Becker Group

Gamma-rays are a part of the electromagnetic radiation with a very short wavelength under 0.5 nm, which are emitted by nuclear processes, like e.g. the radiaktiven decay. With the gamma spectroscopy the statistics of photons by energy selection are detected and allow identification of nuclei and their cascades.  The gamma spectrometer consists of a germanium single crystal detector, which is cooled with liquid nitrogen, in order to minimize the underground signal of the detector. A thick lead block around the detector shields the natural environment radiation, so that extremely small activities can be determined.

In solid chemistry Gamma-spectroscopy is mainly used for investigation of diffusion coefficients. Only very small quantities of the radioactive isotope are dropped on the material and are allowed to diffuse into it under controlled conditions. Subsequently, the activity of the isotope is measured in the detector, while thin layers of the surface are removed by polishing.

The remaining activity provides a profile depended of the activity or concentration profile of the diffusing isotope, which can be described by the laws of Fick. For measuring the self-diffusion coefficients radioactive nuclei of the same atom art are used.

The electrical conductivity is determined by a modified four point technique (separate measurement of U and I) with two electrodes. The sample is clamped between these two platinum electrodes and the contact between sample and electrode is enhanced by a platinum paste. The measurements can be done within a frequency range of 20 Hz to 1 MHz. For the dc conductivity determination, only the data near 20 Hz is analyzed and in case of the impedance spectroscopy the whole frequency range is used. Within the used sample holders solid samples with dimensions of about 5 mm x 5 mm x 20 mm can be measured between room temperature and T = 1375 °C. The temperature is determined by a type-S thermocouple located at a distance of about 3 mm from the sample position.

Additionally, the gas atmosphere can be controlled by a gas mixing unit and therefore the oxygen partial pressure can be set to a desired value. The atmospheres can be mixed from following gases: oxygen, nitrogen, carbon monoxide, carbon dioxide, argon and hydrogen.

The oxygen content is measured by an oxygen sensor which uses a EMF-cell. In our laboratory there exist two identical furnaces and it is possible to measure two samples in parallel. This is established by a four channel switch, which shares the LCR-meter between the two sample holders.

Luminescence describes the light-emission of electronically excited molecules. The nature of excitation can happen in different ways. For example photons (photo-luminescence), sound waves (sonor-luminescence) or even heat (thermal-luminescence) can induce light emission of materials. In nature, one can observe bio-luminescence of flowers or animals.

One knows two different types of luminescence: fluorescence and phosphorescence, that can be distinguished by their emitting state. Excited electrons relax from a metastable first excited singulett state (S₁) or triplett state (T₁) to the ground-state (S₀).

If there was no emission of light due to the absorbance of energy, the energy transfer must have happened in a different way. Besides radationless transfer of energy (quenching) and (photo-) chemical reactions the energy can be distributed inside the molecules (ITC and IC).

Planetary ball mills serve as reactors for homogeneous or heterogeneous mechanochemical reactions. The milled material is crushed by high-energetic impacts and friction of the milling balls and the grinding chamber walls. Important factors for the efficiency of a high-energy ball milling process are the proportion of the masses (powder to milling ball mass ratio), the geometric parameters of the mill, the duration of the milling process and the rotational frequency. Planetary ball mills are usually used for the batched particle size reduction to ultrafine particle sizes of materials (nanomaterial) of various hardnesses, dry or in a suspension, also for mixing and homogenisation of powders (e.g. mechanical alloying), emulsions and pastes. Many parameters have to be controlled to have reproductive results. In modern ball mills for example interval and pause times or a reverse automatic for special applications can be programmed. For laboratory use grinding chambers from 40 to 500 ml of volume are reasonable. For the milling of oxidizable material, special lids for the application of inert gas atmospheres have to be used. Gas pressure and temperature development during the milling process can be observed and recorded using the GTM system.

In the year 1958 the german physicist Rudolf L. Mössbauer discovered (during his Ph.D. work) a resonance phenomenon for which he recieved the Nobel price three years later. The observed phenomenon is about recoilless nuclear resonance absorption and emission of gamma rays. To honour the discoverer this phenomenon was named "Mössbauer Effect", the developed method "Mössbauer-Spectroscopy". With Mössbauer-Spectroscopy nuclei as emitters (recievers) of recoilless emitted (absorbed) gamma rays are observed. Depending on the character of the analyzed species characteristical Mössbauer spectra are recorded. The main paramaters are the isomer shift, the electric quadrupole splitting and magnetic splitting. With these parameters information about oxidation, bond properties and local electric field gradients and magnetic fields can be gained.

The used spectrometers are modular built out of single components from "Halder". Materials that contain the following elements can be examined in the Becker workgroup: iron, tin, europium

The samples must be powders or very thin foils (up to a thickness of 50 micrometers) and can be analyzed under in-situ conditions with variable gas mixtures up to 1000°C. The desired gas mixtures (CO, CO₂, N₂, O₂, Ar, H₂, NH₃) are set using "MKS" gas mixing equipment. To control the gas mixtures an additional commercial sensor (lambda probe) is used.

UV/Vis spectroscopy engages with the interaction of light (electromagnetic radiation) and matter. This technique can be applied to gases, fluids and solids.

The measuring principle is based on the absorption of particular energies of the incident light by electrons. These electrons are excited from the ground state to an excited state and the difference of energy between these two states is subtracted from the incident light, which leads to an attenuation of the intensity of the radiation. In an optical experiment this attenuation is monitored by a detector.

The following list shows the whole range of applications of UV/Vis spectroscopy in our workgroup:

• Samples: Single crystals, powders
• Wavelength range: 200 nm to 3300 nm
• Temperature range: -196 °C to 1300 °C
• Gas atmospheres: O₂, N₂, Ar/H₂, CO, CO₂, O₂, N₂, Ar/H₂

PAC (Perturbed Angular Correlation) is a technique used to study the local environment around probe atoms in solids. Small quantities (in the order of a few ppb) of radioactive probe atoms are introduced in the sample material. The hyperfine interactions between electromagnetic fields produced by the sample material, and the nuclear moments of the probe atoms result in a perturbation of the angular correlation of gamma-rays.

A typical PAC probe, ¹¹¹In, decays to the ground state of ¹¹¹Cd by the successive emissions of two gamma-rays. The hyperfine interaction between the quadrupole moment of the nucleus and, e.g., the local electric field gradient removes the degeneracy of the intermediate energy level and perturbs the angular correlation of the two gamma-rays. By measuring the decay of the intermediate state, one can observe that the lifetime exponential curve is modulated by the perturbation of the angular correlation. All the information that can be extracted from a PAC experiment is contained in the counting rate ratio.

A PAC experiment can be performed at any temperature, there is no signal degradation as a function of temperature. It is very sensitive to small changes in local environment around the probe atoms. A limitation of this technique is the number probe atoms that have the necessary properties to be used as PAC probes: In our laboratory we use the most common PAC probe atoms: ¹¹¹In/¹¹¹Cd and ¹⁸¹Hf/¹⁸¹Ta. ¹¹¹In/¹¹¹Cd is available commercially as a solution of InCl₃ in HCl. ¹⁸¹Hf/¹⁸¹Ta is easily produced by thermal neutron capture by ¹⁸⁰Hf.

The phenomenon of diffraction at the lattice goes generally into action whenever the lattice spacing is in the same order of magnitude of the wavelength. Crystals consist of arranged ion, atomic or molecular lattices, whose lattice spacings lie in the size of X-ray wavelengths. To that three-dimensional lattices X-rays scatter at the electrons of the atoms. They emit spherical waves, which interfere in certain direction constructionally in other destructively.  The lattice can be determined by measuring these diffraction reflexes and recalculating back the structure of the lattice, which is known as X-ray structure analysis.  Already well-known and presumptuous crystalline substances or phase mixtures can be determined by data bases.

For powder measurements Bragg Bretano diffractometer of Phillips (PW1820) with variable divergence slit, a Guinier camera of Seifert with primary monochromator and a Debye Scherrer camera are used. Measurements of powders pellets and frozen liquids at low temperatures (77K), polycrystallines metal panels as well as pure powders are measured with Bragg Bretano diffractometers of Phillips (PW1050) with firmer divergence slit. Texture investigations of polycrystalline solids are measured over pinhole pattern setup. Single crystals can be oriented by means of a Laue camera and a precession camera.