that happens the in the beta degeneration of 137Cs,theproduct (or "daughter") bariumnucleus is left in one "excited" state. This decays in a fewminutes back to the "ground" state that the bariumnucleus,emitting a photon (a quantum the light) of relatively high energy, referred to as a"gamma" beam (technically, gamma beam ofenergy 0.66MeV, corresponding to light through wavelength 0.02 Angstroms. Energy of the gamma rayequals thedifference in between the power of the excited and ground states of 137Ba.)Suchagamma beam interacts weakly through matter, and will commonly penetratethe container and fly right into the room whereby it could be soaked up in yourgeiger respond to detector, especially if you put the sample close to thegeiger counter. But don"t put it too close -- friend mightaccidentallybreak the delicate "window" of the respond to which will damage thecounter.

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prior to you begin counting gamma rays from the137Ba,you should check detector, a box, within which is hidden aGeiger-Müller tube. Readjust the high voltageto a value close to 600 V, and then store it there throughout yourmeasurments. Measure up the room"background"for 10 minutes. Girlfriend will need this information later to correct yourcountingmeasurements of 137Ba decays.

us will different some stormy 137Baatoms from the "parent" 137Cs source. This is done bywashing the 137Cs with dilute hydrochloric acid todissolveout the barium. Us will give you the result solution,containingno cesium, only barium.The technical term because that this is "milkingthecow." The parent 137Cs source is the "cow" and also thesolutionwe give you is the "milk." The milk consists of some excited bariumnuclei which to be probably produced by very recent decays that the cow(Cs) atoms.It additionally contains some barium atoms in the soil state, i beg your pardon wereprobablycreated by less recent decays that cow atoms, and also have already emitted agamma ray and decayed to your ground state. Prior to too long, therest that the barium atom will also decay into their soil state,emittingmore gammas, few of which you deserve to measure v your counters.Thetime forced for half the "excited" 137Ba nuclei(theseare designated as 137Ba* nuclei) to decay into floor state137Banuclei is dubbed the "half-life." This is what I want you tofind out from her measurements.The answer will most likely be a couple of minutes. You must be may be tomeasure the half-life to approximately 10% accuracy. Thisdoesnot average that girlfriend are only 90% competent. If you and also the groupthatcomes after you space both perfect competent, then the group that comesafter girlfriend will probably get solution 10% smaller or bigger than youdid.In order to acquire a an ext accurate answer, girlfriend would need to count manymoredecays.

As quickly as Li Li offers you her sample, placed itin the detector, which must be in the "stop" and "reset" mode,showing zero counts. Then promptlymove the "count" switch of yourdetector to the "continuous mode" and start timing. At then endof every minute, document the full counts reflecting on the detector.

~ one half-life has actually elapsed, only 1/2 the the137Banuclei stay in the 137Ba* form, and also 1/2 will have actually decayedto the floor state. After two half-lives have elapsed, just 1/4of the137Banuclei will certainly be in the 137Ba* form, and also 3/4 will havedecayedto the floor state. After three half-lives, just 1/8 room 137Ba*.Therefore,therate of brand-new counts in the counter will be decreasing withtime by corresponding factors. The number of brand-new counts will beproportionalto the number left in the unstable137Ba* form. If youmake a graph v the horizontal axis gift clock time due to the fact that you startcounting, and also the upright axis gift the total variety of accumulatedcounts inyour counter because you began counting at time "zero", then theverticalaxis will show how many 137Ba decays have been detectedplus how many background events have happened. You should thengraph the accumulated number of counts matches time.

How should youdetermine the half-life from this graph? I will certainly let you thinkabout it. Because that your first try, girlfriend canignore any correction for background.

When you have finished counting, please provide your samples earlier to LiLi. Also, please give him a copy of her data (60 sec, # ofcounts, 120 sec, # of counts, etc.) Li Li will include the resultsof all groupsfor each 1 min interval. We will examine this additive counting grapheither later on in the night or in ~ the next class.

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Figure 1: The 137Cs degeneration chain.

Lab Writeup:

Please begin with an introductory paragraph. The does nothave tobe technical, yet should include the meaning of the crucial terms youwill use, together as: unstable, decay, gamma... Next a brief section top top what friend did. Climate a section on results, consisting of graphs. lastly a quick conclusion. What did you learn? Is thelifetime of one excited Ba nucleus in any means analogous to the lifetimeof a multicelled organism such as yourself? perform you have any ideawhy there is a "background" counting in this experiment?