Munich Research Reactor FRM-II Research Articles

Development of a new time and position resolving detector for the pulsed low energy positron system PLEPS

The pulsed low energy positron system PLEPS at the Munich research reactor FRM-II is a user facility for depth resolved positron lifetime measurements. Besides positron lifetime measurements 2D-AMOC (Two Dimensional Age Momentum Correlation) experiments are also possible. 2D-AMOC provides in coincidence the lifetime of the positron and the longitudinal momentum distribution of the annihilated electron. It would be of great scientific concern to measure simultaneously the entire 3D-momentum distribution of the electron annihilating with the positron and the corresponding lifetime of the positron (4D-AMOC). To perform 4D-AMOC measurements a time and position resolving detector is required in coincidence with a pixelated Germanium detector. Therefore a time and spatially resolving detector is currently developed at our institute with envisaged time resolution of 100 ps (FWHM) and a spatial resolution of about 2.6 mm (FWHM) over an area of 12 cm2. First test measurements have been carried out with a 25 mm diameter MCP (Micro Channel Plate) image intensifier and with special delay-line anode readout for the spatial information. Up to now 178 ps (FWHM) time resolution and on average 3.4 mm (FWHM) position resolution have been achieved with BaF2 as scintillator material and a 60Co source.

Parabolic versus elliptic focusing – Optimization of the focusing design of a cold triple-axis neutron spectrometer by Monte-Carlo simulations

We present Monte-Carlo simulations for the focusing design of a novel cold-neutron triple-axis spectrometer to be installed at the end position of the cold guide NL-1 of the research reactor FRM-II in Munich, Germany. Our simulations are of general relevance for the design of triple-axis spectrometers at end positions of neutron guides. Using the McStas program code we performed ray trajectories to compare parabolic and elliptic focusing concepts. In addition the design of the monochromator was optimized concerning crystal size and mosaic spread. The parabolic focusing concept is superior to the elliptic alternative in view of the neutron intensity distribution as a function of energy and divergence. In particular, the elliptical configuration leads to an inhomogeneous divergence distribution.

Optimization of elliptic neutron guides for triple-axis spectroscopy

In the last decade the performance of neutron guides for the transport of neutrons has been significantly increased. The most recent developments have shown that elliptic guide systems can be used to focus neutron beams while simultaneously reducing the number of neutron reflections, hence, leading to considerable gains in neutron flux. We have carried out Monte-Carlo simulations for a new triple-axis spectrometer that will be built at the end position of the conventional cold guide NL-1 in the neutron guide hall of the research reactor FRM-II in Munich, Germany. Our results demonstrate that an elliptic guide section at the end of a conventional guide can be used to at least maintain the total neutron flux onto the sample, while significantly improving the energy resolution of the spectrometer. The simulation further allows detailed insight how the defining parameters of an elliptic guide have to be chosen to obtain optimum results. Finally, we show that the elliptic guide limits losses in the neutron flux that generally arise at the gaps, where the monochromator system of the upstream instrument is situated.

Status of the pulsed low energy positron beam system (PLEPS) at the Munich Research Reactor FRM-II

The Munich pulsed low energy positron beam system (PLEPS) is now installed at the high intensity positron source (NEPOMUC) at the Munich Research Reactor FRM-II. In order to enhance the performance of the system several improvements have been implemented: two additional collinear detector ports have been installed. Therefore in addition to the normal lifetime measurements it is now possible to simultaneously perform Doppler-broadening, coincident Doppler-broadening and age momentum correlation experiments. An additional chopper was included to periodically suppress pulses and therefore to extend the standard time window of 20 ns for precise measurements of longer lifetimes. First test-experiments have been performed in May and July 2007. With all pulsing components in operation we achieved a count-rate of 1.4 × 10 4 counts per second. The total time resolution (pulsing and detector) was about 240 ps (FWHM) with a peak to background ratio up to 6 × 10 3:1.

Radioactivity distribution in the MAFF beamline

The Munich Accelerator for Fission Fragments (MAFF) facility planned at the research reactor FRM-II in Munich is dedicated to produce, cool and accelerate high-intensity neutron-rich radioactive beams. The radioactive beam of fission fragments results from bombarding a uranium target by the thermal neutrons emerging from the reactor. Part of the nuclides is released from the target/ion source unit in a non-ionized state. This contribution is concerned with simulating the distribution of the non-ionized radioactivity along the MAFF beamline. The calculations are performed with the help of the computer program MOVAK3D. The calculated fraction of non-ionized radioactivity at the border of the experimental area is at the level of about 10 −7, relative to the activity produced in the target.

Measurement of neutron flux and beam divergence at the cold neutron guide system of the new Munich research reactor FRM-II

A sophisticated neutron guide system has been installed at the new Munich neutron source FRM-II to transport neutrons from the D 2 cold neutron source to several instruments, which are situated in a separate neutron guide hall. The guide system takes advantage of supermirror coatings and includes a worldwide unique “twisted” guide for a desired phase space transformation of the neutron beam. During the initial reactor commissioning in summer 2004, the integral and differential neutron flux as well as the distribution of beam divergence at the exit of two representative and the twisted neutron guide were measured using time-of-flight spectroscopy and gold-foil activation. The experimental results can be compared to extensive simulation calculations based on MCNP and McStas. The investigated guides fulfill the expectations of providing high neutron fluxes and reveal good quality with respect to the reflective coatings and the installation precision.

First positron experiments at NEPOMUC

The in-pile positron source NEutron induced POsitron source MUniCh (NEPOMUC) of the new Munich research reactor FRM-II is now operated at the nominal reactor power of 20 MW. Recently, intensity and positron beam profile measurements were performed at 30 eV and 1 keV, respectively. For this purpose, NaI-scintillators detect the 511 keV γ-radiation of positrons that annihilate at a removable target in the beam line. The beam profile is determined with a micro-channel plate detector and a CCD-camera. In the present arrangement of NEPOMUC's instrumentation the positron beam is connected to a coincident Doppler broadening (CDB) facility and to a positron induced Auger electron spectroscopy (PAES) analysis chamber. First experiments were carried out in order to show the performance of these new spectrometers. An overview of the positron beam facility is given and first experimental results of PAES are presented.

Low-energy positrons of high intensity at the new positron beam facility NEPOMUC

In summer 2004 the NEutron-induced POsitron source MUniCh—NEPOMUC—of the new Munich research reactor FRM-II was set into operation at the nominal reactor power of 20 MW. With this novel facility one of the worlds most intense low-energy positron beam is now available. Intensity and positron beam profile measurements were performed at 1 and 30 eV, respectively. Within the first reactor cycle a yield of up to 10 8 moderated positrons per second was achieved. In the present arrangement of NEPOMUC's instrumentation the monoenergetic positron beam is magnetically guided to a coincident Doppler broadening (CDB) facility and to a positron-induced Auger electron spectroscopy (PAES) analysis chamber.

The Munich Accelerator for Fission Fragments - MAFF

At the Munich research reactor FRM-II a radioactive beam facility is planned. About 1014 fissions/s will be induced by thermal neutrons in a target of 235U. Fission products will be ionised, extracted from the neighbourhood of the reactor core, mass analysed and cooled. For some nuclides more than 1011 mass separated ions per second can be obtained. One ion species will be selected in a high-resolution separator and accelerated up to 5.9 MeV/u.

Backscattering spectrometer RSSM for the FRM-II reactor in Munich

The new backscattering spectrometer RSSM is under construction for the research reactor FRM-II in Munich. It uses all possibilities to optimise this type of instrument. The spectrometer uses a neutron with an energy of 2.08 meV . A high energy resolution will be ensured by staying as closely to 90° backscattering as possible. The high-intensity version with unpolished Si(1 1 1) monochromator and analysers will have an energy resolution of <0.9 μeV . A fast Doppler monochromator with two linear motors and air bearings ensures a dynamic range of ±32 μeV . Use of offset monochromator panels with crystals other than Si(1 1 1) allows to shift this range, e.g. with CaF 2 by 24 μeV . The instrument will achieve a high neutron beam intensity of 6×10 5 neutrons per second per cm 2 at the sample by use of a phase-space transformation chopper. To reduce further loss of neutrons, the analyser system covers a large solid angle. Special care is also taken to reduce the radiation background released to the environment. The instrument is expected to be ready for test operation in the third quarter of 2004.

Positron source based on neutron capture

A positron beam based on absorption of high-energy prompt γ-rays from thermal neutron capture in 113Cd was installed at a neutron guide of the High Flux Reactor at the ILL in Grenoble. The positron source consists of platinum foils acting as γ–e +e −-converter and positron moderator. After acceleration to 3 keV by electrical lenses the positrons were magnetically guided through the beamline. Measurements were performed for various source geometries. The beam diameter and moderation characteristics such as positron work function, moderation efficiency and degradation were determined as well. The results lead to an optimised design of the in-pile positron source which will be implemented at the Munich research reactor FRM-II.

The Munich accelerator for fission fragments MAFF

The Munich Accelerator for Fission Fragments MAFF has been designed for the new Munich research reactor FRM-II. It will deliver several intense beams (∼3×10 11 s −1) of very neutron-rich fission fragments with a final energy of 30 keV (low-energy beam) or energies between 3.7 and 5.9 MeV· A (high-energy beam). Such beams are of interest for the creation of super-heavy elements by fusion reactions, nuclear spectroscopy of exotic nuclei, but they also have a potential for applications, e.g. in medicine. Presently the Munich research reactor FRM-II is ready for operation, but authorities delay the final permission to turn the reactor critical probably till the end of 2002. Only after this final permission the financing of the major parts of MAFF can start. On the other hand all major components have been designed and special components have been tested in separate setups.

Two-dimensional position-sensitive gaseous detectors for high-resolution neutron and X-ray diffraction

Two-dimensional position-sensitive gaseous detectors have been developed at the Geesthacht Neutron Facility (GeNF) for high-resolution neutron and X-ray diffractometry. They are multi-wire proportional counters with delay-line readout and sensitive areas of 300 mm×300 mm or 500 mm×500 mm. For detecting X-rays, neutrons and hard X-rays the counters are filled with Ar/CO2, 3He/CF4 and Xe/CO2, respectively. One neutron detector is used at the ARES diffractometer at GKSS, which is dedicated to the analysis of residual stresses. Further ones are used for analysing textures and residual stresses at the hard-X-ray beamline PETRA-2 at HASYLAB, and one detector is being developed for the neutron reflectometer REFSANS at the research reactor FRM-II in Munich, Germany.

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.

Intense positron source at the Munich research reactor FRM-II

The principle and the design of the in-pile positron source at the new Munich research reactor FRM-II are presented. Absorption of high-energy prompt γ-rays from thermal neutron capture in 113Cd generates positrons by pair production. For this purpose, a cadmium cap is placed inside the tip of the inclined beam tube SR11 in the neutron field of the reactor, where an undisturbed thermal neutron flux up to 2×1014 n cm-2 s-1 is expected. At this position the flux ratio of thermal to fast neutrons will be better than 104. Monte Carlo calculations showed that a mean capture rate in cadmium between 4.5 and 6.0×1013 n cm-2 s-1 can be expected. Inside the cadmium cap a structure of platinum foils is placed for converting γ-radiation into positron–electron pairs. The heated foils also act as positron moderators to generate monoenergetic positrons. After acceleration to 5 keV a positron beam is formed by electric lenses and guided by magnetic fields. In the primary positron beam an intensity up to 1010 slow positrons per second is expected.

First platinum moderated positron beam based on neutron capture

A positron beam based on absorption of high energy prompt γ-rays from thermal neutron capture in 113Cd was installed at a neutron guide of the high flux reactor at the ILL in Grenoble. Measurements were performed for various source geometries, dependent on converter mass, moderator surface and extraction voltages. The results lead to an optimised design of the in-pile positron source which will be implemented at the Munich research reactor FRM-II. The positron source consists of platinum foils acting as γ−e +e −-converter and positron moderator. Due to the negative positron work function moderation in heated platinum leads to emission of monoenergetic positrons. The positron work function of polycrystalline platinum was determined to 1.95(5) eV. After acceleration to several keV by four electrical lenses the beam was magnetically guided in a solenoid field of 7.5 mT leading to a NaI-detector in order to detect the 511 keV γ-radiation of the annihilating positrons. The positron beam with a diameter of less than 20 mm yielded an intensity of 3.1×10 4 moderated positrons per second. The total moderation efficiency of the positron source was about ε=1.06(16)×10 −4. Within the first 20 h of operation a degradation of the moderation efficiency of 30% was observed. An annealing procedure at 873 K in air recovers the platinum moderator.

Monoenergetic positron beam at the reactor based positron source at FRM-II

The principle of the in-pile positron source at the Munich research reactor FRM-II is based on absorption of high energy prompt γ-rays from thermal neutron capture in 113Cd. For this purpose, a cadmium cap is placed inside the tip of the inclined beam tube SR-11 in the moderator tank of the reactor, where an undisturbed thermal neutron flux up to 2×10 14 n cm −2 s −1 is expected. Inside the cadmium cap a structure of platinum foils is placed for converting high energy γ-radiation into positron–electron pairs. Due to the negative positron work function, moderation in annealed platinum leads to emission of monoenergetic positrons. Therefore, platinum will also be used as moderator, since its moderation property seems to yield long-term stability under reactor conditions and it is much easier to handle than tungsten. Model calculations were performed with SIMION-7.0w to optimise geometry and potential of Pt-foils and electrical lenses. It could be shown that the potentials between the Pt-foils must be chosen in the range of 1–10 V to extract moderated positrons. After successive acceleration to 5 keV by four electrical lenses the beam is magnetically guided in a solenoid field of 7.5 mT resulting in a beam diameter of about 25 mm. An intensity of about 10 10 slow positrons per second is expected in the primary positron beam. Outside of the reactor shield a W(1 0 0) single crystal remoderation stage will lead to an improvement of the positron beam brilliance before the positrons are guided to the experimental facilities.

High intense positron beam at the new Munich research reactor FRM-II

The Munich Intense POsitron Source (MIPOS) facility for producing a high intense positron beam at the new Munich research reactor (Forschungs-Reaktor München II) FRM-II is presented. Positrons are generated by pair production of high energy prompt γ-rays from neutron capture in cadmium: 113 Cd(n,γ) 114 Cd . A cadmium cap will be located inside a beamtube at an undisturbed thermal neutron flux of 2×10 14 n cm −2 s −1. Model calculation showed that this would lead to a mean capture rate of 1.2×10 13 n cm −2 s −1. Thermal load resulting from absorbed γ-rays is expected to be less than 4 W cm −2. Inside the cadmium cap a structure of platinum and a stack of tungsten foils is placed for converting the γ-rays into positron–electron pairs. Platinum is used as converting material, since the cross section for pair production is even higher (+11%) than in tungsten. The maximum of the energy spectrum of the positrons produced is about 800 keV. The tungsten foils also act as moderator. The positrons will be accelerated by electric lenses and guided by magnetic fields. Various arrangements are tested to improve the efficiency of the system. After remoderation of the positron beam an intensity of about 10 9 slow positrons per second is expected.