* Th and U are radioactive but with such long half-lives that
neutron activation analysis can be used for their determination.
† µg = Microgram (one-millionth of a gram)
HOW AND WHERE TO USE IT
In a Physics Laboratory
The Problem
You are a physicist investigating the properties of semiconductors, which are materials used to make transistors. When you apply a voltage to one specimen of silicon (a semiconductor), it doesn’t behave quite like the others that you’ve studied. The electrical properties of this odd specimen are unusual and interesting and could lead to a new type of transistor. What makes this specimen different from the others? Very small amounts of impurities can cause large changes in the electrical properties of semiconductors. You would like to obtain a chemical analysis of the material, but your colleagues in chemistry tell you they would have to dissolve a good size part of your sample to analyze it and you are reluctant to give it up. How do you do it?
The Solution
You decide to try neutron activation analysis. You realize you won’t be able to detect all the elements, but many of those that might affect semiconductor performance could be detected quite easily.
What will you need? A source of neutrons to activate the material and a gamma-ray spectrometer to measure the radiation from the material afterwards. This spectrometer detects and measures gamma rays and sorts them according to their energy. You find that your friend down the hall, who is a nuclear physicist, has a gamma-ray spectrometer that incorporates a lithium-drifted germanium crystal as a detector and a pulse height analyzer. The germanium detector is a device that senses the gamma rays that enter it and gives electrical signals related to the energy of the gamma rays. It was invented only a few years ago and has a very fine resolution. That is, it can easily “pick out” gamma rays that are only slightly different in energy. For example, for gamma rays with energies of approximately 1 MeV (million electron volts), it is not unusual to distinguish between gamma rays that differ by only 2 or 3 tenths of a percent. The pulse height analyzer is an electronic device that sorts the electrical pulses from the detector according to their energy.