Improved measurements of the neutron beta-decay lifetime

Project Information

Grant agreement ID: INTAS-93-0298

Project closed

Start date 1 October 1994

End date 30 September 1995

Funded under

• International Association for the promotion of cooperation with scientists from the independent states of the former Soviet Union (INTAS), 1993-

Total cost € 0,00

EU contribution € 65 000,00

Coordinated by

Technische Universität München
Germany

Exploitable results

Improved measurements of the neutron beta-decay lifetime

This project aimed at an improved experimental determination of the free neutron lifetime which is of fundamental importance for particle physics and cosmology. The most precise values for neutron lifetime were obtained recently by storage of ultracold neutrons (UCN) in material bottles and the project participants were involved in these experiments. This relatively new method has the potential for significant improvement. At the PNPI (Serebrov's groups) a new UCN trap was developed. The top of the trap is closed by the gravitational force on the neutrons and extra surfaces can be moved into the trap to control wall losses. The experiment is ready to move to the UCN source of the ILL, Grenoble, and a corresponding proposal was submitted to the ILL and accepted. In addition Serebrov's and Strelkov's (Dubna) group performed at PNPI and the ILL experiments on UCN wall losses. They observed anomalous transmission of UCN through beryllium foils. This effect is in contradiction to the standard picture of coherent reflection of UCN at material walls and needs a new theoretical approach. Morosov's group (Kurchtov Inst.)has worked on two kinds of UCN experiments and spent several months at the ILL. The absorption of neutrons by hydrogen and fluorine on beryllium surfaces was studied, whether it lead to an upscattering of UCN or to neutron capture. An enourmously high neutron capture rate was observed which could not be explained by the standard UCN-wall interaction. A common interpretation with the above transmission anomaly seems to be in view. In a second experiment at the ILL Morosov's group used an UCN trap, coated with Fomblin oil, for a measurement of the free neutron's lifetime. The neutrons escaping from the trap were recorded by surrounding counters. Surviving neutrons after different storage times were measured. A detailed evaluation of the data resulted in τn=885.4+/-0.9(stat)+/-0.3(syst) s, which is at present the most precise neutron life time value.