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