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Nuclear Safeguards Education Portal

Delayed Gamma (DG)

Similar to the delayed neutron measurements, delayed gamma techniques utilize measurements of beta-delayed isotopes that are produced in fission events. Delayed gamma rays are emitted from particular fission products following beta-decays, and therefore have distinguishing time scales associated with emission.  The differences in the distribution of fission product yields can be used to quantify and distinguish each fissile element.  The DG system works by interrogating a spent fuel assembly with a neutron source, commonly a Deuterium-Tritium source compatible with most active interrogation techniques, and using a high-purity germanium (HPGe) detector to measure the delayed gamma peaks.  An example of a DG spectra is shown in Figure 28 for a depleted uranium sample (i.e. almost entirely 238U). 

Figure 28. Calculated (bottom) and measured (top) spectra of delayed-gamma from depleted uranium (

In order to isolate the relevant gamma peaks from the spectra, previous knowledge is needed of the energy of the delayed gammas specific to the material being investigated.  The isolation of the DG peaks can be a challenge due to interferences with passive gamma radiation from the spent fuel.  Current work is concentration on minimizing interferences, such as by better optimizing the detector setup.  Other efforts are focused on better characterizing the differences in the DG spectra for unique spent fuel compositions.


  1. V. Mozin, L. Campbell, A. Hunt, B. Ludewigt, "Delayed Gamma-Ray Spectroscopy for Spent Nuclear Fuel Assay," Journal of Nuclear Materials Management 40:3 (2012).
  2. V. Mozin, S. Tobin, L. Cambell, J. Cheatham, C. Freeman, C. Gesh, A. Hunt, B. Ludewigt, E. Reedy,  H. Selpel, L. Smith, J. Sterbentz, J. Vujic, J. White, P. Blanc, S. Croft, J. Conlin, L. Evans, M. Fensin, J. Hu, T. Lee, A. LaFleur, H. Menlove, M. Schear, M. Swinhoe, W. Koehler, J. Richard, N. Sandoval, and S. Thompson, "Determining Plutonium Mass in Spent Fuel with Non-destructive Assay Techniques - NGSI Research Overview and Update on NDA Techniques, Part II," IAEA Safeguards Symposium, IAEA-CN-184/137, 2010. See document.