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Channel electron multiplier (CEM) and microchannel plate (MCP) detectors are routinely used in space instrumentation for measurement of space plasmas. Our goal is to understand the relative sensitivities of these detectors to penetrating radiation in space, which can generate background counts and shorten detector lifetime. We use $662~\hbox{keV} \, \gamma $ -rays as a proxy for penetrating radiation such as $\gamma $ -rays, cosmic rays, and high-energy electrons and protons that are ubiquitous in the space environment. We find that MCP detectors are $\sim 20$ times more sensitive to $662~\hbox{keV} \, \gamma $ -rays than CEM detectors. This is attributed to the larger total area of multiplication channels in an MCP detector that is sensitive to electronic excitation and ionization resulting from the interaction of penetrating radiation with the detector material. In contrast to the CEM detector, whose quantum efficiency ${\varepsilon _\gamma} $ for $662~\hbox{keV} \, \gamma $ -rays is found to be 0.00175 and largely independent of detector bias, the quantum efficiency of the MCP detector is strongly dependent on the detector bias, with a power law index of 5.5. Background counts in MCP detectors from penetrating radiation can be reduced using MCP geometries with higher pitch and smaller channel diameter.

We measure the response of silicon photodiodes to irradiation by H/sup +/, He/sup +/, C/sup +/, N/sup +/, O/sup +/, Ne/sup +/, and Ar/sup +/ ions with energies up to 60 keV. The unique properties of these photodiodes, including an ultrathin SiO/sub 2/ dead layer and 100% internal carrier collection efficiency, allow direct measurement of the total energy lost to nuclear (nonionizing) and electronic (ionizing) energy loss processes, which are important for quantifying effects such as damage and charge deposition. When plotted as a function of E/mZ/sup 1/2/, where E, m, and Z are the incident ion energy, mass, and atomic number, respectively, the responsivity is found to follow a single curve that represents all ion species and energies used in this study. This enables rapid, accurate estimation of damage and charge deposition by an ion as a function of penetration depth in silicon. A comparison of the measurements with the stopping and range of ions in matter (SRIM) Monte Carlo simulation code shows that SRIM significantly overestimates the fraction of the incident energy lost to electronic stopping processes for E/mZ/sup 1/2/

We measure the change in the response of 100% internal carrier collection efficiency silicon photodiodes having 60 /spl Aring/ SiO/sub 2/ passivation layers due to the damage induced by bombardment with 10-60 keV ions of H, He, N, Ne, and Ar. We find an initially exponential decrease in responsivity with increasing ion fluence /spl Phi/ and use this to define a damage constant /spl beta/. The correlation of /spl beta/ with the nuclear stopping power of the incident ion instead of with the total energy lost to nuclear stopping indicates that damage in a channel lying within the n-type silicon near the Si-SiO/sub 2/ interface dominates the radiation-induced change in the photodiode response. We use a fluid model of electron transport in the channel to derive a universal curve to describe the damage as a function of ion fluence and to show that the damage constant /spl beta/ is proportional to the damage cross section. Over the energy range of this study, damage cross sections of N/sup +/, Ne/sup +/, and Ar/sup +/ are 10-100 times that of He/sup +/, and /spl sim/1000 times that of H.

Electron irradiation of 100% internal quantum efficiency silicon photodiodes having a thin (60 /spl Aring/) SiO/sub 2/ dead layer results in measured responsivities ranging from 0.056 A/W at an incident electron energy E/sub 0/=0.2 keV to 0.24 A/W at E/sub 0/=40 keV. By comparing the data to a Monte Carlo simulation of electron interactions with the photodiode over an energy range of 1-40 keV, we derive an average electron-hole pair creation energy of 3.71 eV, in close agreement with other studies. Analysis of electron energy lost to processes that do not contribute to electron-hole pair creation shows that the energy lost in the SiO/sub 2/ dead layer is dominant for E/sub 0/ 1.5 keV. At E/sub 0/=300 eV, the Monte Carlo simulation results show that the electron projected range is significantly less than the dead layer thickness even though the measured response is 0.082 A/W, indicating that electron-hole pairs generated in the oxide dead layer are collected by the junction.