The Internal Contamination Monitor (ICM)

The Internal Contamination Monitor (ICM) is an ⁵⁵Fe source located on the inside of the ACIS door. Through electron capture, ⁵⁵Fe decays to ⁵⁵Mn through the channel:
⁵⁵₂₆Fe + e^-1 $\rightarrow$ ⁵⁵₂₅Mn$^{\ast}$ + $\nu.$The excited Mn atom may then relax to the ground state through fluorescence of a photon of characteristic energy: ⁵⁵₂₅Mn$^{\ast}$ $\rightarrow$ ⁵⁵₂₅Mn + $\gamma_{\alpha}$, { $\gamma_{\alpha}$: K_i, L_j, M_k, $\cdots$ }. It is (mostly) the Mn K and L shell photons that are responsible for the observed spectrum.

The main purpose of the ICM is to monitor the potential accumulation of contaminants on the surface of the CCDs. The most probable contaminants are ice or hydrocarbons, and due their large absorption coefficients at the L-line energies ($\sim$ 640 eV), even a relatively thin layer of contamination would result in the attenuation of the L photons. At the K-line energies ($\sim$ 6 keV), however, the photons have a much larger attenuation length and would not be absorbed by the contaminants, much the way that high energy photons pass through the CCD gate structure and insulating layers. As the fluorescent yield of K to L photons from the radioactive source is constant, an increase of the K:L ratio would indicate the the build-up of contamination.

The initial Mn K and L fluorescence lines and additional lines they excite produce a rich spectrum that allows an accurate gain calculation. Thus, the ICM also has served as a gain calibrator during pre-launch calibration efforts and allowed a check of long-term gain stability of those CCDs illuminated by the ICM. Below, we carefully discuss the spectrum of the ICM and attempt to identify all the features present. We also measure the K:L ratio from three different measurements spanning over a year.