Measurements Of Neutron-induced Cross Sections Research Articles

Neutron-induced cross section measurements

Neutron-induced cross sections represent the main nuclear input to models of stellar and Big-Bang nucleosynthesis. While (n,γ) reactions are relevant for the formation of elements heavier than iron, (n,p) and (n,α) reactions can play an important role in specific cases. The time-of-flight method is routinely used at n_TOF to experimentally determine the cross section data. In addition, recent upgrades of the facility will allow the use of activation techniques as well, possibly opening the way to a systematic study of neutron interaction with radioactive isotopes. In the last 20 years n_TOF has provided a large amount of experimental data for Nuclear Astrophysics. Our plan is to carry on challenging measurements and produce nuclear data in the next decades as well.

Production of powder targets for neutron-induced cross section measurements

Nuclear powder targets for neutron-induced cross section measurements were prepared by pressing. The choice of the production technique was related to the type of nuclear experiment and the quality of the powder. This paper describes the production process of unsintered compacts of 94Mo, 95Mo, 96Mo metal powders and 239PuO2 powder with a thickness as low as possible by uniaxial pressing of powders. Special attention goes to the difficulties encountered during the preparation process of 239PuO2 pellets. Investigation on the presence of impurities and so the quality of the PuO2 material was performed by scanning electron microscopy with energy dispersive X-ray spectroscopy, by inductively coupled plasma mass spectrometry, X-ray diffraction and by thermal analysis.

Initial years’ neutron-induced cross-section measurements at the CSNS Back-n white neutron source * *Supported by the National Key Research and Development Plan (2016YFA0401600) and the National Natural Science Foundation of China (11675155, 11790321)

The Back-n white neutron source (known as Back-n) is based on back-streaming neutrons from the spallation target at the China Spallation Neutron Source (CSNS). With its excellent beam properties, e.g., a neutron flux of approximately 1.8×107 n/cm2/s at 55 m from the spallation target, energy range spanning from 0.5 eV to 200 MeV, and time-of-flight resolution of a few per thousand, along with the equipped physical spectrometers, Back-n is considered to be among the best facilities in the world for carrying out nuclear data measurements. Since its completion and commencement of operation in May 2018, five types of cross-section measurements concerning neutron capture cross-sections, fission cross-sections, total cross-sections, light charged particle emissions, in-beam gamma spectra, and more than forty nuclides have been measured. This article presents an overview of the experimental setup and result analysis on the neutron-induced cross-section measurements and gamma spectroscopy at Back-n in the initial years.

Indirect measurements of neutron cross-secti at heavy-ion storage rings

Cross sections for neutron-induced reactions of short-lived nuclei are essential for nuclear astrophysics since these reactions in the stars are responsible for the production of most heavy elements in the universe. These reactions are also key in applied domains like energy production and medicine. Nevertheless, neutron-induced cross-section measurements can be extremely challenging or even impossible to perform due to the radioactivity of the targets involved. Indirect measurements through the surrogate-reaction method can help to overcome these difficulties.The surrogate-reaction method relies on the use of an alternative reaction that will lead to the formation of the same excited nucleus as in the neutron-induced reaction of interest. The decay probabilities (for fission, neutron and gamma-ray emission) of the nucleus produced via the surrogate reaction allow one to constrain models and the prediction of the desired neutron cross sections.We propose to perform surrogate reaction measurements in inverse kinematics at heavy-ion storage rings, in particular at the CRYRING@ESR of the GSI/FAIR facility. We present the conceptual idea of the most promising setup to measure for the first time simultaneously the fission, neutron and gamma-ray emission probabilities. The results of the first simulations considering the 238U(d,d’) reaction are shown, as well as new technical developments that are being carried out towards this set-up.

A modified unscented transformation method for uncertainty propagation in neutron induced cross section measurement

This paper investigates the application of the modified unscented transformation (MUT) method for uncertainty propagation in neutron induced cross section measurement in Nuclear Science. The method is the improvement on unscented transformation (UT) technique by incorporating more parameters (not random) to increase the number of sigma points.We examined the application of MUT method for covariance analysis of cross section measurement in comparison to the first order sensitivity analysis method and Monte Carlo methods. The MUT method is simple to use and easier to implement in comparison to the sandwich formula. The Monte Carlo simulations had been carried out to illustrate the good performance of the MUT method in covariance analysis in neutron activation cross section measurement.

Preparation and characterization of 10B targets at JRC-Geel

Measurements of neutron-induced cross sections to generate nuclear data are a core activity of the JRC-Directorate G Standards for Nuclear Safety, Security and Safeguards unit in Geel. Thin 10B layers are of great importance in this activity as they are used to measure the absolute neutron flux in the beam using the standard 10B(n,α)7Li reaction cross-section as a reference. After a period of reduced activity and in line with a renewed interest for nuclear data, the demand for high quality 10B targets increased. In this paper we describe the design and features of a new e-beam evaporator specifically customized for the preparation of boron targets as replacement of the old dysfunctional equipment. Several 10B targets of varying thicknesses were prepared and characterized as part of the factory acceptance tests and implementation in the JRC-Geel target preparation laboratory. Differential substitution weighing was applied for mass determination and in order to calibrate the thickness monitor. Comparative time-of-flight measurements relative to 10B and 235U standard targets were conducted at the GELINA neutron time-of-flight facility at the JRC-Geel site as second methodology for the determination of 10B areal density. The morphology of the layers was assessed by means of Scanning Electron Microscopy (SEM). The determination of impurities was realized by means of Energy Dispersive X-ray (EDX). Finally, two boron targets were prepared in the frame of the measurement of the neutron induced fission cross-section of 230Th at the n_TOF neutron time-of-flight facility at CERN.

Neutron induced cross section measurements on gallium isotopes at neutron energy 14.90 ± 0.01 MeV and covariance analysis

The cross sections of the 69Ga(n,p)69Znm, 69Ga(n,2n)68Ga, 71Ga(n,p)71Znm, 71Ga(n,n′α)67Cu and 71Ga(n,2n)70Ga reactions had been measured at 14.90±0.01MeV neutron energy with the 27Al(n,α)24Na as monitor reaction using Neutron Activation Technique. The data analysis was carried out using the latest decay data. Detail uncertainty propagation had been performed and the measured cross sections are reported with their uncertainties and correlation coefficients. The cross sections newly measured in the present work are compared with the earlier reported cross sections available in the EXFOR Database. The measured cross sections reported by various authors had also been studied and discussed in detail. Theoretical calculations had been performed from reaction threshold to 20 MeV and compared along with the evaluated files from ENDF/B-VIII.0 and JENDL-4.0.

Development of a gaseous proton-recoil telescope for neutron flux measurements between 0.2 and 2 MeV neutron energy

Absolute measurements of neutron fluence are an essential prerequisite of neutron-induced cross section measurements, neutron beam lines characterization and dosimetric investigations. The H(n,p) elastic scattering cross section is a very well known standard used to perform precise neutron flux measurements in high precision measurements. The use of this technique, with proton recoil detectors, is not straightforward below incident neutron energy of 1 MeV, due to a high background in the detected proton spectrum. Experiments have been carried out at the AIFIRA facility to investigate such background and to determine its origin and components. Based on these investigations, a gaseous proton-recoil telescope has been designed with a reduced low energy background. A first test of this detector has been carried out at the AIFIRA facility, and first results will be presented.

Development of a Gaseous Proton-Recoil Detector for neutron flux measurements between 0.2 and 2 MeV neutron energy

Absolute measurements of neutron fluence are an essential prerequisite of neutron-induced cross section measurements, dosimetric investigations and neutron beam lines characterisation. Independent and precise neutron flux measurements can be performed with respect to the H(n,p) elastic cross section. However, the use of silicon proton recoil detectors is not straightforward below incident neutron energy of 1 MeV, due to a high background in the detected proton spectrum. A new gaseous proton-recoil detector has been designed to answer the challenge. The detector is described in details and results of the commissioning tests are presented.

General-purpose readout electronics for white neutron source at China Spallation Neutron Source.

The under-construction White Neutron Source (WNS) at China Spallation Neutron Source is a facility for accurate measurements of neutron-induced cross section. Seven spectrometers are planned at WNS. As the physical objectives of each spectrometer are different, the requirements for readout electronics are not the same. In order to simplify the development of the readout electronics, this paper presents a general method for detector signal readout. This method has advantages of expansibility and flexibility, which makes it adaptable to most detectors at WNS. In the WNS general-purpose readout electronics, signals from any kinds of detectors are conditioned by a dedicated signal conditioning module corresponding to this detector, and then digitized by a common waveform digitizer with high speed and high precision (1 GSPS at 12-bit) to obtain the full waveform data. The waveform digitizer uses a field programmable gate array chip to process the data stream and trigger information in real time. PXI Express platform is used to support the functionalities of data readout, clock distribution, and trigger information exchange between digitizers and trigger modules. Test results show that the performance of the WNS general-purpose readout electronics can meet the requirements of the WNS spectrometers.

Reactor neutrons in nuclear astrophysics

The huge neutron fluxes offer the possibility to use research reactors to produce isotopes of interest, which can be investigated afterwards. An example is the half-lives of long-lived isotopes like 129 I. A direct usage of reactor neutrons in the astrophysical energy regime is only possible, if the corresponding ions are not at rest in the laboratory frame. The combination of an ion storage ring with a reactor and a neutron guide could open the path to direct measurements of neutron-induced cross sections on short-lived radioactive isotopes in the astrophysically interesting energy regime.

Development of a gaseous recoil-proton detector for neutron flux measurements between 0.2 and 2 MeV neutron energy

Absolute measurements of neutron fluence are an essential prerequisite of neutron-induced cross section measurements, neutron beam lines characterisation and dosimetric investigations. Precise neutron flux measurements can be performed with respect to the H(n,p) elastic cross section. The use of this technique, with silicon proton recoil detectors, is not straightforward below incident neutron energy of 1 MeV, due to a high background in the detected proton spectrum. Experiments carried out at the AIFIRA facility identified its origin. Based on these investigations, a gaseous recoil-proton detector has been designed, with a reduced low energy background. Preliminary results of the first tests of the developed detector are discussed here.

A Micromegas Detector for Neutron Beam Imaging at the n_TOF Facility at CERN

Micromegas (Micro-MEsh Gaseous Structure) detectors are gas detectors consisting of a stack of one ionization and one proportional chamber. A micromesh separates the two communicating regions, where two different electric fields establish respectively a charge drift and a charge multiplication regime. The n_TOF facility at CERN provides a white neutron beam (from thermal up to GeV neutrons) for neutron induced cross section measurements. These measurements need a perfect knowlodge of the incident neutron beam, in particular regarding its spatial profile. A position sensitive micromegas detector equipped with a 10B based neutron/charged particle converter has been extensively used at the n_TOF facility for characterizing the neutron beam profile and extracting the beam interception factor for samples of different size. The boron converter allowed to scan the energy region of interest for neutron induced capture reactions as a function of the neutron energy, determined by the time of flight. Experimental results will be presented and compared to simulations, performed by means of the FLUKA code.

Neutron-induced Cross Section Measurements of Calcium

To support the US Department of Energy Nuclear Criticality Safety Program, neutron-induced cross section experiments were performed at the Geel Electron Linear Accelerator of the Institute for Reference Material and Measurements of the Joint Research Centers, European Union. Neutron capture and transmission measurements were carried out using a metallic calcium sample. The measured data will be used for a new calcium evaluation, which will be submitted with covariances to the ENDF/B nuclear data library.

Neutron-induced cross sections of short-lived nuclei via the surrogate reaction method

The measurement of neutron-induced cross sections of short-lived nuclei is extremely difficult due to the radioactivity of the samples. The surrogate reaction method is an indirect way of determining cross sections for nuclear reactions that proceed through a compound nucleus. This method presents the advantage that the target material can be stable or less radioactive than the material required for a neutron-induced measurement. We have successfully used the surrogate reaction method to extract neutron-induced fission cross sections of various short-lived actinides. In this work, we investigate whether this technique can be used to determine neutron-induced capture cross sections in the rare-earth region.

Measurements with the high flux lead slowing-down spectrometer at LANL

A Lead Slowing-Down Spectrometer (LSDS) was recently installed at LANL [D. Rochman, R.C. Haight, J.M. O’Donnell, A. Michaudon, S.A. Wender, D.J. Vieira, E.M. Bond, T.A. Bredeweg, A. Kronenberg, J.B. Wilhelmy, T. Ethvignot, T. Granier, M. Petit, Y. Danon, Characteristics of a lead slowing-down spectrometer coupled to the LANSCE accelerator, Nucl. Instr. and Meth. A 550 (2005) 397]. The LSDS is comprised of a cube of pure lead 1.2m on the side, with a spallation pulsed neutron source in its center. The LSDS is driven by 800MeV protons with a time-averaged current of up to 1μA, pulse widths of 0.05–0.25μs and a repetition rate of 20–40Hz. Spallation neutrons are created by directing the proton beam into an air-cooled tungsten target in the center of the lead cube. The neutrons slow down by scattering interactions with the lead and thus enable measurements of neutron-induced reaction rates as a function of the slowing-down time, which correlates to neutron energy. The advantage of an LSDS as a neutron spectrometer is that the neutron flux is 3–4 orders of magnitude higher than a standard time-of-flight experiment at the equivalent flight path, 5.6m. The effective energy range is 0.1eV to 100keV with a typical energy resolution of 30% from 1eV to 10keV. The average neutron flux between 1 and 10keV is about 1.7×109n/cm2/s/μA. This high flux makes the LSDS an important tool for neutron-induced cross section measurements of ultra-small samples (nanograms) or of samples with very low cross sections. The LSDS at LANL was initially built in order to measure the fission cross section of the short-lived metastable isotope of U-235, however it can also be used to measure (n,α) and (n,p) reactions. Fission cross section measurements were made with samples of 235U, 236U, 238U and 239Pu. The smallest sample measured was 10ng of 239Pu. Measurement of (n,α) cross section with 760ng of Li-6 was also demonstrated. Possible future cross section measurements include fission and (n,p) and (n,α) reaction in radioactive samples.

The Neutron Time Of Flight Facility at CERN

The neutron Time of Flight (n_TOF) facility at CERN is a high flux neutron source obtained by the spallation of 20 GeV/c protons onto a solid lead target. The proton beam is delivered by the Proton Synchrotron (PS)1) capable of providing up to four sharp bunches (6 ns) with an intensity of 71012 protons per bunch, within a 14.4 s supercycle. The outstanding characteristics of this facility: high neutron flux of 106 n/cm2/71012 p. at 182.5 m, wide spectral function from 1 eV up to 250 MeV, low repetition rates and an excellent energy resolution of 2 × 10–4 open new possibilities for high precision neutron induced cross section measurements, using samples of modest mass. The facility was commissioned in Nov. 2000 and Apr. 2001, with performances similar to those predicted by Monte Carlo simulations.