The elastic scattering of beam ions is shown to cause a potentially damaging flux of penetrating secondary knock‐on particles. These secondary particles will be more penetrating than the primary ions if the atomic mass of the primary is larger than the secondary. A calculation, using the specific example of helium ions incident on an absorbing material containing hydrogen, reveals that the knock‐on protons have a range greater than five times the range of the primary helium ions, and can cause a preheat problem. Thus, this effect is shown to be an important design consideration.