Graduate

A cryostat has been constructed for the determination of latent heat of vaporization of liquid helium at temperatures from the critical temperature (5.2°K) to below the A point (2.2°K). Some details of the apparatus, hints on low temperature technique and suggestions for future equipment are presented, together with results of some experiments on the apparatus. / Thesis / Master of Science (MS)

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A low temperature scanning tunneling microscope (LTSTM) was built that we could use in an ultra high vacuum (UHV) system. The scanning tunneling microscope (STM) was tested on an existing 3He cryostat and calibrated at room, liquid nitrogen and helium temperatures. We analyzed the operational electronic and vibration noises and made some effective improvements. To demonstrate the capabilities of the STM, we obtained atomically resolved images of the Au (111) and graphite surfaces. In addition, we showed that the stable tunneling junctions can be formed between the Pt/Ir tip and a superconducting thin film PbBi. We observed the atomic corrugation on Au (111) and measured the height of the atomic steps to be approximately2.53Å, which agrees with published values. In our images of the graphite surface, we found both the β atoms triangular structure, as well as the complete α-β hexagonal unit cell, using the same tip and the same bias voltage of 0.2V. The successful observation of the hidden α atoms of graphite is encouraging in regards to the possibility of imaging other materials with atomic resolution using our STM. We also demonstrated that stable tunneling junctions can be formed at various temperatures. To demonstrate this, the superconducting current-voltage and differential conductance-voltage characteristics of a PbBi film were measured from 1.1K to 9K From this data, the temperature dependent energy gap of the superconductor was shown to be consistent with the predictions of the Bardeen, Cooper, and Schrieffer (BCS) theory.

This thesis covers considerations regarding commercially availble magnetic field sensors and the design work and assembly of a 3-axis magnetic field sensor of Hall technology that is tested for use in a cryostat at the FREIA laboratory, Uppsala university. The assembled sensor needs to withstand cryogenic temperatures and a high vacuum environment. A design for a mechanical structure that will be able to move a matrix of multiple sensors inside the cryostat is also covered. A short section discusses magnetoresistance magnetic field sensors for these purposes. The assembled sensor withstood a cryogenic test and a vacuum test separately, but it did start to show an odd behavior in a specific temperature range towards the last cycles of the cryogenic test. The printed ciruit board didn't show any damage on the board or the solder joints after operation in the vacuum environment.

The diploma thesis addresses the design and experimental verification of cooling system and low temperature part of UHV - STM working in temperature range of 20K - 300K. Due to the demand of variable temperature, the flow cooling system with cryogenic (~5 K) helium (He) is used. Two variants of the low temperature part of the microscope are studied. First the version with cooling only the sample holder, and second with cooling of the whole STM. Designed cooling system consists of He flow cryostat allowing to connect it to the Dewar vessel with liquid helium (LHe) using a low-loss transfer line. The cryostat consists of He inlet and outlet, heat exchangers and copper strains (i.e. braids) for the thermal connection of the sample holder/STM and radiation shield around the STM with the heat exchangers. The thesis describes the design of the flow cryostat and its initial tests in the designed vacuum chamber. Heat flow through a spot contact is also discussed to estimate thermal conductance of insulation supports based on thermal resistance of spherical contacts. The thesis was elaborated in collaboration with the Institute of Scientific Instruments of the ASCR, v.v.i.

Ce travail de thèse est porté sur l’étude des interactions protéines-protéines, protéines-peptides ainsi que la stabilité des protéines en faisant appel à la spectroscopie Raman et infrarouge. Dans la première partie, nous nous sommes focalisés sur l’étude de l’interaction entre les différentes protéines d’adrénodoxine et d’adrénodoxine réductase afin d’apporter de nouvelles données pour compléter la compréhension du mécanisme d’échange électronique. Le deuxième objectif à atteindre dans le cadre de ce travail est de suivre les changements conformationnels au niveau de la structure secondaire et tertiaire qui se produisent en présence et en absence des peptides lors de la formation des complexes. Enfin, la dernière partie est dédiée dans un premier temps à une étude comparative de la stabilité des hémocyanines de Limulus polyphemus et Eurypelma californicum issue de deux organismes ayant des conditions de vie différentes en suivant l’effet de la température (294-20 K) et du pH sur la structure secondaire des protéines. Dans un deuxième temps l’étude porte sur l’influence de la teneur en oxygène sur la structure secondaire et sur le site actif des hemocyanines de Limulus polyphemus, Eurypelma californicum et Astacus leptodactylus. / This thesis is focused on the study of protein-protein and protein-peptide interactions as well as the study of proteins stability by means of Raman and infrared spectroscopies. In the first part, we focused on the interactions between different adrenodoxin and adrenodoxin reductase proteins in order to get a better understanding of the electron transfer mechanism. The second part of the thesis concerns the changes in the secondary and tertiary structure of PDZ domains in the presence and absence of peptides during complex formation. The last part is dedicated to a comparative study of hemocyanins originated from organisms living in vastly different conditions such as Limulus polyphemus and Eurypelma californicum. This part of the project concerns the effect of temperature (294-20 K) and pH on the secondary structure of proteins. Finally the influence of oxygen binding on the secondary structure and the active site of Limulus polyphemus, Eurypelma californicum and Astacus leptodactylus was investigated.

This Master thesis aims at designing, assembling and operating a prototypal luminescentbolometer containing a candidate with high Q-value (116Cd and 100Mo)for the study of the neutrinoless double- decay. The crystal is scintillating (with 116CdWO4 and Li2MoO4 compounds). The prototype is designed according to a simple thermal model and cooled down to 18 mK. Data analysis of the 116CdWO4 crystal determines the energy resolution (intrinsicand in the ROI) and the alpha/beta discrimination power. It includes a full interpretation of the background energy spectrum in terms of environmental radioactivity and an evaluation of the crystal radiopurity by the detection of internal contamination of the detector. An evaluation of the potential of a future experiment based on the 116CdWO4developed prototype is performed, ascertaining the feasibility of large scale experiments to search for neutrinoless double beta decay. The use of the thermal model of the detector response to interpret its bolometric behaviour and the study of future optimizations of the detector performance concludes this project.

Ce mémoire de thèse décrit l'implantation et l'étude spectroscopique d'atomes de césium dans un monocristal d'hélium hcp à une température de 1.1 K dont une des applications possible pourrait être une mesure du moment anapolaire du noyau de Cs. Nous avons mis en place un cryostat optique permettant de travailler pendant plusieurs mois avec un monocristal d'hélium orienté. L'implantation des atomes de césium a été réalisée par atomisation laser à partir de grains métalliques, d'abord avec un laser YAG doublé, puis avec un laser Ti:Sa femtoseconde amplifié. Ce nouveau procédé a permis d'obtenir une densité atomique supérieure tout en déposant moins d'énergie dans le cristal. La détection optique des atomes a mis en évidence un allongement notable de la durée de piègeage de la population atomique à 1.1 K par rapport à 1.6 K. La forme de raie de la transition D2 (6S1/2 --> 6P3/2) en absorption est compatible avec une levée de dégénérescence entre les sous-niveaux MJ = ±3/2 et ±1/2 de 200 cm−1, en bon accord avec les prévisions théoriques donnant l'écart en énergie entre ces sous-niveaux dans une bulle atomique présentant une anisotropie statique de 6%. L'analyse de l'intensité relative des composantes de la raie D2 permet d'inférer que la composante haute énergie correspondrait aux sous-niveaux | MJ |= 1/2 et que la déformation de la bulle autour de l'état fondamental serait négative (en forme de coussin). Nous avons cherché à vérifier la dépendance de la forme de raie avec la polarisation de la lumière. La faiblesse des effets observés laisse supposer soit que la lumière incidente est dépolarisée, soit l'axe local de l'anisotropie du cristal autour des atomes est désorienté de manière isotrope.