| When nuclear and radiation events occur, the rescue and treatment of victims urgently depends on quickly triages according to their absorbed doses. This is also a crucial problem to be solved in radiation accident emergency. Tooth dosimetry by conventional electron paramagnetic resonance(EPR) measurement is only available in retrospective dosimetry applications because the sampling process is invasive and it is difficult to get the dose value quickly in the early period after an accident therefore the application is limitd. EPR is an important way to implement dosimetry quickly and in vivo. Modulation magnetic field device which could enhance the sensitivity of measurement was an important component of in vivo spectrometer. In this work, the magnet modulation device special for in vivo use was developed. The device included a driven amplifier, a couple of coil sets, modulation frequency setting and modulation amplitude display unit. The main contents and results contain:1. An EPR magnet modulation power amplifier for in vivo tooth dosimetry was designed. The magnet modulation power amplifier took advantage of switching circuit instead of traditional linearly analogue circuit. This circuit could satisfy the in vivo tooth dosimetry and was used in the circumstance of wide magnetic poles. The magnet modulation power amplifier drive circuit had the character of higher power capacity, higher efficiency, simple structure and high flexibility for frequency setting in 10~100 k Hz. Meanwhile, this device could generate modulation magnet field of 0~2 m T in amplitude in the center of the two magnet poles separating more than 9 cm. Based on multi-MOSFET H-bridge circuit, the control unit of H-bridge had been designed, IR2103 was used in this control unit. When the input signal was changed, this IR2103 could adjust the interval time, which could avert the magnet modulation power amplifier drive circuit from breakdown by avoid opening unilateral N-MOSFETs of H-bridge at the same time. Every single N-MOSFET had its own protection device which could not only forbid breakdown but also prevent damage of circuit from electric leakage. This greatly enhanced the service life and practicability of the magnet modulation power amplifier drive circuit. The performance of this power amplifier drive circuit had been tested. We operated the power amplifier drive circuit with incorrect way, and this power amplifier drive circuit could also function well.2. Modulation coil sets and modulation amplitude display unit for in vivo tooth dosimetry were developed. To meet the needs of limited magnetic pole space and less than the size of magnetic pole, the modulation magnet field coil sets were hollow and round, its diameter was 1cm at the center and 5cm at the edge. The coils had four layers; each layer had thirty loops and there were totally 120 loops. 10 loops independent coils had been coiled inside the modulation magnet field coil sets. The central axis of coil sets was in the center of the detection aperture of the cavity and coil sets separated more than 8 cm. Every coil sets had a modulation amplitude display and therefore there had two display instrument. To observe the status and the amplitude of the modulation magnet field at any time, 10 loops independent coils had been coiled inside the modulation magnet field coil sets, which could generate a special signal in the same frequency by inputting an alternating current into the 120 loops of the modulation magnet field coil sets. This special signal could observe the modulation magnet and verify the amplitude of the modulation magnet field. This method also avoided the phenomenon that the variation of the output state of the amplifier might cause a large deviation between the actual modulation magnetic field value and the preset value. This method deals with the weak signal that will input to the digital display module and digital oscilloscope by induction pre amplifier and amplitude scale, which could display the signal of modulated magnetic field.3. Performance test of modulation magnetic field device.(1) The measurements of DPPH dot sample with different modulation amplitude and different position in the detection aperture of cavity were carried out. The experimental results showed that: The modulation amplitude and frequency of this modulated magnetic field device could be adjusted in a wide range, and the actual modulation amplitude range was 0 ~ 2m T, modulation frequency was 10 ~ 100 k Hz which could be adjusted at any time. Spectral line broadening of DPPH dot sample had been obviously observed in using this modulation magnetic field device, which proved that this device could provide adequate modulation amplitude to the in vivo tooth dosimetry. The distribution characteristic of the modulation amplitude showed that the modulation amplitude distribution was not uniform in the resonator cavity and was stronger in the center of the cavity.(2) The human teeth that were separately irradiated by doses of 1~8 Gy had been measured. The measurement of 6 Gy intact tooth in vitro was carried out by different modulation amplitude, which was 0.11 m T, 0.22 m T, 0.33 m T, 0.44 m T, 0.55 m T, 0.66 m T, 0.77 m T, 0.88 m T. The EPR spectrums were clear and dose-effect relationship was obvious, this demonstrate that modulation efficiency was greatly improved than before and the modulation magnetic field device could meet the needs of the triage in vivo tooth dosimetry situations.Conclusion. Modulation magnetic field device was designed based on the special requirements of in vivo EPR tooth dosimetry. The magnet modulation power amplifier drive circuit, the modulation magnet field coil sets and the modulation amplitude display unit were designed. The magnet modulation power amplifier drive circuit took advantage of switching circuit instead of traditional linearly analogue circuit. It solved the problem of weak modulation amplitude, stability and security of the amplifier had also been solved. The modulation magnet field coil sets could not only operate modulation magnet field but also get and verify the status of the modulation magnet field by 10 loops independent coils coiled inside the modulation magnet field coil sets. The modulation amplitude display unit was calibrated and the status of the modulation magnet field could be showed intuitional. This method also avoided the phenomenon that the variation of the output state of the amplifier might cause a large deviation between the actual modulation magnetic field value and the preset value. The measurements of DPPH dot sample with different modulation amplitude and the measurements of intact teeth had been done by using modulation magnetic field device and the resonator cavity for in vivo dosimetry, which showed that the device had higher modulation efficiency and practicability that could meet the special needs of the in vivo tooth dosimetry. The modulation magnetic field device could improve the measurement sensitivity and the reliability of in vivo EPR tooth dosimetry for the nuclear and radiation emergency effectively. |
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