Monte Carlo calculation of radiation energy absorbed in plastic scintillators

Pergamon

Appl. Radiat. lsot. Vol. 46, No. 6/7, pp. 499-500, 1995 Copyright © 1995 Elsevier Science Ltd 0969-8043(95)00069-0 Printed in Great Britain. All rights reserved 0969-8043/95 $9.50 + 0.00

M O N T E CARLO C A L C U L A T I O N O F RADIATION ENERGY ABSORBED IN PLASTIC SCINTILLATORS RAOL T. MAINARDI AND EDGAR_DO V. BONZI Facultad de Matem~itica, Astronomfa y Ffsica, Universidad Nacional de C6rdoba, Ciudad Universitaria, 5000 C6rdoba - Argentina

Plastic scintillators are finding increased use as radiation dosemeters since they seem to possess some special advantages over air-filled compensated chambers, such as higher detection efficiencies and responses independent of atmospheric factors (pressure, humidity and temperature). These materials do not have a uniform response to x-ray or gamma ray absorbed energy and the so-called air-equivalent types do not behave as such at low energies. Only if the absorbed energy in the scintillator material is proportional to the absorbed energy in air through a factor independent of the photon energy, will we be able to say that this it is an air equivalent material. This property is of importance because it allows the use of radiotherapy calibration protocols straightforwardly. Deviations from this behaviour were first reported by other authors. We set forth to study the response of plastic scintillators by Monte Carlo simulations, which showed that these materials are truly air-equivalent between 100 keV to 1 MeV. Below 100 keV, we found that the response is lower than air, in agreement with other workers. This energy region is of interest in radiology and surface radio therapy and we conclude that a plastic scintillator with truly air equivalent behaviour is of importance to carry out more precise dosimetry. We proposed doping the plastic with elements having higher atomic number and after several trials we considered silicon as the most appropriate one for several reasons. First, it has the K-absorption edge at the lowest energy of interest. Second, it is a suitable d e m e n t to dope a plastic with by organo-metallic chemistry. If this technique is not suitable, there are many other compounds that could be mixed with a scintillator material to increase the average atomic number of the mixture. Besides, only three atoms of Si in every one hundred of C was enough to increase the response below 100 keV to get an airequivalent material. Other elements like fluorine and magnesium were also considered, but were not found to be appropriate since greater atomic concentrations were needed. Figure 1 shows results of calculations of absorbed energy in arbitrary units for air, a plastic scintillator material without silicon, and the same plastic-doped material with the above concentration of 3 atoms of silicon to every 100 carbon atoms. To compare the absorbed energy efficiencies obtained between the different materials and air we calculated, in the energy range of interest, an average of relative deviations (RD)av. In figure 2 we can see the results of (RD),v versus the number of silicon atoms added as dopant. As can be seen the doped plastic has a better equivalence to air than the undoped one, with an overall standard deviation of 0.37% as compared to 5% before doping. 499

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Rafil T. Mainardi and Edgardo V. Bonzi

The (RD),v calculated values include the statistical errors, even though five computer runs with 5000 histories each were averaged to smooth the data. The volume relation to define strictly the air equivalence was found to be 863 _ 1, with a ratio of the masses contained in those volumes of 0.96 + 0.03. That is, for a half litre air ionization chamber, less than one cubic centimetre scintillator absorbs the same amount of energy. The compensation thus achieved at low energies results in the following. For energies below 600 keV the response of the plastic scintillator is equivalent to air, while for energies greater than that the response decreases slowly with respect to air. It is uncertain whether this material will be transparent enough to be used in large volumes, so we intend to use it in rather small pieces (a few cubic millimetre) to overcome this potential disadvantage. Moreover, the ratio of equivalent volumes being 1:863, with very small scintillators we can measure doses with the same accuracy as with a large air chamber. The casting of a piece of this material will make the testing of its properties possible. Finally, we conclude that is possible to design either tissue- or water-equivalent plastic scintillators which might be valuable for use with phantoms in calibrations measurements of electron or photon beams, as well as in practical dosimetry.

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Figure 1: Calculated results of absorbed energy in arbitrary units by air, a commercial brand scintillator and the same plastic doped with silicon, versus photon beam energy. The vertical scale factor was chosen to make the absorbed energy unity at low energies.

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Figure 2: Calculated results of (RD)av (see main text) versus number of silicon atoms used as doping element.

N u m b e r of Si per 1 0 0 0 C a r b o n a t o m s