A chemical dosimeter for monitoring gamma-radiation doses of 1–100 krad

InternationalJournalofAppliedRadiationand Isotopes,1971,Vol. 22, pp. 135--140. PergamonPre~. Printedin NorthernIreland

A Chemical Dosimeter for Monitoring Gamma-Radiation Doses of 1 - 1 0 0 krad W. L. M c L A U G H L I N a n d E. K. H U S S M A N N National Bureau of Standards, Washington, D.C. 20234, U.S.A. H. H. E I S E N L O H K International Atomic Energy Agency, Vienna and L. C H A L K L E Y 6626 Tyrian Street, La Jolla, California

(Received 11 July 1970) A simple chemical dosimeter is described for measuring gamma-ray doses useful for insect sterilization, seed-sprouting inhibition, and food shelf-life extension. The solutions, colorless before irradiation, assume a stable blue-violet color when irradiated to absorbed doses from 1-100 krad. The readout may be made either visually, colorimetrically, or spectrophotometrically. The optical density is linear with dose, and the response does not vary with dose rate. UN DOSIMETRE CHIMIQ.UE P O U R LA D E T E R M I N A T I O N DES DOSES DE RAYONNEMENT G A M M A ENTRE I ET 100 KRAD Les auteurs d~crivent un dosim~tre chimique simple pour la mesure des doses de rayons gamma qui conviennent ~ la st~rilisation des insectes, ~ l'inhibition des germes des v~g~taux et ~ la prolongation de la fralcheur des denr~es alimentaires. Les solutions, qui sont incolores avant exposition aux rayonnements, prennent une couleur bleu-violet stable lorsqu'elles reqoivent des doses absorb~s de 1 ~ 100 kilorad. Le changement de la couleur est obtenue soit par comparison dlrecte, soit par spectrophotometrie. La densit~ optique est une fonction lin~aire de la dose, et la r~sponse ne varie pas avec le d~bit de dose. XHMHHECHIIII ~O3HMETP ~JIH H3MEPEHHH FAMMA-H3JIYI~EHHH HPH MOII~HOCTH ~ O 3 b I OT 1 ~O 100 KH~opa~ IIpI~BO~HTC~ onHcaHj~e npoeroro XI~MHqecI~oro ~OaHMeTpa ~a~ Ha~tepeHH~ ~oa raMMaHa:ryqeHH~, npHMeHHeM~X K~H cTepHa~sa~K HaceHoM~x, TOpMO~'~eHHH npopacTaHHA CeMHH H yBeJItiHeHHH cpOHOB xpaHeHHH Hpo~oBoJIBCTBHH. PaCTBOpI,I, ~eCI~BeTHIJe ~0 O6JIyqeHtla, HpHHHMalOTyCTOfiqI4Bt,IttcHHe~HoJIe'roBHtiI~BeTnpI4 IIOPJIOI~eHHtl~08 06~IyqeHHa OT 1 ~0 100 ~zJxopa~. OTqC~ HoHa3aHHI~ npz6opa MOt-HeT 5LITb c~eJ~aH BHSyaJII~HO HJIH cHem'po~oTOMeTpH'~ecHH. On~rHqecHaHH.TIOTHOCTbJIHHe~Ha BeJIHHIIHe~03I,I, H qyBffrBHTeJIbHOCTb~IpH60pa He ~3aBHCHTOT cHopOCTHH3MeHOHHH~03M. EIN CHEMISCHES D O S I M E T E R Z U R K O N T R O L L E DER ENERGIEDOSIS VON G A M M A S T R A H L U N G ZWISCHEN 1 UND 100 KRAD Die Autoren beschreiben eln einfaches ehemisches Dosimeter zur Messung der Energiedosis yon G~mmastrahlen, das bei Insekten-Sterilisation,bei Verhinderung unerwfinschter Keimung oder Haltbarmachung yon verderblichen Giitern mlttels Bestrahlung verwendet werden kann. * Support for this work was provided by the Division of Isotopes Development, U.S. Atomic Energy Commission. 135

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Die vor der Bestrahlung farblose L6sung entwickelt mater Bestrahlung (1-100 kraal) eine sehr best~ndige blau-violette Verftirbung, die entweder durch direkte Farbtiefenvergleichung oder spektralphotometrisch bestimmt werden kann. Die Extinktion ist der Energiedosis proportional und vonder Dosisleistung unabhtingig. INTRODUCTION SINCE g a m m a radiation will be used more widely for insect control, seed and vegetable sprouting inhibition, and vegetable and fruit shelf-life extension, there is a need for a dosimeter that will measure simply and reproducibly doses between 1 and 100 krad. T h e chemical dosimeter most frequently used in this range is the ferrous-sulfate dosimeter (4-50 krad), ¢1) or variations of it, such as the ferrous-cupric sulfate dosimeter (50-1000 kilorads). ¢~) These systems are considered reliable and accurate if handled carefully, but sometimes impurities or unclean components m a y cause unexpected errors, ts'4) Moreover, the spectroscopic readout is in the ultraviolet, and one is limited to relatively sophisticated instrumentation, with careful control of temperature. I n the radiochromic chemical dosimeter described here, such careful handling is not necessary. T h e readout is m a d e in the visible part of the spectrnm, which has the added advantage that either qualitative visual readings, or more accurate spectrophotometric or colorimetric measurements m a y be made. T h e r e is no need to use ultrapure ingredients, triplydistilled solvents, or super-clean glassware. Another chemical dosimeter has been proposed as a dose indicator for insect control by ionizing radiation. ~5) It consists of an aqueous solution of chloral hydrate plus an indicator dye and provides sensitive visual readings from 0"3 to 30 krad, depending on the concentration of the chloral hydrate. I t is similar to chloral hydrate systems previously studied b y Andrews and coworkers. ~6'7) SPECIAL DOSIMETRY APPLICATIONS I t has been known for m a n y years that Xand g a m m a - r a y doses of the order of 10 krad inhibit the reproductive power of certain harmful insects in the adult stage, without seriously affecting their mating vigor. ~s'9) Irradiating the various insect stages (eggs, larvae, pupae, adults) to doses up to 100 krad is known to cause eventual or immediate

mortality. ~l°'m T h e latter application, particularly in disinfestation of grains and other food stores, has found increasing economic value in recent years. T h e eradication of insect colonies or control of insect overpopulation by the release of radiation-sterilized males still capable of copulating is perhaps of even greater world-wide interest, as indicated by a n u m b e r of recent meetings sponsored by the International Atomic Energy Agency.* O t h e r radiation applications in these dose ranges, also of potential economic value, include inhibiting vegetable and seed sprouting u2~ and prolonging the storage life of fruits and vegetables. ~ls) Each irradiation procedure for a given pest control or food preservation requires some optimum X- or y-ray dose lying between approx. 1 and 100 krad in order to achieve the desired radiation effect. I n the case of the sterile male technique, underdose will not sterilize a sufficient percentage of insects to be economically worthwhile, while overdose will kill or disable too many. As seen in Fig. 1, various dose levels are required for the o p t i m u m sterilization of different pests, and for inhibition of sprouting and storage-life extension. I t is the purpose of this p a p e r to show how the present chemical dosimeter can be used for radiation monitoring in pest control and in the other applications requiring doses up to 100 krad. In order to achieve successful results, a specified radiation dose should in m a n y cases be administered to within 10 or 15 per cent. T h e dye solution described here is more than * IAEA Panel on Control of Livestock Pests by the Sterile Male Technique, Vienna (1968). IAEA Panel on Radiation, Radioisotopes, and Rearing Methods in the Control of Insect Pests, Tel Aviv (1966). IAEA Panel on Application of Sterile Male Technique for Control of Insects with Special Reference to Fruit Flies, Vienna (1969). IAEA Symposium on the Sterility Principle for Insect Control or Eradication, Athens (1970). Training course on the Use of Radiation in Entomology, Turrialba, Costa Rica (1970).

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adequate for measuring dose within these limits, and rough indication of these dose levels can be monitored visually with these systems.

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The dosimeter, a radiochromic organic solution, consists of a 5*mmolar solution of hexa(hydroxyethyl) pararosaniline cyanide* in reagent-grade ethylene glycol monomethyl ether. (14as) Characteristics of the dye precursor molecule are shown in Fig. 2. For brevity it has been given a code name of HE6. Before dissolving the dye precursor, the solvent is made slightly acid by adding 1.0ml of glacial acetic acid per liter of solution in order to achieve greater stability of the irradiated solution. The solutions are sensitive to sunlight or fluorescent lighting and therefore should be stored in amber glass bottles which shield the solutions from actinic ultraviolet radiation. It is suggested that the dosimeter solutions be irradiated in amber glass ampoules of 2-5 ml volume, which is a sufficient amount for transferring to 10-ram pathlength spectrophotometer ceils for readout. A three-day illumination of the dosimeter solution in a 2-ml amber glass ampoule under bright fluorescent lighting resulted in an optical density (absorbanee) change of only 0-01 at the wavelength of maximum dye absorbance (599 nm). In an ordinary glass ampoule this optical * The sensitive dye-precursor salt (HE6) used in this set of experiments was synthesized under Chalkley's patenttX4) and was supplied by the Santa Barbara Division of EG&G, Inc., Goleta, California.

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density would have increased to greater than 3.00 during such a 3-day illumination. Figure 3 shows a series of dye solutions in amber glass ampoules which were irradiated with 6°Co y-rays to various dose levels up to 100 krad. It is seen that the dose levels illustrated can easily be discriminated visually. Differentiation between ampoules is made even easier when viewed in color. A simple field instrument for visual comparison might consist of a binocular device containing on one side a transparency of variable color intensity with a rotating calibrated dose scale, and on the other a slot for inserting unknown irradiated ampoules. T h e unknown dose could be determined to within about 15 or 20 per cent according to the color shade indicated on the comparator scale. A series of absorption spectra of irradiated and control dosimeter solutions (5 m M concentration) is shown in Fig. 4, where the percent

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transmittance is plotted as a function of wavelength in nanometers. Spectra are given for different e°Co y-ray doses in kUorads, the absorbed dose being determined for the solvent, CsHsO=, which is essentially water-equivalent for Compton interaction and electron stopping. It is seen that discrimination of the dose levels of interest in Fig. 1 is readily accomplished by comparing the curves of the figure. Figure 5 shows that the optical density at the absorption peak (599 nm) is linear with absorbed dose in the solvent, for three different concentrations of solution. T h e optical density readings for any one batch of 5 mmolar solution irradiated at 10, 20, 50 and 100 krad showed a standard deviation of 1.4 per cent. At the nominal lower limit of 1 krad., the precision limits of dose readings was found to be 4 - 6 per cent, due primarily to poor reproducibility of spectrophotometer readings through a 10-ram pathlength at 0.025 optical density units. Use of longe r optical pathlengths would provide accordingly lower dose readings at these precision limits. Other characteristics of these dosimeter solutions are as follows: ~15) (1) no variation of response with absorbed dose rate has been found between 1 and 100,000 rad/min for S°Co y-rays; (2) no energy dependence of response in terms of absorbed dose in water has been observed for 24 keV to 1.25 MeV photons; (3) the solutions have a shelf-life of at least a year when stored in amber glass bottles; (4) the color produced by irradiation is stable for at least three months within 4 - 2 per cent (although it may increase outside that limit due to evaporation if the ampoules are not tightly closed); (5) the response increases with temperature during irradiation by NO-3 per cent/°C, from --80°C to + 6 0 ° C ; (6) optical density variations with temperature during spectrophotometry are negligible; (7) the molar extinction coefficient of the dye molecule in this solution was determined to be 4"3 × 104 1 mole-lcm-X; (8) the G-value for dye formation in this solution at room temperature is 0-57 molecules/100 eV for 6°Co and taTCs y-rays; it varies sharply with concentration of the solution as shown in Fig. 5. Certain words of caution should be given regarding the use of these dosimeter solutions.

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140

W. L. McLaughlin et al.

Extensive toxicity tests have not been carried out on the dosimeter materials. I t is therefore recommended that the solutions should be handled with care if the dosimeter is used over long periods of time. I t should be pointed out that, while the v a p o r pressure of the solvent is less than that of water, the flash point is rather low (47°C), and therefore ampoules, if sealed, should be fused with suitable precautions. Best results can be expected by using partly evacuated and sealed ampoules, but it m a y be more convenient to use capped vials or stoppered ampoules, as indicated in Fig. 3. A possible source of error lies in the fact that the color of solutions is bleached somewhat at doses greater than about 150 krad. Figure 6 shows that, although the response is linear up to about 150 krad, there is a sharp reversal at higher doses. Ambiguous readings might result unless comparisons are made between absorption curves at low and high doses. T h e curve for high-dose bleaching has a slightly broader spectrophotometric absorption b a n d than the lower dose curve with the same m a x i m u m absorption, resulting in a mildly discolored solution. SUMMARY A radiochromic dye solution dosimeter has been described, which is inexpensive and is easily prepared and handled for measurement of g a m m a - r a y absorbed doses from 1 to 100 krad. T h e system is easier to make up and use, and has a wider response range, than the ferrous sulfate dosimeter. Because the stable radiationinduced color is in the visible part of the spectrum, the dosimeter lends itself to qualitative visual dose interpretation. T h e system

is envisaged as a versatile dosimeter for radiation disinfestation, shelf-life extension, sprouting inhibition, and insect population control. REFERENCES I. SEHESTED K. Manual on Radiation Dosimetry (Edited by N. W. HOLM and R. J. B~m~Y), p. 313. Marcel Dekker, New York (1970). 2. BJERGBAKKEE. ibid, p. 319. 3. HOLM N. W. and ZAGOnSKIZ. P. Chap. 6, ibid, p. 83. 4. FnXCKE H. and HART E. J. Radiation Dosimetry (Edited by F. H. Aa'rrx and W. C. Rogsca~), Vol. 2, Chap. 12. Academic Press, New York (1966). 5. Moos W. S., NAGL J. and HAman J. Int. J. appl. Radiat. Isotopes 21, 177 (1970). 6. A~REWS H. L. and SHOR~ P. A. J. chem. Phys. 18, 1165 (1950). 7. ANDREWS H. L., MURPHY R. E. and LEBRuN E. J., Rev. scient. Instrum. 28, 329 (1957). 8. Rum~ER G. A. J. agric. Res. 6, 383 (1916). 9. MULLERH. J. Science 66, 84 (1927). 10. D~.SROSIER N. W. and ROSSNSTOCK H. M. Radiation Technology in Food, Agriculture, and Biology, Chap. 14. The Avi Publishing Co., Westport, Conn. (1960). 11. CORNWALL P. B. Massive Radiation Techniques (Edited by S. JEFF~-RSON) Chap. 5. Wiley, New York (1964). 12. M.ACQU~-~NK. F. Radiation Preservation of Foods, Proc. Int. Conf., Boston, 1964. Publication 1273, National Academy of Sciences-National Research Council, Washington, D.C., p. 127 (1965). 13. M~a~g_~is P. and NxeHo~s R. C. Irradiation of fruits and vegetables. Michigan State Univ. Final Rep. to U.S. No, AEC, C00-1592-35 (1969). 14. CHAgKg~Y L. HydrophilicDyeGyanides-Hydroxyalkyl Compounds. U.S. Patent 2, 877, 169 (1959). 15. McLAuoHLXN W. L. Manual on Radiation Dosimetry (Edited by N. W. HOLMand R. J. B~.RRY), p. 377. Marcel Dekker, New York (1970).