Environmental Applications

Transcription

1 Environmental Applications Gamma ray Spectrometry Paul Nolan University of Liverpool

2 Gamma ray spectrometry of environmental samples is a standard technique Germanium detector Programs available for spectrum analysis Nuclear Physics groups run training sessions Liverpool Facility

3 Typical Spectra

4 Lo-salt Potassium chloride (KCl) contains 40 K 1460keV 4

5 Dee Estuary 662 kev 241 Am and 137 Cs MAN MADE 60 kev 5

6 Post Fukushima Ceramic samples from Japan

7 Ceramic piece pre March 2011

8 Ceramic piece pre March Thorium 232 decay chain 609 Uranium 238 decay chain 662- Caesium 137

9 Ceramic piece post March 2011

10 Ceramic piece post March Thorium 232 decay chain 609 Uranium 238 decay chain 604 Caesium Caesium 137

11 Results Activity of ceramic pieces was Bq for each Cs isotope 134 Cs analysis complex decay scheme with many paths coincidence summing 20% correction

12 Environmental applications of nuclear gamma-ray imaging

13 Applications Real-time soil-plant uptake studies Monitoring of radionuclide transport in soils/sediments High-resolution 3D mapping of radioactivity in soils and across surfaces Identifying airborne radioactivity?

14 Challenges Detection Low concentrations, disperse, background noise Quantification Calibration, spatial location, resolution Speciation Unknown systems, interference, low concs Dynamics Measurement time, data loads, processing costs?

15 Images Can we improve detection, speciation, and quantification in this mode? Can we make this detector portable/wearable?

16 Images Can we do this at lower concentrations, for a range of species, without having to go into a hospital, and in 3D? Time-lapse imaging of Tc99m transport through quartz sand (top) and Nirex Reference Vault Backfill, using GE Medical Systems Infinia gamma camera with high resolution collimator (Corkhill, Bridge et al, in review)

17 Environmental Analysis using Lead 210 dating

18 Lake Sediments as Natural Tape Recordings To quite a remarkable extent we may regard lake sediments as Natural Tape Recorders of environmental events. As each layer of sediment is laid down in the bed of the lake it carries a rich diversity of biological and chemical information about the contemporary environment.

19 Dating Recent Sediments by 210 Pb Dating recent sediments is usually done via the natural radionuclide, 210 Pb, a member of the 238 U decay series. 238 U 4.51x10 9 y 226 Ra 1602 y 222 Rn 3.82 d 210 Pb y 206 Pb 210 Pb occurs naturally in most soils. Disequilibrium between 210 Pb and 226 Ra, its parent isotope in the series, arises through diffusion of the intermediate gaseous radionuclide 222 Rn.

20 Dating recent lake sediments by 210 Pb, 137 Cs and 241 Am Fallout radionuclides provide one of the most important means for dating recent lake sediments. The principal means for dating recent sediments is by the decay of 210 Pb (half-life 22.26y), a natural radioactive isotope of lead. This can be used for dating on timescales from years. This is supported by records of 137 Cs (half-life 30.2y) and 241 Am (half-life 432y), two artificial radionuclides. These are used to identify particular fallout events via chronostratigraphic features in the sediment records.

21 Environmental information stored in lake sediment records Pollen grains Atmospheric pollutants such as SO 2, Pb, Hg, DDT Eroded soils Diatoms

22 Some 222 Rn atoms recoil into the interstices of soils and escape into the atmosphere where they decay to 210 Pb. This is removed from the atmosphere by precipitation or dry deposition, falling onto the land surface or into lakes and oceans. 210 Pb falling directly into lakes is scavenged from the water column and deposited on the bed of the lake with the sediments.

23 Measurement of Fallout Radionuclides by Gamma Spectrometry Radiometric dating of sediments is done by gamma spectrometry using Hyper-pure Germanium detectors. Advantages of Gamma Assay include: Measurements are non-destructive Minimal sample preparation Simultaneous determination of a range of radionuclides, including 210 Pb, 226 Ra, 137 Cs, 241 Am, etc.

24 A Typical Gamma System

25 1 Full Energy Peak Efficiency of a Typical Well Detector Full Energy Peak Efficiency η 0.1 Least Squares Fit Efficiency =135 Energy Energy (kev)

26 Number of Counts per Channel Pb 241 Am Full Energy Range 214 Pb ( 226 Ra) 137 Cs Photon Energy (kev)

27 Number of Counts per Channel Cs X -rays Low Energy Range 210 Pb 241 Am Photon Energy (kev)

28 Sample from top of core

29 Sample from bottom of core

30 Dating of a sediment core from Windermere, Cumbria UK Artificial radionuclide records 241 Am (Bq kg -1 ) 241 Am (Bq kg -1 ) Chernobyl (1986) Depth in Sediment Core (cm) Weapons (1963) Weapons fallout (1963) 137 Cs 134 Cs 241 Am Core Core Radiocaesium (Bq kg -1 ) Radiocaesium (Bq kg -1 )

31 (a) (b) Total 210 Pb Activity (Bq kg -1 ) 100 Measurement Total 210 Pb Supported 210 Pb Subtraction Unsupported 210 Pb Activity (Bq kg -1 ) 100 Derived data Depth (cm) Depth (cm) (a) Total and supported and (b) unsupported 210 Pb versus depth in a sediment core from Windermere

32 Depth (cm) Cs/ 241 Am Dates CRS 210 Pb Dates CRS Sedimentation Rates Sedimentation rate (gcm -2 y -1 ) Age (y) CRS model 210 Pb chronology for Windermere core. Also shown are the 1986 and 1963 depths indicated by the 137 Cs and 241 Am records

33 Potential Improvements Better energy resolution for 46 kev peak factor of 3 improvement Use of pulse shape analysis to select low energy events near the surface reduce Compton background

34 Energy resolution

35 Pulse Shape Analysis (PSA) Low energy gammas (46, 60 kev) will deposit all their energy near the surface where the source is placed External background and Compton background will occur through the whole crystal Use PSA to select the low energy gammas

36 Potential Improvements Better energy resolution for 46 kev peak factor of 3 improvement Use of pulse shape analysis to select low energy events near the surface reduce Compton background These should lead to: More accurate dating in the year timescale Dating over a longer period, up to 200 years Dating of samples where small amounts are available

37 Thanks to: Janet Sampson Jonathon Bridge Peter Appleby For slides used in this talk