Franz-Josef Hambsch

The 235U(n, f) prompt fission neutron spectrum at 100 K input neutron energy

Abstract A measurement of the 235U prompt fission neutron spectrum (PFNS) was performed at the Budapest Nuclear Research Reactor at 100 K incident neutron energy. The motivation for this investigation was to verify some literature data measured over the past 20 years that contradict the Los Alamos model, as well as integral data, benchmark (Keff) experiments, and recent spectral data taken at 0.5 MeV incident neutron energy. The measured spectra using three neutron detectors are in excellent agreement with each other. The average spectrum confirms literature data within the error bars in the neutron energy range of 0.7 to 10 MeV. However, the present PFNS shape cannot predict integral experimental data. It seems to be clear now that the disagreement between microscopic and macroscopic data is not connected with a systematic experimental error in the PFNS at low incident neutron energy.

New work on updating and extending the nuclear data standards

An International Atomic Energy Agency (IAEA) Data Development Project was initiated to provide a mechanism for allowing new experimental data and improvements in evaluation procedures to be incorporated into new evaluations of the standards. The work on this project is ongoing. In the past very long periods sometimes occurred between evaluations of the standards. Through the use of this Project, such long periods should not occur. Work on the cross section standards through this Project has included an update of the experimental data to be used in the cross section standards evaluations, a study of the uncertainties obtained in the international standards evaluation, and improvements in the smoothing procedure for capture cross sections. It was decided that this Project should have a broader range of activities than just the cross section standards and thus encompass standards related activities. The following are being investigated: improvements in the gold cross section at energies below where it is considered a standard and work on certain cross sections that are not as well known as the cross section standards but could be very useful as reference cross sections relative to which certain types of cross section measurements can be made. This work includes promptmorexa0» gamma-ray production in fast neutron-induced reactions, and work on the {sup 252}Cf spontaneous fission neutron spectrum and the {sup 235}U thermal neutron fission spectrum. Most of the data investigated through this Project are used in dosimetry applications. (authors)«xa0less

Sensitivity of Prompt Neutron Multiplicity to Fission Fragment Characteristics

Abstract The Point-by-Point (PbP) model as well as the related computer code is a useful tool to provide different prompt emission data [as a function of fragment mass A, fragment charge Z, total kinetic energy (TKE), and total average ones]. The present work focuses on the sensitivity of prompt neutron multiplicity to different properties of the fission fragments. In the construction of the fragmentation range of the PbP treatment, the use of different Z prescriptions affects the multiparametric matrices of different fragment and prompt emission quantities q(A,Z,TKE). The nonnegligible influence of how the most probable charge is considered (as unchanged charge distribution without or with the charge deviations ΔZ as a function of A or an average ΔZ value), as well as the number of Z taken at each A, is discussed. The calculated average prompt emission quantities as a function of A, as a function of TKE, and total average ones depend on the accuracy of experimental Y(A,TKE) distributions. The prompt neutron multiplicity of complementary fragments vpair (A) has a weak dependence on the total excitation energy (TXE) partition between complementary fully accelerated fragments. This assures a good prediction of the average prompt neutron multiplicity as a function of TKE and of the total average one even in the case of a rough or inappropriate TXE partition. The systematic behavior revealed by the experimental ratio vH/vpair as a function of AH together with the weak dependence of vpair(A) on the TXE partition can be exploited—in the absence of experimental v(A) information—for an indirect verification of predicted v(A).

Status of the Neutron Cross‐Section Standards Database

A new evaluation of the neutron cross‐section standards is now underway. This evaluation has been supported by the Working Party on International Evaluation Cooperation (WPEC), the Cross Section Evaluation Working Group (CSEWG), and an International Atomic Energy Agency Coordinated Research Program (CRP). The CRP has had the dominant role in producing these evaluations. An important goal is to produce the standards needed for the upcoming new ENDF/B‐VII library. Since most neutron cross‐section measurements are made relative to neutron cross‐section standards, the standards evaluation is of crucial importance. The standard reactions to be evaluated are: H(n,n), 3He(n,p), 6Li(n,t), 10B(n,α), 10B(n,α1γ), C(n,n), Au(n,γ), 235U(n,f), and 238U(n,f). These standards should receive international acceptance to ensure that all evaluation projects use the same set of standards. The last complete evaluation of the standards dates back almost 20 years. In the meantime quite a number of new and improved measurements h...

Fission of 239Pu with resonance neutrons

A measurement of fission fragment energy and mass distributions in the 239Pu(n, f) resolved resonance region has been performed applying the twin Frisch-gridded ionization chamber technique. An improved pile-up rejection system has been developed to cope with the strong α background. The results for the mass and mean total kinetic energy (TKE) distributions for resolved resonances do not show any strong ‘pattern’ of fluctuations, unlike it was the case for 235U(n, f) in the same neutron energy region. This can be explained in the frame of existing theoretical models, according to which the only possible low-energy spin states available (Jπ=0+, 1+) belong to two well separated (~1.25 MeV) transition state bands with given fission fragment properties. An anti-correlation between TKE and ⊽p was found to be not significant. The obtained small spin dependence of TKE of about 70 keV is in agreement with literature values. A slight correlation was found between TKE versus 1/Γf. But also this was considered not to be significant.

Status of the International Neutron Cross-Section Standards File

A report is given of the progress achieved in an IAEA Co‐ordinated Research Project (CRP) to improve the cross‐section standards. The objectives of the CRP, started in 2002, were initially the understanding of the origin of the strong uncertainty reduction in R‐matrix model fits and the improvement of the evaluation methodology. These aims were extended in 2003 to the preparation of new evaluations for the standard 6Li(n,t), 10B(n,α), 10B(n,α1), 197Au(n,γ), 235U(n,f), and 238U(n,f) reactions. The methodology, codes, and experimental database developed by Poenitz and Hale for the ENDF/B‐VI standards evaluation were taken as the basis for the new evaluation. The major results achieved by the CRP participants include the testing and intercomparison of a number of codes that can be used in the standards evaluation, updating the database of experimental results, analysis of the reasons leading to the strong uncertainty reduction in model fits, and a study of the bias in evaluated data caused by the Peelles’s P...

Fission fragment properties and the problem of the pulse height defect

The pulse height defect (PHD) has been investigated for three different counting gases commonly used in ionization chambers. The PHD introduces an underestimation of the kinetic energy of a charged particle detected with an ionization chamber. Thus, in some cases it is of crucial importance to correct for this effect, e.g. when studying fission fragment properties. A new method was used, applying a waveform digitizer, to study the PHD. The fission fragment properties from spontaneous fission of 252Cf where determined using different counting gases and different ways of correcting for the PHD were evaluated.

Measurements of Prompt Neutron Multiplicity Distributions in Correlation with Mass-Energy Distribution of Fission Fragments in Spontaneous Fission of 252Cf, 244Cm and 248Cm

The number of prompt neutrons emitted in the fission event have been measured separately for each complementary fragment in coincidence with fragment mass and kinetic energies in spontaneous fission of 252Cf, 244Cm and 248Cm. Two high efficient Gd-loaded liquid scintillator tanks were used for the neutron registration. Approximately 3 106 fission events coincident with prompt neutron emission have been accumulated for each isotope. The mean neutron multiplicity, the dispersion and the covariance of the multiplicity distributions have been obtained as a function of fission fragment mass and kinetic energy. The neutron multiplicity data have been corrected for neutron registration efficiency, background and pile-up. Dependencies of the moments of the multiplicity distributions on the fragment mass and total kinetic energy for different mass bins, as well as mass and total kinetic energy distributions of the fission fragments are presented, discussed and compared for the different isotopes investigated. The results showed a different behavior of the moments of the multiplicity distribution depending on the fragment mass asymmetry that reflects changes in the dynamical effects for different fission modes.

Kinematics of the 6Li(n,t)4He reaction and experimental scenarios for cross-section measurement

Abstract An approach is presented for the measurement of the 6Li(n,t)4He reaction cross section based on complementary measurements benchmarked against kinematic simulations. Key aspects of the approach include taking advantage of the particle leaking (PL) effect, and using a one-dimensional time projection chamber (1D-TPC) and an ionization chamber to detect the reaction products from monoenergetic and white neutron beams, respectively. We have derived analytical expressions describing the PL region in both the laboratory and the center-of-mass reference systems. Two complementary 1D-TPC experiments are discussed, using 6LiF deposits onto transparent aluminum foils, in the backward and forward orientations, respectively. The 6Li(n,t)4He reaction kinematics is discussed for 2-MeV neutrons and extended to the energy range from thermal to 3 MeV to reflect the experimental capability of the Institute for Reference Materials and Measurements Van de Graaff and Geel Electron Linear Accelerator facilities.

Prompt Fission Neutron Emission in Resonance Fission of 239Pu

The prompt neutron emission probability from neutron‐induced fission in the resonance region is being investigated at the time‐of‐flight facility GELINA of the IRMM. A double Frisch‐gridded ionization chamber is used as a fission‐fragment detector. For the data acquisition of both the fission‐fragment signals as well as the neutron detector signals the fast digitization technique has been applied. For the neutron detection, large‐volume liquid scintillation detectors from the DEMON collaboration are used. A specialized data analysis program taking advantage of the digital filtering technique has been developed to treat the acquired data.Neutron multiplicity investigations for actinides, especially in resonance neutron‐induced fission, are rather scarce. They are, however, important for reactor control and safety issues as well as for understanding the basic physics of the fission process. Fission yield measurements on both 235U and 239Pu without prompt neutron emission coincidence have shown that fluctuat...