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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.

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.

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 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 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.

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.

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.

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