Cold Neutron Beam Facility Research Articles

Realization of an advanced super-mirror solid-state neutron polarizer for the instrument PF1B at the Institut Laue-Langevin.

In this last of a series of three papers on the development of an advanced solid-state neutron polarizer, we present the final construction of the polarizer and the results of its commissioning. The polarizer uses spin-selective reflection of neutrons by interfaces coated with polarizing super-mirrors. The polarizer is built entirely in-house for the PF1B cold neutron beam facility at the Institut Maxvon Laue-Paul Langevin (ILL). It has been installed in the PF1B casemate and tested under real conditions. The average transmission for the "good" spin component is measured to be >30%. The polarization averaged over the capture spectrum reaches a record value of Pn ≈ 0.997 for the full angular divergence in the neutron beam, delivered by the H113 neutron guide, and the full wavelength band λ of 0.3-2.0nm. This unprecedented performance is due to a series of innovations in the design and fabrication in the following domains: choice of the substrate material, super-mirror and anti-reflecting multilayer coatings, magnetizing field, and assembling process. The polarizer is used for user experiments at PF1B since the last reactor cycle in 2020.

BRAND—A detection system for [formula omitted]-decay correlation measurement

The BRAND experiment aims at the search of Beyond Standard Model (BSM) physics via measurement of exotic components of the weak interaction. For this purpose, eleven correlation coefficients of neutron β-decay will be measured simultaneously. The BRAND detection system is oriented for the registration of charged products of β-decay of polarized, free neutrons. With the measurement of the four-momenta of electron and proton, the complete kinematic of the decay will be determined. Moreover, the transverse spin component of the electron, which is the crucial observable to probe BSM exotic components of weak interaction, will be measured via Mott scattering. The electron detection system features both tracking and energy measurement capability. It is also responsible for the determination of the electron spin orientation. A challenging detection of low-energy protons from the β-decay is performed with a system, which involves the acceleration and subsequent conversion of protons into bunches of electrons. To test the feasibility of the proposed experimental techniques, a small-scale prototype setup was installed at the cold neutron beam facility PF1B at the Laue-Langevin Institute (ILL) in Grenoble, France. In this contribution, the preliminary results of the commissioning run are presented with an emphasis on the performance of individual parts of the detection system.

ANNI – A pulsed cold neutron beam facility for particle physics at the ESS

Pulsed beams have tremendous advantages for precision experiments with cold neutrons. In order to minimise and measure systematic effects, they are used at continuous sources in spite of the related substantial decrease in intensity. At the European Spallation Source ESS these experiments will profit from the pulse structure of the source and its 50 times higher peak brightness compared to the most intense reactor facilities, making novel concepts feasible. Therefore, the cold neutron beam facility for particle physics ANNI was proposed as part of the ESS instrument suite. The proposed design has been re-optimised to take into account the present ESS cold moderator layout. We present design considerations, the optimised instrument parameters and performance, and expected gain factors for several reference experiments.

A new detector system for the measurement of high-energy prompt [formula omitted]-rays for low-energy neutron induced fission

We have developed a new setup to measure prompt fission γ-ray spectra in neutron induced fission up to energies sufficient to reveal the structure associated with giant dipole resonances of fission fragments. The setup consists of multi-wire proportional counters, to detect both fission fragments in coincidence, and two large volume (101.6 mm in diameter and 127.0 mm in length) LaBr3(Ce) scintillators, to measure the γ rays. The setup was used to obtain the prompt fission γ-ray spectrum for thermal neutron induced fission of 235U at the PF1B cold-neutron beam facility of the Institut Laue-Langevin, Grenoble, France. We have successfully measured the γ-ray spectrum up to energies of about 20 MeV, what extends the currently known γ-ray spectrum limit to higher energies by approximately a factor of two.

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.

Identification of excited states and collectivity in Se88

The $\ensuremath{\gamma}$ decays of excited states in the very neutron-rich nucleus $^{88}\mathrm{Se}$ have been observed following the neutron-induced fission of $^{235}\mathrm{U}$. The measurement was performed using the EXILL array of Ge detectors at the PF1B cold-neutron beam facility of the Institut Laue-Langevin, Grenoble. The level scheme of $^{88}\mathrm{Se}$ was established using $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidences. A low $({2}_{1}^{+})$ energy hints at the onset of quadrupole deformation, and the identification of possible members of a $({2}_{2}^{+})$ band provide evidence for $\ensuremath{\gamma}$ vibrations. Shell-model calculations using a $^{78}\mathrm{Ni}$ core reproduce the decay scheme well. Equivalent deformation and $B(E2)$ values have previously been predicted for $^{88}\mathrm{Se}$ using pseudo-SU(3), shell-model, and beyond mean-field frameworks implying that interactions between particles in the $\ensuremath{\pi}{f}_{5/2}p$ and $\ensuremath{\nu}sd$ orbits are mostly responsible for the collectivity present.

Ultracold-neutron production and up-scattering in superfluid helium between 1.1 K and 2.4 K

Ultracold neutrons (UCNs) were produced in a 4 liter volume of superfluid helium using the PF1B cold neutron beam facility at the Institut Laue-Langevin and then extracted to a detector at room temperature. With a converter temperature of 1.08 K the number of accumulated UCNs was counted to be $91,\!700 \pm 300$. From this, we derive a volumetric UCN production rate of $(6.9 \pm 1.7)\,\mathrm{cm^{-3}\,s^{-1}}$, which includes a correction for losses in the converter during UCN extraction caused by a short storage time, but not accounting for UCN transport and detection efficiencies. The up-scattering rate of UCNs due to excitations in the superfluid was studied by scanning the temperature between 1.2-2.4 K. Using the temperature-dependent UCN production rate calculated from inelastic neutron scattering data in the analysis, the only UCN up-scattering process found to be present was from two-phonon scattering. Our analysis rules out contributions from the other scattering processes to $\lesssim 10\%$ of their predicted levels.

Neutron-proton multiplets in the nucleusBr88

Medium spin excited levels in $^{88}\mathrm{Br}$ populated in fission of $^{235}\mathrm{U}$ induced by neutrons have been observed for the first time. The measurement of $\ensuremath{\gamma}$ radiation following fission has been performed using the EXILL array of Ge detectors at the cold-neutron beam facility PF1B of the Institut Laue-Langevin (ILL), Grenoble. The ground state of $^{88}\mathrm{Br}$ is proposed to be ${1}^{\ensuremath{-}}$, changing the adopted $({2}^{\ensuremath{-}})$ value. The low-energy, newly observed levels are members of the $\ensuremath{\pi}{p}_{3/2}\ensuremath{\nu}{({d}_{5/2})}^{3}$ and $\ensuremath{\pi}{f}_{5/2}^{\ensuremath{-}1}\ensuremath{\nu}{({d}_{5/2})}^{3}$ multiplets. A triplet of yrast levels observed at around 2 MeV is interpreted as being due to coupling of the ${g}_{9/2}$ proton to the $({d}_{5/2}{)}^{3}$, seniority 3 multiplet, supporting the presence of collective effects in $^{88}\mathrm{Br}$. The position of the ${g}_{9/2}$ proton intruder in the $^{78}\mathrm{Ni}$ core is determined at 5.7 MeV above the ${f}_{5/2}$ proton level. Shell-model calculations predict the same proton-neutron excitations proposed in $^{88}\mathrm{Br}$.

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.

Neutron capture studies of 206Pb at a cold neutron beam

Gamma-ray transitions following neutron capture in 206Pb have been studied at the cold neutron beam facility of the Budapest Neutron Centre using a metallic sample enriched in 206Pb and a natural lead nitrate powder pellet. The measurements were performed using a coaxial HPGe detector with Compton suppression. The observed \( \gamma\) -rays have been incorporated into a decay scheme for neutron capture in 206Pb . Partial capture cross sections for 206Pb(n,\( \gamma\)) at thermal energy have been derived relative to the cross section for the 1884keV transition after neutron capture in 14N . From the average crossing sum a total thermal neutron capture cross section of \( 29^{+2}_{-1}\) mb was derived for the 206Pb(n,\( \gamma\)) reaction. The thermal neutron capture cross section for 206Pb has been compared with contributions due to both direct capture and distant unbound s-wave resonances. From the same measurements a thermal neutron-induced capture cross section of \( (649 \pm 14)\) mb was determined for the 207Pb(n,\( \gamma\)) reaction.

Isomeric levels in92Rb and the structure of neutron-rich92,94Rb isotopes

The medium-spin structure of the ${}^{92}$Rb nucleus, populated in spontaneous fission of ${}^{248}$Cm and ${}^{252}$Cf, has been studied using the EUROGAM2 and the GAMMASPHERE Ge arrays, respectively. Two isomers have been found in ${}^{92}$Rb at 284.2 and 1958.2 keV with half-lives of ${T}_{1/2}=54(3)$ ns and ${T}_{1/2}=7(2)$ ns, respectively. A measurement of neutron-induced fission of ${}^{235}$U, at the PF1B cold-neutron beam facility of the Institut Laue Langevin, Grenoble, has been performed to confirm the assignment of the ${T}_{1/2}=54(3)$ ns, 284.2-keV isomeric level to the ${}^{92}$Rb nucleus. Excited levels in ${}^{92}$Rb and ${}^{94}$Rb nuclei have been calculated in a large-scale shell-model framework. Isomers observed in these two nuclei have been interpreted as proton-neutron configurations involving the high-$j$ $\ensuremath{\nu}1{h}_{11/2}$ and $\ensuremath{\pi}1{g}_{9/2}$ orbitals.

Determination of the 209Bi( [formula omitted]) 210Bi and 209Bi( [formula omitted]) 210m,gBi reaction cross sections in a cold neutron beam

Gamma-ray transitions following neutron capture in 209Bi have been studied at the cold neutron beam facility of the Budapest Neutron Centre. Measurements have been performed using a coaxial HPGe detector with Compton suppression. Partial capture cross sections at a velocity of 2200 m/s (or a neutron energy of 25.3 meV) have been deduced relatively to the one for the 4055 keV transition after thermal neutron capture in 209Bi. This partial cross section of (8.07 ± 0.14) mb has in turn been obtained from measurements with a bismuth nitrate target relatively to the cross section for the 1884 keV transition after thermal neutron capture in 14N. Shell model calculations have been performed to deduce the half-life of the 3rd excited state at 320 keV and to assign the M1 multipolarity of the 320 keV transition to the ground state. Lower limits of (21.4 ± 0.8) mb and (18.2 ± 0.7) mb for the cross sections to the ground state and to the isomeric state, respectively, have been deduced from the data for the gamma rays feeding these states. The results obtained in this work are in good agreement with experimental data reported in the literature. The thermal total capture cross section has been compared with the contribution due to observed s-wave resonances.

Monte Carlo Study of the abBA Experiment: Detector Response and Physics Analysis.

The abBA collaboration proposes to conduct a comprehensive program of precise measurements of neutron β-decay coefficients a (the correlation between the neutrino momentum and the decay electron momentum), b (the electron energy spectral distortion term), A (the correlation between the neutron spin and the decay electron momentum), and B (the correlation between the neutron spin and the decay neutrino momentum) at a cold neutron beam facility. We have used a GEANT4-based code to simulate the propagation of decay electrons and protons in the electromagnetic spectrometer and study the energy and timing response of a pair of Silicon detectors. We used these results to examine systematic effects and find the uncertainties with which the physics parameters a, b, A, and B can be extracted from an over-determined experimental data set.

Cold neutron PGAA facility developments at university research reactors in the USA

The PGAA applications can be enhanced by using subthermal neutrons, cold neutrons at university research reactors. Only two cold neutron beam facilities were developed at the U.S. university research reactors, namely at Cornell University and the University of Texas at Austin. Both facilities used mesitylene moderator. The mesitylene moderator in the Cornell Cold Neutron Beam Facility (CNBF) was cooled by a helium cryorefrigerator via copper cold fingers to maintain the moderator below 30 K at full power reactor operation. Texas Cold Neutron Source (TCNS) also uses mesitylene moderator that is cooled by a cryorefrigerator via a neon thermosiphon. The operation of the TCNS is based on a helium cryorefrigerator, which liquefies neon gas in a 3-m long thermosiphon. The thermosiphon cools and maintains mesitylene moderator at about 30 K in a chamber. Neutrons streaming through the mesitylene chamber are moderated and thus reduce their energy to produce a cold neutron distribution.

An experiment to measure λ= GA/ GV from a combination of angular correlation coefficients in the beta decay of polarized neutrons

A technique is described which allows us to measure the ratio λ= G A/ G V of the weak axial-vector and vector coupling constants in neutron decay without any measurement of the neutron beam polarization. λ is determined from a combination of the parity-odd angular correlations σ → · p → e and σ → · p → ν between the neutron spin σ → and the electron momentum p → e and the anti-neutrino momentum p → ν , respectively, both of which are measured in a single experiment using the same neutron beam. A description of the experiment and the results of the first run at the cold neutron beam facility at the Institut Laue–Langevin are presented.

On the Design of a Cold Neutron Source

Two major parts of the design study for a cold source for the cold neutron beam facility under construction at the Cornell University 500-kW TRIG A reactor are presented. The reasons are discussed ...