The 7 classes of X-ray Detectors  - by Albert Lim (2000)

There are currently 7 classes of X-ray detectors and they are :

1. Microchannel Plates
   
   

   Fig 01 - Diagram of an Microchannel Plate (MCP). A single X-ray photon enters the MCP from left and goes through a multitude of photon multiplication to the right. The resolution is dependent on the diameter (D) of the individual microchannels.

   
   Microchannel Plates or MCPs are electron multiplier tubes of high gain. These have a very wide variety of applications besides being an X-ray detector. They are perhaps most commonly used as image intensifiers in many night vision viewers and applications. MCPs have large numbers of channels, from a few million to as much as 30 million. Channel diameters may be as small as approximately 10 microns each. Each channel is basically a dinode photomultiplier providing extremely high spatial resolution and distortion free imaging. With lower energy X-ray photons below 5 keV, the interaction is via photoelectric effect and there is no crossover of energy into neighbouring channels. At high energies however, such crossing over phenomena needs to be addressed. MCPs are generally more immune to magnetic fields than other class detectors and astronomers are only just beginning to exploit this property. In general, MCPs are desirable as X-ray detectors because they have high gain (one X-ray photon can produce many millions of electrons), high spatial resolution (channels can now be made very small in diameter resulting in high spatial resolution) and good temporal resolution (MCPs are intrinsically extremely fast detectors). Their disadvantages being their low quantum detection efficiency (only 1% to 10% of X-rays are detected, the higher the energy, the lower the efficiency), essentially no energy resolution (in particular for hard high energy X-rays) and high dark noise  (caused by current generation in the channels) compared to proportional counters for example.
    
2. Proportional Counters
   By far, proportional counters are the most commonly used detectors in X-ray astronomy. These counters work by detecting X-rays through the photo-ionisation of gas. When X-rays pass through the counter gas, it is absorbed by the gas and these absorption levels provide information on the sensitivity or energies of the X-ray.
   

   Fig 02 - The X-ray Position Sensitive Proportional Counter (PSPC) used on board ROSAT. Multiwire proportional counters such as this provide modest energy resolution, high spectral resolution and a relative time resolution of about 130 microseconds.

   
   The process starts with X-ray photons ionising the counter gas through collisions leaving a trial of ionised particles going through numerous cells of the counter. The counters are capable of rejecting charged particles by the number of cells they activate, or by the shape of their analog pulse because their shape differs from that of X-rays. Gas atoms ionised by  protons or charged particles produce free electrons which are attracted to the positive anode wires at the center of the cell. This electron can also ionises more atoms through collisions producing an avalanche of electrons to the anode resulting in an electrical impulse.  
   All X-ray proportional counters are made up of a gas cell enclosed within a window. The cell is subdivided into a number of low and high electric field areas by certain arrangements of electrodes. At energies lower than 50 keV, X-ray photons interact with the gas molecules within the cell via the photoelectric effect, with the immediate release of a primary photo-electron, followed by a cascade of Auger electrons and or with fluorescent photons. Recent developments in proportional counters are geared towards 3 areas - large area low background collimated detectors, imaging detectors and enhanced energy resolution detectors.
    
3. Semiconductor Detectors
   Semiconductor Detectors are rugged solid state detectors distinct from scintillators discussed below. Semiconductor detectors are primarily used in X-ray astronomy because of their high spectral resolution. They act as non-dispersive spectrometers with high energy resolution - a most desired feature.
   
   

   Fig 03 - The 6cm x 6 cm large X-ray pn-CCD detector developed by the Max-Planck-Institute (MPE). This CCD x-ray detector is the heart of the EPIC/XMM camera. It has quantum efficiency of over 80 percent over the entire XMM energy range from 200eV to 12 keV. It is  extremely radiation  resistant and  has excellent time resolutions

   
   Semiconductor detectors consist of Charge Coupled Devices or CCDs as they are commonly called, as well as silicon drift chambers. Both these devices can image with low noise and good energy resolution and thus are frequently used as focal plane X-ray detectors.
   These semiconductor detectors are made of semiconductor materials, usually cooled silicon, germanium or mercuric iodide, subdivided by doping with impurities into areas with different conductivity. Appropriate bias voltages are applied to a set of surface contacts so that a charge collecting field of electrons can be established. This usually results in soft X-ray applications as creation of hole pairs analogous to electron-positive ion pairs in a counting gas.
    
4. Scintillators
   
   

   Fig 04 -  Simple  diagram  of  Scintillator

   
   The principle behind scintillators as X-ray detectors is the basic conversion of high energy X-ray photons into lower energy visible photons. Although scintillators also used bulk crystalline materials such as NaI and CsI, these are distinguished from thin granular layers of rare earth oxysulphides as phosphors as far as X-ray astronomy is concerned. NaI and CsI alkali halides deliberately activated with tiny contaminations of thallium or sodium are usually used as detector materials for scintillators. These contaminants act to produce luminescent centers energetically between the valence and conduction bands of the host crystal. Below 100 keV, X-ray photon interaction of detector material is mainly through the photoelectric effect. Unlike bismuth germanate (BGO) which are only used for small detectors, NaI and CsI can be made into large area crystals suitable for use as large area X-ray detectors.
   Energy conversion efficiency of scintillators crystals varies and is highly dependent on temperature. At 20 degrees C for example, NaI with thallium has a factor of 0.12, while CsI with sodium or thallium has a factor of 0.10 and 0.05 respectively.
   The energy resolution of scintillators are generally not good because it depends heavily on the variation in the numbers of electrons liberated from the Photomultiplier (PMT) photocathode and is essentially a Poisson distribution. As such, scintillators are usually employed where large collecting areas and high quantum efficiency at energies of 20 keV range are required.
    
5. Phosphor X-ray Detectors
   The principle of X-ray conversion into visible light for Phosphor X-ray detectors is essentially similar to that of scintillators discussed above. The function of scintillator as an X-ray detector as opposed to Phosphor is however, entirely different. NaI and CsI used in scintillators are valued for their large area and absorbing power in stopping hard X-rays or even Gamma-rays. Phosphor detectors on the other hand, are thin layered and ideal for high resolution soft X-ray imaging. In theory at least, phosphor detectors should be capable of  achieving  the  highest  possible  spatial  resolution  of  all photon  counting  X-ray  imagers -  the  limit being  the size  of the  phosphor grain at  about a micron. This however is yet to be achieved in practice.
    
6. Negative Electron Affinity Detector (NEADS)
   
   

   Fig 05 - Simple Diagram of a NEAD x-ray Detector

   
   Once touted as the most promising new technology for X-ray detection' in the 70s, NEADS are hardly even referenced in current papers. The reason seems to be the near impossibility of maintaining the extreme vacuum (10^-10 Torr) environment that these detectors need to be housed in order to safeguard against contamination for negative electron affinity to work. Because X-ray astronomy needs large area detectors, these proved to be extremely difficult problems - especially in the 70s.
   Perhaps someday, NEADs may regain significance as technology develops. NEADs utilise semiconducting compounds which are activated to a state of negative electron affinity  by treatment  of the surface with Cesium and Oxygen. Conduction band electrons can therefore escape from it's surface when an X-ray is absorbed, even if the photon interaction is deep within the semiconductor. This is very desirable and provides essentially 100% quantum efficiency for X-rays. On top of that, NEADS also have high spatial resolution and moderate energy resolutions making them highly suitable as X-ray detectors.
    
7. Single Photon Calorimeters
   Single Photon Calorimeters are basically thermal devices. These detectors represent the most promising advance in X-ray detection technology today. Pioneering work and leading research on single photon calorimeters are mostly carried out by researchers at NASA's Goddard Space Flight Center.
   The overall concept of single photon calorimeter relies on measurements of temperature increases in pulses caused by impact of X-rays on a super cooled detector at a fraction of a Kelvin. The rationale is that when an X-ray photon gets hit and stopped the detector, it gives of  all it's energy in one electron. This electron in turn gives energy to other electrons in the detector thereby raising their energy states. These electrons eventually return to 'ground states' by releasing their extra energy gained via scattering or vibrations within the detector itself. These vibration causes heat which shows up an increase in temperature which can then be measured precisely using very sensitive thermometers. Knowing the temperature differential, researchers can relate how much energy the X-ray originally had.
   
   

   Fig 06 Single Photon Calorimeter

   
   Essentially, this class of detectors consist of an absorber capable of detecting an X-ray photon by very low noise absorption and a ultra sensitive thermometer (in effect a thermistor)  operating at less than 0.1 Kelvin range. Both devices are linked through a load resistor to a low noise amplifier. The temperature increase caused by the X-ray absorption process triggers a voltage pulse from which information about it's energy can be extracted. The key is to be able to measure such a small increase in temperature on top of the background noise of intrinsic thermal fluctuations as well as Johnson noise - a fundamental noise associated with resistance. With these overcomed, single photon calorimeters provide the best spectral resolution of any non-dispersive  spectrometer.