γand X rays from the zodiacal dust

Abstract

γ-RAY lines can be produced by MeV protons in interactions with interstellar1–4 and solar coronal matter5–7. Here I show that, in favourable conditions, solar flare protons can produce both observable γ-ray lines and characteristic X-ray lines, when interacting with the material of the zodiacal dust cloud. Of particular interest are narrow γ-ray lines without kinematical broadening (ΔE <1 keV), which are produced by long-lived nuclear excited states (τ ≳; 10−12 s) in solids. A typical zodiacal dust grain is sufficiently large for such narrow line production, and modern germanium detectors can detect and identify these lines. Observation of X rays and γ rays together would yield information on the average interplanetary dust density, grain size, material density, and chemical composition. This might help to determine whether the dust is of Solar System (for example, cometary) or of interstellar origin. Protons ≳ 10 MeV form a probe for which the dust grains are transparent, so that the measured quantity does not depend on grain size and surface properties, in contrast to the zodiacal light.