137Cs-ajoitusmenetelmän tarkkuus ja luotettavuus

137Cs is an artificial isotope generated by fission reactions, whose deposition times are well known both globally and locally. The 137Cs dating method measures the 137Cs activity in sediment layers from different depths, and by utilizing knowledge of the deposition moment dating accuracy can be up to one year. The prerequisites of the dating method are not only the knowledge of precise moments of deposition, but also calm sedimentation and understanding of the sediment disturbances. The best destinations for using the 137Cs dating method are deep basin sediments, where bioturbation is very low and sedimentation is calm. This study looked at the accuracy and reliability of the 137Cs dating of the bottom sediment from five different lakes. There were clear annual varves in four out of five lakes, so the sediment was not mixed and mechanical movements of 137Cs had not occurred. The most significant fall-off affecting Finland in 1986 was the Chernobyl nuclear accident and when analyzing sediment samples in 1986, it could even be traced back to a single varve. Another significant 137Cs fall-off has been the nuclear tests of the 1950s to 1960s. The 137Cs peak of these events was almost invisible compared to the 137Cs activity of 1986. Regardless of the limnological features of the lake, 1986 had also migrated downward in the analysis results. The more porous sediment was, the stronger the spread had occurred. According to the results of this study the 137Cs dating method is still usable, especially in Finland at the time of the Chernobyl nuclear accident of 1986. From 1950 to 1960, the time of nuclear testing, this method is far more uncertain in Finland, although the marker horizon is more globally distinguishable. However, downward migration of 137Cs was clearly evident even in a calm sedimentation environment, which is a major hindrance to the method. Chemical follow-up studies would be useful to clarify the causes of migration.137Cs on keinotekoinen fissioreaktioissa syntyvä isotooppi, jonka laskeumahetket ovat tunnettuja sekä globaalisti että paikallisesti. 137Cs-ajoitusmenetelmässä mitataan sedimenttinäytteessä oleva 137Cs-aktiivisuus eri syvyyksiltä ja hyödyntäen tietämystä laskeumahetkistä voidaan ajoitus tehdä jopa vuoden tarkkuudelle. Ajoitusmenetelmän lähtökohtia ovat tarkkojen laskeumahetkien tietämisen lisäksi sedimentin häiriöttömyys ja rauhallinen sedimentaatio. Parhaimpia kohteita 137Cs-ajoitusmenetelmän käytölle ovat järvien syvänteet, joissa bioturbaatio on hyvin vähistä ja sedimentaatio rauhallista. Työssä tutkittiin viiden erilaisen järven pohjasedimentin 137Cs-ajoittamisen tarkkuutta ja luotettavuutta. Järvisedimenteistä neljässä viidestä oli havaittavissa selkeitä vuosilustoja, joten sedimentti ei ole sekoittunut ja mekaanista 137Cs:n liikkumista ei ole tapahtunut. Suomeen vaikuttanut merkittävin laskeumahetki on 1986 tapahtunut Tshernobylin ydinonnettomuus ja sedimenttinäytteitä analysoitaessa vuosi 1986 voitiin jäljittää jopa lustotasolle. Toinen merkittävä 137Cs:n laskeuma-ajankohta on ollut 1950 - 1960 -lukujen ydinkokeet. Näiden 137Cs-aktiivisuuden piikki oli lähes näkymätön vuoden 1986 137Cs-aktiivisuuteen verrattuna. Riippumatta järven limnologisista piirteistä oli analyysituloksissa vuoden 1986 piikki levinnyt myös alaspäin. Mitä huokoisempaa sedimentti oli sitä voimakkaampaa leviämistä oli tapahtunut. Tulosten mukaan 137Cs-ajoitusmenetelmä on edelleen käyttökelpoinen varsinkin Suomessa vuoden 1986 Tshernobylin ydinonnettomuuden ajankohtaa käyttäen. 1950 - 1960 ydinkokeiden ajankohta on huomattavasti epävarmempi, vaikkakin laskeuma on tasaisempi globaalisti. 137Cs:n migraatiota alaspäin oli selkeästi havaittavissa jopa rauhallisessa sedimentaatioympäristössä. Mahdolliset kemialliset jatkotutkimukset toivottavasti selventävät migraation syitä

Simulation and Efficiency Studies of Optical Photon Transportation and Detection with Plastic Antineutrino Detector Modules

In this work, the simulation of optical photons is carried out in an antineutrino detector module consisting of a plastic scintillator connected to light guides and photomultipliers on both ends, which is considered to be used for remote reactor monitoring in the field of nuclear safety. Using Monte Carlo (MC) based GEANT4 simulation, numerous parameters influencing the light collection and thereby the energy resolution of the antineutrino detector module are studied: e.g., degrees of scintillator surface roughness, reflector type, and its ap- plying method onto scintillator and light guide surface, the reflectivity of the reflector, light guide geometries and diameter of the photocathode. The impact of each parameter is inves- tigated by looking at the detected spectrum, i.e. the number photoelectrons per depositing energy. In addition, the average light collection efficiency of the detector module and its spatial variation are calculated for each simulation setup. According to the simulation re- sults, it is found that photocathode size, light guide shape, reflectivity of reflecting material and wrapping method show a significant impact on the light collection efficiency while scin- tillator surface polishing level and the choose of reflector type show relatively less impact. This study demonstrates that these parameters are very important in the design of plastic scintillator included antineutrino detectors to improve the energy resolution efficiency

Investigation of Spatial and Temporal Aspects of Airborne Gamma Spectrometry: Final Report

A study has been conducted which demonstrates the reproducibility of Airborne Gamma-ray Spectrometry (AGS) and the effects of changes in survey parameters, particularly line spacing. This has involved analysis of new data collected from estuarine salt marsh and upland areas in West Cumbria and SW Scotland during three phases of field work, in which over 150000 spectra were recorded with a 16 litre NaI(Tl) detector. The shapes and inventories of radiometric features have been examined. It has been shown that features with dimensions that are large relative to the survey line spacing are very well reproduced with all line spacings, whereas smaller features show more variability. The AGS technique has been applied to measuring changes in the radiation environment over a range of time scales from a few days to several years using data collected during this and previous surveys of the area. Changes due to sedimentation and erosion of salt marshes, and hydrological transportation of upland activity have been observed

Report on Airborne Radiometric Survey of the Thurso Area, 15th to 17th November 2006

A survey of part of Caithness near Thurso has been conducted as a performance test for a new Airborne Gamma Spectrometry (AGS) system developed at SUERC. The system deployed for this work consisted of a 16 litre NaI(Tl) spectrometer using a 2s measurement time. A total of 7500 spectra were collected in 13.5 hours flight time between the 15th-17th November 2006. The survey comprised a 60x31km area with a 5km line spacing providing an overview of the regional radiation environment, and a 15x20km area at 250m and 500m line spacings providing detailed information on the radiation environment of the Forss Water and River Thurso and their catchments. The survey areas did not include the Dounreay site and the town of Thurso. The data were processed to estimate activity concentrations of 137Cs (kBq m-2), naturally occurring 40K, 214Bi and 208Tl (Bq kg-1) and the gamma ray dose rate (mGy a-1). Maps were produced for the distribution of these activities within both the larger and more detailed survey areas. The gamma dose rate is driven by the natural activity, with no significant contribution from 137Cs activity. The region has considerable areas of saturated peat, suppressing the radiation from the underlying geology. The demarcation between the peat and the flagstone formations with thin soil overlays is clearly seen in the natural series activity and dose rate maps. 137Cs activities of up to 10 kBq m-2 have been observed on the drier land, consistent with Chernobyl fallout, with strongly suppressed signals from the peaty areas indicating that any fallout on these areas has migrated down through the peat. This work confirms the presence of features which accumulate 137Cs in the Forss Water system that had been noted in the survey commissioned by UKAEA in 1998. One feature observed in 1998 is absent in the data recorded in this work. Additional features were also recorded for the first time in the Forss Water upstream of the 1998 survey, and a series of similar featured, potentially of more limited extent, in the River Thurso system. The exploratory survey to the west also showed possible small scale features in Strath Halladale. Additional work would be needed to assess the dynamics of these systems, and their detailed local spatial characteristics and activity profiles, and to explore the extent to which similar features can be found in the fluvial catchments of other areas subjected to radioactive fallout in upper catchments, particularly those with peaty substrates. Time series analysis of these new data in comparison with older airborne data sets from the region would be of value, as would additional work in the future to extend the spatial and temporal boundaries of this survey

Measurement of the Intrinsic Radiopurity of Cs-137/U-235/U-238/Th-232 in CsI(Tl) Crystal Scintillators

The inorganic crystal scintillator CsI(Tl) has been used for low energy neutrino and Dark Matter experiments, where the intrinsic radiopurity is an issue of major importance. Low-background data were taken with a CsI(Tl) crystal array at the Kuo-Sheng Reactor Neutrino Laboratory. The pulse shape discrimination capabilities of the crystal, as well as the temporal and spatial correlations of the events, provide powerful means of measuring the intrinsic radiopurity of Cs-137 as well as the U-235, U-238 and Th-232 series. The event selection algorithms are described, with which the decay half-lives of Po-218, Po-214, Rn-220, Po-216 and Po-212 were derived. The measurements of the contamination levels, their concentration gradients with the crystal growth axis, and the uniformity among different crystal samples, are reported. The radiopurity in the U-238 and Th-232 series are comparable to those of the best reported in other crystal scintillators. Significant improvements in measurement sensitivities were achieved, similar to those from dedicated massive liquid scintillator detector. This analysis also provides in situ measurements of the detector performance parameters, such as spatial resolution, quenching factors, and data acquisition dead time.Comment: 28 pages, 12 figure