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The following is a summary of the nuclear processes in the laboratory experiment on radioactivity:
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| Generating neutrons |
The neutron generator consists of a large plastic barrel of water, in the center of which is a neutron source. This source is a small piece of the man-made element, americium, (24395Am) in contact with a small piece of beryllium (94Be). Both are metals. The americium is alpha radioactive, and spontaneously produces alpha particles (nuclei helium-4):
The alpha particles hit beryllium nuclei, and the following nuclear reaction takes place:
Thus the source generates free neutrons, particles which are almost never present in ordinary matter. |
| Shielding |
The purpose of the barrel of water is to prevent these neutrons from coming into the environment, where they could cause radioactive damage. Water absorbs neutrons so that none come out into the room. (Water is the molecule, H2O, and the nucleus of the hydrogen atom is a proton. What happens is, in fact, a neutron absorption reaction, in which a neutron and a proton combine to form the heavy isotope of hydrogen, 21H:
This heavy hydrogen stays in the barrel as part of the water.) |
| Activating the aluminum |
There is a cylindrical opening in the side of the barrel that allows an experimenter to take a small object and insert it into the center of the barrel, close to the neutron source. In our experiment the object is a disk of aluminum. 2713Al is the common isotope of aluminum. Neutrons are absorbed according to:
In 3 or 4 minutes enough 2813Al is formed to do the experiment. This nucleus is beta radioactive, and decays according to:
Silicon nuclei replace some of the aluminums in the disk. |
| Artificial radioactivity |
We sometimes use the term "artificially radioactive" to describe the nucleus, 2813Al. It doesn't exist in nature, and is only created in an experiment like the one we are doing. In contrast, we call nuclei like 23892U and 21084Po "naturally radioactive".
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| Measuring radioactive decay |
The student places the radioactive disk in the Geiger counter, which detects the emerging high speed electrons. Each individual electron causes an electrical discharge, and these get recorded and summed by the counter. Note: All these nuclear processes are happening to only a small fraction of the material we are using. Only a small fraction of the americium has turned to neptunium, even though the neutron source has been operating continuously for several years. And while the aluminum in the disks is being changed to silicon, only a small fraction of silicon has been produced, even though the disks have been through perhaps hundred of lab periods. |
KEY CONCEPTS
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