Intense laser produced plasmas are known to be novel compact sources of high energy electrons. Electrons deflected by magnetic fields and imaged on phosphorescent plates is a well used diagnostic for electron spectrum measurements. As only negatively charged particles are filtered by the magnetic field, it is a common practise to interpret the measured scintillation entirely as a signal due to the electrons. We show here that such an interpretation can be erroneous. Low energy H− ions arrive concurrently at the same position as high energy electrons and can lead to a wrong assessment of the maximum electron energy measured, which is often used in understanding the underlying physics. Conventional accelerator based experiments prove that the H− ions are indeed detected even when the image plate is covered with a metal foil.
