X-ray astronomy detectors have been designed and configured primarily for energy and occasionally for wavelength detection using a variety of techniques usually limited to the technology of the time.
X-rays span 3 decades in wavelength (~8nm - 8 pm), frequency (~50 PHz - 50 EHz) and energy (~0.12 - 120 keV). In terms of temperature, 1 eV = 11,604 K. Thus X-rays (0.12 to 120 keV) correspond to 1.39 × 106 to 1.39 × 109 K. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft X-rays, and from 0.1nm to 0.01nm (about 12 to 120 keV) as hard X-rays.
Closer to the visible range of the electromagnetic spectrum is the ultraviolet. The draft ISO standard on determining solar irradiances (ISO-DIS-21348)[1] describes the ultraviolet as ranging from ~10nm to ~400nm. That portion closest to X-rays is often referred to as the "extreme ultraviolet" (EUV or XUV). When an EUV photon is absorbed, photoelectrons and secondary electrons are generated by ionization, much like what happens when X-rays or electron beams are absorbed by matter.[2]
The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubes had a longer wavelength than the radiation emitted by radioactivenuclei (gamma rays).[3] So older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m, defined as gamma rays.[4] However, as shorter wavelength continuous spectrum "X-ray" sources such as linear accelerators and longer wavelength "gamma ray" emitters were discovered, the wavelength bands largely overlapped. The two types of radiation are now usually distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.[3][5][6][7]
Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ), can penetrate the air at least for distances of a few meters, the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface (they would have been detected and medical X-ray machines would not work if this was not the case). X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; 90% of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10cm of air.
To detect X-rays from the sky, X-ray detectors must be flown above most of the Earth's atmosphere. There are three main methods of doing so: sounding rocket flights, balloons, and satellites.
An individual medium energy proportional counter on EXOSAT had a front window of beryllium with aluminized kapton foil for thermal protection, a front chamber filled with an argon/CO2 mixture, a rear chamber with xenon/CO2, and a beryllium window separating the two chambers.[9] The argon portion of the detector was optimized for 2-6 keV and the total energy ranges for both detectors was 1.5-15 keV and 5-50 keV, respectively.
The US portion of the Apollo–Soyuz mission (July 1975) carried a proportional counter system sensitive to 0.18-0.28 and 0.6-10.0 keV X-rays. The total effective area was 0.1 m2, and there was a 4.5° FWHM circular FOV.
The French TOURNESOL instrument consisted of four proportional counters and two optical detectors. The proportional counters detected photons between 2keV and 20MeV in a 6° × 6° FOV. The visible detectors had a field of view of 5° × 5°. The instrument was designed to look for optical counterparts of high-energy burst sources, as well as performing spectral analysis of the high-energy events.[10]
X-ray monitor
Monitoring generally means to be aware of the state of a system. A device that displays or sends a signal for displaying X-ray output from an X-ray generating source so as to be aware of the state of the source is referred to as an X-ray monitor in space applications. On Apollo 15 in orbit above the Moon, for example, an X-ray monitor was used to follow the possible variation in solar X-ray intensity and spectral shape while mapping the lunar surface with respect to its chemical composition due to the production of secondary X-rays.[11]
The X-ray monitor of Solwind, designated NRL-608 or XMON, was a collaboration between the Naval Research Laboratory and Los Alamos National Laboratory. The monitor consisted of 2 collimated argon proportional counters. The instrument bandwidth of 3-10 keV was defined by the detector window absorption (the window was 0.254mm beryllium) and the upper level discriminator. The active gas volume (P-10 mixture) was 2.54cm deep, providing good efficiency up to 10 keV. Counts were recorded in 2 energy channels. Slat collimators defined a FOV of 3° × 30° (FWHM) for each detector; the long axes of the FOVs were perpendicular to each other. The long axes were inclined 45° to the scan direction, allowing localization of transient events to about 1°. The centers of the FOVs coincided, and were pointed 40° below the scan equator of the wheel in order to avoid scanning across the Sun. The spacecraft wheel rotated once every 6 seconds. This scan rate corresponds to 1° every 16 milliseconds (ms); counts were telemetered in 64 or 32 ms bins to minimize smearing the collimator response.
The instrument parameters and data yield implied a 3 σ point source sensitivity of 30 UFU in one day's operation (1 UFU = 2.66−12 erg/cm2-s-keV). Each detector was about 0.1 of the area of the Uhuru instrument. The instrument background at low geomagnetic latitudes was ~16 counts/s. Of this background, ~6 counts/s comes from the diffuse cosmic X-ray background, with the rest being instrumental. Assuming a conservative 10% data return, the net source duty cycle in scanning mode was 1.4 × 10−3, implying a source exposure of 120 seconds per day. For a background of 16 counts/s, the 3 σ error in determining the flux from a given sky bin was then 4.5 counts/s, or about 45 UFU, after 1 day. A limiting sensitivity of 30 UFU was obtained by combining both detectors. A comparable error existed in the flux determination for moderately bright galactic sources. Source confusion due to the 5° FOV projected along the scan direction complicated the observation of sources in the galactic bulge region (approximately 30° > l > -30°, |b| < 10°).
A scintillator is a material which exhibits the property of luminescence[12] when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, such as an X-ray photon, absorb its energy and scintillate, i.e. reemit the absorbed energy in the form of a small flash of light, typically in the visible range.
Scintillation crystal surrounded by various scintillation detector assemblies
The scintillation X-ray detector (XC) aboard Vela 5A and its twin Vela 5B consisted of two 1mm thick NaI(Tl) crystals mounted on photomultiplier tubes and covered by a 0.13mm thick beryllium window. Electronic thresholds provided two energy channels, 3-12 keV and 6-12 keV.[13] In front of each crystal was a slat collimator providing a full width at half maximum (FWHM) aperture of ~6.1 × 6.1°. The effective detector area was ~26cm2. Sensitivity to celestial sources was severely limited by the high intrinsic detector background.
The X-ray telescope onboard OSO 4 consisted of a single thin NaI(Tl) scintillation crystal plus phototube assembly enclosed in a CsI(Tl) anti-coincidence shield. The energy resolution was 45% at 30 keV. The instrument operated from ~8 to 200 keV with 6 channel resolution.
OSO 5 carried a CsI crystal scintillator. The central crystal was 0.635cm thick, had a sensitive area of 70cm2, and was viewed from behind by a pair of photomultiplier tubes. The shield crystal had a wall thickness of 4.4cm and was viewed by 4 photomultipliers. The field of view was ~40°. The energy range covered was 14-254 keV. There were 9 energy channels: the first covering 14-28 keV and the others equally spaced from 28 to 254 keV. In-flight calibration was done with an 241Am source.
The PHEBUS experiment recorded high energy transient events in the range 100keV to 100MeV. It consisted of two independent detectors and their associated electronics. Each detector consisted of a bismuth germinate (BGO) crystal 78mm in diameter by 120mm thick, surrounded by a plastic anti-coincidence jacket. The two detectors were arranged on the spacecraft so as to observe 4πsteradians. The burst mode was triggered when the count rate in the 0.1 to 1.5MeV energy range exceeded the background level by 8 σ (standard deviations) in either 0.25 or 1.0 seconds. There were 116 channels over the energy range.[10]
The KONUS-B instrument consisted of seven detectors distributed around the spacecraft that responded to photons of 10keV to 8MeV energy. They consisted of NaI(Tl) scintillator crystals 200mm in diameter by 50mm thick behind a Be entrance window. The side surfaces were protected by a 5mm thick lead layer. The burst detection threshold was 5×10-7 to 5×10-8 ergs/cm2, depending on the burst spectrum and rise time. Spectra were taken in two 31-channel pulse height analyzers (PHAs), of which the first eight were measured with 1/16s time resolution and the remaining with variable time resolutions depending on the count rate. The range of resolutions covered 0.25 to 8s.
Kvant-1 carried the HEXE, or High Energy X-ray Experiment, which employed a phoswich of sodium iodide and caesium iodide. It covered the energy range 15-200 keV with a 1.6° × 1.6° FOV FWHM. Each of the 4 identical detectors had a geometric area of 200cm2. The maximum time resolution was 0.3-25 ms.
Modulation collimator
In electronics, modulation is the process of varying one waveform in relation to another waveform. With a 'modulation collimator' the amplitude (intensity) of the incoming X-rays is reduced by the presence of two or more 'diffraction gratings' of parallel wires that block or greatly reduce that portion of the signal incident upon the wires.
An X-ray collimator is a device that filters a stream of X-rays so that only those traveling parallel to a specified direction are allowed through.
Minoru Oda, President of Tokyo University of Information Sciences, invented the modulation collimator, first used to identify the counterpart of Sco X-1 in 1966, which led to the most accurate positions for X-ray sources available, prior to the launch of X-ray imaging telescopes.[14]
SAS 3 carried modulation collimators (2-11 keV) and Slat and Tube collimators (1 up to 60keV).[15]
On board the Granat International Astrophysical Observatory were four WATCH instruments that could localize bright sources in the 6 to 180 keV range to within 0.5° using a Rotation Modulation Collimator. Taken together, the instruments' three fields of view covered approximately 75% of the sky. The energy resolution was 30% FWHM at 60keV. During quiet periods, count rates in two energy bands (6 to 15 and 15 to 180keV) were accumulated for 4, 8, or 16 seconds, depending on onboard computer memory availability. During a burst or transient event, count rates were accumulated with a time resolution of 1 s per 36 s.[10]
OSO 8 had on board a Graphite Crystal X-ray Spectrometer, with energy range of 2-8 keV, FOV 3°.
The GranatART-S X-ray spectrometer covered the energy range 3 to 100keV, FOV 2° × 2°. The instrument consisted of four detectors based on spectroscopic MWPCs, making an effective area of 2,400cm2 at 10keV and 800cm2 at 100keV. The time resolution was 200 microseconds.[10]
The X-ray spectrometer aboard ISEE-3 was designed to study both solar flares and cosmic gamma-ray bursts over the energy range 5-228 keV. The detector provided full-time coverage, 3π FOV for E > 130 keV, time resolution of 0.25 ms, and absolute timing to within 1 ms. It was intended to be a part of a long-baseline interferometry network of widely separated spacecraft. The efforts were aimed primarily at determining the origin of the bursts through precise directional information established by such a network. The experiment consisted of 2 cylindrical X-ray detectors: a Xenon filled proportional counter covering 5-14 keV, and a NaI(Tl) scintillator covering 12-1250 keV. The proportional counter was 1.27cm in diameter and was filled with a mixture of 97% Xenon and 3% carbon dioxide. The central part of the counter body was made of 0.51mm thick beryllium and served as the X-ray entrance window. The scintillator consisted of a 1.0cm thick cylindrical shell of NaI(Tl) crystal surrounded on all sides by 0.3cm thick plastic scintillator. The central region, 4.1cm in diameter, was filled by a quartz light pipe. The whole assembly was enclosed (except for one end) in a 0.1cm thick beryllium container. The energy channel resolution and timing resolution could be selected by commands sent to the spacecraft. The proportional counter could have up to 9 channels with 0.5 s resolution; the NaI scintillator could have up to 16 channels and 0.00025 s resolution.
CCDs
Most existing X-ray telescopes use CCD detectors, similar to those in visible-light cameras. In visible-light, a single photon can produce a single electron of charge in a pixel, and an image is built up by accumulating many such charges from many photons during the exposure time. When an X-ray photon hits a CCD, it produces enough charge (hundreds to thousands of electrons, proportional to its energy) that the individual X-rays have their energies measured on read-out.
Microcalorimeters
Microcalorimeters can only detect X-rays one photon at a time (but can measure the energy of each).
Transition edge sensors
TES devices are the next step in microcalorimetry. In essence they are super-conducting metals kept as close as possible to their transition temperature. This is the temperature at which these metals become super-conductors and their resistance drops to zero. These transition temperatures are usually just a few degrees above absolute zero (usually less than 10 K).
X-ray fluorescence (XRF) is the emission of characteristic "secondary" X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in geochemistry, forensic science, archaeology and art objects such as paintings.
BeppoSAX was an Italian–Dutch satellite for X-ray astronomy which played a crucial role in resolving the origin of gamma-ray bursts (GRBs), the most energetic events known in the universe. It was the first X-ray mission capable of simultaneously observing targets over more than 3 decades of energy, from 0.1 to 300 kiloelectronvolts (keV) with relatively large area, good energy resolution and imaging capabilities. BeppoSAX was a major programme of the Italian Space Agency (ASI) with the participation of the Netherlands Agency for Aerospace Programmes (NIVR). The prime contractor for the space segment was Alenia while Nuova Telespazio led the development of the ground segment. Most of the scientific instruments were developed by the Italian National Research Council (CNR) while the Wide Field Cameras were developed by the Netherlands Institute for Space Research (SRON) and the LECS was developed by the astrophysics division of the European Space Agency's ESTEC facility.
A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate. Sometimes, the excited state is metastable, so the relaxation back down from the excited state to lower states is delayed. The process then corresponds to one of two phenomena: delayed fluorescence or phosphorescence. The correspondence depends on the type of transition and hence the wavelength of the emitted optical photon.
The Compton Gamma Ray Observatory (CGRO) was a space observatory detecting photons with energies from 20 keV to 30 GeV, in Earth orbit from 1991 to 2000. The observatory featured four main telescopes in one spacecraft, covering X-rays and gamma rays, including various specialized sub-instruments and detectors. Following 14 years of effort, the observatory was launched from Space Shuttle Atlantis during STS-37 on April 5, 1991, and operated until its deorbit on June 4, 2000. It was deployed in low Earth orbit at 450 km (280 mi) to avoid the Van Allen radiation belt. It was the heaviest astrophysical payload ever flown at that time at 16,300 kilograms (35,900 lb).
The Rossi X-ray Timing Explorer (RXTE) was a NASA satellite that observed the time variation of astronomical X-ray sources, named after physicist Bruno Rossi. The RXTE had three instruments — an All Sky Monitor, the High-Energy X-ray Timing Experiment (HEXTE) and the Proportional Counter Array. The RXTE observed X-rays from black holes, neutron stars, X-ray pulsars and X-ray bursts. It was funded as part of the Explorer program, and was also called Explorer 69.
A gamma camera (γ-camera), also called a scintillation camera or Anger camera, is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy. The applications of scintigraphy include early drug development and nuclear medical imaging to view and analyse images of the human body or the distribution of medically injected, inhaled, or ingested radionuclides emitting gamma rays.
A gamma-ray spectrometer (GRS) is an instrument for measuring the distribution of the intensity of gamma radiation versus the energy of each photon. The study and analysis of gamma-ray spectra for scientific and technical use is called gamma spectroscopy, and gamma-ray spectrometers are the instruments which observe and collect such data. Because the energy of each photon of EM radiation is proportional to its frequency, gamma rays have sufficient energy that they are typically observed by counting individual photons.
Yohkoh, known before launch as Solar-A, was a Solar observatory spacecraft of the Institute of Space and Astronautical Science (Japan), in collaboration with space agencies in the United States and the United Kingdom. It was launched into Earth orbit on August 30, 1991 by the M-3SII rocket from Kagoshima Space Center. It took its first soft X-ray image on September 13, 1991, 21:53:40, and movie representations of the X-ray corona over 1991-2001 are available at the Yohkoh Legacy site.
HEAO-1 was an X-ray telescope launched in 1977. HEAO-1 surveyed the sky in the X-ray portion of the electromagnetic spectrum, providing nearly constant monitoring of X-ray sources near the ecliptic poles and more detailed studies of a number of objects by observations lasting 3–6 hours. It was the first of NASA's three High Energy Astronomy Observatories, HEAO 1, launched August 12, 1977 aboard an Atlas rocket with a Centaur upper stage, operated until 9 January 1979. During that time, it scanned the X-ray sky almost three times
Gamma-ray spectroscopy is the qualitative study of the energy spectra of gamma-ray sources, such as in the nuclear industry, geochemical investigation, and astrophysics. Gamma-ray spectrometry, on the other hand, is the method used to acquire a quantitative spectrum measurement.
The International Astrophysical Observatory "GRANAT", was a Soviet space observatory developed in collaboration with France, Denmark and Bulgaria. It was launched on 1 December 1989 aboard a Proton rocket and placed in a highly eccentric four-day orbit, of which three were devoted to observations. It operated for almost nine years.
The last of NASA's three High Energy Astronomy Observatories, HEAO 3 was launched 20 September 1979 on an Atlas-Centaur launch vehicle, into a nearly circular, 43.6 degree inclination low Earth orbit with an initial perigeum of 486.4 km. The normal operating mode was a continuous celestial scan, spinning approximately once every 20 min about the spacecraft z-axis, which was nominally pointed at the Sun. Total mass of the observatory at launch was 2,660.0 kilograms (5,864.3 lb).
The Small Astronomy Satellite 3 was a NASA X-ray astronomy space telescope. It functioned from May 7, 1975 to April 1979. It covered the X-ray range with four experiments on board. The satellite, built by the Johns Hopkins University Applied Physics Laboratory (APL), was proposed and operated by MIT's Center for Space Research (CSR). It was launched on a Scout vehicle from the Italian San Marco launch platform near Mombasa, Kenya, into a low-Earth, nearly equatorial orbit. It was also known as Explorer 53, as part of NASA's Explorer program.
OSO 7 or Orbiting Solar Observatory 7, before launch known as OSO H is the seventh in the series of American Orbiting Solar Observatory satellites launched by NASA between 1962 and 1975. OSO 7 was launched from Cape Kennedy on 29 September 1971 by a Delta N rocket into a 33.1° inclination, low-Earth orbit, and re-entered the Earth's atmosphere on 9 July 1974. It was built by the Ball Brothers Research Corporation (BBRC), now known as Ball Aerospace, in Boulder Colorado.
X-ray emission occurs from many celestial objects. These emissions can have a pattern, occur intermittently, or as a transient astronomical event. In X-ray astronomy many sources have been discovered by placing an X-ray detector above the Earth's atmosphere. Often, the first X-ray source discovered in many constellations is an X-ray transient. These objects show changing levels of X-ray emission. NRL astronomer Dr. Joseph Lazio stated: " ... the sky is known to be full of transient objects emitting at X- and gamma-ray wavelengths, ...". There are a growing number of recurrent X-ray transients. In the sense of traveling as a transient, the only stellar X-ray source that does not belong to a constellation is the Sun. As seen from Earth, the Sun moves from west to east along the ecliptic, passing over the course of one year through the twelve constellations of the Zodiac, and Ophiuchus.
An X-ray space telescope is a type of space telescope used to study X-ray emissions from celestial objects, as part of a branch of space science known as X-ray astronomy. Satellites are needed because X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites.
The history of X-ray astronomy begins in the 1920s, with interest in short wave communications for the U.S. Navy. This was soon followed by extensive study of the earth's ionosphere. By 1927, interest in the detection of X-ray and ultraviolet (UV) radiation at high altitudes inspired researchers to launch Goddard's rockets into the upper atmosphere to support theoretical studies and data gathering. The first successful rocket flight equipped with instrumentation able to detect solar ultraviolet radiation occurred in 1946. X-ray solar studies began in 1949. By 1973 a solar instrument package orbited on Skylab providing significant solar data.
A radionuclide identification device is a small, lightweight, portable gamma-ray spectrometer used for the detection and identification of radioactive substances. These devices are available from many companies in various forms to provide hand-held gamma-ray radionuclide identification. Since these instruments are easily carried, they are suitable for first-line responders in key applications of Homeland Security, Environmental Monitoring and Radiological Mapping. These devices are also useful in medical and industrial applications as well as a number of unique applications such as geological surveys. In the past two decades RIIDs have addressed the growing demand for fast, accurate isotope identification. These light-weight instruments require room temperature detectors so they can be easily carried and perform meaningful measurements in various environments and locations.
Solrad 10, also known Explorer 44, NRL-PL 165 and Explorer SE-C, was one of the SOLRAD series designed to provide continuous coverage of wavelength and intensity changes in solar radiation in the UV, soft and hard X-ray regions. The satellite also mapped the celestial sphere using a high-sensitivity X-ray detector. Information collected was expected to contribute to a better understanding of the physical processes involved in solar flares and other solar activity, and the potential effects of this activity on short-wave communications, as well as on future human space travel. For the period of July 1971 to June 1973, the core memory data of Explorer 44 were used rather than those from Explorer 37. The Explorer 44 core memory failed on 11 June 1973, and Explorer 37 was heavily used until 25 February 1974, when the gas supply of the attitude control system was exhausted.
Explorer 47, was a NASA satellite launched as part of Explorer program. Explorer 47 was launched on 23 September 1972 from Cape Canaveral, Florida, with a Thor-Delta 1604. Explorer 47 was the ninth overall launch of the Interplanetary Monitoring Platform series, but received the launch designation "IMP-7" because two previous "Anchored IMP" flights had used "AIMP" instead.
↑ Henke BL; etal. (1977). "0.1–10-keV X-ray induced electron emissions from solids—Models and secondary electron measurements". Journal of Applied Physics. 48 (5): 1852. Bibcode:1977JAP....48.1852H. doi:10.1063/1.323938.
↑ Adler I; Gerard J; Trombka J; Schmadebeck R; Lowman P; Bodgett H (1972). "The Apollo 15 x-ray fluorescence experiment". Proc Lunar Sci Conf. 2: 2157. Bibcode:1972LPSC....3.2157A.
↑ Leo WR (1994). Techniques for Nuclear and particle Physics Experiments (2nded.). Springer.
↑ Conner JP; Evans WD; Belian RD (1969). "The Recent Appearance of a New X-Ray Source in the Southern Sky". Astrophys J. 157: L157. Bibcode:1969ApJ...157L.157C. doi:10.1086/180409.
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