Answer to Question #5799 Submitted to "Ask the Experts"

Q

How do I determine the activity concentration (in Bq/cm³ or Bq/L of ¹³⁷Cs or ⁹⁰Sr) in water? I have to convert activity concentration in Bq/cm³ to surface activity in Bq/cm².

A

I shall assume initially that your question relates to how to calculate the activity concentration rather than how to determine it by experimental means. The activity concentration, assuming a uniform distribution of activity throughout the water volume, is simply the total activity in the volume divided by the volume. Units may vary for both activity and volume, depending on requirements and/or preferences. You have specified Bq/cm³ as likely units. If activity is given in curies (Ci), you can convert it to Bq by noting that 1 Ci = 3.7 x 10¹⁰ Bq. For example, if it is given that the concentration is 5.00 x 10^-11 Ci/L, this would represent 1.85 Bq/L, or 1.85 x 10^-3 Bq/cm³ (since there are 1,000 cm³ per liter).

To convert to dimensions of Bq/cm² requires knowing the effective depth of the volume of water. If the depth is reasonably uniform throughout the volume, the effective depth is simply the known depth. Thus, for the example here, if the depth were constant throughout the volume and was 150 cm, the number of Bq/cm² would be (1.85 x 10^-3 Bq/cm³)(150 cm) = 0.278 Bq/cm².

Physically, this quantity represents the amount of activity contained in a water column with a cross-sectional area of 1 cm² and a depth of 150 cm. If you know the surface area of the volume of water, and if this area is representative of the cross-sectional area of the volume at all depths, multiplying the surface area by the activity/cm² will produce the total activity in the water volume.

For our example, if the surface area were 5,000 square meters, equivalent to 5 x 10⁷ cm², and the cross-sectional area was constant throughout the depth, then the total activity we would calculate would be (0.278 Bq/cm²)(5 x 10⁷ cm²) = 1.39 x 10⁷ Bq. Since the total volume would then be (5 x 10⁷ cm²)(150 cm) = 7.50 x 10⁹ cm³, the activity concentration could be confirmed as 1.39 x 10⁷ Bq/7.50 x 10⁹ cm³ = 1.85 x 10^-3 Bq/cm³. I hope this answers your question.

If, rather, you need to know how to determine the concentration experimentally, the most direct way to assess ¹³⁷Cs concentration is by taking a known volume of water and counting it sufficiently long on a gamma-spectroscopy system, looking at the 662 keV photopeak. The measured count rate in the photopeak region is converted into activity by dividing by a predetermined counting efficiency for the energy and sample geometry used. The concentration is then obtained by dividing the activity determined by the original sample volume. The ⁹⁰Sr is a pure beta particle emitter. As such, it emits no gamma rays and a different technique is required. The ⁹⁰Sr can be separated from the water by various chemical schemes, often involving fairly simple precipitation schemes. The daughter of ⁹⁰Sr is ⁹⁰Y; it is a rare earth element and is chemically very different from its alkaline earth parent, strontium. There are separate precipitation and/or solvent extraction techniques to separate the ⁹⁰Y from the ⁹⁰Sr. It is often most convenient to separate the ⁹⁰Y, prepare it as a solid deposit on a planchette, and count the high-energy beta radiation that it emits as a way to determine the ⁹⁰Sr content. If you want to read more details concerning radiochemical procedures for these and other radionuclides you can find various sources of information on the Internet. One good site is run by the Radiochemical Society, where you can download manuals that give detailed procedures for radiochemical analyses for radioisotopes of many elements.

George Chabot, PhD, CHP