| counts/minute measured in sample |
| c/m/gNa (step 6.) |
Step 8. Calculate percent sodium in sample.
% sodium =
| grams sodium in sample (step 7.) |
| weight of sample (step 1.) |
THE SENSITIVITY[3] OF NEUTRON ACTIVATION ANALYSIS
There are several factors that determine the sensitivity of the method. Some are variable within limits and some, like the cross section, are fixed. Time is variable to a degree, partially determined by the half-life of the nuclide created and with an upper practical limit determined by how long we want to wait for an analysis.
The crucial step in the analytical procedure is the measurement of the number of radioactive atoms that were created.
1. How do we measure how many radioactive atoms are present?
2. Since there will usually be a mixture of elements in a target, and many of these will be made radioactive, how can we tell one from another?
3. Since radioactive atoms are constantly “disappearing” by radioactive decay, how do we obtain the number of atoms created from a measurement made some time after the bombardment has taken place? And what of those atoms disintegrating while others are still being created in the reactor?
Radioactive atoms almost always decay by emitting negatively charged beta particles usually accompanied by gamma rays. Instruments can detect these kinds of radiation, and it is by measuring the radiation that we determine how many radioactive atoms are present. To do this we have to know the types of radiation emitted by the radioactive atoms we are trying to measure. Fortunately each kind of radioactive atom decays with a unique “pattern” scientists call a “decay scheme”. The [figure on the next page] shows a simplified decay scheme for manganese-56, which is produced by activation of manganese, and a diagram showing what the decay scheme means.
Until a few years ago, it was difficult to measure the number of gamma rays of a particular energy that were being emitted by a mixture of radioactive isotopes unless there were only a few such gamma rays with very different energies. Today instruments are available that can really pick them out of a complex mixture. Thus it is usually possible to “separate” with electronic instruments the radioactive element we are interested in measuring. Some of the examples below will show how this might be accomplished.