Lanthanum-138 (La-138) is a naturally occurring radioactive isotope of lanthanum with an extremely long half-life of about 100 billion years. It decays through two pathways, producing stable barium-138 and cerium-138. Its decay is accompanied by characteristic gamma rays at approximately 789 and 1436 keV, making La-138 identifiable by gamma spectroscopy.
La-138 is mainly of interest in nuclear physics, geochemistry, and geochronology. The La-138 decay system can be used to study the formation and age of ancient rocks and minerals over geological timescales. It is also important in radiation detection because naturally occurring La-138 produces intrinsic background radiation in lanthanum-containing scintillation detectors such as LaBr₃.
Unlike many radioactive isotopes, La-138 occurs naturally. It makes up about 0.09% of natural lanthanum and can therefore be found in minerals and materials containing this element. It is especially noticeable in lanthanum-based scintillation crystals, where its characteristic gamma emissions can appear directly in measured spectra.
La-138
Lanthanum-138
Half-life: 100 billion years Main emission lines: 789, 1436 keV
Decay mode Alpha
Decay mode ec β+
Gamma
| Energy, keV | Intensity, % |
| 1435.795 | 65.5 |
X-rays
| Energy, keV | Intensity, % |
| 32.193 | 19.61 |
| 31.816 | 10.64 |
| 36.304 – 37.350 | 7.22 |
| 3.956 – 5.975 | 6.1 |
| 36.304 – 36.660 | 5.77 |
| 37.249 – 37.261 | 1.45 |
Decay mode ec β−
Beta−
| Avg En., keV | Intensity, % | Decay En., keV |
| 98 | 34.5 | 264 |
Gamma
| Energy, keV | Intensity, % |
| 788.742 | 34.5 |