Session PF: Reactions: (n,gamma) and Fission

Extreme non-statistical effects in $\gamma $ decay of $^{95}$Mo neutron resonances

We obtained unprecedentedly large sets of total radiation widths $\Gamma _{\gamma}$ of $^{95}$Mo neutron resonances for all six $s$- and $p$-wave $J^{\pi}$ values. We demonstrate that the resulting $\Gamma_{\gamma}$ distributions can be used to test and improve nuclear models. In particular, $\Gamma_{\gamma}$ distribution simulations in the framework of the nuclear statistical model yielded results in sharp disagreement with the data. Simulations modified to include doorway effects resulted in much better agreement. These results call into question the reliability of the nuclear statistical model, and demonstrate that high-quality $\Gamma_{\gamma}$ data are a virtually untapped resource for testing and improving nuclear models.   

MANTRA: Measuring Neutron Capture Cross Sections in Actinides with Accelerator Mass Spectrometry

With rising global energy needs, there is substantial interest in nuclear energy research. To explore possibilities for advanced fuel cycles, better neutron cross section data are needed for the minor actinides. The MANTRA (Measurement of Actinide Neutron TRAsmutation) project will improve these data by measuring integral (n,$\gamma$) cross sections. The cross sections will be extracted by measuring isotopic ratios in pure actinide samples, irradiated in the Advanced Test Reactor at Idaho National Lab, using Accelerator Mass Spectrometry(AMS) at the Argonne Tandem Linac Accelerator System (ATLAS). MANTRA presents a unique AMS challenge because of the goal to measure multiple isotopic ratios on a large number of samples. To meet these challenges, we have modified the AMS setup at ATLAS to include a laser ablation system for solid material injection into our ECR ion source. I will present work on the laser ablation system and modified source geometry, as well as preliminary measurements of unirradiated actinide samples at ATLAS.   

Search for Nuclear Excitation by Electronic Transition in U-235

Nuclear excitation by electronic transition (NEET) is a rare nuclear excitation that is predicted to occur in numerous isotopes, including U-235. When a nuclear transition matches the energy and the multipolarity of an electronic transition, there is a possibility that NEET will occur. If NEET were to occur in U-235, the nucleus would be excited to its 1/2$+$ isomeric state that subsequently decays by internal conversion with a decay energy of 77 eV and a half-life of 26 minutes. Theory predicts that NEET can occur in partially ionized uranium plasma with a charge state of 23$+$. A pulsed Nd:YAG laser operating at 1064 nm with a pulse energy of 780 mJ and a pulse width of 9 ns was used to generate the uranium plasma. The plasma was collected on a plate and the internal conversion electrons were focused onto a microchannel plate detector by a series of electrostatic lenses. Depleted uranium and highly enriched uranium samples were used for the experiment. Preliminary results will be presented.   

Neutron-Induced Fission Cross Sections for Uranium-238 Above 100 MeV

The cross section for neutron-induced fission of $^{238}$U is not well known above 100 MeV; only a few published measurements exist between 100 and 300 MeV. We report here new cross section data which span the range from 100 to over 200 MeV. A white neutron beam produced at the LANSCE/WNR facility was incident both on a thin transmission fission chamber, and subsequently on a liquid hydrogen target. Data were simultaneously collected from fission-fragment triggers in the chamber, as well as from n-p elastic scattering events from the cryogenic target. The fragment time spectrum was used to determine the energy of the initiating neutron, while an ADC spectrum from the chamber allowed for a clean separation of alpha-particle backgrounds. Elastic n-p triggers were derived from a coincidence between scattered neutrons and the recoil protons, detected in a plastic-CsI telescope whose time spectrum was used to determine the incident neutron energy. The cross section for n-p scattering is well known, and is used to normalize the fission yields at beam energies above 100 MeV. The new fission cross section data are compared with previous measurements.   

Measurement of the U-238/U-235 (n,f) cross-section ratio with the NIFFTE Time Projection Chamber

Nuclear data play a fundamental role in energy and defense related applications. In recent years, understanding of these systems has become dependent upon advanced simulation and modeling, where uncertainties in nuclear data propagate into calculated performance parameters. It is important therefore that nuclear data uncertainties are minimized and well-understood. To this end, the Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration is developing a Time Projection Chamber (TPC) to measure energy-differential (n,f) cross sections with unprecedented precision. (n,f) cross-section measurements with the NIFFTE TPC take place at the Los Alamos Neutron Science Center (LANSCE) WNR facility, a spallation neutron source which provides a neutron spectrum ranging from hundreds of keV to hundreds of MeV. During the 2012 LANSCE run cycle, data were collected on several actinide samples, including U-238 and U-235. These data, along with those collected during the 2013 LANSCE run cycle, will be used to deduce a U-238/U-235 (n,f) cross-section ratio, to benchmark TPC performance, and to provide high-quality data to the community. A brief overview of the NIFFTE TPC and preliminary analysis of the U-238/U-235 (n,f) ratio data will be presented.   

Techniques for Measuring the $^{239}$Pu(n,f) /$^{235}$U (n,f) Cross Section Ratio using the NIFFTE Time Projection Chamber

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) aims to measure the $^{239}$Pu (n,f) cross-section to better than 1\% across the 250~keV to 20~MeV incident neutron energy range through the use of a purpose-built Time Projection Chamber (TPC). Prior to the 2013 LANSCE run cycle, the active area of the TPC was doubled and now provides 4$\pi$ coverage with nearly 6000 independent readout channels. The current status of the $^{239}$Pu/$^{235}$U (n,f) cross-section measurement, including techniques for handling the data-rates associated with the large spontaneous alpha activity of $^{239}$Pu in the fission TPC will be discussed.   

Measurement of Neutron Induced and Spontaneous Fission in Pu-242 at DANCE

Neutron capture and fission reactions are important in nuclear engineering and physics. DANCE (Detector for Advanced Neutron Capture Measurement, LANL) combined with PPAC (avalanche technique based fission tagging detector, LLNL) were used to study neutron induced and spontaneous fission in $^{242}$Pu. 2 measurements were performed in 2013. The first experiment was done without the incident neutron beam with the fission tagging ability to study $\gamma$-rays emitted in the spontaneous fission of $^{242}$Pu. The second one -- with the neutron beam to measure both the neutron capture and fission reactions. This is the first direct measurement of prompt fission $\gamma$-rays in $^{242}$Pu. The $\gamma$-ray multiplicity, $\gamma$-ray energy, and total energy of $\gamma$-rays per fission in $^{242}$Pu will be presented. These distributions of the $^{242}$Pu spontaneous fission will be compared to those in the $^{241}$Pu neutron induced fission.