BL3 Neutron Lifetime Apparatus

The neutron is a basic constituent of ordinary matter and the majority of the Earth's mass is in the form of neutrons. When freed from the confines of a stable atomic nucleus the neutron is unstable; it decays into a proton, electron, and antineutrino with an average lifetime of about fifteen minutes. Neutron decay played an important role in the early universe; it determined the relative abundances of light elements (hydrogen, helium, lithium, and beryllium) and their isotopes that were formed in the first minutes after the Big Bang.

Due to its simplicity, neutron decay is an ideal system for studying details of the most basic forces of subatomic physics, in particular the weak nuclear force. Such studies improve our understanding of nature and may provide hints of new fundamental physical phenomena yet to be discovered. The neutron lifetime has been measured with an uncertainty of less than one second by individual experiments, but results from the two main experimental methods, the beam method and the ultracold neutron storage method, currently disagree by more than eight seconds.

Resolving this discrepancy is a matter of great importance for nuclear physics. This award supports the design, construction, and testing of a next-generation beam neutron lifetime experiment called BL3. It will employ new, powerful technical features to enable its goals of resolving the discrepancy and providing a reliable measurement of the neutron lifetime to well below one second of uncertainty.

The scope of this project is to design, construct, and test the BL3 apparatus, a new experiment to measure the neutron lifetime using the beam method. It is similar in concept and improves upon previous beam neutron lifetime experiments. It will employ a significantly larger superconducting magnet to accommodate a large area neutron beam and will incorporate many technical improvements, such as much higher counting statistics, a more uniform magnetic field in the trapping region; a large, segmented, ultrathin window silicon proton detector; and a sophisticated neutron time of flight spectrometer.

BL3 has two scientific goals: 1) to investigate and test systematic effects in the beam method that may contribute to the 8.4 s (4 sigma) discrepancy between the beam and ultracold neutron storage experiments; and 2) reduce the total uncertainty of the neutron lifetime beam method to less than 0.3 s. The value of the neutron lifetime has important consequences in nuclear physics, particle physics, astrophysics, and cosmology.

This project provides an excellent opportunity to train undergraduates, graduate students, and postdocs in the methods and theory of neutron science which are applicable to diverse scientific studies in physics, chemistry, materials science, and biology at existing and emerging neutron sources around the world.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.