= Solution
The principal passive-material requirement is a short <radiation length> in centimetres, normally obtained using a dense, high-atomic-number <Sampling calorimeter>[absorber] such as lead or tungsten. It produces and contains an <electromagnetic shower> in a compact detector. Mechanical construction, transverse spread, cost, <Radiation damage>[radiation tolerance] and inactive gaps also affect the choice, but the <Sampling calorimeter>[absorber] is selected for <Electromagnetic shower>[shower] formation rather than signal production.
<Argon>[Liquid-argon] gaps are a suitable active sampling medium. <Ionization> <electrons> drift in an <electric field> and induce a measurable charge signal. Advantages include <Radiation damage>[radiation tolerance], stable response and useful spatial segmentation. Disadvantages include cryogenic infrastructure, high-voltage and purity requirements, signal collection times and electronic-noise control. Only the active fraction is measured directly, so fluctuations of <energy> shared between <Sampling calorimeter>[absorber] and active gaps introduce a sampling contribution to the resolution. A high active fraction and fine sampling help, at the price of construction complexity and additional channels.
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