Single-walled carbon nanotube field-effect transistors (CNT-FETs) were characterized before and after gamma radiation treatment using noise spectroscopy. The results obtained demonstrate that in long channel CNT-FETs with a length of 10 mu m the contribution of contact regions can be neglected. Moreover, radiation treatment with doses of 1x10(6) and 2x10(6) rad allows a considerable decrease parallel to the nanotube parasitic conductivity and even the shift region with maximal conductivity to the voltage range of nearly zero gate voltage that improves the working point of the FETs. The Hooge parameters obtained before and after gamma radiation treatment with a dose of 1x10(6) rad are found to be about 5x10(-3). The parameters are comparable with typical values for conventional semiconductors.