ILL Reactor Research Articles

Preparation of facilities for fundamental research with ultracold neutrons at PNPI

The WWR-M reactor of PNPI offers a unique opportunity to prepare a source for ultracold neutrons (UCN) in an environment of high neutron flux (about 3×10 12 n/cm 2/s) but still acceptable radiation heat release (about 4×10 −3 W/g). It can be realized within the thermal column situated close to the reactor core. With its large diameter of 1 m, this channel allows to install a 15-cm-thick bismuth shielding, a graphite premoderator (300 dm 3 at 20 K), and a superfluid helium converter (35 dm 3). At a temperature of 1.2 K it is possible to remove the heat release power of about 20 W. Using 4π flux of cold neutrons within the reactor column can bring more than a factor 100 of cold neutron flux incident on the superfluid helium with respect to the present cold neutron beam conditions at the ILL reactor. The storage lifetime for UCN in superfluid He at 1.2 K is about 30 s, which is sufficient when feeding experiments requiring a similar filling time. The calculated density of UCN with energy between 50 and 230 neV in an experimental volume of 40 l is about 10 4 n/cm 3. Technical solutions for realization of the project are discussed.

Preparation of a one-curie 171Tm target for the detector for advanced neutron capture experiments (DANCE)

Approximately one curie of 171Tm (T1/2 = 1.92a) has been produced and purified for the purpose of making a nuclear target for the first measurements of its neutron capture cross section. Target preparation consisted of three key steps: (1) material production; (2) separation and purification; and (3) electrodeposition onto a suitable backing material. Approximately 1.5 mg of the target material (at the time of separation) was produced by irradiating ca. 250 mg of its stable enriched 170Er lanthanide neighbour with neutrons at the ILL reactor in France. This production method resulted in a “difficult-to-separate” 1:167 mixture of near-neighboring lanthanides, Tm and Er. Separation and purification was accomplished using high-performance liquid chromatography (HPLC), with a proprietary cation-exchange column (Dionex, CS-3) and alpha-hydroxyisobutyric acid (α-HIB) eluent. This technique yielded a final product of ∼95% purity with respect to Tm. A portion (20 μg) of the Tm was electrodeposited onto thin Be foil and delivered to the Los Alamos Neutron Science Center (LANSCE) for preliminary analysis of its neutron capture cross section using the Detector for Advanced Neutron Capture Experiments (DANCE). This paper discusses the major hurdles associated with the separation and purification step, including scale-up issues related to the use of HPLC for material separation and purification of the target material from α-HIB and 4-(2-pyridylazo)resorcinol (PAR) colorant.

Triaxiality inMo105andMo107from the low to intermediate spin region

The nuclear structure of the odd $^{105}\mathrm{Mo}$ and $^{107}\mathrm{Mo}$ isotopes was reinvestigated in the present work. The excited levels of $^{105}\mathrm{Mo}$ were studied by observing prompt \ensuremath{\gamma} rays emitted after the spontaneous fission of $^{248}\mathrm{Cm}$ with the EUROGAM2 multidetector array. The already well-studied level structure of $^{107}\mathrm{Mo}$ was also completed by a search for microsecond isomers. For this purpose, this nucleus was produced by the thermal-neutron-induced fission reaction at the ILL reactor, in Grenoble. We have shown that the experimental level energies and the \ensuremath{\gamma}-decay patterns are well reproduced by simple particle-rotor calculations, assuming that these nuclei have an asymmetric shape. The shapes of the odd and even Mo are compared in the neutron range $N=62\text{\ensuremath{-}}66$.

First observation of low-lying excited states in the very neutron-richKr95

Microsecond isomers of neutron-rich nuclei in the mass $A=95$ chain were investigated at the ILL reactor in Grenoble. These nuclei were produced by thermal-neutron induced fission of $^{241}\mathrm{Pu}$. The detection is based on time correlation between fission fragments selected by the Lohengrin mass spectrometer and the \ensuremath{\gamma} rays from the isomers. In this article, the decay scheme of an isomer of half-life 1.4(2) $\ensuremath{\mu}$s in $^{95}\mathrm{Kr}$ is reported. It is the first time that excited states of this nucleus have been measured. These data show that this neutron-rich $N=59$ isotone is spherical at low excitation energy.

Neutron-rich In and Cd isotopes close to the doubly magicSn132

Microsecond isomers in the In and Cd isotopes, in the mass range $A=123\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}130$, were investigated at the ILL reactor, Grenoble, using the LOHENGRIN mass spectrometer, through thermal-neutron induced fission reactions of Pu targets. The level schemes of the odd-mass $^{123--129}\mathrm{In}$ and new measurements of the $\ensuremath{\mu}\mathrm{s}$ half-lives of the odd-odd $^{126--130}\mathrm{In}$ are reported. However, the expected ${8}^{+}$ isomers in the even-mass Cd isotopes were not observed. The comparisons between the experimental $B(M2)$ strengths for In and Sn isotopes are discussed. A shell-model study of the heaviest In and Cd nuclei was performed using a realistic interaction derived from the CD-Bonn nucleon-nucleon potential. The calculation predicts values of the half-lives of the first ${8}^{+}$ states in $^{126,128}\mathrm{Cd}$ of $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, which could explain the nonobservation of $\ensuremath{\mu}\mathrm{s}$ isomers. Comparison shows that the calculated levels of $^{130}\mathrm{In}$ and $^{129}\mathrm{In}$ are in good agreement with the experimental values while some discrepancies occur for the lighter In isotopes. The collectivity of $^{126,128}\mathrm{Cd}$ is discussed in the framework of the shell model and in comparison with $^{204}\mathrm{Hg}$.

The Laue diffraction method to search for a neutron EDM. Experimental test of the sensitivity

The feasibility of an experiment to search for the neutron electric dipole moment (EDM) by Laue diffraction in crystals without a center of symmetry was tested. At the PF1A beam of the ILL reactor a record time delay of τ≈2 ms for the passage of neutrons through a quartz crystal was reached for the (1 1 0) plane and diffraction angles equal to 88.5°. That corresponds to an effective neutron velocity in the crystal of 20 m/s, while the velocity of the incident neutron was 800 m/s. It was shown experimentally that the value τN 1/2, determining the method's sensitivity, has a maximum for the Bragg angle equal to 86°. The results allow us to estimate the statistical sensitivity of the method for the neutron EDM. For the PF1B beam of the ILL reactor the sensitivity can reach ∼6×10 −25 e cm per day for the available quartz crystal.

First attempt of neutron quantum state reconstruction

The neutron interferometer set-up S18 at the ILL reactor has been used for the first neutron quantum state reconstruction experiment. By a simultaneous measurement of the coherence function and of the modulated momentum distribution behind the interferometer, the Wigner function of various quantum states can be reconstructed. This is equivalent to knowledge of the particle wavefunction. Its spatial dimension follows from the measurement of the coherence function, which is the autocorrelation function of the wavefunction and the shape in momentum space follows from momentum post-selection experiments. Non-orthogonal traces through the (x, k)-phase space have to be measured for an unambiguous reconstruction. Nearly classical (coherent) and non-classical states in the form of Schrodinger cat-like states have been identified. Comparisons of calculated and measured Wigner functions will be shown.

Heinz Maier-Leibnitz

Heinz Maier-Leibnitz

E9: The new high-resolution powder diffractometer at BENSC

In the past only powder diffractometers with medium resolution were available at BENSC. To satisfy ademand from the user community for crystal structure refinements using the Rierveld method, a new high-resolution powder diffractometer has been installed at the BER2 reactor and it came into user operation during the second half of 1999. The instrument is designed for investigating crystal structures with cell volumes up to 1000 k3 by profile refinements of powder diffractograms. Data sets of sufficient resolution and statistics depenlng on the complexity of the structure are needed to apply the Rietveld method successfully. To fulfill these demands, it was decided to build an instrument based on a classical angular dispersive diffractometer similar to the D2B at the ILL however, with relaxed collimations considering the lower thermal flux at the BER2 compared to the ILL reactor. Beside the standard setting with a take-off angle at the monochromator above 90°, the instrument allows to vary the take-off angle in a large angular range. In combination with the optional use of different reflecting planes of the Ge-monochromator crystals, the diffractometer exhibits a great flexibility in the choice of wavelength and resolution. In the following we willshortly describe the instrument design and list the important instrumental specifications. In particular we will describe the performance of the instrument and focus on the line profile shapes, which are crucial for the quality of the Rietveld refinement. Finally an example is presented which illustrates the possibilities the instrument off ersfor crystal structure refinements.

Inelastic neutron scattering to very high pressures

Progress in high-pressure and neutron scattering methods has recently allowed measurements of phonon dispersion curves of simple solids at high pressures to 10 GPa. In this technique single crystals of 10–25 mm3 volume are compressed by the Paris-Edinburgh cell and the phonon frequencies are measured on high-flux triple axis spectrometers. Detailed studies of the lattice dynamics of low-compressible systems are feasible, including measurements of mode Gruneisen parameters, elastic constants, and precursor effects of phase transitions. We describe the experimental set-up and illustrate its potential by results on semiconductors (Ge and GaSb) and metals (Fe and Zn) obtained at the LLB (Saclay) and ILL (Grenoble) reactor sources.

Neutron fibre diffraction: Recent advances at the ILL

Before the ILL reactor shutdown in 1991 a group from Keele started neutron diffraction studies of DNA on diffractometer D19. Although neutron diffraction had been used before to investigate the structural properties of fibrous polymers these were the first high-resolution studies. Following the restart in 1995 groups from Cambridge, EMBL Grenoble, ILL, Julich, Keele, Parma and Stockholm began studies using this new technique on a wide variety of systems. This report outlines this trend and the technological and methodological advances which are making it possible.

The uranium target and source for the PIAFE project

In the PIAFE project, the uranium source will be exposed to the high neutron flux of the ILL reactor. Fission products will be ionized and accelerated at energies up to 30 keV for further mass selection. A description of the source and the tests on the rhenium container are given.

Startup of the ILL reactor planned

Startup of the ILL reactor planned

Experimental exclusion of neutral resonances in Bhabha scattering at MeV energies

Using an active-shadow technique and a monoenergetic e + beam at the high-flux reactor of ILL, we pursued the search for long-lived (> 5 × 10 −13 s) neutral resonances that could be formed in e +e − scattering around an invariant mass of 1.8 MeV/ c 2. With a significantly improved experimental sensitivity we found no evidence for such a narrow resonance decaying into e +e − pairs. From our whole investigations, the model-independent stringent lower limits (95% CL) of 5 × 10 −11 s and 1 × 10 −10 s can be set on the lifetime of hypothetical J = 0 and J = 1 resonances, respectively. Our conclusive results rule out theoretical proposals that involve neutral free particles or e +e − bound states in this mass range, as an explanation of the e +e − lines observed at GSI.

Free neutron oscillations

The status of the free neutron oscillation search is described and the new experiment at the ILL reactor in Grenoble aiming at investigating the neutron-antineutron oscillation time up to 10 8 s is discussed.

An experimental limit on production of axions in a fission reactor

We report on an experiment conducted at the ILL reactor aimed at searching for coincident gamma-ray events from axion decay. No such events are seen and our results are interpreted, using the calculations by Donnelly et al., as an upper limit of the axion mass of 280 keV.

Absolute measurement of the beta spectrum from 235U fission as a basis for reactor antineutrino experiments

The beta spectrum from fission products in thermal-neutron induced fission of 235U has been measured on line at the ILL reactor with a magnetic spectrometer. In the energy range from E β = 2.0 to 7.5 MeV the spectrum was determined per fission with an accuracy of 5%. The experimental electron spectrum was converted to the corresponding electron antineutrino spectrum to form a basis for the study of antineutrino induced reactions at reactors.