Naomi Porat

Geological Survey of Israel

Geoff Duller and Helen Roberts, Aberystwyth University

• Principles of luminescence dating

• Sampling and lab work

• Obtaining precise and accurate ages

• The results

• Usefulness

The “clock”:

� Uses a physical property that grows

over time as a result of natural

environmental ionizing radiation.

� This radiation creates a signal

(electrons trapped in the crystal

lattice) whose intensity is proportional

to time and radiation.

� In the lab we measure these electrons

that accumulated over time.

� The signals are rapidly reset by

sunlight or heat.

� The method dates the last exposure to

sunlight (or to high temperature).

Physical model

Energy level in the atom

trap photons

Energetically deeper traps will be more stable over geological time scale

Signal measurement

Luminescence TL/OSL Reader (Risø DA-12)

The luminescence signals:

Optically stimulated luminescence (OSL) signal Thermoluminescence (TL) signal

� OSL of quartz, excitation in the green-blue and emission in the near UV.

� Infrared stimulated luminescence (IRSL) of alkali feldspar, excitation in

the infrared and emission in the blue-violet

Bleaching experiments of quartz and feldspar grains

(150-180 µm from southern Israel

The quartz OSL signal is reduced by 90% within 2-3 seconds.

The feldspar IRSL signal is reduced by 90% within 50-80 seconds.

After 10 minutes there is still 5% signal remaining in the feldspar.

0 10 20 30 40

Bleaching time (s)

0

0.2

0.4

0.6

0.8

1

Luminescence (normalized)

18-6 KF

18-10 KF

18-6 QZ

18-10 QZ

0 100 200 300 400 500 600

0

0.2

0.4

0.6

0.8

1

OSL signal

Thermoluminescence signal

Quartz

Feldspar

Experiments in solar signal resetting (“bleaching”)

� Two values are required to obtain an age:

1. The equivalent dose, De.

2. The environmental dose rate, D.

� The age is the ratio between these values De/D

The age equation

The Equivalent dose (Archaeological dose; Paleodose)

Measured by reconstructing the signal by a known laboratory dose

extrapolation - Added dose interpolation - Regenerative dose

From: Duller,G. A. T. 2008, Luminescence Dating: guidelines on using

luminescence dating in archaeology. Swindon: English Heritage.

A 10 mm disc

covered with several

thousands of grains.

Dose-response curve

The single aliquot

regenerative (SAR) dose

protocol (Murray & Wintle

2000).

• De is measured for a large number of aliquots from the same sample.

• Aliquot size varies, may contain from thousands to a few tens of

grains.

• Also single grains can be measured in the same manner.

• The smaller the samples size the greater the scatter.

Errors on individual measurements

include:

• Counting statistics.

• Signal to noise ratio.

• Beta source calibration uncertainties.

• Goodness of fit to the dose response

curve.

• Errors on De values are a measure of

inhomogeneity. Can be reduced by

large number of measurements and

statistics.

Dose distribution

Palaeodose(Gy)3.6 4.0 4.4 4.8 5.2

Relative Probability

N=20

De=4.2±0.1 Gy

Environmental Dose rates:

� Can be measured in the field or the

lab.

� Need to know the sum of

radioactivity from the decay of U, Th

and K in the sediment, and from

cosmic radiation.

� There are α, β and γ particles, each

with different penetration range.

� Burial depth is important for

calculating cosmic dose contribution.

� Water content needs to be known for

calculating γ absorption.

Summary:

� A dosimetric method.

� Usually uses quartz or alkali feldspar in the fine-sand size range.

� OSL is used for dating sediments reset by sunlight, such as aeolian,

fluvial, alluvial and colluvial sediments.

� TL is used for heated material such as burned flint from prehistoric sites

or ceramics from archaeological sites.

� The age range is from several hundred years to several hundred-

thousand years.

� Errors range from 5-10 % of the age.

The complex history of a quartz grain in Megiddo

• There is no quartz in the chalk, limestone and basalt constructing the local

geology of Megiddo.

• The soils in the Jezerael valley soils contains wind-blown quartz; this could

blow onto the site during dust storms or brought by man.

• Quartz may also be blown from far away during dust storms.

• Dust arriving in storms travels long distances and the OSL signal of the

quartz is most likely well bleached.

• The dust settles on the site and then accumulates in open spaces.

• In the case of local source, the quartz might carry a residual OSL signal.

• Another sources of unbleached quartz could be recycled local sediments

(when leveling an area for construction).

• Yet another source is from bricks, which are made of local material,

perhaps brought from the wadi nearby. The quartz in bricks is not

bleached.

• Very likely the quartz at the site has had a complex depositional history.

• The samples collected for OSL dating may have more than one source of

quartz, with varied bleaching histories.

Where to sample?

� Archaeologically, destruction layers are best dated.

� These could have been heated during destruction and then exposed at

the surface before the next layer was deposited.

� Sediment between building stones. This was either placed during

construction or accumulated later from wind, and in this case is well

bleached. Will give a minimum age.

� Sediment from living floors and court yards.

� Sediment from water-lain features.

M 8/M615

ca. 1000 BCEM 414,16

ca. 850 BCEL 313

800-732 BCEL 212

ca. 3000 BCEJ 411

Destruction ca. 1130 BCEK 610

1300-1200 BCEK 89

1400-1300 BCEK 98

Radiocarbon ca. 1400 BCEF 10B7

700-600 BCEH 16

Destruction 732; life, say, 750 BCEH 35

ca. 850 BCEH 54

ca. 900 BCEH 73

ca. 1000 BCEH 92

ca. 1100 BCE H 111

Known datesSample

location

Lab

No.

(MGD)

List of samples

� Samples were drilled from the sections under cover, to prevent

exposure to sunlight.

� Sample preparation was carried out under subdued light.

� The sediment was sieved 74-125 µm. Carbonates were dissolved,

organic matter oxidized, the fines washed, and the quartz fraction

was extracted by magnetic separation. It was then etched with

concentrated HF to remove any feldspars and remove the α-affected

rim.

� Dose rate (d) was evaluated in the field and by chemical analyses.

The concentrations of U, Th and K (the radioactive elements) in the

sediments were measured by ICP-MS or ICP-AES. The gamma and

cosmic dose rates were measured in the field using aluminum oxide

dosimeters or calculated from the radioactive elements and burial

depth.

� The OSL signal was measured and De was determined using the

SAR protocol.

� Up to 42 aliquots were measured from each sample, to obtain dose

distribution.

Laboratory work

Preheat plateau:

Preheat is used to isolate the stable

signal. The De should not be

affected by the selected preheat

over a range of temperatures.

Quality assurance

Recycling Ratio:

One dose point is repeated at the end of

the measurements. It should fall within

5% of the first point. This shows that

the corrections to signal sensitivity

changes have worked.

280

Beta (s x 100)

Lx/T

x

MGD-3

De=163.3±4.5 s

Sample MGD-11, from EB wall, J4

N=18

De = 7.87±0.08 Gy

Age = 4530±180 years (before 2010)

(archaeological age: 3000 BC)

4% error on the age

OSL measurement results:

The OSL signal decays rapidly,

indicating the dominance of a

fast (easily bleached) component.

Sample MGD-12,

from stable floor in L2

N=41

De = 5.8±0.3 Gy

Age = 5100±310 years (before 2010)

Archaeological age: 8th C BC

Using statistics, 20/41

De = 4.76±0.08 Gy

Age = 4150±150 years (before 2010)

Second look – full of collapsed brick

material

Scatter in De results:

• The signal of the Megiddo samples is dominated by the

fast OSL component and is bright, reducing measurement

uncertainties.

• Quality assurance results are very satisfactory, indicating

that the SAR protocol is appropriate and that a thermally

stable signal is measured.

• The main uncertainty is from scatter between individual

De measurements (aliquots).

Solution:

• Measurements of small aliquots and the use of

component statistics allows to isolate the better-bleached

grain population.

• Single grain measurements could improve on this.

Uncertainties in dose rate estimates:

• Has dose rate been constant over time?

• Change in burial depth (affects the cosmic dose).

• Change in moisture contents (probably minor).

• Diagenesis (minor).

• Non-homogenous gamma field (30 cm radius) due to the juxtaposition of

limestone with low radioactivity and soil with high radioactivity.

• Only in-situ measurements can estimate the gamma dose so that these

inhomogeneities are taken into account.

• Aluminum oxides: new type of dosimeters which are light sensitive and can

detect very low doses (short time in the field).

• Dosimeters brought by Geoff Duller and Helen Roberts from

Aberystwyth University.

• Placed in copper tubes in each of the sampling points at a depth of

~30 cm.

• Left there for ~ 6 weeks.

• Removed and measured at the GSI.

• Entails very special calibration of the beta source.

gamma & cosmic

300

400

500

600

700

800

900

1000

300 400 500 600 700 800 900 1000

Al oxides

chemistry+depth 11

5

• A systematic offset for 10 of 12 aluminum oxide dosimeters,

where they give values which are ~ 90 microgray lower than the

chemistry values..

• Could be from source calibration, travel dosimenters or other

factors.

• The gamma+cosmic dose is ~ 50% of the total dose rate. A

change in 10% in this component would change the age by 5%.

Comparison between the dosimeters and dose rate

calculated from chemistry & depth

•Sample MGD-11 taken from soil between

stones. This sediment was also given for

chemistry.

•There are very large differences in dose

rates between stone and sediment: the

limestone is low, the sediment (with clays)

is high.

•Gamma dose rate calculated only from the

sediment would be too high, giving a too-

young age.

•In this case the Al oxide measurements

were used, giving an OSL age of 4530±180

years.

•No clear reason for sample MGD-5.

22304240±150M 8/M615

7702780±130ca. 1000M 414

20304040±130ca. 850L 313

20704080±150800-732L 212

25204530±180ca. 3000J 411

12303240±150Destruction ca. 1130 K 610

19103920±1201300-1200K 89

11903200±1601400-1300K 98

31105120±170Radiocarbon ca. 1400 F 10B7

5602570±150700-600H 16

16603670±140Destruction 732; life 750H 35

18603870±130ca. 850H 54

13903400±120ca. 900H 73

7402750±80ca. 1000H 92

10703080±100ca. 1100H 111

OSL ages

BCE

OSL ages

Before 2010

‘known dates’

BCE

Sample

location

Lab No.

(MGD)

OSL age results

Megiddo ages

2000

2500

3000

3500

4000

4500

5000

5500

2000 2500 3000 3500 4000 4500 5000 5500

Calendar (archaeological) age

OSL age

Only 8 of 15 samples fall

within ±10 % of the

archaeological age (green

lines).

The deviation is mostly

towards older OSL ages.

Could be the result of

incorporation of older

material, such as unfired

bricks.

Comparison between OSL and archaeological ages

123032401130K6

770, 7402780, 2750ca. 1000M4, H9

13903400ca. 900H7

2030, 18604040, 3870ca. 850L3, H5

1660, 20703670, 4080ca. 732H3, L2

OSL ages

BCE

OSL ages

Before 2010

‘known dates’

BCE

Levels:

Destruction layers:

MGD-1

H-11

3080±100

(1100) BC

MGD-3

H-7

3400±120

(900 BC)

MGD-2

H-9

2750±80

(1000 BC)

MGD-5

H-3

3670±140

(750 BC)

AREA H

MGD-4

H-5

3870±130

(850 BC)

AREA K

MGD-8

K-9

3200±160

(1300-1400 BC)

MGD-10

K-6

3240±150

(1130 BC)

MGD-9

K-8

3920±120

(1200-1300 BC)

• There is no particular depositional environment which gives a

better agreement between the OSL and archaeological ages.

• Some destruction layers are better behaved than others.

• Water lain sediments could be OK.

• A large component of the sediments in Megiddo was brought in

from outside, or it has been sitting around for a long time and

constantly reused.

• Need to beware of collapsed brick beds.

Summary

The way forward:

• Sample during excavation season when all sections are clean.

• Careful observations of sedimentary environments.

• Sample from courtyards, where dust collects directly or swept out

from the houses.

• Measure single grains, to isolate the best bleached grain

population.

• Perhaps be able to identify sources of recycled material.

• Measure dose rates by several methods.

• Pottery?