Geiger-Muller counter, radioactive sources which emit different types of radiation (alpha, beta and gamma). 

The three primary types of radiation from nuclei, alpha, beta, and gamma rays, all interact with matter by giving up all or part of their energy to electrons in the material they are passing through. After the energy lost in the interaction, the radiation has reduced energy and may leave the material or interact further with it.

 Alpha particles interact strongly with the electrons of atoms, imparting enough momentum (through an impulse due to the. Coulomb interaction) to ionize the atom. In one interaction with an atom the alpha particle will lose only a small fraction of its energy. Many interactions are required for the alpha particle to lose all its energy. However, it interacts with many of the atoms along its path and loses all its energy in a surprisingly short distance.

 Beta particles are high energy electrons emitted by neutron rich nuclei. Unless the beta particle is emitted by an internal conversion process, the beta particle energy will not be unique and will have a maximum value, such as 0.6 MeV for Sr-90 and about 0.3 MeV for Co-60. Beta particles also ionize atoms, but with less probability than alpha particles, and may travel much greater distances before losing all their energy. Beta particles lose energy by multiple collisions with the electrons of atoms, and by emission of bremsstrahlung (x-rays) radiation (breaking or slowing down radiation). The range* of beta particles is much smaller than the distance these particles travel in matter, due to multiple scattering creating a wandering path as they travel through any material. Experimentally it is found that the beta ray intensity obeys a relationship:

I= I₀exp(- mx)       (1)

where I is the observed intensity, I₀ is the incident intensity, x is the absorber thickness, and m is the mass absorption coefficient. For monoenergetic electrons, the average range will increase with energy. For a real source of non monoenergetic electrons (beta particles), extrapolation of the log of the beta intensity into the background as a function of absorber thickness will give the range for the maximum energy beta particle.

Gamma rays are just short wavelength waves that usually follow (by a few picoseconds) beta ray emission. Gamma radiation may ionize atoms but the interaction with one atom usually consumes all or a large fraction of the energy. However, the gamma radiation interacts with atoms with a much smaller probability than either alpha or beta particles.

Gamma rays of energies less than 1.02 MeV interact with matter through the photoelectric effect (producing a high energy electron) or through Compton scattering (producing an energetic electron and a deflected longer wavelength gamma ray). Above energies of 1.02 MeV, pair production can occur. This allows the gamma ray to spontaneously produce (near a nucleus) an electron-positron pair. The total interaction probability at any photon energy is equal to the sum of the probabilities for the above processes. At low gamma energies, the photoelectric effect dominates the total gamma ray interaction probability. At intermediate energies (up to about 5 MeV) Compton scattering dominates the total interaction probability, and at high energies pair production dominates. The total gamma ray intensity after passing through a material obeys equation (1).

 Of the three types of radiation, alpha particles create the greatest ionization density per path length. Since the ionization of atoms requires energy, all three types of particles lose energy as they pass through matter (ionizing atoms), and eventually are stopped or absorbed completely. We are going to explore how this happens in the exercises below.

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   * The range is the thickness of material (or air) layer at which the radiation intensity is equal to the background signal.

Exercise 1.
Place the Po-210 alpha particle source (alpha particle energy is 5.3 MeV) as close to the GM tube as possible. Court for 2 minutes (about 200 counts) and record the number of counts. Move the alpha particle source farther away from the GM tube (move the planchet holder to the next lowest setting on the Nucleus equipment). Record the number of counts in 2 minutes. Repeat this for several positions. Measure the approximate source to detector window distances at each position. Cover the alpha source with a sheet of paper and re-measure the count rate at one position.

Exercise 2.
Measure the background radiation for two minutes. Then place a Sr-90 source 5 cm away from the GM counter and measure the number of beta particles detected in two minutes. The Sr-90 source should be about 5 cm away from the detector at all times during this exercise. DO NOT MOVED the Sr-90 source during this exercise. Insert plastic absorbers of different thickness between the source and counter. Measure the beta intensity (count for two minutes) for each absorber.

Exercise 3.
Take a Co-60 source which emits a 0.3 MeV beta particles, and cover it with enough plastic so that no beta particles reach the GM tube. You must estimate this thickness from your exercise 2 results. Place this source so that it is about 5 or 7 cm away from the detector. Record the gamma ray intensity for two minutes. Insert lead absorbers of different thickness between the source and the detector. Record the gamma ray intensity for each thickness with a two minute count.

Exercise 1.
Describe the experiment you have done. Find is the range of alpha particles in air. Is an absorber necessary to shield a human from an external alpha particle source?

Exercise 2.
Plot the natural log of the beta intensity as a function of absorber thickness for plastic and determine the maximum beta range. Calculate m from your graph.

Exercise 3.
Plot the natural log of the gamma intensity as a function of lead absorber thickness and determine the maximum gamma range. Calculate m from your graph for gamma radiation.