A nostalgic exercise in radiation measurements

This week I’m running the “Radiochemisches Praktikum” here at the Atominstitut. With only 5 students this week it has been a good experience and some interesting discussions.

Our example today is a very nice one we’ve been using for quite a long time. Way back, Prof. Buchtela built a nuclide generator out of a steel capillary and a 1 ml syringe. The inside of the capillary was covered with the iron (III) salt of hexacyanoferrate (II), or prussian blue. Onto this salt, ¹³⁷Cs ions were absorbed. When ¹³⁷Cs decays, ^137mBa is produced. Since the Ba-ions are smaller than the Cs-ions, they can be eluted with diluted hydrochloric acid. In this way, the short-lived (T_1/2=2.5 minutes) ^137mBa is separated from its much longer lived (T_1/2=30 a) mother nuclide ¹³⁷Cs.

The solution containing the ¹³⁷Ba was put on a small piece of paper and measured repeatedly with a simple GM-counter. As expected, the measured count rate decreases over time. The students then marked the measured values on a piece of semi-logarithmic paper (you remember this?)

That in itself was a bit of a challenge, since they haven’t used something as simple as that in a while. Once they had a nice, almost linear looking plot, they tried to derive the half life of ^137mBa from fitting (eyeballing) a line to their plot. Turns out, they had a really hard time to arrive at a reasonable number. The major problem was to wrap their heads around the concept of a linear fit to an exponential curve 😉

Since this time around, they came up with some really good values, here’s a quick plot and fit that gets the half-life almost right: