1. Marilia I. Savva, Marios J. Anagnostakis
Determination of 7Be, 210Pb and 22Na Activity in Air and Rainwater by Gamma-ray Spectrometry
Marilia I. Savva, Marios J. Anagnostakis
Nuclear Engineering Department
School of Mechanical Engineering
National Technical University of Athens
25th Symposium of the Hellenic Nuclear Physics Society, 3-4 June 2016, Athens, Greece

2. Structure of presentation
Scope
Natural radionuclides such as short lived radon daughters and 7Be have been analyzed in aerosols at NED-NTUA for many years. Scope of this work is to extend NED-NTUA analytical capabilities to detect and analyze 210Pb and 22Na in air and rainwater.
Structure of presentation
Introduction – Radionuclides as tracers
Method – γ-spectrometry
Air sampling and analysis
Rainwater sampling and analysis
Conclusions

3. Radionuclides as Tracers
Radionuclides are used as tracers in many applications:
air mass and water mass transport
atmospheric aerosol studies
soil erosion and sedimentation
solar activity studies
Cosmogenic radionuclides, such as 7Be, 22Na, 14C
Radionuclides of terrestrial origin, such as 210Pb
their production is influenced by the solar circle
Soil characteristics: porosity, humidity etc
As well as fallout radionuclides such as 137Cs
its production is influenced by the concentration of parent 226Ra in the top soil and soil characteristics

4. Cosmogenic radionuclides production
Cosmic rays (mainly protons and neutrons) interact with the atmosphere resulting in the production of a number of cosmogenic radionuclides (7Be, 22Na)
The production rate depends on:
variations in the geomagnetic field of Earth
solar activity, such as the number of sunspots and the 11th year solar circle
the higher the number of sunspots, the lower the production rate of cosmogenic radionuclides

5. Ratio of 7Be/22Na
Beryllium-7 (T1/2=53.22 d) is produced by spallation of light nuclei C, N, O, whereas 22Na (T1/2=2.60 y) is produced by spallation of Ar
Both radionuclides are produced in stratosphere and troposhpere, - mainly in stratosphere - with different production rates
Because of the very different half-lives, the ratio 7Be/22Na can be used as a tracer for the transport of air masses of stratospheric origin to ground level air
7Be is produced mainly in the stratosphere (~70%) and less in the upper troposhpere (~30%)
μικρός λόγος 7Be / 22Na στον επιφανειακό αέρα δηλώνει:
εισβολή αερίων μαζών από τη χαμηλότερη στρατόσφαιρα ή την ανώτερη τροπόσφαιρα ή/και μεγάλο χρόνο παραμονής αερίων μαζών

6. Cosmogenic radionuclides concentrations
Activity concentration in ground level air is relatively low:
7Be  – 10.0 mBq/m3
22Na  of the order of 1.0 μBq/m3
In order to be able to detect such low activities, we need:
large sample volume
low background
high measuring efficiency
long analysis time

7. Radionuclides of terrestrial origin
210Pb in soil
from 222Rn decay in the atmospheric air
unsupported 210Pb
from 222Rn decay in the ground
supported 210Pb
Also widely used for atmospheric studies is radon progeny, mainly 210Pb, 210Bi and 210Po
The unsupported (or excess) 210Pb is used in atmospheric studies
Activity in the air 0.1 – 3.4 mBq/m3

8. Study of 7Be, 210Pb, 22Na at NED-NTUA
analysis with γ-spectrometry
air samples
rainwater samples
chemical treatment

9. γ-spectroscopic analysis
Detector:
XtRa Ge (rel. eff %, carbon epoxy window)
Compton Suppression System
Efficiency calibration by means of Monte Carlo simulation (PENELOPE code), introducing an uncertainty of the order of 4.0% (1σ).

10. Typical γ-spectrum
477.6 keV
7Be
46.5 keV
210Pb
keV
22Na

11. γ-spectroscopic analysis difficulties
Lead-210:
photon energy: keV  self-absorption in the sample
low emission probability (~4%)
detection of 210Pb in the background spectrum
Sodium-22:
very low concentration  very small photopeak
coincidence effects due to β+ emission

12. γ-spectroscopic analysis of 22Na photopeak
keV
22Na
Οι άλλες κορυφές είναι τα 1120keV και τα 1238keV του Bi-214, ενώ αριστερά της φωτοκορυφής του Na-22 είναι τα 1378kev του Bi-214

13. Compton Suppression System
Compton Suppression System for the suppression of background:
NaI(Tl) annulus guard detector (Ø195x267mm)
NaI(Tl) plug detector (Ø103x51mm)
lower background

14. Compton Suppression System
Two spectra are collected simultaneously
In the suppressed spectrum are detected:
Beryllium-7
Lead-210
In the unsuppressed spectrum is detected:
Sodium-22

15. Analysis of air samples

16. Air Sampling at NED-NTUA
High volume air sampler (DH-50810E of F&J)
Fiber Glass filter 8x10” (FP810M of F&J)
Beryllium-7 measurements:
since 2008
concentrations from 1.0 to 10.9 mBq/m3
observation of daily variations with 4h sampling

17. Air Sampling in this study
Sampling time: 4 – 10 days per month
Sampling average flow: 1300 L/min
In total 20 samples were taken:
volume: 7500 – m3
sampling period: July 2014 – April 2016
analysis time: sec
in-house developed analysis code – SPUNAL

18. Air filter γ-spectroscopic analysis
The filter is fold into final dimensions 5.08 × 6.35 × 0.9 cm
The γ-spectroscopic analysis starts at least 2 h after the sampling to allow the decay of radon progeny.
Higher efficiency
Lower background continuum

19. Beryllium-7 measurements in air
Concentration from 2.5 to 9.2 mBq/m3
Uncertainties of the order of 4.0% (1σ)
Να υπενθυμίσω ότι είναι δύο χρόνια η περίοδος που δείχνουμε

20. Lead-210 measurements in air
Concentration from 0.3 to 1.7 mBq/m3
Uncertainties of the order of 5.0% (1σ)
Lead-210 measured is one order of magnitude more than the background

21. Sodium-22 analysis techniques
The photopeak is comparable to the limits of detection
In some cases the levels of significance in the γ-spectroscopic analysis software had to be lowered for the 22Na photopeak to be analyzed
Lowering the levels of significance leads to the detection of more photopeaks with relatively higher uncertainties (acceptable ??)
Even with lower level of significance 22Na could not be detected in 15 samples

22. Sodium-22 measurements in air
Concentration from 1.0 to μBq/m3
Uncertainties greater than 50% (1σ)
Limit of Detection of the order of μBq/m3
~10700 m3
~7500 m3
~18600 m3
~18700 m3
~18400 m3
Sampling : 4d for the first sample, 10d for the following and 7d for the last one

23. Combination of samples for 22Na detection
Three filter samples were combined to study the detection of 22Na
Concentrations from 0.8 to 1.5 μBq/m3
Uncertainties from 31% to 94% (1σ)
~54000 m3
~34000 m3
~52000 m3
The main result that is concluded from this study is that the sampling of 7 days (approximately m3) is not enough in order to detect efficiently 22Na, since 22Na was not detected in the individual samples, but when the samples were combined the analysis shoed detectable amount of 22Na.

24. 7Be/22Na ratio in air
Results only for summer period due to the difficulties in 22Na detection
Ratios from 2×103 to 6×103
Uncertainties from 53% to 93% (1σ)
The activity ratio for 7Be/22Na in ground level air should show a cyclic, seasonal structure with warm periods (spring or summer) maxima and minima in winter or autumn.
However, we measured 22Na only in summer time, so in this phase of the study we cannot extract any conclusions regarding the variation of this ratio.
The values estimated in this study are lower than those reported in the literature (even form the lowest of them), however they refer to a different phase of the solar circle. At this moment we are at the lowest production of cosmogenic isotopes.

25. Analysis of Rainwater Samples

26. Rainwater Sampling
Rainwater collectors placed at the roof of NED-NTUA building
Sampling area per collector: 0.2 m2
4 collectors were used with total area 0.8 m2
In total 12 rainwater samples were collected:
volume: 6 – 64 lt
time period: Sept 2014 – Mar 2015
analysis time: sec
in-house developed analysis code – SPUNAL

27. Rainwater samples treatment (1/2)
Cation exchange resin is added at the rainwater sample
HNO3 to adjust pH ≈ 5.0
5 g resin for every 1 L of rainwater sample
stirring for ~1.0 h
Without any treatment of the sample and analysis of 24 – 48 h the results were not satisfactory, since no Be-7 was detected. Thus, we decided to treat the sample using an ion exchange resin.
The sample is filtered to allow for the retention for the resin

28. Rainwater samples treatment (2/2)
The resin is dried using Silica Gel
drying for ~5 days
The dried resin is packed for γ-spectroscopic analysis
During the drying procedure we noticed that if we leave the resin to dry for only 1 day and then pack it for analysis, after a while mass losses occurred in the resin sample. For this reason we measured the resin sample mass every 2 hours to see when it is completely dried and found that 5 days are enough.

29. Beryllium-7 measurements in rainwater
Concentrations from 0.6 to 2.2 Bq/L
Uncertainties of the order of 4.0% (1σ)
The first measurement is high because the duration of the sampling – rainfall was comparatively shorter that the others. The amount of 7Be from the atmospheric air is deposited independently from the duration of the rain event. So, if the sampling is longer we add water to the sample, diluting the 7Be concentration.

30. Lead-210 measurements in rainwater
Concentrations from 22 to 278 mBq/L
Uncertainties from 5% to 22% (1σ)
The first measurement is high because the duration of the sampling – rainfall was comparatively shorter that the others. The amount of 7Be from the atmospheric air is deposited independently from the duration of the rain event. So, if the sampling is longer we add water to the sample, diluting the 7Be concentration.
Sodium-22 LLD = mBq/lt

31. Conclusions Air samples
The sampling and analysis procedure developed allowed for the detection of:
7Be with total uncertainty of the order of 4% (1σ)
210Pb with total uncertainty of the order of 5% (1σ)
22Na with high uncertainties (>50%), only in warm periods and higher sampled volumes
Rainwater samples
210Pb with uncertainties from 5% to 22% (1σ)

32. Thank you!
Questions?

33. Steinmann et al. (2013),
Switzerland, 2000 – 2011