High-flux Reactor FRM-II Research Articles

Shielding design for the PANDA spectrometer at the Munich high-flux reactor FRM-II

The start-up of the Munich high-flux reactor FRM-II is in progress on. At the beam tube SR-2 the spectrometer PANDA has been installed. It is at three-axis neutron spectrometer looking onto a slightly under-moderated cold neutron source. For polarisation analysis, PANDA is equipped with a vertical cryomagnet producing fields up to 14.5 T for the sample. To get an appropriate shielding of the high-intensity instrument, one has to take into account the large cross section of the primary beam, several restrictions using magnetic materials, limitations in loading the site and finally, has to keep the lateral extent of the shielding small to allow for high-scattering angles. The shielding has been designed on the basis of the results, which were achieved by the combined use of both the Monte Carlo code MCNP-4B2 and an analytical method based on one-dimensional dose transmission functions.

Radioactive ion beam acceleration at MAFF

In April 2003, the German safety commission has given the final approval for the oper- ation of the high flux reactor FRM-II. This is an important step towards the development and installation of the Munich accelerator for fission fragments (MAFF), which will deliver highest intensities of neutron rich fission fragments. The acceleration chain of MAFF [1] consists of a charge breeder, which will deliver the ions with a mass to charge ratio of A/q ⩽ 6.3 irrespective of the mass range, and with a repetition rate of maximum 50 Hz. The LINAC operating at 10% duty cycle is composed of a 101.28 IH-RFQ, which will boost up the energy from 2.5 up to 300 keV/u, three IH-tanks that will deliver an energy of 5.4 MeV/u and 2 seven gap IH-resonators that are used to vary the final energy up to a maximum of 5.9 MeV/u. Currently beam dynamics revisions are in progress especially in the low energy section, since the experimental program has requested specific time structures of the beam for TOF experiments. The status of the beam dynamics studies as well as the status of the single components of the accelerator will be presented in this paper.

Design and performance of a two-dimensional multi-wire neutron detector with a sensitive area of 50×50 cm 2 and a spatial resolution of ∼2×2 mm 2

One two-dimensional position-sensitive multi-wire gaseous detector for small-angle neutron scattering (SANS), reflectometry and high-resolution diffractometry has been developed at the GKSS research center in co-operation with DENEX GmbH. The counter with a sensitive area of 500×500 mm 2 has been designed to be used in the reflectometer REFSANS being built at the new high flux reactor FRM-II. Tests of the detector with an 55Fe-source, cold neutrons and γ-radiation are presented.

The potential of the horizontal reflectometer REFSANS/FRM-II for measuring low reflectivity and diffuse surface scattering

The reflectometer REFSANS being built at the high flux reactor FRM-II in Munich is dedicated to the analysis of surfaces, interfaces and phase boundaries at the air–water interface of liquid/soft matter. Novel time-of-flight design and neutron optics have been developed to make it a most flexible device for the comprehensive analysis of samples with horizontally aligned surfaces. The potential of REFSANS for measuring low reflectivity and off-specular scattering is outlined.

Perspectives for polarised reflectometry at the novel reflectometer REFSANS at FRM-II in Munich/Germany

The reflectometer REFSANS being built at the high flux reactor FRM-II in Munich is dedicated to the analysis of interfaces, phase boundaries and surfaces at the air–water interface of liquid/soft samples. Novel neutron optics have been developed for REFSANS to measure weak diffuse surface scattering most efficiently for comprehensive analyses of lateral surface structures. The machine may further be used for high-resolution SANS of bulk materials with the option of multi-beam focussing in the detector plane. Furthermore, REFSANS will allow of polarised investigations including polarisation analysis. This makes REFSANS a novel and unique tool for investigations on large-scale structures, especially for biological research projects: weak coherent scattering contributions may reliably be separated from incoherent scattering of hydrogen and comprehensive analyses of magnetic materials may be performed.

MAFF — The Munich accelerator for fission fragments

At the new high flux reactor FRM-II in Munich the accelerator MAFF (Munich accelerator for fission fragments) is under design. In the high neutron flux of 1014 n/cm2 s up to 1014 neutron-rich fission fragments per second are produced in the 1 g U-235 target. Ions with an energy of 30 keV are extracted from the ion source. In the mass separator two isotopes can be selected. One of the beams is used for low energy experiments, the other one is injected into an ECRIS (or EBIS) for charge breeding to a q/A≥0.16. A gas filled RFQ cooler is used for emittance improvement. The subsequent LINAC delivers beams with an energy ranging from 3.7 MeV/u to 5.9 MeV/u. New IH structures are being developed at the Munich tandem laboratory. A small storage ring is planned in a further stage to recycle the fission fragments. A thin target foil can be placed into this ring, e.g., for synthesis of super-heavy elements. The through-going beam tube has been installed in the heavy water tank of the reactor. Tests of the target ion source in a special oven to test long term stability and safety tests were in progress.

The novel reflectometer REFSANS for analyses of liquid and soft surfaces at the new research reactor FRM-II in Munich, Germany

The reflectometer REFSANS is being built at the high-flux reactor FRM-II in Munich, Germany. It is dedicated to the analysis of interfaces, phase boundaries and surfaces at the air–water interface of liquid/soft samples. Novel neutron optics have been developed for REFSANS to measure weak diffuse surface scattering most efficiently for comprehensive analyses of lateral surface structures. This makes REFSANS a novel and unique tool for biological research projects. REFSANS will go into operation in 2002 and will be open for national as well as international users.

Optimization of a partially non-magnetic primary radiation. shielding for the triple-axis spectrometer PANDA at the Munich high-flux reactor FRM-II

Monte Carlo simulations have been used to optimize the monochromator shielding of the polarized cold-neutron triple-axis spectrometer PANDA at the Munich high-flux reactor FRM-II. By using the Monte Carlo program MCNP-4B, the density of the total spectrum of incoming neutrons and γ radiation from the beam tube SR-2 has been determined during the three-dimensional diffusion process in different types of heavy concrete and other absorbing material. Special attention has been paid to build a compact and highly efficient shielding, partially non-magnetic, with a total biological radiation dose of less than 10 μSv/h at its outsides. Especially considered was the construction of an albedo reducer, which serves to reduce the background in the experiment outside the shielding.

Production of intense beams of fission fragments at MAFF

At the new Munich high flux reactor FRM-II, the Munich Accelerator for Fission Fragments (MAFF) is under development. The main objective will be the production and study of new very heavy elements (Z>100). To obtain this goal, intense beams of neutron rich isotopes (70<A<160) are required. Thermal neutron induced fission is considered the most suitable method to produce these isotopes due to the large fission cross section and high thermal neutron flux (>1014 n/s*cm2) available at the new reactor. The target ion source design is based on the ANUBIS source at OSIRIS in Studvik, optimized for very high neutron fluxes. Using 1 g of U235 diluted in a graphite target, intensities of several 1011 ions/s for Kr91, Sn132, or Cs144, e.g., are expected after mass separation. These singly charged ions will be charge bred in an electron cyclotron resonance ion source and then injected into the MAFF-LINAC to reach the energies at the Coulomb barrier. The production of intense ion beams of neutron rich isotopes by thermal neutron induced fission, the development of the target ion source, and the development of the fission target will be presented.

RNB production with thermal neutrons

Thermal neutron induced nuclear fission is the most suitable method to produce neutron-rich isotopes (70⩽ A⩽160) due to the large fission cross section and the high thermal neutron fluxes in modern reactors. Intensities of mass separated neutron rich nuclei of some 10 11 ions/s are expected, e.g. for 91Kr, 132Sn or 144Cs from 235U diluted in a porous graphite target. Several front runners with low-energy fission-fragment beams exist like OSIRIS in Studsvik. In order to get beams of neutron-rich nuclei at the Coulomb barrier, the PIAFE project worked out a first concept of production and mass separation of high-intensity beams of fission fragments. At the new Munich high-flux reactor FRMII, the Munich Accelerator for Fission Fragments (MAFF) is under development to make use of post accelerated beams of neutron rich isotopes for experiments in many different fields of nuclear physics, solid state physics and medicine. One key experiment will be the production and the study of very heavy elements. An overview of the production method of neutron-rich isotopes by thermal neutron induced fission, and of the expected yields will be given and the development of target–ion-sources and of the fission targets for MAFF will be characterized.

The REX-ISOLDE-project and the Munich accelerator for fission fragments MAFF

After a general discussion of ISOL-facilities in Europe we focus on the present status of the REX-ISOLDE facility at CERN and the Munich Accelerator for Fission Fragments MAFF. At REX-ISOLDE in 2001 radioactive beams of ISOLDE will be accelerated to (0.8–2.4) MeV/u. At the new Munich highflux reactor FRM-II a production target of MAFF with 10 14 fissions/s is under design. Probably in 2003 intense low-energy beams (≈ 10 11 /s) of very neutron-rich fission fragments will be available. For MAFF a linac is being developed, which will accelerate the ions after charge breeding to energies between 3.7 and 5.9 MeV/u. In the long term a recycling ring with large momentum acceptance will further increase the radioactive beam intensities by a factor of 10 2 – 10 3 for specific experiments.

Design of the triple-axis spectrometer PANDA at the high-flux reactor FRM-II of Garching

A novel triple-axis spectrometer is presented which is at present under construction at the FRM-II reactor of Garching. This instrument, installed at the cold source, largely profits from its vicinity to the reactor and uses state-of-the-art neutron optics. Large in-pile supermirror neutron guide sections allow an enormous increase in neutron flux due to beam focusing in accordance with Liouville's theorem. Full polarization analysis at high neutron flux is achieved by focusing Heusler crystals. A resonant spin-echo device will allow lifetime analysis of dispersive excitations. The spectrometer is designed to obtain an ultra-high neutron flux and to go far beyond the actual limits for cold triple-axis spectrometer in energy resolution and energy transfer.

The Munich fission fragment accelerator

For the new Munich high-flux reactor FRM-II a fission fragment accelerator is under design. The accelerator will deliver intense mass-separated beams of very neutron-rich fission fragments with energies at and beyond the Coulomb barrier. One main physics goal will be the investigation of very heavy neutron-rich nuclei produced in fusion reactions.

Concept of a high-resolution online mass separator for the Munich fission fragment accelerator

A fission fragment accelerator combined with the Munich high-flux reactor FRM-II is under design for the delivery of intense beams of mass separated very neutron-rich fission fragments with energies at the Coulomb barrier. They can be used to produce very heavy neutron-rich nuclei in fusion reactions. The large neutron excess will result in much longer lifetimes of the produced heavy elements, compared to neutron-deficient ones produced in reactions with stable nuclear beams. Thus fast α-chains can no longer be used to identify the heavy reaction products. A new separator will be used consisting of a velocity filter, an ion guide system (IGISOL) and a Penning trap. The velocity filter separates the primary beam and results in a strong background suppression. The IGISOL system improves the emittance and enriches the 1 + ions in the beam, which will then be efficiently transferred into a Penning trap. In the Penning trap the ions are bunched, stopped and mass separated with high resolving power ( R ≈ 10 5).