Linearly modulated optically stimulated ... 0 Linearly modulated optically stimulated luminescence of

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    Linearly modulated optically stimulated Linearly modulated optically stimulated Linearly modulated optically stimulated Linearly modulated optically stimulated

    luminescence of sedimentary quartz: luminescence of sedimentary quartz: luminescence of sedimentary quartz: luminescence of sedimentary quartz:

    physical mechanisms and implications for physical mechanisms and implications for physical mechanisms and implications for physical mechanisms and implications for

    datingdatingdatingdating

    Joy Sargita Singarayer

    Linacre College

    Thesis submitted for the degree

    of

    Doctor of Philosophy at the

    University of Oxford

    Trinity Term 2002

  • 1

    Abstract

    The optically stimulated luminescence (OSL) signal from sedimentary quartz has previously

    been found to be the sum of several physically distinct signal components. In this thesis the

    technique of linearly modulated OSL (LM OSL), in which the stimulation intensity is

    linearly increased during measurement, was employed to further investigate the OSL signal

    components. The method of LM OSL and subsequent fitting procedures used to separate the

    contributions of the components were rigorously tested using specifically developed

    numerical and analytical models.

    In a survey of a number of sedimentary samples five common OSL components were

    observed; the ‘fast’ and ‘medium’ components as identified in earlier studies and three slow

    components ‘S1’, ‘S2’ and ‘S3’. The fast, medium, S1 and S2 components displayed first-

    order characteristics while S3 did not (e.g. dose dependent bleaching rate).

    The behaviour of the components, relevant to optical dating, was empirically examined and

    observed to be markedly different. The fast, medium and S1 components were demonstrated

    to be thermally stable, having lifetimes, τ > 107 years. Component S2 was found to be

    thermally unstable and associated with the TL region at ~280°C. The calculated lifetime of

    S2 at ambient temperatures was calculated to be ~19ka at 20°C, estimated by isothermal

    decay analysis as for the fast, medium and S1 components.

    A single-aliquot regenerative-dose protocol was developed for obtaining component-

    resolved equivalent dose estimates. Examination of the dose response of the components

    demonstrated the potential of component S3 for extending the upper age limit of quartz

    optical dating (D0 > 400Gy). Component S2 was observed to saturate at relatively low doses

    (D0 ~ 30Gy) and the fast, medium and S1 components all showed similar dose response

    characteristics (D0 ~ 200Gy).

    Photoionization cross-section spectra were obtained for the fast and medium components. It

    was found that the difference in the response of the OSL components to photon energy could

    be exploited in several ways; firstly, to separate the components by selection of appropriate

    photon energies/temperatures to successively bleach one component with negligible

    reduction in the next, thereby avoiding the need for complicated, lengthy fitting procedures,

    and secondly, the change of signal form following incomplete resetting, allows identification

    of partial bleaching of sediments.

  • 2

    Acknowledgements

    First and foremost I would like to express my deepest gratitude to my excellent supervisor,

    Dr Richard Bailey, who has tirelessly supported, advised and tutored me during my time at

    Oxford. His genuine enthusiasm and energy for scientific enquiry and for teaching has

    inspired and guided me throughout. I feel very honoured to have been his first student and

    hope very much that this thesis will make him proud.

    I would especially like to thank Dr Ed Rhodes and Dr Stephen Stokes: Dr Rhodes for

    additional supervision and for providing me with interesting samples for this project; and Dr

    Stokes for the use of his many samples, machines and characteristically no-nonsense advice.

    For the short time I spent at the Risø National Laboratory in Denmark I would like to thank

    Professor Lars Bøtter-Jensen and Professor Andrew Murray. I am especially grateful to Dr

    Enver Bulur for his help and valued discussion on LM OSL.

    Dr Von Whitley is thanked for help with fitting procedures, Dr Eduardo Yukihara for

    making measurements on my samples using the laser in Oklahoma State University, and

    both for their interest in my project.

    I am grateful to Dr David Allwright and the scarily intelligent people at OCIAM,

    Mathematical Institute, Oxford University, for the workshops on curve deconvolution.

    I also give my thanks to Professor M. Tite, for additional financial support, M. Franks for

    building the external LED unit and invaluable technical assistance, and J. Fenton, J. Simcox,

    A. Allsop and others at the Research Laboratory for Archaeology for general help. I am

    pleased to have studied with and had the support of the Oxford luminescence group,

    especially Morteza Fattahi and Grzes Adamiec.

    Financial support for this project was provided by the Natural Environment Research

    Council (reference: GT04/98/ES/231).

    I would also like to say a big thank you to the good friends I made at karate for welcome

    distraction from work, inspiration and friendship, especially sensei Phil Stevens, Alison

    Stevens, Alison Jones, and Onofrio Marago.

    And above all to my family; Mum, Dad and Adam, for their endless support and

    encouragement over the years.

  • 3

    Table of contents

    Abstract……………………………………………….………

    Acknowledgements…………………………………….….

    Table of Contents……………………………………….…. List of Figures…………………………………………….…

    List of Tables………………………………………………..

    List of Parameters ………………………………………...

    1 2 3 6

    10 11

    Chapter 1 Introduction …………………………………………….….. 13

    1.1

    1.2

    1.3

    Background to optical dating of sediments………………….

    The OSL components of quartz………………………………

    Thesis scope and format………………………………………

    14

    16

    18

    Chapter 2 Technical Information………………………………….… 19 2.1

    2.2

    2.3

    2.4

    2.5

    2.6

    Introduction…………………………………………….……...

    Sample Collection……………………………………..……….

    Preparation of samples for OSL measurement……….……..

    Measurement Apparatus………………………….……….….

    2.4.1 Measurement of luminescence……………………..……..

    2.4.2 Dose rate determination…………………………….…….

    Noise and background signal components……….………….

    Error analysis………………………………….……………....

    20

    20

    21

    25 25

    29

    30

    31

    Chapter 3 Linearly modulated OSL and deconvolution…..……. 35 3.1

    3.2

    3.3

    3.4

    3.5

    3.6

    Introduction……………………………………………..……..

    LM OSL: concepts and theory…………………………..……

    3.2.1 Comparison of CW and LM measurement techniques..….

    3.2.2 Analytical solutions: CW and LM OSL…………..………

    3.2.3 Factors affecting photoionization cross-section……..…....

    3.2.4 Other forms of modulation…………………………..……

    Review of previous studies using LM OSL………………..…

    Measurement of LM OSL………………………………..…...

    3.4.1 Solutions for non-linear ramping……………………..…..

    Deconvolution of quartz LM OSL curves………………..….

    3.5.1 Formulation of the problem…………………………..…..

    3.5.2 Curve fitting algorithms………………………………..…

    3.5.3 Testing of curve fitting routines………………………..…

    3.5.4 Testing modifications for dealing with empirical data..….

    Discussion………………………………………………..…….

    36

    36

    36

    39

    46

    47

    50

    52

    55

    63

    63

    64

    66

    69

    71

    Chapter 4

    Initial observations of quartz LM OSL and thermal properties of the OSL components……………...……..

    72 4.1

    4.2

    4.3

    Introduction……………………………………………..……..

    Basis of choice of kinetic order for deconvolution……..……

    OSL variability…………………………………………..…….

    4.3.1 Sample variability…………………………………..…….

    4.3.2 Grain-to-grain variability…………………………..……..

    73

    73

    94