## Doctoral Dissertations

Spring 1989

Dissertation

Physics

#### Degree Name

Doctor of Philosophy

#### Abstract

This thesis deals with the measurement of atmospheric gamma ray flux in the 1-6 MeV range at ground level. These measurements were carried out using a Compton gamma ray telescope, developed at the University of New Hampshire. It utilizes the Compton scattering principle to detect and image gamma ray sources. The telescope was used to measure ground level atmospheric gamma rays at four locations (Leadville (10200 ft), Boulder (5430 ft), Mt. Washington (6072 ft) and Durham (80 ft)) which ranged in atmospheric depth from 720-1033 g/cm$\sp2$ and in local cutoff rigidity from 1.4-2.9 GV. Data was collected over a two week period at each location during 1987. The results yielded for the first time statistically atmospheric gamma ray flux values at large depths in the atmosphere.

The analysis provided differential energy flux (photon/cm$\sp2$-s-sr-MeV) at various zenith angles (10$\sp\circ$-40$\sp\circ$) in the 1-6 MeV energy range. The zenith angle dependence of the differential energy flux indicated a cos$\sp{\rm n}\theta$ dependence where n $\approx$ 2.8 at higher altitudes (Leadville and Mt. Washington) and n $\approx$ 2.0 deeper in the atmosphere (Boulder and Durham). The vertical intensity fitted a power law spectrum of index $\approx$ 1.2, with the spectrum softening at large atmospheric depths. The atmospheric depth dependence shows an e-folding depth of 153 g/cm$\sp2$. Using this depth dependence, all existing measurements below 700 g/cm$\sp2$ were normalized to sea level. Good agreement is seen among the normalized sea level flux corresponding to different experiments. Comparing experimental results with existing theoretical and Monte Carlo calculations in the 1-10 MeV range, the measurements indicate a softer power law spectrum, indicating the need to further examine the calculations. Combining UNH results with University of California (Riverside) measurements, indicate a weak rigidity dependence in the vertical atmospheric gamma ray intensity.

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