OpenUCT :: Browsing by Author "Brooks, F D"

Browsing by Author "Brooks, F D"

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Open Access
Alpha decay of excited states of carbon-12
(1970) Shackleton, David; Brooks, F D
A crystal of the carbon-rich phosphor, anthracene, has been bombarded by monoenergetic fast neutrons. Alpha-particle decays of carbon nuclei have been separated from other events by a specialised application of the pulse shape discrimination technique. A numerical analysis of the data has yielded alpha-particle energy spectra over a wide energy range. A counter has been assembled to detect neutrons scattered by carbon nuclei in the crystal, and the measurements repeated in coincidence with these neutrons. The energy range of particle resolving power has been extended. Structure in the resulting alpha-particle and proton energy spectra is attributed to particle decays of specific levels in carbon-12. It is shown how the experiment may be modified and extended to measure decay parameters in absolute terms.
Open Access
Alpha emission from the interactions of 14-200 MeV neutrons on carbon-12
(1994) Nchodu, M R; Brooks, F D
Cross-sections for the production of alpha particles from the ¹²C(n,α)⁹Begs and ¹²C(n,n')3α reactions were measured for incident neutrons in the energy range 25 to 52 MeV. The measurements were reduced to absolute cross-sections by simultaneously observing deuteron recoils from n-d elastic scattering in the scintillator using D(n,n)D elastic scattering cross-section as a reference standard. Neutron energy was determined by time-of-flight and pulse shape discrimination (PSD) was used to identify and separate alphas and deuterons from other reaction products. The feasibility of distinguishing different mechanisms for the ¹²C(n,n')3α reaction by means of PSD was also investigated.
Open Access
Coincident proton decay of the continuum induced by 200 MeV protons on 12C
(1989) Pilcher, John Victor; Brooks, F D
Coincident protons from the reaction ¹²C(p,2p) at an incident proton energy of 200 Me V, have been measured using conventional particle telescopes. Data were acquired at nine pairs of angles (θ₂=20°, 45°, 70°, 95°, 120° and 145° for θ₁ =-20°, and θ₂=35°, 85° and 135° for θ₁ =-45°). An improved model, based on previous theoretical and experimental work, has been proposed to describe the reaction mechanism leading to the observed coincident proton yield from the nuclear continuum. This model considers an initial quasifree nucleon-nucleon interaction - described by a distorted wave impulse approximation (DWIA) treatment - between the incident proton and a single target nucleon, followed by the rescattering of the struck nucleon, which behaves as an intranuclear projectile, from the spectator part of the target nucleus. The validity of the DWIA treatment used in the continuum study, has been tested by analysing the kinematic regions of the coincident proton energy spectra corresponding to the knockout of protons from the outer lp3/2 orbital of ¹²C which do not undergo further interactions with the residual nucleus. Good agreement in shape has been found between the calculated and measured cross sections in the kinematic region of interest to the continuum study. An average spectroscopic factor of 1.1±0.3 was extracted. Except in the -20°,20° case, the DWIA treatment was found to be insensitive to the choice of the initial or final energy prescriptions for the two-body scattering approximation. Nonlocality effects were found to be small, changing the absolute value of spectroscopic factors by a maximum of 20%. Coincident continuum cross sections were extracted from the experimental data corresponding to the detection of protons at the θ₁ angles with energies of 70±5 MeV, 100±5 MeV and 130±5 MeV. Comparisons were made with calculations which modelled the proposed reaction mechanism in terms of a convolution integral over the DWIA cross sections describing the initial quasifree knockout process and experimental inelastic scattering probabilities describing the further multiple scattering interactions undergone by the knocked out nucleons. Good agreement has been found between the normalized calculated and measured continuum cross sections (spectroscopic factors varying from 0.8 to 2.3 between sets of data, depending on the kinematic region sampled and the prescription used for the two-body scattering approximation). It has been shown that contributions from nucleons that are knocked out both in- and out-of-plane in the initial quasifree process should be considered, and that the contribution from neutron knockout in the initial interaction is significant, ranging from 0.13 to 0.62 that of the proton knockout contribution.
Open Access
Determination of energy spectra of proton therapy beams
(2002) Nchodu, M R; Brooks, F D; Buffler, Andy; Jones, Dan T L
A technique has been developed for measuring the energy spectra of high-energy proton therapy beams in situ under conditions similar to those used for radiotherapy at the South African National Accelerator Centre. The method is based on proton elastic scattering, H(p,p)H, in a thin polyethylene radiator and uses two ΔE-E detector telescopes to detect coincident proton pairs. Measurements have been made to investigate the effect of standard beam modification elements on the energy spectra of proton therapy beams produced by a passive scattering system. Monte Carlo simulations of the spectra were computed with the MCNPX 2.1.5 Monte Carlo code to compare with experimental measurements.
Open Access
Energy correlations of prompt neutrons from 252Cf fission
(1983) Whittal, David Mark; Brooks, F D
Measurements have been made of the neutron-neutron energy correlations for coincident neutrons emitted in the spontaneous fission of ²⁵²Cf. The hypothesis that cooling of the fission fragments during the cascade evaporation of prompt neutrons affects the energies of successive neutrons in a cascade has been investigated by a comparison of the energy correlations of coincident neutrons from the same fragment with those of coincident neutrons from opposite fragments in the same fission. It is found that the energies of coincident neutrons are posftively correlated and that the correlation coefficient is greater for neutrons from opposite fragments than for neutrons from the same fragment. This provides a sensitive test of the nature of neutron emission in an evaporation cascade. The energy correlations of random (noncoincidental) neutrons have also been investigated and are found to show similar but smaller correlation trends. It is concluded that there is some evidence for the cascade cooling effect on the energies of prompt fission neutrons.
Open Access
Fast neutron scattering analysis
(1998) Buffler, Andy; Brooks, F D; Aschman, David G
The scattering of a beam of fast monoenergetic neutrons is used to determine elemental compositions of small (0.2-1 kg) samples of materials. Particular emphasis is placed on the measurement of concentrations of the elements H, C, N and O, which are the principal constituents of contraband materials, such as explosives and narcotics. Scattered neutrons are detected by liquid scintillators located at forward and at backward angles, and different elements are identified by their characteristic scattering signatures derived either from a combination of time-of-flight and pulse height measurements or from pulse height measurements alone. Atom fractions for H, C, N, O and other elements are derived from unfolding analyses based on these scattering signatures and used to identify materials. Effects of neutron interactions in surrounding materials, either in the neutron beam or between the scatterer and the detectors, can be detected and allowed for in such a way as not to interfere significantly in the identification of the scatterer. The Fast Neutron Scattering Analysis technique provides a non- intrusive method for detecting and identifying sub-kilogram quantities of contraband materials. Methods for locating the positions of small contraband items in packages of volume up to about 0.5m³ are described and a two-stage screening system for detecting contraband hidden in small packages is proposed.
Open Access
Measurement of neutron radiotherapy spectra in a tissue-equivalent phantom
(1998) Herbert, Mark; Brooks, F D
A new technique was investigated for determining the changes in neutron radiotherapy spectra with position within a tissue-equivalent phantom at the National Accelerator Centre in the energy range 0 to 63 MeV. An deuterated liquid scintillator (NE230) was used as the neutron spectrometer for these measurements. The pulse height spectra measured using the NE230 scintillator were unfolded to give the neutron spectra. The results show beam hardening with depth into the phantom, which are consistent with other reported findings. The unfolding procedures used are described and the results of the tests performed to verify reliability are also presented and discussed.
Open Access
Neutron induced alpha production from carbon between 18 and 22 MeV
(1976) Stevens, Alun Peter; Brooks, F D
Cross sections for neutron induced alpha production in carbon were measured at seventeen energies between 18 and 22 MeV, using a deuterated anthracene crystal as both target and detector. Pulse shape discrimination was employed to separate the alphas and elastically scattered deuterons from the other reaction products. Published (n,d) elastic scattering data were used as a standard to obtain the alpha production cross sections. Comparison with available measurements shows good agreement
Open Access
Neutron spectrum from 252Cf spontaneous fission
(1984) Allie, Muhammed Saalih; Brooks, F D
Measurements have been made of the neutron spectrum from the spontaneous fission of 252Cf in the restframe of the fragment by simultaneous observation of the neutron time-of-flight and the fragment velocity. The fragments were detected by means of thin film plastic scintillators in which the scintillation pulse-height response was verified to be a linear function of fragment velocity. The measurements of the neutron spectrum in the restframe of the fragment are compared with previous work and are found to deviate significantly from the predicted observations of the simple form of the evaporation model.
Open Access
The neutron-deutron breakup cross section between 8 and 22 MeV
(1973) Pauletta, Giovanni; Brooks, F D
The n-d breakup cross section has been measured at eleven energies. between 8 and 22 MeV by integrating the energy distributions of breakup protons in a deuterated scintillator. The breakup protons were separated from the recoil deuterons by pulse shape discrimination. A comparison of these measurements with recent calculations favours those performed with local potentials.
Open Access
Neutron-proton bremsstrahlung at 4.8 MeV
(1979) Whittaker, John; Brooks, F D
The differential cross-section for neutron-proton bremsstrahlung at an incident energy of 4.8 MeV has been measured where the outgoing nucleon angles were θₙ = 35°, θᵨ = 25°. An upper limit to d²σ/dΩₙdΩᵨ of 210 μb/sr² was found. The experimental method was unusual in that the target was an organic scintillation crystal which also served as recoil proton detector, and the bremsstrahlung photon was detected in addition to the two nucleons in a second crystal. An estimate of the cross-section corresponding to this experiment has been made using elastic scattering parameters. The measured upper limit is above this and other theoretical predictions, but represents a marked improvement over previous measurements in the energy range. Bibliography: p. 107-111.
Open Access
The photodisintegration of the deuteron close to threshold
(1986) Smit, F. D.; Brooks, F D
Theoretical calculations predict that deuteron photodisintegration is sensitive to meson exchange current and isobar configuration contributions in the threshold (2.225 MeV) region. These effects are manifested chiefly through Ml transitions, which reach maximum amplitude in this energy region. The calculated size of these effects can be verified from an angular distribution measurement of the photo-products.
Open Access
Polarization in neutron-deuteron scattering from 8 to 22 MeV
(1975) Steinbock, Mary; Brooks, F D
The polarization in n-d scattering has been measured at incident energies 7.9, 16.4, and 21.6 MeV using neutrons from the ⁹Be(α,n) ¹²C and ³H(d,n) ⁴He reactions. The polarization analyzer was a deuterated anthracene scintillation crystal. Recoil deuterons from n-d elastic scattering within the crystal were studied and pulse shape discrimination (PSD) was used to determine the left-right asymmetry of the recoils. The experimental method depends on the fact that the PSD response of the crystal is sensitive to the direction of the recoil deuteron relative to the crystal axes. The results obtained are compared with theoretical calculations from the literature and with other measurements. A comparison of the n-d and p-d polarization data reveals no evidence of breaking of the charge symmetry of nuclear forces.
Open Access
Polarization in neutron-proton elastic scattering at 21.6 MEV
(1986) Simpson, B R S; Brooks, F D
The polarization in n-p scattering has been measured at 21.6 MeV using polarized neutrons from the T(d,ṅ)4He reaction. The measuring instrument was an anthracene scintillation polarimeter. A critical assessment of this polarimeter was made, and extensive refinements to the polarimeter were introduced so as to minimise perceived sources of systematic error. These included improvements both in instrumental design and data reduction techniques.
Open Access
Prompt neutrons from fission of 252Cf and 235U
(1977) Pringle, John Stephen; Brooks, F D
Measurements have been made of the n-n angular correlation for neutrons emitted in spontaneous fission of ²⁵²Cf and thermal-neutron- induced fission of ²³⁵U. The dependence of the angular correlation on the neutron detection threshold energy was investigated. These measurements and other experimental data on the fission spectrum and the neutron angular distribution are compared with Monte Carlo calculations based on the evaporation model. It is apparent that a model assuming isotropic evaporation of all neutrons from the moving fragments cannot adequately explain the observed distributions. No evidence was found favouring the emission of a fraction of the neutrons isotropically at scission, or during the fragment acceleration period.
Open Access
Proton scattering from 32S
(1968) Spitz, Leslie Maurice; Brooks, F D
Open Access
Quasifree knockout of charged particles from ⁴He with 100 MeV protons
(1989) Whittal, David Mark; Brooks, F D
Exclusive measurements have been made of the reactions ⁴He(p,2p ), (p,pd), (p,pt) and (p,ph) at 100 MeV. The primary protons were measured at two angles, 45° and 60°, in coincidence with secondary protons, deuterons, tritons or helions covering a wide range of angles on the opposite side of the beam, from -15° to -90° in plane, and from 0° to 30° out-of-plane. The purpose of the experiment was to explore the validity of the Distorted Wave Impulse Approximation (DWIA) as a description of these quasifree knockout reactions, at all geometries where the knockout cross section is significant. Six particle telescopes, each consisting of two silicon detectors and a NaI detector, were used to measure the coincident particles. The experimental data are presented in the form of energy-sharing distributions, which were generated by projecting the two-dimensional quasifree knockout loci onto the energy axis of the primary proton. The comparison between these energy-sharing distributions and the curves calculated by means of the DWIA is satisfactory for all geometries, except at the most forward secondary angles for the (p,pt) and (p,ph) reactions. It is concluded that the DWIA is an adequate formalism for modelling the quasifree knockout reactions induced by 100 MeV protons on ⁴He. Based on this conclusion, the total contribution of quasifree knockout to the existing inclusive (p,p') spectra has been calculated by integrating the DWIA cross sections over the solid angles of the secondary particles. The spectroscopic factors used were those extracted from the coincidence data. For the (p,pn) reaction, it was assumed that the spectroscopic factors were the same as for (p,2p). It is found that 70 ± 10% of the inclusive continuum yields is attributable to quasifree scattering, with ~30% arising from multiple scattering. At low energies of the scattered proton, the knockout of nucleons constitutes over 90% of the quasifree component, whereas at high energies the knockout of deuterons, tritons and helions dominates the yield. Thus the absence of discernable quasifree peaks in the inclusive spectra is probably due to the cluster knockout contributions filling in the yield at higher energies.
Open Access
A search for muon-catalysed dd fusion in a deuterated organic crystal
(1992) Oliver, Marian Jean; Brooks, F D
The possibility of muon-catalysed dd fusion via a Vesman-type mechanism in an organic crystal of deuterated anthracene is considered. In order to test this possibility the production of muons by 200 Me V protons incident on a 1mm Pb target is investigated. Two experiments to search for evidence of muon-catalysed fusion in a cylindrical deuterated anthracene crystal (9 mm diameter by l20 mm length) are described. The results indicate, tentatively, a fusion rate of 0.2 - 1.1 per incident μ-.
Open Access
Simulation of muon-catalysed fusion experiments
(1991) Henderson, C G L; Brooks, F D
A Monte Carlo code MUGLU has been, written for the simulation of experiments to measure alpha-muon sticking in muon catalysed deuterium-tritium fusion. The experiments simulated are those which measure the ratio of αμ to α from fusion in a low density gas target by detecting collinear alpha-neutron coincidences and relying on the differing stopping powers of the α and αμ ions. The Monte Carlo simulations provide estimates of geometrical and detection efficiency factors required for the calculation of the sticking coefficient from the experimental measurements. Simulations have been made of alternative experimental geometries in order to investigate the α-neutron coincidence signature and other characteristics of existing and proposed systems. The characteristics of a neutron detector used in, one of the current sticking experiments (Rutherford Appleton Laboratory) have been studied experimentally, as well as simulated, using the T(d,n)α reaction to emulate the fusion source. The results obtained show that the dependence of the neutron detection efficiency on the position and angle of neutron incidence on the detector is significant and should be taken into account when determining sticking coefficients from alpha-neutron coincidence measurements.
Open Access
Studies of neutron inelastic scattering in crystal and liquid scintillators
(1967) Bain, Cairns A R; Brooks, F D
An investigation has been made of the feasibility of using crystal, liquid and gel scintillators for the study of neutron inelastic scattering from elements, contained or loaded in the scintillators. The nuclides included those of iodine, cesium, lanthanum, holmium, praseodymium and bismuth. investigations showed that the technique was feasible for crystal and liquid scintillators, and indicated the study or 12 7I iri sodium iodide crystal to be the most suitable for further investigation. A study was then made of inelastic scattering to the 59 keV level of 1 2 7 I for incident neutrons in the energy range 60 to 360 keV. Cross section data obtained were compared with inelastic scattering cross sections calculated using a Hauser-Feshbach model of Auerbach and Moore. Experimental data were; found to be in reasonable agreement with the calculated cross sections based on neutron transmission coefficients given by Auerbach and Perey.