Optical time projection chamber for imaging of two-proton decay of /sup 45/Fe nucleus

We present a design of the gaseous detector for three-dimensional (3-D) measurement of the topology of a recently discovered radioactive decay of ⁴⁵Fe nucleus which involves simultaneous emission of two protons from the ground state. Our apparatus, called an optical time projection chamber (OTPC), consists of several parallel wire-mesh electrodes inside a gaseous medium which form the conversion region and the multistage charge amplification structure. The gas mixture filling the chamber enables strong emission of ultraviolet (UV) photons during avalanche process. Ultraviolet photons from charge avalanches are converted into visible light by means of a wavelength shifter foil. A CCD camera located outside the detection volume records 2-D image of the decay process. Drift time of primary ionization charge toward the amplification stage provides the third coordinate. Correlation of 2-D image with drift-time structure allows 3-D reconstruction. We present results on: the light yield, the charge gain and the electron drift velocity for several gas mixtures consisting of: Ar, He, N₂, and triethylamine vapor. A feasibility of the event topology reconstruction and the energy measurement is addressed