For over 40 years, physicists have considered possible uses for neutrino detectors in nuclear nonproliferation, arms control, and fissile materials security. Neutrinos are an attractive fission signature because they readily pass through matter. The same property makes neutrinos challenging to detect in systems that would be practical for nuclear security applications. This Colloquium presents a broad overview of several potential neutrino applications, including the near-field monitoring of known reactors, far-field monitoring of known or discovery of undeclared reactors, detection of reactor waste streams, and detection of nuclear explosions. Recent detector advances have made near-field monitoring feasible, whereas farther-field reactor detection and waste stream detection monitoring may be possible in some cases with further research and development. Very long-range reactor monitoring and nuclear explosion detection do not appear feasible for the foreseeable future due to considerable physical and/or practical constraints.

A new experimental technique is presented using proton-$\gamma \mathrm{\text{−}}\gamma$ correlations from ${}^{94}\mathrm{Mo}(d,p){}^{95}\mathrm{Mo}$ reactions which allows for the model-independent extraction of the photon strength function at various excitation energies using primary $\gamma$-ray decay from the quasicontinuum to individual low-lying levels. Detected particle energies provide the entrance excitation energies into the residual nucleus while $\gamma$-ray transitions from low-lying levels specify the discrete states being fed. Results strongly support the existence of the previously reported low-energy enhancement in the photon strength function.

The surrogate nuclear reaction method can be used to determine neutron-induced reaction cross sections from measured decay properties of a compound nucleus created using a different reaction and calculated formation cross sections. The reliability of (n,$\gamma${}) cross sections determined using the Weisskopf-Ewing and ratio approximations are explored for the Gd155,157(n,$\gamma${}) reactions. Enriched gadolinium targets were bombarded with 22-MeV protons and $\gamma${} rays were detected in coincidence with scattered protons using the Silicon Telescope Array for Reaction Studies/Livermore-Berkeley Array for Collaborative Experiments (STARS/LiBerACE) silicon and germanium detector arrays. The $\gamma${}-emission probabilities for the Gd154,156,158 compound nuclei were measured at excitation energies up to 12 MeV. It is found that the approximations yield results that deviate from directly measured Gd155,157(n,$\gamma${}) cross sections at low energies. To extract reliable cross sections, a more sophisticated analysis should be developed that takes into account angular-momentum differences between the neutron-induced and surrogate reactions.

Yb171,173(d,p$\gamma${}) reactions are measured, with the goal of extracting the neutron capture cross-section ratio as a function of the neutron energy using the external surrogate ratio method. The cross-section ratios obtained are compared to the known neutron capture cross sections. Although the Weisskopf-Ewing limit is demonstrated not to apply for these low neutron energies, a prescription for deducing surrogate cross sections is presented. The surrogate cross-section ratios deduced from the Yb171,173(d,p$\gamma${}) measurements agree with the neutron capture results within 15%.

The neutron-induced fission cross section of ${}^{238}\mathrm{Pu}$ was determined using the surrogate ratio method. The $(n,f)$ cross section over an equivalent neutron energy range 5–20 MeV was deduced from inelastic $\alpha$-induced fission reactions on ${}^{239}\mathrm{Pu}$, with ${}^{235}\mathrm{U}$($\alpha ,{\alpha }^{\prime }f)$ and ${}^{236}\mathrm{U}$($\alpha ,{\alpha }^{\prime }f)$ used as references. These reference reactions reflect ${}^{234}\mathrm{U}$($n,f)$ and ${}^{235}\mathrm{U}$($n,f)$ yields, respectively. The deduced ${}^{238}\mathrm{Pu}$$(n,f$) cross section agrees well with standard data libraries up to $~$10 MeV, although larger values are seen at higher energies. The difference at higher energies is less than 20%.

The level density and $\gamma$-ray strength function ($\gamma \mathrm{SF}$) of ${}^{243}\text{Pu}$ have been measured in the quasicontinuum using the Oslo method. Excited states in ${}^{243}\text{Pu}$ were populated using the ${}^{242}\text{Pu}(d,p)$ reaction. The level density closely follows the constant-temperature level density formula for excitation energies above the pairing gap. The $\gamma \mathrm{SF}$ displays a double-humped resonance at low energy as also seen in previous investigations of actinide isotopes. The structure is interpreted as the scissors resonance and has a centroid of ${\omega }_{\mathrm{SR}}=2.42\left(5\right)$ MeV and a total strength of $B_{\mathrm{SR}}=10.1\left(15\right){\mu }_N^2$, which is in excellent agreement with sum-rule estimates. The measured level density and $\gamma \mathrm{SF}$ were used to calculate the ${}^{242}\text{Pu}(n,\gamma )$ cross section in a neutron energy range for which there were previously no measured data.