analysis of foods for radioactivity

8/6/2019 Analysis of Foods for Radioactivity

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216 q Environmental Contaminants in Food

present in the body. For the present purpose, weare only concerned that the penetrating nature of gamma radiation allows its direct measurementin foodstuffs, while alpha and beta emitters gen-erally must be separated from the bulk constitu-ents of the sample before measurement is possi-ble.

There are other processes in radioactive disin-tegration that produce emissions. Alpha emissionis usually accompanied by low-energy gammarays that may be used for measurement. X-rayscan be produced by electron capture and somegamma emitters decay by internal conversion, aprocess where a fraction of the gamma rays areconverted to monoenergetic electrons. Theseprocesses do not really modify our measurementconcepts but the decay modes of the significantradionuclides must be known for their accuratemeasurement.

The physical half-life of the radionuclide tendsto control its persistence in the environment. Forexample iodine-l 31, with a half-life of about 8days, is important for only a few weeks, whilecesium-137, with a half-life of about 30 years, maybe a problem for centuries.

A third classification that has some value is thesource that produced the radionuclides. Knowl-edge of the source of radioactive contaminationgives a good indication of the nuclides that are tobe expected in the sample. This is of considerableassistance in planning the analysis since request-ing a complete analysis for all radionuclides oreven for all types of radioactivity in a single sam-ple would lead to a lengthy and expensive opera-

tion. Many radionuclides are not potent healthhazards, particularly those that are not metabo-lized by the body. The general groups of nuclidesto be expected include the natural activities, spe-cifically radioactive potassium and members of the uranium and thorium series, and the artificialfission products, transuranic elements, and otheract ivat ion products that resul t f rom nuclearweapon explosions and nuclear reactor opera-tions.

Fission products are a very complex mixture atthe time of formation but the short-lived radionu-clides die out rapidly and the mixture becomessimpler within a few days, The transuranics (plu-tonium, americium, etc., formed by activation of the basic fissionable material) are of some inter-est because of their high toxicity when incorpo-rated into the body but present evidence indicatesthat their uptake through the gut is relativelysmall and that dietary intake is not a significantproblem. This should be true particularly if therelative hazard of other radioactive contaminantsprobably present in the same sample is taken intoaccount, The other activation products are fre-quently elements that make up steel or othermetal containers or structural elements. Radioac-tive manganese, chromium, cobalt, zinc, and ironare particularly common and result from inter-actions of the materials with neutrons released inthe nuclear reaction. It is worth pointing out thatcontamination of foodstuffs with single nuclides isextremely unlikely, and that more than one mem-ber of any group will probably be present in anysample.

DISTRIBUTI ON OF RADIONUCLIDES

The source of the radionuclides involved willgenerally control their distribution in the environ-ment and their consequent transfer to the foodchain. The sources considered here will includenatural radioactivity, releases from operation of nuclear reactors and processing plants, and fall-out from nuclear weapons tests.

Natural activity may be of concern when it isenhanced by mans intervention, say by mining tobring material to the surface and processing theore to yield either products or wastes that mayconcentrate the radionuclides. Good examplesare radium in uranium tailings, in phosphate rock waste, or in slags from phosphorus production,Radium may enter the food chain by dissolving inground water and transferring through plantroots.

Nuclear reactors in normal operation releasechiefly the radioactive noble gases that are not of interest in considering foods. Reactors do containlarge inventories of fission products, transura-nics, and other activation products, however, andaccidental releases can contaminate vegetationby deposition or through the water pathway. Gas-eous releases would most likely involve the vola-tile elements such as iodine and tritium or thosewith volatile precursors, such as strontium-90and cesium-137, Aqueous releases would followfailure of the onsite ion exchange cleanup systemand any water-soluble elements could be in-volved.

Processing plants could also have either gas-eous or aqueous releases, but only fuel reproc-essing is likely to be a significant contributor. In


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this case, the f ission products are aged beforeprocessing and iodine and the gaseous precursorradionuclides are not released. Tritium and car-bon-14 are the major airborne products, while thewaterborne radionuclides are the same as forreactors.

Atmospheric nuclear weapons tests distributetheir fission products, transuranics, and other ac-tivation products globally, with local deposition

being more or less, depending on the size of theweapon and the conditions of firing (high altitude,surface, underground).

In summary, the deposition of airborne materi-al on vegetation or on soil is the route by whichfoodstuffs become contaminated and the subse-quent behavior of the radionuclide is controlledby its chemical nature, including solubility andplant or animal metabolism.

CONTAMINATION OF FOODSTUFFS

Contaminat ion of foods can occur e i therthrough atmospheric deposition or by transferwith water. In the first case it is possible for theradioactive material to be in the form of insolubleparticulates rather than in a more available formwhere it will follow the chemistry of the elementsinvolved. A knowledge of the pathway is not abso-lutely necessary but it does assist in deciding onthe proper preliminary treatment of the sample of

evaluating exposure. For example, surface con-tamination may have a different significance thanthe same material present in a plant through rootuptake.

Since pathway information is not always avail-able i t is generally considered proper to measureradioactivity in samples that have been preparedas if for eating, so as to approximate the true ex-pected intake. This will usually result in strippingoff or washing off of a considerable fraction of surface contamination. Cooking is generally notpart of the preparation, as the mode of cookingand the use o f j u ices, cooking water, and the likecannot be predicted,

Milk is often recommended as an indicator foodfor studying radioactive contamination. It hasmany advantages:

1. It is available locally at most desired sam-pling locations.

2. It is marketed rapidly, so that short-lived ra-dionuclides. such as iodine-l31, can be eval-uated.

3. It is a major diet component in the UnitedStates, both directly and as an ingredient of prepared foods.

4. There is a lot of background informationavailable on previous contaminating events.

It is worth noting that some of these advan-

ta ge s are the factors that contribute t o the roleof milk as a source of human exposure.Milk is a poor indicator of many contaminants.

The natural activities, the transuranics. and theactivation products have relatively low concen-trations in milk. The first two are low because of poor biological transfer and the last because theirpathways are almost entirely through the aquaticor marine food chain. Thus, the best approach isto know what is in the environment through othermonitoring systems, and to design the food anal-ysis program to fit the circumstances.

Other monitoring data are also necessary infixing the geographical extent of a contaminatingevent. As a general rule, nuclear tests are globalin radionuclide distribution, with enhanced levelsnear the test site. Releases from other nuclear op-erations tend to be more local in their effects an dthe food-monitoring plan can be modified to suit.

SAMPLING

The mechanics of obtaining representative foodsamples will not be considered here since the pro-cedures are common to all types of food analysis.There are certain points that must be consideredhowever. The first is whether the m e a s u r e m e n t s

are being made to determine human intake or thesource of the contamination. In the latter case theearly approach used by FDA(1) for radioactivity isapprop r i a t e. There, each sample was identified

as to its place of origin. For evaluating intake i t ispossible to collect total diet samples for a partic-ular population group and to measure the radio-activity in this composite diet. This approach hasbeen described by FDA(2) and by EPA(3) and is

normally applied in institutions where mass feed-ing is carried out. A more elaborate procedure isto simulate a total diet by measuring a number of component food classes selected on the basis of


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statistical information regarding consumption.This approach was originally used by the AtomicEnergy Commission(4) and was applied originallyto t h r ee major cities. This approach does allowidentification of specific food types that are con-taminated but requires much greater effort andcost in analysis.

The use of indicator foods as an intermediatetype of monitoring is widespread. As mentionedpreviously, most of the systems depend on thesampling of milk which is available in most partsof the country either with specific information asto place of origin or the general area of the milk-shed. Part of the reason for using milk is the easein sampling but it also has significance as a pri-mary food for the youngest and most susceptiblepopulation group and it also does tend to pick upseveral of the fission product nuclides of dietarysignificance such as radiostrontium, radiocesium,

and radioiodine. Most other foods give limitedgeographical or seasonal coverage and are lesssatisfactory.

The preservation of samples in the field duringtransport and in the laboratory awaiting analysisis only an esthetic matter in the case of radionu-elides. Decomposition processes do not changethe radioactivity, and sample contamination byradioactivity is unlikely. Thus, freezing, formalde-hyde addition, or any other method that will main-tain the sample is adequate. Any additive shouldbe checked to assure it does not contain signifi-cant amounts of the radionuclide sought.

For the purpose of this report we will assumethat an adequate quantity of a representativesample is available for analysis and that anotherportion is available for storage, either permanent-ly or until the analytical results are accepted assatisfactory.

SAMPLE PREPARATION

The measurement of radioactivity is a physicalprocess and it is most efficient when the radioac-tivity from a relatively large sample can be placedclose to the detector. This means that direct meas-urements of bulk samples are only useful at rela-tively high levels of contamination and that mostmeasurements are preceded by preparation andpossibly chemical separation to reduce the bulk of the material and to improve the efficiency of themeasurement.

As mentioned previously, sample preparationmay include removal of inedible portions of thefoods or those portions not generally eaten. For

example, citrus rinds, apple cores, outer leaves of leafy vegetables, and aboveground portions of root vegetables would normally be discarded. Thegeneral goal is to prepare the foods as if for cook-ing or consumption.

Foods generally have a high water content anda primary method of bulk reduction is drying atroom temperature, at elevated temperatures, orfreeze drying, Most of the radionuclides of inter-est are not volatile under these conditions andlosses must be considered only for elements suchas tritium and iodine. The dried material can bereduced further by ashing at elevated tempera-tures, by cold ashing with activated oxygen, or bywet ashing with oxidizing acids. The sample dry-ashing process is most likely to lead to loss of volatile elements but with care even cesium, polo-nium, and lead can be retained, The other proc-

esses should not lead to losses of elements of in-terest with the exception of iodine and tritium,mentioned above, and of carbon.

Another approach that is essentially one of re-ducing bulk is to extract either the original sam-ple or the dried or ashed material with acids orother solvents and thus remove the desired ele-ments from the bulk of the sample. This requiresconsiderable testing beforehand to be certainthat the process operates in the desired manner.

All of these procedures reduce the bulk of thesample and in the case of extraction may alsoseparate the desired constituents from some of

the remaining inert material. They do not, how-ever, separate the radionuclides of interest com-pletely from the other radionuclides present inthe sample. Such separations will be covered inthe next section, but they may not be necessary if the measurement technique can provide bothqualitative and quantitative information. In mostcases this limits the possibilities to gamma spec-trometry on the prepared sample.

If the samples are going to be subjected tochemical analysis, the sample preparation mustinclude the dissolution of the dried or ashed mate-rial. This has already been done, of course, inpreparing the wet ashed or extracted solutions.The radionuclides of interest in ashed foodsshould be soluble in strong acids if ashing tem-peratures have not been excessive, and this treat-ment is usually accepted. If there is concern that


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Appendix lAnalysis of Foods for Radioactivity 219

insoluble particulate may be present, it is neces-sary to use more drastic methods such as fusion tobring the sample into solution. This should not benecessary, however, if human hazard is the prob-lem under consideration.

At the time of preparation, if not before, thebasis of measurement must be established. De-pending on the use of the data, wet weight, dry

weight, ash weight, volume, or even numericalcount (e. g., eggs) has to be determined. Frequent-ly, this may have to be done in the field, but for-tunately, relatively crude measures are adequate,

Since unforeseen questions often arise, it is rec-ommended that as many of these quantities bemeasured as is possible.

The requirements for measurement of radionu-clides in foods are such that sample preparationis best handled by the group responsible for therest of the analysis. The chief difference fromother types of analytical work is the initial sam-ple. The usual range is from 1 to 20 kg, and thereduction of this amount of material is a special-ized problem.

RADIOCHEMICAL SEPARATIONS

Radiochemicallate the desired

separations are required to iso-radionuclide both from the re-

maining bulk constituents and from other radionu-clides which would interfere in the measurement.In addition, it is necessary to convert the finalproduct to a form suitable for presentation to thecounter. This may involve elect redeposition, pre-cipitation, or other processes. There are a num-ber of manuals giving the details of specific radio-chemical procedures and these details will not berepeated here. There are a few generalities how-ever that may be of interest.

The actual mass of radionuclide that is meas-ured is almost always vanishingly small. Thismeans that many of the normal chemical reac-tions used in analytical chemistry will not takeplace; for instance precipitates will not form. Forthis reason it is common to add a few milligramsof carrier material which is preferably the inertform of the same element. Where this does not ex-

ist it is frequently possible to use similar elementsas carrier, such as the substitution of barium forradium. The inert form then follows normal chem-istry, carrying the radionuclide with it. It is alsoworth noting that even when the separation tech-nique does not depend on the mass of elementpresent, a carrier may still be useful in prevent-ing unwanted coprecipitation or absorption onglassware.

Because of the high degree of purification re-quired in radiochemistry, it has been customaryto make a number of repeated separations eitheridentical or different to insure purity. To carrythese out in a reasonable time it is better to lose a

small amount of the nuclide sought while remov-ing a large fraction of the undesired materialrather than retaining all of the nuclide and muchof the undesired material. Fortunately, it is possi-

ble to measure these losses in analysis and tomake a correction at the end. One approach is tomeasure the carrier at the end of the analysiseither by gravimetric or instrumental analysis. If there was none of the carrier substance presentin the initial sample, the fraction recovered willbe equal to the fraction of desired radionucliderecovered. Where carriers are not available orwhere they are present in variable amounts in theoriginal sample it is frequently possible to useradioactive tracers, that is radioactive isotopes of the same element or a similar element. The frac-tion of the amount added that is left at the end of the analysis can be used to determine the recov-ery of the radionuclide sought.

In the chemical separations there is consider-able use of classical analytical chemistry basedon precipitation and in most cases it will at leastappear as a final collection step to put the desiredradionuclide in a condition suitable for counting.

In addition the general techniques of ion ex-change, liquid extraction, distillation, and elec-trolysis are used.

Since many of the separations in radiochemicalanalysis are carried out in small volumes the cen-trifuge is widely applied, It has become commonpractice to redissolve and reprecipitate ratherthan to wash precipitates carefully. This may berepeated several times very rapidly and will gen-erally give good decontamination (separationfrom other radionuclides). As noted above, wherethe element sought and the contaminant havesimilar properties it is preferable to use two dif-ferent precipitations, a precipitation followed by

an extraction, or any two widely different steps toachieve good decontamination.One process that is frequently of value is called

scavenging. This term is usually applied to a pre-


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cipitation carried out to remove contaminating ra-dionuclides after the bulk matrix of the samplehas been removed. The process involves the addi-tion of a group carrier, precipitation, and discard-ing of the precipitate. A typical example would bean addition of iron carrier followed by hydroxideprecipitation to remove rare earths and otherheavy metals in a determination of an alkali oralkaline earth. Frequently the scavenging proce-dure is repeated to improve decontamination, butit is only rarely that the scavenge precipitate isredissolved and reprecipitated to recover any of the desired constituent that may have been ab-sorbed.

After suitable radiochemical separations havebeen made, it is necessary to collect and mountthe sample for counting. This is most often doneby precipitation or by elect redeposition. In thesection on measurement, the requirements for

sample preparation are discussed: for example,the sample area and mass should be reproducibleand the sample should be mounted on a sampleholder identical with that used for the counterstandard. Since it is frequently necessary to de-termine the weight of precipitate for recovery de-termination, this factor must also be considered.

The selection of a mounting technique is usual-ly a compromise between convenience and thecounting requirements. Besides consideration of the type and energy of emission, practical matterssuch as the counter size and sample mounts avail-able must be weighed,

In certain cases, the total amount of sampleavailable may be limited and analysis for severalradionuclides may be required. Proceduresshould be on hand for the sequential analysis of single samples, even though separate samples areused for routine work.

MEASUREMENT

The method of measurement to be selected de-pends on the type of radiation, the form of thesample, and to some extent on the amount of ra-dioactivity. It is necessary that the complete ana-lytical procedure be designed so that the sampleis brought to a suitable form for the equipmentand conditions that exist.

It is possible to measure the total gamma, totalbeta, or even the total alpha activity on a sampleof food, Unfortunately, such data are valueless inestimating human exposure. The accuracy of thedetermination is very poor, natural potassiumusually interferes, and the chemical and radia-tion characteristics needed to evaluate possiblehazard are not known. It is possible, however, toset a particular total activity level as a screeninglevel for a specific food. If the measured value isbelow the screening level, no analyses for individ-ual radionuclides are performed. In such a case,the measurements should be considered as inter-nal data only and the numerical results should notbe published. Any report should merely list thesamples as having activities below the statedscreening level.

Qualitative identification of radionuclides onoriginal samples of foods is limited to gamma emit-ters at relatively high levels. The sensitivity canbe increased if some bulk reduction, as describedunder Sample Preparation, is carried out. Theidentification of alpha and beta emitters dependson radiochemical separation for element identifi-cation and the measurement of energy or half-life

on the separated material for radionuclide identi-fication. The latter step may be omitted if otherconsiderations limit the possibility to a single ra-dionuclide.

The equipment available for quantitative meas-urement of radioactivity is sufficiently sensitivefor all foreseeable cases. Instruments for detec-tion of the three major emissions are describedhere, and their applications are shown in table 2 0(chapter VIII) in terms of the detection limits forvarious radionuclides.

It is worth pointing out that a number of the in-struments described are not commercially avail-able. They have been in the past, but the low de-mand has removed them from the market, Thenecessary electronic components are availablebut the mechanical assemblies for the detectorsare not, and the larger laboratories tend to buildtheir own systems.

Alpha Emitters. The measurement of alpha ac-tivity is best carried out on a very thin sample toavoid self-absorption of the alpha particles. Thisis even more true for spectrometry since degrada-tion of the original alpha energy will give a spec-trum with poor resolution. The measurement of the total alpha activity can be carried out eitherin thin-window counters or by scintillation count-ing with zinc sulfide phosphor. Both techniqueshave high efficiency but the scintillation methodcan give a considerably lower background with aconsequently lower limit of detection. Unfortu-nately, neither system is readily available as a


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222 Environmental Contaminants in Food

Figure I-1 .Comparison of Gamma Spectra Taken With a Sodium Iodide Detector (Upper Curve) anda Germanium Diode Detector (Lower Curve). Note that the main single peak in the sodium iodide spectrum is

resolved into 9 peaks for 6 radionuclides with the germanium diode spectrum

o

purities are greatly reduced compared to spectrafrom sodium iodide detectors. The efficiency of the diode is low and, for many analyses, a spec-trometer can only be used for one measurement aday. Another disadvantage is that the detectormust be kept at liquid nitrogen temperature tomaintain its detection capability. Newer diodeshave been developed that do not require storageat liquid nitrogen temperatures; however, theymust be cooled during measurement. Since thesystem is in use most of the time, this may not be asignificant advantage.

Diode spectrometers may also be used to meas-

ure the low-energy gamma-rays that accompanyalpha emission. This allows direct measurementin some environmental samples, but the levels in

foods have not been high enough for this tech-nique.

General Requirements. The choice of a count-ing procedure depends on the precision required.In turn, the relative precision of a quantitativecounting measurement is inversely proportionalto the square root of the number of counts ob-tained. Thus, any improvement in precision mustbe obtained by increasing the number of counts.This can be done by using larger samples, bycounting for longer times, or by using counterswith higher efficiency. A secondary improvementis possible for low-activity samples by decreasingthe background. Each of these improvements hassome drawback, and selection of the optimum bal-ance requires a degree of experience to weigh


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Appendix lAnalysis of Foods for Radioactivity 223

cost, manpower, and quality. For example, han-dling larger samples increases the effort in sam-ple preparation and radiochemistry while longercounting times require more counters, and in-creased counter efficiency or lower backgroundis expensive.

Counting-room operation requires the mainte-nance of detailed records on standardization,background, and sample measurement. This in-formation can frequently allow the recovery of bad data or the correction of calculational errors.

In addition, maintenance of control charts(5) willsignal when instrument problems arise.

Experience with modern nuclear instrumenta-tion has been good, and downtime of the order of 5percent is common. This requires that service beavailable immediately or the next day following abreakdown. The increasing complexity of measur-ing systems tends to preclude in-house servicingin most cases, but some diagnostic capability andcompetence in minor repairs is very valuable.

CONTROLS

An analytical laboratory carrying out measure-ments of radioactivity in foodstuffs will require aprogram to document the validity of the measure-ments, This is necessary in legal cases and is alsohighly desirable when data are being presentedfor use in decisionmaking. A suitable program of quality control should be carried on in addition to

the necessary calibrations and standardizations.Calibration is the determination of the relation-ship between a desired quantity and the responseof a particular instrument. In the case of radioac-tive materials this may mean that the instrumentmust be calibrated with each radionuclide to bemeasured unless there is evidence that the instru-ment response is independent of the energy orother characteristics of the radiation. Alterna-tively a complete response v. energy calibrationmay be substituted. These calibrations, to have alegal standing, should probably be traceable tothe U.S. National Bureau of Standards or compar-able authority. This turns out to be a requirementthat is far from trivial in the effort required.

Fortunately, a complete calibration is not re-quired at frequent intervals and, depending onexperience, may not be needed oftener than everyfew months. In the meantime of course it is neces-sary to have assurance of the proper operation of measuring equipment but this can be done withsimple standards or even with samples that arereproducible over a period of time. For example, asimple counter standard may be run every morn-ing before starting operations just to be sure thatthe counter is working properly. Similar stand-ards should also be available for checking spec-trometer energy response.

An ideal quality control program should in-clude the checking of the complete procedurefrom sample preparation through measurement.

This requires that standard samples be availablefor testing the full procedure. Additional controlswould include running of blind duplicate samplesto test reproducibility of analyses, and blank samples to check on the possibility of laboratoryor reagent contamination. These three types of samples should make up at least 10 percent of thelaboratory output if the quality of measurement isto be followed closely. It is also necessary that thedata be published with the ordinary laboratoryresults and that any deviations from the expectedresults should be used to make suitable correc-tions in laboratory operations.

There is always some danger that a laboratorymay drift out of control in spite of an adequate in-ternal quality control program, This is possiblesince an internal program may emphasize consist-ency while the absolute values drift. Therefore,some part of the quality control effort must bedevoted to intercomparisons with establishedgroups, such as IAEA or EPA. This should be doneon an annual basis, at least.

It must be noted that standard samples are fre-quently not available, and recourse must be hadto spiking, which is the addition of a knownamount of a radionuclide to a sample that is rela-tively free of that nuclide. Recovery of the spike isnot necessarily a good test of a chemical proce-dure, since a natural sample may be more diffi-cult to analyze.

A final component of a good quality control sys-tem is a careful, responsible review of all the dataproduced. This means checking the arithmetic,knowing the characteristics of the equipment and,hopefully, having a sixth sense that recognizesthat certain results do not look right. This isespecially true when relying on automatic count-ers or on computer processing of the data.


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224 . Environmental Contaminants in Food

STAFF AND FACILI TIES

A minimum facility could be designed around astaff of six, including a B.S. or M.S. senior chemistwith experience, a B.S. junior chemist and threetechnicians for chemistry, sample preparation,and counting-maintenance, A- secretary-adminis-trative assistant could handle reports, local pur-chasing, and similar duties. With this small staff,considerable versatility and flexibility would berequired.

The measurement of radioactivity in foods re-quires fairly extensive facilities to handle thevaried analyses that might be called for, In addi-tion to a modest office space, separate roomswould be necessary for sample preparation, wetchemistry, measuring equipment, and for mainte-nance support and storage. The first two lab-oratory rooms would require hoods, chemicalbenches, and laboratory safety devices such asshowers and eye fountains. Since the samples tobe measured are normal foods there are no spe-cial requirements for radiation safety, If the sam-ples were radioactive enough to be a personnelhazard in the laboratory they certainly would notrequire this type of precise measurement.

If the laboratory is to be in continuous opera-tion it is most likely that the principal chemicaland measurement systems will have to be avail-able at least in duplicate. This and similar consid-erations lead to the conclusion that a certainminimum size and sample throughput are not nec-essary if the radioactivity laboratory is part of alarger operation which can furnish support. Onecontinuing problem is the need for electronicmaintenance of radiation measuring equipment.This is rather specialized and must be consideredeither when staffing the laboratory or in locatingit where such services are readily available.

It is not always possible to purchase the idealcounting equipment for a particular purpose. Thetotal demand for many systems is small and com-mercial instrument makers are not interested.Large laboratories can make some of their ownequipment but this is not possible for the groupdescribed here. The construction of alpha andbeta radiation detectors requires services of afirst-class machinist plus sufficient electronicknow-how to transfer the detector signal to theavailable commercial equipment. Where such ca-

pability already exists in the overall organization,it may be fruitful to copy advanced noncommer-cial instruments, but staffing specifically for thisfunction is probably not economical. Thus, mostsmall laboratories must operate with the less-than-optimum commercial equipment,

If significant uses are to be made of gammaspectrometry there will be a need for at leastmodest computing facilities. These can be part of the purchased spectrometer system but this is anexpensive method if suitable computer time isavailable otherwise. The former approach is usedhere, and it might be noted that the spectrometercomputer has capacity for other work.

Additional space is required not only for stock-ing necessary reagents and materials but also forstorage of incoming samples and for storage of residual material from samples that have beenrun. As a general rule it is desirable to take alarger sample than would be required and to setaside a portion of this for possible contingenciesor if the data are later brought into question.

The output of a group this size should run be-tween 1,000 and 4,000 samples ] a year, dependingon the difficulty and the activity level, One limit-ing factor is that, at background levels, eachcounter can only turn out one or two samples perday. Following a contaminating event, it should bepossible to process smaller samples and to countfor shorter times, both of which would allow high-er output. It might be possible to add 3 more lab-oratory staff plus 50 percent more space andequipment dollars and essentially double the out-put. Further increases would probabl y run intoother bottlenecks due to the need for added sup-port.

Table I-1 details the costs for space modifica-tion and furnishings ($140,000), for equipment($200,000), and for supplies ($20,000). These costsare based on a building in place, lights and parti-tions in place, and utility stubs in each room, It isalso assumed that building services are provided.

IA sample is defined as a ~x]mp]ete gamm{i spectral an[]lysisor a single raciionuclicie an[]lysis that requires preparation plusr:idi(x:hemist rv.


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GENERAL REFERENCES

Ra d i o c h e m i c a l M e t h o d s

National Academy of Sciences, Nuclear Science Series(NAS-NS 3001 ff), Office of 1echnical Services,Washington (1960 and later), A Series of Mono-graphs on Individual Elements and Techniques.

WHO-FAO-IAEA, Methods of Radiochemical Analysis,World Health Organization, Geneva ( 1966).

Johns (Editor), National Environmental Research Cen-ter, Las Vegas, Handbook of Radiochemical Analyti-cal Methods, EPA-680/4-75-001, February 1975.

Harley (Editor), Environmental Measurements Labora-tory, DOE, EML Procedures Manual, HASL-300,1972 (Updated Annually).

International Atomic Energy Agency, Reference Meth-ods for Marine Radioactivity Studies (Sr, Cs, Ce, Co,Zn), Technical Reports Series No. 118 (1970).

Ibid. (Transuranics, Ru, Zr, Ag, I)Technical Reports Series No. 169 (1975)

M e a s u r e m e n t o f R a d i o a c t i v i t y

National Council on Radiation Protection and Meas-urements, A Manual of Radioactivity Measure-ments Procedures, Report 58 ( 1977).

National Council on Radiation Protection and Meas-urements, Environmental Radiation Measurements,Report 50 (1976).

International Commission on Radiation Units and Meas-urements, Measurement of Low-Level Radioactiv-ity, Report 22 ( 1972).

International Atomic Energy Agency, Measurement of Short-Range Radiations, Technical Reports SeriesNo. 150 ( 1973).

International Atomic Energy Agency, Metrology andRadionuclides (Symposium), IAEA, Vienna ( 1960).

Crouthamel (Editor), Revised by Adams and Dams, Ap-plied Gamma-Ray Spectrometry, Pergamon, NewYork (1970).

Data Compi la t ions

Lederer and Shirley, Table of Isotopes7th Edition,John Wiley, New York (1978).

Health, Scintillation Spectrometry, Gamma-Ray Spec-t r u m Catolog ( 3 r d E d i t i o n ) , USAEC R e p o r tANCR-lOOO ( 1974).

Chanda and Deal, Catolog of Semiconductor Alpha-Par-ticle Spectra, USAEC Report IN-1261, March 1970,

Niday and Gunnink, Computerized Quantitative Anal-ysis of Gamma-Ray Spectrometry, USAEC ReportUCRL-51061, 4 Volumes (Includes Decay SchemeData Library).

Qual i ty AssuranceKanipe, Handbook for Analytical Quality Control in Ra-

dioanalytical Laboratories, EPA Report 6 0 0 / 7 -77-088 (April 1977),

Ziegler and Hunt, Quality Control for EnvironmentalMeasurements using Gamma-Ray Spectrometry,EPA Report 600/7-77-144 (I)ecember 1977).

See also:Metrology Needs in the Measurement of Environmental

Radioactivity Environment International 1, 7 f f (1978),

GLOSSARY

Activation ProductsThe radionuclides formed ineither fissionable material or in surrounding mate-rial during a nuclear reaction, usually by capture of neutrons,

CarrierA stable element added in radiochemicalanalysis to provide sufficient mass that a radionu-clide can follow various separation steps.

Det ect or An y device that transforms the radiationemitted by a radionuclide into a signal that can behandled electronically. The most usual signal is anelectrical pulse which can be recorded.

Disintegration-The spontaneous process by which aradionuclide gives off energy in the form of nuclearradiation.

Fissionable MaterialAny of the heavy radionuclideswhich can undergo fission, or splitting into two ormore lighter atoms. The fission process is accompa-nied by a large release of energy.

Fission ProductsThe radionuclides formed when fis-sionable material splits into two or more atoms.

Geiger Cou nter A detector based on gas ionizationwhich converts any ionization within the detectorinto a large electrical pulse.

Germanium Diode DetectorA gamma-ray detectorcomposed of very pure germanium, usually acti-vated with lithium.

Indicator FoodA food that is measured to give anestimate of the total dietary intake of man.

Ionization ChamberA radiation detector based ongas ionization which converts any ionization withinthe chamber into an equivalent electrical pulse.

PathwayThe route by which a radionuclide is trans-ferred from the source through the environment toman.

PicocurieSee units of radioactivity. Equal to 2.2 dis-integrations per minute.


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Proportional CounterA detector based on gas ioniza-tion which converts any ionization within the detec-tor into an amplified electrical pulse proportional tothe amount of ionization.

RadionuclideAny atomic species which is unstableand gives off nuclear radiation to attain stability.

Scintillation DetectorA detector which converts nu-clear radiation to a pulse of light. This, in turn, canbe converted to an electrical pulse with a photo-tube.

Sodium Iodide DetectorA scintillation gamma-ray de-tector made up of a single crystal of sodium iodideactivated with silver.

SpectrometryThe measurement of the energy of radi-ation emitted during decay of a radionuclide or mix-ture of radionuclides. Quantitative as well as qual-itative information may usually be derived.

A p p e n d i x I A n a l y s i s o f F o o d s f o r R a d i o a c t i v i t y . 2 2 7

TracerA radionuclide added in radiochemical anal-ysis to follow the distribution of the desired constit-uent in various separation steps.

Transuranic ElementAn artificial element having ahigher atomic number than uranium, formed by ac-tivation, usually with neutrons.

Units of RadioactivityThere are various ways of ex-pressing the disintegration rate of a radionuclide.In this report the disintegrations per minute (dpm)unit is used. In other reports, the curie and its sub-multiple (millicurie, microcurie, picocurie) areused and the new International Standard nomen-clature is the bequerel, equal to one disintegrationper second.

analysis of foods for radioactivity

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