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Unlike the proton, whose lifetime is longer than the age of the universe, a free neutron decays with a lifetime of about 15 minutes. Measuring the exact lifetime of neutrons is surprisingly tricky; putting them in a container and monitoring their decay can lead to errors because some neutrons will be lost owing to interactions with the container walls. To overcome this problem, Pattie et al. measured the lifetime in a trap where ultracold polarized neutrons were levitated by magnetic fields, precluding interactions with the trap walls (see the Perspective by Mumm). This more precise determination of the neutron lifetime will aid our understanding of how the first nuclei formed after the Big Bang. Science , this issue p. 627 ; see also p. 605 Ultracold polarized neutrons are levitated in a trap to measure their lifetime with reduced systematic uncertainty. The precise value of the mean neutron lifetime, τ n , plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls. As a result of this approach and the use of an in situ neutron detector, the lifetime reported here [877.7 ± 0.7 (stat) +0.4/–0.2 (sys) seconds] does not require corrections larger than the quoted uncertainties.

The past two decades have yielded several new measurements and reanalyses of older measurements of the neutron lifetime. These have led to a 4.4 standard deviation discrepancy between the most precise measurements of the neutron decay rate producing protons in cold neutron beams and the lifetime measured in neutron storage experiments. Measurements using different techniques are important for investigating whether there are unidentified systematic effects in any of the measurements. In this paper we report a new measurement using the Los Alamos asymmetric magneto-gravitational trap where the surviving neutrons are counted external to the trap using the fill and dump method. The new measurement gives a free neutron lifetime of . Although this measurement is not as precise, it is in statistical agreement with previous results using in situ counting in the same apparatus.

Here, the precise value of the mean neutron lifetime, τn, plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls. As a result of this approach and the use of an in situ neutron detector, the lifetime reported here [877.7 ± 0.7 (stat) +0.4/–0.2 (sys) seconds] does not require corrections larger than the quoted uncertainties.

A new boron-coated CCD camera is described for direct detection of ultracold neutrons (UCN) through the capture reactions \\(^{10}\\)B (n,\\(\\alpha\\)0\\(\\gamma\\))\\(^7\\)Li (6%) and \\(^{10}\\)B(n,\\(\\alpha\\)1\\(\\gamma\\))\\(^7\\)Li (94%). The experiments, which extend earlier works using a boron-coated ZnS:Ag scintillator, are based on direct detections of the neutron-capture byproducts in silicon. The high position resolution, energy resolution and particle ID performance of a scientific CCD allows for observation and identification of all the byproducts \\(\\alpha\\), \\(^7\\)Li and \\(\\gamma\\) (electron recoils). A signal-to-noise improvement on the order of 10\\(^4\\) over the indirect method has been achieved. Sub-pixel position resolution of a few microns is demonstrated. The technology can also be used to build UCN detectors with an area on the order of 1 m\\(^2\\). The combination of micrometer scale spatial resolution, few electrons ionization thresholds and large area paves the way to new research avenues including quantum physics of UCN and high-resolution neutron imaging and spectroscopy.

The ultracold neutron (UCN) source at Los Alamos National Laboratory (LANL), which uses solid deuterium as the UCN converter and is driven by accelerator spallation neutrons, has been successfully operated for over 10 years, providing UCN to various experiments, as the first production UCN source based on the superthermal process. It has recently undergone a major upgrade. This paper describes the design and performance of the upgraded LANL UCN source. Measurements of the cold neutron spectrum and UCN density are presented and compared to Monte Carlo predictions. The source is shown to perform as modeled. The UCN density measured at the exit of the biological shield was \\(184(32)\\) UCN/cm\\(^3\\), a four-fold increase from the highest previously reported. The polarized UCN density stored in an external chamber was measured to be \\(39(7)\\) UCN/cm\\(^3\\), which is sufficient to perform an experiment to search for the nonzero neutron electric dipole moment with a one-standard-deviation sensitivity of \\(\\sigma(d_n) = 3\\times 10^{-27}\\) \\(e\\cdot\\)cm.

The precise value of the mean neutron lifetime, \\(\\tau_n\\), plays an important role in nuclear and particle physics and cosmology. It is a key input for predicting the ratio of protons to helium atoms in the primordial universe and is used to search for new physics beyond the Standard Model of particle physics. There is a 3.9 standard deviation discrepancy between \\(\\tau_n\\) measured by counting the decay rate of free neutrons in a beam (887.7 \\(\\pm\\) 2.2 s) and by counting surviving ultracold neutrons stored for different storage times in a material trap (878.5\\(\\pm\\)0.8 s). The experiment described here eliminates loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls and neutrons in quasi-stable orbits rapidly exit the trap. As a result of this approach and the use of a new in situ neutron detector, the lifetime reported here (877.7 \\(\\pm\\) 0.7 (stat) +0.4/-0.2 (sys) s) is the first modern measurement of \\(\\tau_n\\) that does not require corrections larger than the quoted uncertainties.

We report on the evaluation of commercial electroless nickel phosphorus (NiP) coatings for ultracold neutron (UCN) transport and storage. The material potential of 50~\\(\\mu\\)m thick NiP coatings on stainless steel and aluminum substrates was measured to be \\(V_F = 213(5.2)\\)~neV using the time-of-flight spectrometer ASTERIX at the Lujan Center. The loss per bounce probability was measured in pinhole bottling experiments carried out at ultracold neutron sources at Los Alamos Neutron Science Center and the Institut Laue-Langevin. For these tests a new guide coupling design was used to minimize gaps between the guide sections. The observed UCN loss in the bottle was interpreted in terms of an energy independent effective loss per bounce, which is the appropriate model when gaps in the system and upscattering are the dominate loss mechanisms, yielding a loss per bounce of \\(1.3(1) \\times 10^{-4}\\). We also present a detailed discussion of the pinhole bottling methodology and an energy dependent analysis of the experimental results.

We report a measurement of the spin-flip probabilities for ultracold neutrons interacting with surfaces coated with nickel phosphorus. For 50~\\(\\mu\\)m thick nickel phosphorus coated on stainless steel, the spin-flip probability per bounce was found to be \\(\\beta_{\\rm NiP\\;on\\;SS} = (3.3^{+1.8}_{-5.6}) \\times 10^{-6}\\). For 50~\\(\\mu\\)m thick nickel phosphorus coated on aluminum, the spin-flip probability per bounce was found to be \\(\\beta_{\\rm NiP\\;on\\;Al} = (3.6^{+2.1}_{-5.9}) \\times 10^{-6}\\). For the copper guide used as reference, the spin flip probability per bounce was found to be \\(\\beta_{\\rm Cu} = (6.7^{+5.0}_{-2.5}) \\times 10^{-6}\\). The results on the nickel phosphorus-coated surfaces may be interpreted as upper limits, yielding \\(\\beta_{\\rm NiP\\;on\\;SS} < 6.2 \\times 10^{-6}\\) (90\\% C.L.) and \\(\\beta_{\\rm NiP\\;on\\;Al} < 7.0 \\times 10^{-6}\\) (90\\% C.L.) for 50~\\(\\mu\\)m thick nickel phosphorus coated on stainless steel and 50~\\(\\mu\\)m thick nickel phosphorus coated on aluminum, respectively. Nickel phosphorus coated stainless steel or aluminum provides a solution when low-cost, mechanically robust, and non-depolarizing UCN guides with a high-Fermi-potential are needed.

The neutron lifetime is important in understanding the production of light nuclei in the first minutes after the big bang and it provides basic information on the charged weak current of the standard model of particle physics. Two different methods have been used to measure the neutron lifetime: disappearance measurements using bottled ultracold neutrons and decay rate measurements using neutron beams. The best measurements using these two techniques give results that differ by nearly 4 standard deviations. In this paper we describe a new method for measuring surviving neutrons in neutron lifetime measurements using bottled ultracold neutrons that provides better characterization of systematic uncertainties and enables higher precision than previous measurement techniques. We present results obtained using our method.

Position-sensitive detection of ultracold neutrons (UCNs) is demonstrated using an imaging charge-coupled device (CCD) camera. A spatial resolution less than 15 \\(\\mu\\)m has been achieved, which is equivalent to an UCN energy resolution below 2 pico-electron-volts through the relation \\(\\delta E = m_0g \\delta x\\). Here, the symbols \\(\\delta E\\), \\(\\delta x\\), \\(m_0\\) and \\(g\\) are the energy resolution, the spatial resolution, the neutron rest mass and the gravitational acceleration, respectively. A multilayer surface convertor described previously is used to capture UCNs and then emits visible light for CCD imaging. Particle identification and noise rejection are discussed through the use of light intensity profile analysis. This method allows different types of UCN spectroscopy and other applications.