report osl

8/10/2019 Report OSL

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A study on OSL Dating of

Sediments and Its

Characteristics Using SAR

Protocol

I n s t i t u t e o f

S e i s m o l o g i c a l

R e s e a r c h

G a n d h i n a g a r

Summer Internship 2014

Submitted by:

Arkaprabha Sarkar

Kurukshetra University Kurukshetra


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Acknowledgement

I would like to acknowledge the Institute of

Seismological Research, Gandhinagar for the

permission and providing me with this opportunity of a

summer internship during June July 2014 at its

premises.

I am indebted to Dr. S. P. Prizomwala for accepting meas an intern under his supervision and providing me with

invaluable guidance throughout my internship.

I would also like to thank Dr. G. Kothari for

accompanying and encouraging me constantly with their

experience and knowledge, especially in learning thetechniques of Luminescence Dating.

My gratitude to the Laboratory Staff of Luminescence

dating for their cooperation and contribution to my work.


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Contents

1.

Preface

2.Introduction

3.Mechanism

4.Single Aliquot Regenerative Dose (SAR) Protocol

4.1. Recycling Test

4.2.

Preheat Test4.3. Dose Recovery Test

4.4. Replicate measurements of De

4.5. Aliquot size

5.Methodology

5.1. Geochemical Treatment

5.2.

Hydraulic Sieving

5.3. Magnetic separation

5.4. Etching with hydrofluoric acid and concentrated

hydrochloric acid

6.Measurement of dose rate

6.1.

Chemical methods6.2. Emission counting

7.References


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Preface

The Quaternary Period was, and is, arguably one of the most important intervals in Earths

history. It has been not only a time of extreme climatic fluctuation, but also an interval during

which modern humans first appeared, the first civilizations formed and farming began, and

humans began to have a profound effect on the natural environment. This significant period has,

however, been difficult to put into an absolute temporal context despite the development of

several dating techniques. This is because most techniques require the presence of a specific,

often uncommon, material that has to occur in the relevant context. Moreover, many dating

methods are only useful over short time scales, the calculated age does not directly date the

desired event, and/or complex age calibration may be necessary which can significantly increase

the uncertainty in the final result.

The most commonly used geochronological technique has been radiocarbon dating which was

also one of the first quaternary dating methods to be invented and studied in detail. Although the

method has contributed greatly to our understanding of the Earths history, its use nevertheless is

limited. In most circumstances, it yields the dates between a few hundred years to 60,000 years;

it usually requires the presence of fossil vegetation and this is often detrital; and if radiocarbon

dates are to be compared to the calendar years, detail knowledge about the past variations in the

concentration of radiocarbon in atmosphere is required.

Other geochronological techniques such as potassium-argon, argon-argon, uranium series andfission track dating requires the presence of in situ volcanic and carbonate sediments.

Dendrochronology, amino acid racemisation and paleomagnetic techniques will give relative

ages unless the age patterns are individually correlated with previously dated sequences.

Clearly, the most useful dating method would be one that directly dates the material of interest,

can be applied to the most ubiquitous materials, has a long time range and can give calendar

dates without complex calibration. The only method that comes close to satisfying all of these

characters is luminescence dating.


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Introduction

Luminescence dating is a chronological method that has been used extensively in archaeologic,

anthropologic contexts and many areas of earth and environmental sciences (Aitken 1998). It is

based on the emission of light, luminescence by commonly occurring minerals, principally

quartz. The method can be applied to a wide range of materials that contain quartz or similar.

The age range over which the method can be applied is from a century or less to over a hundred

thousand years. For pottery and burnt flints and ceramics, the event being dated by this method is

the last heating of the material. While dating sediments, the event being dated is the last exposure

to sunlight.

Radioactivity is ubiquitous in the natural environment. Luminescence chronological technique

exploits the presence of radioactive elements such as Uranium isotopes (235

U and238

U), Thorium

Figure-1

Cartoon illustration of the main environments in which optically stimulated luminescence (OSL) can be applied on timescale of

1 year to ~ 100,000 years, depending on site and sample characteristics. In particular, the method has contributed significantly

to studies in Quaternary glaciation, dune building, fluvial activity and loess records. The applicability of OSL depends on both

environmental factors and sample characteristics. Some applications are well established (e.g. dating dunes or loess) whereas

others are under development (e.g. thermochronology, wildfire reconstruction).


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(Th) and Potassium (K). Naturally occurring minerals such as quartz and feldspars act as

dosimeters and record the amount of radiation to which they have been exposed.

Figure-2

Simple band gap energy model of optically stimulated luminescence (OSL). Light-sensitive (OSL) electron traps are shown in red,

light insensitive (thermoluminescence) traps in blue. (a) Under typical initial conditions, low thermal stability traps close to the

band are kept empty by thermal eviction at ambient temperature but other traps are filled; luminescence centers are available.(b) On light exposure, electrons in OSL traps are evicted and may become trapped in other available trapping sites or may

recombine at hole centers (luminescence centers). After a brief light exposure, all OSL traps are emptied. (c) During subsequent

burial, ionization radiation gradually produces electron-hole pairs, some of which may become trapped, increasing the OSL trap

population. (d) Following collection and mineral separation, intense blue green stimulating light is directed at the sample.

Electrons are evicted from OSL traps and recombine at luminescence centers, emitting UV luminescence centers, emitting UV

luminescence, detected through glass filters with a photomultiplier tube. As the remaining OSL trap populations falls the

emission rapidly decays to a low level


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A common property of some naturally occurring minerals is that when they are exposed to

emissions due to radioactive decay, they are able to store within their crystals a small proportion

of the energy delivered by the radiation. At some later date, this energy is released and in some

minerals, this release of energy is in the form of light. This release of light is termed as

luminescence.

The method is similar to radioactive damage techniques (e.g. fission track dating). It broadly

consists of four major steps:

a) Removal of trapped charge within the mineral grains by exposure to natural daylight

during transfer

b) Gradual build up of charge as buried grains are exposed to environmental radiations

c) Sample collection and OSL measurement of the trapped charge concentration

d) Determination of the environmental dose rate experienced by grains at each sample

location.

The technique has a huge potential for understanding earth and planetary surface processes. OSL

has extraordinary sensitivity to environmental conditions, namely signal erasure in seconds by

daylight or by heating in the range of 200C to 400C. Furthermore, the minerals most

commonly used in ISL, quartz and feldspar, are virtually ubiquitous in the terrestrial surface

settings. Providing an age range from less than a century to more than a hundred thousand years

and with indication that the upper limit will soon be extended by an order of magnitude, the

method helps fill gap between observational process studies and events dated by radiometric

methods that depend on crystallization. The technique complements other Quaternary datingmethods, including radiocarbon (

14C), uranium series method and cosmogenic nuclide techniques

and benefits from an extremely wide range of applicable contexts.

Luminescence dating includes the technique of thermoluminescence (TL) (Aitken 1985), in

which mineral grains are heated during measurement, and optical dating (Huntley et al. 1985,

Aitken 1998), in which samples are exposed to an intense light source to evict charge from only

light sensitive traps.

Optical dating uses the OSL signals of quartz (Smith et al. 1986) or feldspar (Godfrey-Smith et

al. 1988) or the infrared stimulated luminescence (IRSL) (Hutt et al. 1988) signals of feldspar.

With different stimulation wavelengths, a choice of minerals and grain sizes and several

measurement procedures, this related group of techniques can be confusing. However, on

measurement protocol stands out, namely the quartz OSL single aliquot regenerative dose (SAR)

protocol of Murray and Wintle (2000, 2003), owing to its apparent accuracy and reliability

(Murray and Olley 2002, Rhodes et al. 2003).


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Measurements of the brightness of the luminescence signal can be used to calculate the total

amount of radiation to which the sample was exposed during the period of burial. If this is

divided by the amount of radiation that the sample receives from its surroundings each year then

this will give the duration of time that the sample has been receiving energy.

Age = (Total energy accumulated during burial / energy delivered each year from radioactive

decay)

The SI unit of absorbed radiation is the Gray (Gy). It is a measure of the amount of energy

absorbed by one kilogram of the sample and has the units joules per kilogram (Jkg-1

)


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Mechanism

Many naturally occurring minerals will yield luminescence signals, including quartz, feldspars,

calcite and zircon. Quartz is the most suitable material for dating and the single aliquot

regenerative dose (SAR) protocol is now used routinely.

Natural environmental causes charge (electrons and holes) to become trapped when grains are

buried in the ground, as bonding electrons are excited from their valence positions, and a small

fraction become trapped within the crystal lattice. Electron and hole traps are formed at point

defects such as those formed by elemental substitution, for example, where Ti replaces Si in

Quartz. Charge may be compensated by the presence of alkali ions for example species such as

an electron trap [TiO4/Li+], or the hole center [AlO4], known from electron paramagneticresonance studies (Weil 1984). Although several models have been proposed (Itoh et al. 2002),

the detailed structure of the OSL electron traps in quartz continues to be explored; the AL hole

center mentioned above is likely responsible for the quartz OSL emission in the UV region

around 360-380nm (Martini et al. 2009).

Brief heating at 200C-400C or a short daylight exposure (in the range of 1 to 100s) is sufficient

to reduce certain electron trap populations to a low level, effectively resetting the OSL dating

clock. The subsequent gradual increase in trapped charge population in response to the prevailing

radiation flux provides the basis for dating. Trapped charge concentration is assessed by

laboratory OSL measurements whereas natural radiation flux or dose rate may be determined

with a choice of methods.

Luminescence results when some external stimulation ejects electrons from traps. The evicted

electron diffuse around the crystal lattice around the crystal lattice; some of these eventually

become lodged at a similar type of trap or return directly to the ground state, while others find

themselves at a different kind of site, referred to as a recombination center.

When recombination occurs, excess energy is given off either as heat (lattice vibrations or

phonons) or as luminescence (photons); the latter process is referred to as radiativerecombination. This process of electron eviction and recombination is nearly instantaneous. In

any given crystal, there are many kinds of electron traps. Some of these traps can be emptied

very easily when the grains are stimulated by light, but, for others, exposure to light has little or

no effect.


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Most traps, however, can be emptied by exposure to sufficient heat (500C). Moreover, at

ambient temperature (20C), some traps are only able to hold electrons for a few days or less,

while others can hold electrons for a millions of years or more. The latter types of trap are

referred to as deep traps, and these have thermal lifetimes that are sufficiently long for

Quaternary dating.


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Single Aliquot Regenerative Dose (SAR)

Protocol

It comprises a series of cycles. In the first cycle the OSL signal (denoted L) from the aliquot

arises from the radiation dose to which the sample was exposed in nature and hence is given the

term LN. In the second cycle the aliquot is exposed to an artificial source of radioactivity in the

laboratory. The OSL signal is then measured this is the first laboratory generated signal L1.

Subsequent cycles measure L2, L3etc as different regeneration doses (e.g. 10Gy, 20Gy etc) are

given to the aliquot. All of these measurement of luminescence are preceded by a preheat

heating the sample to a fixed temperature (normally between 160C and 300C)and holding it

there for a short period of time (e.g. 10s). This procedure removes unstable electrons that would

have been stored safely through the burial.

The brightness of these

luminescence signals (L1, L2, L3 etc)

can be used to construct a dose

response curve. However, the

luminescence sensitivity of the

aliquotthe amount of light it emits

for each unit of radiation to which itis exposed changes depending on

the laboratory procedures

undertaken (e.g. the temperature and

duration of the preheat treatment)

and conditions during burial (e.g.

ambient temperature). The second

half of each cycle in figure 4

addresses this problem by

measuring the luminescencesensitivity. It is this second half of

each cycle that was the major

breakthrough in SAR; if such

changes are not compensated for

then the calculation of De will be

incorrect.

Figure-3

SAR dose response curve for a quartz aliquot from Kalambo Falls, Zambia,

measuring eight cycles: first cyclethe natural signal (LN) and its response to

a test dose (TN) enabling calculation of the sensitive-corrected OSL (LN/TN);next regeneration doses of 16, 32, 64, 96, 128 and 0Gy give sensitive

corrected values L1/T1, L2/T2, L3/T3, L4/T4, L5/T5and L6/T6 (these data define

the dose response curve for this aliquot). By interpolating the value of LN/TN

onto the dose response curve, Decan be calculated; in this case 25.61.3Gy).

The final cycle repeats the measurement for a regeneration dose of 32Gy

(L7/T7). The ratio of the two measurements for 32Gy (L7/T7divided by L2/T2) is

known as the recycling ratio and for this aliquot has a value of 0.990.05

close to the ideal value of 1.00 and well within the accepted range of 0.9 to

1.1


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The luminescence sensitivity of the quartz is measured by giving a fixed radiation dose

(commonly known as a test dose) in the second half of each cycle then by measuring the

resulting OSL signal (TN, T1, T2etc). The effect of any change in sensitivity can be corrected by

plotting a graph, not of the luminescence (LX) as a function of regenerative dose, but of the

sensitivity corrected luminescence signal (LX/TX), De can then be calculated, based upon this

luminescence signal, corrected for any changes in sensitivity that may have occurred.

The SAR protocol has been widely applied to both heated quartz and unheated quartz (Wintle

and Murray 2006) and has been shown to give ages consistent with independent age control

(Murray and Olley 2002). Figure 13illustrates the idea behind SAR procedure and shows three

cycles of measurement. In practice, more cycles are used, typically between five and ten in total

(Figure 3). These additional cycles are used to define the dose response curve using a variety of

regeneration doses and to enable a number of checks to be made on the behavior of each aliquot

(Wintle and Murray 2006). It is crucial that these checks are made, as it is known that althoughSAR does work on most quartz samples, it does not work on all. In particular, some dim samples

may not be appropriate for analysis using SAR because they lack the part of the OSL signal that

gives the rapid decrease in signal observed during the initial part of the OSL measurement.


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Figure-4

The single aliquot regenerative dose (SAR) procedure applied to quartz: the growth of the signal with dose is characterized by

administering a number of laboratory doses (regeneration doses) of different sizes (10Gy, 30Gy etc) and measuring the resulting

OSL signals L1, L2etc. After each measurement the luminescence sensitivity is measured by giving a fixed dose (here 5Gy) and

measuring the resulting OSL signal (T1, T2 etc). The effect of changes in sensitivity can be corrected for by taking the ratio of the

luminescence signal (Lx) to the response to the fixed dose (T x). The plot of the sensitivity-corrected OSL (Lx/Tx) as a function of

the laboratory dose (bottom) can be used to calculate De(here 22Gy) for that aliquot when the ratio of the initial measurements

on the natural sample (LN/TN) is projected onto the dose response curve..


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Recycling test

Once the dose response curve is defined as described above, it is routine to repeat the

measurement of the luminescence signal resulting from one of the radiation doses. If the

procedure is working appropriately and the SAR corrects for changes in the sensitivity of the

sample, then the luminescence signals (L/T) of the repeated dose (e.g. 32Gy in figure 3) should

be same. The ratio between two sensitivity-corrected luminescence signals is the recycling ratio

ideally 1; in practice, a ratio between 0.9 and 1.1 is deemed acceptable. Values greater than or

less than these limits suggest that the procedure or the sample is inappropriate, and therefore that

results from that aliquot should be rejected.

Preheat test

Before each luminescence

measurement, the aliquot is

heated between 160C and 300C

for 10s to remove unstable

electrons the appropriate preheat

temperature is determined by

making measurements of De

using a range of temperatures.

Figure 5shows the result for one

sample measuring De24 aliquots:three aliquots with a pre heat of

160C, three at 180C, three at

200C etc to a maximum of

300C. This procedure shows that

the same Devalue is obtained for

temperatures from 160C to

260C. At temperatures > 260C

Deincreases, probably because of thermal transfer.

A strength of the SAR procedure is that it works with a range of temperatures. Nevertheless, it is

important to test one or two samples from within a suite of samples to determine if a suitable

preheat is achievable.

Figure-5

Preheat plot, showing De (and associated measurement errors) for beach sand

buried beneath gravel at Dungeness Foreland, measured at preheat temperatures

from 160C to 300C for 24 aliquots (Roberts and Plater 2005; 2007). A plateau in

De values extends from 160C to 260C (blue symbols) and the average value

(dashed red line) was used to calculate the age of the sample(3850180a;

Aber73/BH-3/2). At higher preheat temperatures De increases (grey symbols),

probably due to thermal transfer, so these data are excluded.


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Dose Recovery Test

One of the most powerful tests available commonly undertaken with SAR is the doserecovery test. This involves removing the trapped electron population from the sample, thus

mimicking the resetting of the sample at the time of the event being dated.

For sediments this involves exposing an aliquot of naturally irradiated grains to daylight or to an

artificial light source, irradiating the aliquot with a known dose (e.g. 10Gy), and then treating

this dose as an unknown. The SAR procedure is applied. If it is appropriate then the value of the

calculated dose should match the known laboratory dose. It is common to choose a known

laboratory dose close to the value of De measured for the sample. In this way, complications

introduced as samples become close to saturation at high values are assessed implicitly. If a

sample fails a dose recovery test then it is unlikely that the D ecalculated for the sample will be

correct.

Replicate measurements of De

Single aliquot measurements reduce the

time involved in determining De by

automating much of the procedure and

they make it feasible to make replicate

measurements of De for each sample.

Thus, it is routinely possible to assess

the reproducibility of De within each

sample. Replicate determinations of De

within each sample. Replicate

determinations of Dewould be expected

to form a normal or log-normal

distribution where three conditions hold

true:

a) The sample contains grains that

were completely bleached at deposition

b) Variations in the annual dose

due to small scale differences in the

concentration of uranium, thorium and

potassium are small;

Figure-6

Replicate De measurements for two contrasting samples: (a) 31

measurements of a beach sand sample (Aber73/BH1-1) from

Dungeness, all giving similar values. Since the samples belong to a single

population, the mean Decan be used for age calculation (3.700.06Gy);

(b) 44 measurements for a sample of fluvial sand from Crete

(Aber60/AN2002/4) showing large De variability. Using the average value

(ranked in increasing order) is a point with an error bar of analytical

uncertainty.


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c) No post depositional mixing has occurred

In this situation, averaging the replicate Demeasurements gives an increase in the precision of the

age calculated (figure 6a)

If replicate measurements of De are not similar (as in figure 6b) then this implies that the sample

is more complex and requires further

investigation. The most common cause of

such large scatter is that the trapped

electron population in the different

mineral grains of the sample was not

completely reset during the event being

dated this is known as incomplete

bleaching. The De values obtained for

quartz from a modern sand dune form a

tight cluster close to 0Gy (figure 7a).However, when rivers transport

sediments not all the mineral grains are

exposed to sufficient daylight before

deposition to reset their signal (figure

17b). Thus, the Dethat is measured in the

laboratory will be the sum of the dose

acquired since the sample was deposited,

plus the residual signal remaining at the

time of deposition. In such situations, the

best estimate of the dose that has beenacquired since the event of interest will

be given by those aliquots with the

lowest Devalues. Statistical models have

been developed (Galbraith et al. 1999;

Galbraith 2005), to deal with such

situations and can be applied where the

cause of the distribution in Devalues can

be understood. While much can be

learned from analysis of the distributionof De values alone, it is always

important to consider the depositional

context from which a sample has been

collected when assessing the most likely

cause of scatter in the distribution of De

values.

Figure-7

Histograms of Devalues for two samples, demonstrating the problemof dating fluvial samples. Aliquots containing between 60 and 100

quartz grains (125-180m diam) were analysed. Both samples should

give De values close to zero, as they were collected from sites of

current deposition. (a) Sand dune De values close to zero indicate

that it was exposed to sufficient daylight at deposition to reduce the

luminescence signal to near zeo. In contrast, (b) drom a fluvial

environment produced many Devalues significantly greater than zero

implying that only some of the grains in this sample were exposed to

sufficient daylight (modified from Olley et al 1998)


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The second potential cause of Devariation is dose rate differences in different parts of the sample

different parts of the sample have received different

radiation doses during burial. Fortunately, such variation in

microdosimetry can normally be avoided by careful

sample selection. Situation where this problem is likely to

be most severe are those where sediment contains a small

proportion of highly radioactive grains for instance

zircons or where a sample is very heterogeneous, e.g.

owing to the presence of gypsum with low radioactivity or

wood ash with relatively high radioactivity from a high

potassium content.

Another situation that might produce a wide range of De

values is a sediment sample whose grains are of different,mixed ages. Mixing might have occurred in antiquity

(David et al. 2007), possibly by human activity, or it may

have occurred during sampling. Where samples of

different ages are closely juxtaposed, for example, more

than a single context might have been sampled in error

(Jacobs et al 2008).

Aliquot size

In situations where different mineral grains have different

Devalues, the result of single aliquot analyses will depend

on how many grains are contained in each aliquot. In

typical luminescence measurements, sand sized mineral

grains c0.2mm diameters are measured. For such a grain

size, it is possible to make single aliquots containing about

a thousand grains, hundreds of grains, tens of grains or

even a single grain.

Figure 19a shows the result of making replicate De

measurements on a sample. De values obtained are

relatively similar. These measurements were made using

aliquots of c 1,000 grains. Measurements of the same

sample made using aliquots with c200 grains show more

Figure-8

Potential effect of changing the number of grains in analiquotthree histograms of Deresults for one sample

using different numbers of grains in each aliquot (a) c

1000 grains (b) c200 grains and (c) single grains. Many

grains in aliquot (a) averaged out differences, masking

the real variation; as the number of grains is decreased

(b and c) the variability in the Debecomes obvious. This

sample was not completely bleached at deposition, so

an age based on the large aliquots shown in (a) would

have been an overestimate.


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variation in De (Figure 8b) and measurement of single grains increases the variation further

(Figure 8c). This sample consists of grains with different Devalues, as is clearly shown in the

single grain results. The measurements on aliquots with hundreds or thousands of grains are

masking these variations by averaging the results from many grains. Thus, if a sample is thought

to be affected by incomplete bleaching or mixing, then making replicate measurements on

aliquots with relatively few grains, or even with single grains should be used to test this

possibility. The spread in De shown in Figure 8a as obtained in routine measurements using

aliquots of c1,000 grains would not necessarily cause suspicion of incomplete bleaching.


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Methodology

Sampling for luminescence dating method has a different procedure compared to othertechniques. Luminescence dating samples are highly sensitive to light and hence, are never

exposed to any source of light during the entire process.

Samples of sediments are collected from field and stored in opaque cylindrical metal tubes. The

tubes are sealed from both ends to avoid exposure to light. The sample inside is tightly packed so

as to prevent mixing of samples. The tubes are opened in laboratory under special dim red or

orange lighting condition. Parts of the sample from both the ends are removed as they have been

exposed to daylight during collection from field. The part of the sample from the central portion

of the tube is extracted and kept separately.

The end parts of the sample are used for the determination of dose rate. The central part of the

sample is used for the determination of equivalent dose.

Before the sample is taken to the TL/OSL reader to determine the equivalent dose, it is subjected

to a series of purification methods to extract quartz.

Geochemical treatment

Treatment with hydrochloric acid

The sample is taken in a beaker and 1N HCl is added to it to remove carbonate matter. As

effervescence starts, it demarcates the reaction of calcium carbonate with hydrochloric acid.

CaCO3+ HCl CaCl2+ H2O + CO2

The beaker is kept in ultrasonic bath for 10 minutes and left undisturbed until the effervescence

stops and the supernatant liquid becomes clear. The supernatant liquid is then decanted and 1N

HCl is added. The process is repeated until there is no more effervescence.


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Treatment with hydrogen peroxide

The carbonate free sample is now taken in a beaker and hydrogen peroxide is added to it to

remove organic matter. Hydrogen peroxide oxidizes organic sediments and releases carbon

dioxide. The reaction is demarcated by effervescence.

The beaker is kept in ultrasonic bath for 10 minutes and left undisturbed until the effervescence

slows down and the supernatant liquid becomes clear. The supernatant liquid is then decanted

and hydrogen peroxide is added. The process is repeated until there is no effervescence.

Hydraulic sieving

The left over sample is sieved by hydraulic sieving process. Sediments of particle size less than

150m and greater than 90m are needed for further analysis.

A sieve of size 150m is fitted on top of a sieve of size 90m. Distilled water is added to the

beaker containing the sample. The sample is stirred and slurry is prepared. This slurry is slowly

flowed into the top beaker under a jet of water. The system of sieve is shook sideways for proper

sieving action. The larger particles are time to time removed so as to prevent choking and

damaging of the sieves.

Once this is complete, the system of sieves is dismantled. The amount of sediment collected inthe lower sieve is collected in a petridish by spraying the sieve with distilled water. Excess water

from the petridish is decanted and the sample is kept in the oven for 24 hours to dry.

Magnetic separation

The dry sample is now taken to a magnetic separator machine to remove the magneticallysusceptible minerals. This treatment is done in two phases.

In the first phase, a potential difference of 0.5 volts is used to generate a weak magnetic field

using an electromagnet. The weak magnetic field magnetizes the minerals with high magnetic

susceptibility. Such minerals under the action of magnetic force are separated from the rest. The

magnetic minerals thus separated are removed disposed and the non magnetic part is preserved

for the next phase.


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In the second phase, the non magnetic part of first phase is loaded in the machine for separation.

A potential difference of 1.5 volts is used to generate a stronger magnetic field using the

electromagnet. This strong magnetic field magnetizes minerals with very low magnetic

susceptibility. Such minerals under the action of the strong field are separated from the non

magnetic part. A large fraction of the feldspar and mica content is removed in this phase. Hence,

the non magnetic part consists mainly of quartz with traces of feldspars. The feldspars are

removed during etching in the next step.

Etching with hydrofluoric acid and concentrated hydrochloric acid

The sample after magnetic separation is mainly quarts with feldspars and mica in traces. These

trace minerals are removed using hydrofluoric acid. Also, the hydrofluoric acid etches the quartzgrains. The acid dissolves the outermost layer of the quarts grains.

SiO2+ 6HF H2SiF6+ 2H2O

(water soluble)

Two objectives are achieved by this processthe deeper traps are exposed which have electrons

from the event to be dated and separates two fused grain planes increasing the roundness of the

quartz grains. Any other substance that is present dissolves in the acid. The sample is etched for

80 minutes. After etching, distilled water is added and the acid is washed out.

Once hydrofluoric acid is washed out, concentrated hydrochloric acid is added to complete theprocess. Hydrochloric acid dissolves all by products formed during etching with hydrofluoric

acid. After the process, hydrochloric acid is washed out with distilled water and the sample is

kept in the oven for drying.


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Measurement of dose rate

There are four types of environmental radiation: alpha particles (), beta particles (), gammarays () and cosmic rays. The first three originate from naturally occurring elements in the

sample itself and its surroundings. The most important of these sources are radioactive isotopes

of uranium (U), thorium (Th) and potassium (K). The radioactive isotope of potassium is40

K,

which decays to40

Ca or40

Ar by the emission of beta particles and gamma rays. Both40

Ca and40

Ar are stable.

Uranium and thorium

are more complex238

U

decays to form234Th

,

which itself isradioactive, and will

decay to form

protactinium (234

Pa) and

so on until a stable

isotope of lead (206

Pb) is

reached (figure 9). Such

chain of isotopes is

called a decay series.

Similar decay series

exist for235

U and for232

Th. The U and Th

chains emit alpha, beta

and gamma radiation

and contribute to the

total radiation absorbed

by samples (figure 10)

Cosmic rays originate

from sources far out in

the universe. They are a

type of electromagnetic

radiation similar to

gamma rays derived

from U, Th and K but typically with much higher energy. The Earth is shielded from most

cosmic rays by the atmosphere and by the Earths magnetic field. However, a proportion does

Figure-9

The decay chain of 238U: arrows show the sequence of isotopes produced by emission ofbeta a beta particle (horizontal arrows) or an apha particle (diagonal) with half lives varying

from hundreds of thousands of years to less than a second (Z = atomic number, the numberof protons in the nucleus and hence chemical properties; A = atomic mass (sum of protonsand neutrons), which differentiates isotopes).


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reach the Earths surface and penetrate sediments. The radiation dose received decreases with

depth, particularly rapidly in the uppermost metre.

Two approaches can be used to measure the annual radiation dose provided to a sample from the

radioactive elements

surrounding it; measuring theconcentration of U, Th and K

and using these

concentrations to calculate

the radiation dose received

by the sample; or directly

counting the emission of

radiation.

Chemical methods

A range of geochemical

methods exist to measure the

concentration of the key

elements responsible for

delivering the radiation dose

U, Th and K. For sediments

and bricks the dose rates from the surrounding can be measured from a finely ground sub-sample

of the sediment itself. For pottery and burnt flints, both the sample and the sediment surrounding

the sample need to be analyzed.

Of the three elements measured, K occurs in the greatest concentration, typically 0.5% to 3% by

weight. It can be measured using a range of methods, including atomic absorption spectrometry

(AAS), flame photometry, X-ray fluorescence (XRF) and inductively coupled plasma mass

spectrometry (ICP-MS). In all cases, a precision of c5% or better would be expected.

U and Th occur in much lower concentrations, typically in the range of 1ppm to 10ppm. The

range of measuring methods is more limited, but includes ICP-MS and neutron activationanalysis (NAA).

Three major issues may arise with these measurements:

a) Concentrations of these elements may be close to the detection limits for some analytical

facilities; minimum detectable limits are often quoted and one would want these to be

several times smaller than the concentrations being measured

Figure 10

Environmental sources of radiation, showing ranges for a alpha (), beta () and

gamma () radiation and the decrease in cosmic rays with increasing depth below

ground.


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P a g e | 24

b) Sample preparation ICP-MS and many other methods may rely on dissolving the

sample in strong acids to produce a solution that can be analyzed; in some samples, U

and Th may be associated with highly resistant minerals if these are not completely

dissolved during the analysis then the measured values will be underestimated.

c) These measurements are made on sub samples of only a few mg, and such a small sample

may not be representative of the sediment or brick being used for dating.

Once the concentrations of these three elements are known, conversion factors enable the

calculations of the radiation dose rate (Adamiec and Aitken 1998). For example, 1% potassium

in sediment will produce a gamma radiation dose rate, as the decay of40

K does not result in the

emission of alpha particles. Adding together the alpha beta and gamma dose rates gives the total

radiation dose rate.

This approach makes a number of assumptions. For U and Th decay chains the concentration ofmeasured is that of the parent isotopes. Radiation is emitted at all stages of the chain but ht e

chemical methods described above will only measure the concentration of U they do not

normally differentiate between the different uranium decay series (238

U,235

U) nor do they

normally measure the daughter products. Conventionally, calculation of the dose rate from 1ppm

of U assumes that the ratio of the concentrations of238

U to235

U is about 400 and that all of the

decay products are in secular equilibrium with their parent isotopes.

Secular equilibrium is the term used to describe a decay series when the activities of each of the

daughter products are the same. For luminescence dating this is commonly assumed to be the

case, but in certain geochemical situations, this assumption may not hold (Olley et al 1996). Forexample where water has percolated through sediment, this can selectively remove uranium (e.g.238

U and234

U) while leaving more immobile daughter isotopes such as thorium (e.g.230Th

)

behind. In seasonally wetted environmentssuch as marshes where reducing conditions occur,

radium (Ra) mobility is of concern. Where parts of a decay chain are either lost or gained in

these processes, the decay chain is described as being in disequilibrium, meaning that the dose

rate will change throughout time, making calculation of the luminescence age complex. After

some period of time the rate at which the various isotopes are formed and the decay will come

into balance, and at that stage the sample is said to be in equilibrium. In practice, difficulties

associated with disequilibrium can be avoided by selecting samples from geochemical

environments in which uranium, which is highly soluble and thorium, which is insoluble are notmoving differentially. In particular, sediments that are within 0.3m (the range of gamma rays) of

peat or other highly organic sediments should be avoided. Iron stained sediments, especially

where there is evidence of ground water and samples with evidence for significant movement of

groundwater and samples with evidence for precipitation of calcite should also be avoided.

However, when working in limestone caves such problems may be ubiquitous. Where


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disequilibrium is suspected, high resolution gamma spectrometry (an emission counting

technique) may be appropriate.

Emission counting

Instead of measuring the concentrations of U, Th and K and using these to calculate the dose

rate, a more direct method is to count the alpha, beta and gamma radiation.

Alpha particles can be counted using either a thick source alpha counter (TSAC) or alpha

spectroscopy. Where disequilibrium is suspected in a sample, alpha and gamma spectroscopy are

the best ways of studying the U and Th decay series.

TSAC is able to measure total number of alpha particles and by exploiting the rapid decay of220

Rn (Radon) in the thorium decay series, the relative proportion of the particles that come fromthe U and Th decay series can be calculated separately.

Alpha spectroscopy is more complex analytical, requiring the samples to be dissolved but by

using this method the energy of each alpha particle can be measured which is characteristic of

the isotope undergoing decay. In the same way, the energy of gamma radiation emitted from a

sample is characteristic of the isotope undergoing decay. High purity germanium (HpGe)

detectors, kept at liquid nitrogen temperatures (-196C)are able to measure this energy and

produce a spectrum for each sample (figure 11)

Beta particles, unlike alphaparticles and gamma rays, are

emitted at a range of energies

and thus there is no point in

spectroscopy. However, simply

counting the beta particles is

perfectly feasible, for example

by using a Geiger-Muller based

system, and this directly

assesses the beta dose rate.

One radioactive element, radon

(Rn) is a gas and thus there is

the potential for it to diffuse out

of the sediments. Rn isotopes

occur in the232

Th,235

U and238

U decay chains, but they have different half lives ranging from

3.96s (219

Rn) to 3.83 days (222

Rn). The longer lived isotope222

Rn is part of the238

U decay series

Figure-11

Part of a spectrum obtained from gamma spectroscopy: gamma rays emitted by

specific isotopes have fixed energies, some of the more easily identifiable isotopes

are shown (modified by Aitken 1985)


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P a g e | 26

(figure 9) and because of its longevity there may be time for the gas to move through the pores

between sediment grains and escape into the atmosphere. If this occurs, the daughter isotopes

will be produced away from the sample, thus reducing the radiation dose rate and causing

disequilibrium in the decay chain. This situation will not cause difficulties if emission counting

methods are used, but could be an issue if only parent concentrations (eg U and Th) are measured

using chemical methods. These methods enable alpha, beta and gamma radiation flux to be

measured and alpha and gamma spectroscopy enable assessment of the extent of equilibrium in

the decay chains.

Owing to the distances the travel, laboratory measurements of the alpha and beta flux on a small

(c10g) sub samples are likely to accurately measure the natural flux of the sample. However,

gamma radiation travels up to 0.3m in sediments. Laboratory measurements, based on emission

counting or chemical methods, will only accurately determine the gamma dose id the material

around the sample is homogeneous over a scale of c0.3m in all directions. In many situations,

this will no t be the case and then in situ measurements need to be made.


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