Carlos Granja

Selective detection of secondary particles and neutrons produced in ion beam therapy with 3D sensitive voxel detector

Ion beam therapy is a rapidly developing method for treatment of certain types of cancer. A main advantage of ions is that they deposit most of the energy at the end of their range according to the Bragg curve. Unfortunately, the ion beam often generates a substantial amount of energetic secondary particles with not negligible range. Thus, a fraction of the dose is deposited by other than the primary ions outside of the planned volume. It is, therefore, important to estimate and experimentally verify the distributions of these secondary particles. It is particularly difficult to identify fast neutrons generated by ions in tissue. Fast neutrons are usually detected via their interaction (scattering) with hydrogen nuclei (proton). The proton recoiled by the scattered neutron is subsequently detected by a suitable sensor. The problem is that certain fraction of secondary particles consists of protons as well. Therefore, it is necessary to distinguish protons recoiled by neutrons from protons naturally present in the sample. In this work we present the experimental technique enabling the separation of fast neutrons from protons. The technique uses a 3D sensitive voxel detector composed of several layers of Timepix pixel detectors. These layers are interlaced with a hydrogen rich material (plastic) serving as a convertor of neutrons to recoiled protons. The device records the traces of all interacting radiation providing the time stamp and/or deposited energy for each single particle. A proton passing through the detector creates a trace in all layers, whereas a protons recoiled by neutron originates in the convertor inside of the structure creating a trace in the inner layers only. This way it is possible to distinguish the protons from neutrons with very high selectivity. The technique can be easily extended for detection of slow neutrons. An initial experimental study to register the outcoming neutron radiation was performed at the Heidelberg Ion Beam Therapy Center (HIT) in Germany using medical proton and carbon ion beams. This work is carried out in frame of the Medipix Collaboration.

Detection and track visualization of primary and secondary radiation in hadron therapy beams with the pixel detector Timepix

The interaction of proton and hadron beams of relativistic energy in matter is accompanied by the production of penetrating highly energetic secondary particles and nuclear reaction products which can affect the desired highly localized deposition of energy of the primary high LET particles for radiotherapy purposes. The observation of all ionizing particles arising from and accompanying the primary radiation as well as the direct measurement of particle energy loss, trajectory, local energy deposition and lateral straggling can be directly provided by the semiconductor pixel detector Timepix. This device operates as an active nuclear emulsion providing on-line visualization of particle traces. Results of a pilot experiment are presented with proton and carbon ion beams in the energy range 48–220 MeV/u and 88–430 MeV, respectively produced at the Heavy Ion Therapy HIT facility in Heidelberg.

Compact System for High Resolution X-ray Transmission Radiography, In-line Phase Enhanced Imaging and Micro CT of Biological Samples

Phase imaging visualizes phase shift of photons which passed the sample. Although phase sensitive X-ray imaging offers many advantages it is not routinely used in biological research due to demands of high intensity and highly coherent X-ray beam which is accessible mainly at synchrotron facilities. Phase sensitive imaging can be also carried out with microfocus X-ray tubes. However, the low beam intensity of such systems prolongs the exposure time to such an extent that common digital imaging detectors (CCD, Flat panels) are insufficient due to low efficiency, dark current and noise. This contribution presents a compact phase contrast enhanced imaging system based on a microfocus X-ray tube and the single photon counting pixel detector Medipix2. The spectral sensitivity of the detector together with the polychromatic nature of the beam allows distinguishing an absorption image from a phase image. Spatial resolution of the system can be on the sub micrometer level and measuring times less than a minute. Applications of the system for biological samples are presented. The simplicity of the system allows for routine laboratory work including dynamic in-vivo studies.

Measurement of secondary radiation during ion beam therapy with the pixel detector Timepix

In ion beam therapy the finite range of the ion beams in tissue and the presence of the Bragg-peak are exploited. Unpredictable changes in the patient`s condition can alter the range of the ion beam in the body. Therefore it is desired to verify the actual ion range during the treatment, preferably in a non-invasive way. Positron emission tomography (PET) has been used successfully to monitor the applied dose distributions. This method however suffers from limited applicability and low detection efficiency. In order to increase the detection efficiency and to decrease the uncertainties, in this study we investigate the possibility to measure secondary charged particles emerging from the patient during irradiation. An initial experimental study to register the particle radiation coming out of a patient phantom during the therapy was performed at the Heidelberg Ion Beam Therapy Center (HIT) in Germany. A static narrowly-focused beam of carbon ions was directed into a head phantom. The emerging secondary radiation was measured with the position-sensitive Timepix detector outside of the phantom. The detector, developed by the Medipix Collaboration, consists of a silicon sensor bump bonded to a pixelated readout chip (256 × 256 pixels with 55 μm pitch). Together with the USB-based readout interface, Timepix can operate as an active nuclear emulsion registering single particles online with 2D-track visualization. In this contribution we measured the signal behind the head phantom and investigated its dependence on the beam energy (corresponding to beam range in water 2–30 cm). Furthermore, the response was measured at four angles between 0 and 90 degrees. At all investigated energies some signal was registered. Its pattern corresponds to ions. Differences in the total amount of signal for different beam energies were observed. The time-structure of the signal is correlated with that of the incoming beam, showing that we register products of prompt processes. Such measurements are less likely to be influenced by biological washout processes than the signal registered by the PET technique, coming from decays of beam-induced radioactive nuclei. This work demonstrates that the Timepix detector is able to register ions emerging from the patient during the treatment by carbon ion beams. In future work it will be investigated which information about the incoming beam can be gained from the analysis of the measured data.

Imaging with secondary radiation in hadron therapy beams with the 3D sensitive voxel detector

In this work we present the technique enabling visualization of the field of scattered and secondary ions generated by primary beam. The technique uses a small 3D sensitive voxel detector composed of several layers of Timepix pixel detectors. The device is placed close to the irradiated object (outside of the primary ion beam) recording the traces of all radiation coming from the sample. Detector provides the timestamp and/or deposited energy for each single particle. The shapes of traces are very typical for different particles which allows for separation of ions from other background such as electrons and gamma photons. The 3D information form the voxel detector allows for reconstruction of direction of incoming radiation. Therefore it is possible to distinguish whether particle came with the beam or it was generated or scattered later. The back projection of reconstructed directions for all registered ions can be in suitable geometry used for generation of image of distribution of scattering or fragmentation in the volume of the irradiated object. The initial experimental study to register the out coming ion radiation from testing phantom was performed at the Heidelberg Ion Beam Therapy Center (HIT) in Germany using medical carbon ion beam.

3D measurement of the radiation distribution in a water phantom in a hadron therapy beam

Hadron therapy is a highly precise radio-therapeutic method with many advantages especially in cases when the tumour is close to sensitive organs where standard treatments cannot be used. For reliable treatment planning it is necessary to have calculation tools for maximization of the dose delivered to the targeted tissue and minimization of the dose outside of it. While the main physical processes in material irradiated by hadron beams are known, in reality the processes involved are complex so that analytical computations are impossible. Thus, the planning tools to incorporate simplified models and numerical approximations and an experimental method for high precision verification of the models within phantoms is desired. The development of sensitive, high resolution and online methods for measurement of the radiation environment inside of the irradiated object is the aim of this work. Such measurements are made possible by the resolving power of the state-of-the-art pixel detector Timepix. This quantum counting imaging device is able to record the characteristic shapes of the particle traces including their energies deposited in the detector. All these data recorded for each event allow to estimate the particle type, its energy and direction of flight. Event-by-event analysis is done using pattern recognition of the characteristic traces. The objective of the experiment is the detection and characterization of secondary radiation generated by the primary therapeutic beams in tissue equivalent material (water). Measurements were performed inside of a water phantom irradiated by a carbon beam at the Heidelberg Ion-Beam Therapy Center (HIT).

A comparative study of LaBr3(Ce3+) and CeBr3 based gamma-ray spectrometers for planetary remote sensing applications

The recent availability of large volume cerium bromide crystals raises the possibility of substantially improving gamma-ray spectrometer limiting flux sensitivities over current systems based on the lanthanum tri-halides, e.g., lanthanum bromide and lanthanum chloride, especially for remote sensing, low-level counting applications or any type of measurement characterized by poor signal to noise ratios. The Russian Space Research Institute has developed and manufactured a highly sensitive gamma-ray spectrometer for remote sensing observations of the planet Mercury from the Mercury Polar Orbiter (MPO), which forms part of ESAs BepiColombo mission. The Flight Model (FM) gamma-ray spectrometer is based on a 3-in. single crystal of LaBr3(Ce(3+)) produced in a separate crystal development programme specifically for this mission. During the spectrometers development, manufacturing, and qualification phases, large crystals of CeBr3 became available in a subsequent phase of the same crystal development programme. Consequently, the Flight Spare Model (FSM) gamma-ray spectrometer was retrofitted with a 3-in. CeBr3 crystal and qualified for space. Except for the crystals, the two systems are essentially identical. In this paper, we report on a comparative assessment of the two systems, in terms of their respective spectral properties, as well as their suitability for use in planetary mission with respect to radiation tolerance and their propensity for activation. We also contrast their performance with a Ge detector representative of that flown on MESSENGER and show that: (a) both LaBr3(Ce(3+)) and CeBr3 provide superior detection systems over HPGe in the context of minimally resourced spacecraft and (b) CeBr3 is a more attractive system than LaBr3(Ce(3+)) in terms of sensitivities at lower gamma fluxes. Based on the tests, the FM has now been replaced by the FSM on the BepiColombo spacecraft. Thus, CeBr3 now forms the central gamma-ray detection element on the MPO spacecraft.

Distortion of the per-pixel signal in the Timepix detector observed in high energy carbon ion beams

Within the application of the pixelated semiconductor Timepix detector for ion beam therapy purposes, distortion and non-linearity in the spectrometric pixel response to high energy carbon ions were observed. In this contribution, these effects are studied in detail. A distinct correlation between the arrival time of a particle during the exposure time and the respective detector signal was found. The hypothesis to explain these findings by oscillations in the pixel electronics leading to a second rise of the preamplifier output above threshold is discussed. Depending on the particle arrival time, the distortions can result in an artificially increased counter value and consequently an enlarged detector signal in energy mode. The effect appears when the signal per-pixel is above approximately 1 MeV, therefore becomig especially significant for measurements with heavy ions.

Spatially correlated and coincidence detection of fission fragments with the pixel detector Timepix

Charged-particle coincidence correlated measurements such as angular correlations between rare and main fission fragments measured with conventional detectors provide only partial and limited information (energy cutoff, narrow range of studied ion Z numbers). Many of these drawbacks arise from the standard solid state detectors used so far which can be solved simultaneously by usage of highly segmented single-quantum counting pixel detectors. The Timepix pixel device, which is equipped with energy and time sensitivity capability per pixel, provides high granularity, wide dynamic range and per pixel threshold. This detector operated with integrated USB-readout interfaces such as the USB 1.0 and FITPix devices and the data acquisition software tool Pixelman, both developed for the pixel detectors of the Medipix-family, enables a variety of instrumental configurations, visualization, real-time event-by-event selection as well as vacuum and portability of operation for flexible measurements on different targets and setups. These features combined with event track analysis provide enhanced signal to noise ratio with a high suppression of background and unwanted events. The detector provides multi-parameter information (position, energy and time) for basically all types of ionizing particles in a wide dynamic range of energy (pixel energy threshold ≈ 4 keV), interaction/arrival time (timepix clock step ≥ 100 ns) and position (pixel size = 55 μm). High selectivity is achieved by spatial and time correlation in the same sensor. In addition, several detectors can be run in coincidence. The open and close exposition (shutter) time as well as the readout DAQ can be fully synchronized. For this purpose, we have assembled a modular multi-parameter, tunable and extendable coincidence detector array system based on two and more Timepix devices which can be coupled with supplementary detectors (solid state ΔE detectors and/or ionization chambers) for enhanced ion selectivity. We describe the individual configurations and techniques together with the experiments carried out at several neutron beam/source facilities. We summarize the results and capabilities of application.

Hybrid detectors of neutrons based on 3D silicon sensors with PolySiloxane converter

We report on the first prototypes of hybrid detectors for neutrons from the INFN HYDE project. Devices consist of 3D silicon sensors coupled to PolySiloxane-based converters. The sensor design and fabrication technology are presented, along with initial results from the functional characterization of the devices in response to radioactive sources and neutron beams of different energies.