It's important to keep an understanding of the scales involved here. A gram of Hydrogen contains 6e26 atoms. So a nanogram of material contains on the order of 10^17 atoms. That's ten million trillion atoms.
A "beam of antimatter" which literally annihilates the walls of the vacuum chamber it is contained within may seem dramatic but in this case we are talking about mere dozens of anti-atoms. Against walls of solid metal the damage is so minute as to be undetectable with even the finest instruments. Even if the beam were far, far stronger it would still only cause erosion at the scale of picograms per trillion anti-atoms, which isn't much concern.
It's important to keep an understanding of the scales involved here. A gram of Hydrogen contains 6e26 atoms. So a nanogram of material contains on the order of 10^17 atoms. That's ten million trillion atoms.
A "beam of antimatter" which literally annihilates the walls of the vacuum chamber it is contained within may seem dramatic but in this case we are talking about mere dozens of anti-atoms. Against walls of solid metal the damage is so minute as to be undetectable with even the finest instruments. Even if the beam were far, far stronger it would still only cause erosion at the scale of picograms per trillion anti-atoms, which isn't much concern.