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Electron Spin Relaxation Behaviors of
Radicals Induced in Gamma-irradiated Food
M. Ukai a) , H. Kameya a) , H. Nakamura a) , M. Kikuchi b) and Y. Kobayashi b)
a) Hokkaido University of Education, b) Radiation-Applied Biology Division, QuBS, JAEA
Irradiation of food by various electromagnetic waves or
rays of elementary particles is gaining wide applications 1) .
It has been proved that irradiation eliminates pathogen and
reduces food borne illness. Since irradiation may induce
free radicals, electron spin resonance (ESR) spectroscopy
has been applied to reveal the irradiation history and dosage
in a wide variety of foods. Ukai et al. have reported
radiation response of the irradiated foods using ESR 2-4) . In
order to determine the relaxation behaviors of radicals in
irradiated foods, we varied the microwave strength to obtain
progressive saturation behavior (PSB). These microwave
saturation ESR spectra showed that an organic free radical
signal is quite readily saturated, and thus has a long
relaxation time. The time scale of magnetic relaxations
reflects existence of organic free radicals. We also have
reported the detection protocol of irradiated food based on
4)
radical relaxation phenomena .
The theory for continuous microwave saturation for
homogenously and inhomogeneously broadened
5)
paramagnetic systems was developed decades ago . The
measurement procedures have later been refined and
employed to estimate the spin-lattice and spin-spin
relaxation times T1 and T2 from the saturation curves in
various model cases characterized by inhomogeneous
6)
broadening . The saturation parameters and relaxation
times were obtained by employing diagrams that utilized a
few selected points on the saturation curve. To our
knowledge, an analysis employing all data on a saturation
curve has not been carried out previously.
In the present work, a computer program 7) to analyze the
microwave power dependence of inhomogeneously
broadened ESR lines using all data points on a saturation
curve was therefore applied. Using this theoretical analysis,
the relaxation behaviors of radicals were revealed by
Magnetic field (mT)
Fig. 1 Typical ESR spectra of nutmeg.
JAEA-Review 2010-065
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microwave saturation ESR spectra.
The specimen was commercial available nutmeg which
was proven to be no irradiation history. Irradiation was
carried out at JAEA at room temperature (ca. 300 K). We
used the gamma ray from 60 Co as an irradiation source.
The irradiation doses used for the sample were 1, 3 and
5 kGy. ESR measurements were carried out with an ESR
spectrometer (JEOL, JES-FA3XG, Akishima). All the
ESR spectra were recorded at the X-band (9.3 GHz).
Figure 1 shows the typical ESR spectra of nutmeg. ESR
spectra obtained before irradiation consisted of three signal
components. The sharp intense signal (P1) is due to an
organic free radical. The P2 signal in the vicinity of P1 is
attributable to a signal with hyperfine (hf) interaction of
Mn 2+ . Furthermore, the signal (P3) detected at g = 4.0 may
be originated from the ferric (Fe 3+ ) ion. The signal
intensity of P1 is increased by irradiation. The PSB curves
shows that the intensity of the P1 signal increased linearly
with microwave power and met the threshold at the
4 - 8 mW range. After the thresholds, the P1 signal
intensities decreased. As radiation dosage increased, the
signal intensity became higher. By PSB we could evaluate
relaxation times of T1 and T2 (Table 1). Irradiation
induced radicals of nutmeg yielded relaxation times, T1 in
the sec and T2 in nsec ranges, respectively. Upon the
irradiation, T1 shortened, and T2 lengthened.
References
1) T H. Delincée, Radiat. Phys. Chem. 63 (2002) 455-458.
2) M. Ukai et al., Appl. Magn. Reson. 24 (2003) 1-11.
3) H. Nakamura et al., Spectrochim. Acta 63(4) (2006)
883-887.
4) M. Ukai et al., Spectrochim. Acta 63(4) (2006) 879-882.
5) T. G. Castner Jr., Phys. Rev. 115 (1959) 1506-1519.
6) J. Maruani, J. Magn. Reson. 7 (1972) 207-215.
7) A. Lund, Radiat. Res. 172 (2009) 753-760.
Table 1 Relaxation times (T1 and T2).
Irradiation T1(s) T2 (ns)
non-irradiation
1 kGy
3 kGy
5 kGy
16
7
7
7
90
130
130
130