Author Institution: Department of Biology, Bowling Green State University, Bowling Green, Ohi

Verduin, Jacob

The Ohio Journal of Science

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THE OHIO JOURNAL OF SCIENCEVOL. 61 JANUARY, 1961 No. 1MEASUREMENT OF ENVIRONMENTAL RADIOACTIVITYWITH BASIC COUNTING EQUIPMENTJACOB VERDUINDepartment of Biology, Bowling Green State University, Bowling Green, OhioIn June 1959 the Biology Department of Bowling Green State Universityreceived a grant from the Atomic Energy Commission (Grant No. BM-66-59)to purchase equipment for a radiobiology laboratory. The grant was obtainedfor educational purposes and the proposal specified that the equipment wouldbe devoted primarily to class use. Some of the work reported below, however,was subsidized by a research contract with the Atomic Energy CommissionAT(ll-l)-536.The following list of equipment was purchased: three basic decade sealersequipped with lead shielded manual sample changers, halogen-quenched end-window Geiger-Muller tubes and preset timers; one amplifier type decade sealerequipped with a gas flow counting chamber, a cylinder of Q-gas and pressurereducing valve; one organic-quenched end-window G-M Tube; one portable G-Mcounter; a source kit; and a set of absorbers.The equipment was supplied with instructions for assembly and operation.It was assembled by members of the biology department and every item was inoperation within a few days after its arrival.In utilizing this equipment we have introduced exercises in radiobiology intoexisting courses; for example, the general physiology course introduced an exercisein measurement of photosynthesis using C14. The bacteriology course introducedan exercise comparing C14 uptake from NaHCO3 solution by yeast cells in presenceand absence of glucose. The ecology course introduced an exercise measuringthe level of gross beta radioactivity in ash from natural materials. In the generalbiology course a lecture-demonstration, using the portable G-M counter, wasintroduced. The research techniques course (graduate students) included lab-oratory work with these instruments, and a few students registered for specialproblems utilizing them. During the first 9 months, over 60 students gainedexperience in the techniques and theory of utilizing radioactive isotopes. Thisnumber does not include the students exposed to the lecture-demonstration ingeneral biology. The equipment is installed in a room set aside for this purposeand classes using the room are scheduled in advance to avoid conflicts.A radiobiology workshop has been scheduled for the summer sessions. Thisis the only new course that has been added to the curriculum. It provides astudy of the fundamentals of radiochemistry, the techniques of measuring radio-activity and of using radioactive isotopes in biological experiments.Although the instruments purchased were well supplied with instructionmanuals, these provided no information as to the efficiency of the instruments fordifferent radioactive elements. The efficiency of an instrument is, of course,profoundly influenced by the shape and thickness of the sample, and the distanceof the sample from the window of the G-M tube, but a table of efficiencies forspecific substances of given sample thickness and distance from the tube willprovide a valuable reference source for comparison of different instruments. InTHE OHIO JOURNAL OF SCIENCE 61(1): 1, January, 1961.2 JACOB VERDUIN Vol. 61table 1 such data are provided. Table 1 shows that the beta radiation frompotassium, which has an energy of about 1.3 MEV, is detected with an efficiencyof about 4 to 7 percent with the halogen-quenched G-M tube, if it is within about1 cm of the end of the tube, and a sample of 1 g or less is contained in a planchetof 7 cm2 area. Potassium contains naturally radioactive K40 in such concentra-tion that one gram of potassium undergoes 1660 disintegrations per minute(0.75 x 10~3/xc). It is a most convenient source to use for instrument calibration.It is also revealing for the student to compute the number of K40 disintegrationsper minute in the adult body knowing that the normal potassium concentrationis about 2 g/kg of body weight.The windowless gas flow counter detects about 33 percent of the K40 disinte-grations. The weaker C14 radiations, however, (energy of about 0.14 MEV;Comar, 1955) are detected by the gas flow counter with an efficiency of only 5percent. The halogen-quenched end-window tubes measure only 0.4 percent ofthe radiations emitted by C14. In spite of the low efficiency of the end-windowtubes for C14, we have used them successfully for experiments employing thisisotope. For example, if 25 ml of pond water are labelled with 0.25 /xc of NaC14O3and exposed to natural light for 6 hr, then filtered through a millipore filter, theactivity on the filter will usually register 50 or more counts per minute on an instru-ment having an efficiency factor of 0.004. A five-minute count of such a filterTABLE 1Efficiency of various instruments under specified conditionsRadioactivesubstanceKC1ua«u.C "Type of tubeHalogen Quenchedend window, 1.4mg/cm2aGas flowfHalogen Quenchedend window, 1.4mg/cm2Gas flowjDistancefrom tube (cm)0.40.40.41.2windowless0.9windowlessSample*weight (g)1.020.400.200.870.570.0010.001Efficiency0.0510.0650.0700.0430.330.0040.05*The sample was distributed over an area of about 7 cm2.fThe gas flow counter (Nuclear Chicago D47) was attached to the amplifier type sealer(Nuclear Chicago 186A), the others were attached to basic sealers (Nuclear Chicago 186).will measure the activity with a certainty of about 85 percent. This is similarto the reproducibility of duplicate samples; hence, there is no virtue in prolongingthe counting time of individual samples.Before we obtained the equipment listed above, the physics department ofBowling Green State University permitted us to use its basic sealer equipped witha halogen-quenched end-window G-M counter. Karl Schurr, a graduate studentin biology, collected specimens of various plants and animals, dried and ashedthem, and measured radioactivity of ash samples. Some of the data from hisMaster's Thesis were used to compile table 2. The observed counts per gram perminute are recorded. The disintegrations per gram per minute were computedusing the efficiency factor for K40 (0.0038) and the p/zc per gram were computedusing the relationship 1 /^ uc = 2.22 disintegrations per minute. Leaves fromterrestrial plants showed the highest activities; leaves from aquatic plants showedNo. 1 ENVIRONMENTAL RADIOACTIVITY 3less; and ash from most animal tissues yielded counts hardly distinguishable fromthe background count of the instrument (13 counts/min).The high values observed for Sycamore and Willow leaves are higher than thehighest average values reported by laboratories using more sensitive instruments.Thus, Nehemias and Whipple (1955) found grass leaves to yield the highest grossbeta counts of any organisms they studied. The average concentration observedTABLE 2Radioactivity of ash from plants and animals (1957)MaterialTerrestrial plantsSycamore leavesfruitsbudsMixed wood ashes(fireplace)Willow leavesHackberry leavesTrembling Aspen leavesAquatic plantsPotamogeton leavesVallisneria leavesRhizocloniumCladophoraAnimalDuck liver (Scaup)Mouse legsMollusc fleshEarthwormBlue gillIsopodsSnailscounts/g/min11415141410157502728201119882100disintegrations/g/min3000390370370266015001310670726526290500210210532600135017516616612006755923193272361302159595241200TABLE 3Radioactivity and potassium content of ash from deciduous tree leaves (1958)Radioactivity PotassiumSpecies (disintegrations/g/min) content (%)Cottonwood, Populus deltoidesBasswood, Tilia americanaWhite Birch, Betula papyriferaWhite Oak, Quercus albaSycamore, Platanus occidental-isAmerican Elm, Ulmus americanaCatalpa, C. bignoniodesMaple, Acer saccharinumAsh, Fraxinus americanaAverage140062530502200900180022503200317020664.98.611.46.95.310.68.78.210.08.3in their grass samples was 25 /x/xc per g wet weight; this amounts to about 250 MMCper g of ash, because 10 g of grass leaves yielded about 1 g of ash. During 1958we made further analyses of ash from leaves of deciduous trees. Ralph Sechriest,a graduate student, did this work. Because potassium is an important com-ponent of plant ash, subsamples of ash were sent to the Brookside Research Lab.,4 JACOB VERDUIN Vol. 61Inc., New Knoxville, Ohio, for quantitative spectrophometric analysis of potassiumcontent. The observed radioactivity and potassium content of ash of nine speciesare presented in table 3. The average radioactivity was 2066 disintegrations/g/min and the average potassium content was 8.3 percent. Hence, the observedpotassium content could account for only 200 disintegrations/g/min or about 10percent of the observed radioactivity. A smaller group of samples analyzed forpotassium in 1957 yielded values similar to those of table 3.During the summer and fall of 1959 a broader survey of terrestrial plantleaves was made. Many more species were collected than previously, and col-lections were made at intervals during the season to learn whether the radio-activity of the ash increased as the season progressed. In table 4 the averagelevels of radioactivity observed during 1959 are compared with the averagevalues from 1958 and 1957. The samples collected in June yielded a slightlyhigher average value of radioactivity than the July and September samples, butthe differences are not statistically significant. The significant feature of thisstudy was that the level of radioactivity did not increase as the season progressed.TABLE 4Average radioactivity of ash from terrestrial plant leaves collected at various timesTime Fall '57 Fall '58 June '59 July '59 Sept. '59Gross j8 disintegrations/g/min/21929852066930126056810604771120505TABLE 5A verage radioactivitycorndisintegrations/g/minMMc/gof ash from old cornstalks compared withleaves grown during summer of 1959Old Cornstalks4562051959 Stalks550248cornstalks and1959 Leaves1490670Each value is the average of 8 or more samples. The difference between stalkand leaves is statistically significant beyond the 0.01 level. These data wereobtained with the gas flow counter having an efficiency of 33 percent for K40.The disintegrations/g/min were computed by multiplying counts/g/min by 3.Samples of about 0.1 g in planchets having an area of 7 cm2 were used.The 1959 average values were distinctly lower than those observed in 1958 and1957, and a sample of the willow ash which had been saved from the 1957 collec-tion when counted in 1959 yielded only 1100 disintegrations/g/min. These datasuggest that a considerable fraction of the radioactivity detected in these ashsamples represents debris from weapons testing and that the reduced levels in1959 reflect decay of short-lived components detected in 1957 and 1958. Theabsence of a detectable increase in level of radioactivity during the 1959 seasonsuggests that very little radioactivity was absorbed by the leaves from rain duringthat season; however, the summer was unusually dry.During the winter of 1959-60 a collection of cornstalks and leaves from afield near Bowling Green was studied and compared with a sample of old corn-stalks collected from my barn at Put-in-Bay, Ohio. The old cornstalks werepresent in the barn when I purchased it in 1948; so, they had been stored for morethan 11 years. The radioactivity of ash from these samples is presented in table 5.No. 1 ENVIRONMENTAL RADIOACTIVITYThe 1959 cornstalks revealed a level of radioactivity only 20 percent above thatof the old cornstalks. But the 1959 leaves revealed a level about 3 times higherthan the stalks. The simplest explanation for the higher radioactivity of theseleaves is that they absorbed it from atomic debris in rain that washed over them.If this excess radioactivity were absorbed from the soil it is difficult to understandwhy it would be deposited preferentially in the leaves, and if the minerals absorbedfrom soil in 1959 included considerably larger amounts of radioactive atoms thanwere absorbed by the old stalks, one would expect a larger difference betweenthe old corn stalks and those of 1959.One additional study was carried out with the ash from cornstalks and leaves:a number of discs were cut from an aluminum sheet, measured, weighed, and theirweights per cm2 computed. These were used as absorbers and the transmissionthrough various thicknesses was measured (Harley and Hallden, 1959). Infigure 1 the transmission data for radiation from corn leaf ash are presented.Similar data from a sample of KC1 were used to establish the line so labelled. In10160 200 240 280mg /cm 2FIGURE 1. Semi-log plot of counts per minute versus absorber thickness (mg/cm2). Lowerplot represents ash from corn leaves collected December 1959. Numbers indicateto.l values.analyzing such data it is convenient to draw a line through a set of data and com-pute the weight per cm2 required to reduce the transmission to one-tenth of theinitial intensity (1 cycle on the logarithmic scale). This parameter, which wewill call to.i, is directly proportional to the energy of the beta radiation emitted bythe source. Thus, KC1 with an energy of about 1.3 MEV has a to.i value of about250 mg/cm2, and C14 with an energy of 0.14 MEV has a to.i value of 28 mg/cm2.In the ash from corn leaves there seem to be 3 distinct components: a low energycomponent having a to.i value of about 63 mg/cm2, an intermediate componentwhose to.i value is closely similar to that of K40, and a high energy componentwhose to.i value is about 610 mg/cm2. Similar analyses of the ash from cornstalks,old and new, revealed only a single component, approximating the line of inter-mediate slope in figure 1. It seems likely that the low energy component repre-sents a mixture of soft beta emitters. Cerium144, a beta emitter with energies of56 JACOB VERDUIN Vol. (110.3, 0.2 MEV and Zirconium95 (/3, 0.4 MEV) are prominent products of weaponstests (Van Dilla, 1960) and may be important in this portion of the curve. Theintermediate slope is certainly influenced by natural K40, but this line also mayrepresent a mixture for there are radioisotopes of energies 1 to 2 MEV in weaponsdebris which could not be distinguished from K40 in this portion of the curve.The high energy portion of the curve may be influenced by Rhodium106, thedaughter of Ruthenium106, which is also abundant in weapons debris. Rhodiumhas a half life of only 30 sec so the disintegration Ru106 (/3, 0.04 MEV) wouldbe quickly followed by the decay of Rh106 (0, 3.5 MEV). The daughter ofCerium144, Praseodymium144, also emits a strong beta (3.0 MEV) and Strontium90(/S, 0.5 MEV) has a daughter Yttrium90 whose beta energy of 2.2 MEV wouldplace it on this portion of the curve.An analysis of the kind portrayed in figure 1 will not separate the individualelements in the complex mixture of substances likely to be present in fallout fromweapons tests, but it does provide information about the variety of beta energylevels present in the mixture; and the contrast between stalks and leaves suggeststhat the leaves collect fallout to a much greater extent than do the stalks. Thesimilarity between the radioactivity of ash from old and new cornstalks suggeststhat most of this radioactivity is contributed by naturally radioactive elements,with K40 predominating.The data presented above demonstrate that radioactivity in ash from naturalmaterials is sufficiently high to be measured with equipment (basic sealer andhalogen-quenched end-window G-M tube) now becoming available to manycolleges through educational grants from the Atomic Energy Commission. Al-though measurements of radioactivity in the environment are being made moreaccurately by laboratories equipped with more sensitive (and expensive) equip-ment (Nehemias and Whipple, 1960; Setter and Straub, 1958), a student's under-standing of the problem is improved by carrying out studies like those describedabove.SUMMARYInstruments purchased with an educational grant from the AEC measuredthe radioactivity of ash from materials collected in nature. Ash from plantleaves showed levels of about 1000 MMc/g during 1957 and 1958. Leaves collectedin 1959 showed levels only one-half so high. Ash from corn stalks was onlyone-third as radioactive as that from leaves. Surveys of this kind can be carriedout by undergraduate students.LITERATURE CITEDComar, C. L. 1955. Radioisotopes in Biology and Agriculture. McGraw-Hill. 481pp.Harley, J. H. and N. Hallden. 1959. Analyzing beta absorption graphically to identifyemitters. Nucleonics 13: 32-35.Nehemias, J. V. and G. H. Whipple. 1960. The pre-operational environmental survey for theEnrico Fermi Reactor—1958. J. Am. Indust. Hyg. Assoc. 21: 144—150.Settler, L. L. and C. P. Straub. 1958. The distribution of radioactivity from rain. Trans.Am. Geol. Union 39: 451-457.Van Dilla, M. A. 1960. Zinc-65 and Zirconium-95 in food. Science 131: 659-660.

Measurement of Environmental Radioactivity with Basic Counting Equipment

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Measurement of Environmental Radioactivity with Basic Counting Equipment

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