  We have systematically investigated the energy resolution of a magnetic
micro-calorimeter (MMC) for atomic and molecular projectiles at impact energies
ranging from $E\approx13$ to 150 keV. For atoms we obtained absolute energy
resolutions down to $\Delta E \approx 120$ eV and relative energy resolutions
down to $\Delta E/E\approx10^{-3}$. We also studied in detail the MMC
energy-response function to molecular projectiles of up to mass 56 u. We have
demonstrated the capability of identifying neutral fragmentation products of
these molecules by calorimetric mass spectrometry. We have modeled the MMC
energy-response function for molecular projectiles and conclude that
backscattering is the dominant source of the energy spread at the impact
energies investigated. We have successfully demonstrated the use of a detector
absorber coating to suppress such spreads. We briefly outline the use of MMC
detectors in experiments on gas-phase collision reactions with neutral
products. Our findings are of general interest for mass spectrometric
techniques, particularly for those desiring to make neutral-particle mass
measurements.
