Ultra-cold neutrons (UCN) offer unique opportunities for studying the properties of the free neutron with exceptionally high precision. Properties as its lifetime can be measured with unprecedented accuracy.
UCN sources are under construction at several research centres worldwide in order to tackle the existing count-rate limitations in these kinds of experiments. Low-power reactors, such as TRIGA Mainz with its inherent safety, are strongly competitive due to the possibility to pulse the reactor every five minutes (peak power 250 MW) and produce a high density of UCNs that ideally meet the requirements of storage experiments. Using a super-thermal UCN source [1] at TRIGA a density of 10 UCN/cm3 in a 10 litre storage volume has been achieved [2]. Background interference during data taking is essentially zero since the reactor is off during the measurements. Low magnetic noise is another quality feature of this reactor.
Our long-term goal is to reach a UCN density of ~100/cm3. PRISMA+ provides essential infrastructure to develop and sustain experiments at a facility well suited for UCN-storage experiments. A Helium-liquefier with a capacity of 14 l/h has been in use since autumn 2014. New staff for user operation has been hired. Current activities focus on the upgrade of the UCN-source and UCN densities of ~50/cm3 in the near future is expected.
Another important step is the transfer of UCNs from the source to the experiment. For high transmission efficiency a sputter facility is in operation at the Institute of Physics. Here neutron guide tubes of high quality that guarantee low losses and a large reflective Fermi potential are produced for the experiments at TRIGA as well as for experiments at other UCN facilities. A major highlight is the production of neutron guides internally coated with 58Ni, the material best suited for neutron guidance. The coating of the extraction volume of the UCN source with 58Ni is also a crucial part for its upgrade.
Presently the τSPECT experiment is being set-up in area D of the TRIGA hall. τSPECT will measure the lifetime of the free neutron using the novel technique of neutron magnetic storage. Using an in-situ measurement of the decay curve by detecting the decay products of the neutron decay, the proton and the electron, the neutron lifetime can be determined with much better control over systematics. The rapid development of τSPECT is possible since many components of the successful aSPECT experiment can be reused. Another experiment, a Penning trap and a laser-spectroscopy setup for the investigation of short-lived fission products - TRIGA-SPEC [3] - has also been installed at TRIGA for high-precision measurements of neutron-rich isotopes of astrophysical relevance.
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[1] A. Frei, et al., First production of ultracold neutrons with a solid deuterium source at the pulsed reactor TRIGA Mainz, Eur. Phys. J. A 34, 119 (2007)[2] J. Karch, et al., Performance of the solid deuterium ultra-cold neutron source at the pulsed reactor TRIGA Mainz, Eur. Phys. J. A, 50, 78 (2014)
[3] J. Kahlenberg, et al., Upgrade of the ultracold neutron source at the pulsed reactor TRIGA Mainz, arXiv:1706.07795, (2017)
[4] B. Lauss, D. Ries, et al., Comparison of ultracold neutron sources for fundamental physics measurements, Phys. Rev. C 95, 045503 (2017)
[5] S. Kaufmann, et al,. TRIGA-SPEC: the prototype of MATS and LaSpec, J. Phys. Conf. Ser. 599, 012033 (2015)