The Effect of Particle leaking and its Implications for Measurements of the (n,alpha) reaction on Light Elements Using Ionisation Chambers

A new technique for the spectrometry of (n,alpha) reactions on light elements at MeV energies has been developed and successfully used for the measurement of the 10B(n,alpha)7Li reaction at the 7MV Van de Graaff accelerator of IRMM. The basic elements of the new method are a gridded ionisation chamber, a fast waveform digitizer and advanced off-line analysis. The powerful data visualistion allowed the discovery of the effect of particle leaking. Particle leaking arises from the simultaneous emision of reaction products in forwards angles and the inablity of the detector to resolve multiple particles. It is an inherent property of all GIC spectrometers used for the study of (n,charged particle) reactions on light-element solid targets. The measurement of the cross section strongly benefits from it but the determination of other measurables is negatively affected. Cross sections at seven energies between 1.5 MeV and 3.8 MeV have been obtained using the new technique. Compared to evaluations the IRMM cross sections are close to the JENDL 3.2 and JEF 2.2 but strongly deviate from the ENDF/B-VI data with the exception of very good agreement at 2.5 MeV. Forwards anagular distrivbutions are truncated at large emission angles by the effect of particle leaking and look like depleted of reaction products between a kinematically determined angle theta0 and 90°. it is shown that all values of the branching ratio alpha0/alpha1 of the 10B(n,alpha)7Li reaction published up to now in refereed journals and obtained by using ionisation chambers and face to face surface barrier detectors contain inaccuracies caused by particle leaking which has not been considered.JRC.D.5-Neutron physic

The Cross-section of the 10B(n,alpha)7Li Reaction Measured in the MeV Energy Range

The excitation function of the 10B(n,alpha)7Li reaction was measured between 1.5 MeV and 5.6 MeV within the frame of the IAEA Coordinated Research project (CRP) on "Improvement of the Standard Cross Sections for Light Elements";. An ionisation chamber and signal digitisation were used for the spectrometry of the reaction products, which were forwards emitted by bombarding a thin boron target with neutrons produced at the IRMM Van de Graff accelerator. The neutron flux was determined by measuring the backwards emitted fission fragments from a 238U sample mounted in a back-to-back geometry relative to the boron target. The highlight of this study was the discovery of the kinematic effect of particle leaking, which is very important for the accurate determination of the number of the boron reaction events. The experiment is described and the results are discussed and compared with evaluations and experimental data of other groups.JRC.D.5-Neutron physic

The Effect of Particle Leaking and its Implications for Measurements of the (n,alpha) Reaction on Light Elements Using Ionisation Chambers.

Abstract not availableJRC.D-Institute for Reference Materials and Measurements (Geel

Novel 4π Detection System for the Measurement of the 6Li(n,α)3H Reaction Cross-Section

A dedicated one-dimensional Time Projection Chamber (1D-TPC) was designed and produced at IRMM to determine the 6Li(n,α)3H cross section, in the 0.4-2.8 MeV energy range, aiming at 5% accuracy. The basic TPC components were a twin gridded ionisation chamber (GIC) with interwired electrodes and fast digitisation of the anode and cathode signals. The energy of both reaction products emitted from a thin 6LiF sample at the common TPC cathode was measured. A Kr(97%)CO2(3%) mixture was used as the detector gas at a pressure up to 3.5 bar. A 238U sample mounted on the cathode of an ionisation chamber without grid was used as the neutron flux monitor. Special care was taken to reduce the experimental sources of uncertainty. The beam-monitor 238U sample was characterised at IRMM by low-geometry α-counting with an accuracy of 0.1%. A 6Li sample was produced at IRMM by vacuum evaporation of 6LiF onto transparent aluminium backing. The number of 6Li atoms will be measured via Thermal Neutron Depth Profiling with an expected accuracy of 2% with respect to an IRMM Standard Reference Material. First test measurements were performed using a monoenergetic neutron beam produced by the T(p,n)3He reaction at the IRMM 7MV Van de Graaff accelerator. The experimental method and preliminary results are presented.JRC.D.4-Nuclear physic

Particle Leaking, Cross-Section Ratio 10B(n, )/238U(n,fission), and Excitation Function of the Reaction 10B(n, )7Li at MeV Energies

The 10B(n, )7Li reaction was studied in the energy range between 1.5 MeV and 5.6 MeV at the 7-MV Van de Graaff accelerator of IRMM by using a gridded ionisation chamber, signal digitisation, and an intrinsic 238U neutron monitor. The aim was to obtain accurate data for the IAEA Coordinated Research Project (CRP) on the improvement of standard cross sections for light elements. The effect of particle leaking was discovered and its implications investigated. The determination of the cross section delta( alpha0+alpha 1) strongly benefits from it but measurements of angular distributions, individual cross sections delta(alpha 0) and delta( alpha1), and the branching ratio alpha0/ alpha1 are negatively affected. The correct number of reaction events was obtained by identification of unknown particle signatures in the energy spectra as 10B(n,alpha)7Li events in the form of quasi 7Li+ alpha particles created by particle leaking. The cross-section ratio 10B(n,alpha)7Li/238U(n,fission) was measured and the excitation function of 10B(n,alpha)7Li determined by simultaneously detecting the charged particles from the boron disintegration in the forward hemisphere and the 238U fission fragments in the backward hemisphere. The IRMM cross sections are compared to experimental data of other groups and to predictions of the ENDF/B-VI.8, JENDL-3.3, and JEF-2.2 evaluations.JRC.D.5-Neutron physic

Preparation of 6LiF deposits and characterisation via Monte Carlo simulations and Neutron Depth Profiling

The Institute for Reference Materials and Measurements (IRMM) is measuring the 6Li(n,t)4He cross section aiming at extending its status of standard over the MeV energy range. We developed a protocol to stretch-mount 0.75 micronmeter, 1.5 micronmeter, 8 micronmeter, and 20 micronmeter thick aluminium foils onto 0.5 mm thick tantalum rings. 6LiF samples were produced by depositing, by vacuum evaporation onto the aluminium backings, a layer of lithium fluoride 95.5% enriched in 6Li. We engineered dedicated tools and containers to handle and transport the resulting samples. These were characterised first at IRMM by differential weighing, then by Neutron Depth Profiling (NDP) at the TU Delft. These two measurements were found to be consistent for a selected sample, probed by a thermal neutron beam in three different regions to measure the 6LiF layer thickness and uniformity (defined as variation of the thickness relative to its average). The latter was found to be 0.8%, and the 6Li thickness to be 7.30 +/- 0.12, 7.35 +/- 0.12, and 7.29 +/- 0.12 micronmeter/cm2 in the three regions. We performed Monte Carlo simulations to estimate the uniformity of the 6LiF layer, and benchmarked the calculation against the NDP measurements. They were consistent with respect to the deposit uniformity although the simulations were found to overestimate the thickness of the layer.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

The Cross Section of the 16O(n,alpha) 13C Reaction in the MeV Energy Range

A novel spectrometer was developed and used to measure the cross section for the 16O(n,alpha) reaction at IRMM. The basic parts of the new instrument are an ionisation chamber, a gas oxygen target, and signal digitisation. It is shown that simultaneous digitisation of the anode and cathode signals allows an effective background suppression and the accurate determination of the number of reaction events and the number of atoms in the gas target. Cross section values for the 16O(n,alpha) reaction measured in the energy range 3.95-9.0MeV are presented. None of the existing nuclear data libraries describes well the IRMM data in the entire energy range.JRC.D.5-Neutron physic

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

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.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard

Elastic Recoil Detection analysis of Hydrogen at IRMM

Abstract not availableJRC.D-Institute for Reference Materials and Measurements (Geel

Experimental Study of the (n,alpha) Reaction on a Set of Light Nuclei

An experimental setup based on an ionization chamber with Frisch grid and waveform digitizer was used for (n,a) cross section measurements. Use of digital signal processing allowed us to select a gaseous cell inside the sensitive area of the ionisation chamber with high accuracy. This kind of approach provides a powerful method to suppress background from detector components and parasitic reactions on the working gas. The new method is especially interesting for the study of reactions on elements for which solid target preparation is difficult (e.g. noble gases). Additionally it has the advantage of an accurate determination of the number of nonradioactive nuclei in the selected gas cell. In the present experiments a set of working gases was used, which contained admixtures of nitrogen, oxygen, neon, argon and boron. Fission of 238U was used as neutron flux monitor. The cross section of the (n,a) reaction for 16O, 14N, 20Ne, 36Ar, 40Ar and the branching ratio alpha0/alpha1 of the 10B(n,alpha0) to 10B(n,alpha1) reactions were measured for neutron energies between 1.5 and 7 MeV.JRC.D.5-Nuclear physic