The present invention relates to screening assays for functional analogues and antagonists of a polypeptide fragment of acetylcholinesterase, which is believed to exhibit activity corresponding to non-enzymatic function of AchE in the brain. The device comprises an imaging system with a round lens optics, a low-energy drift space and an accelerator driven by at least one rapidly switchable voltage U, such that the velocity andor trajectory of the particles in the beam is influenced and the particles starting from point with different energies are crossing the image plane in one point, i. A power determining circuit then determines individual power measurements of the speech stored in each shift register element. The memory allows for erase operations that reduce leakage current during initiation of an erase operation.