The primary goal of the CRONUS-Earth Project is to better characterize the various uncertainties affecting our ability to utilize observed in situ cosmogenic nuclides as a dating tool. To verify algorithms in models used to predict cosmic-ray fluxes (and hence, cosmogenic nuclide production rates) as a function of latitude and altitude, data from the global network of neutron monitor stations are used as a reference. Therefore, it is critical to determine an accurate response function relating the observed neutron monitor count rates and the cosmogenic nuclide production rate. One of the primary tasks of the University of Delaware group under this project is to determine an accurate detection efficiency of the neutron monitor to ground level cosmic ray air shower particles. During the two-year effort it has been determined that the photonuclear process contributes a significant number of counts in a neutron monitor from gamma rays and e+ and e-. Understanding this additional source of counts in neutron monitors may have significant implications for the altitude dependence of scaling factors. It is critical we continue to investigate the photonuclear Giant Dipole Resonance in lead (the target material in neutron monitor), so that this issue will not delay the progress of the CRONUS-Earth project. Based on our research to date, the count rate from the electromagnetic (EM) component in cosmic rays at 3.3 km (10,000 ft) is expected be roughly ~4 percent of the total. Significantly, the attenuation of the EM component through the atmosphere is considerably different than that of the hadronic component(which is the dominant source of in situ cosmogenic nuclide production). The primary goal of our proposed research is to evaluate this contribution to neutron monitor count rates as a function of altitude and geomagnetic cutoff rigidity. We also will continue investigating the ship environmental effect on the neutron-monitor latitude-survey data (on which cosmogenic nuclide scaling models are based), including angular effects and electronic dead-times. Neglecting these effects could also bias scaling models. The final results from this 2-year continuation of our CRONUS-Earth research will help to improve the current scaling frameworks, and a report of our findings will be published. The Broader Impacts of this proposal is educational opportunities for graduate students. Through the implementation a LINUX-based Monte Carlo simulation of particle and nuclear interactions in the atmosphere and a neutron monitor, this project will involve problem solving using basic principles in computing, mathematics and physics.
