Gas pixel detectors for X-ray polarimetry applications

We discuss a new class of micro pattern gas detectors, the gas pixel detector (GPD), in which a complete integration between the gas amplification structure and the read-out electronics has been reached. An application-specific integrated circuit (ASIC) built in deep sub-micron technology has been developed to realize a monolithic device that is, at the same time, the pixelized charge collecting electrode and the amplifying, shaping and charge measuring front-end electronics. The CMOS chip has the top metal layer patterned in a matrix of 80 μm pitch hexagonal pixels, each of them directly connected to the underneath electronics chain which has been realized in the remaining five layers of the 0.35 μm VLSI technology. Results from tests of a first prototype of such detector with 2 k pixels and a full scale version with 22 k pixels are presented. The application of this device for Astronomical X-ray Polarimetry is discussed. The experimental detector response to polarized and unpolarized X-ray radiation is shown. Results from a full MonteCarlo simulation for two astronomical sources, the Crab Nebula and the Hercules X1, are also reported.     

Development of a gamma compensated boron lined ionisation chamber for reactor safety and control applications

Boron lined ionisation chambers with an overall diameter of 85 mm and maximum length of 165 mm have been developed and tested. The chamber consists of 34 numbers of parallel plate aluminium electrodes spaced at a distance of 2 mm and mounted on SS rods and radiation resistant polyetheretherketone (PEEK) spacers. One surface of the signal electrode and both the surfaces of the +HT electrodes are dip coated with boron. It is filled with nitrogen gas at a pressure of 128 cm of Hg. Tests at the ⁶⁰Co source facility at gamma fields ranging from 200 R/h to 830 kR/h showed that the chamber required 500 V to obtain 90% of the saturation current at 830 kR/h. The gamma compensation factor was measured as 0.12–7% for various gamma fields for polarising voltages of +400 and −350 V. Neutron measurements at the Apsara Thermal Column showed that the linearity of the chamber response as a function of reactor power was within 2%. The neutron sensitivity was measured as 3.9 fA/nv.     

Electrical properties of Ta2O5 thin films deposited on Cu

The electrical and dielectric properties of reactively sputtered Ta₂O₅ thin films with Cu as the top and bottom electrodes forming a simple metal insulator metal (MIM) structure, Cu/Ta₂O₅/Cu/n-Si, were studied. Ta₂O₅ films subjected to rapid thermal annealing (RTA) at 800°C for 30 s in N₂ ambient crystallized the film, decreased the leakage current density and resulted in reliable time-dependent dielectric breakdown characteristics. The conduction mechanism at low electric fields (<100 kV/cm) is due to Ohmic conduction; however, the Schottky mechanism becomes predominant at high fields (>100 kV/cm). Present studies demonstrate the use of Cu as a potential electrode material to replace the conventional precious metal electrodes for Ta₂O₅ storage capacitors.     

A measuring system with a recombination chamber for neutron dosimetry around medical accelerators

A measuring system for dosimetry of neutrons generated around medical electron accelerators is proposed. The system consists of an in-phantom tissue-equivalent recombination chamber and associated electronics for automated control and data acquisition. A second ionization chamber serves as a monitor of photon radiation. Two quantities are determined by the recombination chamber--the total absorbed dose and the recombination index of radiation quality. The ambient dose equivalent, H*(10), or neutron absorbed dose in an appropriate phantom, can be then derived from the measured values. Tests of the system showed that a 0.5% dose contribution of neutrons to the absorbed dose of photons could be detected and estimated under laboratory conditions. Preliminary tests at the 15 MV Varian Clinac 2300C/D medical accelerator confirmed that the measuring system could be used under clinical conditions. The H*(10) of the mixed radiation was determined with an accuracy of approximately 10%.     

STJ X-ray detectors with titanium sublayer

Superconducting tunnel junctions (STJs) with the structure Ti/Nb/Al, AlO_x/Al/Nb/NbN and corresponding layer thickness 30/100/8/13/150/30 nm were investigated as X-ray detectors at T=1.35 K. STJs with one active electrode in which the response of the other one is suppressed due to trapping layer on the surface opposite to the tunnel barrier have a number of potential advantages. The best line width (FWHM) is 78 eV for 6400 μm² junction. Contribution of the electronic noise is about 50 eV. The intrinsic detector line width is less than 60 eV. The collected charge from inactive electrode is more than 8 times less than that from the active one. Titanium proved to be an appropriate material for a sublayer and a trap.     

Space charge distributions of an electric double layer capacitor with carbon nanotubes electrode

Space charge distributions of an electric double layer capacitors (EDLCs) based on polarizable nanoporous electrodes, containing carbon nanotubes (CNTs) as electrode material, were investigated by a pulsed electro acoustic (PEA) method. The EDLCs samples were prepared using CNTs and carbon black (i.e. acetylene and ketjen black) as electrode materials with different types pore structures. The charge distributions of positive/negative ions were spatially uneven and charge accumulation region concentrated on central part of electrode. The polarizable electrodes with ketjen black and CNTs-5 wt.% had higher space charge density. From the results of discharge characteristics, it is clear that EDLCs based on the ketjen black/CNTs-3 wt.% have better capacitive behavior. The specific capacitance of about 14 F/g of EDLCs using the polarizable electrodes with ketjen black and CNTs-3 wt.% were obtained. It can be found that CNTs plays an essential role in the improvement of the charge density and the electrostatic capacity in EDLCs. The use of PEA method allowed us to perform the direct observations of spatio-temporal charge distributions in EDLCs based on CNTs.     

Optimum thickness of carbon stripper foils in tandem accelerator in view of transmission and lifetime

Optimum thickness of charge stripper foils installed at the terminal of a tandem accelerator has been investigated from the view of (1) charge stripping effect, (2) transmission of ions through accelerator, (3) lifetime of foils for the irradiation of ions. For this purpose, measurements have been done for (a) transmission of H, Li, O, Br and Au ions, passing through 12 UD Pelletron tandem accelerator for carbon stripper foils of 1.8–19.5 μg/cm² thickness, at terminal voltages of 5 and 10 MV, and (b) lifetime of 2–15 μg/cm² thick Tanashi foils developed by Sugai by irradiating Au ions at the terminal voltage of 10 MV. The results obtained are as follows: (a) From the view of above items (1) and (2), the optimum thickness of foils is 10 μg/cm² for ions of Z=1, several μg/cm² for Z=8, and less than a few μg/cm² for heavier ions. (b) From the view of item (3), the lifetime of Tanashi foils by means of new arc-discharge method is demonstrated to be much longer than that of commercial foils for foils thicker than about 5 μg/cm² thick. This superiority rapidly decreases with decreasing foil thickness, and at around 2 μg/cm², the lifetime of Tanashi foils is at the most 2.4 times longer than that of commercial foils.     

Properties of piezoelectric actuator on silicon membrane, prepared by screen printing method

Characteristics of piezoelectric actuator on Si membrane were investigated. Si membranes were fabricated as a function of size using bulk micromachining method. Bottom electrode Ag–Pd and piezoelectric thick films were fabricated using screen printing method, respectively. Piezoelectric thick films were sintered by rapid thermal annealing (RTA). Top electrodes Pt were deposited by DC sputtering system. We analyzed micro structure by scanning electron microscope (SEM) and investigated dynamic properties by MTI2000. Therefore, piezoelectric thick film on Si membrane had P_r of 15.7 μC cm⁻². The maximum displacement of micro actuator had 0.05 μm. We find the combination of thick film printing and MEMS process to form a Si membrane micro actuator.     

Improved fatigue properties of lead zirconate titanate films made on oxygen-implanted platinum electrodes

Pb(Zr_0.3Ti_0.7)O₃ (PZT) thin film capacitors fabricated on an oxygen-implanted Pt bottom electrode were studied. Oxygen was implanted at a low acceleration voltage (40 kV) and dose (1×10¹⁵ cm⁻²). Structural examination by grazing-incident X-ray diffraction (GIXD) and chemical analysis by X-ray photoelectron spectroscopy (XPS) revealed that the implantation generated a very thin amorphous top surface layer (approx. 20 nm), which contained approximately 7% of oxygen that stayed in the film in the form of PtO bonding. The amorphous layer, however, resumed the crystalline structure accompanied by the dissociation of PtO under the rapid thermal annealing at 600 °C for 5 min. The remnant polarization of sol–gel derived Pb(Zr_0.3Ti_0.7)O₃ (PZT) films fabricated on the oxygen-implanted Pt was slightly reduced from 11.92 μC/cm² for the PZT capacitors fabricated on a Pt electrode without implanted oxygen to 9.07 μC/cm². Nevertheless, the fatigue endurance was significantly increased. The switching polarization of PtO_x/PZT/Pt (O-implanted) capacitors remained within 95% of the starting value after 4×10¹⁰ switching cycles, which is comparable to that of PZT capacitors made with other conducting oxides.     

Novel electrochromic devices based on complementary nanocrystalline TiO2 and WO3 thin films

Electrochromic devices were elaborated based on two complementary electrodes made of a nanocrystalline metal oxide thin film deposited on conducting glass. The first electrode holds a 5 μm thick nanocrystalline TiO₂ film derivatized by a monolayer of a phosphonated triarylamine which can be rapidly oxidized by electron transfer to the conducting support followed by charge percolation inside the monolayer. The oxidation in accompanied by a blue coloration due to the absorption band at 730 nm of the stable triarylamminum radical cation. The second electrode bears a 0.2 μm thick nanocrystalline WO₃ film which turns from colorless to blue by reduction and lithium ion insertion. The former electrode reaches an absorbance of at least 3 between 700 and 730 nm after full oxidation (16 mC/cm²) at 1.0 V vs. NHE while for the second, complete reduction at −1.3 V (74 mC/cm²) leads to A=2.4 at 774 nm. An electrochromic device comprising both electrodes separated by an electrolytic solution of 0.1 Li⁺ in 4,7-dioxaoctanitrile reaches an absorbance of 2.2 at 700 nm, 4 s after a voltage step to 1.5 V. The system was shown to sustain at least 14400 coloration-discoloration cycles without degradation.     

Ion-implanted capacitively coupled silicon strip detectors with integrated polysilicon bias resistors processed on a 100 mm wafer

Double-sided silicon strip detectors with integrated coupling capacitors and polysilicon resistors have been processed on a 100 mm wafer. A detector with an active area of 19 × 19 mm² was connected to LSI readout electronics and tested. The strip pitch of the detector is 25 μm on the p-side and 50 μm on the n-side. The readout pitch is 50 μm on both sides. The number of readout strips is 774 and the total number of strips is 1161. On the p-side a signal-to-noise of 35 has been measured using a ⁹⁰Sr β-source. The n-side has been studied using a laser.     

Development of a microcalorimeter for measuring absolute intensity of synchrotron radiation

A total absorption calorimeter was developed to measure the absolute intensity of monoenergetic X-ray beams from 10 keV to 70 keV of synchrotron radiation. Experiments with synchrotron radiation have demonstrated that the heat power above about 1 μW due to monoenergetic synchrotron radiation is measured with an accuracy of about 1%.     

Cryogenic and elevated temperature hypervelocity impact facility

The hypervelocity impact facility at Space Research Institute (SRI), Auburn University has recently completed a facility upgrade that permits the impact testing of space materials within the cryogenic and elevated temperature range. Sample temperatures within the range of 40–450 K have been achieved for polymer films. These wide temperature range capabilities add to the facilities current testing experience with impact initiated plasma discharge testing for solar cell arrays. The facility utilizes a plasma drag gun to accelerate a variety simulated micrometeorite materials in the 50–150 μm range to velocities between 5 and 12 km/s. For each test 5–50 particles impact the surface of the target sample within an impact area of approximately 15 cm in diameter. The test chamber can accommodate samples up to a meter wide for ambient and heated tests, and 48 cm for cryogenic samples. The gun and test chambers are evacuated by He cryopumps and dry roughing pumps to produce a clean, oil free environment. Utilizing a streak camera and PMT detection system, the correspondence between individual particle size, speed and impact site can be determined. Standard post-analysis yields: micrographs of each impact site, dimensions of the pertinent impact characteristics, individual particle velocity and size estimates.     

Study of the ratio of non-neutron to neutron dose components of cosmic radiation at typical commercial flight altitudes

CIEMAT, in close co-operation with Iberia Airlines, carried out an extensive programme of in-flight measurements, covering both hemispheres, during the years 2001 and 2002. Although the instrumentation onboard included different active devices, the results presented here were obtained from a polyethylene/tungsten-moderated rem meter (SWENDI2; Eberline) and an ionisation chamber (RSS-131; Reuter-Stokes) used for measuring the ambient dose equivalent due to the neutron and the non-neutron components of cosmic radiation, respectively. This paper presents a study of each of the dose components mentioned as a function of the vertical cut-off rigidity and the flight altitude. The ratio between the two components is also presented to determine the variations in cosmic radiation composition as a function of the aforementioned parameters. The experimental results have also been compared with those predicted by the code EPCARD3.2 for the non-neutron and the neutron components of the ambient dose equivalent.     

Effect of radiation trapping on the emission properties of Er: I13/2 → I15/2 transition in oxide glasses

The effect of radiation trapping on the emission properties of Er³⁺-doped tellurite and phosphate glasses has been investigated as the function of sample thickness and doping concentration. It was found that radiation trapping exists generally in two glass matrices, even at low doping concentration (0.1 mol% Er₂O₃). The larger effect of radiation trapping in tellurite glasses compared with phosphate glasses is due to its larger emission cross-section at 1.5 μm band and the spectral overlap between the emission and absorption spectra of Er³⁺: ⁴I_13/2 ↔ ⁴I_15/2transition. Due to radiation trapping, the measured lifetime of the Er³⁺: ⁴I_13/2 level in tellurite glasses increases by about 11–37% with increasing the sample thickness at the different erbium doping concentration, while 6–17% for phosphate glasses. And the full-width at half maximum of fluorescence (FWHM) of Er³⁺: ⁴I_13/2 → ⁴I_15/2 transition in tellurite glasses increased by about 15–64% with increasing the sample thickness, while 11–55% for phosphate glasses. It caused a high overestimation on the figure of merits (FOM) for amplifier bandwidth (σ_e × FWHM).     

Comparison of sensing characteristics of ChemFEC devices based on spin-coated thin films of calix[n]arenes

ChemFECs (chemical field effect capacitors) or EIS (Electrolyte-Insulator-Semiconductor) devices were coated with calix[n]arene macrocycles for cation sensors development. Silicon nitride surface electrodes were spin-coated with p-tert-butylcalix[n]arene molecules for n = 8, 9 and 11. Electrochemical capacitance measurements were carried out to evaluate the sensitivity of the calix[n]arene layers towards specific cations as calcium and copper ions. Sensor responses were due to the “host-guest” specific interaction between the rigid cavity of calix[n]arene receptors (ligand) and chemical ion species. The developed chemical sensors have demonstrated good analytical characteristic in terms of: stability, selectivity and lifetime. In addition, experimental results were fitted using the site binding model in order to determine complex association constants, pK (1.9 ≤ pK ≤ 6.39) and surface density of ligands, N_L (9.10¹⁴ ≤ N_L ≤ 2.10¹⁶). All results obtained in this work were compared with those obtained using silicon dioxide EIS electrodes coated with the same calix[n]arene receptors using both spin-coating and thermal evaporation deposit processes.     

Tunable Molecular Electrodes for Bistable Polarization Screening

The polar discontinuity at any ferroelectric surface creates a depolarizing field that must be screened for the polarization to be stable. In capacitors, screening is done by the electrodes, while in bare ferroelectric surfaces it is typically accomplished by atmospheric adsorbates. Although chemisorbed species can have even better screening efficiency than conventional electrodes, they are subject to unpredictable environmental fluctuations and, moreover, dominant charged species favor one polarity over the opposite. This paper proposes a new screening concept, namely surface functionalization with resonance-hybrid molecules, which combines the predictability and bipolarity of conventional electrodes with the screening efficiency of adsorbates. Thin films of barium titanate (BaTiO₃) coated with resonant para-aminobenzoic acid (pABA) display increased coercivity for both signs of ferroelectric polarization irrespective of the molecular layer thickness, thanks to the ability of these molecules to swap between different electronic configurations and adapt their surface charge density to the screening needs of the ferroelectric underneath. Because electron delocalization is only in the vertical direction, unlike conventional metals, chemical electrodes allow writing localized domains of different polarity underneath the same electrode. In addition, hybrid capacitors composed of graphene/pABA/ferroelectric have been made with enhanced coercivity compared to pure graphene-electode capacitors.     

Synthesis of fly ash particle reinforced A356 Al composites and their characterization

A356 Al–fly ash particle composites were fabricated using stir-cast technique and hot extrusion. Composites containing 6 and 12 vol.% fly ash particles were processed. Narrow size range (53–106 μm) and wide size range (0.5–400 μm) fly ash particles were used. Hardness, tensile strength, compressive strength and damping characteristics of the unreinforced alloy and composites have been measured. Bulk hardness, matrix microhardness, 0.2% proof stress of A356 Al–fly ash composites are higher compared to that of the unreinforced alloy. Additions of fly ash lead to increase in hardness, elastic modulus and 0.2% proof stress. Composites reinforced with narrow size range fly ash particle exhibit superior mechanical properties compared to composites with wide size range particles. A356 Al–fly ash MMCs were found to exhibit improved damping capacity when compared to unreinforced alloy at ambient temperature.     

Radiation and cryogenic test results with a monolithic GaAs preamplifier in C-HFET technology

We report test results with a monolithic GaAs preamplifier fabricated in industrial C-HFET technology irradiated with a total dose of 10¹⁴ neutrons/cm² and 100 Mrad γ radiation and operated under cryogenic conditions. The measured gate current of the input transistor of a few nA increases by < 10% after irradiation. For a 330 μm input FET width, the equivalent noise charge (ENC) is typically 145 electrons per pF total input capacitance at a shaping time of 25 ns (bipolar) before irradiation and changes approximately by 10% after irradiation. Under cryogenic operation the corresponding input referred noise decreases by roughly a factor of two.