Search

At the ISIS Spallation Neutron Source at the Rutherford Appleton Laboratory in the UK, lifetimes of tantalum-clad tungsten targets on the Second Target Station (TS-2) have not been as long as expected. Analyses of gamma-ray spectra accumulated in the target services area suggest that erosion within the target is characterised by two exponentially increasing functions of time, one tentatively identified with initial erosion of the tantalum cladding, and the other identified with subsequent leaching of tungsten.

As a by-product of another measurement, ratios of the single-escape (SE) and double-escape (DE) efficiencies relative to the full-energy-peak efficiency (FE) have been measured for two HPGe detectors for 60Co. For a 2.5-cm-thick 95 cm3 crystal the results were SE/FE = 0.000 48 ± 0.000 20 and 0.003 25 ± 0.000 24 for 1173 and 1332 keV gamma-rays, respectively, and DE/FE = 0.000 90 ± 0.000 17 and 0.003 41 ± 0.000 11 for 1173 and 1332 keV, respectively. For a 3.0-cm-thick 84 cm3 crystal the results were SE/FE = 0.000 67 ± 0.000 32 and 0.003 79 ± 0.000 27 for 1173 and 1332 keV respectively, and DE/FE = 0.001 05 ± 0.000 28 and 0.004 29 ± 0.000 16 for 1173 and 1332 keV, respectively. These measurements may be of relevance in connection with Monte Carlo calculations of HPGe detector efficiencies, and may also suggest a path towards improved atomic cross-section measurements.

Thermal power from radioactive decay (‘decay heat’) in an ISIS proton-driven spallation neutron target has been measured in a carefully controlled and consistent way as a function of cooling time between 1 minute and 2 weeks. Overall, results are in good agreement with calculations made with the Monte Carlo program MCNPX.

Thermal powers from radioactive decays (‘decay heat’) within a proton-driven neutron-producing tungsten target on the ISIS Spallation Neutron Source have been measured. Very good agreement is found with calculations using the Monte Carlo code MCNPX.

In favourable circumstances, including the availability of prior knowledge, a potential use of gamma-ray spectroscopy with an HPGe detector for probing highly radioactive items behind thick shields is described.

One-dimensional projections of improved two-dimensional representations of transverse profiles of particle beams are proposed for fitting to data from harp-type monitors measuring beam profiles on particle accelerators. Composite distributions, with tails smoothly matched on to a central (inverted) parabola, are shown to give noticeably better fits than single gaussian and single parabolic distributions to data from harp-type beam profile monitors all along the proton beam transport lines to the two target stations on the ISIS Spallation Neutron Source. Some implications for inferring beam current densities on the beam axis are noted.

The systems for monitoring mildly radioactive air emissions from the ISIS Spallation Neutron and Muon Source are described. The systems use Geiger tubes to measure beta activity (both β – and β + ) sampled from stacks discharging air to the atmosphere, and whilst they are relatively simple systems, they have proved themselves very reliable over the thirty-six years since ISIS began operations. Calculations and measurements of sensitivities of these systems in terms of Bq m –3 per count s –1 are presented, and these calculations and measurements are in good agreement with one another.

By fitting a simple 4-parameter function to twelve sets of HPGe detector efficiency measurements, an approximate ‘universal’ expression good to ±15% is obtained for HPGe absolute full-energy-peak efficiencies as a function of photon energy between ∼125 and ∼1750 keV for a ∼4-to-1 range of crystal volumes and for coaxial and planar crystal shapes.

Measured and calculated activities of 182Ta in an ISIS spallation neutron target irradiated over five years are compared. The measurements were carried out by gamma-ray spectroscopy through the thick lead walls of a storage flask, and the calculations were carried out using MCNPX and CINDER-90. The agreement between measurement and calculation is good.

A simple approximate easy-to-use analytic formula is suggested for the absolute integrated-over-angle energy spectrum of neutrons from a high-Z spallation neutron target driven by 500-1500MeV proton beams. Overall, the formula is probably reliable to within a factor ~2-3, and could be useful for quickly making rough 'back of the envelope' estimates.

The design and operation of an apparatus to measure the beam energy of a radio frequency quadrupole (RFQ) particle accelerator, based on multiple scattering of H− ions in xenon gas, is described. The purpose of the apparatus is to confirm the mean energy and energy spread of the nominal 665 keV beam of H− ions from the ISIS RFQ. This RFQ, after comprehensive testing, is intended to replace the existing Cockcroft–Walton pre-injector on the ISIS spallation neutron source at the Rutherford Appleton Laboratory (RAL). The basis of the apparatus is a set of two cascaded assemblies each consisting of a gas scattering cell, a drift length and three small apertures, which together reduce the peak intensity of the beam current sufficiently to allow a semiconductor charged particle detector to be used to detect individual H− ions and measure their energies.

The materials engineering data base relevant to fusion irradiation is poorly populated and it has long been recognized that a fusion spectrum neutron source will be required, the facility IFMIF being the present proposal. Re- evaluation of the regulatory approach for the EU proposed DEMO device shows that the purpose of the source can be changed from lifetime equivalent irradiation exposure to data generation at lower levels of exposure by adopting a defence in depth strategy and regular component surveillance. This reduces the specification of the source with respect to IFMIF allowing lower risk technology solutions to be considered. A description of such a source, the Facility for Fusion Neutron Irradiation Research, FAFNIR, is presented here along with project timescales and costs., 8 pages, 3 figures

High power proton accelerators (HPPAs) with beam powers in the megawatt range have many possible applications including drivers for spallation neutron sources, neutrino factories, waste transmuters and tritium production facilities. These applications typically propose beam powers of 5 MW or more compared to the highest beam power achieved from a pulsed proton accelerator in routine operation of 0.16 MW at ISIS. The UK's commitment to the development of the next generation of HPPAs is demonstrated by a test stand being constructed in collaboration between RAL, Imperial College London and the University of Warwick. The aim of the RAL Front End Test Stand is to demonstrate that chopped low energy beams of high quality can be produced and is intended to allow generic experiments exploring a variety of operational regimes. This paper describes the status of the RAL Front End Test Stand which consists of five main components: a 60 mA H- ion source, a low energy beam transport, a 324 MHz Radio Frequency Quadrupole (RFQ) accelerator, a high speed beam chopper and a comprehensive suite of diagnostics. The aim is to demonstrate production of a 60 mA, 2 ms, 50 pps, chopped H- beam at 3 MeV.

The materials engineering data base relevant to fusion irradiation is poorly populated and it has long been recognized that a fusion spectrum neutron source will be required, the facility IFMIF being the present proposal. Reevaluation of the regulatory approach for the EU proposed DEMO device shows that the purpose of the source can be changed from lifetime equivalent irradiation exposure to data generation at lower levels of exposure by adopting a defence in depth strategy and regular component surveillance. This reduces the specification of the source with respect to IFMIF allowing lower risk technology solutions to be considered. A description of such a source, FAFNIR, is presented here along with project timescales and costs.

A new electron beam irradiation plant is at present being commissioned at AEA Technology's Harwell site. The principal components of the plant are a single-section travelling wave L-band linac, an energy analysis and beam scanning system, a product conveyor system and a large warehouse. The design of the new linac is based on the first section of the Harwell electron linear accelerator HELIOS originally used for nuclear research. The beam transport system on the new linac incorporates energy analysis, beam focussing and a dual magnet scanning system which ensures that the electron beam is always normal to the surface of the product under irradiation as it passes directly beneath the beam exit window. The product conveyor system is based on an inverted “power & free” chain conveyor. The speed of the section of conveyor beneath the scanned beam is servo-controlled by the linac output, which is measured with a non-intercepting toroidal beam monitor. Dosimetry is based on Far West film for product validation, Harwell 4034 red perspex for routine dosimetry and Riso polystyrene calorimeters for direct electron beam monitoring. The design and first stages of plant commissioning are described.

Explicit expressions are presented for calculating specific activities induced in food by electron and photon irradiation. In addition, sets of calculated values are tabulated for neutron fluxes produced from moderation and thermalisation of photoneutrons generated within the food. Evaluations of the prescriptions presented are compared with existing experimental measurements of activities induced by electron and photon irradiation under artificial conditions to induce measurable activities. Good agreement is obtained.

Measurements are presented of the photofission and photoneutron cross sections of 241 Am and of the mean number v of neutrons per fission between 5 and 10 MeV. The measurements were carried out using a bremsstrahlung beam generated by the Harwell electron linear accelerator HELIOS and a 1.5 g metallic 241 Am target. Neutron multiplicity distributions were recorded using a high-efficiency neutron detector. Yield measurements made in steps of 50 keV show possible evidence for a resonance at 6.1 MeV in the photofission cross section, but the statistical significance is not high.

A set of packages containing carbon, nickel, copper, zinc and gold samples was irradiated with 21.25 MeV endpoint bremsstrahlung on the Harwell electron linear accelarator HELIOS. The induced activities, measured absolutely using a 113 cm3 HPGe detector, were compared with activities calculated by folding the photonuclear cross-sections with an analytic expression for the spectrum of bremsstrahlung as a function of photon energy and angle. Very good agreement was obtained.

A series of experiments have been carried out to try and detect 'cold fusion' in titanium which has been loaded with deuterium at high pressure. No neutrons were detected using a range of counting systems, the most sensitive of which could detect 0.9 neutrons/s if sustained over 100 s. Bursts of more than 30 neutrons occurring in a period of less than 0.1 s could also have been detected using coincidence counting. Evidence for erratic behaviour of proportional counters, which could be mistaken for neutron emission, is presented.

The potential use of photonuclear reactions for a range of applications is described. These are: photonuclear transmutation doping of semiconductors, neutron production from electron linacs, quality checking of radioactive waste, fission product incineration, photoexcitation of isomers for dosimetry, and nuclear resonance fluorescence for materials analysis. Initial brief descriptions of atomic and nuclear interactions of photons and of bremsstrahlung are given.

The ISIS pulsed neutron and muon source is described. Up to now the accelerators have delivered a 145-160 kW proton beam at 50 pps to a tungsten neutron-producing target preceded by a graphite muon-producing intermediate target. However a second target station is now being constructed, and the beam power from the accelerators is being increased so that the first target station will not suffer in performance when the beam has to be divided between two target stations.

The novel Gas Scattering Energy Spectrometer on the ISIS RFQ Test Stand at the Rutherford Appleton Laboratory (Oxfordshire, UK) has been used to measure the energy spectrum of the -665 keV H beam from a four-rod RFQ as a function of RF drive power. Since EPAC'02 changes have been made to the experimental arrangement, which markedly improve the quality of the results that can be obtained. These new results are reported.

The design of an apparatus to measure the beam energy of an RFQ, based on multiple scattering of H/sup -/ ions in xenon gas, is described. The purpose of the apparatus is to confirm the mean energy and energy spread of the nominal 665 keV beam from the ISIS RFQ. This RFQ, after comprehensive testing, is intended to replace the existing Cockcroft-Walton pre-injector on the ISIS spallation neutron source at the Rutherford Appleton Laboratory (RAL). The basis of the apparatus is a set of two cascaded assemblies each consisting of a gas scattering cell, a drift length and three small apertures which together reduce the peak intensity of the beam current sufficiently to allow a semiconductor charged particle detector to be used to detect individual H/sup i/ons and measure their energies.

Currently, design software for power semiconductor devices can only represent carrier lifetime control in the crudest of ways, and there is a growing need for better design tools to enable easier process selection, more tightly focused test matrices and a reduction in developments costs for new products. In this paper we discuss new-generation doping and irradiation technologies and possible modelling approaches aimed at improving the design process. (3 pages)

The aim of the front end test stand (FETS) project is to demonstrate that chopped low energy beams of high quality can be produced. FETS consists of a 60 mA Penning Surface Plasma Ion Source, a three solenoid low energy beam transport, a 3 MeV radio frequency quadrupole, a chopper, and a comprehensive suite of diagnostics. This paper details the design and initial performance of the ion source and the laser profile measurement system. Beam current, profile, and emittance measurements are shown for different operating conditions.

Samples of chicken, prawns, cheeses and spices were irradiated on the Harwell electron linear accelerator HELIOS at 20 MeV to assess mechanisms for the induction of radioactivity. The induced radioactivity was measured using a lead shielded Ge(Li) gamma-ray spectrometer, and the results were compared with activities calculated on the basis of photoneutron and photoproton reactions induced by real and virtual photons. In general, there was good agreement. Bounds were also placed on the induction of radioactivity by capture of neutrons produced in the food samples themselves. Further, the data were used to assess the effects of a gross malfunction of an electron beam irradiation facility; after 1 day, the specific activity of food samples irradiated to 10 kGy at 20 MeV was approximately 0.01 Bq g-1. In addition, food samples were also irradiated at 10 MeV, and irradiated and control samples were analysed for microbiological burden. Reductions in the microbiological burden of the food samples by factors consistent with those found in previous measurements were found.

The (γ, f) and (γ, n) cross-sections and the mean number v of neutrons per fission have been measured for 232Th and 238U between 5 and 10 MeV. A new method of making bremsstrahlung measurements resulted in data having a good combination of energy resolution and statistical accuracy. For 232Th observed subthreshold (γ, f) cross-section resonances and v values suggest the existence of two separate shallow wells superimposed on the double humped fission barrier. For 238U similar resonances between about 5.5 and 6.0 MeV cannot easily be interpreted in terms of the double humped fission barrier.

The standard Penfold-Leiss method of unfolding a photonuclear cross-section from a measured bremsstrahlung yield curve is shown to fail if the cross-section is unfolded in photon energy bins whose width is comparable with the range of any smearing of the bremsstrahlung spectrum near the endpoint. In the case described in the text the smearing is due to the energy spread of electrons striking the bremsstrahlung radiator. A simple modification to the Penfold-Leiss prescription is described which overcomes this failure.

Measurements are presented of the photofission cross section and the mean number ν of neutrons per fission for 232 Th made using bremsstrahlung from the Harwell electron linear accelerator HELIOS and a high efficiency neutron detector to record neutron multiplicity distributions. The cross section is interpreted in terms of a fission barrier with shallow wells in two separate outer barriers. Interesting structure is observed in ν as a function of photon energy.

The differential cross section for the /sup 16/O(..gamma..,p)/sup 15/N process has been measured in the energy range E/sub ..gamma../ = 100--400 MeV at laboratory angles of 45/sup 0/, 90/sup 0/, and 135/sup 0/. The cross sections for the (..gamma..,p) reaction leaving /sup 15/N in its ground state, in its first and second excited states at E/sub x/roughly-equal5.3 MeV, and in its third excited state at E/sub x/ = 6.3 MeV were extracted from the tip region of the proton spectra measured at a series of bremsstrahlung end point energies. The data are compared with calculations based on a direct, single-particle knockout mechanism and on mechanisms involving two particles in an intermediate state.

The p-shell momentum distribution in 12C spanning the range 0–450 MeV/c is derived from experimental data on the 12C(e, e′p) and 12C(γ, p) reactions. In both cases a simplified correction is made for the distortion of the outgoing proton wave by the residual nucleus. The results are compared with a momentum wave function calculated from the Elton-Swift potential.

Measurements of the mean ( v ) and variance (σ2 of the prompt neutron multiplicity distribution for the spontaneous fission of 252Cf and 242Pu have been made. Quantities of 252Cf and 242Pu were incorporated in ionization chambers placed at the centre of a large oil-moderated neutron detector. The dependence of both v and σ2 on the fission-fragment detection efficiency of the ionization chambers was investigated in detail and led to a correction in 242Pu of 0.68% for v and 1.04% for σ2. The values deduced are: v ( 252 Cf ) = 3.752 ± 0.029, σ 2 ( 252 Cf ) = 1.562 ± 0.011 v ( 242 Pu ) = 2.153 ± 0.019, σ 2 ( 242 Cf ) = 1.298 ± 0.008 The ratio v ( Pu ) v ( Cf ) is determined more accurately than either absolute measurement, and is 0.5738 ± 0.0033.

A convenient, analytic representation of the bremsstrahlung spectrum as a function of both photon energy and angle has been developed. The representation accommodates radiators of thicknesses up to the range of the incident electrons, and is primarily intended for electrons in the energy range ∼ 5–20 MeV. The basis is a linear integrated-over-angle intensity spectrum, a Gaussian bremsstrahlung cross section angular distribution, a Gaussian electron multiple scattering angular distribution, a constant electron stopping power, and a linear function representing both photon attenuation within the radiator and the electron transmission function through the radiator, from which is derived an analytic expression involving the exponential integral ei( x ). On comparing with measurements and Monte Carlo computations, the present expression is found to deviate by ∼ 20%.

The angular distribution of the 12C( gamma ,p) reaction has been measured at E gamma =60, 80 and 100 MeV for excitation energies 0-7 MeV in the residual nucleus. The measured cross sections are considerably larger than IPM predictions and are compared with a calculation which uses a Jastrow correlation factor to simulate the effect of two-nucleon interactions.

Formulae are given for the computation of the distribution of energy losses (straggling) in moderately thin absorbers (0.01– 0.05 radiation lengths) for electrons with energies ≳a15 MeV. Energy losses by collision and by radiation are taken into account, using an extension of the Landau straggling theory folded with an expression given by Heitler. The results of these calculations are compared with available experimental data.

An effective momentum distribution in the region from about 300 to 900 MeV/c has been obtained from recent high-energy 16O(γ, p0) data. The result is compared with two different interpretations of other experimental data and with theoretical momentum distributions.

The computation of the bremsstrahlung spectrum at 0° from a 0.1 g cm−2 Au radiator struck by a 5–10 MeV beam from an electron linear accelerator is described. The computation is based on recent partial wave calculations of the integrated-over-angle bremsstrahlung cross-section, and includes energy losses, straggling and multiple scattering of the electrons in the radiator, the variation of the bremsstrahlung cross-section angular distribution with photon energy, and an incident electron energy spectrum appropriate for the analysed beam from an electron linear accelerator. The importance of the various components of the bremsstrahlung spectrum is quantified by determining the sensitivity of a typical threshold photofission cross-section unfolded from a yield curve to these components.

Various parameterizations of the multiplicity distribution of prompt neutrons from nuclear fission are investigated. The currently accepted formalism is found to fail for those fission processes which release on average a low number of neutrons. Based on realistic assumptions regarding binary fission, an improved multiplicity distribution formula gives much better agreement with the experimental data. Explicit details of the practical usage of the new formalism are presented. Finally, it is shown that for spontaneously fissioning isotopes, it is possible to calculate the variance of the neutron multiplicity distribution from a knowledge of the mass yield and the variances of the neutron multiplicity distributions of the individual fission fragments.

Measurements have been made of afterpulses induced by light from a blue LED in EMI 9823B, Hamamatsu R1250, Mullard XP2041 and RCA 8854 fast linear-focussed photomultiplier tubes. The RCA and Hamamatsu tubes were found to produce significantly fewer afterpulses than the EMI and Mullard tubes. More detailed measurements of the Hamamatsu R1250 indicated that the total number of afterpulses is proportional to the magnitude of the initial flash and consists of two components. The first component is independent of the wavelength of the initial light flash, arises from ionization of residual gas in the phototube and can be effectively suppressed by gating the photomultiplier. The second component is wavelength-dependent, dominates the first component at short wavelengths, and cannot be significantly reduced by gating. It is possibly due to fluorescence of the phototube glass envelope or excitation of a metastable state of the photocathode. Similar components were found to contribute to afterpulsing in the RCA 8854 PMT.

The 151 Eu, 153 Eu and 156 Gd photoneutron cross sections in the giant dipole resonance region have been measured using bremsstrahlung from the University of Melbourne betatron. Absolute cross-section values, lorentzian parameters, intrinsic quadrupole moments, deformation parameters and integrated cross-section values are derived. For 151 Eu and 153 Eu the measured cross sections are in good agreement with the predictions of the extended dynamic collective model.

In this paper we present (γ,p) measurements for the lithium, beryllium and carbon nuclei in the photon energy range 60–100 MeV. A single-difference bremsstrahlung unfolding technique is used to produce effectively monochromatic photons of energies 60±1, 80±1 and 100±1 MeV. At 60 MeV proton spectra were obtained at θ p = 45° over a range of residual nuclear excitation energies from 0 to 30 MeV for 6,7 Li and 0–20 MeV for 9 Be and 12 C. Also the angular distribution between 30° and 150° was measured for the highest energy protons from 6, 7 Li and 12 C at E γ = 60 MeV, for 7 Li and 12 C at E γ = 80 MeV and for 12 C at E γ = 100 MeV. Qualitative conclusions concerning the possible reaction mechanism may be drawn from the observed proton spectra, viz. that they support the hypothesis of a direct interaction of the photon with a proton in a shell-model (1s or 1p) orbit. However, simple shell-model wave functions are found to have insufficient amplitude in the high momentum region to account for the data. A phenomenological calculation which includes nucleon-nucleon correlations by means of the Jastrow model represents only approximately the energy and angular dependence of the measured cross sections.

Experiments using three different calorimeter designs and high-efficiency neutron and γ -ray detection on a wide range of materials fail to sustain the recent claims of cold fusion made by Fleischmann et al.1 and Jones et al.2. Spurious effects which, undetected, could have led to claims of cold fusion, include noise from neutron counters, cosmic-ray background variations, calibration errors in simple calorimeters and variable electrolytic enrichment of tritium.

An improvement to the Blunck-Leisegang energy loss straggling distribution is presented by folding the effect of resonant energy transfers into a sum of nine Gaussians which is a good fit to the Landau distribution.

The /sup 16/O(..gamma..,p/sub 0/) cross section has been measured for a series of photon energies between 100 and 300 MeV at proton angles of 45/sup 0/, 90/sup 0/, and 135/sup 0/. Above 250 MeV, the results exceed simple shell-model predictions by several orders of magnitude. The data are compared with a calculation which involves ..delta.. excitation in an intermediate state. (AIP)

Experimental data on the (γ, p) reaction at E γ = 60−100 MeV for targets in the range A = 7−93 are compared with predictions based on a single-particle knock-out mechanism using shell model wavefunctions. The results show that this mechanism is more important than has generally been believed

A recent measurement of 16O(γ,p0) cross sections for Eγ?100 MeV is compared with two theoretical calculations which involve the excitation of the Δ (1232) isobar in an intermediate state.