Spin-echo small-angle neutron scattering development
Başlık çevirisi mevcut değil.
- Tez No: 402375
- Danışmanlar: PROF. DR. G.J. KEARLEY
- Tez Türü: Doktora
- Konular: Elektrik ve Elektronik Mühendisliği, Makine Mühendisliği, Mekatronik Mühendisliği, Electrical and Electronics Engineering, Mechanical Engineering, Mechatronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2003
- Dil: İngilizce
- Üniversite: Technische Universiteit Delft (Delft University of Technology)
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 120
Özet
Özet yok.
Özet (Çeviri)
Spin-Echo Small-Angle Neutron Scattering (SESANS) is a SANS technique which can probe distances in real space from a few nanometers up to microns. With SESANS one can obtain structural information of materials which means the size and the form of the object. In SANS one obtains data as a function of momentum transfer. It is very difficult to measure angles close to zero since then the scattered beam coincides with the incoming beam and because of this the two dimensional detector must be put at a large distance from the sample. However, in SESANS data is obtained as a function of the spin-echo length which corresponds with real space features of the sample. Each data point automatically includes integration over the momentum space. Therefore, small angles are included in an measurement as easy as large angles. Since one measures with SESANS the integrated intensity there is no need for collimating the beam which means high counting statistics. The underlying principle of SESANS is the Larmor precession of polarized neutrons. The setup consists of two precession arms in which the neutrons effectively precess in opposite directions. When there is no scattering the amount of precession in the first arm is equal to the amount of precession in the second arm. This is the echo situation. However, when there is scattering from a sample, the amount of precession in the two parts will not be the same due to the traversed path differences which will lead to depolarization of the beam. The amount of depolarization depends on the structure of the sample. There are four possibilities to realize a SESANS setup. These are: the three foil option, wedge option, resonant field option and four foil option. In all the options dipole magnets are used which generate a magnetic field perpendicular to the neutron beam direction. In this thesis the three foil option is treated extensively. In the three foil option single and double foils which are readily magnetized are used as π/2 and π flippers. The setup consists of three dipole magnets. In the first and last magnets single foils and the middle magnet double foils are mounted. These foils are tilted with respect to the neutron beam direction and form triangular shaped precession regions. The scattering angle is encoded with the precession angle which is proportional to the momentum transfer. The measured quantity is the polarization as a function of the spin-echo length. This length is proportional to the magnetic field value of the magnets, the distance between the sample and magnet and the square of the wavelength. The polarization is proportional to the cosine transform from momentum space to real space of the scattering power of the sample. Multiple scattering can be treated easily in contrast to SANS. In SESANS multiple scattering results in stronger decay of the polarization as a function of the spin-echo length. There are two requirements which have to be fulfilled in a SESANS magnet. Firstly, the magnetic fields in the middle of the magnet in a volume of 30 × 30 × 30 mm3 must homogenous within 1.3 G for a field of 2000 G in the center of the magnet. The reason for this is that the precession angle must be a linear function of the height in order to realize angle encoding. If the field is not homogenous then the precession angle will have a non-linear dependency on the field. The homogeneity of the magnetic field is readily achieved when the width of the pole face of the dipole magnets is 18 cm. To save weight, the radius of the core is brought back to 12 cm, keeping the width of the pole face at 3 cm in order to avoid saturation effects. With these parameters to have 2000 G in the center, a total current of 5000 A is needed through one current package that encloses one pole. Secondly, the line integral must have a homogeneity of 2.2 G.cm over the beam cross-section which cannot be achieved by increasing the pole gap distance. This inhomogeneity is caused by the fringing of the field lines at the entrance and exit opening of the magnets. Inhomogeneities in the vertical component of the magnetic field will not contribute to the line integral errors but it is proportional to the square of the perpendicular components of the field. Two external coil sets are used to homogenize the line integral over the beam cross-section. The first set consists of four coils. These contain four current carrying wires parallel to the mean beam axis. The distances between the wires are chosen in such a way that it gives a quadratically changing field in both perpendicular directions to the mean beam axis. This first coil set actually transforms the variation of the line integral from one perpendicular direction to the other one. The inhomogeneity in this new direction is then corrected with a parabolically shaped coil placed in the neutron beam. For a beam of 15 mm high and 10 mm wide the polarization is increased from 0.25 to 0.86. Model calculations are performed in order to calculate the SESANS correlation function. The length scales present in systems such as spheres, cylinders and ellipsoids can directly be determined. On the other hand determination of Gaussian polydispersity cannot be done with SESANS for spherical systems. The Guinier approximation can be done in the same manner as with SANS and gives information on the Guinier radius. Structure factor effects shows up clearly in the SESANS signal. For hard sphere interaction, oscillations appear around the zero level of the SESANS correlation function. An analytical expression is obtained for systems which can be described by a Debye-Lorentz model. These calculations show that SESANS allows to do interpretations in real space. The SESANS principle is demonstrated by measurement of limestone. The scaling formula for multiple scattering has been applied successfully on limestone measurements of different thicknesses. The first quantitative measurements in Delft are done on latex spheres of 60 and 100 nm. The calculation without any adjustable parameters and the measurement are in good agreement for the 100 nm particles. In case of the 60 nm spheres there is a small difference between the measurement and the calculation which is not yet understood. It would be impossible to these measurements with conventional SANS machines such as at LOQ in ISIS and just possible at D11 in ILL. SESANS can be used very effectively at a pulsed neutron source since the spin-echo length is proportional to the square of the wavelength. Therefore, it will be possible to obtain the whole depolarization curve as a function of the spin-echo length in one single shot. This will make time-dependent measurements on subsecond timescale possible.
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