# The Gyrotrons as Promising Radiation Sources for THz Sensing and Imaging
## 2 Advantages of the Gyrotrons as Powerful Radiation Sources for Sensing and Imaging
### Remote Detection of Concealed Radioactive Materials  
Recently, a new scheme for detecting concealed sources of ionizing radiation by observing the occurrence of a localized breakdown in atmospheric air produced by a focused electromagnetic wave whose electric field intensity surpasses the breakdown field in a small volume surrounding the radioactive material has been proposed [81]. The principle of this promising method (see Figure 8) stems from the fact that any radioactive material emits gamma rays, which ionize the surrounding air and thus produce free electrons. In this technique, the chosen volume (with dimensions on the order of a wavelength) is smaller than that of the naturally occurring free electrons. Since in the absence of  
Figure 7: Simplified diagram of a radioacoustic detection (RAD) spectrometer using gyrotron as a radiation source spectroscopy.  
radioactive materials, the ambient electron density is very low, the probability of finding a free electron that could trigger an avalanche breakdown process is such a small volume is also negligible. Therefore, observing a breakdown there indicates a presence of hidden radioactive material in the vicinity of the focused wave beam. Another specific requirement is that the pulse length of the electromagnetic wave \"must exceed the avalanche breakdown time of approximately 10-200 ns and could profitably be as long as the statistical lag time in ambient air (typically, microseconds)\" [81]. The analysis of the potential sources in the wavelength range 3 mm \(>\lambda>\) 10.6 \(\upmu\)m has revealed that the most appropriate would be a 0.67 THz gyrotron oscillator with an output power of 200 kW and pulse duration of 10 \(\upmu\)s or a Transversely Excited Atmospheric-Pressure (TEA) CO2 laser with 30 MW, 100 ns output pulses. The estimates presented in the cited analysis show that a system employing a 670 GHz gyrotron would have superior sensitivity, while a similar realization based on the TEA CO2 laser could have a longer range of up to 100 m.  
In order to satisfy the formulated requirements, a design of a dedicated 0.67 THz gyrotron that provides a maximum output power of 300 kW (3 Joules in a pulse of 10 \(\upmu\)s) has been proposed [82; 83]. It includes a pulsed solenoid (cooled by liquid nitrogen) with a maximum magnetic field intensity of 28 T. The envisaged detection scheme of a distant breakdown is based on observing the THz signal reflected from the discharge. In the subsequent studies on this technology, many important aspects (e.g., the sensitivity and the range of the sensing) have been investigated in detail, and the components of the system have been optimized [84; 85; 86]. Following the above-mentioned pioneering contributions, several other research groups worldwide have joined the work on this technique [87; 88]. The experimental results obtained using a 0.095 THz gyrotron suggest that detection at distances as large as 1 km should be feasible using an appropriate antenna. Based on these positive developments, it is anticipated that a fully functional system for the remote detection of concealed radioactive materials will be realized soon.