Decay times for second-chance fission of 239U studied by crystal blocking

Neutron-induced fission of ²³⁸U has been studied by the crystal-blocking technique for neutron energies just below and above the threshold for second-chance fission. In agreement with earlier measurements, in this energy range the lifetime for first-chance fission is found to be too short to have an observable effect on the blocking dips. Above the threshold, however, an appreciable filling-in of the dips is observed. The results are analyzed in terms of a two-component lifetime distribution and then indicate an average lifetime of a few fsec for second-chance fission at a neutron energy of E_n ≈ 7.2 MeV, in agreement with results from a simple calculation. It is shown that in this analysis it is important to take into account the anisotropy of the fission-fragment distribution and, in particular, the difference between the angular distributions for first- and second-chance fission.     

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.     

Neutron storage in a magnetic field

The motion of neutrons in magnetic traps is considered for various cases of neutron polarization. The results of implementing such traps in practice and special features of experiments studying magnetic neutron storage are discussed. The problem of neutron losses during injection via magnetic valves can be solved by conjoining a magnetic trap with a converter of cold neutrons into ultracold ones or with a source of ultracold neutrons. Prospects for expanding neutron-storage experiments by invoking a correlation analysis of neutron decay and by using the transport properties of charged particles in a nonuniform magnetic field are analyzed. In such an investigation, the recording of the storage time of neutrons proper can be supplemented with the detection of decay protons and electrons and with a parallel measurement of the asymmetries of proton and electron emission with respect to the magnetic field. A set of relative measurements permits improving the accuracy of an experimental determination of the neutron lifetime and combining this determination with the determination of correlation coefficients. On this basis, it is possible to find directly the ratio of the weak-interaction constants and the constants themselves. The application of the most advanced reactor and accelerator technologies to subcritical electric nuclear devices optimized for generating cold and ultracold neutrons, along with the use of solid deuterium and superfluid helium, creates preconditions for developing a neutron plant and for launching neutron studies at accelerators. Thus, the work that has been done as a development of V.V. Vladimirsky's proposals on magnetic neutron storage is analyzed, and the potential of a further use of ultracold neutrons and magnetic devices for deploying a full-scale precision experiment to study the beta decay of polarized neutrons is demonstrated.     

Measurement of the neutron lifetime using a proton trap

We report a new measurement of the neutron decay lifetime by the absolute counting of in-beam neutrons and their decay protons. Protons were confined in a quasi-Penning trap and counted with a silicon detector. The neutron beam fluence was measured by capture in a thin 6LiF foil detector with known absolute efficiency. The combination of these simultaneous measurements gives the neutron lifetime: tau(n)=(886.8+/-1.2[stat]+/-3.2[syst]) s. The systematic uncertainty is dominated by uncertainties in the mass of the 6LiF deposit and the 6Li(n,t) cross section. This is the most precise measurement of the neutron lifetime to date using an in-beam method.     

Options for the neutron lifetime measurements in traps

Different geometries for the neutron lifetime measurements by the method of ultracold neutron storage in material traps and additional possibilities for the neutron storage in the magnetic storage ring are considered.     

New measurement of the neutron lifetime with a large gravitational trap

The lifetime of the neutron is one of the key physical quantities used to determine the weak interaction parameters and to test predictions of the theory of primary nucleosynthesis. The lifetime of the neutron has been measured in the reported experiment by the method of storing neutrons in a material trap with a gravitational valve. Fomblin grease UT-18 hydrogen-free fluorine polymer has been used as coating. The resistance of the coating to repeated cooling down to 80 K combined with heating up to 300 K has been studied. The probability of losses in the trap is as small as 1.5% of the neutron decay probability. The lifetime of the neutron τ_n = (881.5 ± 0.7_stat ± 0.6_syst)s obtained at the new step is in good agreement with a commonly accepted value of (880.2 ± 1.0) s presented by the Particle Data Group.     

Ultracold neutron detector for the spectrometer of a neutron lifetime measuring

The gas-discharge detector is designed for the neutron lifetime spectrometer. The detector is intended for ultracold neutron flux monitoring in measurement cycles at the specrtometer (ILL, Grenoble, France). The detector has been successively tested with a Pu–Be neutron source under laboratory conditions and as a part of the spectrometer.     

Estimation of scalar and tensor weak-interaction constants on the basis of recent neutron experimental data

The contributions of scalar and tensor interactions associated with right-handed neutrinos to the effective hadron-lepton Hamiltonian are estimated on the basis of modern measurements of the neutron lifetime and correlations in the beta decay of a free neutron.     

Cosmic ray events in January 2005 from the measurements of multiple neutrons in a neutron monitor

Cosmic-ray events has been analyzed by the spectrographic method using measurements of neutron multiplicities in a neutron monitor. The calculations showed an increase in the exponent of the power spectrum of variations and the geomagnetic cutoff rigidity for ground-level enhancements and their insignificant reduction for Forbush decreases.     

Edge localized modes of cold neutrons in periodic condensed media

It is found that for certain energies of discreet cold neutrons, quasi-stationary eigen solutions of the corresponding Schrodinger equation, which are localized in the layer of a periodic medium, exist. The localization time of these solutions is strongly dependent on the layer thickness, being finite for a finite layer thickness and increasing indefinitely upon a infinite growth of the layer thickness as the third power of the layer thickness. The problem has been solved in the two-wave approximation of the dynamic diffraction theory for the neutron propagation direction coinciding with the periodicity axes (normal incidence of the neutron beam on the layer). The expressions for neutron eigenwave functions in a periodic medium, the reflection and transmission coefficients, and the neutron wavefunction in the layer as a function of the neutron energy incident on the layer have been determined. It turns out that for the certain discrete neutron energies, the amplitudes of the neutron wavefunction in the layer reach sharp maxima. The corresponding energies are just outside of the neutron stop band (energies forbidden for neutron propagation in the layer) and determine the energies of neutron edge modes (NEMs) localized in the layer, which are direct analogs of the optical edge modes for photonic crystals. The dispersion equation for the localized neutron edge modes has been obtained and analytically solved for the case of thick layers. A rough estimate for the localization length L is L ~(db N)^–1, where b is the neutron scattering length, d is the crystal period, and N is the density of nuclei in the crystal. The estimates of the localized thermal neutron lifetime show that acheaving of a lifetime close to the free neutron lifetime seems nonrealistic due to absorption of thermal neutrons and requires a perfect large size crystal. Nevertheless, acheaving the localized neutron lifetime exceeding by ~10⁴ times the neutron time of flight through the layer appears as experimentally attainable. The perspectives of the NEM observation are briefly discussed. It is proposed to use NEM for ultrahigh thermal neutron monochromatization by means of NEM excitation in perfect single crystals.     

ST嬗变堆中心柱的中子学及中子辐照效应研究

基于先进核数据库ENDF/B-VI和计算机程序对ST嬗变堆中心柱的中子学及辐照损伤的二维分析计算结果，分别对中心柱导体因中子辐照影响而引起电阻率、电阻、电流和欧姆电阻功率等的沿径向不均匀分布，以及辐照损伤对中心柱热工水力问题及更换寿命的影响进行了分析和计算。结果表明，中子辐照直接改变了中心柱导体材料的电阻率分布。热工-水力学分析和计算表明，电流不均匀分布可显著地延长中心柱的使用寿命，并估算出ST嬗变堆中心柱设计的更换寿命大约8年。     

Analysis of displacement damage effects on bipolar transistors irradiated by spallation neutrons

Displacement damage induced by neutron irradiation in China Spallation Neutron Source(CSNS) is studied on bipolar transistors with lateral PNP, substrate PNP, and vertical NPN configurations, respectively. Comparison of the effects on different type transistors is conducted based on displacement damage factor, and the differences are analyzed through minority carrier lifetime calculation and structure analysis. The influence of CSNS neutrons irradiation on the lateral PNP transistors is analyzed by the gate-controlled method, including the oxide charge accumulation, surface recombine velocity,and minority carrier lifetime. The results indicate that the total ionizing dose in CSNS neutron radiation environment is negligible in this study. The displacement damage factors based on 1-MeV equivalent neutron flux of different transistors are consistent between Xi'an pulse reactor(XAPR) and CSNS.     

Correlations between neutrons with small relative momenta produced in p-Pb interactions at 7.5 GeV/c

Correlations between neutrons with small relative momenta have been studied in p-Pb interactions at 7.5 GeV/c. Two-neutron final-state interaction effects carrying information about spacetime characteristics of a neutron source have been observed. The source lifetime has been found to decrease with increasing mean neutron kinetic energy. This fact is explained by the considerable contribution of the evaporation process to the emission of slow secondaries.     

Theoretical Analysis of Neutron Double-Differential Cross Section of n ＋^19F at 14.2 MeV

A new light nuclear reaction model has been developed and the double-differential measurements of lp shell nuclei have been analyzed successfully. Now, the application of this model is expanded to 19F of the 2s-ld shell nucleus. The double-differential cross section of total outgoing neutron for n ＋^19F reactions at En=14.2 MeV has been calculated and analyzed, which agrees fairly well with the experimental measurements. In this paper, the contributions from different reaction channels to the double-differential cross sections have been analyzed in detail. The calculations indicate that this light nudear reaction model is also able to be used for the 2s-ld shell nucleus so long as the related level scheme couM be provided sufficiently.     

用于中子测量的俘获探测器研究

为了在弱中子场和有限小空间内测量绝对裂变率, 制作了俘获探测器, 研究了俘获探测器的性能。 介绍了用于中子测量的俘获探测器和铅屏蔽室, 以及该探测器系统在特定条件下测量裂变反应率的结果, 并与裂变室测量结果进行了比较。 探讨了铅屏蔽室大小对测量结果的影响。 To detect the absolute neutron flux in a weak neutron field and restricted space, the fission fragment trapping detector was fabricated and the properties of the detector were studied. In this paper, the detector and shielding chamber used in neutron detection were described and the experimental measurements of the fission rate in specific condition were performed with the detection system and the result has been compared with that obtained by fission chamber. The influence of the shielding chamber on the measured results was analyzed.     

Experimental Setup for Neutron Lifetime Measurements with a Large Gravitational Trap at Low Temperatures

Technical Physics - Currently the most accurate measurements of neutron lifetime are being performed at the Petersburg Nuclear Physics Institute with ultracold neutrons (UCNs) stored in a...     

Measurement of the neutron spectrum of a Pu-C source with a liquid scintillator

The neutron response function for a BC501A liquid scintillator (LS) has been measured using a series of monoenergetic neutrons produced by the p-T reaction. The proton energies were chosen such as to produce neutrons in the energy range of 1 to 20 MeV. The principles of the technique of unfolding a neutron energy spectrum by using the measured neutron response function and the measured Pulse Height (PH) spectrum is briefly described. The PH spectrum of neutrons from the Pu-C source, which will be used for the calibration of the reactor antineutrino detectors for the Daya Bay neutrino experiment, was measured and analyzed to get the neutron energy spectrum. Simultaneously the neutron energy spectrum of an Am-Be source was measured and compared with other measurements as a check of the result for the Pu-C source. Finally, an error analysis and a discussion of the results are given.     

Neutron lifetime and density of states of fluoropolymers at low temperatures

Present status of the measurements of the neutron lifetime is shortly reviewed. We report the inelastic neutron scattering measurement of the density of vibrational states G(ω) of two fluoropolymers, which are promising coating materials for the storage of ultracold neutrons (UCNs) in closed volumes covered with polymer film. From determined G(ω), we calculate the expected UCN loss coefficients.     

Neutron lifetime and the background structure in a neutron magnetic trap

A new cycle of processing the 1985–1986 experimental data concerning the storage of neutrons in a magnetic gravitational trap was carried out. It was shown that the count rates determined for the background are multiples of the neutron decay constant. This is explained by the fact that the background in the experiment on the storage of ultracold neutrons was caused by the electron count from background neutrons penetrating through trap walls. Numerous measurements for a small and varying number of background neutrons in the trap make it possible to extract and use data for determining the mean neutron lifetime. This lifetime turns out to be τtn=900.01±0.15 s in a nonuniform magnetic field.     

Analysis and correction of the measurement of the neutron lifetime

Corrections have been introduced into the result τ_β = 885.4 ± 0.9_stat ± 0.4_syst s of our measurements of the neutron lifetime. The corrected value is τ_β = 881.6 ± 0.8_stat ± 1.9_syst s.