control fo r steril ization by irradiation, (underdevelopment). European Committee fo r Standardization.3. ASTM Standard, Standard practice for dosimetry in a high energyelectron beam irradiation facili ty for radiation processing, (underdevelopment). American Society for Tes ting and Materials.4. Miller, A., Kovacs, A. , Calorimetry at industrial electronaccelerators. Nuclear Instruments Methods Phys. Res. B 10/11 (1985)994-997.5. Humphreys, J.C., Mclaughlin, V.L., Calorimetry of electron beams andthe calibration of dosimeters at high doses. Radiat. Phys. Chem. J2,4-6 (1990) 744-749.6. Mehta, K.K., Chu, R. , Van Dyk, G., Electron dosimetry for 10-MeV linac.Radiat. Phys. Chem. 11, 4-6 (1988) 425-434.7. Kovacs, A., Mehta, K.K., Miller, A., Characterization of linearelectron accelerators using reference and routine dosimetry methods.In Proceedings of High Dose Dosimetry for Radiation Processing, lARAVienna, 1990, lARA Publication IARA-SM-314/21 (Vienna: InternationalAtomic Energy Agency), 371-384.8. Mclaughlin, V.L., Boyd, A.V., Chadwick, K.B., McDonald, J.C.,Miller, A. Dosimetry for Radiation Processing, Taylor & Francis,(London), (1989).9. Miller, A., Chadwick, K.H., Dosimetry fo r the approval of foodirradiation processes. Radiat . Phys. Chem. 34, 6 (1989) 999-1004.10. Miller, A., Kovacs, A., Vieser, A., Regulla, D.F., Measurements withalanine and film dosimeters for industrial 10 MeV electron referencedosimetry, Appl. Radiat. Isot. 40, 10-12 (1989) 967-969.11. ICRU, Radiation dosimetry: Electron beams with energies between 1 and50 MeV, Report 35, International Commission on Radiation Units andMeasurements, (1984).12. AAHI Standard, Guideline fo r electron beam radiation sterilization ofmedical devices. Association for the Advancement of MedicalInstrumentation Report, ANSI/AAHI ST31-1990, 1990.Figures:


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diameter 140 mm~ ~~ < H ; ( : , : ; '::;' ;;:.,' "j

FIG.2. Liquid and solid-state dosimeters placed ingraphite phantoms (10)

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FIG.3. Determination of the most probable energy (Ep)of the electron beam by measuring the practicalrange (R,.) in an aluminum wedge.Ep(HeV) = 0.20 + 5.09.R,.(CIl). (11).

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Procedimientos de dosimetria en procesado electrónico.pdf