the happens that in the beta decay of 137Cs,theproduct (or "daughter") bariumnucleus is left in one "excited" state. This decays in a fewminutes ago to the "ground" state of the bariumnucleus,emitting a photon (a quantum of light) of reasonably high energy, called a"gamma" ray (technically, gamma beam ofenergy 0.66MeV, corresponding to light through wavelength 0.02 Angstroms. Energy of the gamma rayequals thedifference between the energy of the excited and ground states of 137Ba.)Suchagamma ray interacts weakly v matter, and also will usually penetratethe container and also fly into the room wherein it can be took in in yourgeiger counter detector, specifically if you put the sample close to thegeiger counter. However don"t placed it also close -- friend mightaccidentallybreak the fragile "window" of the counter which will ruin thecounter.

You are watching: There are neutrons in barium-137.

prior to you start counting gamma rays from the137Ba,you should examine detector, a box, inside which is surprise aGeiger-Müller tube. Adjust the high voltageto a value near 600 V, and also then store it there throughout yourmeasurments. Measure the room"background"for 10 minutes. Girlfriend will need this information later to exactly yourcountingmeasurements the 137Ba decays.

we will different some unstable 137Baatoms indigenous the "parent" 137Cs source. This is excellent bywashing the 137Cs v dilute hydrochloric acid todissolveout the barium. Us will provide you the result solution,containingno cesium, just barium.The technical term for this is "milkingthecow." The parental 137Cs resource is the "cow" and thesolutionwe give you is the "milk." The milk has some excited bariumnuclei which were probably developed by an extremely recent decays the the cow(Cs) atoms.It also contains part barium atom in the floor state, which wereprobablycreated by less recent decays of cow atoms, and have currently emitted agamma ray and decayed to your ground state. Before too long, therest the the barium atom will likewise decay into their ground state,emittingmore gammas, several of which you deserve to measure v your counters.Thetime forced for half the "excited" 137Ba nuclei(theseare designated together 137Ba* nuclei) to decay into ground state137Banuclei is referred to as the "half-life." This is what I want you tofind out from her measurements.The answer will probably be a few minutes. You must be able tomeasure the half-life to around 10% accuracy. Thisdoesnot mean that girlfriend are only 90% competent. If you and also the groupthatcomes after ~ you room both perfectly competent, then the team that comesafter girlfriend will most likely get solution 10% smaller or bigger than youdid.In stimulate to acquire a an ext accurate answer, you would have to count manymoredecays.

As quickly as Li Li provides you your sample, put itin the detector, which must be in the "stop" and "reset" mode,showing zero counts. Then promptlymove the "count" button of yourdetector come the "continuous mode" and start timing. At climate endof each minute, document the full counts showing on the detector.

~ one half-life has elapsed, only 1/2 of the137Banuclei continue to be in the 137Ba* form, and 1/2 will have decayedto the ground state. After 2 half-lives have actually elapsed, just 1/4of the137Banuclei will certainly be in the 137Ba* form, and also 3/4 will havedecayedto the floor state. After 3 half-lives, just 1/8 room 137Ba*.Therefore,therate of 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 through 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 due to the fact that you began counting in ~ time "zero", climate theverticalaxis will present how countless 137Ba decays have actually been detectedplus how countless background occasions have happened. You have to thengraph the accumulated number of counts versus time.

How should youdetermine the half-life from this graph? I will certainly let girlfriend thinkabout it. For your an initial try, girlfriend canignore any type of correction because that background.

When you have finished counting, please give your samples ago to LiLi. Also, please give him a copy of your data (60 sec, # ofcounts, 120 sec, # of counts, etc.) Li Li will add the resultsof all groupsfor each 1 min interval. We will research this additive count grapheither later in the evening or in ~ the following class.

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*
Figure 1: The 137Cs decay chain.

Lab Writeup:

Please begin with an introductory paragraph. The does nothave tobe technical, however should encompass the definition of the necessary terms youwill use, together as: unstable, decay, gamma... Following a quick section on what girlfriend did. Climate a section on results, including graphs. finally a quick conclusion. What did friend learn? Is thelifetime of one excited Ba cell core in any means analogous come the lifetimeof a multicelled biology such as yourself? execute you have any kind of ideawhy over there is a "background" count in this experiment?