= Solution
The <radiation length> $X_0$ is the characteristic length for high-<energy> <electron> <energy> loss by <bremsstrahlung>; smaller $X_0$ means \b[more] radiative loss for the same traversed thickness. To limit that loss, minimize $x/X_0$, choosing less material or a larger $X_0$, rather than minimizing $X_0$ itself. At reactor <Positron> <energies> of a few MeV, ionization and subsequent annihilation matter more than high-<energy> <bremsstrahlung>. A sufficiently large <scintillation counter> measures the deposited <energy>, so containment of the <Positron> and annihilation <photons>, light yield and calibration are the relevant design issues.<Review of Particle Physics by Particle Data Group (2022)>[Sections 34.4.1–34.4.4: collision losses, radiation length, bremsstrahlung and critical energy]{ref}
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