It's possible the particle energies are very homogeneous i.e before the drop they were all just above 0.5MeV with very low std deviation and after the drop they

It's possible the particle energies are very homogeneous i.e before the drop they were all just above 0.5MeV with very low std deviation and after the drop they were all just below 0.5MeV. You cannot tell from the graph what the true drop off is because you cannot tell how their energies were distributed.