Laboratory for dating by the luminescence method

OSL (Optically stimulated Luminescence) and post-IR IRSL (Infra Red stimulated Luminescence).

Currently, the dating laboratory is the only one in Italy run by geologists capable of performing dating on both quartz and feldspar grains. It is in contact with leading European luminescence dating laboratories (Risø-DK, Hannover-DE, Milano Bicocca-IT) with which it has been collaborating for years.

The lab has been fully active since November 2017, serving mainly as a support of research having "climate variations over the last 300,000 years" as its central theme. It involves researchers from the Department of Architecture, Design and Urban Planning, the Department of Chemistry and Pharmacy and the Department of History, Humanities and Education.

The recent need to define as accurately as possible the chronology of geological-environmental events that have occurred over the past thousands of years has prompted the scientific community to search for new absolute dating systems. Among these, the luminescence method is certainly the most recent and under continuous development. The method dates the time of deposition of Quaternary sedimentary rocks (last million years). These are the only ones that have recorded recent climate changes and, therefore, their dating is critical.

The laboratory consists of two parts: sample preparation and analysis.

During preparation, a rock or sediment sample (usually sandstone or sand, the minimum grain size that can be analyzed is 90 microns) is chemically treated (with acids) to extract only the constituent quartz and k-feldspar grains (about 20gr are needed to make an analysis).

The analysis part consists of 2 automatic Reader systems TL/IRSL/Blue OSL, Model Risø TL/OSL-DA-20C/D and a GM gas-flow multicounter unit (Beta counter). The two readers are equipped with two sealed radioactive sources 90Sr/90Y beta source, 1.48 GBq and 90Sr/90Y beta source, 2.96 GBq, respectively. The readers are used to stimulate both with ultra violet and infrared light the quartz and k-feldsapto samples and measure the Dose equivalent (De), which is the total luminescence accumulated in the traps. The Beta Counter is used to measure the Dose rate (Dr), or the amount of radiation that can be accumulated per day.

The grains to be analyzed are placed on steel discs and then on a carousel. The carousel rotating allows a photomultiplier to stimulate the samples. These stimulated emit luminescence that can be measured (natural signal=Lx). Once stimulated, the sample is irradiated by the radioactive source and retested for luminescence (regenerated signal=Tx). The Lx/Tx ratio is used to define the De.

The Dose rate (Dr) is measured by comparing the amount of radioactivity measured by relating it to that of known standards.

Methodology

Luminescence dating makes it possible to determine when mineral grains constituting a sedimentary rock (sandstone or sand) were last exposed to sunlight or significant heating.

Luminescence is a special type of phosphorescence observed in insulators and semiconductors, such as silicon, in which light emission following exposure to ionizing radiation is delayed due to the presence of traps associated with defects. A crystal is an agglomeration of equal atoms or molecules bound together and arranged in a regular manner along the three spatial dimensions. Any deviation from the perfect periodic structure constitutes an imperfection or defect that can behave as a trapping (trap) or recombination center (luminescent center). Minerals are a collection of crystals that can therefore have a number of traps, consequently it is possible to measure the luminescence stored in the various traps by stimulating it optically (OSL), with infrared (IRSL) and thermoluminescence (TL). Quartz and alkali feldspar (potassic=k-feldspar) major constituents of sands and sandstones and many volcanic rocks are used for luminescence dating.

The dating of a rock by luminescence is based on the assumption that the mineral grains that comprise it (quartz and k-feldspar) have been sufficiently "bleached," i.e., have been exposed to sunlight (for quartz, 1-100 seconds is enough) before they are buried and go on to form the sediment that will later give rise to the rock. Exposure to sunlight effectively resets the dating clock for luminescence by discharging the traps from the natural radioactivity in them.

All sediments and soils contain trace amounts of radioactive isotopes of elements such as Potassium, Uranium, Thorium and Rubidium. These decay slowly over time. The ionizing radiation they produce, along with that associated with cosmic rays, can be absorbed by minerals such as quartz and k-feldspar. The absorbed radiation remains within the grains in structurally unstable "electron traps." The trapped charge accumulates over time at a rate determined by the amount of background radiation present at the sample burial site. Stimulating these mineral grains with light (blue or green for OSL on quartz grains, infrared for IRSL on k-feldspar grains) or heat (for TL on ceramics) causes them to emit a luminescence signal, the intensity of which varies with the amount of radiation absorbed during burial and the specific properties of the mineral.

Measuring the total amount (Dose equivalent) and dividing it by the daily amount of radiation (Dose rate) that a rock sample can absorb gives the age of burial of that mineral, that is, the age of formation of that sediment/rock.

The OSL ages of quartz are generally between 100 and 350,000 years BP, those of k-feldspar can be as high as 1 Million years. The differences are due to the significantly higher saturation levels of k-feldspar than those of quartz. The uncertainty, or error, of an OSL age is typically 5-10%.

At the laboratory it is possible to:

• date any material (generally rocks or sediments) containing quartz and/or k-feldspar grains,
• measure the natural radioactivity contained in a sample (of any material, generally, however, rocks).

Dateable rock samples should be no older than 1 million years. The method works very well on sandstone/sandstone samples between 300,000 and 2,000 years old.

Sampling must be done with special techniques that prevent sunlight from contaminating the collected sample. It is recommended that laboratory personnel be consulted in this case.

The time to obtain an age varies greatly depending on the age of the sample itself. On average, it takes about two months.

Geological, Archaeological, Environmental-Landscape.

• OSLD dating
• post-IR IRSLC 
• calculation of De
• Preparation of sample