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F. E. Wietfeldt

Publications and source records attributed to F. E. Wietfeldt.

At least 19 recordsLinked to original sources
arXiv2026

Precision measurement of radiative neutron \b{eta}-decay: methodology and systematic effects

In the Standard Model the free neutron decays to a proton, an electron, and an antineutrino along with a continuous spectrum of photons. In 2016 the RDK II collaboration reported on a measurement of the photon energy spectrum and branching ratio over the range of 0.4 keV to the 782 keV endpoint using two different detector arrays. In the experiment, the radiative decay photons were observed in coincidence with the decay electrons and protons. In this paper, we present details of the analysis, including the determination of the systematic corrections and uncertainties and comparison of measured particle and photon energy spectra to Monte Carlo simulations. We conclude with approaches to improving the precision of these measurements.

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arXiv2025

Detection of molecular hydrogen in a neutron beam lifetime experiment

One method of determining the free neutron lifetime involves the absolute counting of neutrons and trapped decay protons. In such experiments, a cold neutron beam traverses a segmented proton trap inside a superconducting solenoid while the neutron flux is continuously monitored. Protons that are born within the fiducial volume of the trap are confined radially by the magnetic field and axially by the electrostatic potential supplied by trap electrodes. They are periodically released and counted, and the ratio of the absolute number of neutrons to protons is proportional to the neutron lifetime. Systematic error can be introduced if protons in the trap are lost, gained, or misidentified. The influence of molecular hydrogen interactions is of particular interest because of its ubiquitous presence in ultrahigh vacuum systems. To understand how it could affect the neutron lifetime, measurements were performed on the production and detection of molecular hydrogen in an apparatus used to measure the neutron lifetime. We demonstrate that charge exchange with molecular hydrogen can occur with trapped protons, and we determine the efficiency with which the molecular hydrogen ions in the trap are detected. Finally, we comment on the potential impact on a neutron lifetime experiment using this beam technique. We find that the result of the beam neutron lifetime performed at NIST is unlikely to have been significantly affected by charge exchange with molecular hydrogen.

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arXiv2024

Larmor Power Limit for Cyclotron Radiation of Relativistic Particles in a Waveguide

Cyclotron radiation emission spectroscopy (CRES) is a modern technique for high-precision energy spectroscopy, in which the energy of a charged particle in a magnetic field is measured via the frequency of the emitted cyclotron radiation. The He6-CRES collaboration aims to use CRES to probe beyond the standard model physics at the TeV scale by performing high-resolution and low-background beta-decay spectroscopy of ${}^6\textrm{He}$ and ${}^{19}\textrm{Ne}$. Having demonstrated the first observation of individual, high-energy (0.1 -- 2.5 MeV) positrons and electrons via their cyclotron radiation, the experiment provides a novel window into the radiation of relativistic charged particles in a waveguide via the time-derivative (slope) of the cyclotron radiation frequency, $\mathrm{d}f_\textrm{c}/\mathrm{d}t$. We show that analytic predictions for the total cyclotron radiation power emitted by a charged particle in circular and rectangular waveguides are approximately consistent with the Larmor formula, each scaling with the Lorentz factor of the underlying $e^\pm$ as $γ^4$. This hypothesis is corroborated with experimental CRES slope data.

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arXiv2024

Recoil-Order and Radiative Corrections to the aCORN Experiment

The aCORN experiment measures the electron-antineutrino $a$-coefficient in free neutron decay. We update the previous aCORN results to include radiative and recoil corrections to first order, and discuss a key issue in the comparison of results from different $a$-coefficient experimental methods when these effects are considered. The corrected combined result is $\overline{a} = -0.10779 \pm 0.00125\, ({\rm stat}) \pm 0.00133\, ({\rm sys})$, averaged over the full Fermi neutron beta spectrum. The corresponding corrected result for the ratio of weak coupling constants $λ= G_A/G_V$ is $λ= -1.2712 \pm 0.0061$. This improves agreement with previous $a$-coefficient experiments, in particular the 2020 aSPECT result

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arXiv2023

First observation of cyclotron radiation from MeV-scale ${\rm e}^{pm}$ following nuclear beta decay

We present an apparatus for detection of cyclotron radiation that allows a frequency-based beta energy determination in the 5 keV to 5 MeV range, characteristic of nuclear beta decays. The cyclotron frequency of the radiating beta particles in a magnetic field is used to determine the beta energy precisely. Our work establishes the foundation to apply the cyclotron radiation emission spectroscopy (CRES) technique, developed by the Project 8 collaboration, far beyond the 18-keV tritium endpoint region. We report initial measurements of beta^-s from 6He and beta^+s from 19Ne decays to demonstrate the broadband response of our detection system and assess potential systematic uncertainties for beta spectroscopy over the full (MeV) energy range. This work is an important benchmark for the practical application of the CRES technique to a variety of nuclei, in particular, opening its reach to searches for evidence of new physics beyond the TeV scale via precision beta-decay measurements.

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arXiv2023

Neutron interferometric measurement of the scattering length difference between the triplet and singlet states of n-$^3$He

We report a determination of the n-$^3$He scattering length difference $Δb^{\prime} = b_{1}^{\prime}-b_{0}^{\prime} = $ ($-5.411$ $\pm$ $0.031$ (statistical) $\pm$ $0.039$ (systematic)) fm between the triplet and singlet states using a neutron interferometer. This revises our previous result $Δb^{\prime} = $ (-5.610 $\pm$ $0.027$ (statistical) $\pm$ $0.032$ (systematic) fm obtained using the same technique in 2008. This revision is due to a re-analysis of the 2008 experiment that includes a more robust treatment of the phase shift caused by magnetic field gradients near the $^3$He cell. Furthermore, we more than doubled our original data set from 2008 by acquiring six months of additional data in 2013. Both the new data set and a re-analysis of the older data are in good agreement. Scattering lengths of low Z isotopes are valued for use in few-body nuclear effective field theories, provide important tests of modern nuclear potential models and in the case of $^3$He aid in the interpretation of neutron scattering from quantum liquids. The difference $Δb^{\prime}$ was determined by measuring the relative phase shift between two incident neutron polarizations caused by the spin-dependent interaction with a polarized $^3$He target. The target $^3$He gas was sealed inside a small, flat windowed glass cell that was placed in one beam path of the interferometer. The relaxation of $^3$He polarization was monitored continuously with neutron transmission measurements. The neutron polarization and spin flipper efficiency were determined separately using $^3$He analyzers and two different polarimetry analysis methods. A summary of the measured scattering lengths for n-$^3$He with a comparison to nucleon interaction models is given.

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arXiv2022

Comments on Systematic Effects in the NIST Beam Neutron Lifetime Experiment

We discuss issues raised by Serebrov, et al. in a recent paper regarding systematic effects in the beam neutron lifetime experiment performed at NIST. We show that these effects were considered in the original analyses and that our corrections and systematic uncertainties were appropriate. We point out some misconceptions and erroneous assumptions in the analysis of Serebrov, et al. None of the issues raised in Serebrov, et al lead us to alter the value of the neutron lifetime reported previously.

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arXiv2020

Measurement of the neutron decay electron-antineutrino angular correlation by the aCORN experiment

The aCORN experiment measures the neutron decay electron-antineutrino correlation ($a$-coefficient) using a novel method based on an asymmetry in proton time-of-flight for events where the beta electron and recoil proton are detected in delayed coincidence. We report the data analysis and result from the second run at the NIST Center for Neutron Research, using the high-flux cold neutron beam on the new NG-C neutron guide end position: $a = -0.10758 \pm 0.00136 (\mbox{stat}) \pm 0.00148 (\mbox{sys})$. This is consistent within uncertainties with the result from the first aCORN run on the NG-6 cold neutron beam. Combining the two aCORN runs we obtain $a = -0.10782 \pm 0.00124 (\mbox{stat}) \pm 0.00133 (\mbox{sys})$, which has an overall relative standard uncertainty of 1.7 \%. The corresponding result for the ratio of weak coupling constants $λ= G_A/G_V$ is $λ= -1.2796\pm 0.0062$.

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arXiv2020

Geometric Optimization of a Neutron Detector for In-Flight Measurement of the Neutron Lifetime

The recent measurement of the lifetime of the free neutron using the beam method has an 8.7 s (4$σ$) discrepancy with UCN measurements. The goal of the BL3 experiment is to improve the statistical error of this measurement and help rule out systematic uncertainties as an explanation for the discrepancy. A well-characterized neutron flux detector with flat response is essential, since the neutron flux enters linearly into the neutron lifetime. I will present a new detector geometry optimization with uniform acceptance up to sixth order in neutron position.

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arXiv2020

Neutron Polarimetry Using a Polarized 3He Cell for the aCORN Experiment

The neutron polarization of the NG-C beamline at the NIST Center for Neutron Research was measured as part of the aCORN neutron beta decay experiment. Neutron transmission through a polarized 3He spin filter cell was recorded while adiabatic fast passage (AFP) nuclear magnetic resonance (NMR) reversed the polarization direction of the 3He in an eight-step sequence to account for drifts. The dependence of the neutron transmission on the spin filter direction was used to calculate the neutron polarization. The time dependent transmission was fit to a model which included the neutron spectrum, and 3He polarization losses from spin relaxation and AFP-NMR. The polarization of the NG-C beamline was found to be ${\mid}P_\mathrm{n}{\mid} \leq 4\times 10^{-4}$ with 90 % confidence.

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arXiv2019

A Precision Measurement of the Neutron Scattering Length of He-4 Using Neutron Interferometry

We report a 0.08 % measurement of the bound neutron scattering length of $^4$He using neutron interferometry. The result is $b = (3.0982 \pm 0.0021 \mbox{ [stat]} \pm 0.0014 \mbox{ [sys]}) \mbox{ fm}$. The corresponding free atomic scattering length is $a = (2.4746 \pm 0.0017 \mbox{ [stat]} \pm 0.0011 \mbox{ [sys]}) \mbox{ fm}$. With this result the world average becomes $b = (3.0993 \pm 0.0025)$ fm, a 2 % downward shift and a reduction in uncertainty by more than a factor of six. Our result is in disagreement with a previous neutron interferometric measurement but is in good agreement with earlier measurements using neutron transmission.

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arXiv2018

Measurement of the electron-antineutrino correlation in neutron beta decay: aCORN experiment

The aCORN experiment uses a novel asymmetry method to measure the electron-antineutrino correlation (a-coefficient) in free neutron decay that does not require precision proton spectroscopy. aCORN completed two physics runs at the NIST Center for Neutron Research. The first run on the NG-6 beam line in 2013--2014 obtained the result a = 0.1090 +/- 0.0030 (stat) +/- 0.0028 (sys), a total uncertainty of 3.8%. The second run on the new NG-C high flux beam line promises an improvement in precision to <2%.

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arXiv2017

aCORN: an experiment to measure the electron-antineutrino correlation coefficient in free neutron decay

We describe an apparatus used to measure the electron-antineutrino angular correlation coefficient in free neutron decay. The apparatus employs a novel measurement technique in which the angular correlation is converted into a proton time-of-flight asymmetry that is counted directly, avoiding the need for proton spectroscopy. Details of the method, apparatus, detectors, data acquisition, and data reduction scheme are presented, along with a discussion of the important systematic effects.

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arXiv2017

The aCORN Backscatter-Suppressed Beta Spectrometer

Backscatter of electrons from a beta spectrometer, with incomplete energy deposition, can lead to undesirable effects in many types of experiments. We present and discuss the design and operation of a backscatter-suppressed beta spectrometer that was developed as part of a program to measure the electron-antineutrino correlation coefficient in neutron beta decay (aCORN). An array of backscatter veto detectors surrounds a plastic scintillator beta energy detector. The spectrometer contains an axial magnetic field gradient, so electrons are efficiently admitted but have a low probability for escaping back through the entrance after backscattering. The design, construction, calibration, and performance of the spectrometer are discussed.

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arXiv2016

Precision Measurement of the Radiative $\Beta$ Decay of the Free Neutron

The standard model predicts that, in addition to a proton, an electron, and an antineutrino, a continuous spectrum of photons is emitted in the $β$ decay of the free neutron. We report on the RDK II experiment which measured the photon spectrum using two different detector arrays. An annular array of bismuth germanium oxide scintillators detected photons from 14 to 782~keV. The spectral shape was consistent with theory, and we determined a branching ratio of 0.00335 $\pm$ 0.00005 [stat] $\pm$ 0.00015 [syst]. A second detector array of large area avalanche photodiodes directly detected photons from 0.4 to 14~keV. For this array, the spectral shape was consistent with theory, and the branching ratio was determined to be 0.00582 $\pm$ 0.00023 [stat] $\pm$ 0.00062 [syst]. We report the first precision test of the shape of the photon energy spectrum from neutron radiative decay and a substantially improved determination of the branching ratio over a broad range of photon energies.

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arXiv2014

The Neutron Lifetime

The decay of the free neutron into a proton, electron, and antineutrino is the prototype semileptonic weak decay and the simplest example of nuclear beta decay. The nucleon vector and axial vector weak coupling constants G_V and G_A determine the neutron lifetime as well as the strengths of weak interaction processes involving free neutrons and protons that are important in astrophysics, cosmology, solar physics and neutrino detection. In combination with a neutron decay angular correlation measurement, the neutron lifetime can be used to determine the first element of the CKM matrix Vud. Unfortunately the two main experimental methods for measuring the neutron lifetime currently disagree by almost 4 sigma. I will present a brief review of the status of the neutron lifetime and prospects for the future.

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arXiv2014

Determination of the Free Neutron Lifetime

We present the status of current US experimental efforts to measure the lifetime of the free neutron by the "beam" and "bottle" methods. BBN nucleosynthesis models require accurate measurements with 1 second uncertainties, which are currently feasible. For tests of physics beyond the standard model, future efforts will need to achieve uncertainties well below 1 second. We outline paths achieve both.

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