Storage Of Ultracold Neutrons Research Articles

Search for Neutron-to-Hidden-Neutron Oscillations in an Ultracold Neutron Beam.

Models that postulate the existence of hidden sectors address contemporary questions, such as the source of baryogenesis and the nature of dark matter. Neutron-to-hidden-neutron oscillations are among the possible mixing processes and have been tested with ultracold neutron storage and passing-through-wall experiments to set constraints on the oscillation period τ_{nn^{'}}. These searches probe the oscillations as a function of the mass splitting due to the neutron-hidden-neutron energy degeneracy. In this work, we present a new limit derived from neutron disappearance in ultracold neutron beam experiments. The overall limit, given by τ_{nn^{'}}>1 s for |δm|∈[2,69] peV(95.45% C.L.), covers the yet unexplored intermediate mass-splitting range and contributes to the ongoing research on hidden sectors.

A Novel Technique of Extracting UCN Decay Lifetime from Storage Chamber Measurements Dominated by Scattering Losses

The neutron’s lifetime is a critical parameter in the standard model. Its measurements, particularly measurements using both beamline and ultracold neutron storage techniques, have revealed significant tension. In this work, we review the status of the tension between various measurements, especially in light of the insights provided by the β-decay correlation measurements. We revisit the lifetime measurement in a material storage chamber, dominated by losses from scattering off the walls of the storage chamber. The neutron energy spectra and associated uncertainties were, for the first time, well-characterized using storage data alone. Such models have applications in the extraction of the mean time between wall bounces, which is a key parameter for neutron storage disappearance experiments in search of neutron oscillation. A comparison between the loss model and the number of neutrons stored in a single chamber allowed us to extract a neutron lifetime of τn*=880(+158/−78)stat.(+230/−114)sys.s (68.3% C.I.). Though the uncertainty of this lifetime is not competitive with currently available measurements, the highlight of this work is that we precisely identified the systematic sources of uncertainty that contribute to the neutron lifetime measurements in material storage bottles, namely from the uncertainty in the energy spectra, as well as from the storage chamber surface parameters of the Fermi potential and loss per bounce. In doing so, we highlight the underestimation of the uncertainties in the previous Monte Carlo simulations of experiments using the technique of ultracold neutron storage in material bottles.

Time-of-flight spectroscopy of ultracold neutrons at the PSI UCN source

The ultracold neutron (UCN) source at the Paul Scherrer Institute (PSI) provides high intensities of storable neutrons for fundamental physics experiments. The neutron velocity spectrum parallel to the beamline axis was determined by time-of-flight spectroscopy using a neutron chopper. In particular, the temporal evolution of the spectrum during neutron production and UCN storage in the source storage volume was investigated and compared to Monte Carlo simulation results. A softening of the measured spectrum from a mean velocity of 7.7(1)–5.1(1) hbox {m s}^{-1} occurred within the first 30 hbox {s} after the proton beam pulse had impinged on the spallation target. A spectral hardening was observed over longer time scales of one measurement day, consistent with the effect of surface degradation of the solid deuterium moderator.

Characterization of electroless nickel-phosphorus plating for ultracold-neutron storage

Electroless nickel plating is an established industrial process that provides a robust and relatively low-cost coating suitable for transporting and storing ultracold neutrons (UCN). Using roughness measurements and UCN-storage experiments we characterized UCN guides made from polished aluminum or stainless-steel tubes plated by several vendors.All electroless nickel platings were similarly suited for UCN storage with an average loss probability per wall bounce of 2.8 ⋅ 10-4 to 4.1 ⋅ 10-4 for energies between 90neV and 190neV, or a ratio of imaginary to real Fermi potential η of 1.7 ⋅ 10-4 to 3.3 ⋅ 10-4. Measurements at different elevations indicate that the energy dependence of UCN losses is well described by the imaginary Fermi potential. Some special considerations are required to avoid an increase in surface roughness during the plating process and hence a reduction in UCN transmission. Increased roughness had only a minor impact on storage properties.Based on these findings we chose a vendor to plate the UCN-production vessel that will contain the superfluid-helium converter for the new TRIUMF UltraCold Advanced Neutron (TUCAN) source, achieving acceptable UCN-storage properties with η=3.5(5)⋅10-4.

Ultracold neutron storage in a bottle coated with the fluoropolymer CYTOP

The fluoropolymer CYTOP was investigated in order to evaluate its suitability as a coating material for ultracold neutron (UCN) storage vessels. Using neutron reflectometry on CYTOP-coated silicon wafers, its neutron optical potential was measured to be 115.2(2) neV. UCN storage measurements were carried out in a 3.8 l CYTOP-coated aluminum bottle, in which the storage time constant was found to increase from 311(9) s at room temperature to 564(7) s slightly above 10 K. By combining experimental storage data with simulations of the UCN source, the neutron loss factor of CYTOP is estimated to decrease from 1.1(1)times 10^{-4} to 2.7(2)times 10^{-5} at these temperatures, respectively. These results are of particular importance to the next-generation superthermal UCN source SuperSUN, currently under construction at the Institut Laue-Langevin, for which CYTOP is a possible top-surface coating in the UCN production volume.

Ultracold neutron storage and transport at the PSI UCN source

Efficient neutron transport is a key ingredient to the performance of ultracold neutron (UCN) sources, important to meeting the challenges placed by high precision fundamental physics experiments. At the Paul Scherrer Institute’s UCN source we have been continuously improving our understanding of the UCN source parameters by performing a series of studies to characterize neutron production and moderation, and UCN production, extraction, and transport efficiency to the beamport. The present study on the absolute UCN transport efficiency completes our previous publications. We report on complementary measurements, namely one on the height-dependent UCN density and a second on the transmission of a calibrated quantity of UCN over a sim 16 m long UCN guide section connecting one beamport via the source storage vessel to another beamport. These allow us quantifying and optimizing the performance of the guide system based on extensive Monte Carlo simulations.

Superfluid helium film may greatly increase the storage time of ultracold neutrons in material traps

We propose a method to increase both the neutron storage time and the precision of its lifetime measurements by at least tenfold. The storage of ultracold neutrons (UCN) in material traps now provides the most accurate measurements of neutron lifetime and is used in many other experiments. The precision of these measurements is limited by the interaction of UCN with the trap walls. We show that covering trap walls with liquid helium may strongly decrease the UCN losses from material traps. $^4$He does not absorb neutrons at all. Superfluid He covers the trap walls as a thin film, $\sim 10$ nm thick, due to the van der Waals attraction. However, this He film on a flat wall is too thin to protect the UCN from their absorption inside a trap material. By combining the van der Waals attraction with capillary effects we show that surface roughness may increase the thickness of this film much beyond the neutron penetration depth $\sim 33$nm. Using liquid He for UCN storage requires low temperature $T<0.5$ K to avoid neutron interaction with He vapor, while the neutron losses because of the interaction with surface waves are small and can be accounted for using their linear temperature dependence.

Search for explanation of the neutron lifetime anomaly

All results of the neutron lifetime measurements performed in the last 20 years with the ultracold neutron storage method are in a good agreement. These results are also consistent with the latest most accurate measurements of the neutron decay asymmetry within the Standard Model. However, there is a significant discrepancy at the $3.6\ensuremath{\sigma}$ (1% of the decay probability) level between the averaged result of the storage method experiments and the most accurate measurements performed with the beam method. This article addresses the possible causes of the neutron lifetime discrepancy. We focused on finding the spectrum of possible systematic corrections in the beam experiment. Four separate sources of the systematic errors that had not been properly addressed in articles dedicated to the beam technique were considered. Two of those sources are related to the motion of protons in an electromagnetic field and the elastic scattering by the residual gas. These problems are associated with the geometrical configuration of the setup and the proton detector size. The Monte Carlo simulation shows that these effects are negligible and do not affect measurement results. The third error concerns proton loss in the dead layer of the detector. It is shown that the correction for the dead layer requires a more detailed analysis than that given in the papers describing the beam measurement method. The fourth source of the systematic error is the charge exchange process on the residual gas. The influence of the residual gas was neglected in the beam method experiments. We present arguments showing that careful analysis of this issue is necessary since the proposed proton losses correction decreases the measured lifetime, bringing it closer to the storage method results. For the problem of proton charge exchange on a residual gas, the spectrum of possible corrections is investigated. It is shown that for an accurate calculation of the correction, it is necessary to directly measure the concentration and composition of the residual gas inside the proton trap. The analysis reveals that even the presence of only ${\mathrm{H}}_{2}$ molecules inside the proton trap can lead to the significant correction, which is the most probable explanation of the neutron anomaly.

A search for neutron to mirror-neutron oscillations using the nEDM apparatus at PSI

It has been proposed that there could be a mirror copy of the standard model particles, restoring the parity symmetry in the weak interaction on the global level. Oscillations between a neutral standard model particle, such as the neutron, and its mirror counterpart could potentially answer various standing issues in physics today. Astrophysical studies and terrestrial experiments led by ultracold neutron storage measurements have investigated neutron to mirror-neutron oscillations and imposed constraints on the theoretical parameters. Recently, further analysis of these ultracold neutron storage experiments has yielded statistically significant anomalous signals that may be interpreted as neutron to mirror-neutron oscillations, assuming nonzero mirror magnetic fields. The neutron electric dipole moment collaboration performed a dedicated search at the Paul Scherrer Institute and found no evidence of neutron to mirror-neutron oscillations. Thereby, the following new lower limits on the oscillation time were obtained: τnn′>352 s at B′=0 (95% C.L.), τnn′>6s for 0.4μT<B′<25.7μT (95% C.L.), and τnn′/cos⁡β>9s for 5.0μT<B′<25.4μT (95% C.L.), where β is the fixed angle between the applied magnetic field and the local mirror magnetic field, which is assumed to be bound to the Earth. These new constraints are the best measured so far around B′∼10μT and B′∼20μT.

A new neutron lifetime experiment with cold neutron beam decay in superfluid helium-4

The puzzle remains in the large discrepancy between neutron lifetime measured by the two distinct experimental approaches—counts of beta decays in a neutron beam and storage of ultracold neutrons in a potential trap, namely, the beam method versus the bottle method. In this paper, we propose a new experiment to measure the neutron lifetime in a cold neutron (CN) beam with a sensitivity goal of 0.1% or sub-1 s. The neutron beta decays will be counted in a superfluid helium-4 scintillation detector at 0.5 K, and the neutron flux will be simultaneously monitored by the helium-3 captures in the same volume. The CN beam must be of wavelength λ > 16.5 Å to eliminate scattering with superfluid helium. A new precise measurement of neutron lifetime with the beam method of unique inherent systematic effects will greatly advance in resolving the puzzle.

A Search for Neutron to Mirror-Neutron Oscillations

It has been proposed that there could be a mirror copy of the standard model particles, restoring the parity symmetry in the weak interaction on the global level. Oscillations between a neutral standard model particle, such as the neutron, and its mirror counterpart could potentially answer various standing issues in physics today. Astrophysical studies and terrestrial experiments led by ultracold neutron storage measurements have investigated neutron to mirror-neutron oscillations and imposed constraints on the theoretical parameters. Recently, further analysis of these ultracold neutron storage experiments has yielded statistically significant anomalous signals that may be interpreted as neutron to mirror-neutron oscillations, assuming nonzero mirror magnetic fields. The neutron electric dipole moment collaboration performed a dedicated search at the Paul Scherrer Institute and found no evidence of neutron to mirror-neutron oscillations. Thereby, the following new lower limits on the oscillation time were obtained: $\tau_{nn'} > 352~$s at $B'=0$ (95% C.L.), $\tau_{nn'} > 6~\text{s}$ for all $0.4~\mu\text{T} 9~\text{s}$ for all $5.0~\mu\text{T}<B'<25.4~\mu\text{T}$ (95% C.L.), where $\beta$ is the fixed angle between the applied magnetic field and the local mirror magnetic field which is assumed to be bound to the Earth. These new constraints are the best measured so far around $B'\sim10~\mu$T, and $B'\sim20~\mu$T.

Project on searching for neutron-antineutron oscillation at the WWR-M reactor

Supersource of ultracold neutrons on the basis of superfluid helium is under construction in PNPI NRC KI. It must provide UCN density 2-3 orders of magnitude higher than existing sources. For the new source we propose an experiment on search for neutron–antineutron oscillations based on the storage of ultracold neutrons in a material trap. The sensitivity of the experiment mostly depends on the trap size and the amount of UCN in it. The geometric configuration of the experimental setup was developed based on the MC simulations of neutrons trajectories into the trap. To calculate detector efficiency the GEANT4 model of the detector was created.

Disagreement between measurements of the neutron lifetime by the ultracold neutron storage method and the beam technique

Recent measurements of the neutron lifetime performed using a gravitational trap of ultracold neutrons (UCNs) (Konstantinov Petersburg Nuclear Physics Institute, Russia) and a magnetic UCN trap (LANL, USA) have confirmed PNPI’s result of 2005. The results of experiments with stored UCNs agree with each other but differ from those of the beam experiment (NIST, USA) by 3.5σ (corresponding to 1% in the decay probability). This disagreement is currently being discussed in the literature as a ‘neutron anomaly’. We analyze possible reasons for that disagreement and test the experimental data for the neutron lifetime and beta-decay asymmetry within the Standard Model. The test is only passed for neutron lifetime values that are obtained using the UCN storage method.

Specifics of Preparation of Storage Chambers for Ultracold Neutrons and EDM Spectrometer Sensitivity

Storage cells for ultracold neutrons (UCNs) are crucial parts of the experimental setup used to measure the electric dipole moment (EDM) of a neutron. The effect of the technology of preparation and sputtering of elements of UCN storage cells on the limiting operating voltage and on the capacity of these cells for long-term operation under high voltage with polarity switching was studied. A voltage magnitude of ±250 kV was obtained in tests of a neutron storage cell for the EDM spectrometer with a cell height of 9 cm.

Monte Carlo simulations of a neutron lifetime experiment with a big gravitational trap

A Monte Carlo model was developed for our current neutron lifetime experiment using storage of ultracold neutrons (UCN) in a big gravitational trap. The model allows us to calculate neutron trajectories in the given geometry, taking into account gravity, and has been used in simulations that reproduce all stages of the experiment. The simulation was proven able to reproduce the time-dependence of the detected neutron count rates. For investigation of systematic effects, the value of the neutron lifetime resulting from the simulation of the experimental procedure was compared to the value entered as an input parameter. In particular, the systematic uncertainty associated with the method of calculating the effective UCN collision frequency in the trap was found not to exceed 0.1 s. In addition, calculations showed that up to 1% uncoated area of the copper trap has a negligible influence on the neutron lifetime result.

Plan of NNbar experiment at the WWR-M reactor

We propose an experiment to search for neutron–antineutron oscillations based on the storage of ultracold neutrons (UCN) in a material trap. This experiment will be competitive due to a new UCN source which is under construction at the NRC “Kurchatov Institute” – PNPI. It is expected to provide UCN densities by 2–3 orders of magnitude higher than at existing sources. The sensitivity of our experiment mostly depends on the trap size and the stored number of UCN. Simulations show that this can be increased by ∼ 10–40 times compared to the best previous experiment, dependent upon a model of neutron reflection from walls. The project currently includes designs of the UCN chamber, magnetic shielding and the detector system, as well as simulations of neutron storage and detection of annihilations events.

Особенности подготовки камер хранения ультрахолодных нейтронов и чувствительность ЭДМ-спектрометра

AbstractStorage cells for ultracold neutrons (UCNs) are crucial parts of the experimental setup used to measure the electric dipole moment (EDM) of a neutron. The effect of the technology of preparation and sputtering of elements of UCN storage cells on the limiting operating voltage and on the capacity of these cells for long-term operation under high voltage with polarity switching was studied. A voltage magnitude of ±250 kV was obtained in tests of a neutron storage cell for the EDM spectrometer with a cell height of 9 cm.

Measurements of the Neutron Lifetime

Free neutron decay is a fundamental process in particle and nuclear physics. It is the prototype for nuclear beta decay and other semileptonic weak particle decays. Neutron decay played a key role in the formation of light elements in the early universe. The precise value of the neutron mean lifetime, about 15 min, has been the subject of many experiments over the past 70 years. The two main experimental methods, the beam method and the ultracold neutron storage method, give average values of the neutron lifetime that currently differ by 8.7 s (4 standard deviations), a serious discrepancy. The physics of neutron decay, implications of the neutron lifetime, previous and recent experimental measurements, and prospects for the future are reviewed.

Разногласие между методом хранения ультрахолодных нейтронов и пучковым методом при измерении времени жизни нейтрона

Разногласие между методом хранения ультрахолодных нейтронов и пучковым методом при измерении времени жизни нейтрона

Experimental Investigation into the Low-Molecular-Weight Fluoropolymer for the Storage of Ultracold Neutrons

The experimental setup for examining the low-molecular-weight fluoropolymer CF$_{3}$(CF$_{2})_{3}$-O-CF$_{2}$-O-(CF$_{2})_{3}$CF$_{3}$, which is a promising coating material for the walls of storage chambers for ultracold neutrons, is described. The results are detailed. The measurement data are interpreted in the model of a multilayer complex quantum-mechanical potential of the chamber walls.