Weapons Neutron Research Facility Research Articles

Neutron Total Cross Sections of 232Th, 237Np, 235,238U, and 239Pu from 3 to 230 MeV

Neutron total cross-section data for 232Th, 237Np, 235,238U, and 239Pu were measured over the energy range of a few MeV up to 230 MeV at the Weapons Neutron Research Facility of the Los Alamos Neutron Science Center. The results for 235U, 237Np, and 239Pu are the first to be reported over this full energy range. Our data are compared to the ENDF/B-VIII.0 and JENDL-5 evaluations and to selected previous data sets. These results provide neutron cross sections with small statistical uncertainties and low systematic errors to improve theoretical modeling and data evaluation for these nuclei.

Evaluation of fault injection tools for reliability estimation of microprocessor-based embedded systems

Statistical fault injection is widely used to estimate the reliability of mission-critical microprocessor-based systems when exposed to radiation and to evaluate the performance of fault mitigation strategies. However, further research is needed to gain a better understanding of the accuracy of the results and the feasibility of their application under realistic radiation conditions. In this article, an understanding of scenarios in which Instruction Set Architecture simulators or emulators may be relied upon for realistic statistical fault injection campaigns is advanced. An analysis is presented of the results from two simulation-based fault injection tools versus a set of fault emulation results on a real processor. The conclusions of the analysis assist the selection of the most efficient tool and method for testing many different software-based fault mitigation techniques within reasonable time periods and at affordable costs throughout an irradiation campaign. In particular, it was established that a partially ordered set of relations could be defined on the basis of statistical fault injection in relation to the effects of different versions of an application and a given simulator that remained unaltered during the irradiation experiments. The tests were conducted with a Texas Instruments MSP430 microcontroller to perform both fault injection campaigns and irradiation experiments using neutrons at the Los Alamos Neutron Science Center (LANSCE) Weapons Neutron Research Facility at Los Alamos, USA.

Strain prediction and benchmark with measurement in the Spallation Neutron Source mercury target without gas injection

The 1.4-MW Spallation Neutron Source (SNS) currently operates as the world's highest time-average power spallation source and uses mercury as the spallation target material. The mercury circulates through a stainless-steel target vessel and heat exchanger loop that removes the heat deposited from the incident proton beam in the target. The mercury vessel is part of the fixed but replaceable target module. The target module is not safety credited in the SNS design, but reliable long life is vital for meeting operational goals and minimizing cost and waste. SNS is a short-pulse ($\ensuremath{\sim}0.7\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$) neutron source with a repetition rate of 60 Hz with 1.0 GeV protons onto the target. The short-pulse operation introduces two mechanisms that threaten the longevity of the mercury vessel. The pulsed target heating is essentially isochoric, which results in a distributed pressure field in the mercury with each pulse. After each pulse, the pressure propagates, interacts with, and strains the stainless-steel vessel. For intermittent times, this pressure field leads to mercury cavitation. The first mechanism of concern for the target lifetime is high-cycle fatigue to the stainless-steel vessel and the second is cavitation erosion. At the onset of the SNS project, the R program sought to address the need for an accurate and practical method to estimate the fatigue life of the mercury vessel. Around 2000, the R team conducted a series of in-beam mercury target experiments at the Weapons Neutron Research Facility at Los Alamos Neutron Science Center. These experiments successfully produced a small database of pulse response strain data. The data were used to develop and benchmark an explicit dynamic finite element analysis technique for estimating target fatigue life. SNS is now a mature facility with about 15 years of operation and 29 SNS target modules consumed. Since 2015, strain measurements have been collected from actual targets, and abundant target vessel strain data are now available in greater detail than was possible in 2000. Variances in pulse response data have been characterized using tolerance interval analysis to benchmark the simulation technique more thoroughly. The simulated results were found to match measurements generally well, but the method is always conservative. A potential adjustment to the cavitation threshold parameter value was suggested. Variances in the pulse response data are moderate, which is largely explained by the fact that data were acquired in 60-Hz burst mode and by the lasting vibration of the vessel between pulses.

Charged-particle emission in neutron reactions on B10

The $^{10}\mathrm{B}+n$ system has been studied by measuring charged particles from neutron-induced reactions in the 1 to 20 MeV energy range. Protons, deuterons, tritons, and $\ensuremath{\alpha}$ particles were measured at four angles using the pulsed white neutron spectrum at the Los Alamos Neutron Science Center Weapons Neutron Research facility. Differential cross sections for each species are reported. These new data are combined with literature data in an $R$-matrix analysis of the $^{11}\mathrm{B}$ compound system. A comparison is made to the molecular and cluster states candidates observed in these reactions.

Measurement of the U235(n,f) prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons of energy from 1 MeV to 20 MeV

The characterization of fission-driven nuclear systems primarily relies on calculations of neutron-induced chain reactions, and these calculations require evaluated nuclear data as input. Calculation accuracy heavily depends on input nuclear data evaluation accuracy, and thus high precision on the experimental input to the nuclear data evaluation is essential for fundamental quantities like the energy spectrum of neutrons emitted from neutron-induced fission (i.e., the prompt fission neutron spectrum, PFNS). Despite decades of measurement efforts, prior to the measurements described in this work there were only three literature data sets for the $^{235}\mathrm{U}(n,f)$ PFNS at incident neutron energies above 1.0 MeV considered reliable for inclusion in nuclear data evaluations and no reliable data sets above 3.0 MeV incident neutron energy. In this work we report on new measurements of the $^{235}\mathrm{U}(n,f)$ PFNS spanning a grid of 1.0--20.0 MeV in incident neutron energy and 0.01--10.0 MeV in outgoing (PFNS) neutron energy. These measurements were carried out at the Weapons Neutron Research facility at the Los Alamos Neutron Science Center and used a multifoil parallel-plate avalanche counter target with both a Li-glass and a liquid scintillator detector array in separate experiments to span the quoted outgoing neutron energy ranges. The PFNS results are shown in terms of the energy spectra themselves as well as the average PFNS energy ($\ensuremath{\langle}E\ensuremath{\rangle}$) and ratios of $\ensuremath{\langle}E\ensuremath{\rangle}$ at forward and backward angles. The results are compared with literature data and selected nuclear data evaluations. Generally, the data agree with the ENDF/B-VIII.0 evaluation below 5.0-MeV incident neutron energy and more closely with the JEFF-3.3 evaluation above 5.0 MeV, though no evaluations considered for comparison in this work agree with the data across all of the incident and outgoing neutron energies shown, especially in regions where the third-chance fission process becomes available. Additionally, we show a ratio of the present PFNS results for $^{235}\mathrm{U}(n,f)$ with a recent and highly correlated experiment to measure the $^{239}\mathrm{Pu}(n,f)$ PFNS at the same experimental facility and with nearly identical equipment and analysis procedures. Many observations reported in this work are the first of their kind and represent significant advancements for knowledge of the $^{235}\mathrm{U}(n,f)$ PFNS.

A holistic approach to the optimization of neutron beam transport at the LANSCE facility

An advanced neutron beam collimation system has been developed as part of an effort to optimize neutron beam transport at the Weapons Neutron Research (WNR) Facility [1], located within the Los Alamos Neutron Science Center (LANSCE). The goal of this work was to develop and demonstrate techniques to tailor neutron delivery in order to provide maximum available flux on sample with a specific beam profile, while simultaneously reducing unwanted background. A holistic approach was taken, upgrading the facility spallation target, facility metrology infrastructure, and flight path shutter insert to support the implementation of this advanced collimation system. Modern instruments and software were employed to conduct facility surveys, characterization of as-built geometry of critical components, 3D ray tracing and neutron transport calculations. Beam images and flux measurements were acquired after installation to evaluate the performance of the collimation system and to demonstrate consistent agreement with MCNP simulations which showed an 87% increase of flux on sample while providing suppression of background neutron impingement on the sample frame by 103.

Total kinetic energy release in the fast neutron induced fission of actinide nuclei

The total kinetic energy (TKE) release in the fast neutron-induced fission of various actinide nuclei was measured for neutron energies for En=2.6-100 MeV at the Weapons Neutron Research facility of the Los Alamos National Laboratory. The data are compared to the GEF model of the fission process. The variances of the TKE distributions appear to decrease with increasing Z and A of the fissioning systems.

New developments of a fission chamber for very high radioactivity samples

A new fission chamber was developed and built for prompt-fission-neutron spectra (PFNS) measurements in neutron-induced fission of actinides, and is described in Taieb et al. (2016). It allowed us to minimize incident- and outgoing-neutron scattering, and optimize both time resolution and alpha to fission fragment discrimination. The fission chamber was validated and used for 238U and 252Cf PFNS measurements. In order to measure PFNS of samples with a natural alpha activity of the order of 10 MBq, further improvements to the detector were done and are presented in this work. Tests of the improved fission chamber lead to a fission fragment detection efficiency of 91% for a 14.41 MBq 240Pu deposit, and 99% for a 247 kBq 242Pu deposit. The fission chamber was successfully used for 239Pu PFNS measurements at the Weapons Neutron Research facility of the Los Alamos Neutron Science Center (WNR@LANSCE).

Total kinetic energy release in the fast-neutron-induced fission of Np237

The total kinetic energy (TKE) release in the fast neutron-induced fission of $^{237}\mathrm{Np}$ was measured for neutron energies from ${E}_{n}=2.6--100\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ at the Los Alamos Neutron Science Center at the Weapons Neutron Research facility. The post-TKE release decreases nonlinearly with increasing incident neutron energy and can be represented as $\mathrm{TKE}\phantom{\rule{0.16em}{0ex}}(\mathrm{MeV})=(174.38\ifmmode\pm\else\textpm\fi{}0.72)--(5.11\ifmmode\pm\else\textpm\fi{}0.5821){\mathrm{log}}_{10}{E}_{n}$ for ${E}_{n}>1\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$. Analysis of the fragment mass distributions indicates that the decrease in TKE with increasing ${E}_{n}$ is a consequence of the fading out of shell effects at high excitation energies (resulting in an increase of symmetric fission) and the decrease of the total kinetic energy associated with asymmetric fission with increasing ${E}_{n}$.

Empirical Mathematical Model of Microprocessor Sensitivity and Early Prediction to Proton and Neutron Radiation-Induced Soft Errors

A mathematical model is described to predict microprocessor fault tolerance under radiation. The model is empirically trained by combining data from simulated fault-injection campaigns and radiation experiments, both with protons (at the National Center of Accelerators (CNA) facilities, Seville, Spain) and neutrons [at the Los Alamos Neutron Science Center (LANSCE) Weapons Neutron Research Facility at Los Alamos, USA]. The sensitivity to soft errors of different blocks of commercial processors is identified to estimate the reliability of a set of programs that had previously been optimized, hardened, or both. The results showed a standard error under 0.1, in the case of the Advanced RISC Machines (ARM) processor, and 0.12, in the case of the MSP430 microcontroller.

γ -ray measurements in fast-neutron-induced reactions on Tl203

Background: Fast-neutron-induced reactions can be used to characterize reaction mechanisms, investigate nuclear structure, and impose constraints on nuclear models. Because of the difficulty in predicting such effects, experimental data are important to constrain models. Thallium isotopes located close to the doubly magic $^{208}\mathrm{Pb}$ nucleus are important for comparison with shell-model calculations.Purpose: Study the population of excited states in $^{203}\mathrm{Tl}$ and lighter isotopes in such reactions.Methods: $\ensuremath{\gamma}$-ray cross sections were measured. The data were taken by using the Germanium Array for Neutron-Induced Excitations spectrometer. The pulsed neutron source of the Los Alamos Neutron Science Center's Weapons Neutron Research facility provided neutrons in the energy range from 1 to 300 MeV. The time-of-flight technique was used to determine the incident neutron energies.Results: Cross sections for emission of several $\ensuremath{\gamma}$ rays in 20 reaction channels were determined. Candidates for the intruder $9/{2}^{\ensuremath{-}}$ state in $^{203}\mathrm{Tl}$, from the odd proton in the ${h}_{9/2}$ orbital, and the first ${5}^{+}$ states in $^{202,204}\mathrm{Tl}$, from admixture of configurations, were identified.Conclusions:The excitation energy of the candidate $9/{2}^{\ensuremath{-}}, \ensuremath{\pi}{h}_{9/2}$ state is in good agreement with the theoretical prediction from a semi-empirical weak-coupling model and its half-life is in the range of tens of nanoseconds. The feeding of a previously known isomer in $^{202}\mathrm{Tl}$ exhibits similarities with the feeding of other isomers in this mass region.

Prompt-fission-neutron spectra in the Pu239(n,f) reaction

Prompt-fission-neutron spectra from $^{239}\mathrm{Pu}$ ($\mathit{n},\phantom{\rule{0.16em}{0ex}}\mathit{f}$) were measured with respect to $^{252}\mathrm{Cf}$ spontaneous fission for incident neutron energies from 0.7 to $700\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center. A newly designed high-efficiency fission chamber was coupled to the highly segmented Chi-Nu neutron liquid scintillator array to detect neutrons emitted in fission events. The double time-of-flight technique was used to deduce the incident neutron energies from the spallation target and the outgoing-neutron energies from the fission chamber. Prompt-fission-neutron spectra (PFNS) were measured with respect to $^{252}\mathrm{Cf}$ spontaneous fission down to $200\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ and up to about $12\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ for all the incident neutron energies with typical total uncertainties well below $2%$ up to about 7-MeV outgoing-neutron energy. The general trend of PFNS is well reproduced by JEFF3.3 and ENDF evaluations, although a better agreement is found with JEFF3.3. Discrepancies were observed for the low-energy part of the spectra, especially around the opening of the second-, third- and fourth-chance fission. Neutron average kinetic energies as a function of incident neutron energy are obtained experimentally with reported total uncertainties below $0.5%$. The measured values agree with the most recent data. The trend is fairly well reproduced by the JEFF3.3 evaluation, although it fails to reproduce the experimental values within their uncertainties.

Neutron Imaging at LANSCE—From Cold to Ultrafast

In recent years, neutron radiography and tomography have been applied at different beam lines at Los Alamos Neutron Science Center (LANSCE), covering a very wide neutron energy range. The field of energy-resolved neutron imaging with epi-thermal neutrons, utilizing neutron absorption resonances for contrast as well as quantitative density measurements, was pioneered at the Target 1 (Lujan center), Flight Path 5 beam line and continues to be refined. Applications include: imaging of metallic and ceramic nuclear fuels, fission gas measurements, tomography of fossils and studies of dopants in scintillators. The technique provides the ability to characterize materials opaque to thermal neutrons and to utilize neutron resonance analysis codes to quantify isotopes to within 0.1 atom %. The latter also allows measuring fuel enrichment levels or the pressure of fission gas remotely. More recently, the cold neutron spectrum at the ASTERIX beam line, also located at Target 1, was used to demonstrate phase contrast imaging with pulsed neutrons. This extends the capabilities for imaging of thin and transparent materials at LANSCE. In contrast, high-energy neutron imaging at LANSCE, using unmoderated fast spallation neutrons from Target 4 [Weapons Neutron Research (WNR) facility] has been developed for applications in imaging of dense, thick objects. Using fast (ns), time-of-flight imaging, enables testing and developing imaging at specific, selected MeV neutron energies. The 4FP-60R beam line has been reconfigured with increased shielding and new, larger collimation dedicated to fast neutron imaging. The exploration of ways in which pulsed neutron beams and the time-of-flight method can provide additional benefits is continuing. We will describe the facilities and instruments, present application examples and recent results of all these efforts at LANSCE.

The Prompt Fission Neutron Spectrum of 235U(n,f) below 2.5 MeV for Incident Neutrons from 0.7 to 20 MeV

New prompt fission neutron spectrum measurements are reported for 235U(n,f) reactions induced by neutrons with energies from 0.7 to 20 MeV. These measurements cover outgoing neutron energies from 2.5 MeV down to 10 keV, using an array of 6Li-glass scintillators for neutron detection and a double time-of-flight technique. The neutrons were produced at the Weapons Neutron Research facility of the Los Alamos Neutron Science Center. A detailed MCNP® model of the experimental equipment and the surrounding room was used to interpret the experimental results. Backgrounds were measured in situ, making use of the time-dependent singles rates of the various detectors with asynchronous readout from waveform digitizers. The results presented here have been included in a re-evaluation of the fission neutron spectra for this fissioning system, a description of which is presented elsewhere in this issue.

The Los Alamos Neutron Science Center Spallation Neutron Sources

The Los Alamos Neutron Science Center (LANSCE) provides the scientific community with intense sources of neutrons, which can be used to perform experiments supporting civilian and national security research. These measurements include nuclear physics experiments for the defense program, basic science, and the radiation effect programs. This paper focuses on the radiation effects program, which involves mostly accelerated testing of semiconductor parts.When cosmic rays strike the earth's atmosphere, they cause nuclear reactions with elements in the air and produce a wide range of energetic particles. Because neutrons are uncharged, they can reach aircraft altitudes and sea level. These neutrons are thought to be the most important threat to semiconductor devices and integrated circuits. The best way to determine the failure rate due to these neutrons is to measure the failure rate in a neutron source that has the same spectrum as those produced by cosmic rays.Los Alamos has a high-energy and a low-energy neutron source for semiconductor testing. Both are driven by the 800-MeV proton beam from the LANSCE accelerator. The high-energy neutron source at the Weapons Neutron Research (WNR) facility uses a bare target that is designed to produce fast neutrons with energies from 100 keV to almost 800 MeV. The measured neutron energy distribution from WNR is very similar to that of the cosmic-ray-induced neutrons in the atmosphere. However, the flux provided at the WNR facility is typically 5×107 times more intense than the flux of the cosmic-ray-induced neutrons. This intense neutron flux allows testing at greatly accelerated rates. An irradiation test of less than an hour is equivalent to many years of neutron exposure due to cosmic-ray neutrons.The low-energy neutron source is located at the Lujan Neutron Scattering Center. It is based on a moderated source that provides useful neutrons from subthermal energies to ∼100 keV. The characteristics of these sources, and ongoing industry program are described in this paper.

Feeding of Rh and Ag isomers in fast-neutron-induced reactions

Background: In ($n,{n}^{\ensuremath{'}}$) reactions on stable Ir and Au isotopes in the mass $A=190$ region, the experimentally established feeding of the isomers relative to the feeding of the corresponding ground states increases with increasing neutron energy, up to the neutron energy where the ($n,2n$) reaction channel opens up, and then decreases.Purpose: In order to check for similar behavior in the mass $A=100$ region, the feeding of isomers and ground states in fast-neutron-induced reactions on stable isotopes in this mass region was studied. This is of especial interest for Rh which can be used as a radiochemical detector.Methods: Excited states were studied using the ($n,{n}^{\ensuremath{'}}\ensuremath{\gamma}$), ($n,2n\ensuremath{\gamma}$), and ($n,3n\ensuremath{\gamma})$ reactions on $^{103}\mathrm{Rh}$ and $^{109}\mathrm{Ag}$. A germanium detector array for $\ensuremath{\gamma}$-ray detection and the broad-spectrum pulsed neutron source of the Los Alamos Neutron Science Center's Weapons Neutron Research facility were used for the measurement. The energy of the incident neutrons was determined using the time-of-flight technique.Results: Absolute partial $\ensuremath{\gamma}$-ray cross sections were measured for 57 transitions feeding isomers and ground states in $^{101,102,103}\mathrm{Rh}$ and $^{107,108,109}\mathrm{Ag}$. The feeding of the isomers was found to be very similar in the corresponding reaction channels and it is compared to the feeding determined for the ground states.Conclusions: The opening of reaction channels at higher neutron energies removes angular momentum from the residual nucleus and reduces the population of the higher-spin isomers relative to the feeding of the lower-spin ground states. Similar behavior was observed in the mass $A=190$ region in the feeding of higher-spin isomers, but the reverse behavior was observed in $^{176}\mathrm{Lu}$ with a lower-spin isomer and a higher-spin ground state.

Comparison of polystyrene scintillator fiber array and monolithic polystyrene for neutron imaging and radiography.

The neutron imaging diagnostic at the National Ignition Facility has been operating since 2011 generating neutron images of deuterium-tritium (DT) implosions at peak compression. The current design features a scintillating fiber array, which allows for high imaging resolution to discern small-scale structure within the implosion. In recent years, it has become clear that additional neutron imaging systems need to be constructed in order to provide 3D reconstructions of the DT source and these additional views need to be on a shorter line of sight. As a result, there has been increased effort to identify new image collection techniques that improve upon imaging resolution for these next generation neutron imaging systems, such as monolithic deuterated scintillators. This work details measurements performed at the Weapons Neutron Research Facility at Los Alamos National Laboratory that compares the radiographic abilities of the fiber scintillator with a monolithic scintillator, which may be featured in a future short line of sight neutron imaging systems.

New transitions and feeding of theJπ=(8+)isomer inRe186

The spallation neutron source at the Los Alamos Neutron Science Center Weapons Neutron Research facility was used to populate excited states in $^{186}\mathrm{Re}$ via $(n,2n\ensuremath{\gamma})$ reactions on an enriched $^{187}\mathrm{Re}$ target. Gamma rays were detected with the GErmanium Array for Neutron Induced Excitations spectrometer, a Compton-suppressed array of 18 HPGe detectors. Incident neutron energies were determined by the time-of-flight technique and used to obtain $\ensuremath{\gamma}$-ray excitation functions for the purpose of identifying $\ensuremath{\gamma}$ rays by reaction channel. Analysis of the singles $\ensuremath{\gamma}$-ray spectrum gated on the neutron energy range $10\ensuremath{\le}{E}_{n}\ensuremath{\le}25\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ resulted in five transitions and one level added to the $^{186}\mathrm{Re}$ level scheme. The additions include the placement of three $\ensuremath{\gamma}$ rays at 266.7, 381.2, and 647.7 keV which have been identified as feeding the $2.0\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{4pt}{0ex}}\mathrm{yr}$, ${J}^{\ensuremath{\pi}}=({8}^{+})$ isomer and yield an improved value of $148.2(5)\phantom{\rule{4pt}{0ex}}\mathrm{keV}$ for the isomer energy. These transitions may have astrophysical implications related to the use of the Re-Os cosmochronometer.

αand2p2nemission in fast neutron-induced reactions onNi60

Background: The cross sections for populating the residual nucleus in the reaction ${}_{Z}^{A}\text{X}{(n,x)}_{Z\ensuremath{-}2}^{A\ensuremath{-}4}\text{Y}$ exhibit peaks as a function of incident neutron energy corresponding to the ($n,{n}^{\ensuremath{'}}\ensuremath{\alpha}$) reaction and, at higher energy, to the ($n,2p3n$) reaction. The relative magnitudes of these peaks vary with the $Z$ of the target nucleus.Purpose: Study fast neutron-induced reactions on $^{60}\text{Ni}$. Locate experimentally the nuclear charge region along the line of stability where the cross sections for $\ensuremath{\alpha}$ emission and for $2p2n$ emission in fast neutron-induced reactions are comparable as a further test of reaction models.Methods: Data were taken by using the Germanium Array for Neutron-Induced Excitations. The broad-spectrum pulsed neutron beam of the Los Alamos Neutron Science Center's Weapons Neutron Research facility provided neutrons in the energy range from 1 to 250 MeV. The time-of-flight technique was used to determine the incident-neutron energies.Results: Absolute partial cross sections for production of seven discrete Fe $\ensuremath{\gamma}$ rays populated in $^{60}\text{Ni}(n,\ensuremath{\alpha}/2pxn\ensuremath{\gamma})$ reactions with $2\ensuremath{\le}x\ensuremath{\le}5$ were measured for neutron energies $1\phantom{\rule{4pt}{0ex}}\text{MeV}<{E}_{n}<250\phantom{\rule{4pt}{0ex}}\text{MeV}$. Hauser--Feshbach plus pre-equilibrium theoretical calculations are compared to the experimental results.Conclusions: There is good agreement between experimental results and theoretical predictions at lower neutron energies while discrepancies appear at higher neutron energies. The cross section for producing an isotope in fast neutron-induced reactions on stable targets via $\ensuremath{\alpha}$ emission at the peak of the ($n,\ensuremath{\alpha}$) and ($n,{n}^{\ensuremath{'}}\ensuremath{\alpha}$) reactions is comparable to that for $2p2n$ and $2p3n$ emission at higher incident energies in the nuclear charge region around Fe.

Plastic fiber scintillator response to fast neutrons.

The Neutron Imaging System at NIF uses an array of plastic scintillator fibers in conjunction with a time-gated imaging system to form an image of the neutron emission from the imploded capsule. By gating on neutrons that have scattered from the 14.1 MeV DT energy to lower energy ranges, an image of the dense, cold fuel around the hotspot is also obtained. An unmoderated spallation neutron beamline at the Weapons Neutron Research facility at Los Alamos was used in conjunction with a time-gated imaging system to measure the yield of a scintillating fiber array over several energy bands ranging from 1 to 15 MeV. The results and comparison to simulation are presented.