The central experiment with UCNs within PRISMA+ is the τSPECT experiment to measure the lifetime of the neutron. On the one hand, the neutron lifetime is of importance for precision tests of the standard model of particle physics. On the other hand, it plays an important role during the synthesis of the light elements in the universe, i.e. during big bang nucleosynthesis. In this latter context it determines the abundance ratio of the light elements in the cosmos, e.g. of helium to hydrogen.
Besides this fundamental interest in the neutron lifetime, there are experimental reasons why the neutron lifetime is of interest in modern physics: There are several precision measurements of this lifetime which deviate from each other by far more than their uncertainties allow. In addition, there are also significant differences of the results of neutron lifetime measurements using different technologies: Determinations of the lifetime measured at neutron beams seem to yield slightly larger lifetimes than those measured with stored neutrons using material walls. For the clarification of these differences several new experiments are under way using neutron storage in magnetic bottles and online measurement of the decay products of the neutron, which will remove the leading systematic uncertainties of previous experiments.
The τSPECT experiment is being set-up at present. It uses the magnet system of the aSPECT experiment for axial magnetic storage, its UHV vacuum system and the aSPECT detection system for real time detection of the decay products from neutron decay, i.e., protons and electrons. The integration and commissioning measurement for most components will take place in early spring 2015. This will still use a mix of magnetic and material wall storage. During the same time period the multipole magnet for radial magnetic storage will be delivered and tested. In a next step a measurement for full magnetic storage will be performed. After this the systematic uncertainties will be investigated and optimized for a measurement of the neutron lifetime with a precision of 1 s (Phase 1). Successively, τSPECT will be optimized for a precision of 0.3 s (Phase 2).