PHY313/CEI544 Radioactive Decays

    You will be given a geiger counter and a sample which contains a small number of barium (Ba) ions (more specifically, the isotope 137Ba) that have been created by decay of a radioactive isotope of cesium (137Cs).  Cesium is number 55 in the periodic chart of the elements,  which means that it has in its nucleus 55 protons.  There are various "isotopes" of cesium, one of which has 82 neutrons, or 55+82=137 "nucleons".  This isotope is called 137Cs.  A nucleus of  137Cs is unstable and will eventually decay to a lower energy nucleus, 137Ba , which has 56 protons and 81 neutrons.  Essentially you can say that one of the neutrons changes into a proton, and in the process throws out an electron.  The charge on the nucleus increases by 1 quantum of charge, but total charge is "conserved" because the electron carries off -1 quantum of charge.  This process is known as "beta decay" because when the process was first discovered, the emitted electron was not yet identified as an electron and was called a "beta ray."  The emitted electrons do not travel very far, and are absorbed inside the sample or in the walls of the container.  There is also a neutrino emitted, which travels away at the speed of light and is highly unlikely to interact with anything.  It can easily penetrate the earth and travel to distant parts of the universe.

    It happens that in the beta decay of 137Cs, the product (or "daughter") barium nucleus is left in an excited state, which decays in a few minutes back to the ground state of the barium nucleus, emitting a photon (a quantum of light) of fairly high energy, called a gamma ray (technically, g-rays of energy 0.66 MeV.  The energy of the g-ray equals the difference between the energy of the excited and ground states of 137Ba.)  Such a gamma ray interacts weakly with matter, and will usually penetrate the container and fly into the room where it might be absorbed in your geiger counter detector, especially if you put the sample close to the geiger counter.  But don't put it too close -- you might accidentally break the fragile "window" of the counter which will destroy the counter.

    Before you begin counting gamma rays from the 137Ba, you should inspect the Geiger-Müller tube.  Adjust the high voltage to a value in the range 400-600 V.  Measure the room "background"  for 10 minutes. You will need this information later to correct your counting measurements of 137Ba decays.

    We want to separate the unstable 137Ba atoms from the "parent" 137Cs source.  This is done by washing the  137Cs with dilute hydrochloric acid to dissolve out the barium.  We will give you the resulting solution containing no cesium, only barium.  The technical term for this is "milking the cow."  The parent 137Cs source is the "cow" and the solution we give you is the "milk."  The milk contains some excited barium nuclei which were probably created by very recent decays of the cow atoms.  It also contains some barium atoms in the ground state, which were probably created by less recent decays of cow atoms, and have already emitted a gamma ray and decayed to their ground state.  Before too long, the rest of the barium atoms will also decay into their ground state, emitting more gammas, some of which you can measure with your counters.  The time required for half the excited 137Ba nuclei  (these are designated as 137Ba* nuclei) to decay into ground state 137Ba nuclei is called the "half-life."  This is what we want you to measure.  The answer will probably be a few minutes.  You should be able to measure the half-life to approximately 10%  accuracy.  This does not mean that you are only 90% competent.  If you and the group that comes after you are both perfectly competent, then the group that comes after you will probably get an answer 10% smaller or bigger than you did.  In order to get a more accurate answer, you would have to count many more decays.

    After two half-lives have elapsed, only 1/4 of the 137Ba nuclei will be in the 137Ba* form, and 3/4 will have decayed to the ground state.  After three half-lives, only 1/8 are 137Ba*.  Therefore, the number of counts in the counter will be decreasing with time by corresponding factors.  The number of counts will be proportional to the number left in the unstable 137Ba* form.  If you make a graph with the horizontal axis being clock time since you start counting, and the vertical axis being the number of counts in your counter during the previous 20 sec interval, then the vertical axis will be proportional to the number of 137Ba left.  You should start the number of counts diminishing as time goes on.  To improve the accuracy of your answer, you will do the experiment in groups of 2 or 3 each.  When you have all finished, Manuela will add the results of each group for each 20 sec interval.  You should graph both your own results and the collective (added) results.

Cesium decay
Figure 1: The 137Cs decay chain.





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