How many years ago did this water sink below the surface? Express your answer us

Question

How many years ago did this water sink below the surface?

Express your answer using two significant figures.

Nuclear weapons tests in the 1950s and 1960s released significant amounts of radioactive tritium (^3_1H. half - life 12.3 years) into the atmosphere. The tritium atoms were quickly bound into water molecules and rained out of the air, most of them ending up in the ocean. For any of this tritium-tagged water that sinks below the surface, the amount of time during which it has been isolated from the surface can be calculated by measuring the ratio of the decay product,^3_2He, to the remaining tritium in the water. For example, if the ratio of^3_1He to^3_1H in a sample of water is 1:1, the water has been below the surface for one half-life, or approximately 12 years. This method has provided oceanographers with a convenient way to trace the movements of subsurface currents in parts of the ocean. Suppose that in a particular sample of water, the ratio of^3_2He to^3_1H is 4.2 to 1.0.

Explanation / Answer

As given in the question: half life of 31H =12.3 years

and, 32H/31H = 1:1 => water has been the surface for one half life ~ 12.3 years

In the given sample of water the ratio 32H/31H = 4.2

Water been below the surface for = 12.3/4.2 = 2.928 years.

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