- [Show abstract] [Hide abstract] ABSTRACT: The paper describes the results of Monte-Carlo calculations for a coupled moderator on a low power pulsed neutron spallation source. Various options were compared including hydrogen, solid methane, grooving the solid methane and compound moderators made of hydrogen in front of solid methane. To maximize the neutron current at low energies, two strategies appear to emerge from the calculations. For instruments that view a large area of moderator surface, a layer of hydrogen in front of a thin solid-methane moderator is optimum, giving a gain of about a factor 10 relative to the current liquid hydrogen moderator on the existing ISIS tantalum target. For instruments that only view a restricted area, a higher flux corresponding to a gain of 13.5, can be achieved with the use of a single groove or re-entrant hole in the moderator.
- [Show abstract] [Hide abstract] ABSTRACT: Using the method of weighted residuals, a detailed numerical study is made of the neutronics of spherical cold neutron sources composed of liquid hydrogen and liquid deuterium in reactor moderators of light water, heavy water and graphite. Particular attention is paid to the physical effects at the cold source boundary. The computations allow a number of conclusions to be made on the best designs of spherical cold source configurations. It is found that for small spherical sources there is little difference in neutron gain using either normal-hydrogen or para-hydrogen, and that the largest gains are obtainable from large deuterium-filled sources.
- [Show abstract] [Hide abstract] ABSTRACT: An accurate alternative method to energy-multigroup theory for slowing-down problems is considered in which a particular energy range is represented by discrete energy points while the larger part of the energy range is treated by groups. Applications of the technique are made for homogeneous iron-sodium and uranium-graphite mixtures, and for two-region cells composed of these materials. It is demonstrated that by suitable distribution of the energy points within the selected energy range, reaction rates and their temperature coefficients can be predicted with better accuracy than the conventional multigroup method of treating the energy range as a single group.
- [Show abstract] [Hide abstract] ABSTRACT: Detailed numerical studies are made of recent measurements by Sumita and Takahashi of average and most probable slowing down times to thermal energies in graphite. Good agreement is obtained between computed and measured values at 52, 491, 146, 028, 20 and 11 eV. Similar studies are successfully made of the lead slowing down time spectrometer for a variety of detectors covering the energy range 1 to 3 keV. Estimates are made of the effects of data uncertainties on the theoretical values.
- [Show abstract] [Hide abstract] ABSTRACT: A detailed analysis is made of the spatial variation of the effective thermal neutron temperature near temperature discontinuities in large volumes of graphite and light water. For graphite comparisons are made with recent measurements by Day. It is seen that it is important that proper consideration be made for the gradient of the epithermal flux. For water a number of predictions are made for both poisoned and unpoisoned systems and some doubt is cast on the feasibility of such measurements for determining the accuracy of different scattering kernels for light water. It is demonstrated, for both graphite and light water, that additional information can be obtained by extracting neutron beams at angles other than perpendicular to the axis of the system.
- [Show abstract] [Hide abstract] ABSTRACT: A numerical investigation is made of neutron rethermalization effects in light water near physical temperature discontinuities and discontinuities in 1/ν-poisoning. It is shown that calculations of effective neutron temperatures in unpoisoned water systems possessing only temperature discontinuities are insensitive to the details of the scattering model assumed, and that rethermalization occurs within 1 cm of the temperature interface. However, with or without the presence of physical temperature discontinuities but with one region poisoned, effective temperature differences of up to 70 K exist between predictions using the Nelkin model and the Haywood frequency distribution for the water molecule. In this case, rethermalization occurs up to 2 cm into the poisoned region.
- [Show abstract] [Hide abstract] ABSTRACT: The method of weighted residuals is used to produce approximate solutions to the integral form of the Boltzmann transport equation for thermal neutron fluxes near a moderator temperature discontinuity. Both spherical and plane geometries are considered. The validity of the technique is established by using a synthetic scattering kernel and making comparisons with exact transport theory solutions for two-temperature problems. The relative accuracy and importance of the choice of different weighting functions and of the number of trial modes in the flux expansion is investigated. Generally, it is found that using three or more maxwellian distributions for the energy trial modes of the flux produces a significant increase in the accuracy compared with a diffusion theory approximation.
Article: The nuclear physics of fast reactors[Show abstract] [Hide abstract] ABSTRACT: The nuclear physics aspects of fast reactor research and development are reviewed with particular emphasis on the liquid metal fast breeder reactor. The uranium-plutonium breeding cycle is discussed and the sensitivity of the cycle to the competing processes of radiative capture and fission is considered. Measurements of the capture-to-fission ratio alpha are given and methods of defining the breeding ratio of a system are examined. The prompt neutron fission yield and the prompt neutron energy spectrum are mentioned and measurements of fast neutron cross sections are reviewed. Two aspects of reactor safety are dealt with: the effect of delayed neutrons from fast fission on the temporal behaviour of a fast reactor; and the role of the nuclear Doppler effect should the reactor undergo a power excursion under accident conditions. Phenomenological nuclear modelling is discussed with reference to coupled channel optical and fluctuating compound nucleus models.
- [Show abstract] [Hide abstract] ABSTRACT: A study is made of the conditions under which multigroup diffusion theory reasonably approximates multigroup transport theory for the fast neutron penetration problem. Defining the probability that a neutron born in a given energy group should suffer its next collision in that group, it is demonstrated that decreasing the size of the energy group decreases the relative accuracy of a diffusion theory calculation. Examples are given for light water and natural iron.
- [Show abstract] [Hide abstract] ABSTRACT: Using an energy dependent extrapolated endpoint Z0g as a variable parameter, a least-squares technique is used to minimise differences between the solutions of the multi-group transport and diffusion equations for one-dimensional slab geometry. The optimised Z0g values thus obtained are compared with the familiar prescriptions based on one-group theory, particularly the Milne condition cg Sigma trgZ0g=0.71. Applications to some moderating materials show that while the Milne formula is adequate for natural iron, a better prescription for light water would be cg Sigma trgZ0g=0.35. A similar analysis for extrapolation lengths lambda g yields an optimised fit of Sigma trg lambda g=0.62 for light water moderators.
- [Show abstract] [Hide abstract] ABSTRACT: A computational study is made of the effects on various blanket parameters of the neutron moderation produced in the highly compressed deuterium-tritium pellet of an inertially confined thermonuclear reactor based on the Wisconsin SOLASE design. For a pellet in a compressed state of rho R=1.2 g cm-2, studies are made of the blanket neutron heating, gas production and tritium production. Compared with a monoenergetic 14.1 MeV neutron source, the values of these parameters can change by as much as 15%.
- [Show abstract] [Hide abstract] ABSTRACT: It is important for the BNCT community to be able to compare and predict the biological effects of mixed radiation fields at various centres. A quantitative comparison of the beams would allow more valid discussions of radiobiological and clinical data between centres. If a biological system could be irradiated in various BNCT facilities and its radiation response analysed such a comparison will be possible. If the beams at each centre can thus be compared the preclinical and radiobiological studies performed at other centres will not have to be repeated at new centres, such as at Birmingham, and comparison of clinical data will be more meaningful. This report describes the design of an in vitro experiment which fulfils these criteria. Cell survival will be determined in relation to the depth in a phantom following irradiation in a BNCT beam. To overcome the problems of the variable repair of sub- lethal damage from the gamma radiation component of any beam during the irradiation period all irradiations will be performed at 4°C The different components of the total dose rate at each position in the phantom will be estimated using paired ionisation chambers and activation foils. To ensure that a valid comparison is being made, the beam survival curves will be compared to x-ray survival curves produced at each centre in this study.
- [Show abstract] [Hide abstract] ABSTRACT: The RPA is used to obtain the double-differential scattering cross-section for a test particle interacting with a multi-component plasma for a range of temperatures and densities appropriate for inertial confinement fusion (ICF) studies. It is shown that strong interference effects may occur between electron-plasmons and ion-waves. Numerical examples are provided for 3.5 MeV alpha particles in deuterium plasmas and 5 GeV uranium ions in silicon plasmas.
- [Show abstract] [Hide abstract] ABSTRACT: The growth of Rayleigh-Taylor instabilities in heavy-ion-driven spherical inertial confinement fusion (ICF) targets is studied using a simple linear perturbation model. The instability growth at the ablator-pusher interface is shown to be dependent on the width, relative to the target size, of the beams used as the driver. Short-wavelength instability growth at the fuel-pusher interface, with a possible consequent deleterious effect on the thermonuclear performance of the target, is also shown to be dependent on the beam width, indicating that beam-target optimisation studies are necessary.
- [Show abstract] [Hide abstract] ABSTRACT: With a growing interest in the use of accelerator-based epithermal neutron sources for BNCT programs, in particular those based upon the 7Li(p,n)7Be reaction, there is a need to address the question of "what is the best proton energy to use?" This paper considers this question by using radiation transport calculations to investigate a range of proton energies from 2.15 to 3.5 MeV and a range of moderator sizes. This study has moved away completely from the use of empty therapy beam parameters and instead defines the beam quality and optimizes the moderator design using widely accepted in-phantom treatment planning figures of merit. It is concluded that up to a proton energy of about 2.8 MeV there is no observed variation in the achievable therapy beam quality, but a price is paid in terms of treatment time for not choosing the upper limit of this range. For higher proton energies, the beam quality falls, but with no improvement in treatment time for optimum configurations.
- [Show abstract] [Hide abstract] ABSTRACT: The absorbed dose in a phantom or patient in boron neutron capture therapy (BNCT) and boron neutron capture enhanced fast neutron therapy (BNCEFNT) is deposited by gamma rays, neutrons of a range of energies and the 10B reaction products. These dose components are commonly measured with paired (TE/Mg) ion chambers and foil activation technique. In the present work, we have investigated the use of paired tissue equivalent (TE) and TE+ l0B proportional counters as an alternate and complementary dosimetry technique for use in these neutron beams. We first describe various aspects of counter operation, uncertainties in dose measurement, and interpretation of the data. We then present measurements made in the following radiation fields: An epithermal beam at the University of Birmingham in the United Kingdom, a d(48.5) + Be fast neutron therapy beam at Harper Hospital in Detroit, and a 252Cf radiation field. In the epithermal beam, our measured gamma and neutron dose rates compare very well with the values calculated using Monte Carlo methods. The measured 10B dose rates show a systematic difference of approximately 35% when compared to the calculations. The measured neutron+gamma dose rates in the fast neutron beam are in good agreement with those measured using a calibrated A-150 TEP (tissue equivalent plastic) ion chamber. The measured 10B dose rates compare very well with those measured using other methods. In the 252Cf radiation field, the measured dose rates for all three components agree well with other Monte Carlo calculations and measurements. Based on these results, we conclude that the paired low-pressure proportional counters can be used to establish an independent technique of dose measurement in these radiation fields.
- [Show abstract] [Hide abstract] ABSTRACT: Near-field designs of Fresnel and Gabor zone plates are computationally analyzed by using versions that allow the foci to be brought closer to the plate than in the usual far-field applications. It is found that the Fresnel plate has a dominant primary conjugate pair of foci well inside the far-field region and a more intense primary focus and smaller off-focal-plane sidelobes than the near-field Gabor systems, thus yielding a superior imaging performance.
- [Show abstract] [Hide abstract] ABSTRACT: This paper is concerned with the proposed Birmingham accelerator-based epithermal neutron beam for boron neutron capture therapy (BNCT). Details of the final moderator design, such as beam delimiter, shield, and beam exit surface shape are considered. Monte Carlo radiation transport simulations with a head and body phantom have shown that a simple flat moderator beam exit surface is preferable to the previously envisioned spherical design. Dose rates to individual body organs during treatment have been calculated using a standard MIRD phantom. We have shown that a simple polyethylene shield, doped with natural lithium, is sufficient to provide adequate protection to the rest of the body during head irradiations. The effect upon the head phantom dose distributions of the use of such a shield to delimit the therapy beam has been evaluated.
- [Show abstract] [Hide abstract] ABSTRACT: This paper is concerned with the proposed Birmingham accelerator-based epithermal neutron beam for boron neutron capture therapy (BNCT). In particular, the option of producing a therapy beam at an orthogonal direction to the incoming protons is considered. Monte Carlo radiation transport simulations, both with and without a head phantom, have shown that an orthogonal beam geometry is not only acceptable but is indeed beneficial, in terms of a lower mean neutron energy and an enhanced therapeutic ratio for the same useful neutron fluence in the therapy beam. Typical treatment times for various beam options have been calculated, and range from 20 to 48 min with a 5 mA beam of 2.8 MeV protons, if the maximum photon-equivalent dose delivered to healthy tissue is to be 12.6 Gy Eq. The effects of proton beam diameter upon the therapy beam parameters have also been considered.
University of Birmingham
Birmingham, England, United Kingdom
- • School of Physics and Astronomy
- • School of Metallurgy and Materials