Experimental set-up and first measurements of the neutron lifetime using PENeLOPE

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The lifetime of the free neutron tn is one of the fundamental physical constants and plays an important role in many areas of physics. In the experiment PENeLOPE, ultra-cold, very slow neutrons shall be stored magnetically in a trap and tn shall be measured; the magnetic fields are created by superconducting magnets. In this way, not understood losses at material walls are avoided. The magnet for PENeLOPE is currently under construction (DFG project PA 762/8-1). The neuton lifetime will be determined in two ways with this experiment; the neutrons surviving in the trap after the storage cycle will be counted after varying storage times and, most notably, the protons generated in neutron decay will be recorded in realtime. Proton detection combines two advantages: it is independent from a variation in the number of neutrons per filling and requires only a fourth of the measuring time compared with neutron counting to achieve the aspired statistical precision of < 0.1 s. During an earlier DFG project (PA 768/5-2) it was proven that decay protons may be detected with a thin layer of cesium iodide (CsI) evaporated on a light guide and read out from the side via an avalanche photodiode (APD). To this end, it is necessary to accelerate the protons to energies of around 30 keV. In addition, it was verified that electrons and protons may be distinguished and that the detector works in the PENeLOPE environment at cryogenic temperatures and in large magnetic fields. The next logical step is to adapt the detector to the geometry of PENeLOPE: a ring of 180 mm inner and 360 mm outer radius, resulting in a total area of around 3000 cm3. The columnar structure of a thin evaporated layer of CsI turned out to be a big challenge; it impedes the light collection efficiency significantly if read out from the side. Hence, other alternatives to this detector solution shall be investigated: large CsI crystals, directly coupled to the APDs, thin organic (plastic) scintillators, microchannel plates and conversion foils to generate secondary electrons.