Neutron Transmission of Single-crystal Sapphire Filters

An additive formula is given that permits the calculation of the nuclear capture, thermal diffuse and Bragg scattering cross-sections as a function of sapphire temperature and crystal parameters. We have developed a computer program that allows calculations of the thermal neutron transmission for the sapphire rhombohedral structure and its equivalent trigonal structure. The calculated total cross-section values and effective attenuation coefficient for single-crystalline sapphire at different temperatures are compared with measured values. Overall agreement is indicated between the formula and experimental data. We discuss the use of sapphire single crystal as a thermal neutron filter in terms of the optimum cystal thickness, mosaic spread, temperature, cutting plane and tuning for efficient transmission of thermal-reactor neutrons.     

The behavior of HfB2 at neutron irradiation: a simulation study

The behavior of Hafnium di-boride (HfB₂) under neutron irradiation has been simulated in a wide range of energy from 0.025?eV up to 14?MeV. The simulation and the analysis have been carried out using Geant4 and its related database. From the radiation shielding perspective, it was observed that, under thermal neutron irradiation, HfB₂ scatters neutrons with a marginally higher energy than the incident neutrons and also produces prompt gamma rays up to 11?MeV. These results would indicate that, for high-energy neutron 14?MeV, not only is HfB₂ unacceptable as a reasonable neutron absorber but also produces 20?MeV prompt gamma rays.     

Absence of voids in fast neutron irradiated zirconium by small angle neutron scattering

Abstract

The aim of this research was to resolve a difference of opinion in the literature on the presence of voids in fast neutron irradiated zirconium. There is a great interest in the study of zirconium, since zirconium and its alloys are used extensively in modern power reactors, for example in the fuel rods as a containment material for enriched uranium. A polycrystalline sample of zirconium was irradiated in the HERALD reactor at 40°C with 10²⁰ fast neutrons per cm^?2. The neutron scattering from irradiated and unirradiated standard samples was studied over a wide Q range from 0.001 to 1.12 Å^?1 on a D11 Spectrometer at the ILL (France). The defect cross-section (the difference between the scattering of the standard zirconium crystal and irradiated crystal) was nearly flat as a function of Q (momentum transfer vector) with an average value of 8.5 mb/Str/atom. This indicates a point defect concentration of about 1.8%. Thus the absence of any small angle (Q dependent) defect scattering indicates that large damage regions (e.g. voids) are not produced in zirconium by fast neutron irradiation.     

Abnormal features of neutron-nucleus low-energy interaction and the mechanism of inhibition of neutron-induced reactions

The features of the distant interaction of thermal and slow neutrons with nuclei are examined. From the Dirac equation analysis it was shown that this interaction is determined by the potential barrier located outside the nucleus. The height of the barrier is proportional to Z ²/A ^4/3 and reaches 110eV for even-even nuclei like U²³⁸ and 25-65eV for even-odd U²³⁵ , U²³³ , Pu²⁴¹ nuclei. This barrier is connected with the non-linear ponderomotive interaction of the neutron abnormal magnetic moment with the strong electric field of a nucleus. The barrier penetrability for thermal neutrons equals 0.9-0.98. For cold neutrons the penetrability decreases greatly and for ultracold ones it becomes very small. At unlimited decrease of the neutron energy, E \( \rightarrow\) 0 , the cross-section of any neutron-nucleus reaction \( \sigma_{{f(tot)}}^{}\) \( \rightarrow\) 0 . So such reactions become impossible. In this work the existence of separated neutron potential wells symmetrically located at r ₀ \( \approx\) (1.3-4.5)×10^-12 cm \( \approx\) (1.7-5)×R from even-odd U²³⁵ , U²³³ , Pu²⁴¹ nuclei is predicted. These wells with depths 0.1-5eV are the result of combined ponderomotive and pure magnetic interactions of the neutron abnormal magnetic moment with nucleus electric and magnetic fields. The presence of distant wells leads to the possibility of the existence of virtual or quasi-stationary neutron-nucleus molecules. Such wells can be virtual traps for thermal and cold neutrons. It was predicted that the neutron halo phenomenon may be connected with such traps.     

A wide angle neutron spin filter system using polarized He

Polarized ³He neutron spin filters can operate over a wide neutron energy range and provide a large angular acceptance. A compact ³He neutron spin filter system has been developed for the Multi-Axis Crystal Spectrometer at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Sealed ³He cells, polarized by spin-exchange optical pumping, are used as polarizer and analyzer. The polarization of the neutrons incident on the sample is inverted by flipping the polarization of the ³He gas in the polarizer, with only a small effect on the analyzer cells. The cell fabrication process, ³He spin flipper, and the holding magnetic field are discussed and we present the results of a first on-linetest.     

Determination of Moisture Content in Coke with <Superscript>239</Superscript>Pu–Be Neutron Source and BGO Scintillation Gamma Detector

A series of experiments has been conducted at the Frank Laboratory of Neutron Physics (FLNP) of the Joint Institute for Nuclear Research (JINR) in order to study the possibility of determining the moisture content of coke using a standard neutron source. The proposed method is based on a measurement of the spectrum of prompt γ rays emitted when samples are irradiated by fast and/or thermal neutrons. The moisture content is determined from the area of the peaks of characteristic γ rays produced in the radiative capture of thermal neutrons by the proton (E_γ = 2.223 MeV) and inelastic scattering of fast neutrons by ¹⁶O (E_γ = 6.109 MeV). The ²³⁹Pu–Be neutron source (〈E _n 〉 ~ 4.5 MeV) with an intensity of ~5 × 10⁶ n/s was used to irradiate the samples under study. A scintillation detector based on a BGO crystal was used to register the characteristic γ radiation from the inelastic fast neutron scattering and slow (thermal) neutron capture. This paper presents the results of humidity measurement in the range of 2–50% [1, 2].     

Concept of Small-Angle Diffractometer in Classical Configuration at the Cold Moderator of the IBR-2 Reactor

The possible concepts whereby a time-of-flight small-angle diffractometer optimized for a neutron moderator operating in the cold (at 30 K) and thermal (at 300 K) modes can be implemented at the IBR-2 reactor are studied on the basis of numerical calculations. Under cold conditions, the peak of the neutronbeam energy spectrum is shifted toward low energies (long wavelengths). This extends the sensitivity range of the instrument with respect to the sizes of the objects under study (1–100 nm and higher). A classical scheme enabling the separation of thermal/cold neutrons (E ~10^–3–10^–2 eV) from the background (formed mainly by fast neutrons), which is based on bent neutron-optical devices, is discussed. Due to restrictions imposed by the geometry of the beamline within which the instrument is planned to be located, a configuration with a short multichannel mirror device for beam bending (beam bender) is preferable. Simulation and optimization of the proposed small-angle instrument is carried out taking into account the real beamline geometry and the available space in the experimental reactor hall. A comparison of the setup has been made with the facility based on the curved neutron guide and the facility with direct view of the moderator.     

Thermal neutron damage in CdS and CdTe

Photoluminescence and electrical resistivity changes in CdS and CdTe produced by thermal neutrons are discussed. The damage is produced principally by the neutron capture reaction ¹¹³Cd (n,y) ¹¹⁴Cd. Since the reaction product ¹¹⁴Cd is stable, complications arising from impurity introduction is minimal. The cumulative recoil nuclear recoil energy is about 143 eV, but is not the recoil energy at the time atomic displacement occurs. Thermal and fast neutrons enhance the CdS luminescence band at 7200A in the ratio of 28:1, but the resistivity changes are in the ratio of 40:1 Cd interstitial is suggested as the luminescence center. Hall measurements on n-type CdTe suggest that only Cd defects are produced for low thermal neutron doses. The acceptor introduction rate is about 1.0 to 0.6, compared to 0.098 for CdS. These are in good agreement with the values reported by R. O. Chester. The fast neutron effects in high resistivity CdS reported by Johnson indicate the need for further measurement.     

Strahlungseinfang langsamer Neutronen im thermischen und Resonanz-Bereich

Within the framework of the many-levelR-matrix formalism the theory ofLane andLynn allows one to calculate the partial cross sections for radiative capture of thermal and resonance energy neutrons. If the cross section for capture to a given final state of the product nucleus has been measured for a number of neutron energies in the thermal and resonance regions, then the variation of the above cross section with incident neutron energy can be completely determined. This calculation has been carried out for the reaction Mn⁵⁵(n, γ) Mn⁵⁶ for two distinct final states of Mn⁵⁶, where enough data are available to fix the parameters for resonance internal capture.     

Long-range alpha emission inP-wave neutron induced fission of235U

The yield and energy distribution of long-range alpha-particles (lra) emitted from neutron-induced fission of²³⁵U have been measured at neutron energies; thermal, 125±12, 155±11, 185±10, 210±9, 240±9, 365±50 and 480±45 keV. The long-range alpha-particles were detected in cellulose nitrate track detector foils. Results showed an increase of about 50% in the yield at neutron energies in the region 150 keV≤E _n≤220 keV as compared to that of thermal neutrons. A calculation has been carried out to extract thelra to binary fission ratio forp-wave neutron induced fission.     

Measurement of delayed-neutron yield from <Superscript>237</Superscript>Np fission induced by thermal neutrons

The delayed-neutron yield from thermal-neutron-induced fission of the ²³⁷Np nucleus was measured using a sample periodically exposed to a pulsed neutron beam with subsequent detection of neutrons during the time intervals between pulses. The experiment was realized on an Isomer-M setup mounted in the IBR-2 pulsed reactor channel equipped with a mirror neutron guide. The setup and the experimental procedure are described, the background sources are thoroughly analyzed, and the experimental data are presented. The total delayed-neutron yield from ²³⁷Np fission induced by thermal neutrons is ν _d = 0.0110 ± 0.0009. This study was performed at the Frank Laboratory of Neutron Physics (JINR, Dubna).     

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.     

墙壁反射中子对脉冲堆波形的影响

墙壁的反射中子会对快脉冲堆的波形产生明显的影响.堆芯中子泄漏后,经过墙壁的反射有一定的概率返回堆芯,由于能量的差异,泄漏中子的返回时间是一个连续的分布.传统的双区模型只考虑了相互作用概率,而没有时间信息,尽管可以很好地解决稳态问题,而无法解决瞬态问题.本文采用等效的方法,把泄漏中子等效为时间相关的堆芯本征源,建立了含有反射效应的时间关联双区模型.求解得到的脉冲波形与CFBR-Ⅱ的实验结果一致,从而合理解释了脉冲波形后沿衰减变慢和坪功率提高的实验现象.     

Radiation effect of neutrons produced by D–D side reactions on a D–3He fusion reactor

One of the most important characteristics in D–³He fusion reactors is neutron production via D–D side reactions. The neutrons can activate structural material, degrading them and ultimately converting them into high-level radioactive waste, while it is really costly and difficult to remove them. The neutrons from a fusion reactor could also be used to make weapons-grade nuclear material, rendering such types of fusion reactors a serious proliferation hazard. A related problem is the presence of radioactive elements such as tritium in D–³He plasma, either as fuel for or as products of the nuclear reactions; substantial quantities of radioactive elements would not only pose a general health risk, but tritium in particular would also be another proliferation hazard. The problems of neutron radiation and radioactive element production are especially interconnected because both would result from the D–D side reaction. Therefore, the presentation approach for reducing neutrons via D–D nuclear side reactions in a D–³He fusion reactor is very important. For doing this research, energy losses and neutron power fraction in D–³He fusion reactors are investigated. Calculations show neutrons produced by the D–D nuclear side reaction could be reduced by changing to a more ³He-rich fuel mixture, but then the bremsstrahlung power loss fraction would increase in the D–³He fusion reactor.     

Polarizing neutron by light‐irradiated graphene

We study the spin orientation of the neutron scattered by light‐irradiated graphene and calculate the average value of spin z‐component of the neutron in terms of a generating functional technique. Our calculation results indicate that there is a remarkable neutron polarization effect when a neutron penetrates graphene irradiated by a circularly polarized light. We analyse the dynamical source of generating this effect from the aspect of photon‐mediated interaction between the neutron spin and valley pseudospin. By comparing with the polarization induced by a magnetic field, we find that this polarization may be equivalent to the one led by a magnetic field of several hundred Teslas if the photon frequency is in the X‐ray frequency range. This provides an approach of polarizing neutrons.

     

Structure determination of Ls-threonine by neutron diffraction

The structure of the aminoacid, L_s-threonine [NH ₃ ⁺ CH(CHOHCH₃)COO^?], space groupP2₁2₁2₁,a=13.630(5),b=7.753(1),c=5.162(2) Å,z=4, has been determined from neutron diffraction data using direct methods. The intensities of 1148 neutron Bragg reflections were measured from a single crystal. The structural parameters were refined by the method of least squares using anisotropic temperature factors. The finalR(F ²) is 0.068. The structure was also refined from the x-ray data of Shoemakeret al (1950J. Am. Chem. Soc. 72 2328); there is good agreement between the two sets of heavy atom parameters. The parameters of hydrogen atoms are of course more precisely determined in our neutron study. The molecular conformation and the hydrogen bonding scheme are discussed. Weighted average values of bond distances and angles from 14 aminoacid structures with ionized carboxylic groups studied by neutron diffraction at Brookheven and Trombay are also presented.     

基于加速器的多终端硼中子俘获治疗装置的束流整形组件设计

硼中子俘获治疗(Boron Neutron Capture Therapy,BNCT)是一种新型的精准放射治疗方法,束流整形组件(Beam Shaping Assembly,BSA)作为硼中子俘获治疗装置的重要组成部分,对于产生适用于BNCT的中子束至关重要.通过BSA可以将快中子慢化到适当的能量范围,并且减少其他不需...     

Double neutron capture in62Ni

The -radiation following single and double neutron capture in isotopically enriched⁶²Ni was studied at the high flux reactor of the Institut Laue-Langevin, using a pair and Compton suppressed germanium detector. Measurements before and after 170 d of breeding were performed. The -ray fluxes through⁶³Ni and⁶⁴Ni are discussed; several new levels and spin-parity assignments were found. On the basis of the known discrete levels and the low-energy neutron resonances, level density parameters were determined within the Constant Temperature Fermi Gas model. The neutron binding energies were measured asB _n (⁶³Ni)=6837.92(18) keV andB _n (⁶⁴Ni)=9657.64(24) keV. The⁶³Ni (n, ) cross section for reactor neutrons was measured to be =20 _–2 ⁺⁵ b.Supported by Deutsches BMFT under contract 06 GÖ 141     

Effect of adding Ar gas on the pulse height distribution of BF<Subscript>3</Subscript>-filled neutron detectors

Boron trifluoride (BF₃) proportional counters are used as detectors for thermal neutrons. They are characterized by high neutron sensitivity and good gamma discriminating properties. Most practical BF₃ counters are filled with pure boron trifluoride gas enriched up to 96% ¹⁰B. But BF₃ is not an ideal proportional counter gas. Worsening of plateau characteristics is observed with increasing radius due to impurities in gas. To overcome this problem, counters are filled with BF₃ with an admixture of a more suitable gas such as argon. The dilution of BF₃ with argon causes a decrease in detection efficiency, but the pulse height spectrum shows sharper peaks and more stable plateau characteristics than counters filled with pure BF₃. The present investigations are undertaken to study the pulse height distribution and other important factors in BF₃+Ar filled signal counters for neutron beam applications. Tests are performed with detectors with cylindrical geometry filled with BF₃ gas enriched in ¹⁰B to 90%, and high purity Ar in different proportions. By analysing pulse height spectra, a value of 6.1 ± 0.2 has been obtained for the branching ratio of the ¹⁰B(n,α) reaction.