Abstract: Long-lived β-decay nuclides, such as ¹⁷⁶Lu, ⁸⁷Rb, and ¹³⁸La, serve as fundamental “radioactive clocks” in cosmochronology and geochronology, facilitating the dating of stellar nucleosynthesis events and the formation of planetary bodies. The reliability of these chronological studies hinges on the precise determination of the half-lives of these nuclides. However, their extremely low activity presents a significant challenge, requiring detection systems with both high efficiency and an ultra-low background. The design, construction, and validation of a prototype spectrometer developed specifically to address these challenges were detailed in this paper. The spectrometer incorporates two BGO scintillation modules, repurposed from the LAMBDA detection array. These modules were arranged in a compact stack, sandwiching a thin foil sample to ensure optimal geometric acceptance. This design achieves a nearly 4π solid-angle coverage, crucial for maximizing the detection efficiency of low-energy decay radiation. To suppress the background, a multi-layered shielding was implemented. A passive shield composed of 10 cm thick lead bricks, surrounds the detector setup to attenuate ambient gamma rays. An inner 5 cm thick copper lining was used to absorb the characteristic X-rays produced in the lead. Furthermore, an active veto system comprising a large plastic scintillator atop the lead shield was employed. This system operates in anti-coincidence with the BGO modules to effectively reject cosmic-ray muon-induced background events. To validate the spectrometer’s performance, a high-precision measurement of the half-life of ¹⁷⁶Lu was conducted using a natural lutetium foil sample. Data acquisition included a 50 h measurement with the sample and an extended 210 h background measurement. Analysis of the background spectrum revealed dominant contributions from intrinsic ²¹⁰Po α-decay within the BGO crystals, ⁴⁰K γ-decay, and X-rays from the lead shielding. The net ¹⁷⁶Lu decay spectrum was obtained by subtracting the time-normalized background, and the half-life of ¹⁷⁶Lu was determined to be (3.721±0.008)×10¹⁰ a. This result is in excellent agreement with recent high-precision literature values, strongly validating the spectrometer’s performance. The spectrometer achieved a minimum detectable activity of 19 mBq/g in the 30-1 200 keV energy range at a 99% confidence level. This work successfully demonstrates a reliable and effective solution for the precise measurement of micro-level radionuclides. Planned enhancements include replacing BGO crystals with scintillators offering lower intrinsic background and superior energy resolution (e.g., SrI₂), as well as refining detector geometry and shielding to push the boundaries of low-level counting.