Neptunium-239 (Np-239) is a short-lived radioactive isotope of neptunium with a half-life of about 2.36 days. It decays by beta-minus emission to plutonium-239. Its decay produces several characteristic gamma rays, with prominent lines at approximately 106, 228, and 278 keV, making Np-239 readily identifiable by gamma spectroscopy.
Np-239 is mainly used in nuclear research, particularly in studies of neutron capture, reactor physics, and the formation of transuranic elements. Historically, Np-239 was especially important as the first artificially produced neptunium isotope and as an intermediate in the production of Pu-239 from U-238.
Np-239 is produced when U-238 captures a neutron, forming U-239, which rapidly beta-decays to Np-239. It can therefore be found temporarily in irradiated uranium and nuclear reactor fuel. Because of its short half-life, Np-239 does not accumulate in significant quantities and quickly decays to Pu-239.
Np-239
Neptunium-239
Half-life: 2.36 days Main emission lines: 106, 228, 278 keV
Decay radiations
Decay mode β-
Beta-
| Avg En., keV | Intensity, % | Decay En., keV |
| 125.6 | 45 | 438 |
| 92.5 | 44 | 341 |
| 111.5 | 7 | 393 |
| 218.1 | 2 | 713 |
| 56.8 | 1.7 | (210.7) |
Gamma
| Energy, keV | Intensity, % |
| 106.123 | 25.34 |
| 277.599 | 14.51 |
| 228.183 | 10.73 |
| 209.753 | 3.363 |
| 334.310 | 2.056 |
| 315.880 | 1.600 |
| 61.460 | 1.30 |
| 285.460 | 0.794 |
| (227.83) | (0.51) |
| 226.38 | 0.259 |
X-rays
| Energy, keV | Intensity, % |
| 12.123 - 23.086 | 45 |
| 103.741 | 20.8 |
| 99.533 | 13.1 |
| 116.243 - 120.995 | 10.3 |
| 116.243 - 118.017 | 7.6 |
| 120.416 - 120.672 | 2.6 |