The study of radiation at the introductory college level usually includes experiments to determine the half-life of a radioactive isotope and also a study of the absorption of nuclear radiation by metal foils. These experiments are straightforward to do using a Geiger-Müller tube and scaler, and allow for the determination of well-known constants. The underlying theory behind these experiments is also easy to understand, but fails to take into account background radiation that will be present, regardless of the source being used. The purpose of this article is to show how students can measure the effect of background radiation on these experiments by modifying the standard textbook theory.

The Cs-137/Ba-137 Isogenerator may be obtained from any supplier of educational apparatus for nuclear physics experiments, such as Sargent-Welch, Daedalon, or Canberra Industries.
The exact syntax for nonlinear curve fitting will depend on the software being used.
A weighted fitting procedure should be used in this case because the deviation associated with each data point is not constant. This is discussed in texts such as P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences (McGraw-Hill, 1969), pp 108–118.
The range R in mg/cm2 is related to the energy E in MeV of the beta particles by Feather's empirical formula, R = 543E − 160. The energy of the beta particles from Tl-204 is 0.763 MeV.
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