The (U-Th)/He dating method rests on a straightforward nuclear clock. Uranium-238, uranium-235, thorium-232, and samarium-147 decay by emitting alpha particles, and each alpha particle is, in essence, a helium-4 nucleus. Over geological time, these nuclei accumulate in minerals such as apatite and zircon. Determining a date requires measuring both the accumulated radiogenic helium and the remaining parent isotopes. The parent isotopes are routinely quantified by inductively coupled plasma mass spectrometry, but that technique cannot touch noble gases. Helium, therefore, must be measured separately on a gas-source mass spectrometer attached to a high-vacuum extraction line, and the quality of that helium measurement directly controls the quality of the resulting date.

Commercial helium analysis lines exist and serve many laboratories well, but the Colorado team faced a familiar research dilemma. Their existing commercial instrument, an Alphachron, had reliably produced whole-grain helium data for more than a decade, yet its proprietary control software made it poorly suited for developing brand-new protocols, and its busy schedule left no machine time for the experimental iterations that laser-ablation work demands. Closed systems also complicate mundane realities: replacing failed hardware, updating software and operating systems to satisfy institutional security requirements, and adapting analytical routines as methods evolve. Rather than fight those constraints, the researchers decided in 2018 to build their own line from scratch, and to document the process in unusual detail because, as they note, published guidance on constructing a helium line is remarkably scarce, and institutional knowledge of this kind tends to vanish when experienced scientists retire.

The central design philosophy of the Jimbochron is ruthless volume minimization. When a laser ablates a minute pit in a crystal, it liberates roughly a femtomole of helium, and that gas expands into whatever volume is available. A smaller expansion volume means a higher helium concentration, a larger signal at the mass spectrometer, and consequently more stable and more precise measurements. Smaller volumes also evacuate faster, driving down the background helium blanks that would otherwise swamp tiny samples. To squeeze the internal volume down, the team arranged pneumatic valves around two hexagonal vacuum manifolds, a configuration that packs many valves closely together with minimal stainless-steel tubing while leaving spare ports for future additions such as a diode laser, extra pumps, or additional getters. The frame itself is built from T-slotted aluminum bars, chosen because it is inexpensive, easy to cut and tap in-house, light enough to mount on wheels, and directly compatible with the mounting holes on the Swagelok pneumatic valve bodies.

Measuring femtomole helium also demands an uncompromising vacuum. Although helium makes up only about five parts per million of dry air, the helium in a mere hundredth of a microliter of air rivals the radiogenic helium in a typical apatite crystal, so atmospheric contamination must be virtually eliminated. The Jimbochron is conceptually split into a process side, where samples are loaded and which is regularly exposed to atmosphere, and an analysis side, which houses the mass spectrometer and almost never sees unconditioned gas. A dry scroll pump backs a turbo pump on the process side, while an ion pump maintains the analysis side, where pressures typically sit below five times ten to the minus ten torr. Baking the entire line at 200 degrees Celsius for roughly 48 hours drives adsorbed gases from the metal surfaces, and all components are rated for bakeout temperatures of at least 225 degrees. Vibrations from the scroll and turbo pumps, which travel readily through rigid aluminum framing, are isolated by mounting the pumps on a separate wheeled cart and connecting them only through flexible bellows.

Gas handling follows a three-stage logic of liberation, conditioning, and analysis. A short-wavelength excimer laser operating at 193 nanometers ablates small regions of a grain, shattering the crystal lattice and releasing trapped helium in situ; a long-wavelength diode laser will later be added for conventional whole-grain heating and continuous ramped-heating experiments. The liberated gas is then cleaned by getters, devices packed with reactive zirconium-based compounds that sequester every non-noble gas, and simultaneously spiked with a known aliquot of helium-3. Finally, the mixed helium-3 and helium-4 charge is analyzed in static mode by a high-sensitivity Hiden 3F quadrupole mass spectrometer, chosen because manufacturer data indicate it is roughly an order of magnitude more sensitive than the quadrupoles common in (U-Th)/He laboratories, and because its triple-quadrupole filters resolve the helium peaks more effectively. A second, less sensitive residual gas analyzer with a wider mass range is reserved for future ramped-heating work that would expose a spectrometer to large gas loads.

Automation was treated as a first-class design goal rather than an afterthought. Custom LabView software, written as modular subroutines and shared through an online repository, controls valves, pumps, gauges, and the laser through a network of USB and serial hubs. Before every measurement the software checks the health of the system, confirming that the mass spectrometer filament is alive, that pressures are below thresholds, and that helium-3 and helium-4 signals sit at proper blank levels, adjusting in real time to changing conditions. Every action, manual or automated, is logged and time-stamped for quality control, and analytical sequences are loaded as text files, which permits runs of more than a hundred analyses, including automated helium mapping of individual zircon crystals, without the human error of manual re-entry. The design even fails safely: valves default closed, and an isolation valve on the scroll pump inlet snaps shut during power loss, preventing a rush of air into the turbo pump, while an inexpensive uninterruptible power supply and a backup compressed-air reservoir keep the line running through most outages.

Perhaps the most scientifically interesting part of the project is its calibration from first principles. Mass spectrometers measure ion-beam sizes, not moles, so converting signals into absolute helium amounts requires knowing exactly how much gas each spike aliquot delivers. The team built three ten-liter reservoir tanks feeding roughly 0.2-cubic-centimeter pipettes, and determined their volumes through a chain of metrological reasoning. First, the volumes of ultra-pure aluminum rods and silicon cubes were computed from repeated weighings on a calibrated balance and literature densities known to parts per million. Those standard objects were then used in gas-expansion experiments, monitored by an absolute capacitance manometer and corrected for thermal transpiration, to calibrate a known reference volume. That known volume, in turn, calibrated the pipettes and tanks via Boyle’s law, with uncertainties propagated at every step. Because each helium-3 shot slightly depletes its reservoir, the team also tracks tank depletion directly with a dedicated correction tank, preventing small errors from compounding over the roughly five thousand measurements a typical year brings.

The payoff is already evident. The Jimbochron now routinely measures helium amounts below 0.1 femtomoles accurately and precisely, and analyses of secondary standards such as Durango fluorapatite and Juina zircon confirm its performance across the 0.1-to-1000-femtomole range typical of laser-ablation work. Full laser-ablation (U-Th)/He dates agree with expected values, and automated sequences have produced helium abundance maps within single zircon crystals. That sensitivity translates into smaller ablation pits, finer spatial resolution, smaller grains, and higher-precision dates on younger samples. Remarkably, the total component cost came in under 160,000 US dollars, a fraction of what closed commercial systems can cost, and the open documentation, from parts suppliers to archived code, offers a template other laboratories can follow. As emerging techniques like laser-ablation helium chronology mature, instruments built to be understood, modified, and shared may prove just as important as the discoveries they enable.

Subject of Research: Design, automation, and first-principles calibration of a low-volume helium measurement line for (U-Th)/He thermochronology

Article Title: Technical note: Design, construction, automation, and first-principles calibration of a low volume He measurement line

Article References: Metcalf, J. R., & Flowers, R. M. (2026). Technical note: Design, construction, automation, and first-principles calibration of a low volume He measurement line. Geochronology, 8(3), 447-462. https://doi.org/10.5194/gchron-8-447-2026

Image Credits: AI Generated

DOI: 10.5194/gchron-8-447-2026

Keywords: helium dating, U-Th/He chronology, thermochronology, mass spectrometry, laser ablation, vacuum technology, geochronology, isotope dilution, instrumentation, metrology, apatite, zircon