Colloquium: The neutron lifetime

The decay of the free neutron into a proton, electron, and antineutrino is the prototype semileptonic weak decay and is the simplest example of nuclear beta decay. It played a key role in the early Universe as it determined the ratio of neutrons to protons during the era of primordial light element nucleosynthesis. Neutron decay is physically related to important processes in solar physics and neutrino detection. The mean neutron lifetime has been the subject of more than 20 major experiments done, using a variety of methods, between 1950 and the present. The most precise recent measurements have stated accuracies approaching 0.1%, but are not in good agreement as they differ by as much as $5\sigma$ using quoted uncertainties. The history of neutron lifetime measurements is reviewed and the different methods used are described, giving important examples of each. The discrepancies and some systematic issues in the experiments that may be responsible are discussed, and it is shown by means of global averages that the neutron lifetime is likely to lie in the range of 880–884 s. Plans and prospects for future experiments are considered that will address these systematic issues and improve our knowledge of the neutron lifetime.

Figure 1

The apparatus that made the first detection of coincident electrons and protons from neutron decay. The neutron beam passes through the page in the center. Protons are directed upward by the accelerating electrode. The two beta counters are below the beam. From 88.

Figure 2

The Risö neutron lifetime experiment . The decay volume is defined by the beta spectrometer, which consists of two parallel plastic scintilator paddles and a perpendicular magnetic field. From 22.

Figure 3

The Kurchatov neutron lifetime experiment: (1) vacuum chamber, (2) neutron beam, (3,5)  precision proton collimator, (6) retarding grid, (7) focusing electrode, (8) proton detector. From 9.

Figure 4

The NIST beam neutron lifetime experiment. Top: The neutron beam passes through a 16 segment quasi-Penning trap where neutron decay protons are trapped and later extracted through a 9.5° bend in the magnetic field to the proton detector. The neutron beam density is measured by counting ${}^6\mathrm{Li}(n,\alpha ){}^3\mathrm{H}$ reactions in a thin ${}^6\mathrm{LiF}$ deposit. Bottom: The measured neutron lifetime vs the calculated proton backscattering fraction for different detectors and acceleration voltages. A linear extrapolation to zero backscattering gives the result. From 70.

Figure 5

Top: The ultracold neutron bottle used in the MAMBO experiment. Neutrons are admitted through the straight tube in front during filling; they exit through the bent tube during the counting phase. The rear wall can be moved using the rod $\mathsf{M}$ to change the volume to surface ratio of the bottle, and hence the wall loss rate. Bottom: Data from the MAMBO experiment; the measured inverse bottle lifetimes as a function of bottle inverse mean free path, for different storage intervals corresponding to different positions of the rear wall. The data extrapolate to a single point at zero inverse mean free path (infinite volume to surface ratio), giving the neutron beta decay lifetime. From 63.

Figure 6

The original Gravitrap neutron lifetime experiment: (1) ultracold neutron storage bottle, (2) liquid nitrogen screen, (3) neutron distribution valve, (4) neutron guide, (5) neutron filling valve, (6) ${}^3\mathrm{He}$ neutron counter, (7) radiation shielding, (8) rotational control linkage, (9) neutron guide, (10) cryogen lines, (11) cryostat, (12) frozen oxygen system. From 54.

Figure 7

Top: The ultracold neutron bottle in 5 used to measure the neutron lifetime. (1) neutron guide, (2) neutron shutters, (3) neutron detector, (4) polyethylene shielding, (5) cadmium neutron absorber, (6) neutron entrance shutter, (7) inner storage vessel, (8) outer storage vessel, (9) cooling coil, (10) ${}^3\mathrm{He}$ proportional counters, (11) vacuum vessel, (12) Fomblin oil puddle, (13) entrance shutter for the annular vessel, (1a) Fomblin oil puddle, (2a) slit. Bottom: The extrapolated neutron lifetime for nine different runs. The bottle wall temperature (°C) is given on the bottom axis. Note that the collaboration will soon publish a correction to the analysis that will lower the neutron lifetime result. From 8.

Figure 8

Data from the Gravitrap II experiment showing the extrapolation of the neutron storage lifetimes to zero wall loss rate. Dark circles are from the smaller cylindrical bottle and open circles from the larger bottle. From 82.

Figure 9

The NIST ultracold neutron lifetime apparatus. Cold neutrons with energy 1.03 MeV enter from the left and are converted to ultracold neutrons in liquid helium, by the superthermal method, in the trapping region. One spin state is trapped there by the quadrupole racetrack coils and two solenoids and the other spin state is ejected. Trapped neutrons that beta decay ionize the liquid helium, producing scintillation light that is downshifted to visible light and transported by the acrylic light guide to a pair of photomultiplier tubes. From 50.

Figure 10

The magnetic bottle apparatus of 33. A cylindrical array of permanent magnets lining the bottle wall traps neutrons magnetically such that they cannot touch the material walls. The shutter solenoid forms the bottom of the trap, and gravity prevents trapped neutrons from escaping at the top. A moveable lift bottle is used to load the trap. The outer solenoid eliminates zero field points that can cause neutron depolarization and leakage.

Figure 11

A summary of neutron lifetime measurements. Solid circles are beam experiments, open squares are bottle experiments, and diamonds are magnetic trap experiments. The inset shows the eight experiments included in our global averages. See Table  for references.