Intense Beams Of Cold Neutrons Research Articles

EXILL—a high-efficiency, high-resolution setup for γ-spectroscopy at an intense cold neutron beam facility

In the EXILL campaign a highly efficient array of high purity germanium (HPGe) detectors was operated at the cold neutron beam facility PF1B of the Institut Laue-Langevin (ILL) to carry out nuclear structure studies, via measurements of γ-rays following neutron-induced capture and fission reactions. The setup consisted of a collimation system producing a pencil beam with a thermal capture equivalent flux of about 108 n s−1cm−2 at the target position and negligible neutron halo. The target was surrounded by an array of eight to ten anti-Compton shielded EXOGAM Clover detectors, four to six anti-Compton shielded large coaxial GASP detectors and two standard Clover detectors. For a part of the campaign the array was combined with 16 LaBr3:(Ce) detectors from the FATIMA collaboration. The detectors were arranged in an array of rhombicuboctahedron geometry, providing the possibility to carry out very precise angular correlation and directional-polarization correlation measurements. The triggerless acquisition system allowed a signal collection rate of up to 6 × 105 Hz. The data allowed to set multi-fold coincidences to obtain decay schemes and in combination with the FATIMA array of LaBr3:(Ce) detectors to analyze half-lives of excited levels in the pico- to microsecond range. Precise energy and efficiency calibrations of EXILL were performed using standard calibration sources of 133Ba, 60Co and 152Eu as well as data from the reactions 27Al(n,γ)28Al and 35Cl(n,γ)36Cl in the energy range from 30 keV up to 10 MeV.

The Generalized Centroid Difference method for lifetime measurements viaγ-γcoincidences using large fast-timing arrays

A novel method for direct electronic “fast-timing” lifetime measurements of nuclear excited states via γ-γ coincidences using an array equipped with N very fast high-resolution LaBr3(Ce) scintillator detectors is presented. The generalized centroid difference method provides two independent “start” and “stop” time spectra obtained without any correction by a superposition of the N(N – 1)/2 calibrated γ-γ time difference spectra of the N detector fast-timing system. The two fast-timing array time spectra correspond to a forward and reverse gating of a specific γ-γ cascade and the centroid difference as the time shift between the centroids of the two time spectra provides a picosecond-sensitive mirror-symmetric observable of the set-up. The energydependent mean prompt response difference between the start and stop events is calibrated and used as a single correction for lifetime determination. These combined fast-timing array mean γ-γ zero-time responses can be determined for 40 keV < Eγ < 1.4 MeV with a precision better than 10 ps using a 152Eu γ-ray source. The new method is described with examples of (n,γ) and (n,f,γ) experiments performed at the intense cold-neutron beam facility PF1B of the Institut Laue-Langevin in Grenoble, France, using 16 LaBr3(Ce) detectors within the EXILL&FATIMA campaign in 2013. The results are discussed with respect to possible systematic errors induced by background contributions.

The Second Target Station at the ORNL Spallation Neutron Source

After the Spallation Neutron Source (SNS) construction project was completed in June, 2006, development of plans to construct a second target station (STS) at SNS began. These plans have evolved to the establishment of a reference concept for a STS and associated neutron beam instruments, and the evaluation of the expected performance for this station, all of which have been documented in a White Paper. Based on this White Paper, the Department of Energy has approved development of a detailed conceptual design leading to a construction project for the STS. The STS reference design is based on pulse stealing from the 60 Hz SNS accelerator system, with one pulse of every three going to the STS and the other two going to the first target station. The STS would operate in long-proton-pulse mode with no pulse compression in the accumulator ring, and would be optimized for production of intense beams of cold neutrons. The reference concept for the STS and the estimated performance for this concept will be discussed

New perspectives for very short-lived neutron activation analysis

Recent attempts to improve the performance of very short-lived neutron activation analysis are reviewed. It is shown that the combination of an intense cold neutron beam from a research reactor with a beam chopper offers higher signal-to-background ratio, more accurate timing and much simpler sample handling than conventional cyclic activation analysis. Application of a digital spectrum analyzer in data list mode allows for easy determination of the half-life. Hence, time-resolved activation analysis utilizing energy and time information becomes practical.

Thermal neutron scattering and transport in methane at 20K

Abstract A thermal neutron scattering kernel in methane has been suggested which takes into account the contributions from both phonon and libron exchange processes. The observed total neutron scattering cross-section can be explained at 20 K, when as many as fourteen libron exchange processes are considered in the thermal neutron scattering kernel along with phonon exchange processes upto two quanta. Using the suggested scattering kernel, thermal neutron transport under different conditions like time dependent, space dependent and steady state neutron spectra have been studied at 20 K. The results of these studies are quite different from other hydrogenous moderatorslike ice, etc. The production of intense beams of cold neutrons at 20 K and below indicate that normal methane is a better moderator for finite sizes of the assembly while heavy methane is a better source of cold neutrons for infinite sizes of the assembly as compared to other hydrogenous moderators.

T Violation in the weak scalar and tensor interaction: neutron and nuclei

The motivation and status of a search for time-reversal violation in nuclear and neutron beta decay are discussed. A new experiment for free neutron decay is proposed. A hitherto unmeasured amplitude, R, of the directional correlation J·( p× σ) , between the neutron spin J , the electron momentum p and the electron spin σ , will be determined. An accuracy well below 0.01 can be achieved using an intense cold neutron beam and an electron tracking detector, where the spin sensitivity is provided by large angle Mott scattering. This study provides an unique access to the exotic scalar S and tensor T interaction. Finite values, or tight constraints on the time-reversal violating scalar components, can be deduced in a combined analysis of the proposed experiment and a precise determination of the tensor couplings from a recent study of 8Li decay. The great interest in weak scalar interaction is stimulated by a multitude of scalar bosons which are introduced in most extensions of the Standard Model.

Transport of Thermal Neutrons in Different Forms of Liquid Hydrogen and the Production of Intense Beams of Cold Neutrons

From their studies the authors find that the thermal neutron inelastic scattering kernel incorporating the chemical binding energy in liquid hydrogen is able to successfully explain various neutron transport studies such as pulsed neutron and steady-state neutron spectra. For an infinite-sized assembly, D/sub 2/ at 4 K yields a very intense flux of cold and ultracold neutrons. For the practicable finite assembly corresponding to B/sup 2/ = 0.0158 cm/sup -2/, it is found that liquid hydrogen at 11 K gives the most intense beam of cold neutrons.

On the production of intense beams of cold neutrons

Theoretical steady state neutron spectra study on various H 2O–D 2O mixtures of sample size ≈ 0.082 m 3 placed at 21 K in a 1/ E spectrum, shows that the mixture containing 20% H 2O and 80% D 2O is the optimum for the production of intense beams of cold neutrons.