A Brief Aerial Survey in the Vicinity of Sellafield in September 1990

A two day survey exercise was conducted by SURRC from 25th-26th September 1990 with the joint aims of demonstrating the potential of helicopters for emergency response and beginning the definition of baseline levels in the immediate vicinity of Sellafield. The aircraft used for this work was a twin engined AS 355 "Squirrel" helicopter chartered from Dollar Helicopters. It was loaded with an 8 litre NaI gamma spectrometry system at SURRC in East Kilbride on the afternoon of 24th September and flown down to Sellafield the same day. Over the following two days roughly 1300 gamma ray spectra were recorded from an area ranging south from an EW line linking Ennerdale Fell and St. Bees Head to beyond Ravenglass. Operations were conducted from the Sellafield helipad, the aircraft being refuelled at Barrow in Furness. The flights were arranged to provide nominally 1km spaced parallel NS flight lines throughout the survey area, for the purpose of baseline mapping. Supplementary flights to improve spatial resolution are possible at a later stage. In addition a rapid response flight route was rehearsed involving definition of landward arcs at 10km, 5km and 2km radii from Sellafield plus the beachline from St. Bees Head to Ravenglass. The precise path was chosen to be navigable under most weather conditions and took roughly 40 minutes to fly. A survey aircraft arriving from East Kilbride could perform such a survey without pausing to refuel. The results have been stored archivally and used to map the naturally occurring nuclides 40K, 214Bi, 208Tl together with 137 Cs and total gamma ray flux. The maps presented are spatially smoothed, both by the inherent character of the aerial survey technique and by the colour contouring processes. This leads to a tendency to broaden spatial features, while slightly reducing maximum values, and should be taken into account when interpreting the maps. Greater spatial detail could be achieved with closer flight line spacing. Activity due to Sellafield was readily detected in the area and can be seen clearly in the maps. It is noted that the levels observed are generally comparable with those in BNFL annual reports and other research publications. Furthermore, with a few exceptions, naturally occurring radionuclides are the dominant radiation source in much of the survey area, and show considerable variations from place to place. 137Cs due to marine discharges was most pronounced in the Irt, Mite and Esk estuaries and on the beachline close to the plant. The same nuclide was detected at lower levels in terrestrial areas due to a combination of global fallout, aerial discharges from the plant and the Chernobyl accident. 41Ar and 16N activities were detected in the immediate vicinity of Calder Hall. A small feature was detected at Drigg, probably associated with current operations on the site, and equivalent to an enhancement of less than twice the local natural background. Radiation levels due to current activities on the Sellafield site fall off rapidly with distance from the perimeter, approaching natural levels within 0.5-1km at the time of the survey. Those from the marine, estuarine and tide washed environments are mostly attributed to past marine discharges, and can be expected to continue to receive attention in the future

A Brief Aerial Survey in the Vicinity of Sellafield in September 1990

A two day survey exercise was conducted by SURRC from 25th-26th September 1990 with the joint aims of demonstrating the potential of helicopters for emergency response and beginning the definition of baseline levels in the immediate vicinity of Sellafield. The aircraft used for this work was a twin engined AS 355 "Squirrel" helicopter chartered from Dollar Helicopters. It was loaded with an 8 litre NaI gamma spectrometry system at SURRC in East Kilbride on the afternoon of 24th September and flown down to Sellafield the same day. Over the following two days roughly 1300 gamma ray spectra were recorded from an area ranging south from an EW line linking Ennerdale Fell and St. Bees Head to beyond Ravenglass. Operations were conducted from the Sellafield helipad, the aircraft being refuelled at Barrow in Furness. The flights were arranged to provide nominally 1km spaced parallel NS flight lines throughout the survey area, for the purpose of baseline mapping. Supplementary flights to improve spatial resolution are possible at a later stage. In addition a rapid response flight route was rehearsed involving definition of landward arcs at 10km, 5km and 2km radii from Sellafield plus the beachline from St. Bees Head to Ravenglass. The precise path was chosen to be navigable under most weather conditions and took roughly 40 minutes to fly. A survey aircraft arriving from East Kilbride could perform such a survey without pausing to refuel. The results have been stored archivally and used to map the naturally occurring nuclides 40K, 214Bi, 208Tl together with 137 Cs and total gamma ray flux. The maps presented are spatially smoothed, both by the inherent character of the aerial survey technique and by the colour contouring processes. This leads to a tendency to broaden spatial features, while slightly reducing maximum values, and should be taken into account when interpreting the maps. Greater spatial detail could be achieved with closer flight line spacing. Activity due to Sellafield was readily detected in the area and can be seen clearly in the maps. It is noted that the levels observed are generally comparable with those in BNFL annual reports and other research publications. Furthermore, with a few exceptions, naturally occurring radionuclides are the dominant radiation source in much of the survey area, and show considerable variations from place to place. 137Cs due to marine discharges was most pronounced in the Irt, Mite and Esk estuaries and on the beachline close to the plant. The same nuclide was detected at lower levels in terrestrial areas due to a combination of global fallout, aerial discharges from the plant and the Chernobyl accident. 41Ar and 16N activities were detected in the immediate vicinity of Calder Hall. A small feature was detected at Drigg, probably associated with current operations on the site, and equivalent to an enhancement of less than twice the local natural background. Radiation levels due to current activities on the Sellafield site fall off rapidly with distance from the perimeter, approaching natural levels within 0.5-1km at the time of the survey. Those from the marine, estuarine and tide washed environments are mostly attributed to past marine discharges, and can be expected to continue to receive attention in the future

Minority carrier lifetime in silicon photovoltaics : the effect of oxygen precipitation

Single-crystal Czochralski silicon used for photovoltaics is typically supersaturated with interstitial oxygen at temperatures just below the melting point. Oxide precipitates therefore can form during ingot cooling and cell processing, and nucleation sites are typically vacancy-rich regions. Oxygen precipitation gives rise to recombination centres, which can reduce cell efficiencies by as much as 4% (absolute). We have studied the recombination behaviour in p-type and n-type monocrystalline silicon with a range of doping levels intentionally processed to contain oxide precipitates with a range of densities, sizes and morphologies. We analyse injection-dependent minority carrier lifetime measurements to give a full parameterisation of the recombination activity in terms of Shockley–Read–Hall statistics. We intentionally contaminate specimens with iron, and show recombination activity arises from iron segregated to oxide precipitates and surrounding defects. We find that phosphorus diffusion gettering reduces the recombination activity of the precipitates to some extent. We also find that bulk iron is preferentially gettered to the phosphorus diffused layer rather than to oxide precipitates

Spin-wave spectrum in La2CuO4 -- double occupancy and competing interaction effects

The recently observed spin-wave energy dispersion along the AF zone boundary in La2CuO4 is discussed in terms of double occupancy and competing interaction effects in the $t-t'$ Hubbard model on a square lattice.Comment: 4 pages, 2 figure

Broken-Symmetry States in Quantum Hall Superlattices

We argue that broken-symmetry states with either spatially diagonal or spatially off-diagonal order are likely in the quantum Hall regime, for clean multiple quantum well (MQW) systems with small layer separations. We find that for MQW systems, unlike bilayers, charge order tends to be favored over spontaneous interlayer coherence. We estimate the size of the interlayer tunneling amplitude needed to stabilize superlattice Bloch minibands by comparing the variational energies of interlayer-coherent superlattice miniband states with those of states with charge order and states with no broken symmetries. We predict that when coherent miniband ground states are stable, strong interlayer electronic correlations will strongly enhance the growth-direction tunneling conductance and promote the possibility of Bloch oscillations.Comment: 9 pages LaTeX, 4 figures EPS, to be published in PR

Spin Excitations in La2CuO4: Consistent Description by Inclusion of Ring-Exchange

We consider the square lattice Heisenberg antiferromagnet with plaquette ring exchange and a finite interlayer coupling leading to a consistent description of the spin-wave excitation spectrum in La2CuO4. The values of the in-plane exchange parameters, including ring-exchange J_{\Box}, are obtained consistently by an accurate fit to the experimentally observed in-plane spin-wave dispersion, while the out-of-plane exchange interaction is found from the temperature dependence of the sublattice magnetization at low temperatures. The fitted exchange interactions J=151.9 meV and J_{\Box}=0.24 J give values for the spin stiffness and the Neel temperature in excellent agreement with the experimental data.Comment: 4 pages, 1 figure, RevTe

Expanded Vandermonde powers and sum rules for the two-dimensional one-component plasma

The two-dimensional one-component plasma (2dOCP) is a system of $N$ mobile particles of the same charge $q$ on a surface with a neutralising background. The Boltzmann factor of the 2dOCP at temperature $T$ can be expressed as a Vandermonde determinant to the power $\Gamma=q^{2}/(k_B T)$. Recent advances in the theory of symmetric and anti-symmetric Jack polymonials provide an efficient way to expand this power of the Vandermonde in their monomial basis, allowing the computation of several thermodynamic and structural properties of the 2dOCP for $N$ values up to 14 and $\Gamma$ equal to 4, 6 and 8. In this work, we explore two applications of this formalism to study the moments of the pair correlation function of the 2dOCP on a sphere, and the distribution of radial linear statistics of the 2dOCP in the plane