The background effect

Principle

This experiment should bring the students to the realization that radioactivity is a natural process. Ever since its origin, life on earth has been continuously exposed to ionizing radiation. Radioactive materials in the earth are a major source of this natural radioactivity. They either act
externally on the living being as terrestial radiation or are inhaled in the form of the radioactive inert gas Radon. In addition, the surface of the earth is ceaselessly exposed to a stream of ionizing radiation of different types and energies from the sun, and also from distant galaxies. An average
of about 1 million cells of a human body are hit by a ray every second. The human immune system is capable of extensively repairing the cell damage caused, however, so that life on our planet has been capable of development despite the influence of this level of radiation.

As a result of terrestial and cosmic radiation, a Geiger-Müller counter registers a radioactivity count even in the absence of a source of radioactivity. This dose rate is called the background count rate, and must be taken into consideration, especially when weakly radioactive substances are examined. The background count rate registered by a standard Geiger-Müller counter is quite small, and can lead to a wrong estimation of the actual intensity of the ambient radiation, as only about 1% of the gamma quanta which penetrate into the counter tube generate an impulse by the photoelectric or Compton effect.

To get an idea of the actual number of gamma quanta which traverse through the counting tube volume within the time of  measurement, you must multiply the base level registered by a factor of 100. A comparison of the volume of the counting tube with the volume of a human body then helps you to imagine how large the number of gamma quanta hitting the human body is. Such considerations could be chosen as a starting point for an objective discussion on how dangerous radioactive materials are.

Task

To examine if the Geiger-Müller counter registers counts without any source of radioactivity in the classroom.

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