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Physics Questions People Ask Fermilab

You wrote:
I just purchased a CDV-700 Geiger counter from an antique store. It even came with the manual! The manual states that the test spot on the side of the unit (behind a sticker) contains a piece of radium d-e. It also states that it's half life is about 22 years. I have searched all over the Internet and cannot find any info on radium d-e. I am an Electrical Engineer not a Physicist. Any help is greatly appreciated.

Thanks...Stephen

Stephen,

I had to do a bit of research on this one and it will take some space to explain. As you may know, a common manifestation of radioactivity in nature is in the form of the so-called natural series. There are three such series in nature. Each of them starts with a very high atomic number radionuclide which has a half-life that is comparable to the apparent age of the earth. The 3 natural series are headed by Thorium 232 (half-life = 1.4 E10 years) called the thorium series, Uranium 238 (half-life = 4.47 E9 years) called the uranium series, and Uranium 235 (half-life = 7 E8 years) called the actinium series. [In this form of scientific notation, 1E6 years is a million years, 1E9 years is a billion years, etc.]

Each of these series decay through a chain of a dozen or so decay products. The decay products are all radioactive themselves and have much shorter half-lives than does the parent of the series and so are almost always present when the parent is present. (They are "almost" alwas present because one of the elements in the chain is radon, a gas at normally-encountered terrestrial temperatures and so under some conditions the subsequent decay products can "escape".) The final product of all 3 series are 3 isotopes of lead, lead 208, lead 206, and lead 207, respectively. All 3 lead isotopes are not radioactive; that is, they are stable and hence the process stops.

The decay products have been known about and studied in some detail for nearly 100 years. In the early days of these studies, neither the chemical elements nor their atomic weights were known but the various "radiations" were measured and could be identified. To keep this all straight, nomenclature, such as ThC, ThD, RaA, etc. was invented. Since the field of nuclear physics has now been investigated in much detail, the radionuclides are now usually referred to by their chemical symbol and atomic weight, (eg. U-238 for uranium 238). The older names have nearly been forgotten. In fact, I had to dig out an old, and somewhat obsolete, textbook to readily find them. This may explain the dearth of information on the Internet.

The material alleged to be in the Geiger counter under the "test spot" is probably lead (Pb) 210. This radionuclide is a member of the uranium decay chain and is referred to "RaD" in the old nomenclature. It comes from the decay of radium 226, a radionuclide having a 1600 year half life that is famous because it was discovered and studied extensively by Mme. Curie. It was one of the first radiochemicals to be readily isolated in quantity. If one has a sample of Ra 226, in a few days one can obtain a reasonable quantity of Pb 210 that can be extracted using chemical means. The Pb 210 (RaD, likely the "Ra-d" referred to in the original message) has a half life of 22.6 years, the "right" length. It produces beta and gamma radiation that could readily be used to check a meter. It also produces some alpha radiation that probably cannot penetrate the "walls" of the Geiger tube. "RaE" is the next product in this decay, bismuth (Bi 210), which has a half-life of 5 days. It also produces radiation that would make the meter respond. The Bi 210 decays into a longer lived isotopie of polonium (Po 210, "RaF", half-life = 138 days) and then, finally into stable Pb 206 where the decay chain ends. The longer half-life of the RaF means also that it does not decay as fast and is thus less useful for making the Geiger counter respond. Hence, the manufacturer probably did not care to "advertise" its presence, thus limiting the refernce to RaD-E.

One is always wondering if one's instrument is working properly. Even today, small radioactive sources are commonly attached to Geiger counters and other instruments. Pb210 was a common material to use for this purpose in the past. Of course, there may not be much of it left if the Geiger counter is very old, as every 22.6 years it would have decreased by half. Details of what radiation is emitted by these radioisotopes, the readily available HANDBOOK OF CHEMISTRY AND PHYSICS has a table of the isotopes in it.

I hope this helps. I would be happy to answer further questions.

Don Cossairt, cosairt@fnal.gov
Fermilab Physicist

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