Maximiliano Montenegro

Resonant X-ray enhancement of the Auger effect in high-Z atoms, molecules, and nanoparticles: potential biomedical applications.

It is shown that X-ray absorption can be considerably enhanced at resonant energies corresponding to K-shell excitation into higher shells with electron vacancies following Auger emissions in high-Z elements and compounds employed in biomedical applications. We calculate Auger resonant probabilities and cross sections to obtain total mass attenuation coefficients with resonant cross sections and detailed resonance structures corresponding to Kalpha, Kbeta, Kgamma, Kdelta, and Keta complexes lying between 6.4-7.1 keV in iron and 67-80 keV in gold. The basic parameters were computed using the relativistic atomic structure codes and the R-matrix codes. It is found that the average enhancement at resonant energies is up to a factor of 1000 or more for associated K --> L, M, N, O, P transitions. The resonant energies in high-Z elements such as gold are sufficiently high to ensure significant penetration in body tissue, and hence the possibility of achieving X-radiation dose reduction commensurate with resonant enhancements for cancer theranostics using high-Z nanoparticles and molecular radiosensitizing agents embedded in malignant tumors. The in situ deposition of X-ray energy, followed by secondary photon and electron emission, will be localized at the tumor site. We also note the relevance of this work to the development of novel monochromatic or narrow-band X-ray emission sources for medical diagnostics and therapeutics.

[O ii] line ratios

Based on new calculations, we reconfirm the low- and high-density limits on the forbidden finestructure line intensity ratio [O II] I (3729)/I (3726) : limNe→0 = 1.5 and limNe→∞ = 0.35. Employing [O II] collision strengths calculated using the Breit‐Pauli R-matrix method, we rule out any significant deviation due to relativistic effects from these canonical values. The present results are in substantial agreement with older calculations by Pradhan, and validate the extensive observational analyses of gaseous nebulae by Copetti & Writzel and Wang et al. that reach the same conclusions. The present theoretical results and the recent observational analyses differ significantly from the calculations by McLaughlin & Bell and Keenan et al. The new Maxwellian averaged effective collision strengths are presented for the 10 transitions among the first five levels to enable computations of [O II] line ratios.

Monte Carlo simulations and atomic calculations for Auger processes in biomedical nanotheranostics.

We present numerical simulations of X-ray emission and absorption in a biological environment for which we have modified the general-purpose computer code Geant4. The underlying mechanism rests on the use of heavy nanoparticles delivered to specific sites, such as cancerous tumors, and treated with monoenergetic X-rays at resonant atomic and molecular transitions. X-ray irradiation of high-Z atoms results in Auger decays of photon emission and electron ejections creating multiple electron vacancies. These vacancies may be filled either be radiative decays from higher electronic shells or by excitations from the K-shell at resonant energies by an external X-ray source, as described in an accompanying paper by Pradhan et al. in this volume. Our Monte Carlo models assume normal body material embedded with a layer of gold nanoparticles. The simulation results presented in this paper demonstrate that resonant excitations via Kalpha, Kbeta, etc., transitions result in a considerable enhancement in localized X-ray energy deposition at the layer with gold nanoparticles, compared with nonresonant processes and energies. The present results could be applicable to in vivo therapy and diagnostics (theranostics) of cancerous tumors using high-Z nanoparticles and monochromatic X-ray sources according to the resonant theranostics (RT) methodology.

Measuring Model Flexibility With Parameter Space Partitioning: An Introduction and Application Example

A primary criterion on which models of cognition are evaluated is their ability to fit empirical data. To understand the reason why a model yields a good or poor fit, it is necessary to determine the data-fitting potential (i.e., flexibility) of the model. In the first part of this article, methods for comparing models and studying their flexibility are reviewed, with a focus on parameter space partitioning (PSP), a general-purpose method for analyzing and comparing all classes of cognitive models. PSP is then demonstrated in the second part of the article in which two connectionist models of speech perception (TRACE and ARTphone) are compared to learn how design differences affect model flexibility.

Relativistic and correlation effects in electron impact excitation of forbidden transitions of OII

We investigate relativistic and correlation effects in electron impact excitation of singly ionized oxygen using the Breit–PauliR-matrix method. The intermediate coupling close-coupling calculations are carried out using a 16-level target representation dominated by the electronic configurations 1s 2 2s 2 2p 3 ,1 s 2 2s2p 4 , 1s 2 2s 2 2p 2 3s. Resonance structures are delineated in detail to ascertain the effect on averaged collision strengths. Convergence of the partial wave summation is ensured for non-dipole transitions in the R-matrix calculations. The present results differ significantly from the similar Breit–Pauli R-matrix calculations by McLaughlin and Bell (1998 J. Phys. B: At. Mol. Opt. Phys. 31 4317–29), but are essentially in agreement with the LS coupling results of Pradhan (1976a J. Phys. B: At. Mol. Opt. Phys. 9 433–43, 1976b Mon. Not. R. Astron. Soc. 177 31–8). A comprehensive study of the detailed energy behaviour of all forbidden transitions among the five levels of the ground configuration, i.e. 2s2p 3 4 S o/2, 2 D o/2,3/2 , 2 P o/2,1/2 shows that the finestructure collision strengths do not significantly depart from the values obtained from a purely LS −→ LSJ transformation, and relativistic effects are therefore small. We find that the Maxwellian-averaged effective collision strengths for the ten transitions also differ from the previous work, most likely due to more extensive delineation of resonances in the present work. However, the differences are largely systematic and therefore the OII line intensity ratios are not significantly affected. We also obtain an excellent agreement between the present-calculated cross sections for the 4 S o − 2 D o transition and the experimental merged beam measurements.

High-temperature behaviour of the helium-like KαG ratio: the effect of improved recombination rate coefficients for calcium, iron and nickel

It is shown that above the temperature of maximum abundance, recombination rates into the excited states of He-like ions that are calculated using earlier, more approximate methods differ markedly from rates obtained from recent distorted-wave and R-matrix calculations (unified recombination rate coefficients) for Ca, Fe and Ni. The present rates lead to G ratios that are greatly lower than those resulting from the more approximate rates in previous works, by up to a factor of 6 at high electron temperatures. Excellent agreement between the distorted-wave and the R-matrix rates, as well as excellent agreement in the G ratios calculated from them, provides support for the accuracy of these new values which have a broad applicability to the modelling and interpreting of X-ray spectra from a variety of astrophysical and laboratory sources.

Multi-Disciplinary Role of Atomic Astrophysics: From Stellar Interiors to Cancer Research Via Nanotechnology

We discuss the application of atomic physics to two diverse topics: Astrophysical opacities that determine the flow of radiation through the interior of stars, and biomedical research using nanotechnology for novel methodologies for cancer diagnostics and therapy (theranostics). Recent determination of solar abundances suggests that a re-examination of the absolute accuracy of these opacities might be in order. A discussion of the Opacity Project work and possible sources of missing opacity and uncertainties in atomic data is presented with a view to possible solution of the solar abundances problem. Another major application is shaping up in biomedicine and nanotechnology: A paradigm change and transition from conventional broadband X-ray imaging (such as in CT scanners or common X-ray sources) to precision monochromatic spectroscoppy for cancer theranostics. A novel methodology — Resonant Theranostics — is proposed to exploit Kα resonances due to deep inner-shell transitions that trigger Auger processes in heavy elements. The methodology can be used to build laboratory monochromatic X-ray sources for imaging using Kα emission, as well as for therapy using Kα absorption by high-Z nanoparticles or radiosenstizing agents embedded in cancerous tumors. This review of recent work demonstrates the scope and power of multi-disciplinary research in general, particularly highlighting the role of atomic physics as an enabling scientific tool from astronomy to medicine.

Geant4 estimation model of high Z atom concentration for tumor vessel ablation

In our novel technique of tumor vessels treatment, High Z (HZ) contrast atoms are injected into the blood vessel and the tumor region is irradiated with “narrowband” fluorescence photon (FP) beam tuned to the “resonance energies”. Theoretically, this technique guarantees a dose 102 − 103 higher than that achieved in conventional radiation therapy (RT). Meanwhile, this high dose is confined to a region of tens of micrometers. This will minimize the side effects caused by the high dose to the surrounding tissues. The FPs are generated by electrons impinging onto target made of the same material as the HZ contrast. In order to support the experiment, an estimation model has been developed based on Geant4 Monte Carlo (MC) simulation. This model takes into account physical and biological factors, which can be determined separately. In this work, the derivation of the model was described in detail, and four HZ atoms, gadolinium (Gd), platinum (Pt), gold (Au) and uranium (U) were evaluated using the model. The scaling law for the capability to yield FPs from IEs had been deduced for these HZ atoms. The results also showed that the minimum molar concentration required for apoptosis of tumor endothelial cells (ECs) for Gd, Pt, Au and U in normal experimental condition were 220.44 nmol/ml, 55.57 nmol/ml, 49.78 nmol/ml and 9.05 nmol/ml, respectively.

Benchmarking the Resonances in Photoionization of O II

Study of accurate resonant features of O II is crucial in predicting its abundances in nebular plasmas and for its diagnostic role. A well known large discrency exists in the predicted abundance of the ion depending on the radiative or collisional processes taken in to consideration. We will report new calculations for resonant structures in photoionization of O II where we have noted important features not seen before, especially in the low-energy region of the three fine structure components of 2s22p2 3P of core O III. These are expected to make considerable differences in the low temperature recombination. Preliminary comparison of the new results with experiments shows improved agreement.

SU‐GG‐J‐167: Resonant X‐Ray Irradiation of High‐Z Nanoparticles for Cancer Theranostics

Purpose: Broadband X‐ray sources employed in current cancer therapy are indiscriminant of tissue composition. We demonstrate significant dose enhancement by high‐Z (HZ) sensitizing agents at resonant energies below the K‐edge. This is of particular interest in cancer theranostics using agents such as goldnanoparticles embedded in malignant tissue.Method and Materials: Relativistic R‐matrix calculations were performed for radiative transitions and photoionization for many HZ elements from Fe to U. Accurate cross section data and monochromatic attenuation coefficients were computed in energy ranges where the large k‐a resonance complexes occur. Scaling laws up to HZ species such as Pt, Au, and U are studied. Depth dose curves based on photon transport in a numerical model containing muscle, bone, and tumor with 1mg/cc concentration of goldnanoparticles were calculated using the new resonant atomic data and compared with those calculated using standard evaluated data. Results: Giant resonance complexes were identified in all HZ species at energies below the respective K edge. For example, the resonant attenuation coefficient for k‐α transitions in Fe and Au are more than 1000 times higher than at their K edge. The average absorption efficiency in cm2/g of Au at ∼68 keV is 7400, compared to 3.3 at the Au K‐edge, and only about 0.2 for C, N, O and 0.24 for Ca. The gold resonance complexes lie in the range 67–80 keV, below the K‐edge. The calculations reveal preferential dose deposition ratio of 3 to 4 between the goldnanoparticle‐bearingtumor and soft tissue (including surface) or bone. Conclusion: Opportunities exist to specifically target HZ‐bearing tumors while sparing normal tissue at monochromatic or narrowband X‐ray sources tuned to resonant energies. We identify and discuss potential sources for the generation of sufficiently intense K‐alpha resonant radiation, such as laser produced plasma devices and electron‐beam ion traps.