The rate of defect accumulation due to ion irradiation is discussed. Silicon was implanted with 11.5 MeV Au+6 ions, to fluences ranging from 2x10(9) to 1x10(11) cm(-2). We find the measured defect concentrations C can be related to ion fluence phi via C = A phi (n), with A approximate to 1600 and n approximate to1.0. The linear dependence of C on phi is consistent with expectations for the rate of accumulation of defects in the limit of low ion fluences, and is in contrast to previously reported values of n <1, which were obtained using higher fluences, for which significant overlap between individual damage cascades is expected. In addition, an attempt to measure the vacancy formation energy in silicon is described.