EFFECTS ON LUMINESCENCE DATES OF CHERNOBYL FALLOUT_ CORRECTIONS

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EFFECTS ON LUMINESCENCE DATES OF CHERNOBYL FALLOUT_ CORRECTIONS
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  534   NuclearInstruments and Methods in Physics Research A26 (1987) 534 537 North-Holland, Amterdam THE CHERNOBYL FALLOUT IN GREECE  N TS EFFECTS ON THE DATNG OF ARCHAEOLOGCAL MATERALSY LIRITZIS Mnistryof Culture, Archaeometry Division, 3 Kahspen Street, Athens, Greece Received 21 April 1987 The effects of the fallout from the nuclear reactor accident at Chernobyl havebeen monitored at various sites to Greece   Here we present the first estimatesof gamma dose rates, an essential parameter in the datingof archaeological materials by thermolumnes-cence (TL) and ESR methods   The dose rates are derived from the long-lived radionuclidesof 137Cs, 134 Cs, 1°6 Ru and 144Ce  with t112 >- 1 yr The present dose rates vary between 30 and 60 mad/yr, but maximum alues of around 811 mrad/yr have also been recorded, for ground-surfaceexposures   These dose rate values must be regarded asvery significant to TL and ESR dating of samples from now on and acorrectionfactor should be applied   1   Radioactive falloutin Greece - measurements Early on the 2nd May 1986, an increased level of radiation was observed at Thessaloniki University in NorthernGreece from issiontracesin an air filter   By theafternoon of the same day the same was detected in Athens, Southern Greece (Nuclear Research Center, Demokritos)   ratemeterreadingsincreased from the normal4 PR h-1, and in some other areas of Greece it reached levels of 4 1LR h t [1]   From that day onwards many measurements were carried out on food products, air and ground surface   It was thought to estimate the y dose rates in various sites in Greece inordertoappreciate the effect that the accident would haveon the TL or ESR datingof archaeological materials, in particular ceramcs and kilns   The age equation in TL is givenasTotaldose Age = Annual dose where the nomnator is measuredby TL or ESR The denomnator comprises the naturalradioisotopes of uranium thorium and potassium cosmc rays and an insignificant contribution from rubidium It is the annual dose which is affected by the long- lived y emtters such as 137C s t34 Cs 106Ru and 144 Ce  table1 Surface and near-surfacematerials are now con-tamnated to such an extent that we cannot always apply the dose rate methods  TLD portable scintillom eter, ß-counting) wthoutstudying fission productde-cay at the same time   0168-9002/87/ 03 .50 © lsevier Science Publishers B V   (North-Holland Physics Publishing Dvision) The dispersion of radionuclides depends not only on their decay mechanism but also on the physical and chemcal peculiarities of the ground and climatologicalfactors (wnd, rainfall which define the degreeof ab- sorptionof particlecarriersof radioactivity (or the free ions) in the ground and theirability of being washed orseeping deeper intothe ground In addition,further transfers of radioactivity to the ground are made by the trees and plants anda contri- bution is made from percolating groundwaters etc   from other neighbouring areas   Therefore, the dose conversion factorsfor fallout insoils is at first sight somewhat uncertain All the above factors lead to important fluctuations in the radioactivity of the ground Rainfall is the most significant factor forthe local maxima of cesium con- centration   Cesium is released as oxideor hydroxides both of which are very soluble   Asa result high con- centrations of cesium are anticipated in the ground for Table 1 Characteristic nuclides from the fallout a> Measured from 137 B a b) Measured from 1o6Rh   Nuclide Energies Y [MeV] R t1/2 137C S a)   .662 0 .520 30 yr 134 C s 0 .605   662 .062 yr 1  ) RU b) 0 .512   .041 371 d 144 C e 0 .1335   .30 285 d  regions wth heavy precipitation during the days of increased radioactivity   This wasfound to be the case in Greece itself, where, particularly, a higher radioactivity was noted in   W Thessaly, S.W Macedonia andLesbos Island   ower level was noted in W Greece, S Sterea, Peloponissos wth local exceptions and most of the Islands  fig   1 Table 2 shows the radioactive fallout forthe first days of May infive centres   The soil samles wth exceptionally high levels of radioactivity  reduced to 7 May 1986 are as follows : 137Cs   34 CS   1-2 Bq g-1   106 Ru   0.5-0 .7 Bqg -1   144 Ce :   0.1-0 .2 Bq g-1   90 Sr :   0.1-0 .2 Bq g-1   estimated Typical values were 5-10 times lower   The radioactivity of sand was especially lowThe typical levels of radioactivity before the Chernobyl fallout were 100-250 Bq m Z yr -1 whilst in 1964 wththenuclear tests theradioactivity from 137CS in the ground was 2000 Bq m z  approx 2 .8 mad/yr)   The above nuclides together wthothers weremasured wtha coaxial gamma spectromter  HpGe or Ge/Li) ; Fig   1   Map ofGreeceshowng areas where the fallout was recorded Y Lintzis / The Chernobyl fallout to Greece and ts effects Table 2 Radioactive fallout  Bq m z ; mximm values 2-6 May 1986 2 Dose rate conversion factors535efficiency 20-23 energy resolution 1 .7-2 keV low level detection for 137CS = Bq 1 -1 in the NRC De- mokritos, Athens [1]   The dose rate conversion factors for external ex- posure to photons are comuted according to various mthods [2-5]   Itis generally difficult to obtain realistic estimates of external dose froman arbitrarydistribution of radio- nuclides due tofactors such as ground roughness, ter- rain irregularities, and penetration of radionuclides into the ground wthtime   For asource activityof 1 Bq the dose rate factor is in units of Sv yr -1  = 10 5 mads yr-1   per Bq cm 2  = 10 kBq m z   Inour calculationsthe external dose ratefactors are referred to an exposed individual and are obtained from environmntal transport models   These factors are basedon idealized exposure conditions which arenever strictly realized in the environmnt and this leads to potentially imortant limtations when applying the results forthe purpose of obtaining appropriate dose rate estimates for dating The limtations arise from  a the assumtion of uniform radionuclide concentra- tions over source regions which are effectively sem-infinite or infinite in extent ;  b the assumtion that ground-surface exposure occurs to the whole body of an individual ;  c the photon transport calculations are based on an isotropic radiation field   Therefore photondose ratefactors for ground-surface exposure should be applied to environmntal dose as- sessmnts for archaeological dating wth due considera- tion Apartfrom the abovemntioned limtations one should consider also  a thedifferent effective dose rate factors between soils and humn organs  tissue ,  b that the gamma spectrum alters due to a different evaluation of the flux of scattered -1-rays in air and in Nuclide City Athens Thessalo- nlkl Ptole- mida Aliven Megalo- poli 137Cs 2500 1200019000100012000 134 CS 1200 60009000500 6000 106 R u 2000 600012000700 8000 144 Ce 400 16001800 500 4000  536   Y Lirltzls / Fig   2 Schematic diagram showng how archaeological artifacts and mn are exposed to environmental and fallout radiation   the ground  fig   2)   However, it is possible, to a first approximtion, to use the dose rate factors from allout for an individual above the ground, to estimate the dose rateto archaeological materials buriedwthina depth  x of an averageheightof an individual under the ground-surface  fig   2)   To justifythis we took into account thefollowng concepts :  a) the density for tissue is p = .12 g cm 3 which is not significantlydifferent from hat of a typical soil of p = 1 .6 g cm 3  b)the whole-body consists minly of   nd O wth C and   o follow)   The ratio of the mss attenuation coefficient   for water-to-air is1 .03 and that for muscle-to-air is - 1.07, whilstthe mss-energy absorption coefficient, 1L, is1 .1 for both,  c) the p of soil-to-air is 0 .845  and 1 .115 for soil   25 water [7,8]   All ratios of  b) and  c) arefor E =0 662 MeV 137 Cs 3 Calculations and discussion The integrated annualdose forthe yearfollowngthe accident from the radionuclides of tables 1 and 2aregiven in table 3 . The effective dose rates in subsequent years aregivenin table 4 From ables 3 and 4 one Table 3 Integrated y dose rates  mad/yr) after one year forthe data of table 2The Chernobyl fallout in Greece and its effects should note particularlythegreat variation in the dose rates in a number of areas of Greece Dose rates vary from about 1 to 50 mad/yr for the comng years   Wthn the next five years the dose ratesare likelyto be mre significant forthe areas which suffered the effects ofthe initial heavy fallout  e .g   Ptolemida,Thessa- loniki, Megalopolis), but in the succeeding ten years they becom insignificant   The dose rates toburied materials are likely to be lower as -y-rays are attenuated by a factor proportional to p for ground and air . At a depth x under the ground-surface the dose rate is expected to bereasona-bly approximtedbyan exponential function, i .e   R = R e - x °   where R is the emssion rate of primry photons at the surface, p is the mss-energy absorption coefficientfor the soil and d is a dispersion factor chosen on the basis of available depth distribution data One should expect, however, that below say, 30 cm he effect of fallout on the ground would be small   The imortance of dose ratesfor dating is indeed mrked especially if one comares today s rates wth those from variousarchaeological sites in Greece in the 1970s Thesewere 150-170 mad/yr in the Kavala area wth its famus Chalkolithic site of Sitagroi, 90-170 mad/yr in the Volos area, the present capital of Thessaly, aregion heavily settled in the Neolithic to classical times, 80 mad/yr in Delphand 120-150 mad/yr in Crete, the home of the Mnoan civilization [9,10], Thus em ploymnt ofeq  1 for an actual annualdose of, say, 130 mad/yr, anage result, for inclusion dating, of 1960 yr BC becoms 1680 yr BC  ceramc from Sitagroi, STG ZA2Vb [10] ,if an extra y-dose of 40 mad/yr is added from the fallout   For an earlier samle from Sesklo, Thessaly, an ageof 5210 yr BC wll appear as 4600 yr BC 6]   The effect on age is even mre pronounced for materials of the last two mllenia  the error in TL or ESR dating is between ±5 and7 and maximum dose rates   One shouldbe aware ofthe above dose ratefiguresin any forthcomng dating application to archaeologicalmaterials from Greece a) From oil samples   Typicalvalues were 5-10 timeslower, which are of the same order as the dosesrecorded in the dwelling areas   Nuclide City Ah Thess   Ptol Al   MegMaxim a> 137 Cs 3 .5 16 .8 26 .6 1 .0 16 .8 162-324 134 Cs 3 .8 19 .2 28 .8 1 .6 19 .2 148-295 106 R u 0 .74 2.224.44 0 .262.96 127-181 144 C ev   small v   small v   small v   small v   small 5- 11 Total 8 .04 38 .2 59 .82 .86 39 .0 442-811 Subtotal   137 Cs+ 34 Cs) 7 3 36 0 55 .4 2 60 36 .0 310-619  Y Liritzis / The Chernobyl falloutin Greece and its effects   537 Table 4   References Effective dose rates at varying timesinyears afterthe deposi tion of the activities oftable 2 on the ground  mad/yr [11 DEMO eport ofthe Greek Atomc Energy Comm NR Cities   Years   Demcritos, July  1986 in Geek [2] D C Kocher,Health Phys 45 1983 665 1   2   5   10   [31 NRPB Publ   n o   DL 1986 Athens   6 7   5   2 4   1  2   [41   E   Purvis and R S   Foot, in   The NaturalRadiation Thessaloniki   32  2   23   11  9   6 1   Environment, eds   J .A.G Adam and   M Lowder Ptolemaida   50  2   36  4   18 6   9  5   Cambridge Unversity Press, 1964 p747   Aiven   4  07   2  8   1 2   0  53   [5] L Lovborgand P Krkegaard, Rso Report  1975 317   Megalopoli   33  7   23  2   11  9   6 .1   [61 Y Lirtzis and R B   Galloway, PACT J   J   ofthe Europ Study Group on Phys   Chem   Biol   and Math Tech- niques Appl   t o Archaeol . 6  1982 450 [7] Y Liritzis and R B   Galloway, Nucl   Instr   and Meth 174 Acknowedgemnt   1980 593   [8] D R Evans Radiat   Dos I  1968 94   am thankful to Vagn Mejdahl for reading the   [ ] Y Liritzis, Rev Archaeometrie 8  1984 7 manuscript   [101 Y Liritzis, Athens Ann Archaeol   12 1979 208
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