The objective of the CRONUS-Earth Project is to simultaneously address the various uncertainties affecting the production and accumulation of in-situ cosmogenic nuclides, with the goal of producing internally consistent set of parameters that can be used in calculating exposure durations and erosion rates. PI: Fred Phillips

The interaction between cosmic rays and exposed target nuclei on Earth provides a means of extracting information about the last few million years of Earth history. The interaction produces isotopes (both stable and radioactive), some of which are only produced through cosmic ray interactions. The abundance of 'cosmogenic' isotopes in various environments can lead to interpretations about geological processes on or near Earth's surface. These interpretations can lead to answers to questions such as How fast is a mountain eroding? What is the earthquake recurrence interval along a particular fault? How fast did glaciers retreat during the last ice-age?

independent measurements of the cosmogenic production rates of many nuclides may vary considerably. Different geographical positions must be scaled for intercomparison Temporal variation of production rates must also be considered

The accumulation of cosmogenic nuclides such as 3 He, 10 Be, 14 C, 21 Ne, 26 Al, 36 Cl in mineral lattices of exposed rocks can be used for quantitative studies of surface exposure histories. Assuming a detailed knowledge of the rates of nuclide production induced by cosmic rays is understood.

Key question: are scaling factors derived from neutron monitors representative of cosmogenic nuclide production rates? With six decades of research using lead neutron monitors, these instruments provide by far the largest body of data on neutron fluxes. All published emprical scaling factor algorithms (e.g. Lifton, Dunai, Lal, Desilets and Zreda) utilize neutron monitor data to some extent or entirely. A wide range of atmospheric attenuation lengths have been published. Lead neutron monitors tend to give lower attenuation lengths than other methods. Reported atmospheric attenuation lengths at the equator range from 220 g cm -2 to 145 g cm -2. The experiments tend to be fairly precise, so why the spread?

Lead versus granite Darin Desilets et al constructed two geometrically similar NMs. To simulate cosmogenic nuclide production, we used granite instead of lead for the “producer” in one of the NMs. To our knowledge, this is the first ever rock neutron monitor. Note that inner diameters of the lead and granite rings are different. In reality, there is no way to maintain both an identical geometry and mass thickness due to the factor of ~4 difference in material density. Larger granite thickness was chosen to provide additional strength and compensate for the greater density of lead. Granite and lead neutron monitors being assembled in Tucson, AZ

Survey details The survey was conducted from January 20-26, 2010 on the big Island of Hawaii The NMs were equipped with identical pulse processing electronics, detector tubes and data loggers. The NMs were transported in separate but identical cube trucks.. Measurements were simultaneous and at the same locations. Counting times ranged from 20 minutes to 20 hours depending on site.

Survey details Typically 4,000 cts per site (1.5% counting error) for granite NM and 40,000 cts per site (0.5 % counting error) for lead NM. Neutron intensity also recorded with bare and moderated (2.5 cm LDPE) proportional counter tubes. Temperature, humidity and barometric pressure recorded independently in each truck.

40 km

Multiple detections can occur for single incident particle

Due to the photo-nuclear effect gamma rays and electrons have a rather large efficiency. The rise in the positive muons at low energy are due to production muon-decay electrons, however the rise for muon minus is due to K-capture of muon by a Pb nucleus. Calculated detection response for different particle species

Applying the multiplicity and photonuclear corrections to the NM scaling schemes significantly improves the results. However the results over predict the CN production spatial distributions by 15 to 20%. If the spatial scaling schemes incorporate a Monte Carlo calculated particle integral spectra, the consistency with geology observations is better than 5%. It is likely the source of problem is the difference in the energy dependence for n-Pb and n-Targets for producing CN This mystery is currently under investigation.