ABSTRACT

DL-ornithine hydrochloride rods (3 × 10 mm) were studied to be a radiation sensitive material for EPR dosimetry. The rods have specified EPR signal developed under irradiation and its intensity increases with the increase in absorbed dose. The intensity also affected by the concentration of DL-ornithine in the rods. The prepared rods can be used in the dose range from 0.5 - 50 kGy. The obtained number of free radicals per 100 eV (G value) was found to be 0.3551 ± 0.0333. The hyperfine (hf) coupling constant is 2.325 mT at g-factor 2.033. The rods have the advantage of negligible humidity effects during irradiation. The pre and post-irradiation stability was found to be satisfactory.

1. Introduction

Electron spin resonance (ESR) has been successfully applied to measuring intermediate and high doses using alanine as a radiation-sensitive material [1-3].

One of the advantages of ESR dosimetry is that the read out does not affect the spin concentration; the sample can be evaluated many times. Therefore, the signal to noise ratio (S/N) can be improved by repeated reading of the sample [1].

A comparative EPR study of a few simple amino acids has been made in order to identify qualitatively and quantitatively the different radiation induced radicals in amino acids powders [4].

Polycrystalline phenyl-alanine and perdeuterated L-α- alanine were studied as potential high-energy radiationsensitive materials (RSM) for solid state/EPR dosimetry [5].

Electron paramagnetic resonance (EPR) was proposed as a method to measure radiation-induced radicals. Alanine dosimetry has been already used for a long time as a standard method by national metrology institutes like NPL, LNHB or NIST, Also an ISO standard has been published [6]. A new EPR dosimeter was prepared by a simple technique in the laboratory through mixed arginine monohydrochloride with both vinyl acetate copolymer and paraffin wax. The dose response effects of humidity and temperature as well as (pre and post) irradiation stability were discussed [7]. A new simple method for preparation of alanine EPR pellets with a new binding medium has been carried out. The dose response influence of humidity and temperature during irradiation, energy dependence as well as storage at different conditions are discussed [8]. A number of solid materials, including amino acids, in which free-radical populations are formed by irradiation, have been suggested for highdose dosimetry by Electron Paramagnetic Resonance (EPR) analysis. A useful method of high dose measurement is the use of EPR spectrometry of irradiated amino acids, in particular L-alanine, CH₃CH(NH₂)COOH, purified in polycrystalline form [9-12]. A new alanine dosimeter in 2005 was prepared using poly (vinyl butyralco-vinyl alcohol-co-vinyl acetate) copolymer as a binder with ratio 40% L-alanine to 60% binder by weight in the form of pellets 5 mm in diameter and of average height 3.5 mm [13]. Several improvements to reduce the orientation effects can be accomplished by certain time-consuming procedures, e.g. for pellet-shaped dosimeters, averaging the measured values at two different orientations 90˚ apart [14,15] or by using double-integration of the spectrum [16], and for film dosimeters, by attaining maximum amplitude or by averaging the maximum and minimum amplitudes [17].

The objective of the present study is preparation of a new EPR dosimeter by simple technique in the laboratory by mixing DL-ornithine with both vinyl acetate copolymer and paraffin wax. The radiation-induced radicals in the rods were studied and investigated for its application from the dosimetric point of view.

2. Experimental

2.1. Materials

DL-ornithine hydrochloride (product of Sigma) has the molecular formula (C₅H₁₂N₂O₂∙HCl), molecular weight 168.62 g, and a melting point of 291˚C, as well as hot melt stick adhesive based on ethylene vinyl acetate copolymer (Tec-Bond 232/12, Power Adhesives Limited, England) and paraffin wax (congealing point 65˚C - 71˚C, BHD) were used for the rods preparation. The molecular structure diagram of DL-ornithine is represented in the following scheme (Scheme 1).

2.2. Preparation of DL-Ornithine Hydrochloride Rods

An equal weight mixture of paraffin wax and ethylene vinyl acetate copolymer (EVA) was melt at 85˚C - 95˚C in a water bath. 10%, 15% and 20% fine powdered DL ornithine hydrochloride material was added to the hot mixture solution and mechanically stirred for about 15 minutes at the same temperature to obtain a homogeneous mixture. The hot solution is sucked into polypropylene tubes (inner diameter 3 mm) and was left to solidify by cooling. Ornithine mixture rod was obtained by removing the polypropylene tube then cut into rods (3 × 10 mm dimensions). The average mass of the prepared rods was found to be 0.07 ± 0.003 g.

2.3. Irradiation of the Prepared Rods

Co⁶⁰ irradiation facility was used for irradiation of the prepared rods. The absorbed dose rate was about 3.0639 kGy·h⁻¹ overall the time of the experimental part. Five rods were irradiated together at the central position of the sample chamber using a specially designed holder made from polystyrene to ensure electronic equilibrium.

2.4. EPR Measurements

EPR signals were recorded at room temperature by using a Bruker EMX spectrometer (X-Band) product of Bruker, Germany. The operating conditions are, microwave power 5.041 mW, modulation amplitude 6.00 Gauss, modulation frequency 100 kHz, sweep width 250 Gauss, microwave frequency 9.725 GHz, time constant 81.92 ms and conversion time 20.48 ms The bottom of the EPR tube was adjusted at fixed position to ensure reproducible and accurate positioning of the rods in the sensing zone of the cavity.

EPR spectra were recorded at two orientations of each rod in the EPR cavity (0˚ and 90˚). The dose responses of dosimeters were calculated in terms of average peak-

Scheme 1. Molecular structure of DL-ornithine hydrochloride.

to-peak heights of the two orientations (h₀ and h₉₀) per unit weight of dosimeter. Readings were corrected to the peak-to-peak height of the reference standard material DPPH (α-α-diphenyl β picrylhydrazyl), which was recorded before and after each signal spectrum of the samples in order to correct the change in the spectrometer sensitivity. Stability of EPR spectrometer sensitivity was checked before and after each series of measurement using reference irradiated alanine dosimeters.

3. Results and Discussion

3.1. Spectral Features

Spectrum of unirradiated DL-ornithine HCl shows no features, however, the spectrum of irradiated one to a dose of 45 Gy (20%) possesses a sextet signal with peakto-peak linewidth Wpp is about 10.4 mT Figure 1, the g-factor for S₁ 2.039 ± 0.0016, 2.024 ± 0.0014 for S₂, 2.011 ± 0.0023 for S₃, 1.995 ± 0.0009 for S₄, 1.99 ± 0.005 for S₅ and 0.979 ± 0.0012 for S₆. So, this suggests the presence of more than one transition between energy levels present in the sample. The linewidth between signals is 2.56 mT between S₁ and S₂, 1.918 mT between S₂, S₃, 3.13 mT between S₃ - S₄, 0.87 mT between S₄ - S₅ and 1.83 mT between S₅ - S₆. S₃ with magnetic field 34553.2 Gauss was chosen to perform all the work in this study due to its high sensitivity, the hyper fine coupling constant A₁₁ is 2.325 and A_┴ 1.967 mT.

3.2. Power Dependence

The intensity of EPR signals increases in proportion to p^1/2 up to high microwave power. Appropriate setting of power level is necessary for the ESR measurement. Figure 2 shows the microwave power dependence of DLornithine signal intensity. The signal intensity increases as a function of power up to 5.041 mW without reaching saturation.

3.3. Effect of Ornithine Concentration on Signal Intensity

Three different concentrations of DL-ornithine hydro-

chloride namely 10%, 15%, 20% were irradiated to a dose of 15 kGy and the obtained spectra were shown in Figure 3 from which it could be concluded that the peak intensity increases with the increase of DL-ornithine concentration.

Figure 4 represents the EPR spectra of DL-ornithine rods (10%) were recorded after irradiation to doses 1, 5, 12, 20, 30 KGy.

From this figure it can be seen that the EPR signal begins to develop upon irradiation and its amplitude increases gradually with increasing the absorbed dose of γ-ray photons without any change in its shape.

3.4. Dose Response

Figure 5 represents the response curves of the three sets of ornithine rods concentrations (10%, 15% and 20%) to different absorbed dose ranging from 0.5 to 50 kGy. EPR spectra were recorded to establish the response curves (each dose point is represented by the average value of 3