A new ESR dosimeter based on Tris(hydroxymethyl)aminomethane

Appl. Radiat. Isot. Vol.47, No. 11/12,pp. 1539-1540,1996

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A N e w ESR Dosimeter Based on

Tris(hydroxymethyl)aminomethane J. A Z O R I N a n d A. G U T I I ~ R R E Z Department of Physics, Universidad Aut6noma Metropolitana-Iztapalapa, 09340 M6xico, D.F. Mexico Results are presented on the dosimetric characteristics of a new dosimetry system based on the ESR response of tris(hydroxymethyl)aminomethane. The response to gamma radiation was measured in the absorbed-dose range from 10 to 105 Gy and the persistence of signal was monitored along 60 days. There is slight fading when stored at elevated temperatures. Copyright © 1996 Elsevier Science Ltd

Introduction The increasing use of ionizing radiation for processing of food as well as for sterilization of medical instruments and tissues have encouraged the development of free radical dosimetry systems involving ESR and lyoluminescence analyses to measure high radiation doses (Azorin and Guti~rrez, 1991). This paper presents the results of a study of the dosimetric characteristics of a ESR dosimeter constituted by a mixture of tris(hydroxymethyl)aminomethane powder, referred as tris, and a binding substance. The binder used, which has a negligible effect on the dose response, was room temperature vulcanizing (RTV) silicone.

Experimental Procedures The dosimeters were formed into rods of tris mixed with RTV silicone gel prepared according to the method proposed by Flores and Galindo (2991). Preparation was as follows. Tris(hydroxymethyl)aminomethane was mixed into RTV silicone gel in a glass bowl stirring thoroughly until a homogeneous blend was obtained. The ratio of tris/silicone was 1 g of tris per millilitre of silicone. The mixture was then placed into cylindrical molds 2.2 mm diameter and 30 mm length drilled in a polystyrene block. Vulcanization occurred within 24 h. Dosimeters were then extracted from the moulds by pushing the rods from one of the two open ends of the mould with a rod of the same diameter as the mould. Dosimeters were irradiated, under electronic equilibrium conditions, with ~Co gamma radiation in a Gamma-Cell 200 Unit at a dose rate of 1.01 kGy h - ~. ESR measurements were made with a Varian

X-band ESR spectrometer employing 25 kHz phasesensitive detection at a constant microwave level (20 dB) and at room temperature. The dosimeter was placed into the cavity and its ESR spectrum obtained by scanning the magnetic field and registering it on an X-Y recorder. The magnetic field was set at 320 mT and the scan was a 20 mT linear sweep in 8 min.

Results Figure 1 shows the first derivative of the typical ESR spectrum obtained for a gamma irradiated tris dosimeter. This spectrum does not differ from the spectrum obtained for tris powder (Azorin and Guti6rrez, 1993). This signal consists of several peaks due to the hyperfine interactions of the unpaired electrons with the N and H atoms. The unirradiated dosimeters did not give any appreciable ESR signal. The dose response is evaluated as the spin concentration, which is proportional to the ESR signal, i.e. the area under the ESR spectrum, as shown in Fig. 2. In this figure it can be observed that

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Fig. 1. First-derivative ESR absorption spectrum of tris(hydroxymethyl)aminomethane irradiated with 6°Co gamma radiation.

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J. Azorin and A. Guti6rrez

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Dose (kGy) Fig. 2. ESR response of the tris dosimeters as a function of 6°Co gamma radiation.

the ESR response as a function of dose is linear from 5 to 105 Gy, free-radical saturation occurring for doses higher than 105 Gy. Estimated uncertainties are about + 3% (1 tr) for doses up to 102 Gy and _+ 2.1% for higher doses. Each experimental data point was taken as the average of 10 measurements. To study the stability of the free radicals induced by radiation, the dosimeters were stored under dry and dark conditions after irradiation at room temperature and in a climate-controlled chamber at 37 and at 60°C. Curves for the decay of free-radical concentration as a function of storage time are shown in Fig. 3. In this figure it can be seen that the fading of the ESR signal, during 2 months is negligible at room temperature. The fading at 37°C is about 2 and 5% at 60°C, over the same storage period.

The E S R dosimeter based on tris mixed with RTV silicone permits evaluation of absorbed doses in the range from 5 to 105 Gy with an uncertainty of about 3%, exhibiting no fading in 2 months at room temperature. For this reason, these dosimeters provide an appropriate method for use in radiation processing. Acknowledgements--This work was partially supported by CONACYT on the contracts 1574-E9208 and 1577-E9208.

References Azorin J. and Guti6rrez A. (1991) Development of a lyoluminescence dosimetry system to measure high radiation doses (IAEA-SM-314/5). Proc. Int. Syrup. High Dose Dosimetry for Radiation Processing, pp. 65-71. IAEA, Vienna. Azorin J. and Guti6rrez A. (1993) ESR-lyoluminescence correlation studies of tris(hydroxymethyl)aminomethane. Appl. Radiat. lsot. 44, 341-343. Flores J. and Galindo S. (1991) Use of alanine-silicone pellets for electron paramagnetic resonance 7 dosimetry. Radiat. Res. 125, 335-337.