Uncovering the (un-)occupied electronic structure of a buried hybrid interface.

The energy level alignment at organic/inorganic (o/i) semiconductor interfaces is crucial for any light-emitting or -harvesting functionality. Essential is the access to both occupied and unoccupied electronic states directly at the interface, which is often deeply buried underneath thick organic films and challenging to characterize. We use several complementary experimental techniques to determine the electronic structure of p -quinquephenyl pyridine (5P-Py) adsorbed on ZnO(1 0 -1 0). The parent anchoring group, pyridine, significantly lowers the work function by up to 2.9 eV and causes an occupied in-gap state (IGS) directly below the Fermi level E F. Adsorption of upright-standing 5P-Py also leads to a strong work function reduction of up to 2.1 eV and to a similar IGS. The latter is then used as an initial state for the transient population of three normally unoccupied molecular levels through optical excitation and, due to its localization right at the o/i interface, provides interfacial sensitivity, even for thick 5P-Py films. We observe two final states above the vacuum level and one bound state at around 2 eV above E F, which we attribute to the 5P-Py LUMO. By the separate study of anchoring group and organic dye combined with the exploitation of the occupied IGS for selective interfacial photoexcitation, this work provides a new pathway for characterizing the electronic structure at buried o/i interfaces.

Ultrafast exciton population, relaxation, and decay dynamics in thin oligothiophene films.

Femtosecond time-resolved two-photon photoemission spectroscopy is utilized to determine the electronically excited states dynamics at the α-sexithiophene (6T)/Au(111) interface and within the 6T film. We found that a photoinduced transition between the highest occupied molecular orbital and lowest unoccupied molecular orbital is essential in order to observe exciton population, which occurs within 100 fs. In thin 6T films, the exciton exhibits a lifetime of 650 fs. On a time scale of 400 fs, an energetic stabilization is observed leading to the formation of a polaron or electron trapping at defect states. The lifetime of this state is 6.3 ps. Coverage-dependent measurements show that apart from the excited state decay within the film, a substrate-mediated relaxation channel is operative. The present study demonstrates that two-photon photoemission spectroscopy is a powerful tool to investigate the whole life cycle from creation to decay of excitons in an organic semiconductor.

Preliminary study on the use of an inhomogeneous anthropomorphic Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT treatment plans.

An inhomogeneous anthropomorphic phantom of the human thorax including lungs and spine was developed for verification of three-dimensional (3D) intensity-modulated radiotherapy (IMRT). The phantom and spinal cord were filled with undiluted Fricke gel, whereas the lungs were filled with a special low-density Fricke gel. Based on a computed tomography scan of the phantom, an intensity-modulated stereotactic radiotherapy plan for a bronchial carcinoma was calculated using an inverse planning system (KonRad, DKFZ Heidelberg, Germany). The plan consisted of seven beams delivered in a step and shoot technique out of 67 sub-fields. Immediately after irradiation 3D magnetic resonance (MR) imaging of the phantom was performed using a special pulse sequence for T1 relaxometry. From the MR image data maps of the longitudinal relaxation rate R1 = 1/T1 were calculated. The R1 maps were converted to dose-proportional image data and compared to planning data. Measurement and planning show good agreement in regions of standard Fricke gel with an average deviation below 5%. In regions of the low-density Fricke gel, deviations are higher due to a decreased signal-to-noise ratio in the MR measurement. In these areas also a different sensitivity of the dose response was observed as compared to standard Fricke gel. The inhomogeneous thorax phantom has proven to be a useful pre-clinical tool for 3D methodical verifications.

[Diagnostic methods for the quantification of radiation injuries of the lungs]. / Diagnostische Methoden zur Erkennung radiogener Spätschäden der Lunge.

PURPOSE: The aim of this paper is to find techniques for quantifying radiation lung injury after irradiation with lung involvement to improve an early diagnosis. METHODS: The case of a patient with NSCLC was used to demonstrate different methods in order to quantify a developing pneumopathy after radiation treatment. By means of HRCT studies in the follow-up, a procedure was developed by defining a test-ROI in high-dose areas of the lung and evaluating the corresponding HU-histogramm for the parameters of the lung peak. Changes during the follow-up can be derived from the differential HU-histogram by the determination of a parameter called delta HUrel, which quantifies the shift to higher HU values. Alternatively, a Fourier analysis of the lung pattern within the test-ROI results in a Fourier amplitude distribution, which reacts sensitively to changes during the follow-up. Furthermore, a Fourier-frequency histogram can be derived which is independent of the spatial orientation of the density pattern. RESULTS: From the HRCT follow-up study, values for delta HUrel can be derived to be 0.24, 0.44, and 0.50 (56, 100 and 422 days after beginning the treatment). The differential Fourier frequency-histogram presentations demonstrate pronounced pattern changes. CONCLUSION: The presented methods demonstrate possibilities to quantify radiation lung injury. The proven sensitivity can possibly be improved after the introduction of a breath triggered HRCT technique.

[Dosimetry of a blood irradiator]. / Dosimetrie eines Blutbestrahlungsgerätes.

BACKGROUND: Blood and blood products are irradiated to avoid the graft-versus-host disease (GVHD) in immunosuppressed patients and to destroy tumor cells during the intra-operative autotransfusion in tumor surgery. For that purpose more and more dedicated gamma irradiators are used. In most cases the equipment is supplied with a dose calibration factor for a totally filled irradiation canister. As users handle different blood product volumes, it is necessary to investigate the influence of the irradiated blood volume on the absolute dose in a reference point and the dose distribution in the irradiation volume. MATERIAL AND METHODS: The dose rate in the center of an empty irradiation canister of an IBL 437C blood irradiator (CIS Diagnostic) was investigated by means of Fricke solution dosimeters from the Physikalisch-Technische Bundesanstalt (PTB). Using thermoluminescence dosimetry (TLD) this value could be transferred to a situation with an empty or completely filled respectively with 2 blood samples (270 ml each) filled canister. Also essential for the irradiation of blood is the knowledge of the dose distribution in the irradiated volume. The distributions in the empty and the realistic filled canister were measured by positioning the TLD on the plexiglas holder in a regular pattern. The case of a completely filled container was investigated by means of the MR Fricke gel dosimetry. All distributions are presented as dose-volume-histograms (DVH). RESULTS: The TLD-measurement in the center of the completely filled canister yielded a 4.8% higher dose rate value as compared to the suppliers certificate. From the investigations using the Fricke solution dosimeters in air combined with TLD-measurements values for the complete bandwidth of different container fillings could be derived. So the dose rate in the centre of the canister in the boundary conditions empty and full canister as compared to the values for the realistic filling condition (2 bags) are 117.5% and 94% respectively. Axial dose distributions and DVH have been determined for the 3 filling conditions. CONCLUSIONS: We recommend a dose calibration measurement of a blood irradiator to determine the irradiation times for the chosen filling condition, which is typical for the hospital. The DVH presented in this work can be used to derive a value for the dose variance within the irradiated blood.

[Optimization of photon dose distributions with compensation]. / Optimierung von Photonendosisverteilungen mit Kompensatoren.

PURPOSE: A homogeneous dose distribution according to the demands of the ICRU-publication 50 often can only be achieved by the use of compensators. Because of the expense those are seldom applied. The purpose of this work is to find practical methods for the production and verification. MATERIAL AND METHODS: Two procedures for the production of compensators using 3 different materials and suitable dose verification methods were investigated. The first procedure uses a laser system to get the patient contour, from which the compensator can be calculated. In a second method a 3D planning system calculates the dose modulators on the basis of CT-slices. The production is done by the use of a computer driven milling machine either via a founding form or direct milling. Mixtures of a polymer and lead powder, a mixture of tin granules and wax and the commercially available alloy MCP-96 were used. Dose verification was done using film-, TL- and the three-dimensional MR Fricke gel dosimetry as well as a diode array scanner. RESULTS: Though both methods can be used, the CT-based procedure proved to be more appropriate. Among the materials the direct milled MCP-96 compensator is favorable with respect to the handling, mechanical properties and inhomogeneous radiation attenuation. The dose verification has been done in an Alderson phantom for mantlefield and head-and-neck irradiation techniques. Here the dose modulation yielded an improvement of the homogeneity. The dose maxima normalized to the dose reference point could be reduced from 127% to 103% respectively 122% to 104%. Verifications of compensators for patient treatments confirmed the good results from the phantom measurements. CONCLUSION: The demonstrated investigations show a practicable way for the clinical application of compensators. The necessity for the use of them can be derived from the verified decrease of the dose maxima.

[The verification of optimized 3D-dosage distributions with an MR Fricke gel]. / Verifikation optimierter 3D-Dosisverteilungen mittels MR-Fricke-Gel.

PURPOSE: Complex 3D treatment planning techniques require a dose verification throughout the irradiated volume. Conventional dosimetry techniques only unsatisfyingly serve these needs. MATERIAL AND METHOD: The chemical dosimeter FeSO4 solution can be used for measuring spatial dose distributions with the help of magnetic resonance imaging by fixing the iron ions in a gelatin matrix. A 3D dosimetry method was developed for 3D verification in an homogeneous, anthropomorphic phantom. The verification is achieved by juxtaposition or superposition of measured and calculated isodoses. RESULTS: Different gel compositions were studied in view of their applicability as clinical 3D dosimeter concerning dose response, linearity and diffusion behaviour. A gel with 5% gelatin and 1 mM of ferrous ions proved to be the most suitable. The inverse spin-spin relaxation time T2(-1) is an indicator of the ferric ion concentration that showed to be linear with the dose in the range between 0 and 40 Gy (R2 = 0.996). The dose response was 0.057 per second and Gy. The observed diffusion of the iron ions was only influenced little by different gel compositions. To isolate the restrictions in the clinical application, measurements on the disturbing effects like, e.g. the inhomogeneous spatial response and the gel surface effects, were made and eliminated with the subtraction method. The clinical use of the method is demonstrated for the examples of the verification of calculated 3D dose distributions of a shielded 192Ir afterloading vaginal applicator and a head and neck 3-field plan with the use of asymmetric jaws and compensators. CONCLUSION: The study demonstrates the clinical applicability of the method and shows its limitations.

[Approximation errors in the physical planning of radioiodine therapy of the thyroid]. / Näherungsfehler bei der physikalischen Planung der Radiojodtherapie der Schilddrüse.

The approximations in the equations used for the calculation of the radioiodine therapy are reviewed and compared with a reference formula, based on ICRP Publication 30. Applying these equations to two clinical cases, we find pronounced differences compared to the reference method. Investigations of a collective of 314 patients yield in a relatively high fraction delayed maxima in the uptake curves. Simulations prove that serious errors occur when the ascending slope of the uptake curve is neglected. An equation taking care of this effect is presented. Pronounced individual differences are found between uptake curves recorded before and after therapy. The effects on the difference between planned and actually applied dose to the thyroid are shown. The difficulties of the determination of the thyroid mass are discussed and a combined method using scinti- and sonographic data presented.

[Experimental and mathematical check of the moving-strip irradiation technic]. / Experimentelle und rechnerische Uberprüfung der Moving-strip-Bestrahlungstechnik.

The moving-strip irradiation of the total peritoneal region plays an important role in the adjuvant therapy of the ovarian carcinoma. TLD dosimetric and mathematic investigations of this radiotherapeutic method show that 1. the best dose homogeneity in the longitudinal profile is achieved by 60Co units, if the patients are not too stout, 2. a higher relative depth dose has to be estimated than for an open field of the same total field size, 3. an additional 2,5 cm strip has to be inserted in order to achieve an abrupt drop on the border of the total irradiation field, 4. this method offers a good guarantee against false adjustment, if this is not too grave, 5. the conformity between the dose distribution curves calculated and those measured by TLD is excellent.