ADAM – Angular Distribution of charged particles – Atmosphere Measurement

The interaction of cosmic rays with atmospheric molecues results in a particle shower consisting of a large number of various primary and secondary particles. The flux of primary and secondary particles depends strongly on the altitude and the angle of which particles are measured. \textsc{Pfotzer} found that the count rate reaches a maximum (today called Pfotzer-maximum) at approximately 20 km height. The angular distribution of energetic particles at this height is unknown, but of importance for, e.g., future hypersonic aircraft, but also as an analogon for the surface radiation on Mars. The atmospheric pressure at the surface of Mars is comparable to that at \geq 20 km height on earth. The goal of the ADAM team is to measure the angular dependence of the charged particle flux (with respect to the zenith angle) in the earth’s atmosphere above the Pfotzer-maximum.

The particle flux is known to be organised as  \cos{\theta}^2  at low altitudes and to be isotropic (i.e.,  \alpha \; \cos{\theta} ) in free space. Thus, we propose to measure this angular dependence at the relevant altitude of ~25 km, and to determine the power of the  \cos{\theta}  dependence. The particles will be detected by a sensor head composed of several solid-state detectors (SSDs). To determine the angular distribution, the sensor head has a well defined geometric arrangement of SSDs with sufficient angular resolution to match the expected environment. Once a particle passes through one of the SSDs it deposits energy in the semiconductor. The electronics of the experiment register, amplify, and digitize this event and registers the time it was detected. On their way through the sensor head the particles can pass through several SSDs.  Those particles which are detected by more than one detector (by a coincidence trigger) are faithfull tracers of the angular dependence of the charged radiation field in the upper atmosphere. Thus, our experiment called EVA allows us to determine accurately the distribution of incidence angles of charged particles above the Pfotzer-maximum.