The optical emission spectra of pure metals Fe, Zr, Ta, and W were measured in the course of irradiation with Ar+, Kr+, and Xe+ ions (5, 10, 15 and 20 keV). It has been established that the energy and type of an ion in the specified energy range do not have a noticeable effect on the profile of the thermal emission spectra of these targets and, accordingly, on the temperature of the thermal spikes formed in the areas of passage of dense cascades of atomic displacements. It is shown that the constant temperature regime of thermal spikes ( T = const) is realized when the average radius R of the cascade region is proportional to the root of the energy of the incident ion: R ~ and the main part of the thermal energy is released at the periphery of the cascade, which has a fixed length. As a result, the energy per atom of the medium released in this region does not change with a change of the ion energy and the radius of the cascade. Besides it is shown that the temperature of thermal spikes for all types of ions significantly depends on the target material.