Dose and temperature response of “opti-chromic” dosimeters

Radlat Phyx them Vol 31, Nos 4--6, pp 435--440, 1988

0146-5724/88 $3.00 + 0 00 Pergamon Journals Ltd

Int J Radtat Appl Instrum Part C

Pnnted m Great Bntmn

BOSE AND TEMPERATURERESPONSEOF "OPTI-CHROMIC" DOSIMETERS~

M. Sohrabpour, P.H.G. Sharpe, and J.H. Barrett National Physical Laboratory Teddington Middlesex T~ql, OLH, U.K. * GammaIrradiation Center, Atomic Energy Ornanization of Iran, P.O.Box 11365-RA8~ Tehran - Iran

ABSTRACT Opti-chromic dosimetry systems which are commerciallyavailable and consist of radiochromic dye solutions in plastic tubing have been tested for measurementof Co-60 Gammaradiation at dose range of 0-0.3, 0-2.5, 0-30 KGy using detector types of FWT-70-40 and FHT-70-83. The required doses have been delivered from a hot-spot 3000 Irradlator with a dose rate of about a.25 KGy/hr, at two irradiation temperatures of 22 and 50 C respectively. The response of the abo~e detectors has been measuredwith FWT-9~ reader at wavelengths of 600, 656 and 680 nm as appropriate. The results of this study and certain characteristics of this dosimetry system are presented. KEYHORDS Opti-chromic dosimetry, temperatureresponse of Opti-chromic dosemeters, dose response of Optichromic dosemeters. INTRODUCTION Opti-chromic dosemeters consist of leuko-radiochromicdye solutions in polar organic solvents which are contained in flourinated polymer tubing. The polar solvents have slightly higher index of refraction comparedwith the polymer tubing and thus form a reproducible and efficient optical waveguide. The polar solvent also serve the function of being the activator of the colorless leukodye where upon irradiation form color centers. The concentration of the produced coloredcations is maintained by the action of the slightly acidic polar solvent containing a certain amountof dissolved oxygen, thus ensuring the stability of the induced color changes (McLaughlinand Kozanic 1974; Rativanich and Co-workers 1981). The induced absorption band in the dye ~o]ution together with the anomalosdispersion phenomenon(Kronenbergand Co-workers 1981) strongly modify the dosetransparency response at a given wavelength. This effect with selection of several read-out wavelengths at a fixed composition , may give a dose range that spans several decades. The dose range may further be readjusted by variation of the concentration of leuko-dye, changinq of the length of the waveguidetube, and the read-out wavelength (Humpherys and Co-workers 1983). Becauseof the above inherent f l e x i b i l i t i e s of the opti-chromic dosimeters they can be madewith a wide dynamic dose range thus making i t suitable for manyapplications such as radiation processing, Food Irradiation, Radiotherapyetc. Radakand McLaughlln (1984) have reported the results of testing of "Opti-chromic detectors- F~T- 70" dosimetry system using the commercial reading instrument (FFT-Model 98). They have looked at the dose, stabllity, temperatureand the dose-rate responses of these detectors. In thls paper we have also used the samedosimetry system and the reading instrument, the specific detectors used were the low and high dose dosimeter systems designatedas F~K-70-40 and FWT-70-83 respectively. He have in particular looked at the dose response of these systems at temperatures of 22 and 50 c. These correspond to room temperature and a typically elevated temperatureone may encounter in radiation processing. Doseswere delivered at a fixed dose rate and the response has been recorded at appropriate wavelengths. DOSIMETRY SYSTEM As per description in the above reference the dosimeters used were fluorinated polyethylene-polypropylene tubing, FEP, 5 cm long, 0.3 cm O.D. and wall thickness of 0.03 cm. The organic solvent in the dosimeters is a mixture of Dimethyl-Sulfoxide and Triethyl-Phosphate. In which the dye I. Commercial products namedare for identification purooses and are not meant to be an endorsement by the authors.

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precursor, Hexahydroxyethyl Pararosaniline Cyanide is dissolved at 2 mMconcentration with a small amountof added Polyvinyl butyral which acts as a geling substance. The solution is contained within the tubing by insertion of glass balls at either end of the tube. The optical density of dosemeters are measuredin the opti-chromic read-out photometersystem. This instrument uses llqhts at three narrow wavelengths (600, 656, 680 nm) to measure radiation induced increases in the amplitude of the absorption band of the dye. The optical transmission at any given wavelength Is referenced against a wavelength of 750 nm. This photometerconsists of an incandescent lamp used as a l i g h t source, a beams p l i t t e r prism, a series of interference f i l t e r s , silicon photodetectors, and the associated amplifiers circuits and read-out display. A more detailed information on the above dosimeter system can be found in the above listed references as well as in FWT-98 manual producedby the manufacturer (Far West Technology, Inc, Goleta, CA 93117) EXPERIMENTAL PROCEDURE As mentioned in a pre~ious section we have looked at the dose response of two detector types namely FWT-70-40 and FWT-70-83 at three dose ranges of 0-0.3, 0-2.5, and 0-30 KGy respectively. Doses were delivered from a calibrated Co-60 Irradiator (hot-spot 3000) at an absorbeddose rate in water of about 4.25 KGy/hr. Each dose range has been covered with from seven to eleven dose p o i ~ , a n d each set of doses, from a given dose range, were delivered at two irradiation temperatures of 22 c and 50 c. Dosemeterswhichwereto be irradiated at higher temperature were preheated to 50 c before placementinto the temperature controlled irradiation chamber. Each dose point was measured by three different dosemeters that were positioned in a water equivalent block of lucentine in a fixed geometry in the cylindrical irradiation chamber. Dosemeters irradiated at 50 c were cooled down to room temperature before they were read in the photometer (F~fr model 98). The opti-chromic dosemeterswere transported between the irradiation chamberand the measuring station in black polyethylene bags in order to protect the leukodye solutions against exposure to UV l i g h t . Each one of the set of three dosemeterswas marked and set in the same fixed position in the read-out slot. This fixed positioning of the dosemeters in the read out instrument helped to reduce the errors associated with the randomorientation of the detector in the photometer. Each dosemeter's background OD at various wavelengths were recorded prior to irradiation and later subtracted from the gross OD after irradiation. This then gave the net OD corresponding to a dose at a given read-out wavelength. Table 1 gives a summaryof the absordeddose ranges, Irradiation temperature reat out wavelengths, and the detector types that have been studied in this work.

TABLE 1 SUMMARY OF ABSORBED DOSE RANGES IN TWO DETECTOR TYPES, IRRADIATION TEMPERATURE , AND READ OUT WAVELENGTHS, WL/TEM

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Figures the FWT-70-40 detector type corresponding to the low and mediumdose ranges of 0-0.3, 0-2.5 KGy and read out wavelengths of 600 and 656 nm and irradiation temperaturesof 22 c and 50 c respectively.

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DOSE / GY FIg 2. OD vs. dose for FWT-70-~O. 0-2.5 KGv, ]rradlatlon temperature 22 c and 50 c read out WL 656 nm. Likewise the plotted curves in Fig.2. represent least sauares functions of second order f i t t e d to the experimental data. The standard deviations corresponding to the two irradiation temperatures of 22 c and 50 c are IE-2 and 9E-2 respectively. The experimental data of the Ftg.2. At i r r a d i a t i o n temperature o f 50 c seems to show a drop in o p t i c a l density at about 300 Gy. In f a c t OD at t h i s dose tS only 0.03 compared with OD values o f 0.048 and 0.10 f o r doses o f 102 and 201Gy. Respectively. I t almost appears as i f at higher i r r a d i a t i o n temperature the radiochromic s o l u t i o n in the detector behaves as i f i t had a two-component leuko-dye precursor one operating in a l i m i t e d lower dose range and the second one t h e r e a f t e r . This observation a t t h i s notnt however, is only a con4ecture. The resnonse o ~ t h i s Aetectnr under these experimental conditions should be subjected to closer examination to gain a b e t t e r i n s i g h t

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Into the reasons for this non-slngle-valued behavior of the dose calibration curve. Figures 3, 4 and 5 show the dose calibration curves of FWT-70-~3 detector type. The doses covered by this detector type correspond to medium and high dose ranges namely 0-2.5, 0-30 KGy. The read out wavelengths corresponding to these figures are 600, 656 and 680 nm respectively.

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DOSE / G Y Fig. 3. OD vs. dose for FLIT-70-83, 0-2.5 KGv, irradlatlon temperature 22 c and 50 c, read out WL 600 nm In figure 3 third order polynomials are f l t t e d to the optical density data for 22 c and 50 c irradiation temperatures. The standard deviations for these polynomials are IE-2 and 5E-2 respect i vely. t~

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Likewise the linearly f i t t e d functions to the data in figures 4 and 5 have standard deviations of about 8E-3 (656 nm, 22 c), 3.2E-2 (656 nm, 50 c) and 4E-3 (680 nm, 22 c, 50 c) respectively. Constancy of Dosimeter Readings: The net optical density OD for each set of three detectors measuring a given dose was noted to have a few percent variation. However, occasionally much larger deviations from a dosemeter was observed which were disreaarded. S t a b i l i t y of dosimeter reading : Soot checking of OD one month after irradiation when measured at 600 nm for FPT-70-40 and at 630 nm for FPT-70-83 both for 22 c irradiation showed that aging had increased the optical density. This increase in OD however, appeared to be inversely proportional to the absorbed dose. That is the Increased margin of OD in a given dose range (0-30 KGy) almost aymptotically decreased and reached the ordinal OD values say at about mid dose range, this observation in the case of the response of the F~-70-83 detector measured at 680 nm seems to de somewhat different from the results of Radak and McLaughlin where they claim that the above detector type after a storage period of 50 days show an increase in OD whlch is almost proportionally constant over the entire dose range. Temperature Effects : The response of the detectors under two irradiation temperatures of 22 c and 50 c were studled and the results are shown in figures I-5 under hiOher temperature i r r a d i a tion (50 c) the detectors performed best when the irradiation time was kept to a minimum (that is at low absorbed doses or high dose rates). However, over long irradiation times a number of operational problems such as separation of the glass beads from the tubing due to difference in thermal expansion between the solution and the polymer tubing. Bubble formation in the solution, oozing of the solution from the tubing etc. Were observed. I t is therefore f e l t that the use of these detectors at high temperature ambients ( >50 c) and over extended irradiation periods (several hours) may not be without some practical problems. CONCLUSIONS Based on the results of this investigation one may conclude that "opti-chromic" dosemeters have a reasonable performance in the low, medium, and high dose levels (0-0.3, 0-2.5, 0-30 KGy) when irradiated at the room temperature, the performance of these detectors at s l i g h t l y elevated temperatures encountered in radlation processino, say 50 c may be acceptable so long as the i r r a d i a tion time is kept to a minimumthat is for low absorbed doses or higher dose rates. However, at higher Irradiation periods or perhaps higher temperatures the performance of these detectors measured in terms of accuracy of the results and ease of usage suffers to a certain extent. The two detector types have shown non-linear response (Fig 2 and 3), in the medium dose range (0-2.5 KGy) and read out wavelengths of 656 and 600 nm respectively. Results of s t a b i l i t y studies of the two detector types when irradiated at room temperature showed that in general in a given dose range those detectors having recorded lower doses where more amenable to showing higher OD after a certain period of time as comparedwith the higher dose levels for which detectors did not show any change in their OD. The 50 c calibratlen curve for FWT-70-40 at medium dose and read out ML of 656 nm apprears to

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show a non-single-valued behavior as a function of dose in a region about the 300 Gy dose value This interesting behavior may be due to various parameters of leuko-dye type and concentration etc and detailed study of this phenomenonmay be a subject for further investigation. ACKNOWLEDGEMENT One of the authors (M.S.) would like to express his gratitude to the United Nation Development Program, the International Atomic Energy Agency the Atomic Energy Organization of Iran, who provided fellowship and partial support during his stay at the National Physical Laboratory. REFERENCES Hampherys, K.C., N.O. Wilde and A.D. Kantz (1983). Radiat. Phys. Chem.~ 22. No 3-5, 291-294. Kronenberg, S., W. L. McLaughlin, and C. R. Siebentritt (1981). Nucl. Instr. and Meth.~ 190~ 355-368. McLaugh]in, W. L. and M. M. Kosnic !1974). Int. J. App1. Radiat. Isotopes 25. 2~g-262. Radak, B. R., and W. L. McLaughlin (1984). Radiat. Phys. Chem. 23, No. 6, 67§-675. Rativanich, N., B. R. Radak, A. H i l l e r , R. M. Uribe, and H. L. McLaughlin (1981). Radiat. Phys. Chem.~ 18~ No. 5-6, 1001-1010.