ESR dosimetry

AppL Radiat, Isot. Vol. 47, No. 5/6, pp. 525-528, 1996 Copyright© 1996ElsevierScienceLtd Printed in Great Britain.All rights reserved S0969-8043(96)00004-8 0969-8043/96 $15.00 + 0.00

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Improvement of Sensitivity in the Alanine/ESR Dosimetry B. R A K V I N Ruder Bo~kovi6 Institute, P.O. 1016, 10000 Zagreb, Croatia (Received 30 August 1995; in revised form 6December 1995)

Improvement of sensitivity in the alanine/ESR dosimetry is presented by considering spin-lattice and spin--spin relaxation rates of the room temperature radical in irradiated polycrystalline L-alanine. Both relaxation rates exhibit anomalous decrease due to hindered rotation of the methyl group. It is demonstrated that this effect can improve a signal: noise ratio of the spectrum for 30% by detecting the ESR signal at around 200K compared to the room temperature detection. The application of improvement can be expected in the therapy-level dose range where the intensity of detected signal is comparable to a nonlinear background signal.

temperature dependence of the saturation behavior of the spectrum. It was shown that the maximum of intensity of the alanine ESR signal coincides with the anomalous decrease in spin-lattice, T~, and spinspin, T2, relaxation times in the low temperature region around 200 K.

Introduction Paramagnetic centers produced by ionizing radiation in polycrystalline alanine at room temperature exhibit convenient properties which serve as a base for alanine/ESR dosimetry (Regulla and Definer, 1982; Wieser et al., 1993). By employing alanine probes ESR dosimetry can be realized with relatively high precision (SD + 0.5%) in the dose range from 0.1 to 100 kGy. In the low dosage range, which includes the therapy-level dose range (0.5-100Gy), at around 1 Gy this precision decreases (SD + 6%) due to the lower spectral resolution for the standard X-band ESR spectrometer (Wieser et al., 1993). However, besides a low signal:noise ratio which is expected in this region it was found that a weak detection ability has its source mainly in the large contribution of the non-radiation induced ESR background signal from alanine probe within a batch. Recently (Ruckerbauer et al., 1995) the method based on Fast Fourier Transform filtering has been applied to eliminate the low frequency background noise and improve detection of the alanine signal down to the dose of 0.05 Gy. A further improvement in the low dose range can be expected by achieving the highest possible signal:noise ratio for a weak alanine ESR spectrum. The influence of microwave power and other relevant instrumental parameters in order to optimize the signal:noise ratio have been discussed earlier (Wieser et al., 1993). In addition, it can be noted that a detailed knowledge of the frequency dependence as well as temperature dependence of relaxation times for the alanine spectrum represent additional factors which have to be examined in order to obtain the optimal sensitivity. In this work we will focus on

Experimental A polycrystalline L-alanine sample was purchased from Sigma and 7-irradiated with a nominal dose of 100Gy at a dose rate of about l kGy/h in a 6°Co source. The irradiation was performed in air at room temperature. ESR spectra were recorded on a Varian E-9 spectrometer equipped with a Bruker ER4111VT temperature controller unit. In the process of detection microwave power and temperature were variable parameters while all other ESR parameters (0.2 mT modulation amplitude at 100 kHz modulation frequency and magnetic field scan of 20 mT) were fixed for all the recorded spectra.

Results and Discussion A typical ESR spectrum for the radical species in the irradiated powder alanine is dominated by the five lines pattern with intensity ratio 1:4:6:4:1 due to hyperfine coupling of the four equivalent protons at room temperature (Miyagawa and Itoh, 1962; Kuroda & Miyagawa, 1982; Matsuki & Miyagawa, 1982). However, it can be mentioned that the complete set of spectroscopic parameters which can describe all other details of the spectrum is not known yet. The most reliable parameters, such as g-tensor and hyperfine splitting tensors, are known for the 525

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dominant room temperature radical species C H 3 - (~H---COOH, the so called "stable radical" (Kuroda and Miyagawa, 1982; Matsuki and Miyagawa, 1982). The stable radical appears from the transformation of the unstable radical at room temperature while the stable radical arises due to deamination of the anion radical in irradiated alanine. The methyl group rotation in the stable radical is frozen while that of the unstable radical with the same chemical structure as the stable radical is free at 77 K (Kuroda and Miyagawa, 1982; Matsuki and Miyagawa, 1982). Another radical species also detected at room temperature was poorly described with approximate hyperfine tensor and it was ascribed as "second radical" in the ENDOR studies (Kuroda et al., 1982; Matsuki et al., 1982). The second radical has been tentatively assigned as H2NC'HCH 3 cation product which appeared after decarboxylation of the radical cation (Samskog et al., 1980). Therefore, one can expect a complex ESR spectrum of the irradiated alanine at room temperature. Besides that, it was clearly shown that a spectrum of irradiated alanine powder is dominated by the stable radical at low microwave power ( ~ 1 mW) while at higher microwave power (,-~100 mW) the spectral features from the stable radical are saturated and new spectral features can be detected (Van Laere et al., 1989). This is demonstrated in Fig. 1. The

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P(mW) Fig. 2. ESR signal amplitude, h, of polycrystallineL-alanine v-irradiated with an absorbed dose of 0.1 kGy, as function of microwave power, P, detected at two different temperatures: 190 (11) and 290 K (O).

amplitude of the central peak, h, as marked by arrows, which is commonly accepted as the indicator for the absorbed dose, dominates over the spectrum at low microwave power [Fig. l(a)]. It is important to note that at higher microwave power the intensity of the spectrum increases by a factor of two but the central peak is saturated and an additional new spectral feature with amplitude, h', can be seen as indicated in Fig. l(b). The amplitudes of peaks have been measured as a function of microwave power, and different saturation behavior for these peaks can be noted as shown in Figs 2 and 3. Amplitudes h' exhibit almost expected behavior for the saturation curves at high (290 K) and low (190 K) temperature

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B(mT) Fig. l, Typical ESR spectra of polycrystalline L-alanine y-irradiated with an absorbed dose of 0.1 kGy obtained at room temperature. The amplitudes of two types of central peaks, h and h', are denoted with arrows in the spectra, a: Spectrum recorded at 1 mW. b: Spectrum recorded at 100 mW of microwave power; spectrum shifted downwards and plotted in 1/2 of the intensity scale of the a spectrum. The spectra were recorded in the same magnetic field, B, interval.

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P(mW) Fig. 3. ESR signal amplitude, h', of polycrystallineL-alanine -irradiated with an absorbed dose of 0.1 kGy, as function of microwave power, P, detected at two different temperatures: 190 ( I ) and 290 K (0).

Improvement of sensitivity in the alanine ESR dosimetry 4.0

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Fig. 4. ESR intensity (double integral of a total ESR spectrum) of polycrystalline L-alanine y-irradiated with an absorbed dose of 0.1 kGy, as function of microwave power, P, detected at two different temperatures: 190 ( I ) and 290 K (O). showing larger signal amplitudes for the saturation curve at 290 K in the region of the high microwave power. A small anomaly is present in the vicinity of the maximum of saturation curves where the saturation curve at 190 K exhibits slightly larger signal amplitudes than the saturation curve at 290 K. However, the stable radical exhibits in the whole microwave range an anomalous behavior having h amplitudes at 290K smaller than amplitudes at 190K. Moreover, the maximum of the saturation curve at 190K is shifted towards higher microwave power in comparison with the saturation curve at 290 K. This observation is useful in the process of characterization of the radical species and can be also used to obtain the optimum of the spectral intensity for each species in the cumulative spectrum. Maximum of h and intensity (double integral of the total ESR spectrum as shown in Fig. 4) when measured at 190 K increases by 27 and 30% respectively, compared to the values obtained at 290 K. Therefore, these increases in both signal amplitudes and intensities can be employed to improve the signal:noise ratio for this spectrum. Following a simple theoretical description based on Bloch equations (Poole, 1967) the maximum height of the first-derivative ESR lineshape is given as h ~

B (I + 72B~ T, 7"2)3/2

(I)

where 7 is the electronic gyromagnetic ratio, B~ is the amplitude of microwave field, T, is the spin-lattice relaxation time and 7"2 is the spin-spin relaxation time. For the more complex spin system such as a stable radical the Bloch equations should be replaced with the more sophisticated density matrix theory. However, at the present moment we will focus only

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on the qualitative explanation of the saturation behavior in the irradiated alanine. The saturation behavior is characterized by the product of T, T2 and the microwave power incident to the cavity (P~,, oc B~). Therefore the observed shift of the maximum towards the higher microwave power and the increase in intensity can be related to anomalous decrease of the product T l T2. For the stable radical the temperature dependence of T: in the temperature region from 200 to 300 K can be approximately deduced from earlier ENDOR measurements (Brustolon et al., 1986). In this study T2 was described with two terms. The first term continuously decreases with increasing temperature, and the second term shows a T~-type dependence. Generally, T~ of the ordinary paramagnetic centers in the organic crystals exhibits a monotonic increase with decrease in temperature (oc T : ) in the vicinity of the room temperature (Kevan and Schwartz, 1979). Therefore, under these conditions one expects an even more saturated intensity of ESR spectra at lower temperatures. On the other side, an anomalous behavior of T, for these paramagnetic centers can be also expected. For example, a mechanism which contributes to external type of relaxation rates can be related to the effect of methyl group rotation in these paramagnetic centers. It is well known (Miyagawa et al., 1962; Dzuba et al., 1981; Brustolon et al., 1986) that the rotation of the CH 3 group of the stable radical is controlled by hindered potential in the vicinity of the group. For the fast methyl rotation where the value of the rotation rate, 3-~, is comparable with the Larmor frequency, to0, one expects the maximum of relaxation rate, T ? ' , according to the relation (Kevan et al., 1979) 23 T~-' = ~ b ~ i 1 + (too~ )2

(2)

where b~describes a constant of proportionality to the part of interactions that is being modulated by the rotational motion. The relaxation rate is strongly temperature dependent and can be described by the Arrhenius relationship 3 2=3~1e

v..,7'

(3)

where V is the height of the potential barrier. According to relation (2) one expects the most efficient relaxation rate in the temperature interval where the rotation rate reaches the value of the Larmor frequency. Therefore, one can try to achieve the most efficient signal:noise ratio for the stable radical simply by applying the highest portion of the microwave power that will not saturate the signal in the temperature region with the maximum of relaxation rate, 3 ' ~ too- By employing data obtained earlier (Miyagawa et al., 1962; Dzuba et al., 1981; Brustolon et al., 1986) for 3, it is easy to estimate the temperature interval around (250 + 70) K where a maximum in Ti -j can be expected for the stable radical in an X-band experiment. The saturated curves were

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examined in a wide temperature interval (320-170 K), and the maximum shift in saturation as well as the maximum intensity were detected in the vicinity of 190 K as shown in Figs 2--4. The detected maximum intensity in the lower temperature region being higher than expected can be explained by the oversimplified description or by neglecting other possible relaxation mechanisms which are present in this system. Another possible relaxation mechanism which can also exhibit the same functional dependence as relation (2) but now containing the nuclear Larmor frequency, w,, is the electron-nuclear dipolar, END, mechanism. Because of o9~<<~o0 the END mechanism leads to an anomalous increase of the relaxation rate at lower temperature when z - ~,~ ton. These two types of relaxation mechanisms just illustrate a possible description of anomalous saturation behavior in alanine room temperature radical. For the more detailed studies of this phenomenon a density matrix theory as well as more accurate correlation times are required.

Conclusion The saturation experiments performed on alanine indicate that the optimum intensity of the spectrum is related to the anomalous behavior of relaxation times. As documented well before (Brustolon et aL, 1986), relaxation times in this system are modulated by the fast methyl group rotation and lead to the minimum of relaxation times in the temperature region where the rotation rate of the methyl dynamics coincides with the Larmor frequency. The saturation data show that internal motion of methyl groups in the alanine stable radical can enhance the signal:noise ratio by about 30% by detecting the ESR signal at about 200 K as compared with the room temperature. The obtained improvement might be of

great significance in the therapy-level dose range where the detected alanine signal can be comparable to a background signal.

References Brustolon M., Cassol T., Micheletti L. and Segre U. (1986) Methyl dynamics studied by ENDOR spectroscopy: a new method. Molec. Phys. 57, 1005--1014. Dzuba S. A., Salikhov K. M. and Tsvetkov Yu. D. (1981) Slow rotations of methyl groups in radicals studied by pulse ESR spectroscopy. Chem. Phys. Lett. 79, 568-572. Kevan L. and Schwartz R. N. (1979) Time domain ESR, p. 67. Wiley, New York. Kuroda S. and Miyagawa I. (1982) ENDOR study of an irradiated crystal of L-alanine: environment of the stable CH3CHCO~- radical. J. Chem. Phys. 76, 3933-3944. Matsuki K. and Miyagawa I. (1982) ENDOR study of an irradiated crystal of L-alanine: structure and the environment of the unstable CH3CHCOf radical. J. Chem. Phys. 76, 3945-3952. Miyagawa I. and Itoh K. (1962) Electron spin resonance of irradiated single crystal of alanines: hindered rotation of the methyl group in the free radical. J. Chem. Phys. 36, 2157-2163. Poole P. (1967) Electron Spin Resonance. A Comprehensive Treatise on Experimental Techniques, p. 707. Wiley, New York. Regulla D. F. and Definer U. (1982) Dosimetry by ESR spectroscopy of alanine. Appl. Radiat. lsot. 33, 1101-1114. Regulla D. F., Scharmann A. and McLaughlin W. L. (1989) ESR dosimetry and application. Appl. Radiat. Isot. 40, 829-1246. Ruckerbauer F., Sprunck M. and Regulla D. F. (1995) Book of abstracts. 4th International Symposium on ESR Dosimetry and Applications. Miinchen, Germany. Samskog P. O., Nilson G., Lund A. and Gillbopro T. (1980) Primary reactions in irradiated deuterio a-amino acids studied by pulse radiolysis and ESR spectroscopy. J. Phys. Chem. 84, 2819-2823. Wieser A., Lettau C., Fill U. and Regulla D. F. (1993) The influence of non-radiation induced ESR background signal from parafin-alanine probe for dosimetry in the radiotherapy dose range. Appl. Radiat. lsot. 44, 59-65.