Development of a Precision Neutron Lifetime Measurement: Magnetic Trapping of Ultracold Neutrons

The neutron lifetime plays an important role in both nuclear astrophysics and in furthering the understanding of weak interactions in the Standard Model. It is the most important experimental parameter in theoretical predictions of the primordial abundance of 4He in Big Bang Nucleosynthesis. A precision measurement also provides a self consistency check of the unitarity of the CKM mixing matrix which relates the weak and mass eigenstates of quarks in the Standard Model. Our group has successfully demonstrated the trapping of ultracold neutrons in a conservative potential magnetic trap and demonstrated that the measured lifetime was consistent with the present world average value. This work represents a major upgrade of the apparatus assembled at the NIST Center for Neutron Research to address statistical limitations of the former measurement. Our unique approach to this measurement and the advantages it provides over other techniques will be discussed. The major systematics of the technique will be addressed. Tests of the upgrade components and details of the final stages of construction will also be presented.