Table of contents

We examined a PET detector consisting of an LYSO array coupled to a 4 × 4 array of large-size Geiger-mode avalanche photodiode (GAPD). The GAPD coupled to 3 mm × 3 mm × 20 mm LYSO pixel crystal has been investigated for possible use as an MR-compatible PET photosensor. Primary characteristics of a PET detector, such as energy resolution and coincidence timing resolution were measured. Gain variation, count uniformity, and count estimation error of 4 × 4 array of LYSO-GAPD were measured to evaluate the performance parameters relevant for PET imaging. The energy resolution and coincidence timing resolution with 511 keV gamma rays were 18.5 ± 0.7% and 1.6 ns, respectively. The gain variation, count uniformity for all 16 channels were 1.3:1 and 1.3:1, respectively. The count estimation error between adjacent channels measured with an LYSO connected to a GAPD pixel was negligible (0.24 ± 0.04%). Long-term stability results show that there was no significant change in the photopeak position, energy resolution and count rate for 20 days. Cable lengths up to 300 cm, used between the GAPD and preamplifier, did not affect photopeak position and energy resolution. The performance of the LYSO-GAPD detector inside the MRI exhibited no significant change compared to that measured outside the MRI. The MR images acquired with and without the operating LYSO-GAPD detector located on top of the RF coil showed no considerable degradation in image quality. These results demonstrate the feasibility of using the LYSO-GAPD detector as PET photosensors, which could be used for MR compatible PET development.

Polycrystalline chemical vapour deposition (pCVD) diamond detectors are being considered for an upgrade of Forward Calorimetry in the ATLAS detector at the CERN LHC. In order to determine their radiation hardness, four detectors were continuously monitored during irradiation with 500 MeV protons at TRIUMF to a fluence of 2.25 x 10¹⁷ protons/cm². All four detectors worked at the end of the irradiation period. The detector response A as a function of fluence has been fitted to a sum of two exponentials and to a function A = a_o/(1+a₁ϕ) where a_o is the initial response and ϕ is the fluence with a₁ = 9.8 x 10⁻¹⁷ ± 3.2 x 10⁻¹⁸ cm²/proton. At the end of irradiation the response had reduced to five percent of the original value. The time resolution was found to be ∼ 250 picoseconds.

To model the size distribution and composition of interstellar and interplanetary dust grains, and their effect on a wide range of phenomena, it is vital to understand the mechanism of dust-shock interaction. We demonstrate a new laser experiment that subjects dust grains to pressure spikes similar to those of colliding astrophysical dust, and that accelerates the grains to astrophysical velocities. This new method generates much larger data sets than earlier methods; we show how large quantities (thousands) of grains are accelerated at once, rather than accelerating individual grains, as is the case of earlier methods using electric fields. We also measure the in-flight velocity ( ∼ 4.5km/s) of hundreds of grains simultaneously by use of a particle image velocimetry (PIV) technique.

We present a study of a CMOS test sensor which has been designed, fabricated and characterised to investigate the parameters required for a binary readout electromagnetic calorimeter. The sensors were fabricated with several enhancements in addition to standard CMOS processing. Detailed simulations and experimental results of the performance of the sensor are presented. The sensor and pixels are shown to behave in accordance with expectations and the processing enhancements are found to be essential to achieve the performance required.

Subnanosecond X-ray apparatus with a miniature radiator tube, which was connected to a source of high-voltage pulses through a commercial thin flexible radio-frequency coaxial cable (PK50, production of Russia), rated at 50 Ohm and having a length of 1 m, was created. Miniature explosive emission vacuum diodes were used as the picosecond X-ray emitter. The main advantages of the new subnanosecond X-ray apparatus with a flexible cable radiator probe, in comparison with a conventional one, include a small size and a lightweight (from 10 to 32 kg in our performance), as well as the possibility to bring the small-size X-ray tube by means of a long and thin flexible supply cable immediately to an object, which is hard to reach with available X-ray apparatus. The developed X-ray apparatus may find application in medicine for close-focus intracavitary roentgenotherapy, and also in industry for X-ray non-destructive inspection of small diameter lengthy intricate hollow parts.

This work aims to present the current state of simulations of electroluminescence (EL) produced in gas-based detectors with special interest for NEXT — Neutrino Experiment with a Xenon TPC. NEXT is a neutrinoless double beta decay experiment, thus needs outstanding energy resolution which can be achieved by using electroluminescence. The process of light production is reviewed and properties such as EL yield and associated fluctuations, excitation and electroluminescence efficiencies, and energy resolution, are calculated. An EL production region with a 5 mm width gap between two infinite parallel planes is considered, where a uniform electric field is produced. The pressure and temperature considered are 10 bar and 293 K, respectively. The results show that, even for low values of VUV photon detection efficiency, good energy resolution can be achieved: below 0.4 % (FWHM) at Q_ββ = 2.458 MeV.

We present GeFRO (Germanium FROntend) a novel approach to the readout of ionization detectors. The circuit allows to minimize the number of components and the space occupation close to the detector. This way a minimal impact is added on the radioactive background in those experiments where very low signal rates are expected, such as GERDA and MAJORANA. The circuit consists of a JFET transistor and a remote second stage. The DC feedback path is closed using a diode. Only two signal cables are necessary for biasing and readout.

Borexino, a liquid scintillator detector at LNGS, is designed for the detection of neutrinos and antineutrinos from the Sun, supernovae, nuclear reactors, and the Earth. The feeble nature of these signals requires a strong suppression of backgrounds below a few MeV. Very low intrinsic radiogenic contamination of all detector components needs to be accompanied by the efficient identification of muons and of muon-induced backgrounds. Muons produce unstable nuclei by spallation processes along their trajectory through the detector whose decays can mimic the expected signals; for isotopes with half-lives longer than a few seconds, the dead time induced by a muon-related veto becomes unacceptably long, unless its application can be restricted to a sub-volume along the muon track. Consequently, not only the identification of muons with very high efficiency but also a precise reconstruction of their tracks is of primary importance for the physics program of the experiment. The Borexino inner detector is surrounded by an outer water-Cherenkov detector that plays a fundamental role in accomplishing this task. The detector design principles and their implementation are described. The strategies adopted to identify muons are reviewed and their efficiency is evaluated. The overall muon veto efficiency is found to be 99.992 % or better. Ad-hoc track reconstruction algorithms developed are presented. Their performance is tested against muon events of known direction such as those from the CNGS neutrino beam, test tracks available from a dedicated External Muon Tracker and cosmic muons whose angular distribution reflects the local overburden profile. The achieved angular resolution is ∼ 3°-5° and the lateral resolution is ∼ 35-50 cm, depending on the impact parameter of the crossing muon. The methods implemented to efficiently tag cosmogenic neutrons are also presented.

Fast charge sensitive preamplifiers were built using commercial current feedback operational amplifiers for fast read out of charge pulses from a photomultiplier tube. Current feedback opamps prove to be particularly well suited for this application where the charge from the detector is large, of the order of one million electrons, and high timing resolution is required. A proper circuit arrangement allows very fast signals, with rise times down to one nanosecond, while keeping the amplifier stable. After a review of current feedback circuit topology and stability constraints, we provide a "recipe" to build stable and very fast charge sensitive preamplifiers from any current feedback opamp by adding just a few external components. The noise performance of the circuit topology has been evaluated and is reported in terms of equivalent noise charge.

Optical monitoring systems are necessary to manufacture multilayer thin-film optical filters with low tolerance on spectrum specification. Furthermore, to have better accuracy on the measurement of film thickness, direct monitoring is a must. Direct monitoring implies acquiring spectrum data from the optical component undergoing the film deposition itself, in real time. In making film depositions on surfaces of optical components, the high vacuum evaporator chamber is the most popular equipment. Inside the evaporator, at the top of the chamber, there is a metallic support with several holes where the optical components are assembled. This metallic support has rotary motion to promote film homogenization. To acquire a measurement of the spectrum of the film in deposition, it is necessary to pass a light beam through a glass witness undergoing the film deposition process, and collect a sample of the light beam using a spectrometer. As both the light beam and the light collector are stationary, a synchronization system is required to identify the moment at which the optical component passes through the light beam.

Three-dimensional (3D) silicon sensors offer potential advantages over standard planar sensors for radiation hardness in future high energy physics experiments and reduced charge-sharing for X-ray applications, but may introduce inefficiencies due to the columnar electrodes. These inefficiencies are probed by studying variations in response across a unit pixel cell in a 55μm pitch double-sided 3D pixel sensor bump bonded to TimePix and Medipix2 readout ASICs. Two complementary characterisation techniques are discussed: the first uses a custom built telescope and a 120GeV pion beam from the Super Proton Synchrotron (SPS) at CERN; the second employs a novel technique to illuminate the sensor with a micro-focused synchrotron X-ray beam at the Diamond Light Source, UK. For a pion beam incident perpendicular to the sensor plane an overall pixel efficiency of 93.0±0.5% is measured. After a 10^o rotation of the device the effect of the columnar region becomes negligible and the overall efficiency rises to 99.8±0.5%. The double-sided 3D sensor shows significantly reduced charge sharing to neighbouring pixels compared to the planar device. The charge sharing results obtained from the X-ray beam study of the 3D sensor are shown to agree with a simple simulation in which charge diffusion is neglected. The devices tested are found to be compatible with having a region in which no charge is collected centred on the electrode columns and of radius 7.6±0.6μm. Charge collection above and below the columnar electrodes in the double-sided 3D sensor is observed.

A new idea for an experiment to search for neutrinoless tau-decays is presented. The experiment uses the HERA proton beam to produce a large amount of tau-leptons in a beam-dump experiment. With an active target, a spectrometer, and a muon detector it is possible to search for the decay τ → μμμ.

A study of the electron motion in an rf electric field is performed by means of numerical simulations taking into account the spread in electron initial velocities and their angular distribution. It is shown that the absorption of the electrons inside the semi-spherical depressions which in our two-dimensional model are represented by semi-circles, results in a significant reduction of the secondary emission yield. We also find that secondary electron emission is largely sensitive to a rough measure of the fractional surface area of the depressions (in our model the ratio of the sum of the diameters of the semicircles to the length of waveguide). Although the growth of a multipactor is still possible for such a surface, we show that the threshold for the multipactor phenonmenon is higher than that for a flat surface. The surface we have considered in this paper may be considered as a simplified model of sandblasted surfaces, which have already been considered for multipactor reduction.

Silicon devices (both pixels and microstrips) have been widely used in the past years in High Energy Physics experiments and also in other applications involving the detection of ionizing radiation such as medical imaging and dosimetry. The simulation of the silicon devices response to ionizing radiation is an important step needed to understand the performances in terms of signal, noise, spatial and energy resolution as a function of several technology parameters like doping profile, geometrical dimensions, bias voltage. These simulations are routinely carried out using two separate approaches (and tools): radiation interaction with a geometrically segmented silicon material (GEANT4, FLUKA, PENELOPE) taking into account the physical processes and giving as output the deposited energy; transport of generated electron/hole pairs through the device with electronic signal formation (SENTAURUS-TCAD). In this work we propose a new combined approach using both methods, applied to the case of CMOS pixel sensor, to obtain a better understanding of the behavior of the devices.

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