X-ray-Induced Secondary Electron Emission in Porous Materials

Electron emission induced by X-ray radiation in secondary-emission porous materials was investigated. © 2000 MAIK “Nauka/Interperiodica”. Secondary-emission porous materials are widely used as a working substance in electromagnetic and corpuscular radiation detectors [1–5]. The detection process in this case is governed by the interaction between primary radiation and a porous material, fastprimary-electron and slow-secondary-electron emissions, as well as by avalanche generation and transfer of secondary electrons. The use of secondary-emission materials in detecting X-ray radiation poses certain difficulties. First, the possibility of the X-quantum–porous medium interaction may be low; and second, X-ray-induced emission is ambiguously related to structure parameters (pore size, pore wall thickness, and pore shape), the chemical composition of the material, and the quantum energy [5–8]. Optimization of the detector performance (in particular, sensitivity improvement) implies the elaboration of a model for X-ray-induced electron emission in porous materials. In [6–8], electron emission in porous materials was considered for the case of a microchannel plate (MCP) to determine its sensitivity to X-ray radiation. However, the authors used simplified models and failed to discover the effect of the material structure and composition, as well as radiation energy, on the emission. In this work, we suggest a model of X-rays-induced secondary emission in porous materials with both channel-like and closed pores. The study of closed-pore materials seems to be topical in the context of the development of porous insulator technology [2] and a new type of secondary-emission material—microspherical plates [9]. Our model takes into account all structure and composition parameters of the material and possible quantum energies. The probability P of photon-induced electron emission in a porous body is given by (1) where P1 is the probability of quantum–body interaction and P2 is the probability that a photogenerated or P P1P2, = 1063-7842/00/4503$20.00 © 2 0339 Compton electron will escape from the wall of a pore into the free space. The probability P1 is found from the exponential expression [10] (2) where L is the sample thickness and is the mean attenuation coefficient of X-rays in the sample. With regard for porosity, (3) Here, = ρ(1 – V0 /V) is the mean density of the material, ρ is the density of the material, μ is the attenuation coefficient of X-rays in the material, V0 is the pore volume, and V is the total volume of the sample. It can be shown [11] that