Fundamental physics with ultracold neutrons

Superthermal source of ultra-cold neutrons

Ultra-cold neutrons (UCN) have energies in the neV range. They are thus reflected by a number of materials at all incident angles and can be stored either in material bottles or confined by magnetic potential walls. Storage times of several minutes can be achieved in material bottles, while typical beam experiments are only capable of sustaining observation times of several milliseconds. Pulsed reactors with repetition rates of up to 12 per hour and very high pulsed neutron fluxes – as realised at the reactor TRIGA Mainz in collaboration with the institute of nuclear chemistry – are well suited for storage experiments which have a cyclic nature. Thus, owing to its pulse mode the TRIGA Mainz facility will play the leading role for the next generation of precision experiments with stored neutrons. Another advantage of TRIGA Mainz is the short distance (~ 5 m) between the UCN solid deuterium (sD2) converter installed close to the reactor core and the experiments located outside of the biological shield, which reduces UCN transfer losses considerably [1]. Background interference during data taking is essentially zero since the reactor is off during measurements. This UCN source has been set-up and commissioned at the TRIGA Mainz and is routinely used for UCN experiments ...

Experiments with ultra-cold neutrons

Neutron lifetime  

An important measurement that will be done in the first few years at the pulsed UCN source at TRIGA Mainz is the tSPECT experiment, which aims for the world-best determination of the neutron lifetime, with improved statistics and significantly improved systematics compared with existing measurements. This is particularly relevant, as existing determinations of the neutron lifetime using similar techniques differ by 7σ. Also a difference between beam experiments and experiments using material wall storage has to be clarified. This will lead to an improved Deterimination of fundamental parameters of the Standard Model, such as the Cabibbo-Kobayashi-Maskawa (CKM) matrix element Vud in neutron decay.