High Atomic Number Elements Research Articles

MO‐F‐214‐04: Protoyping Compositions of Novel Radiochromic Film Types: Towards Complete Absorbed Dose Energy Independence

Purpose: To develop a new generation energy‐independent radiochromic film over a wide energy range of 50 kVp to 18 MV. The films are preferably symmetric in structure and can be used for measurements in complex spectral distributions. Methods: The overall energy dependence and intrinsic energy dependence were measured for EBT2 and several prototype films of known compositions. The intrinsic energy response is quantified through a measurement of total energy response divided by the Monte Carlo calculated absorbed dose energy response. The measurements consisted of delivering an exact dose of 2 Gy to the sensitive layer of the film at both orthovoltage energies (50kVp, 120kVp, and 180kVp) and 60Co beam. Thus far, two iterations have been performed, whereby; numerical simulations are used to obtain an optimized absorbed dose energy response that will undo for the film intrinsic energy response. DOSRZnrc user‐code of the EGSnrc Monte Carlo code was used for all simulations. AAPM TG51 and TG61 were used to determine the dose‐to‐water and air‐kerma in air in megavoltage and orthovoltage beams, respectively, while Monte Carlo simulated corrections were used to convert these results to the desired dose to film. Results: For EBT2 films the overall energy dependence was found to vary by 35% over 50 kVp to Co‐60 energy range. The latest prototype measured in this work has shown a total net variation of 8% over the same range. High atomic number elements (Chlorine and bromine) were found to affect absorbed dose response drastically. The iterative optimization technique has placed a large focus in accurately determining the elemental composition. Conclusions: We have quantified the intrinsic energy dependence of radiochromic films, and have used the data to numerically optimize the composition of the active layer to produce an energy‐ independent film with an 8% energy variation over a wide energy range.Funding: This work was supported by the Natural Sciences and Engineering Research Council of Canada contract No. 386009.Conflict of interest: ISP is a manufacturing company of GAFCHROMICTM films.

Can X‐ray spectrum imaging replace backscattered electrons for compositional contrast in the scanning electron microscope?

The high throughput of the silicon drift detector energy dispersive X-ray spectrometer (SDD-EDS) enables X-ray spectrum imaging (XSI) in the scanning electron microscope to be performed in frame times of 10-100 s, the typical time needed to record a high-quality backscattered electron (BSE) image. These short-duration XSIs can reveal all elements, except H, He, and Li, present as major constituents, defined as 0.1 mass fraction (10 wt%) or higher, as well as minor constituents in the range 0.01-0.1 mass fraction, depending on the particular composition and possible interferences. Although BSEs have a greater abundance by a factor of 100 compared with characteristic X-rays, the strong compositional contrast in element-specific X-ray maps enables XSI mapping to compete with BSE imaging to reveal compositional features. Differences in the fraction of the interaction volume sampled by the BSE and X-ray signals lead to more delocalization of the X-ray signal at abrupt compositional boundaries, resulting in poorer spatial resolution. Improved resolution in X-ray elemental maps occurs for the case of a small feature composed of intermediate to high atomic number elements embedded in a matrix of lower atomic number elements. XSI imaging strongly complements BSE imaging, and the SDD-EDS technology enables an efficient combined BSE-XSI measurement strategy that maximizes the compositional information. If 10 s or more are available for the measurement of an area of interest, the analyst should always record the combined BSE-XSI information to gain the advantages of both measures of compositional contrast.

Angular Distribution of Fluorescent L X-Rays and Compton-Scattering Photons

ABSTRACT The angular distribution of fluorescent L X-rays and Compton-scattering photons was investigated by measuring differential cross sections for high–atomic-number elements. The measurements were taken simultaneously for L X-ray and Compton-scattering photons using Am-241 radioisotope as photon sources and a Si(Li) detector in various emission angles. The measurements confirm well-known angular dependency of the Compton-scattering differential cross section. L X-ray angular distribution measurements were carried out for the Ll, Lα, Lβ, and Lγ lines. It was observed from measured differential cross sections that Ll and Lα X-ray differential cross sections for the L3 substate depended on the emission angle, meaning that Ll and Lα X-rays had an anisotropic spatial distribution. On the other hand, the Lβ and Lγ X-rays do not show any significant anisotropy.

Dimensionally reduced heavy atom semiconductors as candidate materials for γ-ray detection: the case of Cs2Hg6S7

ABSTRACTWe address the issue of decreasing band-gap with increasing atomic number, inherent in semiconducting materials, by introducing a concept we call dimensional reduction. The concept leads to semiconductor compounds containing high atomic number elements and simultaneously exhibiting a large band gap and high mass density suggesting that dimensional reduction can be successfully employed in developing new γ-ray detecting materials. As an example we discuss the compound Cs2Hg6S7 that exhibits a band-gap of 1.65eV and mobility-lifetime products comparable to those of optimized Cd0.9Zn0.1Te.

SU-GG-T-153: Dosimetric Impact of Titanium Spinal Fixation Devices in Spine IMRT

Purpose: The use of hybrid universal clamp/pedicle screws has become increasingly widespread in the treatment of thoracolumbar spinal diseases. The majority of these spinal cord fixation devices are made of Titanium alloys which compose high atomic number (high‐Z) elements. The dosimetric impacts on the target volume and organs at risk from implantable spinal cord fixation devices for patients with them undergoing spine irradiation were first‐ever evaluated in this study. Method and Materials: Five patients with Titanium thoracic spinal fixation systems who are need spine irradiation underwent extended HU range CT scanning followed by 9 equiangular fields IMRTcomputertreatment planning, where homogeneous and heterogeneous plans based on normal CT to Density table and extended CT to Density table were created respectively for photon beams of energy 6 MV using Collapsed Cone Convolution Superposition algorithm. The DVH based parameters for CTV, spinal cord and lungs were compared. Results: In comparisons for heterogeneous IMRT plans based on normal CT to Density table and extended CT to Density table, the difference on the minimum dose, maximum dose and mean dose for CTV were 0.073%, 3.773% and 1.871%, respectively. The difference on the maximum dose for spinal cord was 0.818% and the difference on the V20 for lungs was 1.804%. In comparisons for homogeneous and heterogeneous IMRT plans based on extended CT to Density table, the difference on the minimum dose, maximum dose and mean dose for CTV were 1.276%, 2.419% and 0.985%, respectively. The difference on the maximum dose for spinal cord was 3.218% and the difference on the V20 for lungs was 2.116%.Conclusion: For organs at risk in this study, the dose differences introduced by high density materials were smaller than that between homogeneous and heterogeneous plans. Further investigation by Monte Carlo simulation should be done to verify it.

High Resolution Scanning Electron Microscope Examination of the Fish Scale: Inspiration for Novel Biomaterials

A Scanning Electron Microscopic study on the fish scale of Cyprinus carpio communis (freshwater carp), depicts remarkable structural and compositional characteristics which may be used as inspiration for novel biomaterial design. The fracture surface structure and sintered fish scale reveals an osseous layer on its dorsal side. It has a compact heterogeneous crystalloid-like structure of assorted shapes and sizes. The ventral side is made of orthogonally arranged mineralized needle like crystalloids template embedded in a fibrillary plate. Frozen scales revealed that this dorsal layer may contain high atomic number elements as it appeared bright with back scattered electron signals. The ventral side consists of collagen plates and a matrix, which are arranged orthogonally in a double-twisted, plywood-like structure. There are alternate crystalloid and matrix which are arranged orthogonally and forming 15-17 layers in between these two sides. This provides useful information of scale composition. Design features from the structure of the fish scale may be useful in the development of functional biomaterials, in various different fields including nano-composites, biopolymers and natural source of hydroxyapatite, used in applied therapeutic, pharmaceutical industries and semiconductor technology. The tolerance of the fish scale to high temperature and very low temperature cooling revealed unique characteristics for biomaterial fabrication. The orthogonally arranged ventral side plates of collagen embedded in proteoglycans may also prove to be a good source of scaffold material for cell culturing for tissue engineering.

Investigation of two heavy element scintillators by Monte-Carlo methods

The aim of this study was to estimate the influence of K-characteristic radiation on the performance of x-ray scintillating screens containing two heavy elements by Monte Carlo methods. K-characteristic radiation is produced within materials of at least one heavy (high atomic number) element. This radiation may result either in spatial resolution degradation or in emission efficiency decrease. The scintillators studied were the following: LYSO (Lu1.8Y0.2SiO5 and LuYSiO5), CsI and YTaO4. All the aforementioned scintillators have two heavy elements, thus the K-characteristic radiation of the high-Z element can produce additional K-characteristic photons on the low-Z element, resulting in further degradation. Scintillator performance was described in terms of the: (a) Probability of generation and reabsorption of a K-characteristic photon (PKR) and (b) Spatial distribution of K-characteristic radiation within the scintillator material. A custom validated Monte Carlo model was used, in order to simulate the transport of K-characteristic radiation within the above scintillator materials. Results showed that, depending on screen thickness (20–100 mg/cm2) and incident photon energy (20–80 keV) the scintillator's emission efficiency may be significantly reduced.

Biological equivalent dose studies for dose escalation in the stereotactic synchrotron radiation therapy clinical trials

Synchrotron radiation is an innovative tool for the treatment of brain tumors. In the stereotactic synchrotron radiation therapy (SSRT) technique a radiation dose enhancement specific to the tumor is obtained. The tumor is loaded with a high atomic number (Z) element and it is irradiated in stereotactic conditions from several entrance angles. The aim of this work was to assess dosimetric properties of the SSRT for preparing clinical trials at the European Synchrotron Radiation Facility (ESRF). To estimate the possible risks, the doses received by the tumor and healthy tissues in the future clinical conditions have been calculated by using Monte Carlo simulations (PENELOPE code). The dose enhancement factors have been determined for different iodine concentrations in the tumor, several tumor positions, tumor sizes, and different beam sizes. A scheme for the dose escalation in the various phases of the clinical trials has been proposed. The biological equivalent doses and the normalized total doses received by the skull have been calculated in order to assure that the tolerance values are not reached.

Glass formation and optical properties of CdGeAs2 alloys

Abstract Cadmium germanium arsenide (CGA), CdGeAs 2 , is a chalcopyrite-structured non-linear optical crystal with a high non-linear optical coefficient ( d 36 =236 pm/V) and a transparency range of 2.3–18 μm. This compound is also known to be a glass-former when quenched from a melt. In this study, the glass-forming ability of CGA alloyed with different amounts of P, Sn, Sb, and Pb is examined, and the optical properties of CGA single crystals are compared to those of the amorphous CGA samples. The glass-forming ability of the alloyed compounds was determined using powder X-ray diffraction, and glass transition and recrystallization temperatures were measured using differential scanning calorimetry. Glass formation ranges were measured to be: 0⩽ x ⩽1 for CdGe(As (1− x ) P x ) 2 , 0⩽ x ⩽0.4 for CdGe (1− x ) Sn x As 2 , 0⩽ x ⩽0.2 for CdGe(As (1− x ) Sb x ) 2 , and 0⩽ x ⩽0.1 for CdGe (1− x ) Pb x As 2 . The glass transition temperature of amorphous CGA decreased with the addition of Sn, Sb, and Pb and increased with the addition of P. Optical spectra were measured for the CGA single crystals and the alloyed glasses. The optical band gaps in all of the CGA glass alloys were measured to be higher than that of the CGA crystal (0.54 eV). The optical band gap of the CGA glass alloys increased with the addition of lower atomic number elements (P), and decreased with the addition of higher atomic number elements (Sn, Sb, Pb).

Polymer gel dosimetry for synchrotron stereotactic radiotherapy and iodine dose-enhancement measurements

Synchrotron stereotactic radiotherapy (SSR) is a radiotherapy technique that makes use of the interactions of monochromatic low energy x-rays with high atomic number (Z) elements. An important dose-enhancement can be obtained if the target volume has been loaded with a sufficient amount of a high-Z element, such as iodine. In this study, we compare experimental dose measurements, obtained with normoxic polymer gel (nPAG), with Monte Carlo computations. Gels were irradiated within an anthropomorphic head phantom and were read out by magnetic resonance imaging. The dose-enhancement due to the presence of iodine in the gel (iodine concentration: 5 and 10 mg ml−1) was measured at two radiation energies (35 and 80 keV) and was compared to the calculated factors. nPAG dosimetry was shown to be efficient for measuring the sharp dose gradients produced by SSR. The agreement between 3D gel dosimetry and calculated dose distributions was found to be within 4% of the dose difference criterion and a distance to agreement of 2.1 mm for 80% of the voxels. Polymer gel doped with iodine exhibited higher sensitivity, in good agreement with the calculated iodine-dose enhancement. We demonstrate in this preliminary study that iodine-doped nPAG could be used for measuring in situ dose distributions for iodine-enhanced SSR treatment.

Mistakes Encountered during Automatic Peak Identification in Low Beam Energy X‐ray Microanalysis

Automated peak identification in electron beam excited X-ray microanalysis with energy dispersive X-ray spectrometry (EDS) is subject to occasional mistakes even on well-separated, high-intensity peaks arising from major constituents. The problem is exacerbated when analysis conditions are restricted to operation in the "low beam energy scanning electron microscopy" (i.e. "low voltage scanning electron microscopy" or LVSEM) regime where the incident beam energy is 5 keV or less. These low beam energy microanalysis conditions force the analyst to use low fluorescence yield L-shell and M-shell peaks rather than higher yield K-shell and L-shell peaks typically selected for elements of intermediate and high atomic number under conventional high beam energy (>10 keV) conditions. Misidentifications can arise in automated peak identification procedures when only a single energy channel is used to characterize an EDS peak. The effect of the EDS measurement process is to convolve the closely spaced Lalpha-Lbeta and Malpha-Mbeta peaks into a single peak with a peak channel shift of 20 eV or more from the Lalpha or Malpha value, which is typically sought in an X-ray database. An extensive list of problem situations encountered in low beam energy microanalysis is presented based upon observed peak identification mistakes as well as likely troublesome situations based upon proximity in peak energy. Robust automatic peak identification requires implementation of peak fitting that utilizes the full peak shape.

Investigating the effect of K-characteristic radiation on the performance of nuclear medicine scintillators by Monte Carlo methods

The aim of this study was to evaluate the effect of K-characteristic radiation on the performance of scintillator crystals incorporated in nuclear medicine detectors (LSO, BGO, GSO). K-characteristic radiation is produced within materials of at least one high atomic number element (e.g. Lu, Gd, Bi). This radiation may either be reabsorbed or it may escape the scintillator. In both cases the light emission efficiency of the scintillator may be affected resulting in either spatial or energy resolution degradation. A computational program, based on Monte Carlo methods, was developed in order to simulate the transport of K-characteristic radiation within the most commonly used scintillator materials. Crystal thickness was allowed to vary from 0.5 up to 15 mm. A monoenergetic pencil beam, with energy varying from 0.60 to 0.511 MeV was considered to fall on the center of the crystal surface. The dominant γ-ray interactions (elastic and inelastic scattering and photoelectric absorption) were taken into account in the simulation. Results showed that, depending on crystal thickness, incident photon energy and scintillator's intrinsic properties (L or K-fluorescence yield, effective atomic number and density), the scintillator's emission efficiency may be significantly reduced and affect spatial or energy resolution.

Measurements of K shell X-ray production cross sections and fluorescence yields of elements in the atomic number range 65 ⩽ Z ⩽ 92 at 123.6 keV

In this paper we report on measurements of Kα 1, Kα 2, Kβ 1′ and K β 2 ′ X-ray production cross sections and K shell fluorescence yields for 21 high atomic number elements. The targets were irradiated with γ-photons at 123.6 keV from 57Co annular source. The K X-rays from different targets were detected using a high resolution Si(Li) semiconductor detector. The measured K X-ray production cross sections were compared with calculated theoretical values. Similarly, the measured K shell fluorescence yields were compared with the experimental and theoretical data in literature. The results have been plotted versus atomic number.

Dosimetric and microdosimetric study of contrast-enhanced radiotherapy with kilovolt x-rays

Kilovolt x-rays are clearly suboptimal compared to MV photon beams for radiotherapy of deep-seated tumours because of the increased attenuation in tissue, causing a rapid dose fall-off. This picture could change drastically when tumours can be labelled with contrast medium, containing high atomic number elements. This causes a significant dose enhancement to the tumour by exploiting the high cross sections for the photo-electric effect for kV x-rays. In this work, we have investigated the dosimetric and microdosimetric characteristics of kV contrast-enhanced radiation therapy (CERT) for different photon energies, contrast-medium concentrations and types (I and Gd). Two idealized patient treatment plans (head and lung) for irradiation with CT-arc beams were calculated. It is shown that the dose enhancement in tumours can be highly significant (up to about sixfold for realistic 80–120 kVp x-ray spectra and an iodine concentration of 50 mg ml−1) but that dose homogeneity in the tumour depends on photon energy, contrast-medium concentration and type, and irradiation scheme. An attempt to optimize the irradiation scheme is discussed. The microdosimetric study of the dose mean lineal energy shows that radiation quality changes in the contrast-medium-labelled region compared to homogeneous tissue are fairly small and limited to 10%. It is concluded that kV-CERT is a promising radiotherapy technique, provided contrast medium can be delivered reliably to tumours.

Dosimetry for synchrotron stereotactic radiotherapy: Fricke gel and Monte Carlo calculations

In Synchrotron stereotactic radiotherapy (SSR) the tumor is loaded with atoms of high atomic number ( Z) element, and exposed to monochromatic X-rays from a synchrotron source (50–100 keV), in stereotactic conditions. This method is under investigation at the European Synchrotron Radiation Facility (ESRF) for the treatment of brain tumors. Conventional treatment planning systems are not suitable for SSR since the beam energy, the irradiation geometry and the presence of high Z elements differ from commercial systems. Therefore, SSR requires the development of new procedures for the evaluation of dose maps. Monte Carlo (MC) method offers the potential of calculating the dose in complicated geometries, made of heterogeneous materials. MC calculations were performed using the MCNP4C general purpose code. Gel dosimetry allows accurate experimental verification of three-dimensional absorbed dose distributions from any irradiation geometries. In this study, the Fricke chemical dosimeter was employed for it is easy to prepare without special facilities. Various phantoms filled with Fricke gel were irradiated in SSR configuration. The dose profiles obtained were compared with MC simulations.

Measurement of vacancy transfer probabilities from K to L shell for high atomic number elements

The probabilities for vacancy transfer from K to L shell, η KL, were obtained by measuring the K β/K α intensity ratios in 25 elements over the range 57 ≤ Z ≤ 92 using a 25 mCi 57Co filtered source for excitation. The K X-rays were measured by using a Si(Li) detector. The theoretical values were calculated via the radiative and radiationless transition rates of these elements. The comparison between present experimental results and theoretical predictions showed that both results agreed well. The experimental and theoretical values were fitted against atomic number ( Z). The measured values of η KL for Tm, Yb, Lu, Hf and Ir are being reported here for the first time.

Synchrotron stereotactic radiotherapy: dosimetry by Fricke gel and Monte Carlo simulations

Synchrotron stereotactic radiotherapy (SSR) consists in loading the tumour with a high atomic number element (Z), and exposing it to monochromatic x-rays from a synchrotron source (50–100 keV), in stereotactic conditions. The dose distribution results from both the stereotactic monochromatic x-ray irradiation and the presence of the high Z element. The purpose of this preliminary study was to evaluate the two-dimensional dose distribution resulting solely from the irradiation geometry, using Monte Carlo simulations and a Fricke gel dosimeter. The verification of a Monte Carlo-based dosimetry was first assessed by depth dose measurements in a water tank. We thereafter used a Fricke dosimeter to compare Monte Carlo simulations with dose measurements. The Fricke dosimeter is a solution containing ferrous ions which are oxidized to ferric ions under ionizing radiation, proportionally to the absorbed dose. A cylindrical phantom filled with Fricke gel was irradiated in stereotactic conditions over several slices with a continuous beam (beam section = 0.1 × 1 cm2). The phantom and calibration vessels were then imaged by nuclear magnetic resonance. The measured doses were fairly consistent with those predicted by Monte Carlo simulations. However, the measured maximum absolute dose was 10% underestimated regarding calculation. The loss of information in the higher region of dose is explained by the diffusion of ferric ions. Monte Carlo simulation is the most accurate tool for dosimetry including complex geometries made of heterogeneous materials. Although the technique requires improvements, gel dosimetry remains an essential tool for the experimental verification of dose distribution in SSR with millimetre precision.

Application of combined micro-proton-induced X-ray emission and micro-synchrotron radiation X-ray fluorescence techniques for the characterization of impact materials around Barringer Meteor Crater

A combined micro-PIXE and micro-SRXRF method has been used for the characterization of impact materials collected at the well-known Barringer Meteor Crater. Elemental maps were recorded and concentrations were determined by micro-PIXE method for the major constituents of samples. Micro-SRXRF technique was used for the complementary measurement of medium and high atomic number elements, especially the siderophilic ones. Altogether, approximately 40 elements were analyzed. These results elucidate many steps of the formation mechanism of the various impact-metamorphosed objects.

Use of back-scattered electron imaging as a tool for examining matrix structure of calcium pectinate

The internal structure of pharmaceutical solid dosage forms is commonly revealed by secondary electron imaging using standard scanning electron microscopy (SEM) technique. In this work we propose a back-scattered electron imaging (BEI) as a new tool for examining the matrix structure of calcium pectinate beads. Imaging samples with back-scattered electrons in the SEM is based on material or atomic number contrast. High atomic number elements, such as calcium, reflect more electrons and appear bright on electron micrographs. The BEI–SEM images of calcium pectinate matrix beads clearly showed net-like structure of calcium pectinate and uniform distribution of drug particles. The matrix compositions were confirmed by energy dispersive analyzer. The result demonstrates the advantageous of BEI for examining the matrix structure of calcium pectinate.

K-shell ionization of high Z elements with low to intermediate velocity Si ions

We report here the measurement of the absolute cross-sections for the K-shell ionization of high atomic number elements (Au and Bi) induced by low to intermediate velocity Si ions. The relativistic wavefunction effects on the ionization cross-sections have been studied by comparing the measured data with different theoretical calculations with and without including the relativistic corrections. The relativistic effect on the K-ionization cross-section is found to depend on the projectile atomic number and velocity.