Backscattered electron emission after proton impact on gold nanoparticles with and without polymer shell coating

This work aims at measuring experimentally proton induced secondary electron energy spectra after interaction with gold nano particles (GNPs) and polymer-coated GNPs. Backscattered electron energy spectra were collected over a 0 to 1000 eV energy range using a retarding field analyzer (RFA). This paper presents the spectra obtained for proton beam energies of 0.5 and 2 MeV and diameter 2.5 and 3.8 nm GNPs. The spectra were also measured for 3.8 nm GNPs after 5 and 10 MeV proton irradiations. GNPs were deposited on a 100 nm carbon film. Each experimental spectrum was compared with dedicated simulations based on existing numerical models used in the TRAX and Geant4 Monte Carlo codes. For 100 nm carbon target, good agreement between experimental, TRAX and Geant4 simulation results can be observed. For 3.8 nm GNPs, the TRAX simulations reproduce with good agreement the electron energy spectra produced after 0.5, 2, 5 and 10 MeV proton irradiations, while Geant4 spectra display a lower secondary electron yield at low energy (<600 eV) for all the studied energies. This underestimation can mostly be explained by the 790 eV threshold applied in the condensed history model used by Geant4 which impacts the secondary electron energy distribution. Results obtained for carbon and gold targets highlight the impact of the secondary electron production threshold for proton ionization process considered in condensed history models. The experimental results demonstrate that the single interaction approach used in TRAX is adapted to reproduce secondary electron emission from GNPs. On the other hand, the standard electron generation threshold implement in G4BetheBlochModel and G4BraggModel condensed-history models used in Geant4 is not adapted to reproduce low energy electron emission in gold targets. Finally, the results highlight that the GNP coating leads to a decrease of the electron yield and mostly affects low energy electrons (<500 eV) emitted from GNPs.

Experimental measurements validate the use of the binary encounter approximation model to accurately compute proton induced dose and radiolysis enhancement from gold nanoparticles

In protontherapy, it has been suggested that nanoparticles of high-Z material like gold (GNP) could be used as radiosensitizers. The origin of this enhancement phenomenon for proton radiation is not yet well understood and additional mechanistic insights are required. Previous works have highlighted the good capabilities of TRAX to reproduce secondary electron emission from gold material. Therefore, TRAX cross sections obtained with the binary encounter approximation (BEA) model for proton ionization were implemented within Geant4 for gold material. Based on the TRAX cross sections, improved Geant4 simulations have been developed to investigate the energy deposition and radical species production around a spherical gold nanoparticle (5 and 10 nm in diameter) placed in a water volume during proton irradiation. Simulations were performed for incident 2 MeV proton. The dose enhancement factor and the radiolysis enhancement factor were quantified. Results obtained with the BEA model were compared with results obtained with condensed-history models. Experimental irradiation of 200 nm gold films were performed to validate the secondary electron emission reproduction capabilities of physical models used in Monte Carlo (MC) simulations. TRAX simulations reproduced the experimental backscattered electron energy spectrum from gold film with better agreement than Geant4. Results on gold film obtained with the BEA model enabled to estimate the electron emission from GNPs. Results obtained in our study tend to support that the use of the BEA discrete model leads to a significant increase of the dose in the near vicinity of GNPs (<20 nm), while condensed history models used in Geant4 seem to overestimate the dose and the number of chemical species for increasing distances from the GNP. Based on discrete BEA model results, no enhancement effect due to secondary electron emitted from the GNP is expected if the GNP is not in close proximity to key cellular functional elements (DNA, mitochondria…).

Micro-Raman spectroscopy and chemometrical analysis for the distinction of copper phthalocyanine polymorphs in paint layers

In art analysis, copper phthalocyanine (CuPc) is often identified as an important pigment (PB15) in 20th century artworks. Raman spectroscopy is a very valuable technique for the detection of this pigment in paint systems. However, PB15 is used in different polymorphic forms and identification of the polymorph could retrieve information on the production process of the pigment at the moment. Raman spectroscopy, being a molecular spectroscopic method of analysis, is able to discriminate between polymorphs of crystals. However, in the case of PB15, spectral interpretation is not straightforward, and Raman data treatment requires some improvements concerning the PB15 polymorphic discrimination in paints. Here, Raman spectroscopy is combined with chemometrical analysis in order to develop a procedure allowing us to identify the PB15 crystalline structure in painted layers and in artworks. The results obtained by Linear Discriminant Analysis (LDA), using intensity ratios as variables, demonstrate the ability of this procedure to predict the crystalline structure of a PB15 pigment in unknown paint samples.

Study of the mechanism of direct laser desorption/ionisation for some small organic molecules (M < 400 Daltons)

Aspects of direct laser desorption/ionisation have been studied for three molecules, aminotriazole (positive ion), dinoterb and ioxynil (negative ion). The samples are deposited on metallic substrates, and a nitrogen laser is used for desorption/ionisation; ion yields are measured with a time-of-flight mass spectrometer. Previous work had shown that ion yields can strongly vary from one substrate to another, and that this variation does not reflect the (calculated) metal surface temperatures. New results obtained in this work indicate that the desorption/ionisation mechanism is linked to the physical state of the substrate surface. Copyright 1999 John Wiley & Sons, Ltd.