edited by GALEN W. EWING Professor Emeritus Seton Hall University Address: P O . Box 2573 Las Vegas, NM 87701
pottery has experienced and to its sensitivity to the acquisition of thermoluminescent energy. The latter is measured by giving the samplea known" .rtifieial" dose of suitable radiation after the original glow curve has been ahtained. The annual duse received can be calculated from measurements of the radioactive contents of the specimen and its In principle' the age in years is directlv obtained as: (TLL (dose), age = -(TL), (dose rate),
CI. Thermoluminescence Part Ill. Application to Archeological Dating Emanuel
P. Manche
Depadrnent of Natural Sciences, York College of The City University of New York,
One application of thermoluminescence is in hasir solid-state research. Thermolummrwence ot#.prreach a w r l r tc,:, length" decay chain 0 7 , frwn I I ~ hN I , . . .I. , .and >, -radintmn emerge. whereas the "'K disintegrates to give mo& 6 with a small percentage of y radiation. The half-lives of decay of the three natural radioisotopes are all well in excess of 10s years so that they do supply a constant dose-rate over the cam~arativelyshort archaeological period. The dosage received, however, is not uniform and is subject to a number of considerations. Far the uraniumand thorium isotopes, the decay schemes indicate that most of the energy is associated with a-particles, hut this radiation is found to be much less efficient than t h e 6 and y radiation a t inducing thermolumineseence; it is only 0.05 t o 0.3 times as efficient as the other radiation in pottery (38). Furthermore, consideration must be given to the relative penetrating power of all three radiations. The a-particles emitted in the uranium and thorium decay series with energies between 4.0 and 8.8 MeV have comparatively short ranges. In pottery fabric, they travel an average of 25 pm (39). &?adiation, on the other hand, can travel approximately 1mm (39). These figures show that Band a radiation have ranges that are, respectively, comparable to or considerably less than the thickness of sherds. They are thus considered the internal sources of radiation. This must he contrasted with the y-radiation which has an approximate range of up to 30 em. As a consequence of this greater range compared t o the dimensions of a typical sherd, only a fraction of the internal y-radiation energy will dissipate within the sherd. On the other hand, i t contributes nearly all of the environmental radiation. Thus, the cosmic and gamma-radiation contributions are generally taken to be the external sources. Figure 11shows a pottery fragment with a typical radioactive content that was buried in soilpossessing the same radioactivity. On an average, the total effectioe dose-rate has been estimated to he around 0.5 rad per year A342 / Journal of Chemical Education
Figure 11. Annual dose (milkads) for a ponery fragment with typical radioactive contents (U = 3 ppm: Th = 12 ppm: K = 1%). burial in soil of the same radioactivity and 5 feet below U?esurlace. The "k-value" is the ratio of the thermoluminescence produced by a given absabed dose han m-pi-particies to the thermaiuminescence produced by the same absorbed dose from ppmicles. (Reproduced from ref. (36) by permission).
It has already been established that for every age determination by this technique it is necessary to obtain, in addition to the natural glow-curve, a t least one artificial glow-curve t o determine the radiation suseeptihihty of the sherd under study. This is necessary since thermoluminescence not only depends upon the concentration of defects and impurities that the lattice of the crystalline inclusions can maintain, hut further depends upon the nature of the crystalline minerals themselves (46).In fact, a particular mineral, such as quartz, may he derived from several sources with different impurities (35). Thus, the sensitivity measurement is made hy subjecting the sample to a known calibrated radiation dose, the resulting thermoluminescence being subsequently measured. When one compares the resultant curves, i t will benoted that theartificialdose will give rise to a more complex curve with peaks in the low-temperature region as shown in Fig-
(40).However, due to the extremevariability of the composition of clays, this value must be ascertained experimentally.
MEASUREMENT OF NATURAL DOSE RATE The determination of the natural dose rate may be grouped under two general headings: relotiue and absolute estimations. I t should be noted that all dating methods attempt to measure chronometric dates and mention of "ahsolute" and "relative" dating in the literature refer, in part, to the method of doserate estimation. The "shsplute" dating arrives a t a date from physical measurements alone; whereas in "relative" dating a knowledge of potterydated by other means is necessary. In relative estimation, the parameter measured is the rate of a-disintegration in the pottery (41.42).This is seen (see Fig. 11) to be the single most important contributor and accounts for approximately half of the effective total dose rate. The assumption in this dating technique is that the radiation damage is proportional to the a-particle homhardment. Experimentally, a few grams of ground sample is measured in a scintillation counter using a ZnS:Ag phosphor screen following the design of Turner et al. (43).The a-emissions are counted in "infinitely thick layers" and because this is low-level detection, several days of counting time are usually needed. In the case of the absolute estimation, an attempt is made t o calculate the energy absorbed by the sherd or by a fraction of that sherd. The usual oraetice has been the determination of the uranium and thorium content by a-counting, in the manner already described above; while the potassium content has been carried out by chemical analysis (36),either using flame photometry (441, or X-ray fluorescence (45). The natural dose rate can he calculated in terms of the total a and 6 energies associated with each complete disintegration in the series u8U 2xPb and 232Th WSPb with the a-particle contribution weighted to allow far its low trap filling efficiency (45).
-
-
oB_ , I .
$00
,r*PrsA,"sL
.C
Figure 12. A, Typical thermoluminescent curves Obtained from ancient pottery. (a) natural thermoluminescence: ( b )thermoluminescence of natural Source 2200 rad of laboratory-applied radiation; ( 0 ) background 'black-body' radiation. The archaealgicai dosage suffered by this 3000-year old sample was 870 rad. The calculated annual dose rate was 0.320 0.065 radlyr. giving a TL age of 2700 i s 5 0 yr. (Reproducedfrom ref. (46a)by permission). 8. The ordinate ratio test to investigatethe stability of thermoluminescent storage during the archawlogicai burial period for the data given in Figwe t2A. (Reproduced with modification from ref. (46a) by permission).
+
*
ure 12A. However, it can he noticed that the shapes of the curves match mare closely as the temperature increases, indicative of the f a d that decay effects eventually disappear a t the higher temperatures.
CURRENT TL DATING TECHNIQUES As we have already seen, dating techniques have been generally termed as "relative" and
"absolute". The former technique has been used mainly by Iehikawa (47,42) and a group a t the University Museum of the University of Pennsylvania (48, 41); while the latter approach has been attempted by Mejdahl (49) and by a group under Aitken a t Oxford Universitv (50) who have ~ uforth t a number of seoar& strateeies which have received
above headings.
"Relative" Dating This dating is generally based on the following equation
where (TL). and (TL), are the natural and artificial thermoluminescenees, respectively, R ,is the a-particle count rate on a sampleof the ootterv and K. an emoirical constant. is
Figure 13. Plat of the specificlhermoluminescence versus knownage sample as used in me "relative" dating technique. Sample migin are: 1. 2.4.5.9. 10. 11. 49. 50 and 51 fram Iran: 8 horn Baluehistan; 6. 7. 12. 13. 14 and 20fram Italy: 22 hom U.S.A.: 71. 73, 74 and 94 horn Turkey. The calibration is based On specimens from samples 800 B.C. old and younger. Older "knownage" dates depend on "C dating of associated carboniferous material. (Reproduced fram ref. (51)by permission).
of the line drawn through the average values of these determinations provides this proportionality constant, The ages of samples may then be calculated. Equation (11) follows from eqn. (10) if one assumes that the a-particle count rate is proportional to the annual dose rate received by the sample under investigation, and also that its sensitivity to a-particles is a constant fraction of the sensitivity to other kinds of radiation. Experimentally, approximately 10-15 g of clean potsherds are crushed to below 30 mesh and then to less than 200 mesh in a small hall mill. A portion of the ground sample is used to measure its relative radioactivity by counting a-emission in "infinitely thick" layers. Another portion is used t o obtain its thermoluminescence. In one laboratory (51) samples are mounted on a small square of thick aluminum foil hy means of silk-screen technique. This is done hy mixing the sample with silicone oil (500 centistokes) to produce a thin uniform layer of 2 em in diameter and
(Continued on page A344) Volume 56, Number 11. November 1979 1 A343
Chemical Instrumentation approximately 0.13 mm thick. The back side of the holder has thermocouples attached and is also coated with a layer of graphite. It is then placed on the furnace, flushed with nitrogen and heated linearly from 70°C up to 450'C and the natural thermoluminescence recorded (TL).. About 20 replicate measurements are made for each potsherd in order to obtain a representative result. After obtaining the natural glow curve, each sample is subsequently irradiated with X-rays (60 see a t 30 kV and 12.7mA). The samples are reheated and the artificial glow curves (TL), are obtained. The specific thermoluminescence in eqn. (11) is then abtained and plotted as a function of known age for each sherd. In Figure 13, the calibration line was plotted using historically dated samples from 1350 A.D. to 800 B.C. (samples number 6,7, 12, 13,14,20 and 22). Other samples, dating to 7000 B.C. were also plotted as samples of "known age", hut these samples were radiocarbon dated from associated carbonaceous material. Thescatter from thecalibration line is approximately i20%; however, in the post-2W0 B.C. range this scatter is only about f10%. But here. the "C-dated samdes were w t rorrrrted fc;r the discrepsniiw rhnt npprnr LO he the result of fluctuations in the production of radiocarbon a t varying times in the past (52,531. ~~
~~
In the fine grain technique developed by Zimmerman in 1971 (44), an extract is made ofrrains withdiameters between 1and Bum.
Supralinearity (57), the fine grain and quartz inclusion techniques are used together. In this method the dating is based on the a dase-rate and hence there is independence of the knowledge of environmental conditions during the burial history of the sample. This methodology is a powerful one in authenticity applications where there is a total lack of environmental history as well as suspicion of clandestine irradiation. However, the method requires a relativelv laree samole size (- 1 e) in the
12%. In the zircon dating technique of Zimmerman (58). the presence of the less abundant mineral zircon with uranium and thorium concentrations of the order of hundreds of ppm, instead of the characteristic few ppm found in most clav. bodies. ~.~leads to a strone rhrrmaluminrwent &yal as a r o n r q w n w o t t h r h ~ x hinternal o-particle dose mtr. 'The technique i\ virtually independent of rhe external radioactive dose rate including burial circumstances, although it requires an initial sample size of about 3 g to furnish the few needed single zircon grains as well as a high level of skill and expertise in both entraction and measurement (59). , ~, The dating techniques mentioned so far require samples with a reasonable time-lapse period (at least 500 years), since young materials lack sufficient light output in the 300'-500°C interval of theglow curve. In an attempt to overcome this limitation, the quartz pre-dose technique was developed (60-62) which has a range of application limited from present-day t o 1500years. The method is based on the faet that quartz, like LiF, displays a sensitivity enhancement after annealing out a pre-irradiation. This change, produced by the combined effeeb of irradiation and heating, has been attributed to an increase in the probability that an electron released from a trap will give rise to luminescent emission, either because of a greater facility in finding the luminescent centers (possibly due to removal of competing nonradiation centers), or because of an increase in the number of activatedluminescent centers (67). The ore-dose characteristics of the mvre m e n r e l w tempemture 1Il1~Cl glow curve penkofquart, ahow remnrkoblv hqh sensitivity changes, a phenomenon that has enabled measurement of the archaeological dosage the artifact has experienced during burial. In a study of Chinese T'ang Dynasty glazed wares, the technique was successful in distinguishing original wares from reproductions and to date the manufacture of some fakes to the second decade of this century (631, thus having the capacity of accurately dating forgeries as well as authentic wares! &
In order to obtain correct "absolute" ages by the thermoluminescent method, allowance must be made for the faet that the radioaetivity within asherd is non-uniform. The bulk of the a-radioactivity from uranium and thorium is in the opaque clay matrix (54). whereas the bulk of the thermoluminescence comes from the less radioactive mineral inclusions, such as quartz, of dimensions of 1 mm to 1pm or less (47,54,46). Because of the short range of the alpha particles (20-50 pm) their contribution to the dosage of a n inclusion is greatly attenuated being almost negligible for inclusions greater than 100 pm. Thus. the arehaeoloeical dose that the inrlu.&ma haw reurivrd wries with their sive. 'l'hiq pnhlem demanded solution before the ages n d d be obtained wlth thrrmoluminescenee using the absolute method. A variety of techniques have been developed for absolute dating. However, only brief mention of the experimental procedure can be made here for each of these techniques. In the quartz inclusion technique developed by Fleming in 1970 (55), grains of approximately 90 t o 105 pm diameter are separated and etched with H F to remove the outer shell of damage due to a-particle bombardment. The relevant dose rate for dating the residual cores is then the @ and y contribution as well as cosmic rays. In this technique the environmental dose rate is emphasized which tends to place importance on burial site conditions. A similar oracedure. the feldsoor
COMPLICATIONS In order to apply the deceptively simple age relation of eqn. (10). a number of cam-
A344 I Journal of Chemical Education
The basic assumption in the age relationship is that thermoluminescence is acquired a t a uniform rate. Unfortunately, this assumption is not strictly justified. For most minerals, the slope of the growth curve initially increases progressively with dose before beaming constant. Supralinearity presents
approach for such a correction is the additive dose technique (64) in which "first elow" measurements are made on portions ofsample which carry artificial TL in addition to the natural TL. Correction for the earl" sunralinear portion of the growth characteristic a done by means of "second glow" measurement.
.
.
~
~~~~~~~
"Absolute" Dating
plicating factors and interfering effects should be taken into consideration. A listing and brief explanation of these factors and effects follows:
Fading of Stored TL Signal 1) Thermal foding-This refers to the gradual decay of stared T L by the thermal release of trapped charges that occur when the sample is held a t a temperature well below that of the glow peak. The mechanism of release is the same as that represented by eqn. (1). For an archaeological sample, in order to establish a region of stability, the "ordinate ratio test" is performed (65). This consists of a plot of the ratioof the natural to the artificial thermoluminescent intensity as a function of elow curve temoeratures as sh
