Gus MacKenzie. Scottish Universities Environmental Research Centre

Transcription

1 From Lucas cells to AMS: 40 years of environmental radioactivity research Gus MacKenzie Scottish Universities Environmental Research Centre University of Glasgow

2 Key dates in developments in radioactivity at Glasgow/SURRC/SUERC Chemical change in α decay. Concept of isotopes: Nobel prize for Chemistry 1921 Frederick Soddy Lecturer in chemistry

3 Sir Samuel Curran MA Natural philosophy 1933 BSc Natural Philosophy 1934 PhD Physics : invention of scintillation counter : lecturer in Physics 1948: invention of gas proportional counter : AWRE Aldermaston : Principal Glasgow Royal College of Science and Technology 1962: University of Strathclyde 1963: established Scottish Universities Research and Reactor Centre (SURRC)

4 Scottish Universities Research and Reactor Centre (SURRC) Scottish Universities Environmental research Centre (SUERC) 1963: Centre opened by Sir John Cockroft UTR 300 reactor operated : Glasgow and NERC radiocarbon labs established 1997: Renamed Scottish Universities Environmental Research Centre (SUERC) Reactor decommissioned and nuclear site de-licensed 2001: 5 MV tandem AMS installed 2008: 250 kv SSAMS installed

5 Radiochemistry in Glasgow University: 1972 Chemistry Department Fluorine chemistry: 18 F tracer studies Surface chemistry: 3 H and 14 C of sorption/desorption reactions Polymers: 14 C tracer studies of polymerisation Organic synthesis: 14 C tracer studies Geochemistry: 14 C dating; natural decay series studies; weapons testing fallout studies Other departments: Neutron activation analysis in: Agricultural Chemistry, Archaeology, Forensic Medicine, Geology University of Glasgow School of Chemistry

6 238 U 234 U 235 U 4.47x10 9 y 2.45x10 5 y 7.04x10 8 y 234 Pa 231 Pa 1.17 min 3.28x10 4 y 234 Th 230 Th 231 Th 227 Th 232 Th 228 Th 24.1 d 7.54x10 4 y 1.06 d 18.7 d 1.41x10 10 y 1.91 y 227 Ac 228 Ac 21.8 d 6.13 h 226 Ra 223 Ra 228 Ra 224 Ra Alpha decay Beta decay 1600 y 11.4 d 5.75 y 3.66 d 222 Rn 219 Rn 220 Rn d 3.96 s 55.6 s 218 Po 214 Po 210 Po 215 Po 216 Po 212 Po 3.0 min 1.6x10-4 s d 1.8x10-3 s 0.15 s 3x10-7 s 214 Bi 210 Bi 211 Bi 212 Bi 19.9 min 5.01 d 2.14 min 1.01 h 214 Pb 210 Pb 206 Pb 211 Pb 207 Pb 212 Pb 208 Pb 26.8 min 22.3 y stable 36.1 min stable 10.6 h stable 207 Tl 208 Tl 4.77 min 3.05 min The Natural Radioactive Decay Series

7 Available equipment in 1972: 222 Rn analysis: home made Lucas cell, pm tube and housing, scaler β - analysis: γ spectroscopy: α spectroscopy: Tracerlab low background anti-coincidence proportional counter 2 NaI(Tl) detector in basic 4 Pb shield; 256 channel analyser surface barrier detectors in home made vacuum chambers, 256 channel analyser. Available equipment in Octete α spectroscopy systems α/β separation liquid scintillation spectrometers Low background HPGe detectors in graded Pb-Cd-Cu shields ICP-MS and MC-ICP-MS instruments 5 MV tandem AMS 250 kv SSAMS

8 Doctoral Research: use of 228 Ra/ 226 Ra ratios as a tracer of water circulation and residence time in the Clyde Sea Area 226 Ra α 222 Rn 1600 y d α Analysis: equilibration then extraction of 222 Rn Detector: Lucas Cell 228 Ra β - β Ac 5.75 y 6.13 h Analysis: Chemical separation of Ra, equilibration then solvent extraction of 228 Ac Detector: Low background, anti-coincidence proportional counter. Problem: wrong half life for 228 Ac Interference:

9 Possible sources of 90 Sr Coulport Holy Loch Faslane Hunterston Windscale: true source of 90 Sr Subsequent interests in anthropogenic radionuclides Weapons testing falllout Sellafield seawater tracer studies sedimentary processes radionuclide geochemistry Dounreay Chernobyl Naval facilities Facilities discharging 14 C Sellafield Mussels 3060 Bq kg -1 Seaweed 1534 Bq kg -1 Marine background 247 Bq kg -1 Amersham, Cardiff Hartlepool? Mussels 913 Bq kg -1 Seaweed 3953 Bq kg -1 Depleted uranium Anthropogenic and natural radionuclide studies: Studies of 25 different radionuclides with half lives ranging from 3.8 days to 1.6x10 7 y.

10 Two selected natural decay series research areas 210 Pb dating (half life = 22.3 y) The most common method for establishing sediment chronologies on the year timescale Covers the period of most intense human impact on the environment and of recent climate change. 238 U- 234 U- 230 Th disequilibrium studies (timescale up to ~ 10 6 y) used in: characterising rock-groundwater interactions, uranium mobility studies applications in radioactive waste repository natural analogue, site selection and site characterisation studies.

11 Atmosphere Land 210 Pb in lacustrine or marine sediment Radioactive decay 222 Rn 210 Pb Detrital minerals with decay series in equilibrium Deposition Radioactive decay 222 Rn 210 Pb Lake or sea water 210Pb (Bq kg-1) Sediment A S A S A S A 0 A 0 e -λ A 0 e -2λ Total 210 Pb Layer of age = n years A S Supported 210 Pb activity A 0 e -nλ Unsupported 210 Pb activity Total activity = supported + unsupported Depth Supported 210 Pb Plot of 210 Pb specific activity versus depth

12 210 Pb dating: constant initial concentration (CIC) model 210Pb (Bq kg-1) ln[210pb (Bq kg-1)] Depth Depth gradient = S λ where S = accumulation rate and λ = 210 Pb decay constant

13 Unsupported 210Pb (Bq kg-1) Depth (cm) 5 Lower sedimentation rate Higher sedimentation rate Variations in specific activity result from variations in sedimentation rate Unsupported 210 Pb profile for Loch Lomond sediment core LLS3A

14 210 Pb dating: constant rate of supply (CRS) model Assumption: 210 Pb inventory decreases exponentially with time 210 Pb inventory (Bq m -2 ) Time I i = I T e -λt Calculation: t i 1 I = ln λ I T i where I T = total inventory I i = inventory for t > i λ = 210 Pb decay constant where t i = age of i th layer

15 210 Pb dating by gamma spectroscopy analysis 226 Ra 222 Rn 218 Po 214 Pb 214 Bi 214 Po 210 Pb 1600 y d 3.11 m 26.8 m 19.9 m 1.6x10-4 s 22.3 y α α α β β α β γ 295 kev γ 609 kev γ 352 kev γ 46.5 kev Supported 210 Pb Total 210 Pb

16 Influence of core length and limit of detection on CRS ages Unsupported 210 Pb is present in sediment below the depth at which the limit of detection, or the bottom of the core, is reached. Thus measured inventories in cores will be lower than the true inventories of 210 Pb in the sediment. However in the equation: t i 1 I = ln λ I I T and I i represent integrations to infinity. T i Therefore, measured inventories of 210 Pb in a core, or below a given depth in a core, do not correspond to I T and I i, respectively.

17 Detection limit 0.1 Bq kg Detection limit 1 Bq kg -1 CRS age (y) CRS age (y) True age (y) True age (y) CRS age (y) CRS age versus true age for theoretical 210 Pb profile for detection limits of 0.1, 1 and 10 Bq kg Detection limit 10 Bq kg True age (y)

18 Where possible, 210 Pb chronologies should be validated by comparison with another independent chronology eg 137 Cs Unsupported 210Pb (Bq kg-1) Cs (Bq kg-1) Chernobyl Depth (cm) 5 Depth (cm) weapons testing fallout Accumulation rate = 32 ± 4 mg cm -2 y -1 Accumulation rate = 31 ± 1.3 mg cm -2 y Unsupported 210 Pb and 137 Cs profiles for Loch Lomond sediment core LLS3A.

19 Applications of 210 Pb dating: Carbon fluxes in peat Atmosphere CO 2 Northern hemisphere peat bogs contain ~4.5 Gt of Carbon (ie ~1/3 of total global soil carbon) Peat Acrotelm Water table fluctuation Catotelm Production CO 2, CH 4 Rapid decay DOC CH 4 Slow decay 210 Pb chronologies: near surface net C sequestration rates of the order of 10 2 g m -2 y C chronologies: catotelm net C sequestration rates of the order of 10 g m -2 y -1 or less. The effects of climate change and other anthropogenic disturbance on peat bog stability and carbon fluxes is uncertain

20 Applications of 210 Pb dating: ash deposition as an indication of temporal variations in air quality 0 Ash content (%) Clean Air Acts Depth (cm) Ash flux (g m -2y-1) Year Ash (inorganic material) content of a peat core from South Drumboy (Fenwick Moor) and temporal trends in ash deposition based upon 210 Pb chronology

21 Pb depositional flux (mg m -2 y -1 ) Applications of 210 Pb dating: temporal variations in contaminant metal deposition Temporal trends in Pb deposition and 206 Pb/ 207 Pb atom ratio derived from a 210 Pb dated sediment core from Loch Lomond Pb/ 207 Pb atom ratio Stable Pb isotopes 204 Pb: primordial 206 Pb; primordial and radiogenic 207 Pb: primordial and radiogenic 208 Pb: primordial and radiogenic 206 Pb/ 207 Pb and 208 Pb/ 207 Pb ratios show characteristic differences between different Pb ores UK indigenous Pb: 206 Pb/ 207 Pb ~ Petrol Pb (Australian) 206 Pb/ 207 Pb ~ 1.075

22 Anomalous Pb deposition records: (i) Loch Tay Pb depositional flux (mg m -2 y -1 ) Pb flux Atom ratio Pb/ 207 Pb atom ratio Year Temporal variations in Pb deposition and 206 Pb/ 207 Pb atom ratio based on a 210 Pb dated Loch Tay sediment core. 1.10

23 Tyndrum Pb/Zn mine Pb/Zn vein discovered: ,000 tonnes of ore extracted Smelter started operation at site to east of mine: 1768 Intermittent small scale mining: Old waste dumps reprocessed: Pb/ 207 Pb ratio of ore: 1.146

24 Tyndrum mine Pb waste

25 Anomalous Pb deposition records: (ii) Loch Long Pb depositional flux (g m -2 y -1 ) Year Pb/ 207 Pb atom ratio Temporal variations in Pb deposition and 206 Pb/ 207 Pb atom ratio for 210 Pb dated Loch Long sediment core

26 0 137 Cs (Bq kg -1 ) Co (Bq kg -1 ) Depth (cm) Depth (cm) Cs and 60 Co profiles for Loch Long sediment core

27 American Navy activities in Holy Loch

28 238 U- 234 U- 230 Th disequilibrium studies 238 U decay series: Nuclide: 238 U 234 Th 234 Pa 234 U 230 Th Half life: 4.5x10 9 y 24.1 d 1.17 min 2.45x10 5 y 7.54x10 4 y Decay series disequilibrium is caused by rock-water interaction resulting in dissolution of uranium as a consequence of : Relatively high solubility of U(VI) under oxidising conditions, but low solubility of Th(IV), resulting in preferential transfer of 238 U and 234 U to aqueous phase Preferential transfer of 234 U to the aqueous phase, relative to 238 U, as a consequence of enhanced dissolution in areas of minerals that have been damaged by 238 U alpha decay. The effect of this is to give: (i) high U concentrations and high 234 U/ 238 U activity ratios in groundwater and in secondary minerals deposited within the last 1 2 million years. (ii) low U concentrations and low 234 U/ 238 U activity ratios in minerals that have been subject to groundwater leaching within the last 1-2 million years.

29 238 U- 234 U- 230 Th disequilibrium studies: matrix diffusion 25 Water bearing fracture Rock matrix Diffusion of radionuclides Rock matrix Uranium concentration (mg kg -1 ) Groundwater (radionuclides) Micro fractures Distance from fracture (cm) Rock matrix Drillcore Activity ratio U/238U activity ratio 230Th/234U activity ratio 1 cm sections 0.5 Fracture K1 drillcore: sampling protocol and analytical results Distance from fracture (cm)

30 U/238U activity ratio Uranium deposition Uranium removal Th/238U activity ratio

31 U/238U activity ratio Sudden U addition in equilibrium Return path to equilibrium Sudden U removal in equilibrium Return path to equilibrium Th/238U activity ratio

32 Return paths towards equilibrium after uranium removal or deposition

33 Redox front migration rates: Poços de Caldas, Brazil

34 Post glacial leaching of uranium from near surface rocks 1.5 Uranium deposition sector Location of Criffel study site 234U/238U activity ratio Complex process sector Biotite muscovite granite Mucovite biotite granite Biotite granite Hornblende biotite granodiorite Clinopyroxene hornblende biotite granodiorite Complex process sector Uranium removal sector Th/238U activity ratio

35 Tono uranium deposit and underground laboratory

36 238 U 234 U/ 238 U 230 Th/ 238 U 226 Ra/ 238 U 210 Pb/ 238 U (Bq kg -1 ) U specific activity and 238 U series activity ratios for Tono sample 99SE m 206 Pb/ 238 U age (Ma) without correction for 226 Ra and 222 Rn loss 207 Pb/ 235 U age (Ma) 206 Pb/ 238 U age (Ma) with correction for 226 Ra and 222 Rn loss Calculated U-Pb ages for Tono sample 99SE m