[en] For the fission neutron spectrum measurement, the neutron energy is determined in a time-of-flight experiment by the time difference between the fission event and detection of the neutron. Therefore, the neutron energy resolution is directly determined by the time resolution of both neutron and fission detectors. For the fission detection, the detector needs not only a good timing response but also the tolerance of radiation damage and high α-decay rate. A parallel-plate avalanche counter (PPAC) has many advantages for the detection of heavy charged particles such as fission fragments. These include fast timing, resistance to radiation damage, and tolerance of high counting rate. A PPAC also can be tuned to be insensitive to particles, which is important for experiments with - emitting actinides. Therefore, a PPAC is an ideal detector for experiments requiring a fast and clean trigger for fission. In the following sections, the description will be given for the design and performance of a new low-mass PPAC for the fission-neutron spectrum measurements at LANL.

[en] Precision neutron-induced reaction data are important for modeling the network of isotope production and destruction within a given diagnostic chain. This network modeling has many applications such as the design of advanced fuel cycle for reactors and the interpretation of radiochemical data related to the stockpile stewardship and nuclear forensics projects. Our current funded effort is to improve the neutron-induced reaction data on the short-lived actinides and the specific goal is to improve the neutron capture data on ²³⁸Pu with a half-life of 87.7 years. In this report, the fabrication of a ²³⁸Pu target for the proposed measurement using the DANCE array at LANL is described. The ²³⁸Pu target was fabricated from a sample enriched to 99.35%, acquired from ORNL. A total of 395 (micro)g was electroplated onto both sides of a 3 (micro)m thick Ti foil using a custom-made plating cell, shown in Fig 1. The target-material loaded Ti foil is sandwiched between two double-side aluminized mylar foils with a thickness of 1.4 (micro)m. The mylar foil is glued to a polyimide ring. This arrangement is shown partially in Fig. 2. The assembled target is then inserted into an aluminum container with a wall thickness of 0.76 mm, shown in Fig. 3. A derlin ring is used to keep the target assembly in place. The ends of this cylindrical container are vacuum-sealed by two covers with thin Kapton foils as windows for the beam entrance and exit. Shown in Fig. 4 is details of the arrangement. This target is used for phase I of the proposed measurement on ²³⁸Pu scheduled for Nov 2010 together with the DANCE array to address the safety issues raised by LANL. Shown in Fig. 5 is the preliminary results on the yield spectrum as a function of neutron incident energy with a gate on the total γ-ray energy of equivalent Q value. Since no fission PPAC is employed, the distinction between the capture and fission events cannot be made, which is important for the higher neutron incident energy. However, it indicates that a cross section of less than one barn can be measured. The second phase of this experiment will be carried out in 2011 by assembling a PPAC with the ²³⁸Pu target to extend the measurement to higher neutron incident energies by distinguishing the capture from fission events. The fission cross section becomes dominant for neutron incident energies above 30 keV. This PPAC was developed in FY2010 under the NA22 funding and performed very well for the ²³⁹Pu and ²⁴¹Pu measurements. A new ²³⁸Pu target will be fabricated for the phase II measurement using the same electroplating technique.

[en] The Detector for Advanced Neutron Capture Experiments (DANCE) consists of 160 BF₂ crystals with equal solid-angle coverage. DANCE is a 4π γ-ray calorimeter and designed to study the neutron-capture reactions on small quantities of radioactive and rare stable nuclei. These reactions are important for the radiochemistry applications and modeling the element production in stars. The recognition of capture event is made by the summed γ-ray energy which is equivalent of the reaction Q-value and unique for a given capture reaction. For a selective group of actinides, where the neutron-induced fission reaction competes favorably with the neutron capture reaction, additional signature is needed to distinguish between fission and capture γ rays for the DANCE measurement. This can be accomplished by introducing a detector system to tag fission fragments and thus establish a unique signature for the fission event. Once this system is implemented, one has the opportunity to study not only the capture but also fission reactions. A parallel-plate avalanche counter (PPAC) has many advantages for the detection of heavy charged particles such as fission fragments. These include fast timing, resistance to radiation damage, and tolerance of high counting rate. A PPAC also can be tuned to be insensitive to α particles, which is important for experiments with α-emitting actinides. Therefore, a PPAC is an ideal detector for experiments requiring a fast and clean trigger for fission. A PPAC with an ingenious design was fabricated in 2006 by integrating amplifiers into the target assembly. However, this counter was proved to be unsuitable for this application because of issues related to the stability of amplifiers and the ability to separate fission fragments from α's. Therefore, a new design is needed. A LLNL proposal to develop a new PPAC for DANCE was funded by NA22 in FY09. The design goal is to minimize the mass for the proposed counter and still be able to maintain a stable operation under extreme radioactivity and the ability to separate fission fragments from α's. In the following sections, the description is given for the design and performance of this new compact PPAC, for studying the neutron-induced reactions on actinides using DANCE at LANL.

[en] The main goal of the test measurement was to determine the feasibility of the ²⁴³Am(p,t) reaction as a surrogate for ²⁴⁰Am(n,f). No data cross section data exists for neutron induced reactions on ²⁴⁰Am; the half-life of this isotope is only 2.1 days making direct measurements difficult, if not impossible. The 48-hour experiment was conducted using the STARS/LIBERACE experimental facility located at the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory in August 2011. A description of the experiment and results is given. The beam energy was initially chosen to be 39 MeV in order to measure an equivalent neutron energy range from 0 to 20 MeV. However, the proton beam was not stopped in the farady cup and the beam was deposited in the surrounding shielding material. The shielding material was not conductive, and a beam current, needed for proper tuning of the beam as well as experimental monitoring, could not be read. If the ²⁴⁰Am(n,f) surrogate experiment is to be run at LBNL, simple modifications to the beam collection site will need to be made. The beam energy was reduced to 29 MeV, which was within an energy regime of prior experiments and tuning conditions at STARS/LIBERACE. At this energy, the beam current was successfully tuned and measured. At 29 MeV, data was collected with both the ²⁴³Am and ²³⁸U targets. An example particle identification plot is shown in Fig. 1. The triton-fission coincidence rate for the ²⁴³Am target and ²³⁸U target were measured. Coincidence rates of 0.0233(1) cps and 0.150(6) cps were observed for the ²⁴³Am and ²³⁸U targets, respectively. The difference in count rate is largely attributed to the available target material - the ²³⁸U target contains approximately 7 times more atoms than the ²⁴³Am. A proton beam current of ∼0.7 nA was used for measurements on both targets. Assuming a full experimental run under similar conditions, an estimate for the run time needed was made. Figure 2 shows the number of days needed as a function of acceptable uncertainty for a measurement of 1-20 MeV equivalent neutron energy, binned into 200 keV increments. A 5% measurement will take 3 days for U, but 20 days for Am. It may be difficult to be the sole user of the LBNL cyclotron, or another facility, for such an extended period. However, a 10% measurement will take 19 hours for U, and 5 days for Am. Such a run period is more reasonable and will allow for the first ever measurement of the ²⁴⁰Am(n,f) cross section. We also anticipate 40% more beam time being available at Texas A and M Cyclotron Institute compared to LBNL in FY2012. The increased amount of beam time will allow us to accumulate better statistics then what would have been available at LBNL.

[en] The goal of this year's effort is to measure the ²³⁸Pu(n,f) and ²³⁸Pu(n,2n) cross section from 100 keV to 20 MeV. We designed a surrogate experiment that used the reaction ²³⁹Pu(a,a(prime)x) as a surrogate for ²³⁸Pu(n,x). The experiment was conducted using the STARS/LIBERACE experimental facility located at the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory in January 2010. A description of the experiment and status of the data analysis is given. In order to obtain a reliable ²³⁸Pu(n,x) cross section we designed the experiment using the surrogate ratio technique. This technique allows one to measure a desired, unknown, cross section relative to a known cross section. In the present example, the ²³⁸Pu(n,x) cross section of interest is determined relative to the known ²³⁵U(n,x) cross section. To increase confidence in the results, and to reduce overall uncertainties, we are also determining the ²³⁸Pu(n,x) cross section relative to the known ²³⁴U(n,x) cross section. The compound nuclei of interest for this experiment were produced using inelastic alpha scattering. For example, ²³⁶U(a,a(prime)x) served as a surrogate for ²³⁵U(n,x); analogous reactions were considered for the other cross sections. Surrogate experiments determine the probabilities for the decay of the compound nuclei into the various channels of interest (fission, gamma decay) by measuring particle-fission (p-f) or particle?gamma (p?g) reaction spectra. By comparing the decay probabilities associated with the unknown cross section to that of a known cross section it is possible to obtain the ratio of these cross sections and thus determine the unknown, desired cross section.

[en] The main goal of this measurement is to determine the ²⁴²Am(n,f) and ²⁴¹Am(n,f) cross sections via the surrogate ²⁴³Am. Gamma-ray data was also collected for the purpose of measuring the (n,2n) cross-sections. The experiment was conducted using the STARS/LIBERACE experimental facility located at the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory the first week of February 2011. A description of the experiment and status of the data analysis follow.

[en] Radiochemical analysis of post-ignition debris inside the National Ignition Facility (NIF) target chamber can help determine various diagnostic parameters associated with the implosion efficiency of the fusion capsule. This technique is limited by the ability to distinguish ablator material from other debris and by the collection efficiency of the capsule debris after implosion. Prior to designing an on-line collection system, the chemical nature and distribution of the debris inside the chamber must be determined. The focus of our current work has been on evaluating capture of activated Au hohlraum debris on passive foils (5 cm diameter, 50 cm from target center) post-shot. Preliminary data suggest that debris distribution is locally heterogeneous along the equatorial and polar line-of-sights.

[en] The neutron activation of gold is the basis of an implosion performance diagnostic at the National Ignition Facility at Lawrence Livermore National Laboratory. In support of this diagnostic, a series of γ-ray spectrometric measurements of the decay of ¹⁹⁶Au^m2 (J ^π = 12⁻) was performed to improve the currently accepted literature values of the nuclear data associated with its half-life, γ-ray energies, and γ-ray intensities. It was determined that ¹⁹⁶Au^m2 decays with a half-life of 9.603 h ± 0.23%. The relative intensities of the γ rays emitted during its decay were also measured, and an absolute decay branch of 0.3352 ± 2.9% was determined for the emission of the 188.2 keV photon, which arises from a nuclear transition whose multipolarity is predominantly M1. Properties of other products arising in the reaction of ¹⁹⁷Au with fast neutrons were measured, as were selected production cross sections. The ¹⁹⁶Au^m2/¹⁹⁶Au^g isomer ratio measured in the ¹⁹⁷Au(n, 2n) reaction at 14.1 MeV was found to be 0.0731 ± 2.6%. (paper)

[en] The National Ignition Facility (NIF) is the world's premier inertial confinement fusion facility designed to achieve sustained thermonuclear burn (ignition) through the compression of hydrogen isotopic fuels to densities in excess of 10³ g/cm³ and temperatures in excess of 100 MK. These plasma conditions are very similar to those found in the cores of Asymptotic Giant Branch (AGB) stars where the s-process takes place, but with a neutron fluence per year 10⁴ times greater than a star. These conditions make NIF an excellent laboratory to measure s-process (n,γ) cross sections in a stellar-like plasma for the first time. Starting in Fall 2009, NIF has been operating regularly with 2-4 shots being performed weekly. These experiments have allowed the first in situ calibration of the detectors and diagnostics needed to measure neutron capture, including solid debris collection and prompt γ-ray detection. In this paper I will describe the NIF facility and capsule environment and present two approaches for measuring s-process neutron capture cross sections using NIF.

[en] The neutron spectrum from neutron-induced fission needs to be known in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security application. The experimental data base of fission neutron spectra is very incomplete and most present evaluated libraries are based on the approach of the Los Alamos Model. To validate these models and to provide improved data for applications, a program is underway to measure the fission neutron spectrum for a wide range of incident neutron energies using the spallation source of fast neutrons at the Weapons Neutron Research (WNR) facility at the Los Alamos Neutron Science Center (LANSCE). In a double time-of-flight experiment, fission neutrons are detected by arrays of neutron detectors to increase the solid angle and also to investigate possible angular dependence of the fission neutrons. The challenge is to measure the spectrum from low energies, down to 100 keV or so, to energies over 10 MeV, where the evaporation-like spectrum decreases by 3 orders of magnitude from its peak around 1 MeV. For these measurements, we are developing two arrays of neutron detectors, one based on liquid organic scintillators and the other on ⁶Li-glass detectors. The range of fission neutrons detected by organic liquid scintillators extends from about 600 keV to well over 10 MeV, with the lower limit being defined by the limit of pulse-shape discrimination. The ⁶Li-glass detectors have a range from very low energies to about 1 MeV, where their efficiency then becomes small. Various considerations and tests are in progress to understand important contributing factors in designing these two arrays and they include selection and characterization of photomultiplier tubes (PM), the performance of relatively thin (1.8 cm) ⁶Li-glass scintillators on 12.5 cm diameter PM tubes, use of 17.5 cm diameter liquid scintillators with 12.5 cm PM tubes, measurements of detector efficiencies with tagged neutrons from the WNR/LANSCE neutron beam, and efficiency calibration with ²⁵²Cf spontaneous fission neutrons. Design considerations and test results are presented.