Experimental gas amplification study in boron-lined proportional counters for neutron detection

Based on theoretical considerations related to the microscopic behaviour of electrons in gases in conjunction with a macroscopic hypothesis, a semi-microscopic gas gain formula for conventional proportional counters has been already developed and was afterwards successfully tested on gas gain data obtained in Ar–CH₄- and He–isobutane-filled proportional counters, using an X-ray radiation source. In the present work, it is verified that the semi-microscopic formula works as well in case of the detection of spread energy-spectrum of ionizing charged particles, such as involved within boron lined proportional counters, commonly used for thermal neutron detection. Indeed, we investigate experimentally the validity of the formula for the parameterization of the gas amplification factor data obtained in two boron lined proportional counters, filled with Ar–CH₄ (10%) and Ar–CO₂ (5%). The obtained results show clearly the ability of the gas gain formula to perfectly describe gas gain characteristics in such counters independently of the filling gas mixture used. Consequently, this provides further evidence of generality of the underlying gas amplification semi-microscopic model.     

高能X射线聚焦组合透镜的理论研究

X射线聚焦组合透镜是一种利用折射效应对X射线辐射聚焦的新型元件。针对X射线波段的特性,综合考虑折射和吸收效应得出组合透镜的衍射屏函数,并利用衍射理论推导出X射线组合透镜的设计理论。利用统计学理论中一阶、二阶原点矩表征像面处的光学性能,使设计更简洁。该理论方法可方便地推广到双凹折射单元情况。通过数值计算分析了像向处强度分布的二阶中心矩以及焦点处辐射强度随组合透镜结构参最的变化关系,给出了一种铝基X射线组合透镜的结构参量设计结果。对于设计的组合透镜结构参量,计算了当X射线辐射能量分别为0．93 keV．9．89 keV和29．78 keV时,X射线辐射经过组合透镜后的焦点强度分布。     

The SAX mission for wide-band X-ray astronomy

Summary In the framework of past and future X-ray missions the SAX satellite, to be launched at the end of 1995, stands out for its very wide spectral coverage from 0.1 to 200 keV. The sensitivity of the scientific payload will also allow the exploitation of the full band of SAX for weak sources (1/20 of 3C273), opening new perspectives in the study of spectral shape and variability of several classes of objects. In this paper we briefly describe the main aspects of the mission, the instruments, the scientific objectives and the operations. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

Preliminary investigation to test extragalactic hypothesis for gamma-ray bursts

Summary Recent observations of CGRO satellite challenged the almost universally accepted galactic origin of gamma-ray bursts and have revived the extragalactic, cosmological models. This hypothesis should imply some variations of the morphological and energetic characteristics of the weaker events to which we think the farthest ones might belong. The existence of such variations due to the cosmological red-shift should be tested. In this paper we examine the possible correlations between temporal morphology of the gamma-ray bursts, peak intensity and photon energy spectra when the red-shift effects do not yet occur. We examine the peak intensities and the energy spectral-hardness distributionversus rise times for events observed by previous space missions, with a logN-logS shape free of cosmological effects. The results of this analysis suggest that a wide range of values of the temporal and spectral characteristic parameters is an intrinsic property of the sources. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

The origin of the continuum of Active Galactic Nuclei

Summary The high-energy continuum of active galactic nuclei can be interpreted by nonthermal models in which electron-positron pairs play a crucial role. The produced primary γ-rays interact with softer photons in the inner, high photon density regions of the source. The created pairs are relativistic, and contribute to the emission, especially in the X-ray band. When the photon density is large enough, therefore for large values of the luminosity-to size ratio, the spectrum produced by the reprocessing due to pairs is predicted to have a characteristic spectral index of one. This power law continuum partly impinges on cold matter, possibly in an accretion disk. The resulting Compton reflected radiation contributes to the X-ray emission, and the predicted overall spectrum is in agreement with the observed shape of the high-energy continuum. The proposed model is easily testable: observations in the hard—X-ray band above (30÷50) keV are crucial to discriminate these nonthermal models against thermal ones. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

An analysis of the red noise in the power spectrum of X-ray binaries as a shot noise process

Summary We have simulated, using a Monte Carlo method, time series representing the aperiodic variability typically observed in some X-ray sources. We have supposed that such red-noise features are generated by a shot noise process. The resulting time series have been analyzed by fast Fourier transform techniques. Using different sets of parameters we have been able to generate power spectra of different shapes. Some preliminary results of these simulations are discussed. Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992.     

X-ray spectral signatures of very thick cold matter in the Spectra of seyfert galaxies

Summary Over the last few years, GINGA observations of several bright Seyfert galaxies have revealed the existence of a new flat component in the X-ray spectrum of these objects above 8 keV. This component (the ?high-energy bump?) is probably produced by the reprocessing of the intrinsic emission (through Compton scattering and photoelectric absorption) by a very thick and cold medium in the environment of the central source, such as an accretion disk. An alternative explanation is provided by partial covering and reprocessing by thick clouds, whose existence is supported by the latest results on the broad line region obtained by line reverberation measurements in the optical-ultraviolet band. The presence of iron fluorescent lines at 6.4 keV with an equivalent width ≅ (100÷200) eV in all those galaxies supports both scenarios. The implications of the presence of the high-energy bump on the spectral shape of the intrinsic continuum, the level of the soft X-ray excess and the X-ray background are briefly pointed out. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Temporal structure of the gamma-ray bursts GRB881024 and GRB880810

Summary In this paper we analyse two particularly significant cosmic gamma-ray bursts recorded by the French-Soviet experiment APEX on board the space mission PHOBOS. The October 24th 1988 event, lasting more than 16 seconds, is among the strongest events observed up to now. The very high temporal resolution available for its light curve, 1/8 second, allows us to study the power spectrum density (PSD) up to a frequency range more extended than in the past. The August 10th 1988 event, lasting about 1 second, recorded with a temporal resolution 1/128 second, higher than in the precedent one, permits us to reach still higher frequencies. We discuss the results of our analysis, comparing them with the results obtained for other events recorded by precedent space missions, and giving interpretative suggestions. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Estimate of the black-hole mass in NGC 6814 from a relativistic accretion disk scenario

Summary By using a fully relativistic model for the spectral line produced by a Keplerian disk orbiting a Schwarzschild black hole, we study the temporal behaviour of the line intensity in response to a continuum variation at an extended central source. We compare our results with the observed properties of the Seyfert galaxy NGC 6814, whose X-ray flux has been observed to decrease by a factor of two in ≈ 50 s, while the iron line intensity variations lag continuum variations by 250 s, at the most. Taking the stationary values of the iron line centroid energy and width, and assuming that the line comes from high-ionisation stages of iron (as several indications suggest) we derive that the inner radius of the line-emitting region is between 6 and 30r _g (r _g =GM/c ²), the inclination is ≤40°, while the mass of the central object is constrained to 8·10⁴ L ₄₃<M<3.9·10⁶ M _⊙(L ₄₃ is the accretion luminosity in units of 10⁴³ erg s⁻¹). Paper presented at the 6th Cosmic Physics National Conference, Palermo, 3–7 November 1992. Affiliated to ICRA.     

Coordinated observation of the massive system 4U1700+24=HD154791

Summary We report results from multifrequency observations of the X-ray source 4U1700+24/HD154791. This source was observed simultaneously with the EXOSAT and IUE satellites on March 25, 1985, and it is continuously monitored in the optical band at the Bologna Astronomical Observatory since July 1984. Our observations show source features which are different from those measured during previous observations. The X-ray spectrum is significantly softer than in previous measurements, and much more absorbed; high excitation lines in UV spectra (C IV and N V) have disappeared; the optical spectrum, despite a fairly good photometric stability of the M giant HD154791 in V and B bands also shows differences with respect to previous observations. We discuss our results in the framework of both a binary model and a coronal model. We point out that our observations strengthen the hypothesis that the X-ray emission is due to mass accretion onto a compact object orbiting around the M giant star HD154791. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

X射线管激发X荧光光谱连续本底扣除方法研究

X射线管是目前X射线荧光光谱分析中最常采用的激发源,它所产生的原级谱成为了X荧光光谱中本底成分的主要来源,在对这种光谱进行进一步的分析处理之前需要对其本底进行扣除,对本底估计的准确性直接影响后续处理步骤的效果。对射线管激发X荧光光谱的成分进行了分析,针对其本底特点构造了一种本底强度的估计方法,并根据实测谱线构建了理论测试谱线以便对光谱处理算法的效果进行评价。该方法利用测得X射线荧光光谱中不包含特征峰的谱段对X射线管原级谱造成的本底成分进行估计,使用只包含连续本底的谱段对整个测量谱段进行插值,从而避免了谱线特征峰重叠或对半高宽估计不当时所产生的影响。利用构建的测试光谱对SNIP法、傅里叶变换法和本文的本底估计方法的使用效果进行了比较,使用该方法估计的本底与理论本底更加接近。结果表明使用的方法对X射线管激发的X荧光光谱的本底估计准确,可以采用这种方法对连续本底进行扣除,在对实际测得的X射线荧光光谱的本底扣除中取得了较好的应用效果。     

X射线成像的一种点扩展函数模型

X射线成像系统可以通过其点扩展函数来表行,其点扩展函数分为一次射线点扩展函数和散射点扩展函数两部分。在分析点扩展函数各个影响因素的基础上,建立了以物体厚度、物体到探测器距离以及成像几何设置为参量的解析模型。利用该模型推导出了特定入射射线能谱和射线源到探测器距离情况下散射比的计算公式。它是以物体厚度和物体到探测器距离为变量的函数。在利用实验数据对模型参量进行最优估计的基础上,利用散射比实验验证了模型的正确性。为散射和几何不清晰度的消除提供了一种实用的模型依据。     

X射线皮秒分幅相机在强X射线情况下的应用研究

扫描型变象管Ｘ射线皮秒分幅相机在强Ｘ射线情况下，由于空间电荷效应，像质变坏，甚至无法工作。本文简述了ＭｇＦ光阴极材料的性能。首次在Ｘ射线皮秒变象管分幅相机上的应用结果：降低了强Ｘ射线源情况下分幅管内空间电荷效应的影响，并获得了清晰的物理图像。     

高能X射线组合透镜聚焦性能的实验结果

高能X射线组合透镜是一种基于折射效应的新型X射线光学元件。报道了这种新型X射线光学元件聚焦性能的最新实验结果。设计的高能X射线组合透镜为聚甲基丙烯酸甲酯(PMMA)材料,包括40个相同的、顺序排布的平凹折射单元,折射单元的球面半径为200μm,组合透镜的几何口径约为500μm,长度约为8 mm,焦距约为1.3 m。简要描述了PMMA材料X射线组合透镜的制作技术和制作过程。并给出了X射线组合透镜聚焦性能测试实验系统和实验条件。最后给出8 keV单色X射线辐射下,PMMA材料X射线组合透镜的聚焦性能的测试结果,对实际测试结果进行了分析和讨论,得出结论,焦距等参量与理论计算结果基本吻合。     

临边成像光谱仪的发展现状与进展

介绍了临边观测模式的探测原理及其特点,给出了美国、欧洲等发达国家关于临边成像光谱仪的研究状况,并列出了各临边成像光谱仪的核心设计方案及具体的技术指标。最后,介绍了国内唯一一例临边成像光谱仪的开展情况,包括设计方案到技术指标。通过对比分析国内外临边成像光谱仪的研究进展,为我国进一步提高临边成像光谱仪的性能及技术指标提供了理论基础。     

类锂硅软X射线激光模拟

对１１６ｐｓ，功率密度为５．４×１０（１２）Ｗ／ｃｍ２的激光脉冲和硅平板靶的相互作用进行了数值模拟，得到了类锂硅离子４ｆ粒子波居数和４ｆ－３ｄ跃迁增益系数的时空变化，并与实验结果进行了比较。     

X-ray signatures of cold accretion disks in AGNs

Summary We have calculated, by means of Monte Carlo simulations, the properties of the radiation emerging from an X-ray illuminated cold accretion disk. These calculations can apply to AGNs, for which there is evidence of cold, optically thick matter near the central black hole. The resulting spectra show an intense iron fluorescent line and a high-energy bump above about 10 keV, where the scattering becomes important with respect to the photoabsorption; these features have been observed by GINGA in some bright Seyfert 1 galaxies. Our detailed results are indicative of the type of future observations which would lead to estimates of the geometrical parameters of the disk, its iron abundance and the mass of the black hole. Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990.     

Variability of GX3+1 observed with EXOSAT

Summary We report results from an observation of the X-ray source GX3+1 carried out with the EXOSAT satellite on 1983, October 23. The source showed a strong X-ray variability in excess of the counting statistics noise. This variability increased toward low frequencies (?red noise?) and was different in different energy bands, with the lower energy X-rays showing a more pronounced increase of the variability at low frequencies. The variation function of the source is also shown to depend on energy, with the highest values associated with the highest energies. A comparison of the time variability properties of GX3+1 with those of other bright low mass X-ray binaries is presented. To speed up publication, the proofs were not sent to the authors and were supervised by the Scientific Committee.     

Design parameters and technology optimization of 3He-filled proportional counters for thermal neutron detection and spectrometry applications

Technological aspects and key features related to the development and design optimization of ³He-filled proportional counters for thermal neutron detection and neutron spectrometry applications are presented in this article. Basic operation features are reviewed in details by focusing on their determinant impact upon the definition of the consistent design parameters. Previous Monte Carlo simulations, where both wall effect and electron attachment effects due to occlusive electronegative impurities were taken in consideration, were carried out in ³He-filled proportional counters for the study of the pulse height distribution and gamma rays influence. From interpretation of the main quantitative results, basic parameters for design optimization requirements were drawn-up. Some laboratory developed ³He-based proportional counter prototypes are then described. The characteristics of a set of gas composition fillings that were experimentally investigated are then compared to those obtained with a commercial specimen that was taken as a reference standard. Finally, a brief conclusion underlines some consistent insights toward the improvement of quality performances during future development tasks of helium-filled proportional counters intended for thermal neutron detection and neutron spectrometry applications.