An intermediate energy neutron source

A neutron source having an average energy of 0.532 keV has been established using a SbBe source in a polythene moderator and a B₄C absorber assembly. The source strength ratio of the moderated to the bare source has been accurately measured using a precision long counter and agrees well with the theoretical value obtained by a Monte Carlo code.     

PKUNIFTY: A neutron imaging facility based on an RFQ accelerator

A neutron imaging facility, PKUNIFTY, based on a radio frequency quadrupole (RFQ) accelerator-driven compact neutron source, presently under construction at the Peking University, is described. It consists of a deuteron linear accelerator, a neutron target-moderator-reflector assembly, and a thermal neutron imaging system. Neutrons are generated via the deuteron-beryllium reaction with an expected fast-neutron yield of 3×10¹² n/s. The thermal neutron flux on the imaging plane is 5×10⁵ n/cm²/s at a nominal L/D ratio of 50, and the L/D ratio can be selectable over a range of 25-200. The corresponding n/γ ratio is close to or higher than 1×10¹⁰ n/cm²/Sv. The field of view is 20 cm×20 cm at 2 m downstream of the collimator entrance aperture where the thermal neutron flux uniformity is better than 7%. The effective Cd ratio can be tuned by using a light chopper and neutron time-of-flight technique without excessive sacrifice of the thermal neutron flux.     

Preliminary design of a Gd-NCT neutron beam based on compact D-T neutron source

Gadolinium has been recently proposed, as neutron capture agent in NCT (Neutron Capture Therapy), due to both the nuclide high neutron capture cross section, and the remarkable selective uptake inside tumour tissue that Gd-loaded compounds, can provide. When a neutron external source is supplied, different Gd nuclear reactions, and the generated Auger electrons in particular, cause a high local energy deposition, which results in a tumour cell inactivation. Preliminary micro- as well as macrodosimetric Monte Carlo computational investigations show that the tumour-to-healthy tissue biological damage ratio is in close relation to the neutron beam energy spectrum. The results points out that the optimum neutron spectrum, to be used for Gd-NCT, seems to lie in the 1 to 10 keV energy range. In order to 'tailor' such spectra, an original, accelerator-driven, neutron source and spectrum shaping assembly for hospital-based Gd-NCT are presented and preliminary results are reported.     

An approximate analysis of steady state axisymmetric deformations of viscoplastic targets

Steady state axisymmetric deformations of a viscoplastic target being penetrated by a rigid cylindrical penetrator with a hemispherical nose are analyzed. The presumed kinematically admissible velocity field satisfies all of the boundary conditions on the target/penetrator interface, and also the balance of mass. The unknown parameters appearing in the admissible velocity field are found by minimizing the error in satisfying the balance of linear momentum. The solution so obtained is found to be very close to the finite element solution of the problem. An advantage of the present technique is the enormous savings in the computational effort and resources required to analyze the problem.     

一种提高稳态误差及收敛速度的回声消除算法

以研究回声消除为背景,在分析比较了三种LMS自适应滤波器算法的基础上,提出了改进的LMS自适应滤波器算法。并利用Matlab进行仿真分析,通过绘制误差曲线、学习曲线,证明了改进后的算法在稳态误差以及收敛速度方面都优于之前的算法。该算法能较好地应用到IP电话中的回声消除的自适应滤波问题。     

The steady state in conducting systems

The author examines a system through which passes a flux of a certain extensive quantity created by two constant sources. The properties and the characteristic steady state of this system are studied, as well as the process of transition to the steady state.     

Crack propagation during steady state creep

A model is proposed tor the steady propagation of a creep crack under steady state creep conditions. Creep is envisaged to take place everywhere in the solid, though higher creep rates at the crick tip lead to a local concentration of the creep strain. A critical local strain criterion is used to describe the condition for crack advance. Local damage is envisaged to accumulate at the crack tip as a result of, or in parallel with the creep strain. The model correctly predicts a dependence of crack propagation rate with the nett section stress varied to the power m, where m is the exponent of stress in the creep equation, for large values of m. An approximate dependence of propagation rate on the elastic stress intensity factor is also shown. Tnese predictions are in accord with experimental work.     

Galerkin based generalized ANOVA for the solution of stochastic steady state diffusion problems

This work presents a novel approach, referred here as Galerkin based generalized analysis of variance decomposition (GG-ANOVA), for the solution of stochastic steady state diffusion problems. The proposed approach utilizes generalized ANOVA (G-ANOVA) expansion to represent the unknown stochastic response and Galerkin projection to decompose the stochastic differential equation into a set of coupled differential equations. The coupled set of partial differential equations obtained are solved using finite difference method and homotopy algorithm. Implementation of the proposed approach for solving stochastic steady state diffusion problems has been illustrated with three numerical examples. For all the examples, results obtained are in excellent agreement with the benchmark solutions. Additionally, for the second and third problems, results obtained have also been compared with those obtained using polynomial chaos expansion (PCE) and conventional G-ANOVA. It is observed that the proposed approach yields highly accurate result outperforming both PCE and G-ANOVA. Moreover, computational time required using GG-ANOVA is in close proximity of G-ANOVA and less as compared to PCE.     

Water-extended polyester neutron shield for a 252Cf neutron source

A Monte Carlo study to determine the shielding features to neutrons of water-extended polyester was carried out. During calculations, (252)Cf and shielding were modelled and the neutron spectra as well as the H(10) were calculated in four sites. The calculation was extended to include a water shielding, the source in vacuum and in air. Besides neutron shielding characteristics, the Kerma in air due to gammas emitted by (252)Cf and due to capture gamma rays in the shielding were included.     

A general steady state mathematical model for fin-and-tube heat exchanger based on graph theory

Fin-and-tube heat exchangers are widely used in air conditioners, chillers, etc. A lot of factors, including arrangement of refrigerant circuits, configure specification of fins and tubes, and operating conditions, have significant influence on the performance of fin-and-tube heat exchangers. For the purpose of fast design of high performance heat exchangers, a simulator reflecting the influence of these factors is necessary. In this paper, a general steady state mathematic model based on the graph theory is presented. With the help of the directed graph and graph-based traversal methods (Breadth-first search and Depth-first search), this model is capable to describe any flexible refrigerant circuit arrangement, and quantify the refrigerant distribution in the refrigerant circuit and heat conduction through fins. An alternative iteration method is also developed to solve the conservation equations, which can shorten the simulating time effectively. The model is verified with the experimental results, and the maximum error is within ±10.0%. A simulator based on this model has been used for designing practical fin-and-tube heat exchangers.     

Field ion source development for neutron generators

An ion source based on the principles of electrostatic field desorption is being developed to improve the performance of existing compact neutron generators. The ion source is an array of gated metal tips derived from field electron emitter array microfabrication technology. A comprehensive summary of development and experimental activities is presented. Many structural modifications to the arrays have been incorporated to achieve higher tip operating fields, while lowering fields at the gate electrode to prevent gate field electron emission which initiates electrical breakdown in the array. The latest focus of fabrication activities has been on rounding the gate electrode edge and surrounding the gate electrode with dielectric material. Array testing results have indicated a steady progression of increased array tip operating fields with each new design tested. The latest arrays have consistently achieved fields beyond those required for the onset of deuterium desorption (∼20 V/nm), and have demonstrated the desorption of deuterium at fields up to 36 V/nm. The number of ions desorbed from an array has been quantified, and field desorption of metal tip substrate material from array tips has been observed for the first time. Gas-phase field ionization studies with ∼10,000 tip arrays have achieved deuterium ion currents of ∼50 nA. Neutron production by field ionization has yielded ∼10² n/s from ∼1 mm² of array area using the deuterium-deuterium fusion reaction at 90 kV.     

Fusion neutron detector calibration using a table-top laser generated plasma neutron source

Using a high intensity, femtosecond laser driven neutron source, a high-sensitivity neutron detector was calibrated. This detector is designed for observing fusion neutrons at the Z accelerator in Sandia National Laboratories. Nuclear fusion from laser driven deuterium cluster explosions was used to generate a clean source of nearly monoenergetic 2.45 MeV neutrons at a well-defined time. This source can run at 10 Hz and was used to build up a clean pulse-height spectrum on scintillating neutron detectors giving a very accurate calibration for neutron yields at 2.45 MeV.     

An iterative algorithm for singular Cauchy problems for the steady state anisotropic heat conduction equation

In this paper, we propose an alternating iterative algorithm to solve a singular Cauchy problem for the anisotropic heat conduction equation. The numerical algorithm is based on the boundary element method (BEM), modified to take into account the form of the singularity, without substantially increasing the amount of computation involved. Two test examples, the first with a singularity caused by an abrupt change in the boundary conditions and the second with a singularity caused by a sharp re-entrant corner, are investigated. The numerical results obtained confirm that provided an appropriate stopping regularization criterion is imposed, the iterative BEM is efficient in dealing with the difficulties arising from both the instabilities produced by the boundary condition formulation and the slow rate of convergence of standard numerical methods around the singular point.     

Measurements of the neutron dose near a 15 Mv medical linear accelerator

In this work, simplified recombination methods for routine estimation of dose equivalent in mixed (gamma and neutrons) radiation field outside the irradiation field of linear medical accelerators is considered. The author's earlier reported method of H(10) measurements, involving determination of the recombination index of radiation quality, Q(4) by tissue-equivalent recombination chamber was combined with the new method for determination of the photon to neutron dose ratio D(X)/D(n) from the ratio of ion collection efficiencies measured in the investigated radiation field and in two reference fields of gamma and neutron radiations. The method is suitable when the neutron contribution to the total absorbed dose, D(n)/D, is >3%.     

Ionisation chamber containing boron as a neutron detector in medical accelerator fields

A combination of the recombination principle of H(10) measurements with the use of the ionisation chambers containing boron has been presented, in order to increase the relative sensitivity of the chamber to neutrons by a factor close to the radiation quality factor of photoneutrons. Three types of the chambers were investigated. Two of them were filled with BF(3) and the third one contained electrodes covered with B(4)C. All the chambers were placed in paraffin moderators. The response of the chambers was investigated, depending on gas pressure and polarising voltage. The results showed that it was possible to obtain nearly the same response of the chamber to H(10) for photons and neutrons in a restricted energy range; however, further investigations are needed to make an optimum design.     

On the steady state unavailability of standby systems

In this paper we study the steady state unavailability of standby systems comprising n identical components of which n − 1 are normally operating and one is in standby, with one repair facility. The components are assumed to have constant failure rates, but arbitrary repair time distribution. Attention is given to the case that the mean time to repair MTTR is relatively small compared to the mean time to failure MTTF, which is the most common situation in practice. Simple approximation formulae are presented and compared with the standard Markov expressions. It is demonstrated that the Markov expressions produce relatively large errors when the coefficient of variation of the repair time distribution is not close to 1. Some easily computed error bounds of the approximation formulae are established.     

On variational approaches to steady state heat conduction

Summary Several aspects of the variational approach to steady state heat conduction are examined.     

Shielding design studies for a neutron irradiator system based on a 252Cf source.

This study aims to investigate a shielding design against neutrons and gamma rays from a source of 252Cf, using Monte Carlo simulation. The shielding materials studied were borated polyethylene, borated-lead polyethylene and stainless steel. The Monte Carlo code MCNP4B was used to design shielding for 252Cf based neutron irradiator systems. By normalising the dose equivalent rate values presented to the neutron production rate of the source, the resulting calculations are independent of the intensity of the actual 252Cf source. The results show that the total dose equivalent rates were reduced significantly by the shielding system optimisation.     

An active personal neutron dosemeter based on microdosimetric principles: CIME

Over the last few years IPSN has been developing a small, tissue equivalent proportional counter (TEPC) with multielement geometry for personal radiation protection monitoring. This paper presents the last prototype, which is insensitive to microphony, and the experimental results. Numerical modelling results using CERN codes are partly presented and allow an understanding of the nuclear and electrostatic physics involved in a TEPC.