During ion implantation a stage can be reached in which a continuous layer within the implanted crystal changes into amorphous material. This happens when the implanted dose reaches a value of DA, which depends on the implantation conditions. The change begins, however, when the implanted dose attains a certain threshold value (DT). It is shown here that in-situ reflectivity (R) versus dose (D) measurements, made during the implantation of 31p+ into Si single crystal substrates, yield a new type of result, not published so far, regarding the threshold point. They show that both DT and the work (WoT) required to induce the threshold conditions within the implanted crystal increase with the implantation energy (E). It is also shown that the change in reflectivity ΔRi, which occurs in the dose range that contains DT, is a function of E and peaks at 140 keV, the energy at which the damaged layer becomes buried. Finally, the ratio DA/DT is found to be a decreasing function of E.