High-Energy Astronomy (Astronomy In Space)

We now discuss the techniques and instruments used by astronomers observing at wavelengths shortward of 91.2 nm. Temperatures in excess of 105K, or non-thermal mechanisms, are required if significant emission is to occur at these wavelengths, and thus we may consider ourselves dealing with high-energy astronomy. Extreme ultraviolet (100 —10nm), soft X-ray (10 —1nm), X-ray (1—0.01nm), soft gamma-ray (0.01—0.001 nm) and gamma-ray (below 0.001nm) radiations can only be directly detected from above the bulk of our atmosphere. High-energy radiations are completely absorbed by even a small depth of air, and indeed it is this ionizing effect of the solar extreme ultraviolet and X-radiation that creates the ionosphere around the Earth. A few parsecs of interstellar hydrogen are opaque at wavelengths just shortward of 91.2nm and so observations even from above the atmosphere are fairly hopeless. The absorption is roughly proportional to the third power of the wavelength, and thus reasonable distances (~ 100pc) may be probed at wavelengths of 102nm. Even shorter wavelength X-rays and gamma rays can penetrate through most of the Universe unimpeded.

The Sun was the first celestial object detected by its short wave¬length radiation, by exposure of a suitably filtered photographic plate on a US-launched V2 rocket in 1947. The first cosmic X-ray source, Scorpius X-l, was discovered during a flight intended to detect fluorescent X-rays from the Moon in 1962. Fluorescent X-rays produced from solar X-rays interacting writh lunar material were detected from several Apollo command modules when orbiting the Moon, but they are still undetectable from Earth. The Crab Nebula pulsar has been observed at all wavelengths up to 10-5nm.

It is convenient in high-energy astronomy to speak in terms of photon energies, and we henceforth use electronvolts (eV) as a measure of these energies. One kiloelectron volt (keV) is equivalent to L24nm, and the photon-energy scales inversely with wave¬length. Cosmic X-radiation is thus typically observed between 0.1 and 100keV, and gamma rays above 100keV.

To understand the mechanism of detectors of high-energy radiation we must first consider the interaction of such radiation with matter. High-energy photons have sufficient energy to ionize atoms, and thus an important absorption process is that due to photoelectric ionization. On encountering an atom, the photon can be absorbed by ejection of an electron, often from the inner shells of the atom. Another electron rapidly fills in the gap left by the ejected electron and fluorescent radiation is emitted. In lighter elements, another electron (Auger electron) can be ejected as the atom de-excites itself, and no fluorescent radiation need be emitted. The net effect is either two electrons, or one electron and a photon, the total energies of which equal that of the initial photon. Photo¬electric interactions are most effective at low photon energies and in high atomic weight materials. The photons may also scatter off the electrons in the material, giving them energy by the Compton effect. At high energies this is a more important energy loss than photoelectric absorption. If the photon energy exceeds twice that of the rest-mass energy of an electron, electron-positron pair production can dominate (in the presence of nuclei). In all these processes, the energy of the incoming photon is converted to kinetic energy of electrons or electrons plus photons (which may themselves be further absorbed).

The fast moving primary electrons (or positrons) produced by the photon-matter interaction can then be detected by means familiar to high-energy physicists in their studies of nuclear particles and cosmic rays. The electrons stream through matter leaving a characteristic trail of ionization in their wake. This might further be detected by the light then emitted or by electrostatic means. In both cases the material may be either a solid, liquid or gas, but a solid or gas is preferred for space applications. The capability of any detector to detect radiation in the required range will depend upon how effective it is in stopping the photon. Solids are of course most effective, but lower energy X-rays are stopped very (dose to their surfaces. This is only useful in SOLID-STATE, DETECTORS, which are made out of semi-conductors such as germanium or silicon and are unlikely to be very large. In most other solids, the primary electrons produced by an incoming low-energy photon produce so few ions in being stopped that the capability of determining the incoming photon energy is severely degraded. A primary electron in a gas, on the other hand, produces many ions and the statistical variations in the number of ions finally produced (after the primary electron has created secondary electrons and so on until the individual electron energies are insufficient for further ionization) is proportional to the energy of the incident photon. The scatter in that number then determines the energy resolution of the detector.

Gas-filled detectors are most widely used in X-ray astronomy. The efficiency at high-photon energies is determined by the gas depth, and at low energies by the thickness of the window needed to retain the gas. Such windows are commonly made of thin sheets of organic plastics (perhaps 1 ?m or so in thickness). A continuous supply of gas may be required to compensate for that which diffuses through this window. The ionization produced in the chamber is detected by an electric field, usually applied between the body of the chamber, and a thin conducting anode running down the centre. The electric field then rapidly increases close to the anode, and electrons attracted there gain sufficient kinetic energy to produce further ionization. This results in amplification of the initial number of electrons. The ions are swept back out to the chamber walls thereby creating a detectable electronic signal. If the voltage applied is in a certain range, the resulting electrical impulse is directly proportional in amplitude to the initial photon energy, provided that no fluorescent radiation escaped. An ionization chamber operated in this manner is known as a PROPORTIONAL COUNTER. At higher voltages, immediate electrical breakdown occurs whenever a particle ionizes the gas, and the device becomes a GEIGER COUNTER. A proportional counter has an energy resolution 20 per cent of that of a typical X-ray photon, but Geiger counters have no such capability. Resistive anodes, which may be constructed as flat plates, together with some sophisticated electronic circuitry may be used to determine the position at which the incoming photon produced the ionization. This is of great value if placed at the focus of an X-ray telescope.

Inorganic SCINTILLATION COUNTERS rely on detection of the light released when secondary electrons are captured by impurities in the crystals. Materials such as sodium iodide doped with a suitable impurity may be employed. The crystal must be transparent to the light flashes produced, which usually occurs in the visible wave¬length; they are detected by conventional photo tubes. Gamma rays may be suitably studied by systems using Compton interactions and high-energy gamma rays can be detected by SPARK CHAMBERS, such as carried on the European satellite COS-B . Here the photon is energetic enough to interact in a thin lead sheet producing fast electrons which then pass through several parallel layers of thin, electrically charged, conducting plates. The result¬ing ionization in the intervening gas causes breakdown between these plates. Ionization tracks can then be observed by the light or sound produced, or by electronic means if the plates are composed of fine wire meshes.

The number of cosmic photons incident on any satellite-sized detectors at energies exceeding a few hundred MeV is exceedingly small, and may be as low as a few per day. All known celestial bodies emit fewer and fewer photons as the energy increases. This might mean that the prospects for detecting photons of energies 1GeV and higher is small. However, it then becomes possible to use the atmosphere itself as the detector. The high-energy photons produce very energetic particles at the top of the atmosphere. These move at nearly the speed of light through the air and emit Cerenkov radiation in a cone about their direction of motion. This radiation is detectable on the ground. A light collector with a field of view of, say, 5° can view a region 3km square of the atmosphere at a height of 30km. Significant results have been derived from such methods.

Having discussed the detection of high-energy X-radiation, we must investigate the means for COLLIMATING this radiation. By this we mean a device capable of restricting the field of view of the detector, or better still, forming an image. Simple X-ray proportional counters may make use of thin slats of material placed in front of the window. These MECHANICAL COLLIMATORS can be constructed to give a field of view down to fractions of a degree, but become massive and unwieldy, especially at higher energies, if finer resolution is required over a large detector. The X-ray satellite UHURU made use of such collimators . MODULATION COLLIMATORS code the spatial information present in the incoming photon directions into a pattern that varies with time. Two wire grids, one above the other may either be rotated at a steady rate or oscillated ,in separation. The moire fringes formed by these grids will vary regularly and regions in the sky will be alternately visible and then invisible according to a known pattern. This pattern depends upon the source position and becomes impressed in the count rate from a cosmic X-ray source. The data can then be searched for such patterns. Determination of track positions in Compton telescopes or spark chambers yields source positions to a precision of a degree or less.

Conventional imaging telescopes are useless at X-ray or gamma-ray wavelengths owing to the ease with which these radiations are absorbed. Total internal reflection can, however, allow X-ray telescopes to be constructed for use up to a few keV. The refractive index of X-rays in materials is less than unity, and thus X-radiation incident on a metallic surface is totally internally reflected if the angle of incidence exceeds the critical angle (typically 87° or more). Such reflection is referred to as glancing angle or GRAZING-INCIDENCE REFLECTION. Grazing-incidence telescopes can be constructed to give an image if two reflecting elements are used in any of three standard ways. The paraboloidal elements are essentially made of the outer rim of a very deep ordinary paraboloidal mirror such as might be used in optical astronomy. The centre part is useless owing to the grazing incidence requirement. The total effective collection area is small compared with the total aperture, but nesting of several telescopes inside each other can increase this factor. The reflecting elements may also be assembled in a slat-like manner. Images from grazing-incidence telescopes enable astronomers to map the X-ray sky up to wavelengths of a few keV. Beyond that, collimation systems must be used. FRESNEL ZONE PLATES, making use of the properties of diffraction through masks of alternately opaque and transparent material can be used in the formation of images, but their application is limited by their extremely small size. Only the Sun produces a sufficient quantity of photons to make their use worthwhile.

Grazing-incidence telescopes have been used to observe the Sun from Skylab and three small non-imaging X-ray telescopes were carried on Copernicus and several later satellites. The second US High-Energy Astronomical Observatory (HE A O -B), will probably carry four nested telescopes with “a maximum diameter of 55 cm. The total effective area of these telescopes varies according to the energy of the incident X-rays but is on average several hundred square centimetres below 1keV. The angular resolution of such a telescope would be limited by diffraction to less than a milliarcsecond. but of course this cannot be achieved. Scattering from the surface of the mirrors will limit the resolution to a few arc seconds. The focal length of the telescope will be about 3.4m. Detectors are thus required to be capable of resolving X-rays incident less than one thirtieth of a millimetre apart. Micro-channel plates producing electron cascades on impact by photons will achieve such resolution. A position-sensitive proportional counter will give one arc minute resolution over a field one degree square.

Spectroscopy of X- and gamma-ray sources is of interest, since {atomic lines are expected to occur in the range up to 10 keV or so, J and nuclear lines may occur up to a few MeV. Crude spectral resolution is achieved from proportional counters, scintillation detectors and spark chambers. This has been sufficient to resolve a few atomic lines, but much finer resolution is desirable. Diffraction of an X-ray beam by a regularly-spaced array of scattering elements will suffice, and transmission gratings have been constructed with 100 lines per millimetre. The atoms of a crystal also satisfy our requirements. Constructive reflection from the surface of a crystal occurs at specific angles for certain energy ranges. CRYSTAL SPECTROMETERS have been produced and operated on this principle. A major drawback of this sort of spectrometry is its low efficiency. Crystal reflection is polarization dependent and this is made use of in the design of X-RAY POLARIMETERS. Little can be done to improve the resolution of detectors beyond 10keV or so, and the results rest mostly on the inherent capabilities of the applicable detectors

With the conclusion of the program to place man on the Moon the thrust of space astronomy may have appeared to lose direction slightly. This is not true however. Plans are clearly laid down for the continuation of astronomy in space for many years to come. International cooperation is enabling the necessary funding to be raised. What will happen after the Space Telescope? That instrument will definitely find new types of cosmic object – it can scarcely fail since it will see so much more than Earth-based instruments. The long-term aim must be to get a large observatory working as a permanent station on the Moon, probably on the far side. The advantages of such an observatory are so great that it will surely be constructed eventually, probably by many nations pooling their resources.

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