BL3 research project

BL3 The Beam Lifetime (BL) experiment at the National Institute of Standards and Technology (NIST) measures the neutron lifetime using a technique different from UCNtau by observing neutron decay in flight. The neutron lifetime is determined by comparing the rate of beta-decay protons captured in a Penning trap to the rate of beam neutrons passing through the trap. The neutron lifetime measured using the beam method differs from that measured using the bottle method (UCNtau) by ~10 seconds. The continued disagreement hints at new physics, including neutron oscillations and low-energy physics in the dark sectors. 

Much careful work has been done to determine the detector efficiencies and study the associated systematic corrections.The BL3 experiment scales up the BL2 experiment, which is currently deployed at the NIST reactor, to reduce the uncertainties of the lifetime measurement. It will improve the counting statistics by increasing the neutron beam diameter from 10 mm to 35 mm. With the interest to solve the neutron lifetime puzzle, we recently joined the BL3 collaboration to address the technical challenges specific to the beam technique. The BL3 project has been funded by NSF to construct a new magnet, a new proton trap, and new detector systems. 

Our group is tasked to lead the work package to construct the Alpha-Gamma (AG) Device, for calibrating the efficiency, ε0, of the neutron fluence monitor. This device will be using a separate monochromatic neutron beam, independent from the main lifetime apparatus. The design is based on a previous proof-of-principle work, which improved the systematic uncertainty of the BL result by a factor of 2. This scaled-up AG device, using an array of Passivated Implanted Planar Silicon (PIPS) detectors and High-Purity Germanium (HPGe) detectors, will cross-reference the neutron capture rates on the lithium and boron isotopes. Foils of varying thickness will be mounted on a sample ladder inside the vacuum chamber to carry out the 4-step calibration procedure, developed by Yue and Greene. The anticipated increase in the counting statistics should enable a coincidence method, using alpha and gamma emitted through neutron capture on 10B, for absolute flux determination. We are optimizing the detector placement to mitigate systematic effects due to finite beam profile and position offsets. If the construction of the BL3 apparatus is funded, over the next grant cycle we plan to construct, assemble, and test a new AG Device to support the upcoming BL3 measurements.

We are also awarded by the NIST Precision Measurement Grant Program to develop an in-situ proton detection by measuring image current induced on the ground electrodes. With this in-situ detection, we will be able to calibrate the efficiency of the external proton detector used in the previous BL experiments and study the effect of residual gas interactions, which might cause protons to got lost inside the Penning trap.

Faculty on this project