Neutron Penetration Research Articles

Monte Carlo calculations of epithermal boron neutron capture therapy with heavy water

Much work over the past decade has centred upon the development of epithermal neutron beams for boron neutron capture therapy (BNCT) in an effort to increase thermal-neutron flux penetration and dose homogeneity throughout the brain. While heavy water has been used extensively to improve neutron penetration associated with thermal neutron beams, the effects of heavy water with epithermal neutron beams remain largely unexplored. Applying the Monte Carlo code MCNP to a heterogenous ellipsoidal skull/brain model, the effects of heavy-water replacement are studied for the JRC/ECN Petten HFR epithermal neutron beam. Thermal neutron flux and induced gamma depth dose distributions are calculated for 20% D2O replacement in comparison to standard brain and skull materials. Results are presented for both unilateral and bilateral irradiation. With bilateral irradiation, thermal-neutron flux homogeneity is substantially increased with 20% D2O replacement, thus improving the potential to give lethal doses to boron-10-loaded, disseminated cancer cells whilst avoiding local 'hot spots' to healthy tissue. Additionally, the induced gamma dose is reduced by up to 30%, substantially lowering the background dose to healthy tissue. With bilateral irradiation, 20% D2O replacement increases the therapeutic ratio from 2.25 to 2.75 for over 4 cm depth centred at the midline of the brain. These calculations use documented tumour and blood 10B concentrations for boronophenylalanine (BPA) in humans and recently documented neutron relative biological effectiveness (RBE) values.

The influence of heavy water on boron requirements for neutron capture therapy

Neutron penetration in tissue is a major limitation of thermal NCT, as such much work has centered upon the epithermal neutron beam in an effort to improve this situation. Further gains in neutron flux penetration, and thus therapeutic ratios, are possible if natural water is replaced with heavy water prior to therapy. Applying MCNP to a heterogeneous ellipsoidal skull/brain model, advantage depth and therapeutic depth parameters are studied as a function of heavy water replacement for a range of tumor to blood boron ratios. Both thermal (0.025 eV) and epithermal (2-7 keV) ideal neutron beams are analyzed. Using 10B ratios in the range of documented human uptake, the thermal advantage depth improved by approximately 0.7 cm for 20% D2O replacement, however, the therapeutic depth increased by less than half this value. For the epithermal beam, both the advantage depth and the therapeutic depth increased by over 1 cm. Effects of heavy water replacement on 10B requirements to therapeutically treat the midline of the brain are also evaluated.

Experimental investigation of neutron and photon penetration and streaming through iron assemblies

Experimental investigation of neutron and photon penetration and streaming through iron assemblies

Experimental investigation of neutron and photon penetration and streaming through iron assemblies

Nuclear shield parameters of fusion reactors such as ITER are determined by the neutron and photon fluxes within and behind the shield. Iron is the main element of shields. In order to verify calculational tools for the blanket design a solid Fe slab of dimensions 100 cm × 100 cm × 30 cm in thickness was irradiated with 14 MeV neutrons, and the neutron and photon fluxes penetrating the assembly were measured in wide energy ranges. They are compared with calculated fluxes using the three-dimensional Monte Carlo code mcnp and nuclear data of the European Fusion File EFF-1. The calculation underestimates the neutron flux at low and at high energies. It is discussed in connection with the underestimated photon flux. The investigation was extended to Fe assemblies with a gap at several positions.

Compound detector with selective sensitivity to intermediate neutrons

A compound detector based on the23Na(n,γ) reaction in a boron shield is investigated. The detector cross section was calculated assuming that the spectrum inside the shield is a sum of two components, one due to direct penetration of neutrons through the shield wall and neutrons scattered in the shield mass. Also investigated was the effect of hydrolyzing varnish in the shield and the size of the activation detector on the detector readings.

Neutron Penetration through Iron and Concrete Shields with the Use of 22.0- and 32.5-MeV Quasi-Monoenergetic Sources

For the first time, spectra in the range of 34 MeV down to 0.4 eV for neutrons that penetrated through iron and concrete shields were measured with the use of quasi-monoenergetic sources generated from a [sup nat]Li(p,n)Be reaction. The source neutrons were produced by the bombardment of 35- and 25-MeV protons on a 2-mm-thick natural lithium target and were measured by the time-of-flight method. The neutrons that penetrated through the shields were measured by an NE-213 scintillator, a proton recoil proportional counter, and a spherical multimoderator spectrometer, i.e., the Bonner Ball, and were analyzed to energy spectra by the unfolding methods. The attenuations of a peak fluence, a total fluence, and a dose equivalent were also analyzed. Monte Carlo calculations using the MORSE-CG code were carried out with the multigroup cross-section library DLC-119/HILO86 and the original version, DLC-87/HILO, to evaluate the cross sections by comparison with the measurement. The results of the calculation indicated the following. On the whole, the calculated neutron spectra agreed with the experimental spectra above 10 odd MeV but gave an underestimation below that value. The peak fluence attenuation length for 22-MeV neutrons that penetrated through the iron shield was overestimated by DLC-87 but corrected bymore » DLC-119. The dose-equivalent attenuation lengths almost agreed with the measurement for the concrete shield. On the other hand, those for the iron shield were underestimated.« less

Non-Invasive Temperature Measurements by Neutron Diffraction in Aero-Engine Components

Abstract A requirement exists in the aeronautical industry for measuring temperature non-invasively in critical components, such as the turbine disc in an operating engine. Neutron diffraction, unique among nuclear techniques, offers the possibility of measuring both temperature and strain within an operating engine by virtue of the high penetration of neutrons through industrial materials. Static diffraction experiments on Waspaloy and Ti6A14V showed, by comparison with thermocouples, that both the diffraction peak position and the peak intensity can measure the tempeiaturc to within ±6 K aL 800 K. Measurements on a rotating Waspaloy disc, heated from its rim, showed that temperature gradients could be determined accurately by lattice parameter measurements. The large grain size in Waspaloy prevented accurate peak intensity measurements in this dynamic test. Finally, the structures within a small Pratt & Whitney engine in its equatorial plane were mapped by mounting the engine on an X-Y translator on the diffractometer and moving it through a grid of positions. Possible future directions in the field will be discussed.

Observed Penetration of 14-MeV Neutrons in Various Materials

Experimentally observed 14-MeV neutron removal cross sections are presented for 16 materials. At a scattering energy loss of 4MeV, the effective macroscopic cross section per unit mass (in square centimeters per gram) for a nucleus of mass A is given by 0.25A[sup [minus]1.78] + 0.10A[sup [minus]0.47]. Such information provides a simple method for estimating neutron transport in systems driven by thermonuclear neutrons.

Dosimetric Effects of Beam Size and Collimation of Epithermal Neutrons for Boron Neutron Capture Therapy

A series of studies of "ideal" beams has been carried out using Monte Carlo simulation with the goal of providing guidance for the design of epithermal beams for boron neutron capture therapy (BNCT). An "ideal" beam is defined as a monoenergetic, photon-free source of neutrons with user-specified size, shape and angular dependence of neutron current. The dosimetric behavior of monoenergetic neutron beams in an elliptical phantom composed of brain-equivalent material has been assessed as a function of beam diameter and neutron emission angle (beam angle), and the results are reported here. The simulation study indicates that substantial differences exist in the dosimetric behavior of small and large neutron beams (with respect to the phantom) as a function of the extent of beam collimation. With a small beam, dose uniformity increases as the beam becomes more isotropic (less collimated); the opposite is seen with large beams. The penetration of thermal neutrons is enhanced as the neutron emission angle is increased with a small beam; again the opposite trend is seen with large beams. When beam size is small, the dose delivered per neutron is very dependent on the extent of beam collimation; this does not appear to be the case with a larger beam. These trends in dose behavior are presented graphically and discussed in terms of their effect on several figures of merit, the advantage depth, the advantage ratio, and the advantage depth-dose rate. Tables giving quick summaries of these results are provided.

Measurement of Neutron Dose Equivalent and Penetration in Concrete for 230 MeV Proton Bombardment of Al, Fe, and Pb Targets

Secondary neutron production from protons striking accelerator beam delivery components and the patient constitute the principal radiation hazard for 70–300 MeV accelerators used in proton radiation therapy. Because of the large mean free path of these high energy neutrons, neutron attenuation requires massive shields. To this end, we measured neutron dose as a function of emission angle and depth in concrete for the radiation environment produced by 230 MeV protons striking stopping targets of aluminium, iron, and lead. By using microdosimetric instrumentation, dose equivalent values were deduced. From these data, dose equivalent penetration as a function of depth in concrete and neutron emission angle were determined. Neutron production was found to vary rapidly with emission angle, while differences in dose equivalent values per incident proton as a function of depth and angle depended only slightly on target material.

Measurement of Neutron Dose Equivalent and Penetration in Concrete for 230 MeV Proton Bombardment of Al, Fe, and Pb Targets

Secondary neutron production from protons striking accelerator beam delivery components and the patient constitute the principal radiation hazard for 70–300 MeV accelerators used in proton radiation therapy. Because of the large mean free path of these high energy neutrons, neutron attenuation requires massive shields. To this end, we measured neutron dose as a function of emission angle and depth in concrete for the radiation environment produced by 230 MeV protons striking stopping targets of aluminium, iron, and lead. By using microdosimetric instrumentation, dose equivalent values were deduced. From these data, dose equivalent penetration as a function of depth in concrete and neutron emission angle were determined. Neutron production was found to vary rapidly with emission angle, while differences in dose equivalent values per incident proton as a function of depth and angle depended only slightly on target material.

Collimated Neutron Probe for Soil Water Content Measurements

AbstractThe use of uncollimated (undirected) neutron moisture meters is common in the biological and geophysical sciences. A collimated neutron probe was designed to enable measurements in specific directions from the access tube. To determine the size and shape of soil volume affecting the neutron counts, experiments were conducted to evaluate: (i) the vertical distance of soil above and below the probe that influences neutron counts, (ii) the horizontal distance away from the probe into the soil that influences neutron counts, (iii) the angle of soil viewed by the probe from the collimator, and (iv) the three‐dimensional thermal‐neutron density field. The distance and the angular dimensions of the volume of influence were defined as the horizontal distance of neutron penetration from the edge of the probe, the vertical distance above and below the center of the effective measurement point of the probe, and the angle from the center of the probe, which would allow the determination of relative water content to within 95%. The vertical distance was ≈0.5 m, the horizontal distance was ≈0.2 m, and the angle of soil viewed by the probe from the collimator was ≈120°. Thermal neutrons detected from distances or angles larger than these values influence the determination of relative water content by 5% or less.

NEUTRON DIFFRACTION STUDIES ON INDUSTRIAL MATERIALS

Neutron diffraction can be a valuable tool in the study of industrial materials because of a)sample size in relation to industrial scaling; b)neutron penetration through machinable metals, and consequent ease of construction of environmental chambers; c)neutron atomic contrast (e.g. scattering by Al or Si); and d)magnetic scattering possibilities. Four examples illustrating points (a)-(c) will be given in this paper. The first example involves the high temperature solid state synthesis of a zeolitic pigment, ultramarine. A dynamic diffraction study of the process is difficult since the radiation must penetrate beyond the “surface” 10 mm where excess boiling of sulphur takes place: such a study is however possible with neutron radiation. Because of the differing neutron atomic scattering lengths of Al and Si, neutron Rietveld refinement has been able to distinguish between two alternative schemes of Al,Si-site occupation on the sodalite cage. The unique and surprising result found is that ultramarine, with Si/Al ∼ 1, displays a random (non-Loewenstein) occupation of the Al,Si-sites. This prompts the question as to why most zeolites appear to be otherwise. Thus interesting (dynamic) experiments have been devised, involving both solid state and hydrothermal pathways, to investigate the kinetic aspects of zeolite synthesis. One of the exciting outcomes from this pursuit has been the construction of an autoclave cell for time-resolved neutron diffraction studies: its application to the synthesis of zeolites and autoclave-curing of cements will be described.

Transport calculations of the influence of physical factors on depth-dose distributions in boron neutron capture therapy

Distributions of thermal neutron fluence and capture gamma ray absorbed dose rates were evaluated, taking into consideration various physical factors relevant to boron neutron capture therapy. The use of a larger neutron irradiation aperture was associated with an increase in thermal neutron fluence and capture gamma ray absorbed dose rates. Radiation leakage was more significant with smaller phantoms. Attenuation of thermal neutron fluence rates by 10B suggested that there was an optimal 10B concentration (<100 PPM) for a given tumour. Deuteration of water allowed better penetration of thermal neutrons with less capture gamma rays and is potentially applicable for the treatment of deep-seated brain tumours.

Calculations of Microdosimetric Spectra for Low Energy Neutrons

Improved calculations of microdosimetric spectra for low energy neutrons have been carried out by separating the elastic and non-elastic nuclear reaction channels. Ion yield calculations for low energy neutrons have been improved by using a new set of W values at low energies. A program for the Monte Carlo calculation of the penetration of low energy neutrons has been developed and the results have been used to show the effect of the thickness of TEPC build-up caps on the nuclear reactions which contribute to the energy deposition and ionisation produced in the chambers.

Experiment and analysis of 14 MeV neutron deep penetration in a type 316L stainless steel assembly

Neutron spectrum measurements in a cylindrical assembly of 316L stainless steel were carried out for DT neutrons to assess the multigroup neutron cross section libraries and the calculational method for fusion neutronics. Energy spectra of scalar flux in the assembly above 0.8 MeV were measured at the positions of 8 to 88 cm in depth using a small spherical NE213 liquid scintillator with an unfolding code FORIST. The measured spectra were analyzed with the two-dimensional transport code DOT3.5 by using three multigroup cross section libraries originated from the ENDF/B-III, -IV, and JENDL-2 and -3PR1 nuclear data files. The calculated spectra with the 42-group library from ENDF/B-III are not consistent with the measured ones in shape. Calculation with the 125-group library from ENDF/B-IV overestimates the measured spectra in the whole energy region at every position, because of the uncertainty in the non-elastic cross section for Cr and the elastic and non-elastic cross sections for Fe. The best agreement is obtained between the measurement and the calculation using the 125-group library from JENDL-2 and -3PR1.

An introduction to small-angle neutron scattering

Neutron and X-ray small-angle scattering provide, along with electron microscopy and diffraction, the principal techniques for the microscopic characterization of materials. Neutron, X-ray and electron beams each have quite different properties. In fact, each has unique advantages. The penetration of neutrons through most materials is responsible for many applications. The ever-increasing intensity of available X-ray beams is opening new fields. The advantage of electron beams is their ability to work in both real and reciprocal space. The problems of transforming the results of an experiment in reciprocal space to give an interpretation in real space are central to small-angle scattering, and are discussed. Several examples will be given of the successful use of small-angle neutron scattering applied to problems where other techniques have failed to make a decisive contribution.

Fission neutron penetration in iron and sodium—I. Activation measurements

EURACOS II is an irradiation facility with a high-intensity fission neutron source (6.12 × 10 exp 11 n s −1). It consists of an irradiation chamber being sufficiently large to accommodate representative mock-ups of reactor radiation shields. This installation has been used to study neutron penetration in homogeneous materials which are among those most commonly used in the construction of advanced reactors: Fe and Na. Using special techniques (like the burning of S detectors in conjunction with a sophisticated separation of the background contributions) fluxes up to 130 cm in Fe and up to 390 cm in Na have been measured.

Fusion reactor neutronics.

Basic neutron behavior in the fusion reactor blanket and shield materials has been described using the terms of neutron moderation, diffusion, penetration and reflection. Special attention has been paid to explain the tritium breeding mechanism by the fusion neutrons in the blanket system. Status reviews on the neutron transport calculation techniques and relating researches to assess their accuracies have been also made.

A multi-function materials facility for a Spallation neutron source

A neutron beam instrument is described which will allow the simultaneous study of the microstructural, crystalline phase, internal stress, defect, local order, texture, diffusional and vibrational properties of materials. The penetration of neutrons permits all these properties to be studied in a bulk specimen in situ during a heat treatment of chemical reaction as a function of time. The possibility of using narrow incident and scattered neutron beams allows the simultaneous monitoring of these properties as a function of position across the sample. Only by using the polychromatic neutron beam from a pulsed source, can all these properties be studied at the same time and at the same point on the specimen. The instrument is designed for installation on the Spallation Neutron Source recently commissioned at the Rutherford Appleton Laboratory. Its performance is evaluated for a series of key experiments in applied science and shown to permit a complete neutron examination of the sample within minutes.