Determination of the Neutron Lifetime Using Magnetically Trapped Neutrons

We report progress on an experiment to measure the neutron lifetime using magnetically trapped neutrons. Neutrons are loaded into a 1.1 T deep superconducting Ioffe-type trap by scattering 0.89 nm neutrons in isotopically pure superfluid ⁴He. Neutron decays are detected in real time using the scintillation light produced in the helium by the beta-decay electrons. The measured trap lifetime at a helium temperature of 300 mK and with no ameliorative magnetic ramping is substantially shorter than the free neutron lifetime. This is attributed to the presence of neutrons with energies higher than the magnetic potential of the trap. Magnetic field ramping is implemented to eliminate these neutrons, resulting in an 833−63+74s trap lifetime, consistent with the currently accepted value of the free neutron lifetime.     

Progress towards magnetic trapping of ultra-cold neutrons

We report progress towards magnetic trapping of ultra-cold neutrons (UCN) in preparation for a neutron lifetime measurement. UCN will be produced by inelastic scattering of cold (0.89 nm) neutrons in a reservoir of superfluid ⁴He and confined in a three-dimensional magnetic trap. As the trapped neutrons decay, recoil electrons will generate scintillations in the liquid He, which should be detectable with nearly 100% efficiency. This direct measure of the number of UCN decays vs. time can be used to determine the neutron beta-decay lifetime.     

Neutron Lifetime Experiment Based on an Accordion-Like UCN Storage Volume Coated With “Low Temperature Fomblin”

A new type of per-fluorinated polymer, “Low Temperature Fomblin,” has been tested as a wall coating in an ultracold neutron (UCN) storage experiment using a gravitational storage system. The data show a UCN reflection loss coefficient η as low as ≈ 5 × 10⁻⁶ in the temperature range 105 K to 150 K. We plan to use this oil in a new type of neutron lifetime measurement, where a bellows system (“accordion”) enables to vary the trap size in a wide range while the total surface area and distribution of surface area over height remain constant. These unique characteristics, in combination with application of the scaling technique developed by W. Mampe et al. in 1989, ensure exact linearity for the extrapolation from inverse storage lifetimes to the inverse neutron lifetime. Linearity holds for any energy dependence of loss coefficient µ(E). Using the UCN source at the Institut Laue Langevin we expect to achieve a lifetime precision below ±1 s.     

Measurement of the Neutron Lifetime by Counting Trapped Protons

We measured the neutron decay lifetime by counting in-beam neutron decay recoil protons trapped in a quasi-Penning trap. The absolute neutron beam fluence was measured by capture in a thin ⁶LiF foil detector with known efficiency. The combination of these measurements gives the neutron lifetime: τ_n = (886.8 ± 1.2 ± 3.2) s, where the first (second) uncertainty is statistical (systematic) in nature. This is the most precise neutron lifetime determination to date using an in-beam method.     

Measurement of the Parity-Violating Neutron Spin Rotation in 4He

In the meson exchange model of weak nucleon-nucleon (NN) interactions, the exchange of virtual mesons between the nucleons is parameterized by a set of weak meson exchange amplitudes. The strengths of these amplitudes from theoretical calculations are not well known, and experimental measurements of parity-violating (PV) observables in different nuclear systems have not constrained their values. Transversely polarized cold neutrons traveling through liquid helium experience a PV spin rotation due to the weak interaction with an angle proportional to a linear combination of these weak meson exchange amplitudes. A measurement of the PV neutron spin rotation in helium (φ_PV (n,α)) would provide information about the relative strengths of the weak meson exchange amplitudes, and with the longitudinal analyzing power measurement in the p + α system, allow the first comparison between isospin mirror systems in weak NN interaction. An earlier experiment performed at NIST obtained a result consistent with zero: φ_PV (n,α) = (8.0 ±14(stat) ±2.2(syst)) ×10⁻⁷ rad / m[1]. We describe a modified apparatus using a superfluid helium target to increase statistics and reduce systematic effects in an effort to reach a sensitivity goal of 10⁻⁷ rad/m.     

First Ever Storage of Ultracold Neutrons in a Magnetic Trap Made of Permanent Magnets

Further improvement in the accuracy of any neutron lifetime experiment by means of ultracold neutrons (UCN) in material bottles is limited due to unavoidable systematic effects when the UCN are reflected from the walls. However, such effects can be excluded in principle if magnetic trapping of UCN is used. The storage of UCN in a small magnetic trap made of permanent magnets was demonstrated for the first time ever. The measured storage time in this feasibility study was (882 ± 16) s. At this level of accuracy no depolarization was observed.     

Neutron lifetime measured by monitored storing of ultra-cold neutrons

The method of measuring the neutron β-decay lifetime τ_β by storage of ultra-cold neutrons (UCN) with simultaneous recording of inelastically scattered neutrons is presented. The result of the measurement is τ_β [s]=885.4±0.9_stat±0.4_syst.     

Commissioning of the NPDGamma Detector Array: Counting Statistics in Current Mode Operation and Parity Violation in the Capture of Cold Neutrons on B4C and 27Al

The NPDGamma γ-ray detector has been built to measure, with high accuracy, the size of the small parity-violating asymmetry in the angular distribution of gamma rays from the capture of polarized cold neutrons by protons. The high cold neutron flux at the Los Alamos Neutron Scattering Center (LANSCE) spallation neutron source and control of systematic errors require the use of current mode detection with vacuum photodiodes and low-noise solid-state preamplifiers. We show that the detector array operates at counting statistics and that the asymmetries due to B₄C and ²⁷Al are zero to with- in 2 × 10⁻⁶ and 7 × 10⁻⁷, respectively. Boron and aluminum are used throughout the experiment. The results presented here are preliminary.     

Precision Neutron Polarimetry for Neutron Beta Decay

The abBA collaboration is developing a new type of field-expansion spectrometer for a measurement of the three correlation coefficients a, A, and B and the shape parameter b. The measurement of A and B requires precision neutron polarimetry. We will polarize a pulsed cold neutron beam from the SNS using a ³He neutron spin filter. The well-known polarizing cross section for n-³He has a 1/v dependence, where v is the neutron velocity, which is used to determine the absolute beam polarization through a time-of-flight (TOF) measurement. We show that by measuring the TOF dependence of A and B, the coefficients and the neutron polarization can be determined with a small loss of the statistical precision and with negligible systematic error. We conclude that it is possible to determine the neutron polarization averaged over a long run in the neutron beta decay experiment with a statistical error less than 10⁻⁴. We discuss various sources of systematic uncertainty in the measurement of A and B and conclude that the fractional systematic errors are less than 2 × 10⁻⁴.     

High-Precision Determination of the Neutron Coherent Scattering Length

The neutron coherent scattering length b_c has been determined interferometrically to an uncertainty of about 5 × 10⁻⁵ by measuring the nondispersive phase. We propose improving the uncertainty to about 10⁻⁶ by optimizing various parameters of the interferometric experiment. Any uncertainty in the b_c determination arising from possible variations in the constitution of the ambient air can be eliminated by performing the experiment in vacuum. When such uncertainty is attained, it becomes necessary to account for the neutron beam refraction at the sample-ambient interfaces, to infer the correct b_c from the observed phase. The formula for the phase used hitherto is approximate and would significantly overestimate b_c. The refractive index for neutrons can thus be determined to a phenomenal uncertainty of about 10⁻¹².     

Identification of a new escape channel for UCN from traps

Ultra-cold neutrons (UCN) can be stored in a trap if their energy is lower than the trap wall potential. It is well known that the neutron density in a trap decreases due to neutron beta-decay, upscattering and absorption on surfaces but we have identified a complementary escape channel. This arises from a small increase in the energy of UCN during their interaction with a surface. Higher-energy neutrons can then escape into the bulk material or penetrate through the trap wall if it is thin enough.     

Simulations of neutron deceleration in a multistage UCN turbine using a multilayer monochromator

A neutron turbine is a neutron decelerator with neutron reflectors on a rotor. The multistage neutron turbine using multilayer monochromators has three rotors to decelerate very cold neutrons to ultra-cold neutrons. Reflecting blades on the rotors are flat and the incident direction of neutrons is perpendicular to the mirror surface. The use of a multistage turbine makes the incident velocity to be about 150 m/s which is faster than the existing neutron turbines and the three rotors make the velocity change smaller in one stage than the Doppler shifter employing Bragg reflection. It simultaneously improves the neutron extraction efficiency from a cold neutron source and the neutron deceleration. The peak deceleration efficiency assuming unit reflectivity in this three-stage turbine is about 0.71 from 150 m/s to UCN, and that of the final stage is about 0.81 from 50 m/s to UCN.     

A Superconducting Magnet UCN Trap for Precise Neutron Lifetime Measurements

Finite-element methods along with Monte Carlo simulations were used to design a magnetic storage device for ultracold neutrons (UCN) to measure their lifetime. A setup was determined which should make it possible to confine UCN with negligible losses and detect the protons emerging from β-decay with high efficiency: stacked superconducting solenoids create the magnetic storage field, an electrostatic extraction field inside the storage volume assures high proton collection efficiency. Alongside with the optimization of the magnetic and electrostatic design, the properties of the trap were investigated through extensive Monte Carlo simulation.     

The Fundamental Neutron Physics Beamline at the Spallation Neutron Source

The Spallation Neutron Source (SNS), currently under construction at Oak Ridge National Laboratory with an anticipated start-up in early 2006, will provide the most intense pulsed beams of cold neutrons in the world. At a projected power of 1.4 MW, the time averaged fluxes and fluences of the SNS will approach those of high flux reactors. One of the flight paths on the cold, coupled moderator will be devoted to fundamental neutron physics. The fundamental neutron physics beamline is anticipated to include two beam-lines; a broad band cold beam, and a monochromatic beam of 0.89 nm neutrons for ultracold neutron (UCN) experiments. The fundamental neutron physics beamline will be operated as a user facility with experiment selection based on a peer reviewed proposal process. An initial program of five experiments in neutron decay, hadronic weak interaction and time reversal symmetry violation have been proposed.     

Measurement of Parity Violation in np Capture: the NPDGamma Experiment

The NPDGamma experiment will measure the parity-violating directional gamma ray asymmetry A_γ in the reaction n→+p→d+γ. Ultimately, this will constitute the first measurement in the neutron-proton system that is sensitive enough to challenge modern theories of nuclear parity violation, providing a theoretically clean determination of the weak pion-nucleon coupling. A new beam-line at the Los Alamos Neutron Science Center (LANSCE) delivers pulsed cold neutrons to the apparatus, where they are polarized by transmission through a large volume polarized ³He spin filter and captured in a liquid para-hydrogen target. The 2.2 MeV gamma rays from the capture reaction are detected in an array of CsI(Tl) scintillators read out by vacuum photodiodes operated in current mode. We will complete commissioning of the apparatus and carry out a first measurement at LANSCE in 2004–05, which would provide a statistics-limited result for A_γ accurate to a standard uncertainty of ±5 × 10⁻⁸ level or better, improving on existing measurements in the neutron-proton system by a factor of 4. Plans to move the experiment to a reactor facility, where the greater flux would enable us to make a measurement with a standard uncertainty of ±1 × 10⁻⁸, are actively being pursued for the longer term.     

Permanent-magnet trap for ultracold neutron storage

The design of a permanent-magnet trap for ultracold neutron (UCN) storage is described. This trap excludes neutron collisions with the walls and, hence, eliminates anomalous losses. Used in the experiments on the neutron lifetime determination such UCN traps remove the main systematic error related to the anomalous neutron losses on the walls.     

Measurement of the Neutron Lifetime Using a Gravitational Trap and a Low-Temperature Fomblin Coating

We present a new value for the neutron lifetime of 878.5 ± 0.7_stat. ± 0.3_syst. This result differs from the world average value by 6.5 standard deviations and by 5.6 standard deviations from the previous most precise result. However, this new value for the neutron lifetime together with a β-asymmetry in neutron decay, A₀, of −0.1189(7) is in a good agreement with the Standard Model.     

粘结碳化硼材料中子屏蔽性能及制备工艺研究

中子准直器在中国散裂中子源(CSNS)的谱仪上起着限制束流截面与发散度的作用,准直器的结构设计中通过在中子飞行管上间隔设置B4C阻挡块可以起到阻挡、吸收屏蔽杂散中子的作用,为此研制了粘结碳化硼中子阻挡块,并对其成型工艺进行了研究,确定了成型最佳粉体尺寸、粘结剂含量和固化温度.对该中子屏蔽材料的中子衰减系数计算结果表明,波长为0.1×10-10、1×10-10和15×10-10m中子通过该材料的线性衰减系数分别为3.42、30.4和449.9 cm-1.力学冲击和热重实验结果也表明该材料符合在CSNS谱仪中子束线屏蔽使用中的性能要求.     

A new superleak to remove He for UCN experiments

The production of superleaks to remove He³ in helium for UCN experiments is described. Using one of these superleaks, He³/He⁴ ratio was found to be less than 3 × 10⁻⁹ as indicated by the UCN storage lifetime.