Conventionally, attosecond pulses are obtained using generation of high-order optical harmonics upon excitation of various media by femtosecond laser pulses with the central frequency in the infrared range. Here, we consider theoretically an alternative possibility of obtaining an isolated extreme ultraviolet (XUV) attosecond pulse in a thin layer of helium atoms excited by a pair of half-cycle (quasi-unipolar) X-ray pulses. The approach is based on the free polarization decay of an atomic medium. An analogy of the processes under consideration with the phenomenon of superradiance is discussed.