Lunatic I Mass Spectrometer

The overall accession in the NMAH Modern Physics Collection consists of components of Lunatic I Mass Spectrometer (accession no. 2009.0133). In their original laboratory setting at the California Institute of Technology, the components were mounted on either of two adjoining metal frames: the spectrometer frame ( or "main bench") seen in the image foreground; and the magnet frame (or "magnet stand") seen in the image behind the main bench. The accession does not include the supporting electronic, computational, or control systems for Lunatic I.
Summary of the scientific contributions of the Lunatic I mass spectrometer by Gerald J. Wasserburg, Emeritus Professor of Geology and Geophysics, California Institute of Technology and co-inventor of the Lunatic I
The following summary (text edited for stylistic consistency and completeness) was provided by Prof. Wasserburg in response to a request from the NMAH curators. The summary is not dated, but is assumed to have been prepared in 2008, at the time that the curators were submitting documentation, including this summary, to the NMAH Collections Committee to justify acquisition of Lunatic I.
The Lunatic I mass spectrometer (LI) is the first fully digital mass spectrometer with computer-controlled magnetic field scanning and rapid switching. This system also had excellent ion optics. Data output is digital and processed on line (originally with an IBM 1800 computer). This system was patented (US Patent # 3,601,607; Inventors G.J. Wasserburg, C.A. Bauman, E.V. Nenow, and D.A. Papanastassiou) and described in Reviews of Scientific Instruments 1969, v. 40, issue 2, pp. 288-295 [authors same as above inventors]. It was developed and built at CalTech for the purpose of obtaining high precision isotopic measurements on the lunar samples to be obtained by the Apollo missions. The 2 sigma precision achieved was better than +/-6x10E-5 in the isotopic ratio of Sr-87/Sr-86 and other isotopic pairs. The precision obtained was about a factor 30 better than previously obtained for surface ionization systems and with a corresponding great increase in sensitivity. This led to a new unit of representing data, "the epsilon unit," which is the fractional deviations of isoptopic ratios relative to a standard in units of 10E-4. This data representation is widely used today. The Lunatic system then permitted measurement of very small isotopic differences on very small samples of many elements (typically limited by ion counting statistics). It served as the international standard of excellence for high precision isotopic measurements for extra-terrestrial and terrestrial materials for three decades. The performance was not exceeded until about the year 2000.
The LI, in combination with a new generation of pure ultra-low level blank chemistry and refined mineral separation techniques as developed in the Lunatic Asylum Laboratory at CalTech, was the main approach used in cosmochemical and planetary science studies of lunar and meteoritic materials and in terrestrial samples.
The LI was first used to determine the ages of basaltic achondrites (D.A. Papanastassiou and G.J. Wasserburg; 1969, Earth and Planetary Science Letters v. 5, pp. 361-376) and establish the primitive Sr in the early solar system. It was then used to determine the ages of a wide variety of lunar rocks from all of the Apollo missions and the Soviet Luna 16 and Luna 20 missions. NASA awarded a medal for the establishment of "an isotopic chronology for lunar events" (1972). These studies established the time scales for lunar evolution and, with the advanced Pb-U measurements, discovered the clear evidence of a late heavy bombardment of the Moon, the Earth, and the whole inner solar system (F. Tera, D.A. Papanastassiou and G.J. Wasserburg; 1974, Earth and Planetary Sciences v22, pp1-21). This late bombardment (the terminal lunar cataclysm) took place 500 million years after the formation of the Earth-Moon system due to special disturbances in the asteroidal belt.
The LI was used to measure the cosmic ray exposure ages of lunar rocks and soils by measuring the isotopic shifts in Gd, and [to determine] the age of deposition of a debris blanket on the Moon.
The LI mass spectrometer was used in the discovery of short lived Al-26 in the Allende meteorite which proved that there was a substantial level of some short-lived nuclei injected in the early Solar System by some stellar source (T. Lee, D.A. Papanastassiou, G.J. Wasserburg; 1976, Geophysical Research Letters, v3, pp109-112; 1977, Astrophysical Journal, v211, pp107-110). Further work on extinct short-lived nuclei using the LI led to the discovery of Pd-107 (W.R. Kelly and G.J. Wasserburg; 1978, Geophysical Research Letters, v5, pp1079-1082). Both Al-26 and Pd-107 are two key short-lived nuclei which play a major role in trying to understand the types of stars that contributed material to the early solar nebula. The LI was used to discover isotopic anomalies in a host of elements (Mg, Ca, Ti, Ba, Sm, Nd, Fe, Sr, Cr) which reflect incomplete mixing of different stellar sources in the solar nebula. These discoveries stimulated intense astrophysical intererest. Following on the work on monitoring neutron capture in extraterrestrial material, the techniques were extended to other rare earth elements, and then applied to terrestrial materials with emphasis on the Sm-147-Nd-143 systems. This has developed into a major research field with implications on the internal structure of the Earth, and the existence of large volumes of the interior that have been chemically separated and segregated for large parts of Earth's history (D. DePaolo and G.J. Wasserburg; 1976, Geophysical Research Letters, v. 3, pp. 743-746). This approach was extended to study the sources of rare earth elements in sea water, and as a tracer of sea water movement from the Atlantic to the Pacific [Ocean] (D.J. Piepgras and G.J. Wasserburg: 1982, Science v. 217, pp. 204-214). The concentration of Nd in sea water [is] ~10E-12 gram [of] Nd per gram of water, [and] with the LI it was possible to routinely obtain measurements [with] a precision of ~5x10E-5 for [the ratio] Nd-143/Nd-144.
The high sensitivity of the LI spectrometer was again applied to measuring time scales in the marine environment. Coral ages could be measured by mass spectrometry using the Th-230-U-234 system with ten times the precision that had been obtained before, [and] in a [measurement] time a factor of 1000 less than [using] earlier techniques. This has now become a major scientific enterprise and permits age determinations from ~50 years to 150,000 years with high precision, and is actively used in studies of sea level and climate change (R.L. Edwards et. al.; 1987, Science, v. 236, pp. 1547-1553).
By reversing the polarity on the LI spectrometer so that negative rather than positive ions could be detected, it was possible to measure Os and other platinum group elements with high precision and extremely high sensitivity by TIMS [thermal ionization mass spectrometry] (R.A. Creaser, D.A. Papanastassiou and G.J. Wasserburg; 1991, Geochimica et Cosmochimica Acta, v. 55, pp. 397-401). This then removed a major impediment in studying Re-Os chronology on meteorites and terrestrial materials, and opened the field to broad use.
The above list is a short summary of the scientific and technical accomplishments that were achieved with the Lunatic I mass spectrometer. [In later generations of the Lunatic system,] the basic ion optical system is unchanged, and only the computer control systems have been changed [to be compatible] with the [performance] improvements in computers.
Currently not on view
Object Name
Wasserburg mass spectrometer
Physical Description
aluminum (overall material)
stainless steel (overall material)
plastic (overall material)
bench overall: 50 1/4 in x 83 1/16 in x 34 3/8 in; 127.635 cm x 210.97875 cm x 87.3125 cm
ID Number
catalog number
accession number
Science & Scientific Instruments
See more items in
Medicine and Science: Modern Physics
Science & Mathematics
Measuring & Mapping
Modern Physics
Data Source
National Museum of American History, Kenneth E. Behring Center


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