RADIOACTIVITY

Film Loop: Radioactivity

Length(min.):4:00, Color: No Sound: No

Matter is radioactive when it emits radiation. There are three kinds of radiation that are easily detected and are the object of this Film-Loop. They are invisible to the unaided eye, but they are detectable with a variety of instruments including geiger counters, proportional counters, electroscopes, and photographic film. Each kind of radiation - alpha, beta and gamma - has unique physical properties. When you have learned to recognize these properties it is relatively easy to detect and to identify the radiation even when two or three kinds are present at once.

All three kinds of radiation ionize the molecules of matter along their paths. The detection device used, called a "probe", is simply a gas-filled chamber in which molecules of a gas are ionized by radiation. The ionized gas conducts an electric current which is measured by the meter to which the probe is connected; the greater the degree of ionization the greater the current reading. Full-scale deflection reads 1500.

When no radioactive source is held in front of the probe the meter does not go to zero; it only falls to 100. Some radiation is still being detected. Cosmic rays and very tiny amounts of radioactivity in the surroundings, far too little to be a health hazard, are being detected constantly by this sensitive instrument. Such constant radiation is called "background" and must be subtracted from the meter reading before you compare one meter reading with another. Since we are only interested in comparing readings with one another, the actual units on the meter scale are not important.

The alpha source emits a stream of particles that Rutherford showed to consist of the nuclei of helium atoms. Notice that the source has to be held close to the probe since the range of alpha particles in air is only a few centimeters. (Q1) What subatomic particles are helium nuclei made of? (Q2) From what you have seen do you think alpha particles will penetrate through your skin? Why do you think so?

The beta source emits a stream of electrons. (Q3) How does the penetrating power of beta particles (electrons) compare with that of alpha particles? (Q4)Can beta particles penetrate the roll of lead sheet? Why do you think so? (Q5) How does a magnetic field affect the path of beta particles? Support your answer with evidence from the film.

Gamma radiation is a stream of electromagnetic waves. It is light of a short wavelength. As a wave it has properties that are quite different from those of alpha and beta particles. (Q6) Name one distinctive property of gamma radiation illustrated in the film.

Neither alpha nor gamma rays are shown being bent by a magnet. (Q7) Why? From the properties of the three kinds of radiation illustrated here you should now be able to identify the unknown radiation being detected at the very end of the film. (Q8) What is it?