We use time-resolved thermometry to monitor the decay of nonequilibrium quasiparticles in superconducting Al in the temperature range from $0.3$ to $1.2\mathrm{K}$. The quasiparticle lifetime at higher temperatures ($T>0.7\mathrm{K}$) agrees well with the calculated energy flow from electrons to phonons, but at lower temperatures it is significantly shorter than the theory predicts. We show well-defined internal equilibrium of quasiparticle system in the studied thermal transients, which implicates that quasiparticle-quasiparticle relaxation is much faster than electron-phonon interaction.

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• Heat transfer
• Quantum thermodynamics
• Low-temperature superconductors
• Nanowires
• Superconducting devices
• Weak links
• Thermometry