  We report on the 29Si nuclear spin decoherence time at room temperature for a
pure (99.99999%, 7N) and carrier-less (p-type, doping level of 10^15cm-3)
silicon in which 29Si nuclei are naturally abundant (4.7%). Despite the
experimental difficulties coming from the extremely long spin-lattice
relaxation time T1 (of the order of 10^4 s), we have successfully observed a
series of spin-echoes of which the time dependence is characterized by the
decoherence time T2. We found that the decoherence process deviates from a
single Gaussian or Lorentzian function but is well-reproduced by a
bi-exponential function with the shorter T2S = 15\pm5 ms and the longer T2L =
200\pm20 ms.
