In this review paper, we discuss the current status of the physics of charged particle swarms, mainly electrons, having plasma modelling in mind. The measurements of the swarm coefficients and the availability of the data are briefly discussed. We try to give a summary of the past ten years and cite the main reviews and databases, which store the majority of the earlier work. The need for reinitiating the swarm experiments and where and how those would be useful is pointed out. We also add some guidance on how to find information on ions and fast neutrals. Most space is devoted to interpretation of transport data, analysis of kinetic phenomena, and accuracy of calculation and proper use of transport data in plasma models. We have tried to show which aspects of kinetic theory developed for swarm physics and which segments of data would be important for further improvement of plasma models. Finally, several examples are given where actual models are mostly based on the physics of swarms and those include Townsend discharges, afterglows, breakdown and some atmospheric phenomena. Finally we stress that, while complex, some of the results from the kinetic theory of swarms and the related phenomenology must be used either to test the plasma models or even to bring in new physics or higher accuracy and reliability to the models. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0022-3727/42/19/194002/pdf

Measurement and interpretation of swarm parameters and their application in plasma modelling

A review on gas proportional scintillation counters (GPSCs) is presented. Recent achievements towards the portability of simple, inexpensive and compact GPSCs are discussed. Compensation of solid angle effects with the curved grid technique can be used to produce non-focused GPSCs with medium-sized radiation windows, at least up to 80% of the photosensor active diameter, without degradation of detector performance. Low power-consuming and compact vacuum UV photosensors that can operate in direct contact with the scintillation gas, as an alternative to photomultiplier tubes, are now available. Small gettering devices with a low-power built-in heating elements have been shown to be sufficient for the required gas purification in GPSCs assembled with simple and inexpensive techniques, such as the use of epoxies for ceramic-to-metal joints.

/pdf/development-of-portable-gas-proportional-scintillation-2e2jxc30ic.pdf

Development of portable gas proportional scintillation counters for x-ray spectrometry

In this paper we discuss the methods of calculation of electron ionization coefficient data. First we give a summary of the data for analytic parametrization of ionization coefficients. Such data may be useful for particle and radiation detectors and for studies of breakdown voltages in gas dielectrics. We have applied extended Townsend formula to fit the experimental data for a number of gases and shown that it provides excellent fits for the entire range of E/N where data are available. We also tested the application of the common E/N and the common mean energy combination of data for pure gases to obtain ionization coefficients for mixtures. The standard combination procedure gives poor results in general but the common mean energy procedure provides an extended region of reasonable usefulness. Test calculations were made for Ar–CH4 mixtures which were found to be the most difficult combination of the selected gases for the application of the mixture law.

On parametrization and mixture laws for electron ionization coefficients

An uniform-field argon-filled gas proportional scintillation counter, using a large-area avalanche photodiode, is presented. The excitation and ionization thresholds for argon were found to be approximately 0.7 and 3.7 V cm/sup -1/ torr/sup -1/, respectively. An upper limit of 0.30/spl plusmn/0.04 was measured for the argon Fano factor for 5.9-keV X-rays. An energy resolution of 12.5% full-width-at-half-maximum was achieved for 5.9-keV X-rays. The ultraviolet large-area avalanche photodiode proved to be suitable for reading the scintillation light of an argon-filled gas proportional scintillation counter, providing a quantum efficiency of about 50% at 128 nm. Good photodiode performance is already achieved for gains about 100.

An argon gas proportional scintillation counter with UV avalanche photodiode scintillation readout

We report on the studies of quenching effects in N 2 scintillation. Our main interest is to find a practical gas that reduces the scintillation yield in air substantially. We have studied O 2 , CO 2 , and CH 4 in N 2 , and have determined their Stern–Volmer constant. When two quenching gases are used in a mixture, the effects are found to be additive. The quenching mechanism of these gases is discussed and its practical application is reported.

Quenching effects in nitrogen gas scintillation

An attempt is made to develop a semi-empirical expression for light gain factors in single wire anode (SWA) gas scintillation proportional counters (GSPC) as a function of various other counter parameters like operating voltage, anode and cathode radii, filling gas mixture and its pressure. Experimental data of charge gain and light gain factors has been taken from four GSPCs having anode diameters ranging from 25 μm to 1.7 mm and operated with Ar or Ar + 2.5% N2 mixture. First the quality of our experimental data has been investigated by fitting the values of charge gain factors from one of the four GSPCs to four different semi-empirical expressions selected from the literature which have been developed for charge gain factors in ordinary ionization proportional counters and which are suitable for experimental verification. Subsequently, assuming appropriate functional forms of electron excitation and ionization coefficients and using derived or assumed values of some other unknown parameters, the relevant integrals have been evaluated. Then the assumed values of unknown parameters are adjusted in such a way that the calculated values of light gain factors are comparable to the experimental values. Thus a correlation between the light gain factors and other counter parameters is established. The values of the effective electron excitation coefficients as a function of electric field strength per unit pressure in Ar + 2.5% N2 from these GSPCs having considerably different electric field gradients have also been estimated.

Development of a semi-empirical expression for light gain factors in gas scintillation proportional counters

Pulse shape discrimination technique has been applied to improve the energy resolution of a single wire anode (SWA) gas scintillation proportional counter (GSPC). Resolution at 5.9 keV photon energy is improved from 33% to 16% by suppressing pulses due to interactions in the scintillation production region (SPR) in a 1.7 mm diameter rod anode cylindrical design GSPC operated with Ar + 2.5% N 2 in continuous flow. For the optimum operating conditions of the GSPC the rise time of the pulses due to 5.9 keV X-rays was typically 300 ns for interactions outside the SPR and a broad continuum from 300 ns down to about 40 ns for interactions inside the SPR. In order to select a certain narrow time window a special time to amplitude converter unit was developed with a time resolution capability to distinguish pulses having rise times differing only by a few ns.

S.P. Garg

Papers

Development of a semi-empirical expression for light gain factors in gas scintillation proportional counters

Application of pulse shape discrimination technique to improve the energy resolution of a gas scintillation proportional counter