Gianfranco Paternò

Design study of a Laue lens for nuclear medicine

A Laue lens is an ensemble of crystals capable of focusing, through diffraction in transmission geometry, a fraction of the photons emitted by an X- or γ-ray source onto a small area of a detector. The present study facilitates a thorough understanding of the effect of each system parameter on the efficiency, the resolution and the field of view of the lens. In this way, the structure and the size of the crystals can be set to achieve a compact lens capable of providing a high-resolution image of the radioactivity distribution lying inside a restricted region of a patients body. As an application, a Laue lens optimized at 140.5u2005keV, the γ-line emitted by 99mTc, has been designed. The lens is composed of ten rings of Ge crystals with curved diffracting planes and focuses the photons onto a detector 50u2005cm apart from the source with 1.16 × 10−5 efficiency and 0.2u2005mm resolution. The combination of these two important figures of merit makes the proposed device better performing than pinhole single photon emission computed tomography, which is the technique employed for top-resolution images in nuclear medicine. Finally, the imaging capability of the designed lens has been tested through simulations performed with a custom-made Monte Carlo code.

High-efficiency focusing of hard X-rays exploiting the quasi-mosaic effect in a bent germanium crystal

A germanium crystal was bent through a grid of superficial grooves, manufactured on the sample surface. The resulting diffraction planes were bent thanks to quasi-mosaicity, which is an effect of mechanical anisotropy in crystals. High integrated diffraction efficiency was achieved in symmetric Laue geometry with a monochromatic X-ray beam set at 150 and 300u2005keV. It is demonstrated that the sample is capable of efficiently focusing X-rays. Such crystals can be used as optical components to focalize X- and γ-rays in a high-resolution Laue lens.

Laue lens to focus an X-ray beam for radiation therapy

A Laue lens is an optical component composed of a set of crystals that produce a convergent beam exploiting X-ray diffraction in transmission geometry. Employment of a system formed by a properly designed Laue lens coupled with an X-ray unit to selectively irradiate tumours is proposed. A convergent beam leads to a depth dose profile with a pronounced peak at the focal depth, which may result in a high precision of the dose delivery. Using a custom-made Monte Carlo code and the GAMOS code, we carried out a design study to determine the geometry and the optimal features of the crystals composing the lens. As an application, a Laue lens capable of focusing a 80u2005keV beam 50u2005cm downstream of the lens has been designed. The lens is composed of an ensemble of Si crystals with curved diffracting planes. The lens produces a focal spot of 2u2005mm enclosing 7.64 × 106 photons for an electron charge of 1u2005mC impinging on the surface of the X-ray tube anode. The combination of these important figures of merit makes the proposed system suitable for irradiating both sub-cm and larger tumour masses efficiently. A dose of 2u2005Gy can be delivered to a small tumour in a few seconds, sparing at the same time the surrounding tissues.

High-efficiency diffraction and focusing of X-rays through asymmetric bent crystalline planes

The grooving technique was employed for manufacturing a self-standing curved Ge crystal. The crystal focuses hard X-rays with high efficiency by diffraction in Laue geometry through asymmetric bent planes. The sample was tested at the Institut Laue–Langevin (Grenoble, France), undergoing two types of characterization. A monochromatic and low-divergence γ-ray beam was used to test the curvature of asymmetric planes, showing a diffraction performance better than for any mosaic crystal under equal conditions. Then, the focusing capability of the crystal was probed through a polychromatic and fine-focus hard X-ray beam. Asymmetric (220) planes were chosen for analysis because of the impossibility of obtaining a curvature along this family of planes via any symmetric configuration in focusing crystals. A method for calculating the curvatures induced in any family of lattice planes is also presented.

Ion implantation for manufacturing bent and periodically bent crystals

Ion implantation is proposed to produce self-standing bent monocrystals. A Si sample 0.2u2009mm thick was bent to a radius of curvature of 10.5u2009m. The sample curvature was characterized by interferometric measurements; the crystalline quality of the bulk was tested by X-ray diffraction in transmission geometry through synchrotron light at ESRF (Grenoble, France). Dislocations induced by ion implantation affect only a very superficial layer of the sample, namely, the damaged region is confined in a layer 1u2009μm thick. Finally, an elective application of a deformed crystal through ion implantation is here proposed, i.e., the realization of a crystalline undulator to produce X-ray beams.

Homogeneous self-standing curved monocrystals, obtained using sandblasting, to be used as manipulators of hard X-rays and charged particle beams

A technique to obtain self-standing curved crystals has been developed. The method is based on a sandblasting process capable of producing an amorphized layer on the substrate. It is demonstrated that the amorphized layer behaves as a thin compressive film, causing the curvature of the substrate. This procedure permits the fabrication of homogeneously curved crystals in a fast and economical way. It is shown that a sandblasted crystal can be used as an X-ray optical element for astrophysical or medical applications. A sandblasted bent crystal can also be used as an optical element for steering charged particles in accelerator beamlines. Several samples were manufactured and bent using the sandblasting method at the Sensor and Semiconductor Laboratory of Ferrara, Italy. Their curvature was verified using interferometric profilometry, showing a deformation in agreement with the Stoney formalism. The curvature of the machined samples was also tested using γ-ray diffraction at the Institut Laue–Langevin (ILL), Grenoble, France. A good agreement with the dynamical theory of diffraction was observed. In particular, the experiment showed that the crystalline quality of the bulk was preserved. Moreover, the method allowed curved samples to be obtained free of any additional material. Finally, a crystalline undulator was produced using sandblasting and tested using γ-ray diffraction at the ILL. The crystal showed a precise undulating pattern, so it will be suitable for hard X-ray production.

Geant4 implementation of inter-atomic interference effect in small-angle coherent X-ray scattering for materials of medical interest

An extension to Geant4 Monte Carlo code was developed to take into account inter-atomic (molecular) interference effects in X-ray coherent scattering. Based on our previous works, the developed code introduces a set of form factors including interference effects for a selected variety of amorphous materials useful for medical applications, namely various tissues and plastics used to build phantoms. The code is easily upgradable in order to include new materials and offers the possibility to model a generic tissue as a combination of a set of four basic components. A dedicated Geant4 application for the simulation of X-ray diffraction experiments was created to validate the proposed upgrade of Rayleigh scattering model. A preliminary validation of the code obtained through a comparison with EGS4 and an experiment is presented, showing a satisfactory agreement.

Abstract ID: 176 Geant4 implementation of inter-atomic interference effect in small-angle coherent X-ray scattering for materials of medical interest

Advanced applications of digital mammography such as dual-energy and tomosynthesis require multiple exposures and thus deliver higher dose compared to standard mammograms. A straightforward manner to reduce patient dose without affecting image quality would be removal of the anti-scatter grid, provided that the involved reconstruction algorithms are able to take the scatter figure into account [1]. Monte Carlo simulations are very well suited for the calculation of X-ray scatter distribution and can be used to integrate such information within the reconstruction software. Geant4 is an open source C++ particle tracking code widely used in several physical fields, including medical physics [2,3]. However, the coherent scattering cross section used by the standard Geant4 code does not take into account the influence of molecular interference. According to the independent atomic scattering approximation (the so-called free-atom model), coherent radiation is indistinguishable from primary radiation because its angular distribution is peaked in the forward direction. Since interference effects occur between x-rays scattered by neighbouring atoms in matter, it was shown experimentally that the scatter distribution is affected by the molecular structure of the target, even in amorphous materials. The most important consequence is that the coherent scatter distribution is not peaked in the forward direction, and the position of the maximum is strongly material-dependent [4]. In this contribution, we present the implementation of a method to take into account inter-atomic interference in small-angle coherent scattering in Geant4, including a dedicated data set of suitable molecular form factor values for several materials of clinical interest. Furthermore, we present scatter images of simple geometric phantoms in which the Rayleigh contribution is rigorously evaluated.

Origin of quasi-mosaic effect for symmetric skew planes in a silicon or germanium plate

Bent silicon and germanium crystals are used for several modern physics applications, above all for focusing of hard X-rays and for steering of charged particle beams by means of channeling and related coherent phenomena. In particular, anisotropic deformations are effectively exploited for these applications. A typical anisotropic deformation that is used is the quasi-mosaic (QM) curvature. It involves the bending of crystallographic planes that would be otherwise flat in the case of an isotropic medium. Here, the curvature the {110} planes was obtained through the quasi-mosaic effect in the symmetric configuration for the first time. This achievement is important because the {110} family of planes is highly efficient for both the applications mentioned above. Until now, the curvature of {110} planes in the QM configuration has not been used because it vanishes if the direction of the planes is aligned with the applied moment that bends the crystal plate. Indeed, to obtain the curvature of this particular family of crystallographic planes, the h110i direction has not to be aligned with respect to the imparted moment that bends the plate, i.e. the {110} planes have to be skew planes. Experimental verification of the quasi-mosaic curvature for the {110} planes was provided through hard X-ray diffraction at beamline ID15A of the European Synchrotron Radiation Facility in Grenoble, France, showing good agreement with the theoretical expectation.