Recent results on the properties of two-phase argon avalanche detectors

The characteristic properties of two-phase Ar avalanche detectors, including those obtained with CsI photocathode, are further studied. Such detectors are relevant in the field of coherent neutrino-nucleus scattering and dark matter search experiments. The detectors investigated comprised a 1 cm thick liquid Ar layer followed by a triple-GEM multiplier. In these detectors, typical gains reaching 10000 were obtained with good reproducibility and a stable operation for at least one day was demonstrated. Amplitude and pulse-shape characteristics are presented under irradiation with X-rays, gamma-rays and neutrons from different radioactive sources. The detection of both primary scintillation and ionization signals at higher gains, at a deposited energy of 60 keV, has been demonstrated.Comment: 6 pages, 11 figures. Presented at Xth Int. Conf. for Collid. Beam Phys., Feb 28 - March 6, 2008, Novosibirsk, to be published in Nucl. Instr. Meth.

Infrared scintillation yield in gaseous and liquid argon

The study of primary and secondary scintillations in noble gases and liquids is of paramount importance to rare-event experiments using noble gas media. In the present work, the scintillation yield in gaseous and liquid Ar has for the first time been measured in the near infrared (NIR) and visible region, both for primary and secondary (proportional) scintillations, using Geiger-mode avalanche photodiodes (G-APDs) and pulsed X-ray irradiation. The primary scintillation yield of the fast component was measured to be 17000 photon/MeV in gaseous Ar in the NIR, in the range of 690-1000 nm, and 510 photon/MeV in liquid Ar, in the range of 400-1000 nm. Proportional NIR scintillations (electroluminescence) in gaseous Ar have been also observed; their amplification parameter at 163 K was measured to be 13 photons per drifting electron per kV. Possible applications of NIR scintillations in high energy physics experiments are discussed.Comment: 6 pages, 5 figures. Submitted to Europhysics Letter. Revised Figs. 3 and

Minimizing memory effects in OFDM transmitters using adaptive baseband equalization

This paper presents a simple and effective approach for eliminating memory effects in OFDM transmitters. It uses advantages of OFDM systems to provide pre-compensation of the frequency-dependent distortions, which are results of the power amplifiers (PA) memory effects. The process of memory effects quantification is carried out in this paper by obtaining a frequency-dependent PA gain, phase shift and intermodulation products. The memory effects are eliminated at baseband using equalization of the IDFT signal. Implementation of the equalization procedure at baseband makes the process of minimizing memory effects simple and effective, because no additional RF components or feedback loops are used. Memory effects are compensated in DSP part using simple multiplication of the frequency-domain digital signal by coefficients, which are calculated adaptively for each OFDM sub-carrier frequency and input power. The approach is tested with Motorola MOSFET MRF9742 power amplifier model in Advanced Design System (ADS). Simulations show significant improvement in minimizing memory effects. Received constellation of the 16-QAM OFDM signal after implementing baseband pre-compensation technique looks alike ideal one, whereas without pre-compensation it shows high dispersion due to the presence of PA memory

Linearization of power amplifiers by baseband digital predistortion for OFDM transmitters

For the signals with high peak-to-average ratio (PAR), such as WiMax OFDM, power amplifiers need to operate in a compression mode. It results in a non-linear distortion of the output signal. To compensate for this distortion, linearizers are used. However, they decrease efficiency of amplifiers. This paper proposes to use a digital predistortion of baseband signals, which is characterized by a small impact on efficiency and a good linearizing performance, for linearization of OFDM transmitters. The paper describes design of a baseband predistorter and verifies its performances by WiMax OFDM simulations in ADS. Proposed predistorter brings significant improvements in AM/AM characteristic, eliminates spectrum re-growth, decreases adjacent channel power ratio (ACPR) and error vector magnitude (EVM)

A new approach for non-linear analysis of power amplifiers

This paper presents a new approach for non-linear analysis of power amplifiers based on the n-order polynomial modelling. A general expression for the fundamental frequency output is developed in this paper. It allows considering any order of polynomial model and obtaining analytical expression for the fundamental-frequency output without complex computations, using developed formulas. This model significantly simplifies non-linear analysis of power amplifiers and the process of designing linearizers. This paper also presents a simple approach for obtaining polynomial model from a frequency response of power amplifier. The proposed method is verified by experiments. Obtained polynomial model shows simulation results similar to the measurements for real amplifier. Developed n-order output modelling and proposed model extraction method can significantly simplify, optimize and enhance power amplifiers’ non-linear analysis and behavioural modelling. Proposed modelling approach can be also used to design linearizers

On the low-temperature performances of THGEM and THGEM/G-APD multipliers in gaseous and two-phase Xe

The performances of THGEM multipliers in two-phase Xe avalanche mode are presented for the first time. Additional results on THGEM operation in gaseous Xe at cryogenic temperatures are provided. Stable operation of a double-THGEM multiplier was demonstrated in two-phase Xe with gains reaching 600. These are compared to existing data, summarized here for two-phase Ar, Kr and Xe avalanche detectors incorporating GEM and THGEM multipliers. The optical readout of THGEMs with Geiger-mode Avalanche Photodiodes (G-APDs) has been investigated in gaseous Xe at cryogenic temperature; avalanche scintillations were recorded in the Near Infrared (NIR) at wavelengths of up to 950 nm. At avalanche charge gain of 350, the double-THGEM/G-APD multiplier yielded 0.07 photoelectrons per initial ionization electron, corresponding to an avalanche scintillation yield of 0.7 NIR photons per avalanche electron over 4pi. The results are compared with those of two-phase Ar avalanche detectors. The advantages, limitations and possible applications are discussed.Comment: 22 pages, 14 figures. Revised Figs. 10,11 and Table 1. To be published in JINS

The teaching profession in Europe : historical and sociological analysis

A possible effect of direct CP violation in D -> K-S(0)pi(+)pi(-) decay on the gamma measurement from B-+/- -> DK +/-, D -> K-S(0)pi(+)pi(-) Dalitz plot analysis is considered. Systematic uncertainty of gamma coming from the current limits on direct CP violation in D -> K-S(0)pi(+)pi(-) is estimated, and a modified model-independent procedure of B-+/- -> DK +/-, D -> K-S(0)pi(+)pi(-) Dalitz plot analysis is proposed that gives an unbiased gamma measurement even in presence of direct CP violation in charm decays. The technique is applicable to other threebody D decays such as D-0 -> (KSK+K-)-K-0, D-0 -> pi(+)pi(-)pi(0), etc

GEM operation in helium and neon at low temperatures

We study the performance of Gas Electron Multipliers (GEMs) in gaseous He, Ne and Ne+H2 at temperatures in the range of 2.6-293 K. In He, at temperatures between 62 and 293 K, the triple-GEM structures often operate at rather high gains, exceeding 1000. There is an indication that this high gain is achieved by Penning effect in the gas impurities released by outgassing. At lower temperatures the gain-voltage characteristics are significantly modified probably due to the freeze-out of impurities. In particular, the double-GEM and single-GEM structures can operate down to 2.6 K at gains reaching only several tens at a gas density of about 0.5 g/l; at higher densities the maximum gain drops further. In Ne, the maximum gain also drops at cryogenic temperatures. The gain drop in Ne at low temperatures can be reestablished in Penning mixtures of Ne+H2: very high gains, exceeding 10000, have been obtained in these mixtures at 50-60 K, at a density of 9.2 g/l corresponding to that of saturated Ne vapor near 27 K. The results obtained are relevant in the fields of two-phase He and Ne detectors for solar neutrino detection and electron avalanching at low temperatures.Comment: 13 pages, 14 figures. Accepted for publishing in Nucl. Instr. and Meth.

Advanced digital predistortion of power amplifiers for mobile and wireless communications

This research work focuses on improving the performances of digital predistorters while maintaining low computational complexity for mobile and wireless communication systems. Initially, the thesis presents the fundamental theory of power amplifiers, overview of existing linearisation and memory-effects compensation techniques and reveals the current issues in the field. Further, the thesis depicts the proposed solutions to the problems, including the developed in-band distortion modelling technique, model extraction methods, memoryless digital predistortion technique based on distortion components iterative injection, baseband equalisation technique for minimising memory effects, Matlab-ADS co-simulation system and adaptation circuit with an offline training scheme. The thesis presents the following contributions of the research work. A generalized in-band distortion modelling technique for predicting the nonlinear behaviour of power amplifiers is developed and verified experimentally. Analytical formulae are derived for calculating predistorter parameters. Two model extraction techniques based on the least-squares regression method and frequency-response analysis are developed and verified experimentally. The area of implementation and the trade-off between the methods are discussed. Adjustable memoryless digital predistortion technique based on the distortion components iterative injection method is proposed in order to overcome the distortion compensation limit peculiar to the conventional injection techniques. A baseband equalisation method is developed in order to provide compensation of memory effects for increasing the linearising performance of the proposed predistorter. A combined Matlab-ADS co-simulation system is designed for providing powerful simulation tools. An adaptation circuit is developed for the proposed predistorter for enabling its adaptation to environmental conditions. The feasibility, performances and computational complexity of the proposed digital predistortion are examined by simulations and experimentally. The proposed method is tuneable for achieving the best ratio of linearisation degree to computational complexity for any particular application