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Technology for neutron instrumentation

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A new version of the Si/Gd detector for thermal neutrons has been developed, namely a 1-dimensional model of the microstrip Si sensor coupled to the fast and parallel VA32/TAN32 ASICS (IDE AS Norway) for signal processing. The detector consists of a 40 x 70mm{2} Si microstrip sensor with double Gd converter, 0.55mm space resolution and 128 parallel readout channels. The sensor is mounted directly on the PCB, close to the preamplifier/shaper system, four VA32 ASICS with 75ns minimum peaking time and a CR-RC shaping function. The output signals from the sensor are AC-coupled to the preamplifier inputs by 1nF capacitors. The 128 shaper outputs are converted into digital pulses by four TAN32 asics, which include a two-level discriminator with different thresholds for each channel. Electrical tests of the detector were successful with excellent characteristics of signal to noise, linearity and cross talk. The high space resolution and high counting rate, typical of the Si detector, are appealing features for operation under intense neutron fluxes. Specific to this detector model, are the very high speed, which improves the maximum instantaneous data rate achievable, and the rather high space resolution, which could be exploited in single crystal neutron diffraction applications.
New diffractive optics, called Zone Plates, was developed. This optics exploits the Fresnel lens concept and was designed to focus and manipulate thermal and cold neutron beams. These devices are new in the field of neutron optics, and their fabrication took benefice from the advances in the modern nano-lithographic techniques. The neutron Zone Plates would have ideal applications in the optimisation of that spectrometer which needs a low-divergence neutron beam with a small spatial size. All the neutron scattering facilities are potential end users. The current production procedures are so advanced that systems of lenses with a complex design can be fabricated. The simplest among compound systems are the assemblies of several lenses with different characteristics (ordinarily used in visible-light optics). A further device, which exploits a more complex design, would be the matrix of high performance small lenses, which can easily reach linear dimensions up to several centimetres. A device like a matrix can be produced in different sizes with characteristics (magnification and working neutron wavelength), which can be optimised for applications in high-resolution neutron imaging and radiography.
A new technology has been developed for producing large area neutron sensitive ZnS scintillator detectors with a one dimensional 3 mm spatial resolution. The current generation of ZnS scintillator detectors have been very successful at ISIS, but its position resolution is limited to 5 mm. In the design stage of ENGIN-X, the new diffractometer for engineering studies at ISIS, it became clear that detectors with a 3 mm position resolution would be required and a new technology for constructing this type of detector was developed. This technology has been transferred to industry and industry has produced (by early 2004) 10 of these full size detector modules covering a 1.4 m2 area. Evaluation has shown that the build quality of these modules is very high. The large area detector array has been cost effective to produce, easy to install and the maintenance costs proved to be very low. Utilisation of the 3 mm detector is not limited to ENGIN-X, though, and this new type of scintillation detector could also be beneficial to other beam lines at ISIS or neutron scattering facilities around the world. Scientists and engineers around the world are currently using ENGIN-X and their results will be widely published.
One method to polarise cold neutrons is to use supermirrors (SM), which transmit neutrons of one spin state and reflect the others. Normally these SM polarisers require for an external magnetic field to keep the magnetisation and for a spin flipper behind the polariser, which allows choosing the spin state. The use of remanent SM has the advantage that no strong magnetic fields are required, that may interfere with the sample or the sample environment. Furthermore, it allows switching the magnetisation of the polarisatior and thus the transmitted spin state. In this case a flipper is a surplus. In the recent years, polarisers of this type have been designed and finally produced on the sputtering plant in our laboratory. Because the polarisers will be used in transmission geometry, material combinations with low absorption have been chosen. The most promising results with respect to the magnetic behaviour were obtained with the system Fe/Si. Several magnetically remanent supermirror coatings were produced. Of these the following two examples are already in use: - The V-shaped white-beam transmission polariser installed in 2002 in the first collimator tube of SANS-I gives polarisations of 96% (Lambda= 4.7 Angstrom) to 98% (Lambda > 6 Angstrom). - A prototype spin analyser for the neutron reflectometer and diffractometer Morpheus at SINQ consists of a Si-wafer, coated on both sides with a Fe/Si SM (m =3 with 599 layers). This waver is mounted inside the gap of a small electromagnet which provides the guide field and allows to switch the magnetisation. For this device the polarisation efficiencies are 92% < P (T) < 95% and 95% < P (R) < 98% (H=15Oe).
The novel low-pressure micro-strip gas chamber (MSGC) detector type with composite 157Gd/CsI converter foil, which was developed at HMI in TECHNI up to laboratory prototype scale, improves the performance of present thermal neutron detectors considerably and delivers an adequate economic solution for fast, high-resolution large-area detectors, even for the strongly rising requirements in experiments at next generation pulsed spallation neutron sources like ESS. By replacing the composite 157Gd/CsI converter with 1 µm thick columnar CsI secondary electron emitter layers by a thicker CsI converter X-rays can be detected instead of neutrons. Two-dimensional position resolutions of better than 0.3mm (FWHM), time-of-flight resolutions of a few ns and count rates >106 cps per detector segment can be reached with a novel cost-effective high-rate multi-hit delay line readout technique developed at HMI based on sub-divided delay lines connected via impedance matching amplifiers. A high-speed, high-resolution data acquisition board type developed in collaboration with JINR Dubna for this detector type is operational.
Currently the resolution of neutron spin-echo spectrometers is mainly limited by the ability to achieve very high homogeneity of the magnetic field integrals within a beam, i.e. a bundle of paths. Especially if a large solid angles for sample illumination and in particular also for detection of the scattered neutrons shall be covered. In principle the combined effect of different lengths and variations of magnetic fields from a set of cylindrical coils can be “homogenized” by three correction elements. The correction elements represent radial current distributions in the neutron beam. The following milestones have been achieved: - Finishing to build test stand for the cooling test. - First prototype can be presented and a series of four test planoconcave FC pieces has been ordered with optimised manufacturing method (splitted backplane). - Single crystal material has been ordered for the backplate. - Manufacturing of first prototype of biconcave Fresnel coil underway. - Simulation of contact finger hints to biconcave Fresnel coils (decrease current). - Cooling test apparatus has been finished (CCD etc.) - optimising air and water-cooling mechanism for highest current carrying capability - Simulation of finite thickness Fresnel coils - better simulation of the reality - Developing of amagnetic highly accurate positioning devices necessary.
A neutron detector based on microstrips technology has been developed. Its size is 100x200 mm². The resolution is 1.3mm in the large direction and 2mm in the other one. It is dedicated for diffraction of disordered systems. The electronics associated is based on charge division. A set of 8 detectors can be linked on the same acquisition card.
A new low gamma-sensitive image plate detector has been developed. A new approach to the construction of image plate detectors of medium resolution (of about 1 mm), which fits to requirements of neutron scattering, was suggested. Different storage phosphors and neutron converters have been tested to find an appropriate mixture for the neutron image plate with low gamma-sensitivity. It was observed for the first time that the mixture KX:Eu(2+) (X=Cl, Br) (storage phosphor) and 6LiF (neutron converter) is a very attractive detector materials to be used in a high gamma-background environment. Calculations and measurements were performed to optimise the image plate parameters. The main processes that affect image plate detector performance were investigated and a mathematical model was developed. Experimental tests for preparations of such pixelated neutron image plates were carried out. First test experiments were performed at a neutron beam port at the reactor DIDO at the FZ Julich. Discussions with TU Delft on Gd nano-particles resulted in a change to 6Li, which is much more attractive from the point of view of neutron sensitivity and gamma-ray insensitivity. It was shown, that optimal size of the absorbing 6Li particles is about 40 µm, the optimal size of the storage phosphor is about 10µm. MCNP calculations have been performed to study the problem of the light propagation in the image plate aiming to find optimal conditions (reflecting walls and bottom) for high effective read-out process. A prototype of the scanner with the high light collection efficiency was constructed and tested; the test neutron images have been scanned and parameters of the scanner have been preliminary defined. An optimisation of parameters of the pixelated image plate was carried out. MCNP calculations have been performed to estimate the expected resolution and efficiency as a function of the pixel parameters and wall coatings. A prototype of pixelated image plate with pixel size of 1 x 1mm{2} was prepared and successfully tested at the neutron beam. The first large scale, 100 x 100mm{2}, image plate was produced. The laser image plate scanner with the high light collection efficiency was constructed and the optical system and the control software were tested. The optical scanning system shows the resolution better than 0.03mm.
The Multi-PSPC is made of 128 Neutron Position Sensitive Proportional Counters (PSPC), mounted side by side in a detection plan which has a 1 m{2} sensitive area. It has been designed to overcome the count rate limitation of Multi Wire Proportional Chambers (MWPC) without compromising other detection parameters (efficiency, position resolution, and gamma suppression). An intrinsic drawback of the Multi-PSPC is that the uniformity in the direction perpendicular to the tubes varies due to the variation of the gas thickness. This is not a limitation for applications, like Small Angle Neutron Scattering, where the neutron signals are generally spread over several tubes. A new PSPC had to be developed in collaboration with an industrial company. It has a length of 1m, an external diameter of 8 mm, and contains a gas mixture at a pressure of 15 bars. A fast electronics has been developed at the ILL to measure the position along the tube by charge division. It is based on the conversion into an address of the charge signals measured on both ends of the tubes. The resolution obtained, 6 mm FWHM, is within the specifications (8mm), and the maximum count rate, above 3 MHz, is at least a factor 20 better than the maximum count rate sustainable with a MWPC. The detector has been installed on the D22 ILL instrument and is now used for routine experiments. This technique can ideally replace MWPCs traditionally used on SANS instruments, as it is cheaper and much faster. It could also be used on neutron reflectometers. The charge division electronics can be used on any type of particle detectors (neutron, X-Ray, MIP).
Identification and characterisation of materials suitable for the construction of neutron-efficient image plates with reduced gamma-ray sensitivity and their use in the manufacture of large plates for a working diffractometer. Examination of the a-sensitivity of Eu(2+) doped BaFBr and BaSrFBr phosphors containing varying quantities of Gd(2)O(3) as neutron converter: Samples of phosphor with differing proportions of Gd(2)O(3) and BaSrFBr were exposed to known neutron and Fe K X-ray fluxes, and the PSL levels measured using a red laser (lamda=635nm ). While the neutron sensitivity of the plates has a broad maximum between 40/60 and a 60/40 mixture of Gd(2)O(3) and BaSrFBr, the gamma -sensitivity reduced montonically with increased Gd(2)O(3) content to zero for 100% Gd(2)O(3). In this system it is therefore preferable to use a 60/40 or a 70/30 ratio of Gd(2)O(3) to BaSrFBr to significantly reduce gamma-sensitivity while keeping a relatively high neutron DQE. Measurement of the thermo-luminescent yield of various inorganic borate storage phosphors which are intrinsically neutron sensitive and contain few atoms of high atomic number: A large series of storage phosphor materials of the type M(2)B(5)O(9)X:Ce(3+), R+ were studied, with M = Sr, Ca; X = Br, Cl; R = Na, K or no extra dopant. These were found to have a much lower gamma-ray sensitivity than the traditional BaSrFBr: Eu mixed with Gd(2)O(3), and the most promising phosphor, Sr(2)B(5)O(9)Br: Ce(3+), was found to give more PSL counts per incident neutron than Gd(2)O(3) doped BaSrFBr. The neutron absorption coefficient for this material is significantly less than for BaSrFBr: Eu/Gd(2)O(3) and requires a thick phosphor layer. This has the effect of reducing the neutron DQE, as the PSL emitted from deep within the phosphor is unable to reach the detector. It was therefore decided to produce large image plates using a 70/30 ratio of BaSrFBr:Eu/Gd(2)O(3) as the storage phosphor. Using a spraying technique for the phosphor / converter, plates of 400mm x 800mm were made and used for neutron diffraction studies of the protein systems lysozyme and glucose isomerase.
New perspectives for epithermal neutron spectroscopy have been opened by the development of the resonant detector (RD) for use on inverse geometry time of flight spectrometers at spallation sources. The RD was first proposed in the eighties and it is now, with the prototype developed in the TECHNI project, brought to a performance level exceeding conventional neutron-sensitive Li-glass scintillator detectors. It features a photon counter coupled to a neutron analyser foil. Resonant neutron absorption in the foil results in the emission of prompt gamma rays that are detected in the photon counter. The dimensions of the RD set the spatial resolution that can be achieved, ranging from a fraction of a cm to several cm. It can thus be tailored to the construction of detector arrays of different geometry. The prototype RD design is based on a combination of YAP scintillation photon counter and uranium or gold analyser foils. This detector has already been selected for application in the upgrade of the VESUVIO spectrometer on ISIS. A special application is the Very Low Angle Detector (VLAD) bank, which will extend the kinematical region for neutron scattering to low momentum transfer (<10Angstroms(-1)) whilst still keeping energy transfer > 1eV, thus allowing new experimental studies in condensed matter systems.
A new device, the Neutron Silicon Lens (NSL), for imaging thermal neutrons has been designed and tested successfully. The NSL consists of multiple layers of different thickness, each layer being a wafer of single crystal silicon coated with a neutron supermirror coating. An optimal stacking arrangement (which has been determined) allows, in principle, the reflection of all neutrons incident upon the lens within an appropriate angular divergence range. By reflecting each neutron once (and only once) the stack of mirrors can focus a line source to a line image of good quality. A combination of two NSL should be able to image a point source. A practical device has been tested with a focal length of around 500mm.
This is a spin-off by-product of the novel hybrid low-pressure micro-strip gas chamber (MSGC) detector developed and of the dedicated high-speed, high-resolution data acquisition (DAQ) board type. For this detector type, a simpler board type based on the same technology was developed. The aim was the readout of multiwire proportional chambers (MWPC) for neutron and X-ray detection. A first series of these boards has been fabricated and tested together with a portable package of dedicated DAQ software.
A coarse adjustable radial collimator for the RITA-2 neutron spectrometer has been designed and constructed. The RITA-2 spectrometer has a multi-blade crystal analyser and a large position-sensitive detector, giving the possibility for performing simultaneous measurements in a triple-axis geometry. In an open geometry, however, there is a risk of neutron �cross-talk� (neutrons from one analyser blade hitting the detector area corresponding to another blade). The �cross-talk� problem can be eliminated by a coarse flexible radial collimator with one vane for each analyser crystal. A first prototype collimator was constructed and tested at SINQ, revealing a few problems. These problems were solved and a final collimator was constructed. However, the subsequent tests could not be performed within the TEHCNI project due to a long shut-down of SINQ. The main innovation in the project was the design of the moving blades in the flexible collimator. The design can be used in connection with other neutron instruments with multiple-analyser systems. With a minor design change, a �real� radial adjustable collimator with a much tighter angular collimation could also be produced for use in e.g. strain diffractometers. Another significant progress was the identification of a generally useful multiple-analyser measuring scheme: The monochromatic imaging mode.

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