Investigation of Dead Times and Dead Zones in Wire Chambers Operated in the Self-Quenching Streamer Mode

The large saturated pulses of the type referred to as "self-quenching streamer" promise to provide an inexpensive method of radiation detection. We have continued our investigations of the performance of wire chambers operating in this mode, with particular emphasis on measurements of dead times and dead zones. The results of these measurements and their effect on the application of such chambers in high rate environments and for digital gas sampling calorimetry are discussed.     

Significance of Ionization-Track Contribution to Self-Quenching Streamer (SQS) Formation Induced by Alpha-Rays

Positive-parity collective band structures of low-lying levels in even-even actinide nuclei were analyzed based on an extension of the Davydov-Chaban soft rotator model, which accounts for the rotation and β- and γ-vibrations of even-even nuclei with non-axial quadrupole deformation. The parameters to reproduce the 4-bands, i.e., the ground-state rotational band, the K≈2 band, and the n _β=1 and n _γ=l bands, were obtained, and their systematic trends were deduced. Based on this result the unassigned band having a sequence of 0⁺, 2⁺, 4⁺,…, which is observed in many actinide nuclei, was assigned likely to be the n _β=1 band. The systematic trends of the parameters found in this work could be a guide to estimate the collective band structure of nuclei for which such data are poorly known. The correct assignment of collective levels was found to be important for the calculation of neutron inelastic scattering cross sections.     

Effect of Methylal Quenching Gas for Self-Quenching Streamer (SQS) Tube with Ar-isoC4H10-Methylal Mixture

Abstract

Properties of a self-quenching streamer (SQS) tube with an Ar-isoC₄H₁₀ (75:25) mixture containing 2.5% methylal vapor have been investigated for uniform α-ray irradiation. The effect of electro-negative methylal as the second quenching gas has been studied for avalanche sizes, pulse shapes of current signals, dead zones, and counting-rate characteristics of the SQS tube. The dead zone and counting-rate characteristics are improved very much by the use of the mixture containing methylal.     

SPECT: Single Photon Emission Computed Tomography

Interest in single photon emission computed tomography (SPECT) has been renewed as a result of the successful application of transmission (x-ray) CT to diagnostic radiology. Many aspects of SPECT are different from those encountered in transmission CT, and often are more difficult to overcome. Examples of two major aspects encountered are 1) the limitations on the available photon flux imposed mainly by dose considerations to the patient and, 2) the internal attenuation of gamma rays within the patient prior to detection. Progress has been made recently in overcoming the quantum limitation by designing SPECT systems using special collimation and large active detector areas. High efficiency systems have been designed and built using both multiple-scanners and also using multiple large-field-of-view scintillation cameras. Much progress has also been made in compensating for the problem of gamma ray attenuation using iterative and analytical approaches. This paper reviews the history of single photon emission tomography, characterizes the physical attributes of SPECT, describes some solutions to the inherent problems encountered, and also reviews a few selected approaches in designing SPECT systems to provide high quality, artifact-free reconstructed images. It is anticipated that future developments will allow SPECT systems to more nearly attain the ultimate goal of determining absolute regional radionuclide concentration as a function of time. These systems, coupled with newly developed physiological radiopharmaceuticals, can provide useful research and clinical information.     

Self-Quenching Streamers in Mixtures with Photoionization Vapours

The basic properties of self-quenching streamers in a proportional counter with a thick anode wire are investigated. In particular, the influence of mixing benzene and triethylamine to argon-isobutane mixtures is studied regarding thresholds for streamer production, pulse shapes and dead time. There is a lowering of the high voltage threshold for streamer production associated with low concentrations of triethylamine and benzene. Higher concentrations of benzene increase that thre? shold. Triethylamine leads to narrower pulses. Stable operation is observed for a much broader range of high voltages than in the absence of photoionization vapours.     

Photon Temperatures of Hard X-Ray Emission of LHCD Plasmas in HT-7 Tokamak

A detailed study of photon temperatures （Tph） of hard X-ray emission in lower hybrid current drive （LHCD） plasmas is presented. The photon temperature increases with the increase in plasma current and decreases with the increase in plasma density. In lower hybrid power and phase scanning experiments; there is no appreciable change in the photon temperature. The numerical results based on ray-tracing calculation and Fokker-Planck solver gives reasonable explanation for the experimental observation. Both experimental and numerical results reveal that the photon temperature depends mainly on global effects of the fast electron population, synergy between the fast electron and the loop voltage and the Coulomb slowing down.     

Scintillating Drift Chambers?the Nature of the Emission Process in Ar/CH4

A mechanism involving optical emission from excited molecular fragments is proposed as the origin of the photon emission from scintillating drift chambers which contain Ar/CH4 mixtures. This explains a spectral feature observed at ~ 4300 ? and also the appearance of spurious photo-electric emission from the cathode wall material.     

光子谱仪的光子判别研究

本工作用JPCIAE事件产生器模拟5．5TeV／A^208Pb ^208Pb对撞产生的7种粒子n、n^-、p、p^-、K^ 、π^ 和光子入射的环境下，对ALICE(A Large Ion Collider Experiment)光子谱仪(PHOS)的光子判别的效率和纯度进行了模拟研究。在簇射形状分析中，采用主成分分析方法将7参数分析问题约简为2参数，给出不同能区中的光子判别效率与纯度的关系。     

Electronic Excitation Temperature in DC Positive Streamer Discharge

The electronic excitation temperature in a direct current positive streamer discharge based on ultra-thin sheet electrodes was measured by optical emission spectrometry in order to deposit materials for potential future applications. It was remarkable that the electronic excitation temperature （Text） did not vary monotonically with the discharge current, but demonstrated a peak at a certain position. In a mixture of oxygen and argon （80% oxygen）, the maximum Texc reached about 6300 K at an average current of 600 pA. Both the positive ions accumulation in the discharge region and the increase of the local temperature around the streamer channel caused by Joule heating are considered to be the main reasons for the variations of Texc.     

High-Accuracy Position Measurements with Drift Chambers and Proportional Chambers

By constructing multiwire drift chambers with a uniform electric field saturating the drift velocity, a perfectly linear response is obtained over all the chamber. The accuracy is close to ? = 100 ?m. A similar accuracy is obtained for inclined tracks and in magnetic fields of the order of 1 tesla. By measuring the centre of gravity of avalanches a very high accuracy is obtained in the measurement of the two coordinates of avalanches in a single gap of a proportional chamber. The applications of the method to the mapping of spatial distributions of soft X-rays are discussed.     

Recent Developments in Spark Chambers

The parallel developments of a variety of spark-chamber schemes have produced many workable systems with success in experimental applications. Major efforts have developed several methods for direct electrical readout of spark-chamber data. These include the use of magnetic cores for direct digitizing of information from wire spark chambers, as well as the use of Vidicon cameras to observe sparks, and of current division and acoustic transit time in analogue devices to measure spark position. For such systems the on-line use of computers, and (or) the construction of significant electrical interfacing circuitry, are important considerations in efforts to maximize the event rate. Conventional spark-chamber systems using film to record data have benefited from the continued development of automatic off-line scanning and measuring systems. There are also many experiments that will use the unique capabilities of wide-gap spark and streamer chambers.     

High Resolution Cryogenic Spark Chambers

A cluster of four narrow gap (2.5 mm) spark chambers containing 8 wires per millimeter has been built and operated at low temperatures (as low as 118°K). Results obtained from these chambers indicate that the width of the spark column is reduced at lower temperatures due to the increased gas density. Electron diffusion and deviation of the spark column from the true particle trajectory are greatly reduced at lower temperatures with narrow gap spark chambers. Space resolutions less than ? = 70 microns (HWHM) has been achieved and even better resolutions are predicted from the observed temperature dependence.     

塑料（PVC）流光管批量制造及质量影响因素的研究

本文描述了塑料流光管的批量制造工艺及影响性能质量的各种因素的研究结果,讨论了用Ar-CO_2作为自猝灭流光管工作气体的可能性问题。