Fundamental Neutron Physics at Spallation Sources

Since their discovery, ultracold neutrons (UCNs) have been a unique tool for the investigation of fundamental properties of the free neutron and its interactions. The succesfull installation of a new pulsed superthermal source for ultracold neutrons based on solid deuterium at the TRIGA Mainz reactor is described. In a combination of solid hydrogen acting as pre-moderator and a solid deuterium converter of around 160 cm3, this new UCN source should provide at the experimental area ≥ 370000 UCN to the storage volume with the reactor operated in the pulse mode. In a storage experiment, a UCN density of 18 ± 2 UCN/cm3 was measured applying 1.5 $ pulses with a nominal power of 7 MJ. Assuming a linear behaviour of the UCN output as a function of reactor power this corresponds to a UCN density of 25 ± 3 UCN/cm3 for 2 $ (10 MJ) pulses.

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.

The era of ime domain and multi-messenger astronomy is not only leading to the development of a much broader set of detectors and instruments for astrophysical observations, but is also providing the means for astronomy to tie directly to cutting-edge studies in physics. In this manner, fundamental physics (theory and experiment) coupled with a strong theoretical understanding of astrophysical phenomena (guided by high-performance computing simulations) can tie directly to the amazing new observations in astronomy. This paper discusses how physics, astrophysical models, and observations can not only help astronomy probe fundamental physics but guide the needs for next-generation astrophysical missions.

A technique for establishing the total neutron rate of a highly-collimated monochromatic cold neutron beam was demonstrated using an alpha–gamma counter. The method involves only the counting of measured rates and is independent of neutron cross sections, decay chain branching ratios, and neutron beam energy. For the measurement, a target of 10B-enriched boron carbide totally absorbed the neutrons in a monochromatic beam, and the rate of absorbed neutrons was determined by counting 478 keV gamma rays from neutron capture on 10B with calibrated high-purity germanium detectors. A second measurement based on Bragg diffraction from a perfect silicon crystal was performed to determine the mean de Broglie wavelength of the beam to a precision of 0.024%. With these measurements, the detection efficiency of a neutron monitor based on neutron absorption on 6Li was determined to an overall uncertainty of 0.058%. We discuss the principle of the alpha–gamma method and present details of how the measurement was performed including the systematic effects. We also describe how this method may be used for applications in neutron dosimetry and metrology, fundamental neutron physics, and neutron cross section measurements.

We make the following points: First, if one is interested in obtaining (without moderation) low-energy neutrons in the energy range of say 0-100 eV, then the reaction Be/sup 9/ + omicron ..-->..Be/sup 8/ + n - 1.664 Mev is of no interest. Rather, one should concentrate on the reaction H/sup 2/ + omicron..-->..H/sup 1/ + n - 2.224 Mev. Second, if as a result of future advances in accelerator technology it becomes possible to produce intermediate energy (E about1 GeV), high current electron beams with both very high energy purity (..delta.. E/E<10/sup -5/) and low divergence (..delta..theta < 10/sup -6/), then it should be possible to produce MeV photon beams with similarly outstanding properties (as regards energy purity and angular divergence) by passing the electron beam through an electromagnetic undulator consisting of a high-power, pulsed wave of CO/sub 2/ laser radiation travelling counter to the electron beam. And third, such a narrow-band, highly-directional photon beam--when tuned to the proper energy above the photonuclear reaction threshold--should permit the generation of low-energy neutrons having a high degree of correlation between energy and angle of emission, provided that this correlation is not destroyed by the irreducible zero-point motion of the target atoms.

Recent experiments in neutron interferometry and optics at NIST

Quantum physics experiments in space using entangled photons and satellites are within reach of current technology. We propose a series of fundamental quantum physics experiments that make advantageous use of the space infrastructure with specific emphasis on the satellite-based distribution of entangled photon pairs. The experiments are feasible already today and will eventually lead to a Bell-experiment over thousands of kilometers, thus demonstrating quantum correlations over distances which cannot be achieved by purely earth-bound experiments.

Exact nuclear data uncertainty propagation for fusion neutronics calculations

This project encompassed fives years of effort in three areas of precision measurements with low energy neutrons: a) hadronic weak interactions, b) decay correlations, and c) electric dipole moment. These experiments encompass the scientific program of the Fundamental Neutron Physics Beamline (FnPB) at the SNS, ORNL. This grant supported our responsibilities in each of the approved experiments at the FnPB (in order of schedule): NPDGamma (post run), n-³He (taking data), Nab (Jan 2016), and nEDM, in addition to the BL3 beamline lifetime experiment at NIST. We focused on three principal activities: a) development of techniques for the design a fabrication of low-field precision surface current electromagnetic coils, b) development of high-speed digital acquisition systems involving real-time digital pulse processing, and high-volume offline waveform processing, and c) experimental data analysis.

Summary form only given. The transfer of ultra-stable frequencies between distant laboratories is a fundamental issue for a wide range of high-sensitivity experiments in advanced metrology and fundamental physics. Since a few years, several experiments of optical frequency transfer were reported over dedicated fiber, with a record distance of 920 km [1] recently demonstrated in Germany. The challenge is now to build a wide frequency dissemination network connecting laboratories and research agencies in Europe.For that purpose we exploit the existing fine meshing of the Internet fiber network, connecting laboratories via the National Research Networks (NRENs). In that scheme, the ultra-stable frequency signal is copropagating simultaneously with the Internet traffic in the same fiber using a dedicated channel in a dense wavelength division multiplexing (DWDM) approach. Optical add-drop multiplexers enable to insert and extract the ultra-stable frequency signal in and from the network.We have demonstrated an ultra-stable optical link over 540 km, including 470 km of public fiber network of the French NREN (RENATER) simultaneously carrying Internet data traffic [2]. The signal bypasses four amplification stages and two Data Center Facilities using multiplexers and custom-made bidirectional erbium-doped fiber amplifiers. One specific point of this experiment is that with Internet fibers, we have a very limited control on the attenuation and noise. For instance the Erbium fiber doped bidirectional amplifiers gain is drastically limited to avoid lasing oscillations due to parasitic reflections due to connectors, splices and more fundamental Rayleigh backscattering. Nevertheless the results are comparable with those obtained with dedicated fibers. Figure 1 shows the measured fractional frequency stability, we obtain 6×10-15 at 1 s and below 10-18 after 10000 second integration time. Such optical link will enable a broad range of high-sensitivity measurements, including the search for fundamental constants variation and gravitational mapping. This concept of fiber link has been very recently used to implement a giant fiber gyroscope [3]

Single crystal lithium ferrite (LiFe) spheres of sub-mm dimension are examined at mK temperatures, microwave frequencies, and variable dc magnetic field, for use in hybrid quantum systems and condensed matter and fundamental physics experiments. Strong coupling regimes of the photon-magnon interaction (cavity magnon polariton quasiparticles) were observed with coupling strength of up to 250 MHz at 9.5 GHz (2.6%) with magnon linewidths of order 4 MHz (with potential improvement to sub-MHz values). We show that the photon-magnon coupling can be significantly improved and exceed that of the widely used yttrium iron garnet crystal, due to the small unit cell of LiFe, allowing twice the spins per unit volume. Magnon mode softening was observed at low dc fields and, combined with the normal Zeeman effect, creates magnon spin-wave modes that are insensitive to first-order magnetic-field fluctuations. This effect is observed in the Kittel mode at 5.5 GHz (and another higher order mode at 6.5 GHz) with a dc magnetic field close to 0.19 tesla. We show that if the cavity is tuned close to this frequency, the magnon polariton particles exhibit an enhanced range of strong coupling and insensitivity to magnetic field fluctuations with both first-order and second-order insensitivity to magnetic field as a function of frequency (double magic point clock transition), which could potentially be exploited in cavity QED experiments.

Concerted efforts are underway to establish an infrastructure for a global quantum Internet to realise a spectrum of quantum technologies. This will enable more precise sensors, secure communications, and faster data processing. Quantum communications are a front‐runner with quantum networks already implemented in several metropolitan areas. A number of recent proposals have modelled the use of space segments to overcome range limitations of purely terrestrial networks. Rapid progress in the design of quantum devices have enabled their deployment in space for in‐orbit demonstrations. We review developments in this emerging area of space‐based quantum technologies and provide a roadmap of key milestones towards a complete, global quantum networked landscape. Small satellites hold increasing promise to provide a cost effective coverage required to realise the quantum Internet. The state of art in small satellite missions is reviewed and the most current in‐field demonstrations of quantum cryptography are collated. The important challenges in space quantum technologies that must be overcome and recent efforts to mitigate their effects are summarised. A perspective on future developments that would improve the performance of space quantum communications is included. The authors conclude with a discussion on fundamental physics experiments that could take advantage of a global, space‐based quantum network.

Promising experiments from the Moon using particle detectors are discussed, noting the advantage of the large flux collecting power Pc∼AΩt(m2‐sr‐years) offered by the remote, stable environment of a lunar base. An observatory class of particle experiments is presented, based upon proposals at NASA’s recent Stanford workshop. They vary from neutrino astronomy, particle astrophysics, and cosmic ray experiments to space physics and fundamental physics experiments such as proton decay and ‘‘table‐top’’ arrays. This research is background‐limited on Earth, and it is awkward and unrealistic in Earth orbit. It is particularly suited for the Moon where Pc can be quite large and the instrumentation is not subject to atmospheric erosion as it is (for large t) in low Earth orbit.

Capacitive transducers are widely used in fundamental physics experiments, seismology, Earth or planetary observations, and space scientific and technical applications because of their high precision, simple structure, and compatibility with various measurements. However, in real applications, there is a trade-off between their resolution and dynamic range. Therefore, this paper is aimed at enlarging the dynamic range while ensuring high resolution. In this paper, a noise analysis of a capacitive transducer is presented, which shows that the amplitude noise of the carrier wave is the main limiting factor. Hence, a new method of generating a carrier wave with lower-amplitude noise is proposed in the paper. Based on the experimental verification, it is found that the carrier wave produced through the new method performed significantly better than the typical digital carrier wave when they were compared in the same sensing circuit. With the carrier wave produced through the new method, the dynamic range of the capacitive transducer can reach 120.7 dB, which is 18.3 dB greater than for the typical direct digital synthesis (DDS) method. In addition, the resolution of the carrier wave is mainly limited by the voltage reference components.

The invention of the 3He/4He dilution refrigerator opened a new chapter in experimental ultra-low temperature physics. Dilution refrigerators became essential for providing ultra-low temperature environments for nuclear demagnetisation experiments, superconducting-qubit quantum processors and highly sensitive bolometers used in fundamental physics experiments. Development of dilution refrigeration technology requires thorough understanding of the quantum mechanical processes that take place in liquid helium at ultra-low temperatures. For decades the quantum fluids research community provided valuable information to engineers and designers involved in the development of advanced dilution refrigerators. However, the lack of methods that allow the measurement of physical parameters of liquid helium during the operation of a dilution refrigerator was hindering development of the technology. Here we show direct imaging of an operational dilution refrigerator using neutron radiography. This allows direct observation of the dilution process in 3He/4He mixtures and opens an opportunity for direct measurement of the 3He concentration. We observe the refrigerator behaviour in different regimes, such as continuous circulation and single shot, and show that our method allows investigation of various failure modes. Our results demonstrate that neutron imaging applied to the study of dilution refrigeration processes can provide essential information for developers of ultra-low temperature systems. We expect that neutron imaging will become instrumental in the research and development of advanced dilution refrigerators.