View source for Optically stimulated luminescence thermochronometry

{{Short description|Dating method}}
[[File:G6000.....png|alt=|thumb|OSL signal is a function of trapped electrons in quartz or feldspar. In the natural environment, these minerals store [[Elementary charge|electron charges]] as they cool below their closure temperatures. The schematic illustration show [[electron]] trapping and detrapping process at the crystal-scale (A) Typical [[crystal structure]] showing [[Crystallographic lattice|crystallographic sites]] or vacancies (B) Trapping of [[electron-hole pair]] as the [[Valency bonds|valence band]] breaks when it is irradiated by an ionizing source (C) release of light during [[Electron-hole pair]] recombination. Electron leaves its site and recombines with hole by thermal vibration of the [[Lattice structure|lattices]]. The process generate [[luminescence]] signal.]]

'''Optically stimulated luminescence (OSL) thermochronometry''' is a dating method used to determine the time since [[quartz]]<ref name=":02">{{Cite journal|last=Herman|first=Frédéric|last2=Rhodes|first2=Edward J.|last3=Braun|first3=Jean|last4=Heiniger|first4=Lukas|date=2010b|title=Uniform erosion rates and relief amplitude during glacial cycles in the Southern Alps of New Zealand, as revealed from OSL-thermochronology|journal=Earth and Planetary Science Letters|volume=297|issue=1–2|pages=183–189|doi=10.1016/j.epsl.2010.06.019|issn=0012-821X|bibcode=2010E&PSL.297..183H}}</ref><ref name=":1">{{Cite journal|last=Guralnik|first=B.|last2=Ankjærgaard|first2=C.|last3=Jain|first3=M.|last4=Murray|first4=A.S.|last5=Müller|first5=A.|last6=Wälle|first6=M.|last7=Lowick|first7=S.E.|last8=Preusser|first8=F.|last9=Rhodes|first9=E.J.|date=2015a|title=OSL-thermochronometry using bedrock quartz: A note of caution|journal=Quaternary Geochronology|volume=25|pages=37–48|doi=10.1016/j.quageo.2014.09.001|issn=1871-1014}}</ref> and/or [[feldspar]]<ref name=":2">{{Cite journal|last=King|first=G.E.|last2=Herman|first2=F.|last3=Lambert|first3=R.|last4=Valla|first4=P.G.|last5=Guralnik|first5=B.|date=2016a|title=Multi-OSL-thermochronometry of feldspar|journal=Quaternary Geochronology|volume=33|pages=76–87|doi=10.1016/j.quageo.2016.01.004|issn=1871-1014}}</ref><ref name=":5">{{Cite journal|last=Guralnik|first=Benny|last2=Jain|first2=Mayank|last3=Herman|first3=Frédéric|last4=Ankjærgaard|first4=Christina|last5=Murray|first5=Andrew S.|last6=Valla|first6=Pierre G.|last7=Preusser|first7=Frank|last8=King|first8=Georgina E.|last9=Chen|first9=Reuven|date=2015c|title=OSL-thermochronometry of feldspar from the KTB borehole, Germany|journal=Earth and Planetary Science Letters|volume=423|pages=232–243|doi=10.1016/j.epsl.2015.04.032|issn=0012-821X|bibcode=2015E&PSL.423..232G|url=http://eprints.whiterose.ac.uk/109132/1/Guralnik_et_al_2015_EPSL_KTB_GOA.pdf}}</ref> began to store [[Electron charge|charge]] as it cools through the effective [[closure temperature]].<ref name=":02" /> The [[closure temperature]] for quartz and Na-rich K-feldspar is 30-35&nbsp;°C<ref name=":02" /><ref name=":12">{{Cite journal|last=Guralnik|first=Benny|last2=Jain|first2=Mayank|last3=Herman|first3=Frédéric|last4=Paris|first4=Richard B.|last5=Harrison|first5=T. Mark|last6=Murray|first6=Andrew S.|last7=Valla|first7=Pierre G.|last8=Rhodes|first8=Edward J.|date=2013|title=Effective closure temperature in leaky and/or saturating thermochronometers|journal=Earth and Planetary Science Letters|volume=384|pages=209–218|doi=10.1016/j.epsl.2013.10.003|issn=0012-821X|bibcode=2013E&PSL.384..209G}}</ref> and 25&nbsp;°C<ref name=":5" /> respectively. When [[quartz]] and [[feldspar]] are beneath the earth, they are hot. They cool when any geological process e.g. [[Erosion|focused erosion]] causes their exhumation to the earth surface.<ref name=":4">{{Cite journal|last=Rhodes|first=Edward J.|date=2011|title=Optically Stimulated Luminescence Dating of Sediments over the Past 200,000 Years|journal=Annual Review of Earth and Planetary Sciences|language=en|volume=39|issue=1|pages=461–488|doi=10.1146/annurev-earth-040610-133425|issn=0084-6597|bibcode=2011AREPS..39..461R}}</ref>  As they cool, they trap [[electron charge]]s originating from within the [[crystal lattice]]. These charges are accommodated within [[crystallographic defect]]s or vacancies in their [[crystal lattices]] as the mineral cools below the closure temperature.<ref name=":3">{{Cite journal|last=King|first=Georgina E.|last2=Guralnik|first2=Benny|last3=Valla|first3=Pierre G.|last4=Herman|first4=Frédéric|date=2016c|title=Trapped-charge thermochronometry and thermometry: A status review|journal=Chemical Geology|volume=446|pages=3–17|doi=10.1016/j.chemgeo.2016.08.023|issn=0009-2541|bibcode=2016ChGeo.446....3K|doi-access=free}}</ref>

During detrapping of these electrons, [[luminescence]] is produced.<ref name=":3" /> The luminescence or light emission from the mineral is assumed to be proportional to the trapped electron charge population.<ref name=":3" />  The age recorded in standard OSL method is determined by counting the number of these trapped charges in an OSL detection system.<ref name=":4" /><ref name=":3" /> The OSL age is the cooling age of the quartz and/or feldspar.<ref name=":02" /> This cooling history is a record of the mineral's thermal history, which is used to reconstruct the geological event.<ref name=":02" /><ref name=":33">{{Cite journal|last=King|first=Georgina E.|last2=Herman|first2=Frédéric|last3=Guralnik|first3=Benny|date=2016b|title=Northward migration of the eastern Himalayan syntaxis revealed by OSL thermochronometry|journal=Science|language=en|volume=353|issue=6301|pages=800–804|doi=10.1126/science.aaf2637|issn=0036-8075|pmid=27540169|bibcode=2016Sci...353..800K}}</ref>

The sub-[[Quaternary]] period (10<sup>4</sup> to 10<sup>5</sup> years) is the geological age where OSL is a favourable dating technique<ref name=":02" /><ref name=":4" /> because of low [[closure temperature]] of quartz and feldspar used in this technique. The Quaternary period is marked by intense crustal erosion particularly within active mountain ranges, leading to high exhumation rate of [[crust (geology)|crustal]] rocks<ref name=":33" /> and formation of sub-[[Quaternary]] sediments. Previous techniques (e.g. Apatite [[Fission track dating|Fission Track]], Zircon [[Fission track dating|Fission Track]], and (Uranium-Thorium)/ [[Helium dating]]) could not adequately track the geological age records particularly in the last ~300 thousand years.<ref name=":02" /><ref name=":3" /><ref name=":7">{{Cite journal|last=Herman|first=Frédéric|last2=King|first2=Georgina E.|date=2018|title=Luminescence Thermochronometry: Investigating the Link between Mountain Erosion, Tectonics and Climate|journal=Elements|language=en|volume=14|issue=1|pages=33–38|doi=10.2138/gselements.14.1.33|issn=1811-5209}}</ref>  OSL dating is currently the only dating method that has been successfully applied to understand the cooling ages of the geological events.<ref name=":02" /><ref name=":33" /><ref>{{Cite journal|last=Wu|first=Tzu-Shuan|last2=Jain|first2=Mayank|last3=Guralnik|first3=Benny|last4=Murray|first4=Andrew S.|last5=Chen|first5=Yue-Gau|date=2015|title=Luminescence characteristics of quartz from Hsuehshan Range (Central Taiwan) and implications for thermochronometry|journal=Radiation Measurements|volume=81|pages=104–109|doi=10.1016/j.radmeas.2015.03.002|issn=1350-4487|bibcode=2015RadM...81..104W}}</ref><ref>{{Cite journal|last=Sarkar|first=Sharmistha|last2=Mathew|first2=George|last3=Pande|first3=Kanchan|last4=Chauhan|first4=Naveen|last5=Singhvi|first5=Ashok|date=2013-12-01|title=Rapid denudation of Higher Himalaya during late Pliestocence, evidence from OSL thermochronology|journal=Geochronometria|volume=40|issue=4|pages=304–310|doi=10.2478/s13386-013-0124-7|issn=1897-1695|doi-access=free}}</ref><ref>{{Cite journal|last=Valla|first=Pierre G.|last2=Lowick|first2=Sally E.|last3=Herman|first3=Frédéric|last4=Champagnac|first4=Jean-Daniel|last5=Steer|first5=Philippe|last6=Guralnik|first6=Benny|date=2016|title=Exploring IRSL 50 fading variability in bedrock feldspars and implications for OSL thermochronometry|journal=Quaternary Geochronology|volume=36|pages=55–66|doi=10.1016/j.quageo.2016.08.004|issn=1871-1014|url=https://hal-insu.archives-ouvertes.fr/insu-01355727/file/valla-2016.pdf|doi-access=free}}</ref>

== Theoretical concepts of electron trapping and detrapping for OSL measurement ==
In natural environment, [[crystal lattices]] of quartz and/or feldspar are bombarded with radiation released from [[radiogenic heating|radiogenic source]]<ref name=":3" /> such as ''in -situ'' [[radioactive decay]].<ref name=":02" /><ref name=":4" /> As the crystals are [[Irradiation|irradiated]], [[Electron charge|charges]] are stored up in their [[crystallographic defect]]s. The [[Electron charge|charge]] trapping process involves atomic-scale [[ion]]ic substitution of both electron and hole within the [[crystal lattices]] of [[quartz]] and [[feldspar]].<ref name=":3" /> The [[Electrodiffusion|electron diffusion]] happens in response to [[ionizing radiation]] as the minerals cools below their [[closure temperature]].<ref name=":12" /><ref>{{Cite journal|last=Dodson|first=Martin H.|date=1973|title=Closure temperature in cooling geochronological and petrological systems|journal=Contributions to Mineralogy and Petrology|volume=40|issue=3|pages=259–274|doi=10.1007/bf00373790|issn=0010-7999|bibcode=1973CoMP...40..259D}}</ref>

If quartz or feldspar gains are exposed to natural light source such as the sun, the trapped charges will be evicted in form of [[luminescence]].<ref name=":3" /> This natural process is called bleaching. Any other process that could heat up the sample will also cause the trapped electrons to escape from the crystal lattice known as thermal bleaching. Optical bleaching of the mineral leads to eviction of trapped charges in the minerals,<ref name=":4" /><ref name=":3" /> hence, careful sampling and handling must be followed to avoid using bleached sample for OSL thermochronometry. To artificially produce luminescence in the laboratory for luminescence study of the mineral, these two processes are adopted.<ref name=":3" />

== Kinetic or rate equations for trapping and detrapping processes ==
A wide range of kinetic models have been developed to explain trapping and detrapping processes in quartz and feldspar crystals.<ref name=":1" /><ref>{{Cite book|last=Chen|first=Reuven|last2=Pagonis|first2=Vasilis|date=2011|title=Thermally and Optically Stimulated Luminescence|doi=10.1002/9781119993766|isbn=978-1-119-99376-6}}</ref> Two of these models are particularly useful in determining the cooling histories of quartz or feldspar<ref name=":3" /><ref name=":7" /> These models are known as the general order kinetic model<ref name=":5" /> and band tail model.<ref name=":8">{{Cite journal|last=Poolton|first=N. R. J.|last2=Ozanyan|first2=K. B.|last3=Wallinga|first3=J.|last4=Murray|first4=A. S.|last5=Bøtter-Jensen|first5=L.|date=2002|title=Electrons in feldspar II: a consideration of the influence of conduction band-tail states on luminescence processes|journal=Physics and Chemistry of Minerals|volume=29|issue=3|pages=217–225|doi=10.1007/s00269-001-0218-2|issn=0342-1791|bibcode=2002PCM....29..217P}}</ref><ref name=":9">{{Cite journal|last=Li|first=Bo|last2=Li|first2=Sheng-Hua|date=2013|title=The effect of band-tail states on the thermal stability of the infrared stimulated luminescence from K-feldspar|journal=Journal of Luminescence|volume=136|pages=5–10|doi=10.1016/j.jlumin.2012.08.043|issn=0022-2313|bibcode=2013JLum..136....5L|url=http://ro.uow.edu.au/cgi/viewcontent.cgi?article=1261&context=smhpapers}}</ref> The two models consider three major processes to characterize the mineral luminescence, which are: trapping process, thermal detrapping process and athermal detrapping process. Each of the processes are guided by different equations discussed below. These models are useful for the determining of cooling history of the mineral, which involves subtracting the differential sum of thermal detrapping and athermal detrapping from the trapping process<ref name=":7" /> (i.e. Trapping – (Thermal detrapping + Athermal detrapping).

=== Rate equations ===
{| class="wikitable"
|
==== Trapping process ====
The process describes the rate at which a mineral acquires charge from its natural environment.<ref name=":3" /> The process is defined by the equation below.

<math>\frac{d\tilde{n}}{dt}=\frac{D_R}{D_o}(1-\tilde{n})^\alpha</math>

where <math>\tilde{n}</math> is the ratio of electrons that are trapped (n) and the full storage capacity  (N) in the mineral crystal. <math>D_R</math> is the charge stored per unit time, known as dose rate (unit is Gy/ky). <math>D_o</math> is the dose that correspond to the maximum storage capacity of the mineral. The probability that the mineral will get filled defines the <math>\dot{D}</math> and <math>D_o</math> relationship. <math>\alpha</math> is an exponential factor greater than or equal to 1.

|
==== Thermal detrapping ====
The equation describes how charges are evicted in form of luminescence from the mineral system when the mineral absorbed heat energy. This is where the general order kinetic model and the band tail model equations differs.

1. For the general order kinetic Model<ref name=":5" />          2. For the band tail Model<ref name=":8" /><ref name=":9" />

<math>\frac{d\tilde{n}}{dt}=-s\tilde{n}^\beta\exp\left ( \frac{-E}{kT} \right )</math>                      <math>\frac{d\tilde{n}}{dt}=-s\tilde{n}^\beta\exp\left ( \frac{-(E_t-E_b)}{kT} \right )</math>

Where <math>s</math> frequency factor measured in per second (s<sup>−1</sup>), <math>\beta</math> is the kinetic order, which is usually less than or equal to 1; <math>E</math> is the [[activation energy]] measured in (eV), <math>E_t</math> and <math>E_b</math> are the activation energies at the [[conduction band]] and the [[valence band]] respectively in the [[Crystal structure|crystal lattices]] of the mineral.  <math>k</math> is the [[Boltzmann constant]] (e''V/K)'' and <math>T</math> is the [[Thermodynamic temperature|absolute temperature]] (K).
|
==== Athermal detrapping ====
This equation describes the luminescence recorded as the signal fades out known as 'anomalous fading'.<ref>{{Cite journal|last=Huntley|first=D J|date=2006|title=An explanation of the power-law decay of luminescence|journal=Journal of Physics: Condensed Matter|volume=18|issue=4|pages=1359–1365|doi=10.1088/0953-8984/18/4/020|issn=0953-8984|bibcode=2006JPCM...18.1359H}}</ref> This is determined without heating the mineral during fading experiment<ref name=":3" /> The relationship is defined by this equation.

<math>\frac{d\tilde{n}}{dt}=-s\tilde{n}^\beta\exp\left ( -p'^{-\tfrac{1}{3}}r'\right )</math>

where <math>p'</math> represents the density of the crystal's recombination centers and r is the distance between the recombination centers and the electron traps.

|}

=== Determination of cooling history from the kinetic equations ===
[[File:BlessJoy1.png|alt=|thumb|A sketch diagram of OSL detection system. The commonly used is an automated Ris<small><math>\varnothing</math></small>Thermal Luminescence Reader (e.g. TL-DA-20). The Infra-red and Blue LEDs ([[Light-emitting diode|Light Emitting Diode]]) provide the optical stimulation depending on the mineral under study. The photomultiplier tube collects, converts the incident light (light blue rays) to electron charges as the OSL signal emitted by the mineral.]]By combining the four equations above, a single differential equation is developed to convert the luminescence into cooling rate. We have:

<math>\frac{d\tilde{n}}{dt}=\frac{D_R}{D_o}(1-\tilde{n})^\alpha-s\tilde{n}^\beta\exp\left ( \frac{-E}{kT} \right )-s\tilde{n}^\beta\exp\left ( -p'^{-\tfrac{1}{3}}r'\right )</math> for the general order kinetic model; and

<math>\frac{d\tilde{n}}{dt}=\frac{D_R}{D_o}(1-\tilde{n})^\alpha-s\tilde{n}^\beta\exp\left ( \frac{-(E_t-E_b)}{kT} \right )-s\tilde{n}^\beta\exp\left ( -p'^{-\tfrac{1}{3}}r'\right )</math> for the band tail model.

Any of the models can be used because the same series of laboratory experiments are followed for the estimation of all the parameters involved in the equations.<ref name=":7" /> The inversion of measured <math>\tilde{n}</math> for a range of temperature -time history or T-t path can be used to determine the cooling rate.<ref name=":7" /> Sufficient number of T-t paths conducted in the laboratory is used to build a probability density function, which will help to determine the most likely cooling histories undergone by the mineral.<ref name=":7" />

== Sample preparation ==
Bedrock samples from earth surface or boreholes are required earth materials for OSL dating.<ref name=":3" /> Minerals ([[quartz]] and/or [[feldspar]]) are usually separated from the rock or sediment samples under regulated laboratory lighting system similar to procedures used in [[Archaeology|archaeological]] OSL dating.<ref name=":02" /><ref name=":4" /> The light source is usually a controlled red light condition to avoid luminescence signal resetting.<sup>[7]</sup> Crushing of sample are gently carried out to avoid generating heat that is strong enough to reset OSL signal in the minerals.<ref name=":3" /> Crushed samples are separated by means of a [[sieve]] to get fine-grained. A range of values varying from 90 – 125 microns,<ref name=":4" /> 100 – 200 microns and 180 – 212 microns<ref name=":3" /><ref name=":33" /> can be used for OSL measurement. The selected grains are chemically treated with HCl to digest carbonates and with H<sub>2</sub>O<sub>2</sub> to remove organic materials<ref name=":3" /> that can contaminate the sensitivity of OSL signal during measurement. Feldspar and quartz with densities of < 2.62 g cm<sup>−3</sup>  and < 2.68 g cm<sup>−3</sup> respectively are separated from other heavier minerals by density separation.<ref name=":3" /> Inclusions of zircon, apatite and feldspar in quartz as well as alpha-particles irradiated grain edges that can contaminate OSL signal are removed by etching in hydrofluoric acid (HF).<ref name=":1" /><ref name=":4" /><ref name=":3" />

== OSL signal detection system ==
OSL ages are commonly measured using an automated Ris<small><math>\varnothing</math></small>[https://www.earthsciences.hku.hk/research/research-group/facilities/thermoluminescence-tl-optically-stimulated-luminescence-osl-reader Thermal Luminescence Reader (e.g. TL-DA-20)]. It contains an internal beta-source (e.g. <sup>90</sup>Sr/<sup>90</sup>Y) with optical stimulation emitted through laser diodes (LEDs). The reader also has a detection filter for transmission of stimulated luminescence signals. During this measurement, the mineral grain (quartz or feldspar) is glued on a heater strip (stainless-steel discs) using adhesive (commonly silicone spray). The mineral grain is stimulated with the light source.<ref name=":4" /> This light is the series of [[Light-emitting diode|light emitting diode]].<ref name=":4" /> This bombardment stimulates the electrons, which are trapped and begin to [[Recombination (physics)|recombine]] in the crystal.<ref name=":3" /> During this process, they give the OSL signal, which is collected or recorded in the ray sensitive [[photomultiplier]] tube.<ref name=":4" /> The photomultiplier tube converts all the incident [[photon]]s (i.e. light) to electronic charge. This is the basic principle of how the luminescence (light) emission from the minerals under investigation is measured.

== OSL age determination ==
[[File:BleJoy.png|alt=|thumb|Plot A shows the decay curve of an OSL signal emitted by an [[wikt:Aliquot|aliquot]]. Plot B shows sensitivity correction of OSL signal using the single aliquot regenerative (SAR) technique. The signal is recorded in the photomultiplier tube (OSL counter). The OSL signal is displayed in a computing system attached to the OSL detection system. The natural dose OSL sensitivity (N<sub>L</sub> / N<sub>T</sub>) is shown by the red circle. The natural dose will plot directly on the vertical axis because SAR dose has not been introduced. The regenerative dose signals (R<sub>i</sub> / R<sub>T</sub>) are represented by the red circles and the regenerative dose curve is the line that passes through the dose signals (i.e. 5 blue circles) as labelled. To determine the equivalent dose (<math>D_E</math>), a line is drawn horizontally from the natural OSL signal to intercept with the regenerative curve. The horizontal value at the point of intercept corresponds to the equivalent dose (<math>D_E</math>). The unit is Gray (Gy). ]]
To determine the OSL age of the sample, the [[Absorbed dose|dose rate]], (<math>D_R</math>) and the equivalent dose (<math>D_E</math>). A [[Absorbed dose|dose]] is the quantity of natural radiation or energy absorbed by a mineral.<ref name=":4" /> The [[dose rate]] is the effective radiation absorbed from naturally occurring [[Ionizing radiation|ionizing source]] per unit time.<ref name=":4" /><ref name=":3" />

The age is calculated by determining the ratio of equivalent dose (<math>D_E</math>) and the dose rate (<math>D_R</math>) using the equation below.

<math>A = \frac{D_E}{D_R}</math>

where <math>A</math> is the age (yr), <math>D_E</math> is measured in Gray (Gy). Note that 1&nbsp;Gy is equivalent to 1 J.kg<sup>−1</sup> (Joule per kilogram) and <math>D_R</math> is Gy year<sup>−1</sup><ref name=":4" />

=== Dose rate determination ===
For a single grain of mineral, the dose rate (<math>D_R</math>) can be determined by measuring the concentrations of [[uranium]], [[potassium]] and [[thorium]] by direct [[Mass spectrometry|mass spectrometric]] analysis of quartz or feldspar grains.'''<ref name=":4" />''' Ge-Gamma, INAA, X-ray flourescnce and [[Inductively coupled plasma mass spectrometry|ICP-MS]] or [[Inductively coupled plasma atomic emission spectroscopy|ICP-OES]] are [[spectrometer]]s that can could be used.<ref name=":4" /> Other methods for the determination the dose rate include: (1) overburden [[Cosmic ray|cosmic]] dose rate estimation, (2) water content attenuation method, and (3) disequilibrium dose rate correction method.<ref name=":4" /> An average dose rate is usually calculated as representative of the dose rate.<ref name=":4" />

=== Equivalent dose determination ===
The equivalent dose (<math>D_E</math>) is also known as the dose response is determined from the dose response curve (see Plot B).<ref name=":3" /> The single-aliquot regenerative (SAR) protocol is a commonly used method for the determination of the equivalent dose.<ref name=":0">{{Cite journal|last=Murray|first=A.S.|last2=Wintle|first2=A.G.|date=2003|title=The single aliquot regenerative dose protocol: potential for improvements in reliability|journal=Radiation Measurements|volume=37|issue=4–5|pages=377–381|doi=10.1016/s1350-4487(03)00053-2|issn=1350-4487|bibcode=2003RadM...37..377M}}</ref> The protocol involves series of laboratory measurement of OSL signal (see Plot A), which is emitted by the [[wikt:Aliquot|aliquot]] after it has been optically stimulated at a known beta dose within a given time in seconds. The beta-source may be <sup>90</sup>Sr/<sup>90</sup>Y in an automated Ris<small><math>\varnothing</math></small>[https://www.earthsciences.hku.hk/research/research-group/facilities/thermoluminescence-tl-optically-stimulated-luminescence-osl-reader Thermal Luminescence Reader] . During SAR protocol, the difference in the measurement for quartz and feldspar is mainly on the degree of heat required per time and the source of stimulation.

The first stage involves determination of the natural dose (see Plot B) preheating the aliquot to about 160 -130&nbsp;°C (for feldspar)<ref name=":0" /> for 10&nbsp;s or 160-300&nbsp;°C (for quartz)<ref name=":10">{{Cite journal|last=Murray|first=A.S.|last2=Wintle|first2=A.G.|date=2000|title=Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol|journal=Radiation Measurements|volume=32|issue=1|pages=57–73|doi=10.1016/s1350-4487(99)00253-x|issn=1350-4487|bibcode=2000RadM...32...57M}}</ref> when the natural [[luminescence]] signal (i.e. natural dose) is still intact. This is done to remove unstable signals in the mineral. After preheating, the aliquot is optically stimulated by [[Infrared]] [[Light-emitting diode|light emitting diode]] (for feldspar) or Blue [[Light-emitting diode|light emitting diode]] (for quartz) depending on which mineral (see OSL detection system) for 40&nbsp;s at 125&nbsp;°C (for feldspar) or 100 s at 125&nbsp;°C (for quartz) and the natural OSL signal (N<sub>L</sub>) is measured and recorded in the photomultiplier tube. For the second stage, the aliquot is [[Irradiation|irradiated]] with a fixed known test dose (beta dose).<ref name=":6" /> The aliquot is preheated at temperature less than 160&nbsp;°C. The IRSL signal measurement is taken as a test dose IRSL response (N<sub>T</sub>) after it has been optically stimulated for 40&nbsp;s at 125&nbsp;°C (for feldspar) or 100 s at 125&nbsp;°C (for quartz).<ref name=":0" /><ref name=":10" /><ref name=":6">{{Cite journal|last=Wallinga|first=Jakob|last2=Murray|first2=Andrew|last3=Duller|first3=Geoff|date=2000|title=Underestimation of equivalent dose in single-aliquot optical dating of feldspars caused by preheating|journal=Radiation Measurements|volume=32|issue=5–6|pages=691–695|doi=10.1016/s1350-4487(00)00127-x|issn=1350-4487|bibcode=2000RadM...32..691W}}</ref> At this stage the aliquot is completely bleached.<ref name=":6" /> A regenerative test dose is then started after bleaching.<ref name=":6" />

The same procedure as described above is followed but a range of regenerative dose is given at different temperature for sensitivity correction of OSL signal (See Plot B). For the regenerative dose measurement, the aliquot is [[Irradiation|irradiated]] with a known dose before preheating at 160-130&nbsp;°C for 10&nbsp;s or 160-300&nbsp;°C for feldspar or quartz respectively while the signal response (R<sub>i</sub>) is measured. A fixed test dose is by irradiating the aliquot and a preheating of the aliquot is carried out at a temperature less than 160&nbsp;°C. The aliquot is optically stimulated at the same rate and the IRSL signal (R<sub>T</sub>) is measured. The steps are repeated for range of different regenerative dose including zero test dose.<ref name=":6" /> During each of the tests, all OSL signals are recorded in the [[photomultiplier tube]] and the OSL counts are plotted against the OSL exposure time in seconds as shown in the OSL signal curve (first graph).<ref name=":6" />

For sensitivity correction, N<sub>L</sub> is plotted against N<sub>T</sub> representing the natural OSL signal while the plot of R<sub>i</sub> against R<sub>T</sub> representing regenerative dose test (see Plot B). The natural dose is along the vertical axis because no laboratory dose is given at the stage. The regenerative dose measurement will vary with respect to the given dose at each stage. The equivalent dose (<math>D_E</math>) is determined by drawing a line (red discontinuous line in Plot B) from the natural dose to intercept with the regenerative dose curve. The point of interception with the curve represent the equivalent dose by reading its value on the horizontal axis (See Plot B).<ref name=":0" /> The corresponding dose value at the horizontal axis is recorded for the equivalent dose (<math>D_E</math>).<ref name=":6" />

== Applications. ==
[[File:ABBSABBS.png|alt=|thumb|Schematic description of OSL ages estimated from samples collected along [[South Island|Whataroa-Perth]] catchment area in the [[Southern Alps]] of [[New Zealand]]. The [[Topography|topographic]] profile represents the line connecting all the sampling locations. The estimated OSL ages are plotted in blue circles. The variation in the OSL ages depicts rock cooling history to changes in the relief of the [[valley]] system of [[South Island|Whataroa-Perth]] catchment area.]]

=== General applications ===
OSL finds application in all low-temperature (<50&nbsp;°C) tectonic and sedimentary processes. These studies are mainly captured within the sub-Quaternary period including, but not limited to focused fluvial and/or [[glacial erosion]], [[rock exhumation]] and evolution of [[topography]] in active tectonic regions.<ref name=":02" /><ref name=":33" /> Other applications include glaciation deposits, [[lagoon deposit]]s, storm surge and tsunami deposits, [[lake deposit]]s including shoreline migration history, fluvial erosion deposits, [[loess]] deposit records.<ref name=":4" /> For example, the rate of slip on a normal faults plane can also be modelled, the rate of glacial or fluvial erosion of the earth surface can also be modelled as well as when sedimentary deposits are found within the sub-[[Quaternary]] period.<ref name=":33" />

In active tectonics regions, the application of OSL dating is very useful in tracking the thermal history and rate of rock exhumation towards the Earth's surface.<ref name=":02" /><ref name=":33" /> The closer the cooling ages, the higher the rate of erosion and/or exhumation of the rock unit under investigation.<ref name=":02" /> When the OSL age of quartz or feldspar is known, the obtained ages are coupled with the existing thermal-mechanical equations e.g. Pecube<ref name=":33" /><ref>{{Cite journal|last=Braun|first=Jean|date=2003|title=Pecube: a new finite-element code to solve the 3D heat transport equation including the effects of a time-varying, finite amplitude surface topography|journal=Computers & Geosciences|volume=29|issue=6|pages=787–794|doi=10.1016/s0098-3004(03)00052-9|issn=0098-3004|bibcode=2003CG.....29..787B}}</ref> to reconstruct the thermal-mechanical history.

The OSL ages (see diagram), cooling ages, elevation data are plotted against the horizontal distance where samples and elevation data were collected to interpret the exhumation rate of rock or the evolution of the relief system through time.<ref name=":02" /> For example, OSL dating has been applied in determined the cooling histories of some rapidly eroding active regions at sub-Quaternary time-scale (i.e. 10<sup>4</sup> to 10<sup>5</sup> years). These examples are [[South Island|Whataroa-Perth]] catchment area in the [[Southern Alps]] of [[New Zealand]]<ref name=":02" /> and Namche Barwa-Gyala Peri dome in eastern Himalaya.<ref name=":33" /> In the Namche Barwa-Gyala Peri dome, river erosion was prevalent<ref name=":33" /> while glacial erosion was the main active process in the [[South Island|Whataroa-Perth]] catchment area.<ref name=":02" /> In both studies, the rate of exhumation and evolution of the relief systems were estimated by inversion of OSL thermochronological ages.<ref name=":02" /><ref name=":33" />

== See also ==
* [[Luminescence dating]]
* [[Optically stimulated luminescence]] (physics)

== References ==
<references />

[[Category:Dating methods]]