Photoelectron Tracks Research Articles

Measurement of X-ray polarization using optical imaging detector with capillary plate

The performance of the optical imaging capillary gas proportional counter (CGPC) was investigated using polarized X-rays with an energy of 15 keV at SPring-8. The CGPC with a fill gas of Ar+8% CH 4+2% TMA at 1 atm was operated at a gas gain of 15 000. The CGPC was irradiated by a polarized X-ray beam in the center region using a slit of 300 μm×300 μm. Images of single photoelectron tracks were obtained by an image-intensified CCD camera combined with the CGPC through a lens system. The average distance between the starting point and the barycenter for single photoelectron tracks was 1.1 mm. The angular distribution is obtained from the directions of the lines connected between the starting point and the barycenter for each image of the photoelectron tracks. From a least-squares fitting, the modulation factor was 25.1±2.0% for the X-rays with a polarization degree of 99%.

Characteristics of an X-ray imaging detector with double capillary plates

A capillary gas proportional counter (CGPC) with the double capillary plates has been constructed to investigate the image of X-ray photoelectron tracks at high gas gain. The double CGPC filled with a Penning gas mixture of Ar+8%CH 4+2%TMA at 1 atm can operate at a gas gain of up to 4×10 4. The light yield was proportional to the gas gain. The electron extraction rate is 1.2% from the upper capillary to the lower one. The energy resolutions of the double CGPC were 35% for charge signals and 40% for light signals for 5.9 keV X-rays. With the double CGPC, clear images of photoelectron tracks were obtained for 22 keV X-rays at a gas gain of 2×10 4. From an analysis of the energy deposit along a photoelectron track, it was indicated that the track has two bright portions and a saddle portion with low brightness. The structure of a photoelectron track image obtained at high gas gain is important for application to an imaging type X-ray polarimeter.

A photoelectric polarimeter based on a Micropattern Gas Detector for X-ray astronomy

Polarimetry of cosmic X-rays is one of the possible observational approaches, together with spectroscopy, photometry and imaging, to study celestial sources. It can provide a general tool to explore the structure of compact sources and derive information on mass and angular momentum of supermassive objects. Comparing with the other three modalities, the development of X-ray polarimetry is modest, due to the inefficiency of traditional X-ray polarimeters. In this paper a new instrument (the Micropattern Gas Detector) to measure the linear polarization of X-ray sources with high efficiency is presented. It is based on the photoelectric effect. Angle and amount of polarization is computed from the angular distribution of the photoelectron tracks, reconstructed by a finely segmented gas detector. The device has pixel read-out allowing a full 2-D image reconstruction. The improvement in sensitivity is nearly two orders of magnitude with respect to traditional polarimeters. At the focus of a large X-ray telescope, in orbit, it can detect low level of polarization in galactic and extragalactic sources.

Photoelectron track image of capillary gas proportional counter

The segmented pixel gas detector has been developed with an optical imaging capillary gas proportional counter (CGPC) to obtain an image of an X-ray photoelectron track. The pixel signal is read out with an image-intensified CCD camera combined with the CGPC through the optical system of mirrors. Using the optical imaging CGPC with a penning gas mixture of Ar+8% CH4+2% TMA at 1atm, we have successfully detected clear images of photoelectron tracks for 22keV X-rays. The detection efficiency of the photoelectron track was approximately 95% for the images. The evaluated range of 5.2mm for 22keV X-ray photoelectron was consistent to the published ranges. The optical imaging CGPC with the high-resolution capability is available for an imaging type of X-ray polarimeter and for the field of medical science.

Detection of photoabsorption point with capillary imaging gas proportional counter

We have successfully detected the images of photoelectron tracks of 2.7and 5.9-keV X-rays utilizing a capillary imaging gas proportional counter (CIGC). The images of the photoelectron tracks were extracted by systematic analysis based on clustering of the tracks, eliminating the spurious noise cluster due to an image-intensified charge-coupled device camera. The characteristics of the images were investigated in terms of the relationship between the light yield and the area of the cluster. The extent of cluster spreading was approximately 1.3 mm on average for 2.7-keV photoelectrons. The light yield of the clusters for 5.9-keV X-rays was distributed at 74% full-width at half-maximum around a peak that is approximately two times larger than that for 2.7-keV photoelectrons. The area of the clusters for 5.9-keV X-rays was approximately 1.5 times larger than that for 2.7-keV photoelectrons. The results imply that the photoabsorption point is detectable for X-rays above 10 keV.

Optical imaging capillary gas proportional counter with Penning mixtures

An optical imaging gas proportional counter with a capillary plate has been tested using a gas mixture of Ar+8%CH/sub 4/+2%TMA.. Both charge and light signals were investigated. The gas gain was found to be approximately 70 times greater than that previously obtained in Ar+10%CH/sub 4/ and the new mixture operates at a gap voltage 220 V lower than the Ar+10%CH/sub 4/, indicating the Penning effect of the trimethylamine (TMA) additive. The energy resolutions obtained for the charge signal and the light signal were 20.8% and 26.6% for /sup 55/Fe 5.9-keV X-rays, respectively. Through the relationship between gain and energy resolution for the light signal it was estimated that the light production rate was 1.2 photons per electron. Finally, from a cluster analysis of the photoelectron tracks due to 22-keV X-rays, it was inferred that the capillary gas proportional counter (CGPC) operating with Ar+8%CH/sub 4/+2%TMA could detect the starting point of the track, a necessary requirement for its eventual use as an imaging polarimeter.

Micropattern gas detectors: the CMS MSGC project and gaseous pixel detector applications

We report recent results from the development and testing of two types of micropattern gas detectors—micro-strip gas chambers and GEM-based devices with two types of pixel read-out. Thirty-two micro-strip gas chambers were tested in a high intensity hadron beam as a milestone for CERN's Compact Muon Solenoid (CMS) experiment. The detectors were operated with voltage settings corresponding to 98% hit detection efficiency at CMS for a total high intensity exposure period of 493 h. All of the requirements expected by the milestone—gain stability, number of lost strips, spark rate, etc.—were met, with wide margins. In a separate investigation, we have coupled PCB pixel read-out planes to GEM foils. In one case, 2 mm×2 mm pixels were fanned out to individual discriminators and scalers to provide very fast (2 MHz/pixel) read-out; this system has been used as an imaging device to provide diagnostic information in fusion experiments. The second type of device used smaller pixels (200 μm squares) and a Flash-ADC read-out system to reconstruct individual photoelectron tracks. The angular distribution of the tracks allows the polarisation direction of polarised X-ray sources to be identified, with possible applications for future space experiments studying celestial X-ray emissions.

Astronomical X-ray polarimetry based on photoelectric effect with microgap detectors

The polarisation of X-ray photons can be determined by measuring the direction of emission of a K-shell photoelectron. Effective exploitation of this effect below 10 keV would allow the development of a highly sensitive X-ray polarimeter dedicated in particular to X-ray astronomy observations. Only with the advent of finely segmented gas detectors it was possible to detect polarisation sensitivity based on the photoelectric effect in this energy range. Simulation and measurements at 5.4 and 8.04 keV with a microgap gas counter, using both a polarised and an unpolarised X-ray source, showed that the photoelectron track in a neon-based gas mixture retains the memory of the polarisation of the incoming photons. Possible experiments aimed at galactic/extragalactic sources and solar flares are considered and their sensitivity to these sources is calculated.

The form of X-ray photoelectron tracks in a capillary gas proportional counter

We have successfully detected images of tracks of photoelectrons from the 22 keV and 30 keV X-rays of /sup 109/Cd and /sup 241/Am, utilizing an optical imaging gas detector with a CCD camera. Most of the 30 keV photoelectrons have track lengths greater than 1 mm in xenon gas. The images of the tracks contain clusters of pixels, and the brightest of these clusters, for each track, were investigated in detail. It was found that the average shape of these clusters was oval-like and that the geometrical center differs from the point of maximum energy loss. These data have important implications for X-ray polarimetry.

A new photoelectron imager for X-ray astronomical polarimetry

A new photoelectron imager for X-ray astronomical polarimetry (PIAP) has been developed and tested at the Frascati laboratories (LNF-INFN). A charge-coupled device (CCD) is placed on one of the two conjugate foci of a Cassegrain reflective optics onto which are focused UV photons emitted by means of gas scintillation. This X-ray detector has been built to image the angular distribution of the photoelectron tracks, whose anisotropy measures the X-ray polarization. First tests, performed by using mixtures based on argon gas and benzene at low pressure, show events which are candidate tracks of photoelectrons and Auger electrons produced by a 55Fe source.

Optical imaging chamber for x-ray astronomy

The light emitted by electron avalanches in a parallel plate chamber can be used to image the tracks of photoelectrons liberated by the interaction of an incident X-ray with the gas filling the chamber. The differing morphologies of photoelectron tracks and high-energy charged particle tracks can be used for background rejection. The initial direction (before scattering) of the liberated photoelectron also contains information about the polarization of the incident radiation. We have built a small test chamber with which we have imaged photoelectron tracks using an intensified CCD camera. Our results show that optical imaging could be used in a hard X-ray imaging polarimeter useful for astronomy.

Track effects during the accumulation of F-centers in KBr and KCl whiskers exposed to X rays

It is shown that as F-centers are reestablished during repeated x irradiation of KCl and KBr whiskers, effects appear which are due to the localization of energy-absorbed in the tracks of x-ray photoelectrons. The parameters of the kinetic recovery curves are calculated, and an estimate is made of the volume Ω of the track of a photoelectron with an energy E=8–9 keV.

Effectiveness of 0·3 keV Carbon Ultrasoft X-rays for the Inactivation and Mutation of Cultured Mammalian Cells

Carbon K characteristic ultrasoft X-rays of energy 0.278 keV were found to be effective in inducing inactivation and mutation to thioguanine resistance in cultured V79 Chinese hamster cells and human diploid fibroblasts. These X-rays act as a probe of the sensitive sites within the cells since they produce low-energy photoelectron tracks of range about 7 nm; this is an order of magnitude smaller than those produced by the 1.5 keV aluminium X-rays used in previous studies. A detailed interpretation of the results requires assumptions to be made about the positions of the sensitive sites within the cells but, for any reasonable set of assumptions, the carbon X-rays are found to be more effective than gamma-rays and are probably at least as effective as long tracks of helium ions of similar LET. These observations extend the conclusions previously drawn from the observed effectiveness of aluminium X-rays regarding the sizes of the subcellular sites involved in inactivation and mutation. They imply that the sensitive sites smaller than about 7 nm, and that highly localized energy depositions consisting of less than or approximately 14 ionizations are sufficient to produce biological effects. These results are also in contradiction to models of radiation action which require relatively large sites, such as the usual form of the 'theory of dual radiation action'.

The angular distribution of photoelectrons from theKshell

In an investigation with the cloud chamber on the properties of X-rays, Wilson (1923) observed that although photoelectrons due to a narrow pencil of X-rays were ejected from the gas atoms in all directions, there was, especially in the case of high-energy electrons, a preponderance with forward components in their velocities of ejection. Wilson’s pictures threw light on results obtained earlier by workers in X-ray fields, who found an excess of electrons ejected forwards from thin metal foils by X-rays, and have since inspired a number of experiments designed to determine with precision the angular distribution of photoelectrons. A survey of the different methods developed for this purpose has been given by Compton and Allison (1935). Of these the cloud expansion chamber has proved the most satisfactory, in spite of the inevitable disadvantages associated with statistical methods. If a suitable gas is chosen, the shells from which the electrons are ejected can usually be identified, and the directions of ejection can be determined from measurements relating to the initial portions of the cloud tracks. No trouble is ever encountered in distinguishing photoelectron tracks from those due to recoil electrons. The method has been criticized by Watson (1927), who suggested the possibility of nuclear scattering within the radius of the first drop. This objection has been answered by Kirchner (1927), who was able to show from an application of Rutherford’s theory of nuclear scattering to the problem, that under normal conditions of observation the effect of nuclear scattering on the initial directions of ejection as measured from track photographs could be neglected. The most likely source of error appears to lie in the systematic exclusion of tracks in certain angular intervals determined by the experimental arrangement (e. g. positions of cameras, lighting, etc.), and much thought has been given by different workers (Kirchner 1927; Williams, Nuttall and Barlow 1928) as to the best means for ensuring representative samples of tracks for measurement. In this respect our experience leads us to believe that a requirement of fundamental importance is sufficient and uniform illumination throughout the whole chamber.

The Auger effect in xenon and krypton

In an earlier paper an account was given of a determination of the efficiency of emission of K series radiations from argon atoms ionized in the K shell. This efficiency, usually termed the K yield, was calculated from a statistical count of ordinary K photoelectron tracks and Auger pair tracks observed in a Wilson expansion chamber. These experiments have been extended to xenon and krypton, and the present paper describes the measurement of the K yields of these two gases. Determinations of the K yields for elements of high atomic number with the cloud expansion chamber should be of especial value, as other methods which depend on the measurement of X-ray energy by ionization currents encounter considerable difficulties owing to the increasing importance of scattering with short wave-length X-rays. The need for new measurements for the heavier elements has been emphasized recently by the striking divergence between the results of two extensive series of measurements, one due to Berkey and the other to Arends, both of whom used an ionization method. Berkey found a well-defined maximum in the K yield-atomic number curve in the neighbourhood of atomic number 44 after which the K yield suddenly decreased with increasing atomic number, an effect which is in direct contradiction with the experiments of Arends and existing theory. In connexion with the latter, too, measurements of the K yields for elements of high atomic number are of importance as calculations made recently by Massey and Burhop show that a relativistic treatment of the Auger effect yields rather lower values of the K yield for the higher atomic numbers than was expected by Burhop from his non-relativistic treatment.

An Optical Method for Analysing Photographs of α—Ray Tracks

MR. L. F. CURTISS, writing in NATURE of April 6, describes a method for examining stereoscopic photographs of α-ray tracks taken by two cameras at right angles. The method which we have been using for some years for the measurement of the lengths and initial directions of emission of β—ray tracks (originally suggested to us by Prof. C. T. R. Wilson) depends on the same essential principle as that described by Mr. Curtiss, and our experience confirms his observation of its accuracy and convenience. We described the method in a paper on “The Ranges of Secondary β—rays” (Phil. Mag., 2, p. 1110; 1926) as follows: “The lengths of the tracks were obtained from the stereoscopic photographs by replacing the photographic plates in the cameras, illuminating them and tracing out the common image which coincides in space with the original track”. We have also used the same method in an examination of the initial directions of emission of photoelectron tracks (Proc. Roy. Soc., A, 121, p. 6l2; 1928). In the case of observations with β—rays, since the track is not in one plane, the use of the translucent screen (as described by Mr. Curtiss for α—rays) is not applicable.

Direction of Photo-Electron Emission

Direction of emission of photoelectrons from hydrogen, air and argon irradiated by Mo $\mathrm{K}\ensuremath{\alpha}$ x-rays.---Photographs of the photo-electron tracks in a Wilson expansion chamber through which a narrow beam of monochromatic Mo $\mathrm{K}\ensuremath{\alpha}$ x-rays were passed were taken with a stereoscopic camera. The direction of the paths with respect to the x-ray beam was determined from measurements made with a specially designed stereoscopic comparator. 445 tracks were studied of which 231 were in argon, 123 in air and 91 in hydrogen. Curves showing the frequency of occurrence of different angles of ejection have a fairly sharp maximum for angles of about 70\ifmmode^\circ\else\textdegree\fi{} with the forward direction of the x-ray beam. The curves are similar for the three gases studied except that the maximum in the case of hydrogen is somewhat sharper than for the other two gases.

Theory of the Number of Beta-rays Associated with Scattered X-rays

In a recent paper by Compton and Simon the ratio of the number of recoil electron tracks to that of photo-electron tracks has been found equal to the ratio of scattering coefficient $\ensuremath{\sigma}$ to true absorption coefficient $T$ for short wave-length x-rays, as predicted by Compton and Hubbard, but for long wave-lengths the experimental ratio is distinctly smaller. In the present paper a correction factor is applied to $\frac{\ensuremath{\sigma}}{T}$ by taking into account the motion and the binding energy of the scattering electron in its Bohr orbit and also the minimum energy which the recoil electron must have in order to produce a visible track in a Wilson cloud apparatus. Assuming a minimum energy of 630 volts for a recoil electron to produce a visible track, the correction factor for a primary wave-length of 0.71A is 0.36, thus making the theoretical value of the ratio of recoil to photo-electron tracks 0.097 as against an experimental value of 0.10. For other wave-lengths the agreement is equally good.

Direction of Ejection of Photo-Electrons by Polarized X-rays

Direction of ejection of photo-electrons by polarized x-rays.---Stereoscopic photographs were obtained, by Wilson's cloud expansion method, which show the ionized tracks of photo-electrons ejected by plane polarized x-rays. The polarized x-rays, scattered by a paraffin block at 90\ifmmode^\circ\else\textdegree\fi{} to an unpolarized primary beam of hard x-rays, were directed horizontally through the expansion chamber of a Wilson cloud apparatus in which they produced the photo-electrons. Exploded tungsten wires furnished the instantaneous illumination of the droplets. The photographs, taken with the plate at 90\ifmmode^\circ\else\textdegree\fi{} to the polarized beam, show two types of asymmetry in the direction of ejection of the photo-electrons. Lateral asymmetry. There is a strong concentration of photo-electrons ejected nearly in the direction of the electric vector of the plane polarized radiation performing the ejection. Longitudinal asymmetry. Stereoscopic examination of the photographs shows one-sixth of the photo-electrons ejected with a component opposite to the beam, one-third ejected approximately at right angles to the beam, and one-half ejected with a component along the beam. Theoretical interpretation according to the classical and quantum theories. The results are in accord with the classical theory. To explain them on the quantum theory we must assume that the quantum is a vector bundle of energy, for it explodes, so to speak, at right angles to its direction of motion.

The Vector Quantum

THE shift, recently discovered by A. H. Compton, in the spectral lines due to the scattering process, or rather the explanation of the shift proposed (Phys. Rev. p. 483, vol. 21, No. 5, May 1923) by Compton on the basis of the scattering of one quantum by one electron, furnishes considerable support for a corpuscular theory of radiation. On the basis of a corpuscular theory, an effect, recently discovered by the present writer (NATURE, September 8, 1923) takes on a new significance. The effect was discovered by taking photographs by Wilson's cloud expansion method, of the tracks of photo-electrons ejected by plane polarised X-rays. Such photographs bring out the fact that most of the photo-electrons are ejected nearly parallel to the electric force of the radiation. Variations in direction of ejection on all sides of the electric force may be accounted for by the initial momentum of the electron in its atomic orbit.