UsingAiuhentic

RADI ISOTOtoTeach .Half-LifeStudents use a radiation monitor^ computer, andradioisotope generator in this real-time lab activity

Scott Liddicoat and John Sebranek

New technology and equipment needed to per-form lab work has made it easier to teachnuclear science in the three levels of chemistryclasses at our high school. Improved technol-

ogy has, for example, replaced tossing pennies as a model forteaching half-life. We perform several lab activities with ra-diation monitors, but our half-life lab—using an isogeneratorto produce an actual radioisotope—works well at every levelof our chemistry program. This safe real-time lab activity isrelatively easy to perform and enables students to determinethe half-life of a genuine radioisotope.

36 The Science Teacher

4 ./

Students observing the radioactive decay of'

as data is collected 0

Teaching nuclear chemistryTraditionally nuclear chemistry appears in the last fewchapters of chemistry textbooks and is not normallyconsidered a mainstream topic. In addition, some scienceteachers lack the training or equipment to teach nuclearchemistry. Yet nuclear chemistry is a very important topicthat should be taught in all chemistry classrooms.

Standards rtlatmg to nuclear chemistry are found inthe National Science Education Standards (NRC 1996, p.62), most state science standards, and in our school dis-trict curriculum (Green Bay Area Public School District2005). Learning about nuclear chemistry concepts andinvestigating radiation in the laboratory helps studentsovercome their sometimes irrational fear of radiation.This lab helps students develop the thinking necessaryto consider societal issues, such as the irradiation offood, the true relative risk of an x-ray, or the future ofnuclear power. It may also open up productive vocation-al choices to them—millions of jobs in today's economyeither use or depend on radiochemistry (NMC 2002).

The concept of half-lifeDifferent radioactive isotopes decay and emit radiation atdifferent rates. Scientists have determined a convenientway to measure and report how fast various radioactiveisotopes (radioisotopes) decay. The rate of decay of aradioisotope is measured by its half-life. One half-life isthe time it takes for one-half of the atoms of a radioactiveisotope to decay to its product (Figure 1).

The halt-lite of a radioisotope plays a role in almostevery application of nuclear chemistry. Radioisotopes areused in medical research, diagnosis, and treatment; nuclearpower; consumer product research, analysis, and produc-tion; the preservation of tresh food; age estimates of ancientorganisms and artifacts; power for deep-space probes; ster-ilization of medical instruments; and much more.

For example, technetium (Tc) is widely used in

FIGURE 2

FIGURE 1

Some radioisotopes and their half-lives(Ser way 1990).

Radioi5OtoDe raic^TvM)

U-238 (a)

Po-214(a) .^ . ,,̂ .̂

Bi-210(P)

C-14 (p)

Po-210 (a)

Sr-90 (P) '""^"^

Co-60 (|3,y)

Ba-137m (y)

4,468,000,000 years

0.000164 second

5 days

5730 years

138 days

28.6 years

5.27 years

153 seconds

Student eluting Ba-137m isotope into awell plate from an isogenerator capsulewith dilute salt solution.

medical applications. With a half-Hfe of six hours,technetium is used by many hospitals as a diagnostictool. Technetium isotopes can be tagged to a substanceso that when they are ingested orally or injected intothe body, the isotopes will travel to the region of thebody that doctors want to study. For instance, to studyblood flow through a patient's liver, a doctor mightmix the technetium with a colloid because one of thefunctions of the liver is to remove large macromol-ecules such as colloids from the blood. A nuclear scan-ner interfaced to a computer generates clear images ofthe blood flow inside the body, in this case the liver.

FIGURE 1

Students print a graph of their decaydata and determine the half-life oftheBa-137m isotope.

38 The Science Teacher

FIGURE 4

Graph of isogenerator decay data plotting count rateversus time.

Radiation Count Rate vs. Time

100

c\

D

o

^ 5 0

y = A*exp(-C*x)+B

A: 95.443 B: 0,000e+000C: 0.004MSE: 33.704 Root MSE: 5.806

100 200 300Time (s)

400 500 600

FIGURE 5

Graph of isogenerator decay data plotting natural logof count rate versus time.

Radiation Count Rate vs. Time

3-\4 - 'c=)oco

"§2

y=mx+bm: -0.00442b: 4.558.5Cor: -0.94928

^ ^n

^ r I ri r TIQ Uri ty [rrn • • _ •n rr Fl V. m rrn n n

D niTi i[ in rri n nH D D p '̂snj:] a D Dnn Dm m ds^] n rn D i;

n 1 1 1 1 M IMHIl 1 JP 1 1 nil II 1 1[ 1 1 1n 1 11 1 n hM|i in I N N

• mu n mn-irnNj nm •• • • m • iVtpi-r

U U ILJ M i l l ULLJOs

n rrm m QE

m n m

n n a n n i m

D

D

100 200 300Time (s)

400 500 600

This noninvasive approach to medi-cal diagnosis has revolutionized themedical industry and eliminated theneed for risky exploratory surgery. Mostnuclear scanners detect best duringthe first half-life period of the isotope.Doctors use these images to make ac-curate diagnoses and treatment plans(Oiswoid 2004). The six-hour half-lifeof technetium is short enough to keepthe radioactive risk to the patient to aminimum, hut long enough to scan tor agood image within the hody.

Determining half-life ofBarium-137m in the labDetermining hall-lite experimentallyis hard to (Jo in an educational setting.It is difficult to isolate the radioactivityfrom just one radioisotope m a decayseries, antl student safety is a concernwith the use ot radioisotopes. However,the materials anil equipment usedto perform the following lab help toovercome these difficulties.

The technetium used in hospitalsis "milked" from a molybdenum (Mo)source in much the same way as teach-ers "milk" Barium-137m (Ba-137m)in the classroom from a mixture ofbarium and cesium isotopes storedin what is called an isogenerator. Anisogenerator is a small plastic capsulethat contains a series of radioisotopesthat decay to a meta-stahie isotope ofbarium (Ba-137m) (Canberra Corpo-ration). We use an "eluting solution"to "milk" or "wash" just the Ba-!37maway from all of the other radioiso-topes in its decay series. The Ba-137mcan be easily and repeatedly elutedfrom an isogenerator capsule usinga dilute salt solution (O.IOM NaCl)(Figure 2).

Ba-137m decays hy gamma emis-sion to a stable, nonradioactive isotopeof barium, Ba'l37. The short half-lifeof Ba-137m (153 seconds) (Lide 2004)means that after one half hour the re-sidual activity in the solution virtuallydisappears, making it safe for disposalusing standard methods for solublebarium salts. The initial activity of theBa-I37m sample eluted from the iso-generator is less than 2 |aCi (Spectrum

December 2005 J9

Techniques Corporation) and its radioactivity only a fewminutes later is considerably lower.

The isogenerator contains enough of the parent iso-topes to be used repeatedly for many years. Only lowintensity gamma radiation is emitted from the elutedsolution of Ba-I37m. This is detected using a radiationmonitor connected to a computer or calculator-baseduiterface. In 10-20 minutes students can obtain an enor-mous data set they can analyzx directly or with a com-puter or graphing calculator.

The data collected can be analyzed in different waysto determine the half-life ot Ra-137m. The methodselected will depentl on class goals and the teachingtime. At our school, students who do not plan to ma-jor in a science-related (iel<] in college take conceptualchemistry. Students taking this elective science classuse a straightforward ajiproach lo calculating half-life.These students make a prmtout of their ilata, ontowhich a curve ot best tit has been superimposed. Ra-diation count rates are plotteil on the y-axis and timeis plotted on the x-axis. By dividing initial radioactivecount rates in half, and halt again, and half again onthe graph, students may determine the half-life of Ba-137m with a good degree ai accuracy (Figure 3, p. 38).

For students taking regular (college-preparatory)chemistry and Advanced Placement chemistry, a dif-terent approach tor determining halt-life is typicallyused. The probeware's data management softwareprogram is used to collect decay data on a computer.Students place an exponential curve fit to the decaydata collected in the program to see how well theirrelationship fits a first-order rate equation (I'igure4, p. 39). Then students create a calculated columninside the program where they take the natural log ofthe count rate data. The natural log data is then plot-ted on the y-axis against time on the x-axis to createthe graph in Figure 5 (p. 39).

A plot of the natural log versus the count rate yieldsa scatter plot of data that appears very linear. Plotting aregression line through the data gives the slope or decayconstant for the graph. This regression plot can be ap-plied to the Hrst-order rate equation to get:

In A

In A-- = kt ={r\ A - In A = kt, which can be written in the torm

In A =-kt+In A , In A = y, ̂ = -m, r = x, In A = b in y ^ -mx + b.

The y-axis in Figure 5 represents the natural log of thecount rate. The initial reading is symbolized by (AJ. Thenatural log of the count rate at time / is equal to A . Basedon our example above, tbe slope (m) is equal to tbe decayconstant (^) wbere f( = 0.693

The slope or decay constant (̂ ) is negative, reflecting thedecrease in counts as the decay progresses. Many text-books write halt-life as a positive quantity, so we willwrite the half-life as a positive quantity in seconds for theloss of Ba-I37m as it decays to Ba-137 stable isotope:

(. ^0.693 _ 0.693

'-'' k ~ 0.00442s--157s.

The accepted bait-life for fia-137m is l.'^'yl minutes or153 seconds. ('Jearly this lab can generate very accurateresults. Over tbe years student data has been used to cal-culate the half-lite of fia-l37m witb under 5% error.

Student assessmentHow do we know that our students understantl theconcept of halt-life.'' Our chemistry students facepaper-and-pencil assessments in which students plotraw data and determine half-life. Our laboratorywrite-ups contam questions progressively secpienced inditticulty to cballenge our students. The questions thatarise trom stutlcnts working witb tbis assessment toolinformally tell us that students usually understand theconcept of halt-life very well. (Write-ups with prelab,lab procedure, and postlab questions are available onour School Science website at http:llhome.new.ry.comlswscience.)

Beyond tbis. iiuich is up to the teacher, the timeavailable, and curriculum requirements for nuclearchemistry. The lab experience is much richer, moreenjoyable, and more easily understood when taught inthe cf)ntext ot a well-planned nuclear chemistry unit,rather than taught as a stand-alone lab. The implica-tions ot tbe concept of balf-Hfe are easier for studentsto grasp when tbe teacher can take tbe time to talkabout medical, business, and consumer applications.For example, most students have known someone wbobas received radiation treatments for cancer or some-one who has undergone a nuclear scan. They usuallywant to kn(»w more about these applicati<tns. Whenstudents can correctly tell us why one radioisotope isbetter to use than another in a particular application,or wbat length of halt-life is appropriate in another ap-plication, we know they understand half-life decay.

Difficulties and concernsP.lach isogenerator can be eluted numerous timesover many years. However, tbe isogenerator needsadequate time (about an hour) to regenerate Ba-137mafter each use. For this reason the teacher with justone isogenerator may wish to do this activity as ademonstration, or at least wait several minutes betweeneach elution. While waiting between elutions, theteacher must recognize that tbe maxiinum number otradioactive counts for each lab group after tbe tirst willbe much lower, which increases the likelihood and range

40 The Science Teacher

Despite the enormous number ofnuclear science applications foundaround us, many of the fundamentalconcepts surrounding nuclear chemistryare misunderstood and even feared.

of error in calculating the half-life of Ba-137m. We have()\ercome this difficulty by gradually purchasing anisogenerator tor eacb lab station (approximately $23(1).Because the isogenerators are used with students at allthree levels ot chemistry, this is not as expensive as itmay seem.

This laboratory exercise is sate tor students to pertorm..'Vs mentioned earlier, Ba-I37m bas a very short half-life(153 seconds) and it decays quickly to a stable, nonra-dioactive isotope of barium. This makes for relativelyrisk-free student bantlling and safe tlisposal after 30 min-utes. Tbe quantity of barium disposed of in this way isconsidered insignificant from botb an environmental andnuclear standpoint (NMC 2002; Canberra ('orporation;Spectrum Techniques (Corporation; Flinn Scientific 21)05).

,.. Students should not get any of the liquitl on their• \ hands during the elution process as Ba-137m emits

|,)\y intensity gamma radiation. When studentswear chemical-resistant gloves and follow procedures (seeSchool Science at http://home.neu'.rr.com/su'science, for thedetailed laboratory procedure), tbere is little or no chanceof spilling. We recommend that the teacher either per-form the elution at each lab station, or personally super-vise students as tbey tlo it. Tbis will prevent any misuse ottbe isogeneratt>r.

Fxposure to radiation during the lab activity isbarely above background levels (Spectrum Tecbniques('orporation; Flinn Scientific 2005) for most studentsstanding a couple of feet away trom the Ba-i37misotope during the lab. Tbis can be demonstratedto students witb tbe mstructor hokling a radiationmonitor a couple of feet away from the Ba-137m. Onlybackground levels of radiation will be detected. Thereis virtually no radioactive risk to the student (and noradioactive risk to the developing child of a studentwho may be pregnant). For a population of well-trained students, this lab is \ery safe to pertorm. Nospecialized student safety equipment is needed beyondstudent goggles and vinyl gloves. Atter completing theradioactivity experiment, the teacher should store theisogenerator in a secure cabinet, which can be lockedand is used tor no other purpose (Spectrtun Techniques(Corporation). The radiation level inside the isogenera-

tor has been tested by Spectrum Techniques to be lesstban 8 fiCi. Tt requires no special storage or handling.It is standard practice to store cbemicals and radioac-tive materials in separate, locked cabinets (Flinn Scien-titR-201)5).

Reviving nuclear science educationNuclear science has taken a back seat in today's cbemistrycurriculum. Despite tbe enormous number ot nuclearscience applications found around us, many ot thetundamental concepts surrounding nuclear chemistryare misunderstood and even feared. Tbis balf-lifelali—especially wben integrated into a well-planned uniton nuclear cbemistry—-will provide stiulents with theknf)v\'ledge to overcome misunderstantling anti irr:itionalfear. Students will be hitting important standards andbencbmarks for tbe teacbing of science and better preparingthemselves tor understanding and living in today's world. •

Scott Liddicoat (sliddico@greenbay.kl2.wi.us) and John Se-branek (jsebrane@greenbay.kl2.wi.us) are chemistry teachersat Green Bay Southwest High School, Uil Packerland Drive,Green Bay, WI 54304.

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Decennber 2005 41