Dissertação para obtenção do Grau de Doutor em
Engenharia Física; The role of ionising radiation as a source of damage to living tissues and cells has been recognized as a key issue regarding cellular DNA integrity and, ultimately, mutagenesis. The lethal effect of radiation, despite being most of the time undesired, can sometimes be useful, as is the case of
radiation therapy. However, still the major concern in medicine is that only the cancerous cell material should be destroyed, keeping as much as possible healthy tissue unaffected. One way to control this damage seems to be the application of radiosensitizers that are incorporated into cancer cells. The cancer tissue doped with these radiation sensitizing molecules may be destroyed preferentially under radiation exposure, in very well defined places and even with radiation doses which may be low
enough to prevent healthy cell material to be affected in the surrounding medium. This leads to nanodosimetry and so the sorts of interactions have now to be described at the molecular level. Upon irradiation, the most abundant secondary species produced along the radiation track are low energy electrons and so the study of electron induced damage to biological relevant molecules seems
The research described in this thesis covers for the first time the study of electron transfer on two halouracils (5-chlorouracil and 5-fluorouracil) and isolated DNA/RNA basis (thymine and uracil)by atom-molecule collisions. In order to investigate such molecules...
Room-temperature phosphorescence characteristics of benzo[b]naphtho[2,3-d]thiophene (BNT) were studied under several experimental conditions. Best phosphorescence was obtained using Tl(I) and Pb(II) as heavy atom enhancers, both in the presence of SDS as surface modifier. Under these two conditions of maximum phosphorescence, working curves with three orders of magnitude were achieved and absolute limits of detection and quantification of respectively 0.6 and 2.0 ng in the presence of Tl(I) and 3.2 and 10.7 ng in the presence of Pb(II). The RTP potential for selective trace-level determination was tested using a sediment sample spiked with BNT and using Pb(II) as the selective heavy atom enhancer. The recovery of BNT was very satisfactory (101.5 ± 7.4%, n = 6) indicating that the method can be properly applied for the analysis of complex samples. The interference potential from other polycyclic aromatic hidrocarbons (pyrene, chrysene, carbazole, 7,9-dimethylbenz[c]acridine and dibenzothiophene) was evaluated and the method was considered selective in the presence of these substances.
We present results on a free-space atom interferometer
operating on the first order magnetically insensitive
|F = 1,mF = 0› → |F = 2,mF = 0› ground state transition of Bosecondensed
87Rb atoms. A pulsed atom laser is output-coupled from a
Bose-Einstein condensate and propagates through a sequence of two
internal state beam splitters, realized via coherent Raman transitions
between the two interfering states. We observe Ramsey fringes with a
visibility close to 100% and determine the current and the potentially
achievable interferometric phase sensitivity. This system is well suited to
testing recent proposals for generating and detecting squeezed atomic states.; Article written under name D. Döring
A pulsed atom laser derived from a Bose-Einstein condensate is used to probe a second target condensate. The target condensate scatters
the incident atom laser pulse. From the spatial distribution of scattered atoms, one can infer important properties of the target condensate and its interaction with the probe pulse. As an example, we measure the s-wave
scattering length that, in low energy collisions, describes the interaction
between the |F = 1,mF = −1> and |F = 2,mF = 0> hyperfine ground states in 87Rb.; Article written under name: Döring, Daniel.
If atom lasers are to transform many fields of physics, as well as find applications in industry, the most essential development needed is increased atom flux. In this thesis we report on the experimental observation of fundamental limits on the flux of continuous atom lasers. The flux produced by the commonly used radio frequency outcoupler, can not be arbitrarily increased by increasing the strength of the outcoupling. Above a critical Rabi frequency, the atom laser shows large classical density fluctuations, due to coupling between the multiple Zeeman states. In our system this critical Rabi frequency is about 1 kHz. Futhermore, at large coupling strengths (> 4 kHz for our experimetnal parameters), the continuous atom laser output completely switches off, as a result of the atoms remaining trapped in a bound state. In addition to the investigations of the radio frequency outcoupler, we have demonstrated the first continuous Raman outcoupled atom laser. Preliminary data indicates that the Raman outcoupler will be flux limited in a similar way to the radio frequency outcoupler.
In this thesis we theoretically model a continuously pumped atom laser using the mean-field description. We find that it is unstable below a critical scattering length. Above the critical scattering length, the atom laser reaches a steady state, the stability of which increases with pumping. Below this limit the atom laser does not reach a steady state. We show that this instability results from the competition between gain and loss for the excited states of the lasing mode, and show how the nonlinearities stabilise the system. The requirement for a minimum scattering length will determine a fundamental limit for the linewidth of an atom laser beam. We propose a method of stabilising the system below the critical scattering length and investigate its effectiveness.; no
A multimode model of a continuously pumped atom laser is shown to be unstable below a critical value of the scattering length. Above the critical scattering length, the atom laser reaches a steady state, the stability of which increases with pumping. Below this limit the laser does not reach a steady state. This instability results from the competition between gain and loss for the excited states of the lasing mode. It will determine a fundamental limit for the linewidth of an atom laser beam.
Measurement is our fundamental tool for learning about the world around us. It is from observing trends in measurements that we develop the theories that enable us to predict future behaviour, and it is against measurements that we determine the validity of these theories. Increases in the precision of our measurements are fundamental to our understanding.
Atom interferometry is a new method for performing precision measurements that uses the matter-wave nature of atoms to perform interferometry experiments analogous to those performed with photons. However, in contrast to optical interferometry, which uses coherent sources of photons, atom interferometry uses thermal atomic sources, in part due to the unavailability of high-flux coherent atomic sources. Although pulsed coherent atomic sources are presently available, continuous sources are not. Creating a truly continuous coherent source for atoms is tricker than for photons. One of the largest challenges is that atom number is conserved. A source of atoms is therefore necessary to produce a truly continuous atom laser. This source must be used to replenish the lasing mode of the atom laser, and the process must operate without significantly disturbing the coherence properties of the lasing mode. It is this replenishment or pumping process that has been investigated theoretically in this thesis.
There are only two choices for the reservoir that makes the replenishment (or pumping) process of an atom laser irreversible: the empty modes of the optical field...
Bose-Einstein condensates can be regarded as sources of coherent matter. When atoms are extracted from a trapped Bose-Einstein condensate, a coherent monoenergetic atomic beam is generated. Such a source is commonly referred to as an atom laser. Previous atom lasers were perturbed by external magnetic fields, since they were based on atoms which populate magnetic-field sensitive states.
In this work, a novel type of atom laser is demonstrated. A coherent atomic beam is generated by outcoupling of atoms from a magnetic field-insensitive Bose-Einstein condensate. The here developed technique does not require magnetic shielding of the apparatus in order to create a quasi-continuous beam.
The presented experiments are based on quasistatic optical dipole traps for cold Rubidium atoms (87Rb), which are trapped in a tightly focussed laser beam (with a measured beam waist of 27 µm) generated by a CO2-laser. Because of the extreme laser detuning with respect to the atomic resonance, the trapping potential is independent on external magnetic fields. Atoms in different spin-projections can be confined. In order to reach the quantum degenerate regime, the temperature of the rubidium cloud is decreased with evaporative cooling technique. The applied method is accomplished by reducing the depth of the optical dipole trap.
At the end of this process...
The field of atom optics has progressed rapidly over the past 20 years since the realisation of Bose-Einstein condensation, such that the wave behaviour of atomic gases is now routinely demonstrated. Furthermore, the study of quantum atom optics, which
goes beyond a ‘mean-field’ description of quantum systems to consider the behaviour
of single particles, has demonstrated both the similarities between photons and massive
species, and their differences as a result of the internal structure and external interactions of atoms. An important class of observable quantities which allow such effects to be measured are nth order correlation functions, which can be interpreted as a result of either particle or wave behaviour. These functions provide a statistical description of fluctuations in n-tuples of particles in a source, which rigorously defines concepts such as coherence. The quantum statistics of a Bose-Einstein condensate should be the same
as that for an optical laser, while an ideal thermal Bose gas matches the behaviour of
incoherent light. However, correlation measurements can also be used to quantify the
influence of interactions, dimensionality, confining potentials and waveguides, and the
difference in quantum statistics between fermions and bosons...
Solvent-free peroxide-initiated polymer modifications are widely used to improve the physical and/or chemical properties of commodity plastics and elastomers. Although the reactions that underlie polymer grafting are known, our understanding of H-atom transfer reactions in this context is incomplete. Fundamental questions remain unanswered, such as the difference in reactivity between different polymers (polyethylene versus polypropylene and polyisobutylene) and differences in the regiochemical outcomes of grafting reactions upon them. Herein, experimental data pertaining to the H-atom transfers involved in polyolefin graft modifications were obtained to improve our fundamental understanding of these reactions by using radical-trapping techniques and quantum chemical calculations. In this project, experimental measurements of the efficiency of H-atom abstraction by t-butoxyl radicals from polyolefins, and suitable model compounds such as pentane, 2,4-dimethylpentane and 2,2,4,4-tetramethylpentane were determined. Insight is gained from alkyl-trapping experiments to quantify the relative reactivities of the primary, secondary and tertiary positions of the model compounds. Experimental data were compared to quantum chemical calculations...
We investigate the quantum many-body dynamics of dissociation of a Bose-Einstein condensate of molecular dimers into pairs of constituent bosonic atoms and analyze the resulting atom-atom correlations. The quantum fields of both the molecules and atoms ar
We propose and analyze a scheme for measuring the quadrature statistics of an atom laser beam using extant optical homodyning and Raman atom laser techniques. Reversal of the normal Raman atom laser outcoupling scheme is used to map the quantum statistics
We report the experimental realization of a multibeam atom laser. A single continuous atom laser is outcoupled from a Bose-Einstein condensate via an optical Raman transition. The atom laser is subsequently split into up to five atomic beams with slightly
9 pages, 7 figures -- PACS numbers: 79.20.Rf, 03.65.Sq, 03.65.Bz; The method of quantum trajectories proposed by de Broglie and Bohm is applied to the study of atom diffraction by surfaces. As an example, a realistic model for the scattering of He off corrugated Cu is considered. In this way, the final angular distribution of trajectories is obtained by box-counting, which is in excellent agreement with the results calculated by standard S-matrix methods of scattering theory. More interestingly, the accumulation of quantum trajectories at the different diffraction peaks is explained in terms of the corresponding quantum potential. This non-local potential "guides" the trajectories causing a transition from a distribution near the surface, which reproduces its shape, to the final diffraction pattern observed in the asymptotic region, far from the diffracting object. These two regimes are homologous to the Fresnel and Fraunhofer regions described in undulatory optics. Finally, the turning points of the quantum trajectories provide a
better description of the surface electronic density than the corresponding classical ones, usually employed for this task.; This work was supported by DGES (Spain) under contracts No PB95-71, PB95-425 and PB96-76. A.S. Sanz also acknowledges the Universidad Autónoma de Madrid for a doctoral grant.; Peer reviewed
7 pages, 3 figures -- PACS numbers: 03.65.-w, 03.65.Ta, 79.20.Rf; The transition to the classical limit in atom-surface diffraction is studied using the de Broglie-Bohm causal formalism. In particular, we focus on rainbow scattering, which is a well-defined effect in classical mechanics and has a clear counterpart in quantum mechanics. In order to achieve this limit, we consider the scattering of particles with increasing masses off a Cu(110) surface. Although the classical limit seems strictly unreachable, quantum trajectories mimic the characteristics of the classical intensity distribution, and their use allows to unveil the mechanism by which the quantum rainbow condition takes place.; This work was been supported in part by DGES (Spain) under contracts No PB95-71, B96-76, PB98-115 and BFM2000-347. A.S. Sanz also acknowledges the Consejería de Educación y Cultura of the Comunidad Autónoma de Madrid (Spain) for a doctoral grant.; Peer reviewed
6 pages, 8 figures.-- PACS nrs.: 03.67.Bg; 03.65.Ud; 42.50.Ct.-- ArXiv pre-print available at: http://arxiv.org/abs/0811.2647; We analyze the entanglement dynamics of a system composed by a pair of neutral two-level atoms that are initially entangled, and the electromagnetic field, initially in the vacuum state, within the formalism of perturbative quantum field theory up to the second order. We show that entanglement sudden death and revival can occur while the atoms remain spacelike separated and therefore cannot be related with photon exchange between the atoms. We interpret these phenomena as the consequence of a transfer of atom-atom entanglement to atom-field entanglement and vice versa. We also consider the different bipartitions of the system, finding similar relationships between their entanglement evolutions.; This work was supported by Spanish MEC FIS2005-
05304 and CSIC 2004 5 OE 271 projects.; Peer reviewed
Mayak Production Association, East Ural, Russia, was established to produce weapons-grade plutonium. Routine discharges and accidents at Mayak PA contaminated large areas, including the Techa River. The objectives of the present work were to study atom ratios for plutonium and, for the first time to our knowledge, uranium isotopes in water, soil, grass, and aquatic biota samples from Reservoir 11 and the Asanov Swamp, downstream from Mayak PA. Atom ratios (240Pu/239Pu, 236U/235U, 235U/238U) were determined using accelerator mass spectrometry to confirm radionuclide source characteristics and calculate activities and concentration factors for the studied samples. The lowest 240Pu/239Pu atom ratios were consistently found in Asanov Swamp samples (∼0,019), indicating a major contribution from early discharges of weapons-grade Pu. 240Pu/239Pu atom ratios in Reservoir 11 were higher, indicating influence from more recent civil reprocessing. The presence of 236U is usually indicative of fuel irradiation; 236U/235U ratios increase from weapons to civil sources. Our new data show that Asanov samples had lower 236U/235U ratios than Reservoir 11 samples (0.0005-0.0045 for Asanov compared with 0.0074-0.0153 for Reservoir 11) in agreement with Pu results. Pu and U concentration factors calculated for vegetation and biota samples at Mayak were comparable with corresponding values found in the literature.
A non-Markovian steady state analysis of a fully quantum-mechanical atom laser model was carried out. It was demonstrated that a self-consistent Markov approximation is valid provided the laser is operating in a linewidth narrowing regime and the reservoir correlation time is sufficiently short. Furthermore, it was shown that the difference between the Born-Markov approximation and the more exact treatment is that the system frequency, occupation number, and effective damping may be shifted by the coupling process.
Occupational therapy today needs to re-position itself in context to be relevant to the traumatised psyche of the South African nation. The social atom of the profession needs to relate more congruently to groups, communities and populations. Addressing the secondary causal issues underlying impairment, means our practice will increasingly become multi-modal, using experience and theory from medical, social, environmental, phenomenological and ethnographic models. Occupational therapy in South Africa needs to take the imperative of contextual relevance forward by attending to the traumatic subtexts of practice. Four propositions are suggested. 1. Inclusive, holistic practice applies to both individuals and collectives. 2. Occupational therapy is located in a social atom that enables it to be a significant role-player for social change. 3. The social atom of occupational therapy paves the way for working with the traumatised individuals and communities. 4. Service providers should manage vicarious traumatisation proactively. My hope is that these four propositions will galvanise debate about the contribution of occupational therapy to the healing of a society traumatised by violence and other forms of oppression and abuse.