## 3 Results and Discussion  
The methodical reactor experiments for tuning the methods for measuring the spectral and spectral-temporal characteristics of gas plasma radiation at IGR reactor have been conducted using the in-pile device similar to the abovementioned ID in design, but without the use of mirrors. The experiments were carried out to study the spectral-temporal characteristics of such gaseous media, as argon (Ar) as well as a mixture of gases He-Ar upon excitation by the \({}^{6}\)Li(n,\(\alpha\))\({}^{3}\)H nuclear reaction products. Optical radiation was registered using the QEPro-abs optical spectrometer, PMT module, connected to oscilloscope, DET100A and DET025A photodiodes for the visible spectra regions and InGaAs-photodiode for an infrared region according to the scheme presented in Figure 3a. As the experimental device with lithium source of gaseous media excitation was primarily designed to register laser radiation, a lens to gather the light flux was located at a distance of about 1.2 m from the reactor core so the intensity of lines on spectrometer were insignificant during the study of spontaneous radiation. Argon at a pressure of 305 Torr was irradiated in the \"Burst\" mode with a duration of 0.13 s and a power of 6.6 GW (Figure 6), which corresponds to a thermal neutron flux of 4.6\(\cdot\)10\({}^{16}\) n/cm\({}^{2}\)s.  
The argon emission spectrum is shown in Figure 7. The spectrum shows that two lithium lines predominate at 610.4 and 670.8 nm (Figure 7). The argon lines are at the background level. Optical radiation intensity vs. time curve (Figure 6) almost follows the reactor power vs. time with a shift of about 15-20 ms. It seems that the shift in time is explained by thermal radiation of the lithium layer due to the additional heating occurred under irradiation with thermal neutron flux and further cooling. Argon radiation pulse duration at 750.4 nm wavelength (\(\approx\)95 ms, Figure 8) is slightly shorter than the duration of pumping pulse and the integral optical radiation flux in the sensitivity range of the Si-photodiode (Figure 6). Radiation maximum in the infrared area is also shifted by approximately 50 ms with respect to the maximum at 750.4 nm (Figure 8). Figure 6: Time dependence of reactor power and signal from Si–photodiode DET100A. Argon is at a pressure of 305 Torr, and lithium layer temperature before reactor startup is 710 K.  
The spectrum was sharply dominated by two lines: 750.4 nm and 912.3 nm in the He(702 Torr) + Ar (1.7 Torr) mixture (Figure 9). High intensity of 750.4 nm line is explained by the low quenching with helium level 2p\({}_{1}\). The prevailing of line 912.3 nm is caused by the fact that the transition begins from the lowest level 2p\({}_{10}\), to which excitation of all high-lying levels is transferred during the quenching by helium and own gas. The weak lines on 810.3 and 842.5 nm are also irradiated from the low-lying levels 2p\({}_{7}\) and 2p\({}_{8}\). Figure 8: Oscillogram of PMT Signals, (1), and InGaAs Photodetector (2). Gas-argon under pressure of 305 Torr, and lithium layer temperature before reactor start-up is 710 K. The scale is expanded vertically in 10\({}^{4}\) for the line (2). Figure 7: Emission spectrum in argon medium. Initial pressure of Ar is 305 Torr, and lithium layer temperature before the reactor start-up is 710 K.  
Thus, it can be concluded from the emission spectrum that the He-Ar mixture offers a chance to achieve laser action at the 2p\({}_{1}\)-1s\({}_{2}\) transition. Similar research has been carried out by other research organizations involved in nuclear-pumped gas lasers. In [17], for instance, the authors present a description of the experimental setup and a measurement technique, as well as the results of the spectral-luminescent properties of noble gases and their mixtures, excited by uranium fission fragments, conducted at the VIR-2M water pulse nuclear reactor.