²¹⁰Pb, ¹³⁷Cs

Solid-phase activities of ²¹⁰Pb and ¹³⁷Cs as a function of depth are determined in the upper 30 cm of sediment cores for the purpose of assisting in the determination of rates of sediment accumulation and mixing. The measurement of both ²²⁶Ra-unsupported ²¹⁰ and ¹³⁷Cs is accomplished by the application of ultra-low level gamma spectrometry.

Sediment slices from a Multi-corer are placed into standard plastic Petri-dishes (approximately 8 cm diameter). It is important that the sediments are evenly distributed through the Petri dish and that the Petri dish is full (i.e., constant geometry). The Petri dish is carefully and thoroughly sealed using electrical tape. The samples are stored (room temperature is appropriate) for at least 15 days, thus insuring that the short-lived daughter products of ²²²Rn have grown into secular equilibrium with ²²⁶Ra. The sealed sample is counted in Bremen on a Eurysis Ge Coaxial Type N gamma detector for 1 to 3 days.

The detector is calibrated using the CANMET reference standard DL1a (Canada Centre for Mineral and Energy Technology) which is a U-Th ore derived from a pale yellow arkose sandstone. The standard is in secular equilibrium with respect to ²¹⁰Pb and ²²⁶Ra.

Sample self absorption of the low-energy ²¹⁰Pb gamma (47 keV) by the calibration standard and sample was made using the method of Cutshell et al. (1983). The sample self absorption is calculated using the expression:



Where S is the sample absorption factor; which is the sample photon emission rate, A, divided by the attenuated sample output, O. To calculate A/O we measure the attenuated, T, and unattenuated intensities of a standard source placed over the sample. A gamma reference standard containing 10 kBq of ²¹⁰Pb is placed on top of the sample. The count rate at 47 keV is measured over 300 seconds, T, and compared to the count rate for an equivalent, empty Petri dish, I.

²²⁶Ra unsupported 210Pb (or ²¹⁰Pb_excess) is calculated by subtracting the activity of ²²⁶Ra from the total calculated ²¹⁰Pb activity. ²²⁶Ra is estimated by averaging the calculated activities form its daughters ²¹⁴Bi (609 keV) and ²¹⁴Pb (295 and 352 keV) peaks. ¹³⁷Cs was determined from the photopeak at 662 keV.

Reference:

Cutshell, N.H., Larsen, I.L., and Olsen, C.R. (1983) Direct analysis of ²¹⁰Pb in sediment samples: self-absorption corrections. Nucl. Instr. and Methods 206, 309-312.

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