A new technique is presented for measuring the half-lives of resonances in the picosecond range using presently available invariant-mass apparatus and a thick target. For short-lived resonances, the energy losses of the decay fragments in the target material change their relative velocities, giving rise to a poor invariant-mass resolution. However, for longer-lived resonances that travel through the remaining target material before decaying outside of the target, this dominant contribution to the resolution is eliminated. The measured invariant-mass peak will then display a narrow peak from these events which sits on a wider structure associated with the remaining events that decay in the target. The fraction of the events in the narrow peak is related to the probability that the resonance leaves the target before decaying and hence is related to the lifetime. This technique is exercised for the 2.685-MeV resonance in ${}^{33}\mathrm{Cl}$ which decays by emitting an $f$-wave proton. A half-life of $T_{1/2}=5.7_{-2.5}^{+4.1}$ ps was measured, which is consistent with values inferred from other known properties of this resonance.

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• Lifetimes & widths
• Nuclear structure & decays
• Unstable nuclei induced nuclear reactions
• 20 ≤ A ≤ 38
• Radioactive beams
• Spectrometers & spectroscopic techniques