alanine dosimeter—power dependence of the X-band spectrum

Appl. Radiat. ht. Vol. 42, No. I, pp. 665-668, ht. J. Radiat. Appl. Instrum. Part A

1991

0883-2889/91

$3.00 + 0.00

Pergamon Press plc

Printed in Great Britain

The ESR/Alanine Dosimeter-Power Dependence of the X-band Spectrum J. M. ARBER’,

P. H. G. SHARPE’, H. A. JOLY’, J. R. MORTON’ and K. F. PRESTON’

’ Division of Radiation Science and Acoustics, National Physical Laboratory, Sciences, Middlesex TWI 1 OLW, England and * Steacie Institute for Molecular Council, Ottawa, Canada KlA OR9 (Received

26 October

Teddington, National Research

1990)

Satellite lines which accompany the central feature of the X-band ESR spectrum of a-alanine dosimeters are shown to be due to forbidden “spin-flip” transitions associated with methyl protons on nearby molecules. At microwave powers in excess of 1 mW the satellites increase in intensity relative to the central feature, and thus measurements at higher microwave powers must be based on experimentally determined calibration curves at the appropriate power levels.

1. Introduction

2. Experimental

It has been known for many years (Miyagawa and Gordy, 1960; Morton and Horsfield, 1961) that the ESR spectrum of irradiated a-alanine is due to the species CH,CHCO; trapped in the crystal lattice. As long ago as 1962 the use of irradiated cc-alanine was suggested as a solid-state dosimeter with ESR readout (Bradshaw et al., 1962). In recent years (Regulla and Deffner, 1982) investigations of the intensity and stability of the spectrum have led to proposals for its use as a secondary standard for radiation dosimetry. As a result of these investigations, the ESR/alanine dosimeter is rapidly emerging as the preferred method in the dose range 100 Gy to 50 kGy. Following approval by the IAEA (Nam, 1988) the use of the ESR/alanine dosimeter for monitoring the irradiation of food is rapidly gaining acceptance. There is now a need for a dosimeter that is functional in the low, or “clinical”, dose range below 1 Gy. The sensitivity of modern ESR spectrometers is such that below about 1 Gy the signal from a typical alanine dosimeter pellet is comparable to the spectrometer noise at high gain. Since the signal-to-noise ratio improves as the square-root of the microwave power (so long as the signal does not saturate), there is a temptation to increase the power at the sample in order to monitor lower and lower dosages. However, as the present article shows, the dominance of satellite lines at high powers limits this option for obtaining higher sensitivity with the ESR/alanine dosimeter at X-band.

Alanine dosimeter pellets were prepared at NPL as follows. High purity L-cc-alanine (BDH Chemicals Ltd) was mixed with 10% w/w paraffin wax (m.p. 98°C). Pellets 5 mm in diameter, ca 2.5 mm thick, weighing an average of 55 mg, were made in a Manesty Hand Tabletting machine and then annealed in an oven at 100°C for 0.5 h. Some experiments were carried out with 50 mg samples of powdered c(-alanine recrystallized twice from water enriched to 99 atom percent in 2H. The alanine dosimeters were given a radiation dose of ca 1 kGy, and examined at room temperature with ESR spectrometers at NPL and NRC. The spectrometer at NPL was a Bruker ESP-300 operating at X-band (9.6 GHz), and using a synthetic quartz sample holder, whereas at NRC a Bruker ESP-300 operating at Q-band (33.8 GHz) was used. The object of the experiments was to explore the spectrum of CH,CHCO, in irradiated alanine as a function of microwave power and frequency. Intensities of the central line and its low-field satellite were assumed to be proportional to the product f (AB)2, where 1 is the peak-to-peak displacement of the first derivative presentation, and AB is the peak-to-peak line width.

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3. Results The ESR spectrum of CH,CHCO; in irradiated, powdered cc-alanine at X-band is well known (Fig. 1); it consists of a quintet of rather broad, poorly

J. M.

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ARBER et al.

0

I I

9 sig”a’ 200

p

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100 mW

/k

50 mW

/k

20 mW

A-5 mW

the signal line, at high powers the peak-to-peak displacement of the satellites from the baseline exceeds that of the signal. The spectrum of freshly irradiated and examined r-alanine recrystallized from DzO was identical to that obtained from commercial a-alanine. It has been reported (Itoh and Miyagawa, 1964) that after two years at room temperature, there is evidence that the CH,CHCO; is partly converted to CH,CDCO, and finally to CD,CDCO; We saw no evidence of this phenomenon because of the much shorter time-frame of our experiments. At Q-band the spectrum of irradiated x-alanine powder is somewhat different from that at X-band. Firstly, the satellites apparent at X-band on the central feature have disappeared, and the central feature is a poorly-resolved triplet at all power levels, with a line-to-line separation of ca 0.4 mT. Moreover, as Fig. 2 shows, there is no significant change in the appearance of the Q-band spectrum as the microwave power is raised from 11 PW to 106 mW.

1 mW i:;-

I

5 mT

I

k

Fig. 1. The ESR spectrum at X-band of irradiated a-&mine showing the increasing intensity of the satellites A and B relative to the central signal as the microwave power increases. The 100 kHz modulation was 0.2 mT throughout, and the magnetic field scan-rate was 2.5 mT/min.

resolved lines separated by about 2.5 mT. This hyperfine structure is due to four protons whose magnetic moments interact with that of the unpaired electron. The four interacting protons are the three methyl protons and the a-proton. For the purposes of dosimetry the size of the central peak is usually accepted as the “readout” of the dosimeter. The central peak is accompanied on either side by a pair of satellites (A and B in Fig. l), each separated from the main peak by ca 0.5 mT. These satellites have often been ascribed to secondary species, although no identification of such a species has been offered, neither were secondary species apparent in the single crystal spectra of cc-alanine referred to earlier. In Fig. I the X-band spectrum of irradiated ;I-alanine is shown as a function of increasing microwave power, the latter varying from 1 mW at the bottom of the figure to 200mW at the top. It will be seen that the relative intensity of the central feature (the “signal”) to that of the satellites decreases rapidly until at 100 mW the latter are the strongest features of the spectrum. Below microwave powers of 1 mW, the ratio of the signal to that of one of the satellites was almost constant (ca 7 k 1); above 1 mW, the signal to satellite ratio decreased steadily to about 1.5 at 100 mW (Fig. 1, top). Since the satellite lines are sharper than

4. Discussion A comment first on the Q-band spectrum. At the higher microwave frequency (33.8 GHz) of the Q-band spectrometer, anisotropy in the g-matrix

4 I\

v

u

Fig. 2. The ESR spectrum at Q-band of irradiated z-alanine at various Dower levels showing the absence of satellites. Experimental conditions similar to Fig. I.

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ESR/alanine dosimeter becomes resolvable. As far as can be discovered, the principal values of the g-matrix of CH,CHCO; in cr-alanine have never been reported, although Miyagawa and Gordy (1960) reported its diagonal elements, i.e. values of the g-factor when B, was placed in turn along each of the crystallographic axes of the orthorhombic a-alanine crystal. These diagonal values were: 2.0040, 2.0047 and 2.0033. At 33.8 GHz, these g-factors are separated by 0.42 mT, whereas at X-band (9.6 GHz) their separation is only 0.12 mT, i.e. less than the line-width. The triplet separation in the central feature at Q-band is therefore assigned to anisotropy in the g-matrix. The behaviour of the satellites at X-band is characteristic of “spin-flips” (Trammel1 et al., 1958; Zeldes and Livingston, 1959; Poole and Farach, 1971). These are forbidden transitions in which the electron spin has a purely dipolar interaction with a nearby proton or protons. No unpaired spin population resides on such protons. Spin-flip transitions involve a simultaneous change in the electron spin quantum number and the proton spin quantum number, and they are separated from the main transition by gNpNBo/gB, where g,&., is the product of the proton g factor and magneton (0.042577 MHz/mT), B, is the applied magnetic field in mT, /l is the Bohr magneton (13.99611 MHz/mT) and g is the average g-factor, 2.0040. At B, = 340 mT, this separation is 0.48 mT, as observed for the satellites A and B. The satellites are forbidden transitions, whereas the central line is an allowed transition; therefore the satellites may power saturate at a different level than does the central line. Usually, the main line saturates at a lower power level than the satellites (Miyazaki et al., 1990) and this is the case here. The central line begins to saturate at 1 mW at both X- and Q-band, and at higher powers loses intensity relative to the satellites. The relative intensity of the central line to that of one of the satellites contains information relating the number n of interacting nuclei to their average distance, (r(n)), from the unpaired electron. If n is known, (r(n)) can be calculated, and vice versa (Bales et al., 1975).

of 0.2fKl.25 nm is gratifying, and it is therefore probable that the satellites A and B are due to methyl protons on an adjacent molecule. Finally, at Q-band the spin-flip satellites are: (a) weaker by a factor of (9.6/33.8)2, or 0.08 and (b) 1.8 mT on either side of the central line. The much lower relative intensity of the satellites explains their apparent absence in the Q-band spectra of Fig. 2.

5. Conclusions The satellites which accompany the central feature of the ESR spectrum of irradiated cc-alanine have been identified as “spin-flip” transitions associated with methyl protons on nearby molecules in the lattice. Insofar as these forbidden transitions “steal” intensity from the central features, caution should be exercised in using the latter in its customary role as the dosimeter readout. It is preferable when operating the ESR spectrometer at X-band to use power levels of less than 1 mW, above which the central feature loses intensity relative to the satellites. Thus, any use of higher power levels as a means of monitoring low doses must be based on calibration curves obtained at the operating power level. The alternatives, although probably both impractical, would be: (1) to use deuterated a-alanine; (2) to use Q-band spectrometry.

or

References Bales L. B., Bowman M. K., Kevan L. and Schwartz R. N. (1975) Separation of isotropic and anisotropic hyperfine constants in disordered systems by analysis of electron paramagnetic resonance lines at two different microwave frequencies. J. Chem. Phys. 63, 3008-3014. Bradshaw W. W., Cadena D. G., Crawford G. W. and Spetzler H. A. W. (1962) The use of alanine as a solid dosimeter. Radial. Res. 17, 11-21. Itoh K. and Miyagawa I. (1964) Electron spin resonance of irradiated single crystals of alanine: protondeuteron exchange reaction of a free radical in the solid state. J. Chem. Phys. 40, 3328-3334.

(r(n))

= 0.73 (gp/B,)‘!3[2nZig/l,,l]“6

where p is the Bohr magneton, 0.927 x lo-l9 ergs/mT and 1 erg/(mT)2 = lOI nm3. For example, with a signal to satellite ratio of 7, and n = 1, (r(n)) = 0.20 nm; for n = 3, (r(n)) = 0.25 nm. The results of the experiments with a-alanine recrystallized from D,O were surprising: the fact that the satellites persisted after deuteration of the NH: function proves that they have their origin with x- or methyl protons on nearby lattice molecules. In an ENDOR study of irradiated a-alanine crystals, Kuroda and Miyagawa (1982) demonstrated that the distance of the a-carbon atom of the radicals from one of the methyl protons on a neighbouring molecule had decreased to 0.23 nm from 0.39 nm in the undamaged crystal (Lehmann et al., 1972). The agreement with the above estimate

Kuroda S. I. and Miyagawa I. (1982) ENDOR study of an irradiated crystal of L-alanine: Environment of the stable CH,CHCO; radical. J. Chem. Phys. 76, 3933-3944. Lehmann M. S., Koetzle T. F. and Hamilton W. C. (1972) Protein and nucleic acid components. I. The crystal and molecular structure of the amino acid L-alanine. J. Amer. Chem. Sot. 94, 2657-2660.

Miyagawa I. and Gordy W. (1960) Electron spin resonance of an irradiated single crystal of alanine: second-order effects in free radical resonances. J. Chem. Phys. 32, 255-263.

Miyazaki T., Iwata N., Fueki K. and Hase H. (1990) Observation of ESR spin flip satellite lines of trapped hydrogen atoms in solid H, at 4.2 K. J. Phys. Chem. 94, 1702-1705. Morton J. R. and Horsfield A. (1961) Electron spin resonance spectrum and structure of CH,CH(CO,H). J. Chem. Phys. 35, 1142-l 143. Nam J. W. (1988) Standardization of high doses in radiation processing. IAEA Bull. 4, 41-43.

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Poole C. P. and Farach H. A. (1971) Influence of the nuclear Zeeman term on anisotropic hyperfine patterns in electron spin resonance. J. Magn. Resonance 4, 3 12-321. Regulla D. F. and Deffner U. (1982) Dosimetry by ESR spectroscopy of alanine. Int. J. Appl. Radiat. Isof. 33, 1103-1114.

Trammel1 G. T., Zeldes H. and Livingston R. (19%) Effect of environmental nuclei in electron spin resonance spectroscopy. Phys. Reo. 110, 630634. Zeldes H. and Livingston R. (1959) Paramagnetic resonance study of irradiated glasses of methanol and ethanol. J. Chem. Phys. 30, 40-44.