Network measurements of natural and artificial targets Print E-mail

Network measurements of natural and artificial targets and TCN-cross calibrations as a basis for accurate scaling factors for all routinely used TCN In order to derive accurate scaling factors for TCNs (3He, 10Be, 21Ne, 26Al, 36Cl) we will use network measurements on natural and artificial targets and a series of cross calibration sites.

The network measurements will deliver a scaling grid for 3He, 21Ne and 36Cl. At cross calibration sites 10Be and 26Al will be linked to this grid. We expect that in the course of the project 10Be measurements on minerals other than quartz will become feasible for routine analysis (e.g. olivine and pyroxene see Development of chemical routines, utilisation of additional minerals ) such that a scaling grid for 10Be will be obtained from the same material. Both natural and artificial target networks will provide point information in the form of a grid. Advanced numerical modelling will be used to obtain scaling factors that allow accurate interpolation between these grid points.

Network of natural targets:

This network will consist of basalt samples from independently dated lava flow tops. Targets of this grid are basalt (for 36Cl) and its phenocrystic/xenocrystic mineral phases (olivine and pyroxene for 3He and 21Ne). The rationale behind using basalt flows for this network is the availability of preserved flow tops at a wide altitude and latitude range which is generally not available for other types of rock surfaces that are suitable for calibration (e.g. moraines, glacial polish, landslides). The sites will span a base line close to sea-level that will be supplemented by several altitude transects to form a grid. In addition to the availability of independent age constraints, the preservation of characteristic flow top features indicating the absence of significant erosion is an important site selection criterion. Consequently, we will sample young lava flows (~10 kyr) on Iceland and the Azores , and flows of various ages (10-100 kyr) on the Canary Islands , Aeolian Islands , Cape Verde Islands and Ascension Island for the sea level base line. Flows that cover a large altitude range on the Azores, Canary Islands, Cape Verde Islands, Etna and East Africa will be used for altitude transects. Independent age constraints will be obtained from radiocarbon and Ar-Ar dating.

Deliverables of the natural target network are:

  1. a scaling grid for 3He, 21Ne and 36Cl and
  2. crucial information of changes of time-integrated production rates of these TCNs through the analysis of samples of different ages. A scaling grid for 10Be will be obtained from the material collected once the necessary chemical routines are developed (10Be in olivine, see C).
  3. Furthermore we will obtain a first order result will be the systematics of the 3He/36Cl-production ratio. This ratio is very sensitive to changes in the neutron energy spectrum (3He is produced by high energy neutrons >100 MeV and 36Cl by low energy neutrons <100 MeV) and will provide the first test of whether isotope-specific scaling factors are required. The scaling grid of natural samples will also be a first benchmark test for existing scaling factors.

Network of artificial targets:

We will expose artificial targets on a grid similar to that of the natural samples (sea-level baseline & altitude transects from sea level up to 4500m), focussing on the latitudes between 20-50? as both sea level production rates and attenuation coefficients for neutron flux in the atmosphere show the largest changes in this region. For optimum results the deployment of targets will be coordinated with the natural target network and neutron monitor experiments planned by the sister initiative CRONUS-Earth. Various target materials will be exposed. A pilot study with quartz sand has demonstrated its suitability for 3He and 21Ne analysis. We will use inclusion-free synthetic quartz, synthetic brucite (Mg(OH)2) and boehmite (Al(OH)3). The combination of these mineral targets with tritium-free water targets obtained from old groundwater will allow the determination of the production rates and ratios of 3He and 21Ne from the four (O, Mg, Si, Al; for 3He) and three most important target elements (Mg, Si, Al; for 21Ne), respectively. Exposure times will be between 1 and 2 years. Typical target size will be between 1 and 4 kg dependent on the altitude. Most samples will be analysed at the ETH-Zürich on a mass spectrometer equipped with a newly developed ion source (gases are compressed by a molecular pump into the ionisation volume) that has a hundred times larger sensitivity for He and Ne compared to conventional machines (Baur, 1999).

The deliverables of the artificial target network will be:

  1. a scaling grid for 3He and 21Ne for the most relevant target elements which will allow a
  2. cross-validation with the natural target network, and
  3. filling of gaps in the natural target network where the spacing between suitable sampling sites is large.