Section 7.3.3.2. Detection via solid converter and gas ionization: the foil detector

This mode of detection is generally used for monitors. In a typical design, a ¹⁰B deposit of controlled thickness, for example t = 0.04 µm giving a capture efficiency of 10⁻³ at λ = 1 Å, is made on a thin aluminium plate (see Fig. 7.3.3.2 ). One of the two particles (α, Li) produced in the solid by the capture reaction is absorbed by the plate; the other escapes and ionizes the gas. The electrons produced are collected by the aluminium plate, itself acting as the anode, or by a separate anode wire, allowing the use of the proportional mode. The detection efficiency is proportional to the deposit thickness t, but t must be kept less than the average range r of the secondary particles in the deposit (for ¹⁰B, r_α = 3.8 µm and r_Li = 1.7 µm), which limits the efficiency to a maximum value of 3–4% for λ = 1 Å. The fraction of the secondary particle energy that is lost in the deposit reduces the detector current, i.e. the signal-to-noise ratio, and worsens the amplitude spectrum (see Section 7.3.4).

Figure 7.3.3.2| top | pdf | Typical design of a 10B-foil detector.