Neutron Lifetime Measurements Research Articles

Spin flip loss in magnetic confinement of ultracold neutrons for neutron lifetime experiments

We analyze the spin flip loss for ultracold neutrons in magnetic bottles of the type used in experiments aiming at a precise measurement of the neutron lifetime, extending the one-dimensional field model used previously by Steyerl $\textit{et al.}$ [Phys.Rev.C $\mathbf{86}$, 065501 (2012)] to two dimensions for cylindrical multipole fields. We also develop a general analysis applicable to three dimensions. Here we apply it to multipole fields and to the bowl-type field configuration used for the Los Alamos UCN$\tau$ experiment. In all cases considered the spin flip loss calculated exceeds the Majorana estimate by many orders of magnitude but can be suppressed sufficiently by applying a holding field of appropriate magnitude to allow high-precision neutron lifetime measurements, provided other possible sources of systematic error are under control.

Total cross sections for ultracold neutrons scattered from gases

We have followed up on our previous measurements of upscattering of ultracold neutrons (UCNs) from a series of gases by making measurements of total cross sections on the following gases hydrogen, ethane, methane, isobutene, n-butane, ethylene, water vapor, propane, neopentane, isopropyl alcohol, and He3. The values of these cross sections are important for estimating the loss rate of trapped neutrons due to residual gas and are relevant to neutron lifetime measurements using UCNs. The effects of the UCN velocity and path-length distributions were accounted for in the analysis using a Monte Carlo transport code. Results are compared to our previous measurements and with the known absorption cross section for He3 scaled to our UCN energy. We find that the total cross sections for the hydrocarbon gases are reasonably described by a function linear in the number of hydrogen atoms in the molecule.Received 12 July 2016DOI:https://doi.org/10.1103/PhysRevC.95.015501©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectroweak interactions in nuclear physicsNeutron physicsNucleon induced nuclear reactionsTechniquesNeutron scatteringNuclear Physics

Research of fundamental interactions with use of ultracold neutrons

Use of ultracold neutrons (UCN) gives unique opportunities of a research of fundamental interactions in physics of elementary particles. Search of the electric dipole moment of a neutron (EDM) aims to test models of CP violation. Precise measurement of neutron lifetime is extremely important for cosmology and astrophysics. Considerable progress in these questions can be reached due to supersource of ultracold neutrons on the basis of superfluid helium which is under construction now in PNPI NRC KI. This source will allow us to increase density of ultracold neutrons approximately by 100 times in respect to the best UCN source at high flux reactor of Institute Laue-Langevin (Grenoble, France). Now the project and basic elements of the source are prepared, full-scale model of the source is tested, the scientific program is developed. Increase in accuracy of neutron EDM measurements by order of magnitude, down to level 10-27 -10-28 e cm is planned. It is highly important for physics of elementary particles. Accuracy of measurement of neutron lifetime can be increased by order of magnitude also. At last, at achievement of UCN density ∼ 103 – 104 cm-3, the experiment search for a neutron-antineutron oscillations using UCN will be possible. The present status of the project and its scientific program will be discussed.

Neutron lifetime measurements and effective spectral cleaning with an ultracold neutron trap using a vertical Halbach octupole permanent magnet array

Ultracold neutron (UCN) storage measurements were made in a trap constructed from a 1.3 T Halbach Octupole PErmanent (HOPE) magnet array aligned vertically, using the TES-port of the PF2 source at the Institut Laue-Langevin. A mechanical UCN valve at the bottom of the trap was used for filling and emptying. This valve was covered with Fomblin grease to induce non-specular reflections and was used in combination with a movable polyethylene UCN remover inserted from the top for cleaning of above-threshold UCNs. Loss due to UCN depolarization was suppressed with a minimum 2 mT bias field. Without using the UCN remover, a total storage time constant of $(712 \pm 19)$ s was observed; with the remover inserted for 80 s and used at either 80 cm or 65 cm from the bottom of the trap, time constants of $(824 \pm 32)$ s and $(835 \pm 36)$ s were observed. Combining the latter two values, a neutron lifetime of $\tau_{\rm n} = (887 \pm 39)$ s is extracted after primarily correcting for losses at the UCN valve. The time constants of the UCN population during cleaning were observed and compared to calculations based on UCN kinetic theory as well as Monte-Carlo studies. These calculations are used to predict above-threshold populations of $\sim 5\%$, $\sim 0.5\%$ and $\sim 10^{-12}\%$ remaining after cleaning in the no remover, 80~cm remover and 65~cm remover measurements. Thus, by using a non-specular reflector covering the entire bottom of the trap and a remover at the top of the trap, we have established an effective cleaning procedure for removing a major systematic effect in high-precision $\tau_{\rm n}$ experiments with magnetically stored UCNs.

A new permanent-magnet trap for ultracold neutron storage

The design of a new permanent-magnet trap for ultracold neutron storage in an experiment on determining the neutron lifetime is presented. Particular attention is paid in this new trap design to the neutron confinement system. Optimization of this system provides an opportunity to raise the strength of the confinement magnetic field and thus increase considerably the number of neutrons stored within the trap and the statistical accuracy of neutron lifetime measurements.

LiNA – Lifetime of neutron apparatus with time projection chamber and solenoid coil

Among measurements of the neutron lifetime, there is a 1% difference between proton-counting and neutron-counting methods. In this paper, a new electron-counting method with a magnetic field aiming for a 0.1% accuracy is proposed, which would possibly allow to resolve the current discrepancy between the different neutron lifetime measurements.

Ultracold neutron detector for neutron lifetime measurements

The gas-filled detector of ultracold neutrons has been designed and constructed for the spectrometer of the neutron lifetime measurements at the ILL, Grenoble, France. The detector has been successfully tested and is currently being used at this spectrometer. We could show that minimization of the “wall” effect is a key factor to ensure efficient background suppression and to maximize the detection efficiency. This effect is primarily related to the composition of the gas mixture, which crucially depends on the neutron velocity spectrum.

20pAH-8 J-PARC/BL05における中性子寿命測定実験 : シミュレーションを用いたベータ崩壊事象の検出効率の評価

20pAH-8 J-PARC/BL05における中性子寿命測定実験 : シミュレーションを用いたベータ崩壊事象の検出効率の評価

Self-triggering readout system for the neutron lifetime experiment PENeLOPE

PENeLOPE is a neutron lifetime measurement developed at the Technische Universität München and located at the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) aiming to achieve a precision of 0.1 seconds. The detector for PENeLOPE consists of about 1250 Avalanche Photodiodes (APDs) with a total active area of 1225 cm2. The decay proton detector and electronics will be operated at a high electrostatic potential of −30 kV and a magnetic field of 0.6 T. This includes shaper, preamplifier, ADC and FPGA cards. In addition, the APDs will be cooled to 77 K. The 1250 APDs are divided into 14 groups of 96 channels, including spares. A 12-bit ADC digitizes the detector signals with 1 MSps. A firmware was developed for the detector including a self-triggering readout with continuous pedestal calculation and configurable signal detection. The data transmission and configuration is done via the Switched Enabling Protocol (SEP). It is a time-division multiplexing low layer protocol which provides determined latency for time critical messages, IPBus, and JTAG interfaces. The network has a n:1 topology, reducing the number of optical links.

Neutron lifetime measurement on setups with gravitational trap

Currently, the best accuracy of neutron lifetime measurements has been attained in the experiment with a gravitational trap for ultracold neutrons (UCNs), performed at the Petersburg Nuclear Physics Institute (PNPI); the measured lifetime was 878.5 ± 0.8 s. A new setup with a big gravitational trap has been designed to continue the methods and approaches used in the previous experiment. It is planned to reduce the measurement error to 0.2 s, i.e., improve the existing accuracy by a factor of 4. The spectrometer was designed at PNPI and installed on the PF2/MAM beam at the Institute Laue–Langevin. Test experiments have been performed.

20pAH-10 J-PARC/BL05における中性子寿命測定実験 : ^3He数密度と中性子数評価

20pAH-10 J-PARC/BL05における中性子寿命測定実験 : ^3He数密度と中性子数評価

Development of time projection chamber for precise neutron lifetime measurement using pulsed cold neutron beams

A new time projection chamber (TPC) was developed for neutron lifetime measurement using a pulsed cold neutron spallation source at the Japan Proton Accelerator Research Complex (J-PARC). Managing considerable background events from natural sources and the beam radioactivity is a challenging aspect of this measurement. To overcome this problem, the developed TPC has unprecedented features such as the use of polyether-ether-ketone plates in the support structure and internal surfaces covered with 6Li-enriched tiles to absorb outlier neutrons. In this paper, the design and performance of the new TPC are reported in detail.

Precision neutron flux measurement with a neutron beam monitor

Neutron beam monitors are regularly used in various neutron beam experiments to compare two or more sets of data taken in different experimental conditions. A neutron lifetime experiment at BL05, the NOP beamline, in J-PARC requires to monitor the initial neutron intensity with an precision of 0.1% to measure the neutron lifetime with the same accuracy. The performance of a thin 3He gas neutron beam monitor used for the experiment was studied to estimate the systematic uncertainties in the neutron lifetime measurement.

Improved Determination of the Neutron Lifetime

The most precise determination of the neutron lifetime using the beam method was completed in 2005 and reported a result of τ(n)=(886.3±1.2[stat]±3.2[syst]) s. The dominant uncertainties were attributed to the absolute determination of the fluence of the neutron beam (2.7 s). The fluence was measured with a neutron monitor that counted the neutron-induced charged particles from absorption in a thin, well-characterized 6Li deposit. The detection efficiency of the monitor was calculated from the areal density of the deposit, the detector solid angle, and the evaluated nuclear data file, ENDF/B-VI 6Li(n,t)4He thermal neutron cross section. In the current work, we measure the detection efficiency of the same monitor used in the neutron lifetime measurement with a second, totally absorbing neutron detector. This direct approach does not rely on the 6Li(n,t)4He cross section or any other nuclear data. The detection efficiency is consistent with the value used in 2005 but is measured with a precision of 0.057%, which represents a fivefold improvement in the uncertainty. We verify the temporal stability of the neutron monitor through ancillary measurements, allowing us to apply the measured neutron monitor efficiency to the lifetime result from the 2005 experiment. The updated lifetime is τ(n)=(887.7±1.2[stat]±1.9[syst]) s.

New installation for measuring a neutron lifetime with a big gravitational trap of ultra cold neutrons

At present the highest precision of neutron lifetime measurements has been achieved by the experiment made in Petersburg Nuclear Physics Institute (PNPI) with a gravitational trap of ultracold neutrons (UCN). A new installation with a big gravitational trap is an advanced development of methods and approaches applied in the previous experiment. We are planning to attain the measurement precision of 0.2 s which is four times better than the existing level of precision. A model of the experiemnt has been made for simulation by the Monte Carlo method. This model allows one to imply the concrete value of the neutron lifetime, then to reproduce an experiemental procedure and to see if there is any difference between the implied value and the measured one. As a result of modelling one has determined systematic uncertainty related to the procedure of calculating the efficient frequency of collisions of UCNs. It is equal to 0.1 s. We have also carried on modeling of different construction units of the installation.

Improvements in the characterization of actinide targets by low solid-angle counting

In a paper presented in the Proceedings of the 19th World Conference of the INTDS, 1998, the results of a blind comparison between groups at NIST and the IRMM showed excellent agreement in characterization of actinide targets by alpha-particle counting with low solid-angle geometry. At that time the most accurate NIST results estimated an uncertainty of 0.03 % for the solid-angle determination. The NIST Neutron Physics Group now has improved the accuracy of its solid-angle determination by about a factor of 4, based on an improved aperture design, improved metrology, and comparisons among independent metrology techniques at NIST. Applications of this improved technology are expected to include improved NIST measurements of the free neutron lifetime and a recalibration of the NIST standard for neutron emission rate, NBS-I.

Study of levitating nanoparticles using ultracold neutrons

Physical adsorption of atoms, molecules and clusters on the surface is well known. It is linked to many other phenomena in physics, chemistry and biology and has numerous practical applications. Due to limitations of the analytical tools usually used, studies of adsorption are limited to the particle sizes of up to ∼102–103 atoms. Following a general formalism developed in this field, we apply it to even larger objects and discover qualitatively new phenomena. A large particle is bound to the surface in a deep and broad potential well formed by van der Waals/Casimir–Polder forces appearing due to the particle and surface electric polarization. The well depth is significantly larger than the characteristic energy of a particle's thermal motion; thus such a nanoparticle is settled in long-living states. Nanoparticles in high-excited states form a two-dimensional gas of objects bound to the surface but quasi-freely traveling along the surface under certain conditions. A particularly interesting feature of this model consists in the prediction of small-energy-transfer inelastic scattering of ultracold neutrons (UCN) on solid/liquid surfaces covered by such levitating nanoparticles/nano-droplets. The change in UCN energy is due to the Doppler shift induced by UCN collisions with nanoparticles/nano-droplets; the energy change is almost as small as the UCN initial energy. We compared theoretical estimations of our model to all existing data on inelastic scattering of UCN with small energy transfers and found that they agree quite well. As our theoretical formalism provides robust predictions of some data and the experimental data are rather detailed and precise, we conclude that the recently discovered intriguing phenomenon of small heating of UCN in traps is due to their collisions with such levitating nanoparticles. Moreover, this new phenomenon might be relevant to the striking contradiction between the results of the neutron lifetime measurements and the smallest reported uncertainties, as it might cause major false effects in these experiments; thus, it affects fundamental conclusions concerning precision checks of unitarity of the Cabibbo–Kobayashi–Maskawa matrix, cosmology and astrophysics. Dedicated measurements of UCN inelastic scattering on specially prepared surfaces and nanoparticles/nano-droplets levitating above them might provide a unique method for studying surface potentials.

Colloquium: The neutron lifetime

The decay of the free neutron into a proton, electron, and antineutrino is the prototype semileptonic weak decay and is the simplest example of nuclear beta decay. It played a key role in the early Universe as it determined the ratio of neutrons to protons during the era of primordial light element nucleosynthesis. Neutron decay is physically related to important processes in solar physics and neutrino detection. The mean neutron lifetime has been the subject of more than 20 major experiments done, using a variety of methods, between 1950 and the present. The most precise recent measurements have stated accuracies approaching 0.1%, but are not in good agreement as they differ by as much as $5\ensuremath{\sigma}$ using quoted uncertainties. The history of neutron lifetime measurements is reviewed and the different methods used are described, giving important examples of each. The discrepancies and some systematic issues in the experiments that may be responsible are discussed, and it is shown by means of global averages that the neutron lifetime is likely to lie in the range of 880--884 s. Plans and prospects for future experiments are considered that will address these systematic issues and improve our knowledge of the neutron lifetime.

New evaluation of neutron lifetime from UCN storage experiments and beam experiments

The analysis of experiments on measuring neutron lifetime has been made. The latest most accurate result of measuring the neutron lifetime [Phys. Lett. B 605, 72 (2005)] 878.5±0.8s differs from the world average value [Phys. Lett. B 667, 1 (2008)] 885.7±0.8s by 6.5 standard deviations. In view of this both the analysis and the Monte Carlo simulation of experiments [Phys. Lett. B 483, 15 (2000)] and [Phys. Rev. Lett. 63, 593 (1989)] have been performed. Systematic errors about –6s have been found in both experiments. The table of results of neutron lifetime measurements is given after corrections and additions have been made. A new world average value of the neutron lifetime of 880.0±0.9s is given. Here is also presented a separate analysis of experiments on measuring neutron lifetime with UCN and with beam experiments. The average neutron lifetime for experiments with UCN is equal to 879.3(0.6) s, while for beam experiments it is equal to 889.1(2.9) s. The present difference of average values for both groups is 3.3σ and needs consideration. The contribution of beam experiments to the world average is not high, therefore it does not influence our analysis. However, it is an independent problem to be solved. It seems desirable that the precision of beam experiments should be enhanced.

Neutron lifetime from a new evaluation of ultracold neutron storage experiments

The analysis of the data on measurements of the neutron lifetime is presented. A new most accurate result of the measurement of neutron lifetime [Phys. Lett. B 605 (2005) 72] 878.5 +/- 0.8 s differs from the world average value [Phys. Lett. B 667 (2008) 1] 885.7 +/- 0.8 s by 6.5 standard deviations. In this connection the analysis and Monte Carlo simulation of experiments [Phys. Lett. B 483 (2000) 15] and [Phys. Rev. Lett. 63 (1989) 593] is carried out. Systematic errors of about -6 s are found in each of the experiments. The summary table for the neutron lifetime measurements after corrections and additions is given. A new world average value for the neutron lifetime 879.9 +/- 0.9 s is presented.