Energy calibration of a cosmic ray ionization spectrometer

Abstract

The NASA/GSFC High Energy Cosmic Ray Experiment was calibrated at the AGS at Brookhaven National Laboratory during the summer of 1970 using protons and pions with energies from 9.3 GeV to 17.6 GeV. The best measure found for the energy E of an incoming primary particle is ΣI, the total number of ionizing particles observed in the instrument, summed over the various iron modules. The resolution in the calibration energy range is about ±30% (s.d.) over a wide range of incident angles and positions. The calibration function may be parameterized as E = ∑ I/ K, where K is predominantly a function of the location of the first interaction and the trajectory of the incoming particle. To a fair approximation the geometrical dependence of K can be encompassed by writing K as a function of d, the distance from the first interaction along the primary ray to the edge of the instrument. Empirically, K=5.83 [1−exp(− d/ λ)] cos θ, where λ is a characteristic length which is a slowly varying function of energy. The value of K, and thus the average energy values calculated from the experimental data are accurate to about ±10% under calibration conditions.