STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
TLD USING IN CERAMICS DATING
C.Cosma and V. Benea
Faculty of Physics, Babes-Bolyai University, 3400-ClujNapoca, Romania
Thermoluminescence dating is based on the measurement of the amount
of light that is released upon thermal or optical stimulation, by minerals
such as quartz and feldspar. The light signal is a measure of the radiation
dose that has accumulated in these minerals through time.
In our work we will present some preliminary results connected with the
samples and device (Harshaw-2000) preparation for thermoluminescence
of feldspar separated from ceramic samples with age well known. For
external doses was used TLD-100 with high sensibility (LiF-Mg-Cu-P)
1. INTRODUCTION
Thermoluminescence (TL) dating is a radiometric method based on the fact
that trace amounts of radioactive atoms, such as uranium and thorium, in rock, soil,
and clay produce constant low amounts of background ionizing radiation. The
atoms of crystalline solids, such as pottery and rock, can be altered by this
radiation. Specifically, the electrons of quartz, feldspar, diamond, or calcite
crystals can become displaced from their normal positions in atoms and trapped in
imperfections in the crystal lattice of the clay molecules. These energy charged
electrons progressively accumulate over time. When a sample is heated to high
temperatures in a laboratory, the trapped electrons are released and return to their
normal positions in their atoms. This causes them to give off their stored energy in
the form of light impulses (photons). This light is referred to as
thermoluminescence. What is actually determined is the amount of elapsed time
since the sample had previously been exposed to high temperatures. In the case of
a pottery vessel, usually it is the time since it was fired. For the clay or rock lining
of a hearth or oven, it is the time since the last intense fire burned there.
The effective time range for TL dating is now about 300,000 years down to a few
decades. Theoretically, this technique could date samples as old as the solar
system, if we could find them.
2. PRINCIPLES OF TL DATING
Thermo luminescence dating is based on the measurement of the amount
of light that is released upon thermal or optical stimulation, by minerals such as
quartz and feldspar. The light signal is a measure of the radiation dose that has
accumulated in these minerals through time.
When they are exposed to sunlight during transportation in the air the
latent thermoluminescence signal in the quartz and feldspar grains is bleached
C. COSMA AND V. BENEA
down to a negligible level and the thermoluminescence "clock" is set to zero. After
deposition of the grains and burial under new sediment, their latent
thermoluminescence signal accumulates again because they absorb the natural
ionising radiation that is emitted by the surrounding sediment. The flux of ionising
radiation (α -, β -, γ -rays) is produced by the very low concentrations of uranium
(235U, 238U), thorium (232Th), potassium (40K) and 87Rb in the sediments. A small
amount is cosmic radiation. The total radiation dose that is accumulated in this way
is called the palaeodose.
The age that is determined corresponds to the time span between the
removal of the thermoluminescence signal by sunlight just before deposition and
the removal of the newly accumulated palaeodose by thermal or stimulation in the
laboratory. Stimulation by heat is called thermoluminescence (TL).
The age equation can be written as:
Thus, luminescence dating involves the determination of two major
parameters: the palaeodose and the annual dose.
Reliable age determinations can be obtained from 0.1ka to 200 ka. Typical
errors are in the range of 5 to 10 %..
Essentially, the palaeodose is evaluated by comparing the natural TL
intensity of the sample with the increase of TL output induced by known amounts
of additional radiation. By extrapolating the growth curve until it intersects the
initial TL intensity of the sample, the dose accumulated since the initialising event
can be found.
In this procedure a complication arises because, in general, the growth
curves are not straight lines. While it is possible to observe the form of the growth
curve for TL intensities greater than the natural level, the manner in which the TL
grew from its initial state is not revealed. This difficulty is answered by measuring
a second growth curve, using portions of the sample in which the initial state of the
TL has been recreated by heating or exposure to light. The form of this second
growth curve is then taken as the correct line to use when extrapolating the first
growth curve. It should be noted that this assumption is ultimately untestable by
scientific investigation, but can only be justified through the dating of known age
samples
The purpose of environmental dosimeter in TL dating is to measure the
gamma and cosmic ray contribution to the total radiation dose received by the TL
sample during its burial at the archaeological or geological site. In the case of flint
and calcite, this contribution is commonly a major part of the total dose. Moreover,
it is often the uncertainties in determining this contribution that govern the overall
error limits of the TL date. The on-site environmental dosimeter is therefore an
important part of the dating procedure.
In contrast to alphas and betas, the range of gamma rays is approximately
0.3 m, and therefore much greater than the dimensions of most TL samples. This
means that the contribution to the total radiation dose, which comes from gammas,
TLD USING IN CERAMICS DATING
is dependent mainly upon the radioactive content of the sediment surrounding the
TL sample, and only slightly upon the radioactivity of the sample. Like the alpha
and beta rays, gamma radiation derives from the decay of naturally occurring
radionuclides present in the ground, such as potassium-40 and members of the
uranium and thorium decay series. Together with the contribution from cosmic
rays, which is a minor one, the gamma component of the total radiation dose is
referred to as the environmental dose.
The purpose of environmental dosimetry is to estimate the mean rate at
which the TL sample has received its environmental dose during burial. Ultimately
this can only be an estimate, because measurements record only the present day
radiation environment. One concern is therefore the temporal variability of the
burial conditions, which, as discussed below, is often associated with changes of
ground water content. The other main concern is spatial variability, since it is rare
to find sites that are homogeneous in their radioactivity. It is these spatial variations
that are the principal subject for investigation in the field
3. EXPERIMENTAL METHOD
The method most commonly used for environmental dosimetry is the small
copper capsules containing a sensitive TL phosphor (LiF-Mg-Cu-P). These
capsules are left buried in the sediment for a period of several months, and then
returned to the laboratory where the TL acquired by the phosphor during its burial
is measured.
An alternative method used for environmental dosimetry is the portable
gamma spectrometer. This usually consists of a crystal scintillation detector with
electronics for sorting gamma rays according to the quantity of energy they deposit
in the crystal. The detector is housed in a cylinder of 55 mm diameter, and is
inserted to a depth of 0.3 m or more into the sediment under study using an auger.
Counting times are generally between 15 mins and an hour, depending on the
radioactivity of the environment. The resulting spectrum of gamma energies is
analysed to determine the concentrations of uranium, thorium and potassium in the
sediment.
Upon arrival in the laboratory TL samples normally consist of two parts:
the sample to be dated and a modern analogue sample for the surface residual
correction. Both specimens are carefully sieved to separate the 90 - 125 micrometre
grain size fraction, chemically cleansed in dilute HCl, etched in 40% w/w HF and
finally subjected to heavy liquid separation. The sample so prepared consists of
better than 99% pure quartz grains.
The quartz from the specimen under investigation is divided into two parts
one of which is heavily bleached under a UV sunlamp. This exposure effectively
removes all of the previously acquired TL leaving only what is termed as the
"unbleachable TL". Aliquots of both the bleached and the unbleached quartz are
deposited onto a series of aluminium planchettes and a number of these are
incrementally irradiated using a calibrated 90Sr plaque source. Each planchette,
complete with its sample aliquot, is heated to 500oC at a controlled rate and in an
C. COSMA AND V. BENEA
oxygen free atmosphere. The light emitted (TL) is recorded and in this way it is
possible to a establish TL growth curve which relates TL output and the absorbed
radiation dose. With reference to this curve the measured naturally accumulated
sample TL may be converted to absorbed radiation units (Palaeodose P). The
surface residual TL correction is determined from the modern analogue sample by
means of a similar procedure and this correction is applied to the palaeodose value.
In the absence of a suitable modern sample the laboratory induced unbleachable TL
level is assumed which has the effect of maximising the resultant TL age
determined. In the case of an older sample this correction may only represent a
small proportion of the total age.
The radiation dose received annually by the sample is measured by means
of calibrated thick source alpha counting which determines the specific activity of
the uranium and thorium decay chains assuming that secular equilibrium exists.
This process requires that the sample be crushed to an extremely fine grain size
such that all of the short range alpha particles may be detected. The crushed sample
is placed in immediate contact with a scintillation screen which is sealed in an
alpha counting cell which in turn is positioned on a photomultiplier tube assembly.
Because certain of the daughters within the uranium and thorium decay chains are
gaseous it is necessary to wait a period of three weeks before introducing the cell
into the counter. This period allows the decay chains to be re-established. The
amount of potassium present in the sample is determined by means of atomic
emission spectroscopy and the rubidium content by X-ray fluorescence. Thus,
assuming the cosmic contribution and applying a correction for the modifying
effect of the sample moisture content, the radiation dose received upon an annual
basis (ARD) may be computed and the depositional age of the sample determined
from the equation shown.
4. RESULTS AND DISCUSSION
Table 1
Calibration of LiF: Mg, Cu, P detectors exposure to
TLD
Dose rate
(Gy/min)
Exposure
time
(min)
0.25
0,5
LiF:Mg,Cu,P
(TLD100H)
0,1557
1
2
4
60
Co gamma rays
Absorbed
Backgraund
Averge TL
dose (Gy )
signal (μC)
signal (μC)
28,5
0,0778
0,1557
0,3113
0,6226
57,5
0,245
117,4
235,7
459,8
TLD USING IN CERAMICS DATING
Figure 1 Thermoluminescence (TL) against exposure to
LiF:Mg,Cu,P (TLD-100)
60
Co gamma rays for
Table 2
Ceramic 1
Ceramic 2
Ceramic 3
Ceramic 4
Determination TL age
Paleodose
Annual dose
TL age
(rad)
(rad/yr)
(BP)
1190 (±4.7%) 0.512 (±6.9%) 2570 ± 190
1650 (±6.1%) 0.453 (±6.4%) 3640 ± 320
2570 (±7.2%) 0.613 (±5.1%) 4190 ± 370
3150 (±4.7%) 0.539 (±4.2%) 5840 ± 350
C-14 age
(BP)
2320 ± 80
4000 ± 65
5230 ±200
As we can see from Table 1 and Fig.1 an exact linear dependence of TLD signal on
absorbed dose was obtained. This result determines the constant of calibration of
370 µC/Gy which will be used for these chips for archeological background of
buried samples.
In table 2 a comparison between TL and C-14 method of age determination is
shown [2].
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