Session D05: Neutron Beams

Precision measurements of cold neutron flux using the Alpha-Gamma device

The Alpha-Gamma device at the National Institute of Standards and Technology utilizes the interaction of neutrons with a totally absorbing ¹⁰B target to precisely measure the flux of a monochromatic neutron beam.  This apparatus has been used to calibrate the ⁶Li(n,α)³H based flux monitor which the NIST neutron beam lifetime experiment BL2 used in its measurement of the most precise in-beam neutron lifetime to date.  The unique geometry and measurement technique of Alpha-Gamma allows for a competitive measurement of neutron interaction cross sections, such as the uranium-235 fission cross section (n,f).  A description of Alpha-Gamma design, recent improvements in the measurement of systematic effects, and further applications of this technique will be discussed.   

Summary of Recent Prompt Fission Neutron Spectra Measurements at LANSCE

Over the last decade, the Chi-Nu project has measured the Prompt Fission Neutron Spectra (PFNS) for neutron-induced fission of the major actinides, ^235,238U and ²³⁹Pu. These measurements have been carried out at the Los Alamos Neutron Science CEnter (LANSCE), over incident neutron energies from below 1 MeV to 20 MeV. The target foils and fission detectors were provided by LLNL, and two LANL neutron detector arrays were used to cover outgoing neutron energies from 10 keV to 20 MeV. The measurements will be described, and the resulting PFNS data will be presented. In addition, PFNS measurements have now been extended to ²⁴⁰Pu(n,f) and ^240,242Pu(sf). Comparisons among the PFNS of these isotopes will be discussed.

LA-UR-23-27458   

The Nab Experiment: Present Status

The Nab experiment, currently taking data at the Spallation Neutron Source, will precisely measure the neutron beta decay correlation coefficients $a$, relating the momenta of the produced electron and antineutrino, and $b$, the Fierz interference term. Precise extraction of these two parameters can probe physics beyond the Standard Model by investigating CKM unitarity and the existence of scalar and tensor currents in the weak interaction. The Nab experiment uses the world's largest cryogen-free superconducting magnet to guide electrons and protons from neutron decays into one of two silicon detectors. The electron energy and the difference in time of flight between electrons and protons in coincident decays can be used to determine $a$ and $b$. This talk will describe Nab's principles of operation and introduce some of the systematic considerations required for a precision measurement. We will discuss Nab's ongoing commissioning, and will provide a first look at neutron decay data taken during the most recent campaign.   

Characterization of Silicon Detectors at the Manitoba Proton Source for Low-Energy Particle Physics

The Neutron-a-b (Nab) experiment hosted at Oak Ridge National Laboratory’s Spallation Neutron Source is a sensitive probe of the weak interaction via free neutron beta decay. With a goal of 1.2⋅10^-3 relative precision on a and 3⋅10^-3 precision on b, this experiment will speak to the discrepancies in λ, the ratio of axial-vector to vector coupling, and the unitarity of the CKM matrix via a determination of Vud. Energy and timing of the decay electrons and protons are quantified with a pair of large-area, thick, segmented silicon detectors. The low–energy steerable proton source located at the University of Manitoba was used to simulate neutron decay protons, in tandem with some low-energy X-ray and beta sources. This talk will overview the capabilities of the facility, and the various studies performed to determine optimal detector running conditions within the Nab experiment.   

Compensated Ferrimagnetism Measurements and Exotic Polarized Neutron-Polarized Electron Interaction Searches in Terbium Iron Garnets

Rare earth iron garnets are known to be ferrimagnetic. In these materials, the magnetism associated with the electron spins undergoes a temperature-dependent orbital cancellation. This provides a dense ensemble of polarized electrons in an environment with no net magnetization and an opportunity to search for exotic interactions if the cancellation occurs at the unit cell level. We have studied the ferrimagnetic state of terbium iron garnet (Tb3Fe5O12) with neutron spin echo spectroscopy and neutron imaging to determine if the cancellation of magnetic moments occurs at the unit cell level or is the result of inhomogeneously distributed domains. The same techniques can be used to conduct searches for beyond-the-Standard Model spin-dependent neutron-electron interactions. We discuss the experimental apparatuses and results.   

Boosting the UCN density at the LANL UCN source using a low enriched uranium fission plate

The unique properties of ultracold neutrons (UCN) make them ideal for precision measurements of neutron properties. With high enough UCN densities, precision measurements can access Beyond the Standard Model (BSM) physics that rivals or surpasses capabilities of high energy physics[1] and other areas of nuclear physics[2] experiments. While measurements at UCN sources with the highest UCN densities[3] have begun to surpass the precision of those made with cold neutrons, discrepancies [4] have arisen that are difficult to resolve with our current sensitivities. Here we present the idea of using a low enriched uranium fission plate to boost the density of the UCN source [5] at Los Alamos National Laboratory (LANL). Preliminary simulations based on minimal modifications to the existing UCN source suggest that a gain of 3 to 10 is possible. The design does present some challenges and we will discuss how we can address these at LANL.

 

 

[1] Y. T. Chien, et al., JHEP 1602, 011 (2016)

[2] Hardy, J. C. & Towner, I. S., arXiv:1807.01146. (2018)

[3] Gonzalez, F. M. et al. Phys. Rev. Lett. 127, 162501 (2021)

[4] Castelvecci, D., Nature 598, 549 (2021) [5] T. M. Ito et al., Phys. Rev. C 97, 012501(R) (2018)

[5] T. M. Ito et al., Phys. Rev. C 97, 012501(R) (2018)   

Toward attaining near zero neutron beam polarization in the Nab experiment at SNS

High precision measurement of beta decay observables presents means to look for beyond standard model physics. Specifically, measurement of beta-neutrino correlation coefficient(a), in neutron beta decay can provide input for Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity condition. In this regard, the Nab experiment stationed at Spallation Neutron Source (SNS) in Oak Ridge National Lab aims to measure 'a' and Fierz interference term(b) in an unpolarized free neutron decay with the relative uncertainty of ∆a/a < 10⁻³ and ∆b/b < 3×10⁻³, respectively. One of the major sources of systematic uncertainty in such a measurement is the degree of unwanted polarization of the neutron beam. To reach the precision goal of Nab we must be able to restrict the unwanted polarization of the beam below 3×10⁻⁵, i.e, ∆P ≤ 3 × 10⁻⁵. To achieve the small beam polarization experimentally, we employ a ³He cell to measure the polarization of the SNS beam, and an Adiabatic Fast Passage(AFP) neutron spin flipper to reduce the beam polarization to the Nab requirements. In this talk, we present details on the process of doing this measurement and the results from the last measurement.   

Enabling the orbital degree of freedom for neutron experiments

Neutrons have been highly successful in precision measurements and experimental explorations of fundamental physics using the three easily accessible degrees of freedom: spin, path, and energy. Our recent work has enabled the use of orbital angular momentum in neutron beams, which essentially provides an additional quantized degree of freedom. We have demonstrated methods that are practical with the existing technologies and show the experimental achievement of neutron helical wavefronts that carry well-defined orbital angular momentum values. In this talk I will describe the work and several proposed applications for searches of new interactions with the orbital degree of freedom. We hope that these advances open the doors for the next generation of high impact neutron experiments.   

Precise measurement of 3He absorption cross sections for neutrons using the J-PARC pulsed neutron source

The decay lifetime of neutrons is an important parameter used in Big Bang elemental synthesis theory and in the unitarity calculation of the CKM matrix. Decay lifetimes have been measured by two methods, the UCN storage method and the proton count method, but there is a discrepancy of 9.5 s (4.5σ) between the results of each method. To solve this problem, lifetime measurements are being performed using a new method at J-PARC in Tokai-mura, Ibaraki Prefecture. This experiment aims to achieve a measurement accuracy of less than 0.1%.

In the above experiment, the 3He neutron absorption cross section is used to determine the neutron flux. The current value is 5333±7 barn, which is problematic because there are only two old measurement points. To update this value, a precise measurement of the absorption cross section was performed at J-PARC in June 2023 with a target accuracy of 0.06%.

Absorption cross sections are inversely proportional to neutron velocity. In order to measure absorption cross sections precisely, calibration experiments of the TOF method were performed at BL05 with a coupled moderator and at BL10 with an uncoupled moderator. Sharp spectra were obtained at BL10, and we decided to use BL10 as the beamline for the absorption cross section measurement experiments.

Absorption cross sections were measured at J-PARC BL10 using the transmission method. This method is based on counting the number of neutrons transmitted through a gas-filled cell and a vacuum cell.

 I report on these experiments.   

Measuring Silicon and Germanium Structure Factors with Pendellösung Interferometry

Dynamical diffraction describes waves inside perfect crystals when an incident neutron wave nearly satisfies the Bragg condition. The interference of these waves, called pendellösung, can be observed by the intensity modulation of the forward diffracted or reflected beams. Pendellösung can be used to determine structure factors (describing the interaction of the neutron with the unit cell) with relative uncertainties of 10^-5, to investigate interactions Beyond the Standard Model, measure the internal structure of the neutron, and provide information on thermal motion of the atoms in a lattice. While neutron-silicon structure factors have been measured for the (111), (220), and (400) reflections, quality data do not exist for high-order reflections and no data exists for germanium. Progress in measuring additional structure factors in silicon, measuring germanium structure factors, and systematic improvements are discussed. The pulsed neutron source at the VULCAN beamline located at ORNL’s SNS was used to pursue higher order reflections. Leveraging VULCAN to measure structure factors simultaneously will reduce systematic uncertainties associated with the previous experiment. Using the BT-8 Diffractometer at the NCNR to measure high-order reflections in silicon will also be discussed. These measurements will allow for the study of anharmonic contributions, increase the precision of the determined neutron charge radius, and provide further constraints on an atomic length scale fifth force.   

NCNR Neutron Interferometry Facilities Post Cold Source Upgrade

The NIST Center for Neutron Research (NCNR) operates a 20 MW reactor for material and nuclear science.  Perfect-crystal neutron interferometry (NI) has proven to be a precise technique for measuring the quantum mechanical phase of a neutron caused by a potential energy difference between two spatially separated neutron paths.   These paths can be separated in space by several centimeters allowing for precise control of the neutron’s wavefunction along each path using macroscopic elements.

What makes perfect-crystal interferometry a compelling technique is its simple-to-interpret results.  Perfect-crystal NI has been used to study nuclear physics, quantum information, orbital angular momentum, gravity, and place limits on cosmological models.    There are 2 NI facilities at the NCNR.   One of these focuses on high-precision phase measurements in a one-of-a-kind environmentally isolated enclosure. The second facility is based on advanced measurement techniques to suppress noise in a more accessible platform.  

The reactor is currently down for unplanned maintenance. Following the reactor returning to operations, the NCNR plans to undergo an upgrade of its cold source shifting the peak neutron intensity to slower neutrons. Impacts and changes to the neutron interferometry facilities at the NCNR post cold source will be discussed.