Academic literature on the topic 'Gaseous photomultiplier'

A portable instrumental system was designed for the routine environmental monitoring of toxic volatile organic compounds (VOCs) in atmospheric conditions based on changes in the photoluminescence emission of semiconductor nanoparticles (quantum dots) entrapped in a sol-gel matrix as the solid sensing material. The sol-gel sensing material displayed a long-lived phosphorescent emission, which is quenched in the presence of trace levels of a volatile organic compound (acetone) in gaseous atmospheres. The developed instrument could measure and process the changes in the photoluminescence of the sensing material after exposure to gaseous acetone. The developed prototype device consists of a deep-ultraviolet ligtht-emitting diode (UV LED), which excites the chemical sensing material; an optical filter to remove scattered light and other non-desirable wavelengths; a photomultiplier tube (PMT) to convert the phosphorescence emission of the sensor phase to an electrical signal; and a microcontroller to correlate the signal with the analyte concentration. The developed prototype was evaluated for its ability to measure low levels of gaseous acetone in contaminated atmospheres with high sensitivity (detection limit: 9 ppm). The obtained results show the feasibility of this type of instrument for environmental analytical control purposes.

Mestrado em Engenharia Física
A new position sensitive gas photomultiplier for the Vacuum Ultraviolet (VUV) region is presented in this work. The detector is composed by two THGEMs, followed by a 2D-THCOBRA being operated in Ne/CH4(5%), at 1 bar pressure in single photon mode. The 2D-THCOBRA is an hybrid microstructure which combines the robustness and the resistance to discharges of a THGEM with the two independent charge multiplication stages and the position discrimination of the 2D-MHSP. In this work the 2D-THCOBRA influence in the charge gain and IBF values was studied. The position resolution of the entire system was also studied. The achieved results shown a charge gain of 106 and, for this gain values, an IBF value of about 20%. Position resolutions below 300 μm were also obtained.
O presente trabalho baseia-se no desenvolvimento e estudo de um fotomultiplicador gasoso na região do Utra-Violeta de Vazio (UVV) e com capacidade de discriminação de posição. O detector é constituído por duas THGEM seguidas de uma 2D-THCOBRA, a operar em Ne/CH4(5%) à pressão de 1 bar e em modo de fotão único. A 2D-THCOBRA é uma estrutura híbrida, que resulta da combinação entre uma THGEM e uma 2D-MHSP, beneficiando da robustez e resistência às descargas da primeira e dos dois estágios de multiplicação e da capacidade de discriminação da posição da 2D-MHSP. Neste trabalho foi estudada a influência dos potenciais aplicados aos eléctrodos da 2D-THCOBRA no ganho e no refluxo de iões (IBF) do detector. Foi ainda avaliada a resolução em posição deste detector. Foram medidos ganhos da ordem de 106 e, para estes valores de ganho, IBF na ordem dos 20%. Obteve-se ainda resoluções em posição inferiores a 300 μm.

Large area photodetectors with single photon counting ability motivates to develop gaseous photomultiplier (GPM), that combine thin film photocathode with electron multipliers for efficient detection of UV photon. This involves extensive research on photocathode preparation, characterization and photoemission properties of UV photocathodes; detector’s operating parameters which influence avalanche process, electron and ion transport properties and hence performance of the detector. Thick Gas Electron Multiplier (THGEM) based photomultipliers offer high gain, fast response, high rate capability and affordable costs with large detection area and efficient detection of light at single photon level and is therefore motivation for studying such detector and related technologies. In this thesis, a detailed study that concerns the development and performance of UV photon detector, realized by coupling THGEM with semi-transparent CsI photocathode is presented. The initial part of the thesis contains simulation studies carried out to understand the effect of gas properties and geometrical parameters of THGEM on detector’s performance. The simulation tools, ANSYS and Garfield++ allow a detailed calculation of electron avalanche developed across the THGEM hole. The CsI photocathode, used to convert UV photons into electrons is prepared by thermal evaporation technique and characterized to analyze its morphological, structural and optical properties. The challenges such as humid air exposure and damages due to electron bombardment faced during characterization are discussed here. The effect of substrate heating, during CsI film deposition, in enhancing the efficiency and increased surface area coverage, and their optical and photoemission properties are discussed in the work presented here. An improvement of 5%-20% in surface area coverage is observed with the heat treatment during deposition. A major portion of the thesis focuses on the single electron spectra obtained from the detector realized by coupling THGEM in double-stage mode with semi-transparent photocathode. The dependence of electron spectrum and performance parameters on operating parameters such as drift field, multiplication voltage and transfer field is studied in detail. Detectors are operated in the gain range of 104-105 without any spark or discharge. Simulations are carried out to interpret the experimental observations. In the next section of the thesis, position sensing capability of the THGEM UV detector has been explored using simulation. Detailed simulation study enables to optimize the parameters affecting spatial resolution for a given THGEM based imaging detector achieving better performance. Last section of the thesis deals with electroluminescence light produced during avalanche in THGEM UV detector. The light, produced in gas medium by excitation and de-excitation of gas during the electron multiplication across the THGEM holes is simulated using Garfield++. The performance of the detector is evaluated in terms of electroluminescence (EL) yield. It is shown that the EL yield depends on the gas pressure, type of gas and the applied voltage. At atmospheric gas pressure, the EL yield is found to be 105 at a gain of 104. These studies are important to analyse the scintillation capability in order to build position sensitive and large area UV detector with optical readout. The study reported here enhances understanding of THGEM based UV detectors enabling us to develop highly efficient/sensitive UV detectors for variety of applications in particle physics and astronomy.

Tese de doutoramento em Engenharia Física, na especialidade de Instrumentação, apresentada à Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Desde cedo que a comunidade científica compreendeu que gases nobres em liquido são excelentes meios de detecção de radiação, combinando a sua elevada densidade, elevado grau de homogeneidade e de elevado rendimento de cintilação. Para além destas características inerentes, estes têm a potencialidade de fornecer ambos sinais de ionização – criando electrões livres – e cintilação em resposta à interacção com radiação ionizante e, tendo em vista a sua aplicação em experiências de eventos raros relacionados com física de neutrinos ou matéria–escura, a capacidade de autoblindagem garante a exclusão de eventos induzidos por radiação de fundo. O facto de não absorverem a sua própria luz, emergente dos eventos de cintilação, permite a expansão deste tipo de detectores até grandes volumes, sendo que as colaborações mais recentes propõem detectores com dezenas de toneladas de xénon em estado liquido. As experiências actuais que usam gases nobres em estado líquido empregam xénon ou árgon numa só fase (estado líquido) ou em dupla-fase (estado líquido + gasoso) e as suas aplicações abrangem desde as já referidas experiências de procura de eventos raros, passando por imagiologia médica tais como detectores de radiação gama para PET ou câmaras Compton “3- γ” em combinação com PET, passando também por aplicações de segurança como sistemas de inspecção para detecção de material físsil e, finalmente, em câmaras Compton para aplicações de astrofísica. Em ambas as configurações a leitura dos sinais de cintilação é geralmente feita através de um grande número de dispendiosos fotomultiplicadores de vácuo agrupados. A presente tese de doutoramento é dedicada aos fotomultiplicadores gasosos de grande área para aplicações criogénicas desenvolvidos no contexto do programa doutoral, tendo em vista a sua eventual aplicação como um dispositivo complementar aos métodos existentes de detecção de cintilação, para aplicação em futuras experiências de grande escala. Esta pesquisa foi direccionada tendo em vista o desenvolvimento de eficientes fotomultiplicadores gasosos de grande área, potencialmente mais económicos por unidade de área, baseados em “Thick Gas-Electron Multipliers” (THGEMs). Combinando fotocátodos de alta eficiência com multiplicadores gasosos de electrões capazes de atingir elevado ganho em carga obteve-se assim um dispositivo com elevada sensibilidade para a detecção de fotões únicos, com a possibilidade de discriminação em posição com resolução espacial inferior a um milímetro e com resolução temporal da ordem de poucos nano segundos. Contrariamente ao que sucede com a tecnologia de vácuo actualmente, com este dispositivo a localização em posição de fotões em grandes áreas é feita num único dispositivo integrando electrónica habitualmente utilizada em experiências de rastreamento de partículas. Neste trabalho o fotomultiplicador gasoso desenvolvido consiste numa cadeia de THGEMs combinados com um fotocátodo de iodeto de césio (CsI) sensível ao ultravioleta enquanto que os testes criogénicos foram realizados na Time Projection Chamber (TPC) de dupla fase de xénon líquido recentemente desenvolvida no Weizmann Institute of Science (WILiX). Relativamente ao GPM desenvolvido foram medidos ganhos máximos em carga de ~8×10^5 em misturas de Ne/CH4(5%) e de ~3×10^5 em misturas de Ne/CH4(20%), a uma pressão de 0.7bar à temperatura de ~180K, para fotões únicos. Foi obtida uma probabilidade de descarga com uma mistura de Ne/CH4(5%) a um ganho em carga de 1×10^5 e com sinais de cintilação secundária S2, induzidos por partículas alfa, a uma taxa de 40Hz de cerca 10^-6. Foram medidos desvios no ganho em carga de cerca de 7–15% durante um período de dois meses, operando em modo selado com 0.7bar de Ne/CH4(20%) a uma temperatura de ~190K, não se registando mudanças significativas tanto para fotões únicos induzidos por uma lâmpada UV como para sinais de cintilação primária S1 induzidos por partículas alfa, indicando a estabilidade da mistura em modo selado assim como para a estabilidade da eficiência quântica do fotocátodo de iodeto de césio. Para além disso foi obtido um valor de 1.2 – 1.3ns para a resolução temporal do GPM em resposta a sinais de cintilação, induzindo cerca de 170–200 fotoelectrões iniciais no GPM. Para a leitura em posição de eventos o GPM foi equipado com um ânodo segmentado em “pads” individuais, cujo desenho foi precedido por extensas simulações em GEANT4 que forneceram dados importantes para a optimização da geometria dos ânodos e qual a resolução espacial esperada para diferentes configurações. Das simulações pôde-se determinar que a resolução espacial esperada do GPM será ~5mm para sinais de cintilação devido a 10 electrões numa TPC de xénon liquido de dupla fase. Nos testes realizados determinou-se que para ~1.3×10^5 fotoelectrões iniciais a posição calculada apresenta um desvio menor que um milímetro da posição real. Os estudos iniciados em Coimbra combinando multiplicadores gasosos de electrões com regiões de indução micrométricas (GEM–MIGAS) fomentaram o desenvolvimento de um análogo com THGEMs e regiões de indução sub–milimétricas. Estes estudos envolveram simulações electrostáticas de forma a compreender a relação entre os campos eléctricos no interior de THGEMs e da região de indução enquanto que o trabalho experimental demonstrou que com misturas de Ne-CH4, sendo mais eficientes em absorver a radiação UV das avalanches, permitem atingir ganhos mais elevados em condições estáveis. Foram também testadas misturas à base de hélio uma vez que podem potencialmente apresentar uma boa alternativa para misturas à base de néon, devido ao ganho em carga elevado, aplicando tensões de operação mais reduzidas, semelhante eficiência de extracção de fotoelectrões e custos mais reduzidos. Em modo de corrente, um detector com um estágio apresentou ganhos superiores a 10^5.
Liquid noble gases are known to be excellent detection media due their characteristics of high density, high homogeneity and high scintillation yield. They provide both ionization and scintillation signals when transversed by ionizing particles and efficient background suppression due to self-shielding. They are transparent to their own scintillation light and allow expansion to large detector masses. Current noble liquid detectors employ either liquid argon or liquid xenon, in single-phase (liquid only) or double-phase (liquid and gas) configurations and the present application ranges from detection of rare scattering events like Dark Matter search or neutrino physics, to medical imaging like in gamma detectors for PET and LXe Compton Telescope for “3-γ imaging” in combination with PET, gamma/neutron imaging detectors for radionuclide security inspections and Compton Cameras for applications in astrophysics. Both configurations rely on measuring the scintillation light emitted from the liquid-phase or from the liquid and gas-phases with costly large arrays of vacuum photomultipliers. This Ph.D. thesis is dedicated to the large-area cryogenic gas-avalanche photomultipliers (GPMs) prototypes developed within the Ph.D. program, envisioning their application as a complimentary scintillation detection method for current and future large scale experiments. The research and development efforts aimed for a potentially economic and efficient large-area GPM based on Thick Gas-Electron Multipliers – THGEMs – combining a high efficiency photocathode with a high-gain gas-avalanche electron multiplier, providing high single-photon sensitivity and the possibility for localization of the photons with sub-mm spatial resolution and few-ns temporal resolution. Unlike current vacuum devices, photon localization over large areas can be made in a single device, using integrated electronics developed for particle tracking. The GPM consists on a cascade of THGEMs combined with a cesium iodide UV-photocathode and the cryogenic tests were performed coupling it to a double-phase liquid xenon detector (a Time Projection Chamber; TPC), in the recently developed Weizmann Institute Liquid Xenon – WILiX – cryogenic system. Moreover, for the successful use of a cesium iodide photocathode, techniques for the production, characterization and transportation were developed and implemented which allowed systematically reproducing photocathodes whose quantum efficiency ranged from 24% to 30% for a wavelength of 175nm, corresponding to the liquid xenon scintillation light, and assembling them to the GPM successfully. Within the Ph.D. thesis it is shown that the maximum gain obtained at 0.7bar and 180K was ~8×105 for Ne/CH4(5%) and ~3×105 for Ne/CH4(20%), for single-photons. With Ne/CH4(5%) at a gain of 1×105 and alpha particle-induced S2 signals at a rate of 40Hz, the discharge probability was found to be of the order of 10-6. Over a period of two months, operating in sealed mode at 0.7bar of Ne/CH4(20%) at a temperature of ~190K, gain measurements were consistent within 7–15%, showing no significant change both for the UV–lamp induced signals and alpha-induced S1 signals, indicating that there were no significant changes in either the gas composition or the CsI quantum efficiency. In terms of the time resolution of the GPM – defined as the temporal spread, or jitter, of the GPM response pulse to a scintillation signal – it was found that for scintillation signals producing ~170 – 200 photoelectrons the resolution was on the nanosecond scale, approaching ~1.2 – 1.3ns at a gain of 3×105. For position sensitive capability the GPM was equipped with position sensitive anodes. Their design was preceded by extensive and CPU–time intensive GEANT4 simulations and analysis that provided valuable information on the expected spatial resolution for different conditions. For the selected pixel size and geometry one can expect a position resolution of ~5mm for scintillation signals due to 10electrons in a double–phase LXe TPC. The tests performed with a second GPM showed that for ~1.3×105 initial photoelectrons the calculated centers–of–gravity are in very good agreement (sub-millimeter) to the actual UV source positions. Furthermore, following the promising studies performed in Coimbra with Gas Electron Multipliers with a micro-induction gap amplifying structure – GEM–MIGAS – an analogous configuration consisting of a THGEM coupled to a submillimetric induction gap was investigated to eventually obtain a GPM configuration capable of reaching higher gains with lower biasing voltages. The investigation combined extensive simulation work showing an interdependence of hole/induction region electric fields while experimental results showed that Ne-CH4 mixtures, having a more effective UV quenching than Ne-CF4 mixtures, allowed achieving higher charge gains in stable operating conditions. Helium based mixtures were also tested in terms of charge–gain and photoelectron extraction efficiency, since they can present a good alternative to Ne-based mixtures for the potential higher gains, with lower applied voltages, similar photoelectron extraction efficiency and lower costs. A Single-THGEM detector was operated in He/CF4 and He/CH4 mixtures reaching effective charge-gains well above 105, measured in current mode, applying relatively low voltages, when compared to Ar mixtures.

Radiations can be classified as either ionizing or non-ionizing according to whether it ionizes or does not ionize the medium through which they propagate. X-rays photons and gamma rays are the typical examples of ionizing radiations whereas radiowave, heat or visible light are examples of non ionizing radiations. UV photons have some features of both ionizing and non-ionizing radiation. Both ionizing and non-ionizing radiation can be harmful to living organisms and to the natural environment. Hence the detection and measurement of radiation is very important for the well being of living organisms as well as the natural environment. Not only for safety reasons, have radiation detectors found their applications in various fields including medical physics, nuclear and particle physics, astronomy and homeland security. Industrial sectors that use radiation detection include medical imaging, security and baggage scanning, the nuclear power industry and defense. Gas electron multiplier (GEM) is one of the most successful representatives of gaseous detectors used for UV photon and X-ray photon detection. Recently there is a growing demand for large area photon detectors with sensitivity reaching to the level of single photon. They are used in spectroscopy and imaging in astronomy high energy physics experiments etc. Thick GEM (THGEM) is a mechanical expansion of standard GEM. It has all the necessary requirements needed for large area detector and offers a multiplication factor that permits efficient detection of light. Hence, the development and performance study of THGEM based radiation detector is chosen as the topic of study in the present thesis. The initial part of the thesis contains simulation studies carried out for the understanding the working of the detector and the effect of various design parameters of THGEM for the above said applications. Different steps for the fabrication of THGEM and the technical challenges faced during the process are discussed. In the view of application of the fabricated THGEM for UV photon detection, cesium iodide photocathode is prepared using thin film technology and characterized. The performance of the photocathode under various operating conditions is studied in terms of its photoemission property. The effect of vacuum treatment on the photoemission property of the photocathode exposed to moist air is studied in detail. A major portion of this thesis focuses on maximizing the detection efficiency of the UV photon detector realized using the fabricated THGEM coupled with the cesium iodide photocathode. Simulations are used at different stages to interpret the experimental observations. The electron spectrum obtained from the detector under study was analyzed. The dependence of secondary effect like photon feedback on the operating parameters is also discussed. The last portion of the thesis deals with the application of THGEM as an X-ray detector. The performance is evaluated in terms of the gain and energy resolution achieved. The thesis is organized as follows: Chapter 1 is divided into two sections. Section A gives a general introduction to different types of radiation detectors found in the present day and their working principles. This is followed by discussion about gas ionization based detector and its working principle in detail. A brief literature survey of the different types of micropattern gas detectors is also given in this section. In Section B of this chapter GEM and THGEM are introduced with discussion about their working principle and areas of application. Chapter 2 deals with the simulation study of THGEM undertaken to have a clear understanding of the detector’s working. Section A of this chapter gives an overview of the simulation tools used for the present thesis in particular ANSYS and GARFIELD. Section B presents the results of the simulation study highlighting the effects of different geometrical and operating parameters on the electric field distribution in and around the THGEM aperture. The relevance of the study to the detectors performance is discussed vividly for all the cases. In Chapter 3, the details of the different steps involved in THGEM fabrication are given. Design aspects involved, fabrication of the THGEM using standard PCB technology coupled with photolithography technique are discussed in this chapter. This is followed by an elaborate description of the test setup used for all the performance study. Preface In the view of application of THGEM as a UV photon detector, cesium iodide photocathode was prepared and characterized. Chapter 4 discusses about the CsI photocathode preparation and its characterization for the above said application. Photoemission property of the photocathode was analyzed under various operating parameters. The effect of vacuum treatment on the photocathode performance is a new aspect of this thesis. Its correlation with the microstructure of the film is reported for the first time. Chapter 5 deals with the application of THGEM as a UV photon detector. The study mainly focuses on the improvement of the detection efficiency of the detector. The effect of drift parameters on the electron transfer efficiency and hence on the detection efficiency of the detector is a major contribution of this thesis. There are no literature available which discusses this aspect of a UV photon detector. The experimental study has been supported with simulation results. In addition to the study on detection efficiency, electron spectrum has also been acquired from the UV photon detector. The spectrum has been analyzed under various operating conditions. Discussions about secondary effects like photon feedback prevailing in the detector output are also present in this chapter. Chapter 6 presents the results of THGEM as an X-ray detector. The performance of the detector has been evaluated in terms of the effective gain and energy resolution achieved under different operating conditions. The gain instability with time and its uniformity across the THGEM area are also studied. The effect of drift field on the energy resolution and its correlation with ETE is a new aspect of this work. Chapter 7 summarizes the salient features of the work presented in this thesis. Also the scope of future work based on this thesis is discussed at the end of the chapter.

Radiations can be classified as either ionizing or non-ionizing according to whether it ionizes or does not ionize the medium through which they propagate. X-rays photons and gamma rays are the typical examples of ionizing radiations whereas radiowave, heat or visible light are examples of non ionizing radiations. UV photons have some features of both ionizing and non-ionizing radiation. Both ionizing and non-ionizing radiation can be harmful to living organisms and to the natural environment. Hence the detection and measurement of radiation is very important for the well being of living organisms as well as the natural environment. Not only for safety reasons, have radiation detectors found their applications in various fields including medical physics, nuclear and particle physics, astronomy and homeland security. Industrial sectors that use radiation detection include medical imaging, security and baggage scanning, the nuclear power industry and defense. Gas electron multiplier (GEM) is one of the most successful representatives of gaseous detectors used for UV photon and X-ray photon detection. Recently there is a growing demand for large area photon detectors with sensitivity reaching to the level of single photon. They are used in spectroscopy and imaging in astronomy high energy physics experiments etc. Thick GEM (THGEM) is a mechanical expansion of standard GEM. It has all the necessary requirements needed for large area detector and offers a multiplication factor that permits efficient detection of light. Hence, the development and performance study of THGEM based radiation detector is chosen as the topic of study in the present thesis. The initial part of the thesis contains simulation studies carried out for the understanding the working of the detector and the effect of various design parameters of THGEM for the above said applications. Different steps for the fabrication of THGEM and the technical challenges faced during the process are discussed. In the view of application of the fabricated THGEM for UV photon detection, cesium iodide photocathode is prepared using thin film technology and characterized. The performance of the photocathode under various operating conditions is studied in terms of its photoemission property. The effect of vacuum treatment on the photoemission property of the photocathode exposed to moist air is studied in detail. A major portion of this thesis focuses on maximizing the detection efficiency of the UV photon detector realized using the fabricated THGEM coupled with the cesium iodide photocathode. Simulations are used at different stages to interpret the experimental observations. The electron spectrum obtained from the detector under study was analyzed. The dependence of secondary effect like photon feedback on the operating parameters is also discussed. The last portion of the thesis deals with the application of THGEM as an X-ray detector. The performance is evaluated in terms of the gain and energy resolution achieved. The thesis is organized as follows: Chapter 1 is divided into two sections. Section A gives a general introduction to different types of radiation detectors found in the present day and their working principles. This is followed by discussion about gas ionization based detector and its working principle in detail. A brief literature survey of the different types of micropattern gas detectors is also given in this section. In Section B of this chapter GEM and THGEM are introduced with discussion about their working principle and areas of application. Chapter 2 deals with the simulation study of THGEM undertaken to have a clear understanding of the detector’s working. Section A of this chapter gives an overview of the simulation tools used for the present thesis in particular ANSYS and GARFIELD. Section B presents the results of the simulation study highlighting the effects of different geometrical and operating parameters on the electric field distribution in and around the THGEM aperture. The relevance of the study to the detectors performance is discussed vividly for all the cases. In Chapter 3, the details of the different steps involved in THGEM fabrication are given. Design aspects involved, fabrication of the THGEM using standard PCB technology coupled with photolithography technique are discussed in this chapter. This is followed by an elaborate description of the test setup used for all the performance study. Preface In the view of application of THGEM as a UV photon detector, cesium iodide photocathode was prepared and characterized. Chapter 4 discusses about the CsI photocathode preparation and its characterization for the above said application. Photoemission property of the photocathode was analyzed under various operating parameters. The effect of vacuum treatment on the photocathode performance is a new aspect of this thesis. Its correlation with the microstructure of the film is reported for the first time. Chapter 5 deals with the application of THGEM as a UV photon detector. The study mainly focuses on the improvement of the detection efficiency of the detector. The effect of drift parameters on the electron transfer efficiency and hence on the detection efficiency of the detector is a major contribution of this thesis. There are no literature available which discusses this aspect of a UV photon detector. The experimental study has been supported with simulation results. In addition to the study on detection efficiency, electron spectrum has also been acquired from the UV photon detector. The spectrum has been analyzed under various operating conditions. Discussions about secondary effects like photon feedback prevailing in the detector output are also present in this chapter. Chapter 6 presents the results of THGEM as an X-ray detector. The performance of the detector has been evaluated in terms of the effective gain and energy resolution achieved under different operating conditions. The gain instability with time and its uniformity across the THGEM area are also studied. The effect of drift field on the energy resolution and its correlation with ETE is a new aspect of this work. Chapter 7 summarizes the salient features of the work presented in this thesis. Also the scope of future work based on this thesis is discussed at the end of the chapter.

A novel neutron diagnostic method is presented based on optical readout from scintillation in gases excited by recoil charged particles for fusion research. According to the method, we have designed a new gas scintillation chamber filled with scintillation gas CF4 and installed with a multi-wires structure to provide strong electric field. The electric field distributions of cylindrical multi-wire structures with different geometrical parameters are analyzed systematically. By building up an optimal multi-wire structure, the optical readout characteristics are also studied with energetic charged particles. Electron-induced avalanche in the electric field excites gas molecules to emit numerous scintillation photons, which are readout using photomultiplier tubes, and measured using multichannel analysers. The results show that the light signals and charge signals are enhanced simultaneously and the rise time of the light signal hardly changes, approximately 12ns, as the applied voltage increases. More importantly, the typical pulse height spectrum from the light signals is acquired, and the obtained resolution is 8.8% for 6.5MeV protons.

The frequency response of a confined premixed swirled flame is explored experimentally through the use of describing functions that depend on both the forcing frequency and forcing level. In these experiments, the flame is forced by a loudspeaker connected to the bottom of the burner in the fresh gas region or by a set of loudspeakers connected to the combustion chamber exhaust tube in the burnt gas region. The experimental setup is equipped with a hot-wire probe and a microphone, both of which located in front of each other below the swirler. The forcing level is varied between |v′0|/v̄0 = 0.10 and 0.72 RMS where v̄0 and v′0 are respectively the mean and fluctuating velocity at the hot-wire probe. An additional microphone is placed on a water-cooled waveguide connected to the combustion chamber backplate. A photomultiplier equipped with an OH* filter is used to measure the heat release rate fluctuations. The describing functions between the photomultiplier signal and the different pressure and velocity reference signals are then analyzed in the case of upstream and downstream forcing. The describing function measured for a given reference signal is shown to vary depending on the type of forcing. The impedance of the injector at the hot-wire location is also measured using the hot-wire and microphone signals for both upstream and downstream forcing. For all describing functions investigated, it is found that their phase lags do not depend on the forcing level whereas their gains strongly depend on |v′0|/v̄0 for certain frequency ranges. It is furthermore shown that the Flame Describing Function measured with respect to the hot-wire signal can be retrieved from the specific impedance at the hot-wire location and the describing function determined with respect to the signal of the microphone located in front of the hot-wire. This relationship is not valid when the signal from the microphone located at the combustion chamber backplate is considered. It is then shown that a ID acoustic model allows to reproduce the describing function computed with respect to the microphone signal inside the injector from the microphone signal located at the combustion chamber backplate in the case of downstream forcing. This relation does not hold for upstream forcing because of the acoustic dissipation across the swirler which is much larger compared to downstream forcing for a given forcing level set at the hot-wire location. This study sheds light on the differences between upstream and downstream acoustic forcing when measuring describing functions. It is also shown that the upstream and downstream forcing techniques are equivalent only if the reference signal used to determine the flame describing function is the acoustic velocity in the fresh gases just before the flame.