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TRANSCRIPT
Determination of 137Cs Concentrations in Soil as aConcentrations in Soil as a
Function of Depth using Gamma Ray Spectroscopy
August 6, 2012
S. Landsberger, C. Lu, C. Brabec, B. Canion, J. Hashem, D. Millsap
O tliOutline• Introduction• Introduction• Theory• Procedure• Procedure• Data Analysis & Results
C l i• Conclusion
I t d tiIntroduction• 137Cs decays to 137Ba with a 661 7 keV gamma ray• 137Cs decays to 137Ba with a 661.7 keV gamma ray• Gamma yield = 85.10%• T = 30 07 years• T1/2 = 30.07 years• 6.2% production yield from
fission of 235Ufission of U• Released en masse during
atmospheric testing of nuclear p gweapons and nuclear reactor accidents
T il ill h t t f 137C• Topsoil will show trace amounts of 137Cs• 137Cs is often used for soil erosion studies
ThTheory• 137Cs adheres very well to soil and has a low• 137Cs adheres very well to soil and has a low
translocation capability once deposited• Undisturbed soil will show an exponentialUndisturbed soil will show an exponential
decrease in 137Cs concentration with depth• Disturbed soil withDisturbed soil with
show a mixing trend• 137Cs adheres best
with sandy/subtropical soil• Concern for the region• Concern for the region
surrounding Fukushima Daiichi nuclear plant
Th D t tiTheory – Detection• High purity germanium (HPGe) detector was used• High-purity germanium (HPGe) detector was used
to conduct gamma ray spectroscopy• Compton suppression was utilizedCompton suppression was utilized
for lower detection limits• Main interaction within detector is
Compton scattering• Annular NaI detector was used to
detect scattered gamma raysdetect scattered gamma rays
Th A ti itTheory – Activity• Comparator method used to determine 137Cs• Comparator method used to determine 137Cs
activity• Two certified standard reference materials withTwo certified standard reference materials with
known 137Cs concentrations used with the following equation:g q
standard
standardsamplesample C
ACA
• Activity concentration can then be determined:standard
Aam
a
P dProcedure• A column of soil ~25-30 cm deep was excavatedA column of soil 25 30 cm deep was excavated
from the ground• For every 5 cm, a sample was cut outy p• Each sample dried and processed with a 250 μm
sieve to produce an average of 27.3 g of dry soil l d th t i ll 500 f lper sample depth – typically 500 g of sample are
needed• Samples were evenly distributed over a 28 3 cm3Samples were evenly distributed over a 28.3 cm
Petri dish® and counted for 24 hours• Reference materials, IAEA-375 and IAEA-6, were
also counted
P dProcedure
D t A l iData Analysis Standard & ReferenceS a da d & e e e ce
CountsCount Time Count Rate Current
Activity ConcCounts Time (h) (cps) Activity Conc.
(Bq/kg)IAEA‐375 91,880 ± 306 12 2.127 ± 0.007 3318.7 ± 215.8IAEA‐6 983 ± 41 18 0.0152 ± 0.0006 27.4 ± 2.9Table 1: 137Cs Counts and Current Activity Concentrations of IAEA-375
Standard and IAEA 6 ReferenceStandard and IAEA-6 Reference
• Using the comparator method, the activity concentration of IAEA-6 was found to beconcentration of IAEA 6 was found to be 23.67 ± 1.83 Bq/kg – very good agreement
D t A l i S tData Analysis - Spectra
R lt S il S lResults – Soil Samples
Depth from Surface (cm)
Activity Concentration (Bq/kg) Mass of 137Cs (ng)
0 00 5 08 1 99 ± 0 28 (1 61 ± 0 22) 10 50.00 – 5.08 1.99 ± 0.28 (1.61 ± 0.22) × 10‐5
5.08 – 10.16 2.31 ± 0.36 (1.73 ± 0.27) × 10‐5
10 16 15 24 1 41 ± 0 35 (1 23 ± 0 31) × 10‐510.16 – 15.24 1.41 ± 0.35 (1.23 ± 0.31) × 10 5
15.24 – 20.32 0.66 ± 0.27 (0.64 ± 0.26) × 10‐5
20 32 – 25 40 < 0 93* < 0 70 × 10‐5*20.32 25.40 < 0.93 < 0.70 × 10Table 2: 137Cs Counts and Current Activity Concentrations of IAEA-375
Standard and IAEA-6 Reference*Detection limit determined to be 0 93 Bq/kgDetection limit determined to be 0.93 Bq/kg
R lt S il S lResults – Soil Samples
R lt D t ti Li itResults – Detection Limit• Lower limit of detection was determined to 95%• Lower limit of detection was determined to 95%
confidence using the following equation:
L 654712• Where μB is the background count
BD ..L 654712
• Detection limit for the 24 hour counts was determined to be 0.93 Bq/kg
C l iConclusion• Majority of the 137Cs is evenly distributed in the topMajority of the Cs is evenly distributed in the top
15.24 cm of soil• Rapidly decreases with increasing depth• Demonstrated pattern of disturbed soil• Further supported by acknowledgment that soil was
disturbed by construction activitiesdisturbed by construction activities• The experiment was conducted in a highly
replicable manner and can be applied to nearly any location where soil samples can be taken
• Very good educational experience for a design-style graduate lab in radiochemistrystyle graduate lab in radiochemistry