Modelling gamma spectrometry systems for use in beach monitoring near Sellafield. Part II: effect of 137Cs background and simulation of finds

Cresswell, A. and Sanderson, D. (2011) Modelling gamma spectrometry systems for use in beach monitoring near Sellafield. Part II: effect of 137Cs background and simulation of finds. Scottish Universities Environmental Research Centre.

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<p>The response of large volume Airborne Gamma Spectrometry (AGS) detectors has been modelled using Monte Carlo methods as part of an assessment of ways to enhance monitoring of beaches and other coastal areas for radioactive particles. Airborne survey methods are capable of rapid surveys of large areas, and of covering diverse environments effectively, including areas where ground based work is difficult to implement safely. They have potential for increasing the effectiveness of ground based surveys, working in a complementary manner to identify areas where detailed work including recovery operations would be targeted. The study presented here is a continuation of an earlier study defining point source detection limits for airborne gamma spectrometry in natural radioactivity backgrounds representative of the West Cumbria beaches. Here anthropogenic backgrounds in the area are also reviewed together with their impact on detection limits. Similarly the signals that would be expected from particles already recovered from the beaches of West Cumbria are assessed in comparison with observed background for 137Cs to appraise the extent to which those particles recovered may have contributed to past airborne surveys of the beaches. Theoretical sensitivities have been assessed using airborne systems operated under two survey designs: a “rapid” design at 75 m (200 ft) ground clearance and a 70 knot (30 m s-1) survey speed, capable of surveying > 60km of shore in a 2h survey with multiple lines; and also a “slow and low” design based on 15 m (50 ft) ground clearance and 15 knot (5 ms-1) forward velocity, suitable to surveys of ~5km of beach in a 2h flight. Additional consideration is given to surveys at further reduced height and speed, to meet a detection criterion of 105 Bq sources at 10cm burial depth.</p> <p>Monte-Carlo methods were used to simulate the full spectral responses for a series of 137Cs and 241Am point sources placed in simulated natural background environments with varying 137Cs background distributions. The MCII Monte Carlo code developed at SUERC, which has been extensively validated in the past and incorporates statistical estimation procedures to simulate spectra at airborne source-detector separations, was used.</p> <p>A review of the distribution of dispersed 137Cs on the beaches showed areas of beach with negligible 137Cs activity, areas with approximately uniform 137Cs backgrounds, and areas with patches of dimensions of 5-10m with elevated 137Cs activity. Samples collected from the higher activity areas of the beaches had activity concentrations of ~50 Bq kg-1. Spectra were simulated for 137Cs background distributions for single and multiple patches of activity with 10m dimensions and 50 Bq kg-1 concentrations, and uniform distributions of different concentrations, at 75 and 15m heights. The analyses for point sources with the medium natural background conducted in the earlier study were repeated with these different 137Cs backgrounds.</p> <p>The data for the location, depth and activity for the recovered particles were used to simulate the spectra that an airborne system would observe from these particles, and compared with the spectra recorded during earlier surveys of the beaches of West Cumbria. Each particle recovered is below the detection limit of a 75m survey, and even cumulatively the recovered finds would not result in any observed signal in the past surveys. The past airborne survey data show that theronsiderable 137Cs inventory on the beaches between St Bees and Duddon), of which the recovered s repre001% of the activity on the beaches they h recovered from. The nature and distribution of theng activity haen comprehensively reviewed in this work, they clearly represent the major anthropogenic so The have shown that in the presence of approxuniform natural and anthropogenic background radiatio typical for the sand and gravel beaches, “rapid” AGS surveys would have a detection limit for superficial 137Cs sources of 5-10 MBq, anled “low and slow” its of 200-300 kBq. Detection limits for sources at 10cm burial are a factor of 4 greater. “Low and slow” surveys have detection limits for superficial 241Am of 1-2 MBq. The patches of activity simulated here generate almost identical flight line profiles as point sources of similar total activity. At ground level the smaller field of view compared with airborne systems assists such discrimination for activity patches of the dimensions simulated here, although the same ambiguity would exist for smaller patches of dispersed activity. The use of collimation in the airborne system to reduce the field of view could be considered. A moderately dense pattern of patches (eg: >25% of the area) approximates a uniform distribution of activity. In this situation, detection limits for a uniform 137Cs background of 50 Bq kg-1 are increased by a similar factor to changing from a medium to high natural activity background.</p> <p>Reducing ground clearance would increase full energy peak count rates, by an order of magnitude at 5m compared to 15m. Thus surveys at these ground clearances would significantly increase the significance of full-energy peak counts from sources in comparison with the 15m specification. To achieve a detection limit for 137Cs sources at 10cm depth of 105 Bq in uniform background radiation fields it is estimated that it would be necessary to reduce ground speed to below 5 knots (2 m s-1) and ground clearance to 5-10m. A data processing methodology that utilises the scattered part of the spectra (>80% of the total count rate from buried sources) might potentially further improve detection efficiency, and provide an estimate of source burial depth. Detector collimation may allow better discrimination between point sources and small distributed patches of activity.</p> <p>The analysis here has related to detection of sources in single measurements, or summed consecutive measurements treated as single measurements. In survey design and data analysis, observations are combined from multiple survey lines. Methods which utilise interpolation across several observations on multiple survey lines have the potential to lower detection limits compared to approaches that utilise information within single survey lines. However such analysis would be retrospective, which may have implications for implementation in source searches.</p> <p>This work has confirmed that airborne methods are capable of providing useful levels of sensitivity, with survey rates which can not be practically achieved at ground level. Airborne methods can demonstrate the absence of strong sources within large areas where anthropogenic background distributions are approximately uniform, and identify areas where patchy anthropogenic distributions would require further ground based investigations to confirm the absence of sources. AGS has an important role in identifying where valuable ground-based resources should be focussed. It is suggested that practical work be initiated to verify the theoretical detection limits presented here and that consideration be given to developing the AGS role in future programmes. In addition to validating the sensitivity and detection limits demonstrated in this theoretical study, this would provide an opportunity to evaluate the practical limits for operating helicopters at low speed and ground clearance over public beaches while complying with safety and regulatory requirement.</p>

Item Type:Books
Glasgow Author(s) Enlighten ID:Sanderson, Professor David and Cresswell, Dr Alan
Authors: Cresswell, A., and Sanderson, D.
Subjects:Q Science > QC Physics
Q Science > QE Geology
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Publisher:Scottish Universities Environmental Research Centre

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