Magnetically Trapped Neutron Lifetime Experiment

It is designed to measure the neutron beta-decay lifetime t_n using a substantially new technique.  Our method confines Ultra Cold Neutrons (UCN) within a three-dimensional magnetic trap. Cooling of the neutrons occurs within the conservative trap when 12 K neutrons (0.89 nm) down-scatter in superfluid ⁴He to near rest via single phonon emission (superthermal production). The UCN then interact only with the magnetic field via their magnetic moment and when the spin is anti-parallel to the magnetic field, they will seek to minimize their potential energy by moving towards low field regions. By cooling the trap to temperatures of approximately 100 mK, the population of UCN becomes thermally detached from the helium bath allowing accumulation of UCN to a density as high as Pt, where P is the superthermal production rate and is the UCN lifetime in the source. Neutron decay is detected by turning off the cold neutron beam and observing the scintillation light resulting from the beta-decay electrons. When an electron moves through liquid helium, it ionizes helium atoms along its track. These helium ions quickly recombine into metastable He_2* molecules. About 35 % of the initial electron energy goes into the production of extreme ultraviolet (EUV) photons from singlet decays, corresponding to approximately 22 photons/keV. These EUV photons are frequency down-converted to blue photons using the organic fluor tetraphenyl butadiene (TPB) coated onto a diffuse reflector surrounding the trapping region. This light is transported via non-imaging optics to room temperature and detected by two photomultiplier tube (PMT)s operating in coincidence. This unique trapping and detection method allows us to observe neutron decay events in situ, and therefore directly measure the decay curve.

The decay of the free neutron is the simplest nuclear beta decay and is the prototype for all charged current semi-leptonic weak interactions.  The decay parameters, the neutron lifetime in particular, provide essential inputs to investigations of the weak interaction.  A precise value for the neutron lifetime is required for several internal consistency tests of the SM including searches for right-handed currents and tests of the unitarity of the CKM mixing matrix.  Measurements of neutron decay coefficients provide information on the vector and axial-vector coupling constants g_v and g_a.  The neutron lifetime is also an essential parameter in the theory of Big Bang Nucleosynthesis.

At present, there is a 6.5 s discrepancy between the two most precise UCN bottle experiments that is not understood.  It is essential to resolve this disagreement, which can best be accomplished through measurements using systematically different techniques.  As beam-type experiments are limited by measurements of the neutron flux, and material bottle experiments are complicated by wall interactions, magnetic trapping techniques offer the best possibility for both solving this discrepancy and improving the precision of the neutron lifetime.

In addition, the work performed over the course of this program has played a major role in the development and design of a number of other significant experiments, including the neutron EDM effort and CLEAN, a neutrino experiment that seeks to both directly measure the rate of pp reactions in the sun and search for dark matter events. We have also developed and tested new technologies, for example, a long wavelength neutron monochromator the basis of which is being used at the Spallation Neutron Source and at the ILL.   We have also tested a variety of methods of detecting light at cryogenic temperatures and are pushing the development of accelerator mass spectrographic methods to measure the isotopic abundance of helium samples.

Finally, as this and other neutron lifetime experiments relying on magnetic trapping move forward, one must fully understand the dynamics of neutrons in magnetic traps.  Our studies of marginally trapped neutrons are helping to guide the design of other experiments in the field.

We have now completed the extensively upgraded apparatus. All systems have been independently tested and perform according to expectations.We are currently taking data using conservative field strength of 50%.  We expect to begin production data collection beginning next reactor cycle. Estimates of background and increased counts rates indicate that we should reach a sensitivity of 2-3 s with approximately three cycles of data collection.