Detection and Spectrometry of Faint Light
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This was the first identification of an isotope. The mass spectrum of methane presented in Fig. Concerning mass resolution, a big step forward was the double focussing spectrograph of Aston, which is explained in Fig. This instrument also utilizes evaporation of probes inside the discharge tube. The canal rays produced in the right tube of the instrument pass through a small hole in the cathode and a narrow slit in front of the deflection plates, - the latter being the entrance slit of the spectrometer. After traversing the bending magnetic field, which is perpendicular to the electric field, the rays impact onto the photographic film.
This film can be turned away to allow visual observation of the canal-ray patch on a Willemit screen through the window F. The instrument of Aston compensated the wide energy dispersion in the electric field by the contrary dispersion in the magnetic field. Double focussing requires a certain arrangement of ion source, bending fields and detection device. The functional dependence of the spectrometer parameters has been derived, for instance, by Wien in his handbook .
With Aston's instrument, the mass spectrometry of canal rays had reached a culminating point. It seems that, up to , canal rays were plainly the general source of ions. Further improvements of mass resolution required ion sources with more homogeneous ion energies than canal rays can provide. A way out of this dilemma was to apply evaporation of the material of interest leading to a thermal energy spectrum and then to accelerate the thermally generated ions. One of the first spectrometers using such ion sources was that of Dempster .
He managed to use only magnetic bending. Goldstein had discovered canal rays by the light they emitted when travelling through gases and by the fluorescent patch they produced on the wall of the discharge tube. The study of this light indeed made a major contribution to the understanding of canal rays and how they reacted with matter. It provided information on the nature of the excited atoms.
An extremely important discovery made by Stark in  was that of the Doppler shift. In addition to the unshifted lines, broad stripes appeared in the direction of the longer wavelengths which corresponded to the atoms moving towards the observer see in Fig. The Doppler shift allowed the observer to differentiate moving canal ray ions and neutral particles from static gas atoms excited by canal rays.
Wien thus examined the question of whether the light was emitted by charged or uncharged atoms or molecules. In doing so, he studied the light decomposed in spectral lines and emitted from deflectable and non-deflectable rays and found, for example, that the Balmer series of hydrogen came from the neutral atoms and the spark-line of oxygen from the ions.
Wien carried out numerous other experiments on light emission; three shall be briefly described in the following section. Firstly, Wien  used the Doppler effect for measuring the absolute energy of light associated with a single spectral line emitted by canal rays. As it is sketched in Fig. This light was observed behind the cathode in direction of the canal rays and compared with the radiation of a black-body having the same intensity and the same wavelength. Applying the radiation laws, the integral radiation energy was calculated, which accounts for the number of emitted photons.
This leads to the number of exciting atomic collisions and subsequently to the cross section for excitation or emission, respectively, of the H b light. Regarding the atomic excitations by collisions as a statistical process, Wien developed a theory using the mean free path as the basic parameter. He also employed the concepts of mean free path and cross section to charge exchange processes, i.
Such processes were indicated by the fact that a certain fraction of the canal rays experienced a smaller deflection than pure ion beams. Wien noticed charge exchange first, when canal rays passed through two subsequent regions with magnetic fields: the first field did not influence the neutral component of the rays. This component, however, turned out to be partially charged, when passing through the second field. Obviously, originally neutral particles were ionized on their way between the two magnets.
He recognized that charge exchange depended strongly on the pressure in the observation tube and that also negatively charged particles were formed. It was used, for instance, to fill the two tubes with different gases or to remove water vapor from the canal rays. In order to prove his theory of collision statistics, Wien built the apparatus shown in Fig.
Behind the cathode, the canal rays fly first through the capillary separating discharge and observation area and pass then a series of deflection plates, which remove successively the charged particles from the canal rays. Loading the condensers one after the other, the relative number of particles moving in the straight canal-ray beam was measured by means of a thermocouple T. Thus, it was possible to determine the ratio of charged to neutral particles behind the last loaded condenser.
Provided that the velocity of the canal rays remains constant over the series of condensers, this ratio should be independent of the number of loaded condensers. It turned out that reliable results were obtainable only with canal rays being generated in very pure gases with a very narrow velocity distribution. By this method, Wien determined, for instance, the mean free path of hydrogen atoms for charge exchange in 0. He got a value of 0. Such results were compared with radii deduced from Bohr's atom model.
Stark performed numerous optical experiments on canal rays. One of his peculiar findings was that the light of some spectral lines is observable beyond the sharp border of a canal-ray bundle. His interpretation of this phenomenon was that due to thermal motion some gas atoms escape from the bundle still emitting light with decreasing intensity.
This did not happen to all spectral lines. An example for decreasing light emission are the lines of the Balmer series. Wien  tried to measure the decay constant for these Balmer lines. He let the canal rays penetrate the observation tube through a short, little capillary. The observation tube was evacuated to the lowest possible pressure; this means only a few atomic collisions occurred in the expanding plume. The canal ray bundle was then visible only over a short distance as seen in Fig. The light of this short gleaming strip was split into spectral lines and then used to make a photograph of the strip.
A wedge-shaped absorber in front of this slit had weakened the light exponentially over the length of the slit. By tuning the intensity and the exponential decrease of the comparative light, Wien was able to determine the decay constant of the light emitted from the atoms streaming with a certain velocity into the observation tube. The velocity was measured with help of the Doppler effect.
For the light of the Balmer series he obtained a decay constant of 6. This corresponds to In , Stark  made another important discovery concerning light emission of canal rays: he found that many spectral lines - in particular those of hydrogen - split, when the light emitting gas is located in a static electric field. As an example, Fig. Two series of polarized lines are seen, - one polarized parallel P to the field direction, the second one perpendicular S. One year after Stark's discovery, Wien  tried to prove if the splitting of spectral lines observed in static electric fields occurs in the same way also, when the light emitting atoms move in a magnetic field.
The corresponding splitting was compared by Wien with the splitting caused in a static Coulomb field. The canal rays used by him had a velocity of 0. Therefore, Wien could compare his splitting with results obtained by Stark. The canal rays generated in the discharge tube R fly through the capillary C fixed between the poles of an electromagnet.
The canal-ray light emerging from a slit in the capillary is examined through a central hole drilled in one of the magnet poles by means of an optical spectrograph. This means, the direction of observation is perpendicular to the velocity of the canal rays and parallel to the magnetic field and therefore transversal to the electrodynamic field as in case of the experiments performed with the electrostatic field.
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In order to allow observation of the two differently polarized components, the light passes a lime spar. Wien investigates the H g and the H b lines of hydrogen. The splitting was in fact hardly visible, - probably due to the broad velocity distribution of the hydrogen atoms, - but the width of the patch agreed well with values published by Stark, who applied a static electric field, and also with theoretically expected values. Two years later , Wien succeeded to observe also the actual splitting of the component, which is polarized perpendicularly to the field.
Two splitted lines are seen in Fig. This was one of Wien's most beautiful and smartest experiments. Polished metals became rough after the tube had been in use for a certain period. This erosion of the cathode surface has an important effect on the composition of the canal rays, as the sputtered material mixes itself with the rays. Canal rays in the observation area also cause a sputtering of the material they collide with.
The amount eroded could be calculated by weighing the cathode, for example. Wien did not carry out any experiments on sputtering himself. The various experiments all showed that the sputtered amount is proportional to the voltage of the cathode fall, i. A dependence such as this corresponds to the sputtering energy dependency observed today of metals in the keV range.
Less explicit was the dependency of the atomic numbers and masses of the ions and irradiated metals. A comparison with yields won through application of our contemporary sputtering theory shows that the yields measured then were considerably more dependent on atomic numbers and masses than the sputtering theory would expect. As Wien observed in his first experiments, slow positive ions are emitted by the anode, which are presumably also produced through sputtering with negative ions or electrons which reach the anode from the cathode. This effect was used to produce canal rays of substances which were otherwise difficult to produce in the discharge tube in gas form.
Alkaline compounds deposited on the anode are particularly effective. Secondary ion mass spectroscopy SIMS was first practiced using an ion source such as this. These canal rays, made up of slow particles coming from the anode see Fig. The work of Wien and his assistants has largely contributed to explaining the nature of canal rays and their cause electrical discharge in rarefied gases.
Wien's experiments always focussed more on their physical aspects and less on the practical application of the information gained through them. This is also expressed in the often very detailed theoretical analyses of his experiments. The diagram of canal rays and their origin Wien sketched out in his handbook on canal rays ; it is not, however, the result of a "gas discharge theory".
Only certain details of these rays, such as their charge exchange, light emission or the effect of electric and magnetic fields on them, are dealt with theoretically. XXII, from , among other sources. Wien developed the following ideas on the nature of canal rays in his book : "The place canal rays are formed is what is known as the dark cathode space, where the largest potential difference can be found".
This is where ionization and excitation largely take place by way of electron collision and acceleration of the ionized gas atoms. Negative cathode rays, which move in the opposite direction of the actual canal rays, are chiefly produced as secondary ions at the cathode. They can also produce positive ions, i. These are joined by photon-induced ionization processes which are responsible for the "Nebel Strahlen" fog rays , for example, which clad the actual canal rays. The canal rays behind the cathode mainly consist of neutral, partially-excited atoms or molecules which are formed in front of the cathode or behind it by charge exchange from ions.
The gas density in the canal ray is usually so high that charge exchange also occurs behind the cathode. This charge exchange and the wide space in front of the cathode, where ions are accelerated, give rise to a broad spectrum of canal ray particle velocity. From these characteristics of the canal rays we can deduce that they are not pure ion beams. In order to become so, they must be passed into a vacuum of under 10 -4 mm Hg and separated from the neutral particles. Ion sources with currents of ca. By way of post-acceleration, ion energies were obtained which were sufficient for the first nuclear reaction experiments.
As a rule, modern ion sources no longer use canal rays directly, but make use of processes which contribute to these in discharge tubes, such as ionization by electron impact, sputtering, secondary ion emission and ionization through photo-effect. From this, we can conclude that Wien first understood the canal rays to be inert, electrically-charged, atomic particles a hundred years ago were in fact the origin of ion beams.
Henning Dwinger, my aunt and my cousin, for the conversations I had with them about their father and grandfather in Munich, where Willy Wien last worked as a professor of Physics. They provided me with a great deal of literature about him, and also with photographs and offprints of his scientific articles. Further material and also an original canal ray apparatus were available at the Deutsches Museum in Munich, whom I would also like to thank for their assistance.
Goldstein, Sitzungsbericht der Berl. Juli ; Wied. Marx, Leipzig Akad. Goldstein, Verh. Wien, Verh. Wien, Ann. Physik 65 , Thomson, Phil. Harms, Wien. Thomson, Rays of positive electricity and their application to chemical analyses , Monographs on Physics, Longmans, London, 1st ed.
April Hammer, Phys. Dempster, Phys. III, No.
Aston, Phil. Stark, Phys. Stark in: Atomionen der Chem. Elemente , Springerverlag, Berlin Stark, Ann. Stark und G. Wendt, Ann. Wien, Berliner Berichte 22 Reichenheim, Bericht d. Reichenheim, Ann. Services on Demand Journal. Figure 2. Figure 3. Figure 4. Gas discharge tube with a mesh as the cathode behind which canal rays can be observed. The diagram is taken from an article written by Wien . The lower section of the tube up to the level of the cathode was placed in a grounded tin box.
Figure 5. The fields are perpendicular to one another. The canal rays pass from tube b through a hole in the iron screen aa into tube C.
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The electric field between the plates aa is parallel to the magnetic field between the poles N and S. The back wall of the observation tube is within the magnetic field. The equations beside Fig. Both diagrams are taken from articles written by Wien [6, 2]. Figure 6. The lower section of the tube up to the level of the anode was placed in a grounded tin box.
Figure 7. Sketch of a parabola-image spectrograph constructed by Thomson for canal rays in K cathode, F capillary between discharge and observation tube, S observation screen, AA electric deflection plates, NS magnet, P iron shielding. At the left side of the apparatus, the pattern of the bright patch on S is sketched. Number 1 corresponds to the spot of atomic hydrogen, number 2 to that of molecular hydrogen. Both drawings are taken from the handbook of Radiology .
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Figure 8. The trajectory of canal rays in parallel electric and magnetic fields. The rays belong to ions of constant mass. The bended rays form a parabolic fluorescent strip at the observation screen. The formula of the deflections y e and y m by the electric and magnetic fields, respectively, have been taken from ref. Figure 9. Part of the first time-of-flight spectrometer constructed by Hammer  for canal rays. The drawing has been taken from the handbook of Wien . Figure Left: discharge and observation tubes contain air p E and p B are the corresponding pressures in mm Hg.
Right: the filling is CO 2. Another example of canal ray spots taken by Retschinsky  with oxygen filling. Parabola-image spectrograph equipped with a glowing filament K for evaporation. The apparatus has been constructed by Dempster . The ions are detected by means of a Faraday cup F behind a slit Sp. The poles of the magnet S and N are also the electric deflection plates. M is an iron shielding. Current of the Faraday cup as a function of the magnetic field strenght. The curve was measured with the spectrograph shown in Fig. A mass spectrum of positive canal rays of methane measured with an parabola-image spectrograph.
The photography has been made by Conrad see Thomson's book . Mass spectrograph of Aston . The upper part of the figure shows the ion trajectories in the electric field between the deflection plates C and in the magnetic field M being perpendicular to the electric field. Theoretical calculations concerning the ion optics of this arrangement can be found in the handbook of Wien . Each place where the radiation is counted is called a pixel picture element , and modern detectors can count the photons in millions of pixels megapixels, or MPs.
Among these are many small moons around the outer planets, icy dwarf planets beyond Pluto, and dwarf galaxies of stars. CCDs also provide more accurate measurements of the brightness of astronomical objects than photography, and their output is digital—in the form of numbers that can go directly into a computer for analysis. Observing the universe in the infrared band of the spectrum presents some additional challenges. Figure 2. Infrared Eyes: Infrared waves can penetrate places in the universe from which light is blocked, as shown in this infrared image where the plastic bag blocks visible light but not infrared.
Hurt SSC. To solve this problem, astronomers must protect the infrared detector from nearby radiation, just as you would shield photographic film from bright daylight. Since anything warm radiates infrared energy, the detector must be isolated in very cold surroundings; often, it is held near absolute zero 1 to 3 K by immersing it in liquid helium.
The second step is to reduce the radiation emitted by the telescope structure and optics, and to block this heat from reaching the infrared detector. To infrared eyes, everything on Earth is brightly aglow—including the telescope and camera Figure 2.
Detection and Spectrometry of Faint Light - John Meaburn - Google книги
The challenge is to detect faint cosmic sources against this sea of infrared light. Another way to look at this is that an astronomer using infrared must always contend with the situation that a visible-light observer would face if working in broad daylight with a telescope and optics lined with bright fluorescent lights. More than half of the time spent on most large telescopes is used for spectroscopy. The many different wavelengths present in light can be separated by passing them through a spectrometer to form a spectrum. The design of a simple spectrometer is illustrated in Figure 3.
Light from the source actually, the image of a source produced by the telescope enters the instrument through a small hole or narrow slit, and is collimated made into a beam of parallel rays by a lens. The light then passes through a prism, producing a spectrum: different wavelengths leave the prism in different directions because each wavelength is bent by a different amount when it enters and leaves the prism.
A second lens placed behind the prism focuses the many different images of the slit or entrance hole onto a CCD or other detecting device. This collection of images spread out by color is the spectrum that astronomers can then analyze at a later point. As spectroscopy spreads the light out into more and more collecting bins, fewer photons go into each bin, so either a larger telescope is needed or the integration time must be greatly increased—usually both.
Figure 3. Prism Spectrometer: The light from the telescope is focused on a slit. A prism or grating disperses the light into a spectrum, which is then photographed or recorded electronically. In practice, astronomers today are more likely to use a different device, called a grating , to disperse the spectrum.
High Performance Raman Spectroscopy
A grating is a piece of material with thousands of grooves on its surface. While it functions completely differently, a grating, like a prism, also spreads light out into a spectrum. Visible-light detectors include the human eye, photographic film, and charge-coupled devices CCDs.