ESR Dosimetric Study on Gamma Induced Radicals in DL-Ornithine Hydrochloride

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.


Introduction
Electron spin resonance (ESR) has been successfully applied to measuring intermediate and high doses using alanine as a radiation-sensitive material [1][2][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].
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, en-ergy 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, CH 3 CH(NH 2 )COOH, purified in polycrystalline form [9][10][11][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.molecular formula (C 5 H 12 N 2 O 2 •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).

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.

Irradiation of the Prepared Rods
Co 60 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.to-peak heights of the two orientations (h 0 and h 90 ) 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.

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 S 1 2.039 ± 0.0016, 2.024 ± 0.0014 for S 2 , 2.011 ± 0.0023 for S 3 , 1.995 ± 0.0009 for S 4 , 1.99 ± 0.005 for S 5 and 0.979 ± 0.0012 for S 6 .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 S 1 and S 2 , 1.918 mT between S 2 , S 3 , 3.13 mT between S 3 -S 4 , 0.87 mT between S 4 -S 5 and 1.83 mT between S 5 -S 6 .S 3 with magnetic field 34553.2Gauss was chosen to perform all the work in this study due to its high sensitivity, the hyper fine coupling constant A 11 is 2.325 and A ┴ 1.967 mT.

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.

Effect of Ornithine Concentration on Signal Intensity
Three different concentrations of DL-ornithine hydro- 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.

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   rods measurements, the response curves obtained for the irradiated samples in terms of average peak-to-peak amplitude normalized to dosimeter mass (peak height/mass) versus the absorbed dose.It can be seen that the radiation sensitivity of ornithine rods increases with the increase of absorbed dose and ornithine concentration in the rods.This increase in radiation sensitivity is mainly attributed to the increase in number of free radicals induced by radiation in the rods.

Radiation Sensitivity G-Value
The efficiency of a dosimeter is expressed by a "Gvalue", i.e., the number of radicals or valency change of ions produced by the absorbed radiation per 100 ev.The radical formation efficiency was determined by double integration of the first derivative spectra of DPPH (α-αdiphenyl β picrylhydrazyl) and compared with those of irradiated materials .The absolute spin concentration was estimated by using the following equation [18,19].where, A sample , A strong-pitch , n strong-pitch and m are the areas of the integrated signals of the sample and strong pitch, number of spin in strong pitch and the mass of the sample, respectively.The G value of the whole area was found 0.3551 ± 0.03.

Pre-Irradiation Stability
The pre irradiation stability of DL-ornithine rods was investigated by measuring the EPR signal of DL-ornithine rods with concentration 20% during a storage period of 50 days.The rods were conditioned at 35% RH and at room temperature (25˚C ± 3˚C) in dark and laboratory light, it was noticed that no detectable EPR signal was produced during the storage period at different storage conditions.

Stability of Ornithine Powder
DL-ornithine powder (0.0231 gm) was irradiated to a dose of 5 kGy and then measuring the signal intensity at different time intervals of 150 days as shown in Figure 6.
The peak intensity values shows slight decrease in the first 5 days, then tends to be stable till the end of the storage period.

Long Term Post Irradiation Stability of
Ornithine Rods The post irradiation stability has been studied for DLornithine rods for a period of 60 days.The ornithine concentration is 20% stored at 35% relative humidity (RH) at room temperature (25˚C ± 3˚C) at dark.Three rods were irradiated to a dose of 30 KGy.The EPR signal intensity of the rods was recorded immediately after irradiation.The rods are then stored for about two month and were measured using EPR at different time intervals during this storage period.The post irradiation stability curve of these rods is shown in Figure 7. Excellent post irradiation stability is obtained where the response decreases only with around 1% at the end of the 2 month storage period.

Humidity during Irradiation
The effect of relative humidity during irradiation on the response of DL-ornithine dosimeters of concentration 10% was investigated by the irradiation of the rods to a dose of 30 kGy at different humidity values.The rods were stored before irradiation for a 3 days period under the same relative humidity as when irradiated, so that equilibrium moisture content in the rods has been established during irradiation.Figure 8 shows the variation in EPR signal intensity as a function of relative humidity during irradiation relative to the response value at 33% relative humidity.It can be concluded that DL-ornithine rods has negligible humidity effect in the range of humidity from 0% -100% not more than 5%.

Conclusions
From the data presented in this study, the following conclusion can be drawn  A new ornithine rod has been prepared by a simple technique in the laboratory using ethylene vinyl acetate copolymer and paraffin wax where the binding materials do not show interference or noise in the EPR signal before or after irradiation. The obtained number of free radicals per 100 eV (G value) was found to be 0.3551 ± 0.0333. The prepared rods have significant EPR signal developed upon irradiation and its intensity increases with the increase of absorbed dose and DL-ornithine concentration. The useful dose range for these rods was found to be 0.5 -50 kGy. The response of this dosimeter has a negligible effect of humidity during irradiation (not more than 5%). These Dosimeters are fairly stable before irradiation and show little fading between 2% and 5% over a 60 days storage period depending on the storage conditions.

Figure 4 .Figure 5 .
Figure 4.The EPR spectra of DL-ornithine hydrochloride rods irradiated to different absorbed doses.