DRI Luminescence Laboratory

Lab Description

The DRI E.L. Cord Luminescence Laboratory (DRILL) is located at DRI’s northern campus in Reno, NV and offers a broad spectrum of luminescence dating capabilities, including optically stimulated luminescence (OSL) and thermoluminescence (TL) techniques. The DRILL is a research laboratory dedicated to fundamental investigations in the luminescence properties of earth materials, and to the application of luminescence dating techniques to geomorphological, geological, and archeological problems.

The DRILL welcomes collaboration with research institute and university faculty, consultants, and government agency researchers. The DRILL research staff can collaborate on proposals, contribute to grant writing, and consult on study design. We can also arrange training for undergraduate and graduate students, post-docs, and visiting researchers.

What is Luminescence Dating?
Luminescence dating typically refers to a suite of radiometric geologic dating techniques whereby the time elapsed since the last exposure of some silicate minerals to light or heat can be measured. In the luminescence process, when naturally occurring minerals are exposed to low level, ambient, ionizing radiation emissions associated with the decay of U, Th, and K, electrons become stored and collected within defects in crystal lattices referred to as ‘trapping centers’ or ‘traps’. When dosed minerals are then re-exposed to light or heat, they release the stored electrons, emitting a photon of light that is referred to as luminescence. This ‘bleaching’ process empties the electrons stored in the traps and resets or ‘zeroes’ the signal.

Conduction band diagram
Above: In silicate minerals, when radiation interacts with the crystal (Irradiation), energy pushes an electron into the conduction band and leaves a ‘hole’ in the valence band. The electron may become trapped at a defect site (T1, T2 etc) for some time (Storage). When the crystal is stimulated by light or heat, the electrons in the traps are evicted into the conduction band (Eviction). From there, they can recombine with holes at the luminescence centers (L), resulting in the emission of a photon of light – the luminescence signal that is observed in the laboratory. (Modified from Aitken, 1990; Duller, 2008)

The burial age is calculated as a ratio of the equivalent dose (total energy accumulated during burial) and the dose rate (energy per year delivered by radioactive decay): Age (yr) = Equivalent dose (Gy)/Dose rate (Gy/year)

Through controlled experiments the emission of luminescence can be controlled and measured and then used to estimate the equivalent dose (De). The dose rate (Dr) is the amount of energy absorbed per year from radiation in the environment surrounding the sample material and is estimated by measuring the amount of radioactivity directly or by chemically analyzing the surrounding material and calculating the concentration of radioisotopes.

Rechargeable battery
Above: The build-up and resetting of luminescence signals is similar to a rechargeable battery. When mineral grains are exposed to light or heat, energy stored in the form of trapped electrons is released, similar to emptying a battery of its charge. During burial, energy builds, recharging the signal. In the lab, mineral grains are stimulated to release the stored energy in the form of light. The brightness of the luminescence signal is related to the amount of energy stored in the mineral. (Adapted from Duller, 2008)

References
Aitken, M.J., 1985. Thermoluminescence Dating. London: Academic Press. (Out of print.)

Duller GAT. 2008. Luminescence Dating: Guidelines in Using Luminescence Dating in Archaeology. Swindon: English Heritage.

The DRILL is equipped to conduct all current techniques for luminescence sample preparation and analysis for geological, geomorphological, and archeological materials, primarily targeting quartz and/or feldspar.  The most commonly used approaches in luminescence dating are listed below. If you are interested in a preparation or analytical approach that is not listed, or if you have questions, please contact us. Please note that the DRILL does not accept samples without prior arrangement with the lab director, Amanda Keen-Zebert.  You may contact her at akz@dri.edu.  

Multi grain discs

Multi-grain aliquot discs on the carousel that holds the discs within the Riso TL/OSL Reader

Preparation:

  • Fine grained sediments
  • Coarse grained sediments
  • Artifacts
  • Solid objects, for example, rock. 

Analysis:

  • Optically Stimulated Luminescence (OSL)
  • Infrared Stimulated Luminescence (IRSL)
  • Multigrain
  • Single grain
  • Pulsed-diode
  • Linearly modulation
  • Thermoluminescence (TL)
  • Dose rate measurement

The configuration of TL/OSL readers varies by maker, but typically consist of a photon detector, usually a photomultiplier tube, fitted with detection filters; an irradiator or radiation source, a heater plate, and a light stimulation source fitted with emission filters, for example blue LEDs for multigrain quartz stimulation, Infrared LEDs for multigrain feldspar, and lasers for single grains.      

Analytical equipment and capabilities at the DRILL

The DRILL operates in a dark-room facility equipped to conduct all necessary sample preparations including coarse and fine grained quartz or feldspar separations of sediments or solid objects. Analytical equipment to conduct luminescence measurements includes:

  • Two OSL/IRSL  DA-20 Risø TL/OSL Readers both equipped with the latest capabilities including green and IR laser single-grain dating attachments, pulsed-diode and linearly modulated addition capabilities for both blue and IR optical stimulation, and automated beta (Sr-90) and mini X-Ray Varian VF-50JWS (max. 50 kV, 1 mA) irradiation attachments. 
  • One Multi- or single-aliquot OSL/IRSL Daybreak 2200 Reader with an embedded irradiation attachment and a beta (Sr-90) source.
  • One multi-aliquot TL and IRSL Daybreak 1150 Reader.

Supporting equipment includes two automated stand-alone evacuable alpha (Am-241) and beta (Sr-90) irradiators. 

Dose rates are measured by thick source alpha counting with five operational Daybreak 582 alpha counters for U and Th content, and K content is determined through ICP-AES at external laboratories.

Sample Preparation
Sample preparation involves removing carbonates and organic material from the sample and isolating the grain size and mineral of choice (typically quartz or feldspar). For coarse grain analysis, hydrofluoric acid is used to etch the outer surface of the grains that is affected by alpha radiation. Single- or multi-grain aliquots are then mounted on 9 mm stainless steel or Al discs that are placed in a carousel that holds the sample within the luminescence reader.

Measurement of equivalent dose (De)
Because there is no systematic relationship between luminescence brightness and radiation dose, the luminescence response of each sample is calibrated through a set of laboratory measurements that are used to derive a measure of De. Although there are many approaches to De determination (see summaries by Duller (2004) and Lian and Roberts 2006), the single aliquot regenerative dose (SAR) procedure (Murray and Wintle, 2000) is now the most widely used owing to its accuracy and broad applicability to quartz, feldspar, and polymineral fine grain OSL, and IRSL. Replicate measurements of De are made for each sample and then a variety of statistical models (e.g. Galbraith and Green, 1990; Galbraith et al, 1999; Galbraith 2005) are applied to the population of De to estimate the total De (sometimes ‘Db’) used in age calculation.

SAR procedure
Above: The SAR protocol uses a regeneration approach to sample calibration whereby the natural signal (Ln) is measured and reset, then given a known laboratory dose by exposure to an artificial source of radioactivity regenerating the luminescence signal; the regenerated signal is then measured (L1, 2, 3,…). This procedure is repeated through a series of cycles of heating, light exposure and signal measurement, and irradiation at different regeneration doses. Because the sensitivity of an aliquot (the amount of light emitted per unit of radiation exposure) can change over the course of the measurement, it is monitored by including a small fixed radiation dose or ‘test dose’ and measuring the OSL signal (T1, 2, 3…) during the second half of each SAR cycle. The sensitivity corrected measurements of luminescence signals (Lx/Tx) can be used to construct a dose response curve from which to estimate De. Adapted from Duller, 2008.

Measurement of dose rate (Dr)
Luminescence age calculation requires accurate assessment of both the equivalent dose and the dose rate (the radiation dose received per year by a sample). The annual radiation dose can be partitioned into the cosmogenic dose rate and the sediment dose rate. The contribution to the total dose rate from cosmogenic radiation is small and is typically estimated following equations of Prescott and Stephan (1982) and Prescott and Hutton (1994) which use latitude and sample depth in the calculations. The sediment dose rate comes from low level, ambient, ionizing radiation associated with 40K, 87Rb (to a very small degree), and the radioactive isotopes in the decay series of 238U, 235U, and 232Th both in the sample and in the material surrounding it. Decay of these long lived radionuclides results in the emission of alpha particles (α), beta particles (β), and gamma rays (γ). Alpha particles are relatively large and travel only 0.03 mm or less from their emitting nucleus. Beta particles and gamma rays travel a few millimeters to up to 0.3 m respectively.

Environmental sources of radiation
Above: Environmental sources of radiation showing the travel distances of alpha and beta particles and gamma rays and an extinct giant wombat for scale. Adapted from Duller, 2008.

Measuring the dose rate from a small sub-sample will determine the gamma dose accurately only if the material within 0.3 m of the sample is homogeneous. In many natural settings this is not the case, for example, the unit of interest could be very small or poorly sorted and made up of mixed sized sediments. In these situations, in situ measurements of gamma dose are important in determining the dose rate. Another important consideration in the assessment of dose rate is whether the system is in secular equilibrium. Secular equilibrium in the decay series of U and Th is often assumed but is not a valid assumption in all environments especially where the parent material is un-weathered (Olley et al., 1996).

References

Duller, G.A.T., 2004. Luminescence dating of Quaternary sediments: recent advances. Journal of Quaternary Science 19, 183-192.

Duller GAT. 2008. Luminescence Dating: Guidelines in Using Luminescence Dating in Archaeology. Swindon: English Heritage.

Galbraith, R.F., 2005. Statistics for Fission Track Analysis. Interdisciplinary Statistics, Chapman and Hall/CRC.

Galbraith, R.F., Green, P.F., 1990. Estimating the component ages in a finite mixture. Nuclear Tracks and Radiation Measurements 17, 197-206.

Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H. and Olley, J.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium Rock Shelter, Northern Australia: Part I, Experimental design and statistical models, Archaeometry 41: 339–364.

Lian, O.B., Roberts, R.G., 2006. Dating the Quaternary: progress in luminescence dating of sediments. Quaternary Science Reviews 25, 2449-2468.

Murray, A.S., Wintle, A.G. 2000. Luminescence dating of quartz using an improved single aliquot regenerative dose protocol. Radiation Measurements 32: 57–73.

Olley, J.M., Murray, A.S., Roberts, R.G. 1996. The effects of disequilibria in the uranium and thorium decay chains on burial dose rates in fluvial sediments. Quaternary Science Reviews 15: 751-60.

Prescott JR, Stephan LG. 1982. The contribution of cosmic radiation to the environmental dose for thermoluminescent dating. Latitude, altitude and depth dependences. PACT J. (Council of Europe) 6:17–25. (Not available online).

Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term variations. Radiation Measurements 23, 497-500.

Applications, Advantages, Age Limits, and Accuracy

Wells Creek

Vertical sequence of OSL samples in a fill terrace within Wells Creek in the Buffalo National River watershed in northern Arkansas.

Applications
Luminescence dating techniques are applicable to a wide range of geological, geomorphological, paleoenvironmental, paleoseismological, and archaeological problems. Quartz and feldspar are the primary minerals that are used but other silicate minerals are known to produce a luminescence signal. In the case of sediments, the last exposure to light is dated; in the case of pottery or burnt stones, the last exposure to heat is dated.

Advantages
Luminescence has several advantages including that the minerals making up the sediments themselves are dated rather than some material within the sediment such as organic material or volcanic ash that may not be present within every sample of interest or may be reworked. Unlike radiocarbon, calibration is not necessary.  The technique is reliable for dating deposition over a large range from decades to ~200,000 years or more.

Age Limits
The age limit of luminescence generally ranges from years to hundreds of thousand years. The lower age limit is restricted by the efficacy of signal resetting, signal sensitivity, and thermal transfer components (signals generated by heating during analysis). The upper age limit is controlled by the capacity of the crystal lattice to store electrons (the number and nature of traps) and the dose rate of the environment. Dose saturation refers to the complete filling of traps such that continued exposure to emissions from radiation decay results in no more accumulation of electrons and thus no increase in luminescence signal. The properties of minerals that control signal sensitivity and dose saturation (as well as other luminescence characteristics) vary even within minerals of a single composition and the dose rate varies in different environments.

Accuracy and Precision
Luminescence typically has good agreement with age comparisons to samples of known age and to ages derived from independent methods (e.g. Barnett, 2000; Bailiff, 2007; Murray and Olley, 2002; Rhodes et al., 2003; Rittenour, 2008). Combined uncertainty in the measurement of De and of the dose rate typically ranges from 5-10% including random and systematic error. Age estimates are typically reported as a central value with one standard deviation (68% confidence interval) uncertainty in years before the measurement. 

References

Bailiff, I. K. ,2007. Methodological developments in the luminescence dating of brick from English late-medieval and post-medieval buildings. Archaeometry 49, 827-851.

Barnett, S.M., 2000. Luminescence dating of pottery from later prehistoric Britain. Archaeometry 42, 431-457.

Murray AS, Olley JM. 2002. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometria 21:1–16. (Links to PDF.)

Rhodes E.J., Bronk-Ramsey C., Outram Z., Batt C., Willis L. 2003. Bayesian methods applied to the interpretation of multiple OSL dates: high precision sediment age estimates from Old Scatness Broch excavations, Shetland Isles. Quaternary Science Reviews 22: 1231–44.

Rittenour, T. M., 2008. Luminescence dating of fluvial deposits: applications to geomorphic, palaeoseismic, and archaeological research. Boreas 37: 613-635.

Amanda Keen-Zebert, Ph.D. – Director of DRI E.L. Cord Luminescence Laboratory
Division of Earth and Ecosystem Sciences
Desert Research Institute, Reno, NV
Phone: 775-673-7434
Email: AKZ@dri.edu

Christina M. Neudorf, Ph.D.
Research Scientist and Luminescence Laboratory Manager
Division of Earth and Ecosystem Sciences
Desert Research Institute, Reno, NV
Phone: 775-673-7407
Fax: 775-673-7485
E-mail: Christina.Neudorf@dri.edu

Kathleen D. Rodrigues
Graduate Research Assistant (PhD)
Division of Earth and Ecosystem Sciences
Desert Research Institute, Reno, NV
Email: Kathleen.Rodrigues@dri.edu

CONTACT

Amanda Keen-Zebert, Ph.D.
Lab Director
Amanda.Keen-Zebert@dri.edu
775.673.7434

LAB LOCATION

Desert Research Institute
2215 Raggio Parkway
Reno, NV 89512

DIVISION

Earth and Ecosystem Sciences