J. G. Correia

Feasibility of a novel design of high resolution parallax-free Compton enhanced PET scanner dedicated to brain research

A novel concept for a positron emission tomography (PET) camera module is proposed, which provides full 3D reconstruction with high resolution over the total detector volume, free of parallax errors. The key components are a matrix of long scintillator crystals and hybrid photon detectors (HPDs) with matched segmentation and integrated readout electronics. The HPDs read out the two ends of the scintillator package. Both excellent spatial (x, y, z) and energy resolution are obtained. The concept allows enhancing the detection efficiency by reconstructing a significant fraction of events which underwent Compton scattering in the crystals. The proof of concept will first be demonstrated with yttrium orthoaluminate perovskite (YAP):Ce crystals, but the final design will rely on other scintillators more adequate for PET applications (e.g. LSO:Ce or LaBr3:Ce). A promising application of the proposed camera module, which is currently under development, is a high resolution 3D brain PET camera with an axial field-of-view of approximately 15 cm dedicated to brain research. The design philosophy and performance predictions based on analytical calculations and Monte Carlo simulations are presented. Image correction and reconstruction tools required to operate this transmissionless device in a research environment are also discussed. Better or similar performance parameters were obtained compared to other known designs at lower fabrication cost. The axial geometrical concept also seems to be promising for applications such as positron emission mammography.

Tunning pore filling of anodic alumina templates by accurate control of the bottom barrier layer thickness

The role of the alumina barrier layer thickness (δ(b)) on the growth of Ni nanowires (NWs) in porous anodic alumina (PAA) has been revealed. By varying the final anodization voltage to form dendrites at the bottom of the nanoporous structure, we are able to optimize δ(b) (in the 2-16 nm range), allowing us to obtain a Ni pore filling percentage (f(p)) of almost 100% for δ(b) = 10 nm. However, deviations from this optimal δ(b)-value led to a strong decrease of f(p). Moreover, an increase of the electrodeposition efficiency (EE) and NW homogeneity was also verified for δ(b) up to 10 nm. Such increase in nominal δ(b) leads to a consistent growth rate in all pores and consequently a complete and uniform nanopore filling. On the other hand, the decrease in electrodeposition efficiency visible for δ(b) > 10 nm is related with hydrogen evolution and dielectric breakdown of the insulator layer due to the required high deposition voltages. Non-uniform NW growth is then visible, with the consequent decrease in f(p). The control of the pore filling and length homogeneity of the fabricated 1D metallic nanostructures, combined with the ability to adjust the pore dimensions of PAA, can bring novel approaches for the fabrication of nano-objects and thus exciting new applications.

New phase transition in the Pr1-xCaxMnO3 system: evidence for electrical polarization in charge ordered manganites.

In this Letter a detailed study of the electric field gradient (EFG) across the Pr(1-x)Ca(x)MnO(3) phase diagram and its temperature dependence is given. Clearly, distinct EFG behavior for samples outside or inside the charge order (CO) region are observed. The EFG temperature dependence evidences a new phase transition occurring over the broad CO region of the phase diagram. This transition is discontinuous and occurs at temperatures between the charge ordering and the Néel temperatures. The prominent features observed in the EFG are associated with polar atomic vibrations which eventually lead to a spontaneous local electric polarization below CO transition.

Emission channeling studies of Pr in GaN

We report on the lattice location of Pr in thin film, single-crystalline hexagonal GaN using the emission channeling technique. The angular distribution of β− particles emitted by the radioactive isotope 143Pr was monitored by a position-sensitive electron detector following 60 keV room temperature implantation of the precursor isotope 143Cs at a dose of 1×1013 cm−2 and annealing up to 900 °C. Our experiments provide direct evidence that Pr is thermally stable at substitutional Ga sites.

Lattice location and thermal stability of implanted Fe in ZnO

The emission channeling technique was applied to evaluate the lattice location of implanted Fe59 in single-crystalline ZnO. The angular distribution of β− particles emitted by Fe59 was monitored with a position-sensitive electron detector, following 60 keV low dose (2.0×1013cm−2) room-temperature implantation of the precursor isotope Mn59. The emission patterns around the [0001], [1102],[1101], and [2113] directions revealed that following annealing at 800 °C, 95(8)% of the Fe atoms occupy ideal substitutional Zn sites with rms displacements of 0.06-0.09 A.

Implantation site of rare earths in single-crystalline ZnO

The lattice location of rare-earth 167mEr in single-crystalline hexagonal ZnO was studied by means of the emission channeling technique. Following 60-keV, room-temperature implantation of the precursor isotope 167Tm at doses of 1.3–2.8×1013 cm−2 and annealing up to 900 °C, the angular distribution of conversion electrons emitted by the radioactive isotope 167mEr was measured by a position-sensitive electron detector. The conversion electron emission patterns from 167mEr around the [0001], [1_102], [1_101], and [2_113] directions give direct evidence that the large majority of Er atoms (75%–90%) occupies substitutional Zn sites.

Direct evidence for Sb as a Zn site impurity in ZnO

The lattice location of ion implanted antimony in zinc oxide has been determined by means of β− emission channeling from the radioactive S124b isotope. Following 30 keV implantation of S124b into a single-crystalline ZnO sample to a fluence of 1×1014 cm−2, the angular-dependent emission rate of β− particles around several crystallographic directions was measured with a position-sensitive Si detector. The majority of Sb was found to occupy Zn sites, with the possible fraction on O sites being at maximum 5%–6%.

Electron emission channeling with position-sensitive detectors

Abstract Electron emission channeling allows direct lattice location studies of low doses of radioactive atoms implanted in single crystals. For that purpose the anisotropic emission yield of conversion electrons from the crystal surface is measured, most conveniently by use of position-sensitive detectors. We discuss characteristic features of this method, including quantitative data analysis procedures, which are achieved by fitting simulated two-dimensional emission distributions for different lattice sites to the experimental patterns. The capabilities of this approach are illustrated by the case of rare earth atoms (Er, Tm, Yb) in Si, where we were able to do lattice location experiments down to implanted doses which are 150 times lower compared to previous RBS studies.

Lattice location study of implanted In in Ge

We report on emission channeling experiments to determine the lattice location and the thermal stability of implanted I111n atoms in Ge. The majority of the In atoms was found on the substitutional site, which is a thermally stable site at least up to 500 °C. We also found strong indication that directly after implantation, a fraction of the implanted I111n atoms occupies the bond-centered (BC) site. This fraction disappears after annealing at 300 °C. From comparison with ab initio calculations, electrical studies, and perturbed angular correlation experiments, the In atoms on the BC site can be related to In-vacancy and In-self-interstitial defect complexes. The activation energy for dissociation of this BC related defect was found to be below 1.6 eV.

Melanocortin-1 receptor-targeting with radiolabeled cyclic α-melanocyte-stimulating hormone analogs for melanoma imaging.

Melanoma is a type of skin cancer known for its high aggressiveness, early dissemination of metastases, and poor prognosis once metastasized. Thus, early diagnosis of melanoma is a key issue for increasing patient survival. The overexpression of melanocortin-1 receptors (MC1R) in isolated melanoma cells and melanoma tissues led to the radiolabeling of several linear and cyclic MC analogs for melanoma imaging or therapy. Cyclization of α-melanocyte stimulating hormone (α-MSH) peptides has been successfully used to improve binding affinity and in vivo stability of peptides. Herein, we describe the different peptide cyclization strategies recently reported for radiolabeled α-MSH analogs and discuss how such strategies affect MC1R binding affinity, pharmacokinetic profile, and MC1R-melanoma imaging. This review also highlights how the nature of the radiometal and labeling approach influence those properties. Among the cyclized α-MSH peptides reported, (99m)Tc/(111)In-labeled metal-cyclized and lactam bridge-cyclized peptides displayed the highest melanoma and lowest renal uptake values in B16/F1 melanoma-bearing mice and became the most promising tools to be further explored as potential melanoma imaging probes.