What is Gamma-ray detector
Here we briefly describes what is meant by gamma-ray detector in the scope of gamma-ray astronomy, gamma-rays are extremely penetrating form of radiation in the electro magnetic spectrum, material will be absorb the all or some of the energy of the gamma-ray when they interact with the matter, therefore when using a gamma-ray detector it should use a absorber which the energy deposited and produce a measurable change which could be calculated ,there will be number of form this measurable change could happen some of form are
- A chemical change
- A small increase in temperature
- The emission of uv or visible photons
- The production of electronic change
Since our mission in this section to prove that the Integral used the best detector system which was available at the time, we consider detector techniques which can be use out of space, therefore more important thing is to calculate change of the absorber in the gamma-ray detector which could be converted to an electrical signal and it helps to store it as data on-board such data could be transmitted to the earth at a later date. Even though, it is not a hard task to build a detector which is capable of counting the gamma-ray photons which arrived to the detector. For a space mission there are more properties of the gamma-ray needed to be measured for instance energy of the photons arrived, where did the photon come from and when did the photon arrive? , to reveal these questions simple gamma-ray detection system will not be helpful, to fulfill this task position sensitive detector system is use in gamma-ray astronomy ,now we have a brief idea what sort of gamma-ray detector we should use for gamma-ray astronomy(BOOK)
Performance of a detector
In gamma-ray astronomy detecting a arrival time of a photon is very important, it will depend on a gamma-ray interaction which ensue the electronic readout, however in an astrophysical source time investigation is a hard task, because it has to be done in milliseconds, a small error in the method use to detect timing of a arrival photon will cause a significant error of the photon arrival time, therefore it is crucial to have a feasible and accurate method of calculating a arrival time of a photon.
When consider spatial resolution most important thing is to reveal the position of the gamma-ray photon which arrived to the detector, to accomplished this task position sensitivity of a detector is crucial, this could be achieved in two different ways ,these detector techniques generally referred to as
- Pixellated detector plane- this is the detection plane system used in Integral which could be consider as the most appropriate detector plane system for gamma-ray astronomy mission ,in this system array of small detectors used to form a large detection plain, when interaction occurred with a detector, detection plane made to be position sensitive .
- Continuous detector plane - this is the second method we could pay attention when considering detector systems, both methods have used in gamma-ray astronomy as aforesaid most appropriate is the Pixellated detector plane ,in this method it use by constructing a single detector not like constructing an array of small detectors in pixellated, using this single detector it will be feasible to determine the gamma-ray interaction site.
Spectral resolution - Spectral resolution is one of the main features that a gamma-ray detector should have because it determines how accurately a detector can determine the energy of an incident photon. But some of the basic gamma-ray detectors don't have this ability therefore in these detectors all information about the photon energy is lost, which is one of the very important data for scantiest that consider in gamma-ray astronomy, for this reason in gamma-ray astronomy detectors which are use have the spectral resolution capability for instance high resolution spectrometers based on germanium detector can determine the photon energy up to good extend but energy resolution shows that not all signals received from the detector will perfectly match the input photon energy. However there are number of contributions to the energy resolution of a detector which later discussed.
When considering the detection techniques there will be lot of different techniques, here most applicable detection techniques will be discuses for gamma-ray astronomy like gas counters, semiconductor detectors and scintillation counters. Combination of these various detector system will helpful to understand the practical detector system used in gamma-ray astronomy and specially in Integral mission detector system and techniques
In general gas counters are limited to operation in range below ~ 150 Kev and there construction is the same, when a gamma-ray interacts in the gas, ionisation results for instance electron iron pairs are formed then motion of these ions in the applied field able to generate a detectable signal. In modern gamma-ray astronomy the technology of gas counters is not largely used, the only gas detectors which could be mentioned are the multi-wire proportional counter (MWPC) and the spark chamber, MWPC is commonly used in X-ray and soft gamma-ray telescope, Spark chambers operate as a large array of small 'local' Geiger-Muller tubes and capable of detecting very high energy gamma-rays (>20 MeV)
With comparing to the previously discussed Gas counter detection techniques Semiconductor detectors have major advantages, the absorber material of the semiconductor detectors are much higher-Z and denser, therefore these detectors are capable of detecting higher photons energies with good efficiency and other important thing about the semiconductor detectors are absorber material which use for detection is a semiconductor, because of this reason energy requires for produce an electron-hole pair is considerably lower than their gas-filled counterparts. Therefore charge signal of given energy will be much vast and the relative statistical uncertainties much lover than the gas counters.
Performance of a semiconductor detectors will be vary according to the type of semiconductor detector material used, most commonly used semiconductor detector materials are Germanium, Silicon, Cadmium, Mercuric as aforesaid there will be deference in the performances when using different detector material, for this reason it is worth to discuss concisely how the detector will behave when using different detector materials.
Germanium detectors gives good spectral resolutions and it only requires miniscule amount of energy to produce an electron-hole pair, modern germanium detectors are build from High-Purity Germanium and the particular High-Purity Germanium has a major benefit over the old germanium detectors, these old lithium-drifted germanium detectors had to kept at low temperatures at all times but the High-Purity Germanium detectors only need to be cooled during operation.
There are two shapes of detectors; they are namely planar and co-axial, there are two main issues using these two configurations of detectors, the main obstacle with the planar germanium detector is, it only could use in limited size, therefore it doesn't have a large active volume for instance maximum active volume for planar germanium detector is ~ 30cm2 , for a application which need more detection of gamma-rays will require many detectors to give a sensible area to fulfill the task, because of this reason planar germanium detectors is not appropriate for serious application like Integral hence it needs many detectors to give a sensible area for detection and its maximum thickness of 2 cm cause major issues for detection efficiency.
The co-axial germanium detector has much larger active volume compare to planar germanium detector, the main issue with the co-axial detector when use in space mission it need to operate at low temperatures, therefore when it use in space mission coolers should be used, most appropriate cooling method for co-axial germanium detector Stirling Cycle cooler, but these coolers are heavy and need to be operated in pairs to avoid vibrations they cause, however operational range of these type of detectors is sufficient for the Integral mission therefore Germanium detectors are suitable for the Integral mission.
When considering Silicon detectors for gamma-ray detection there are major problems that should draw attention, even thought these detectors are operate very much alike germanium detectors the low-Z nature of silicon cause ~ 50 times lower photoelectric cross section than the germanium and the maximum thickness of the silicon which only successfully reverse biased is 2mm, anything over 1mm is unusual . Therefore available depth for interactions is very small; furthermore the band-gap is larger in silicon detector and the energy level is not good as the germanium detectors, because of these issues silicon cannot be consider as a good gamma-ray detector, it is more suitable for detection of low energy photons. Since Integral science goals for detect gamma-ray (10keV - 5 MeV) silicon detectors also not applicable for Integral observatory.
Cadmium telluride detector
Cadmium telluride detector doesn't have the low band gap which gives Germanium detectors its attractive spectral resolution, on the other hand Germanium detectors need to be cooled during the operations which is the biggest problem, this problem will not occur with Cadmium telluride detectors (CdTe) because it is one material could use at room temperatures and currently the best room-temperature semiconductor detectors. As aforesaid CdTe has a large band gap compared to the Germanium detectors which cause slightly worse spectral resolution than the Germanium detectors but it could be disregard since Cadmium telluride detectors be able to use at room-temperature, and larger band gap may be greater because thermal production of noise charges will be much lower compared to Germanium detectors.
There are two major problems with CdTe one issue is growing quality large crystals. Even if consider aid of best manufactures in the world, to have a reasonable size of detector plane requires thousands of detectors, other problem with the Cadmium telluride detector is, it endure a problem called charge trapping, which cause by the low mobility of the electrons and holes within the material, even though it could be corrected there are limits to the technique which could use, therefore charge trapping problem of the Cadmium telluride detector reduce its detection efficiency. Because of aforesaid major issues with the CdTe Germanium detector could be consider as the best for Integral mission even though it has a problem of cooling the detector during the operations.
Mercuric Iodide detectors
Mercuric Iodide detectors has a higher -Z, higher -density material than cadmium telluride detectors and required energy to create an electron-hole pare is slightly lower. Therefore Mercuric Iodide detector should have better detection efficiency than cadmium telluride detectors and other fact is Mercuric Iodide semiconductors are also able to use at room-temperature and could be considered for space applications, for these reasons Mercuric Iodide detectors should have a better over role performance than the Cadmium telluride detectors. Unluckily Mercuric Iodide detectors endure from charge trapping problems to an even greater extend than Cadmium telluride detectors therefore energy resolution of the detector is poor even it has some good features over CdTe.
Scintillation Counters generally works in two steps when the gamma-ray photon deposits its energy in the scintillator it is converted in to a number of photons of uv or visible light by scintillation, then in the subsequent stage detection of those scintillation photons with a appropriate visible photon detector which could convert an incident gamma-ray into an electrical pulse. There are two types of scintillators organic scintillators and inorganic scintillators, when considering organic scintillators only few are good enough for design of gamma-ray detectors, therefore plastics scintillatros are made using some of organic scintillators, these plastic scintillators have very limited use for direct gamma-ray detection because these scintillators are likely to be of very low density. Anyway they are used in active shielding systems, number of inorganic crystals are also scintillate compare to organic scintillators they are much more dense and capable of better stopping power for gamma-ray photons, because of this quality inorganic scintillators are used in both the primary detection elements and in the shielding system of a detector.
After scintillation event major task is to collect the uv or visible light photons which have been produce in the scintillation process. To full fill this task suitable visible photon detectors like photomultiplier or photodiode could use.
It is worth to have a basic idea how the photomultiplier works, it is a solid-state instrument which converts a pulse of optical photons in to a measurable current. It has a photocathode which is a semiconductor those coverts photons to electrons. The efficiency of creation of these photons is been questioned since it has a low quantum efficiency compare to photodiodes. By an applied voltage photoelectron which comes from the photocathode are accelerated towards the dynode, using several dynodes more new electrons are made, because of this method even low light levels can be detected, as aforesaid main draw back is the low quantum efficiency at the photocathode which could result limitation in the detector energy resolution. Despite of its quantum efficiency it is used in gamma-ray detection but there are more disadvantages which should consider, in photomultiplier applied voltage usually in the range of 1-2 kV which could cause engineering problems, it is also vulnerable to applied magnetic fields, and photomultipliers are rather huge and not hard compare to its terrestrial counterparts (book) therefore it has not been consider in Integral mission.
Photodiode has a much better readout compared to the previously discussed photomultiplier, since it doesn't have any problem with magnetic fields and only need of 50 V power supply compare to the photomultiplier power requirement of 1-2kV, also have a good quantum efficiency which is considerably higher than the photomultiplier, only problem with the photodiode is that there is no intrinsic gain therefore photodiode is not capable of detection very low light levels.
Position-sensitive scintillation counters
we have discuss about the scintillation detectors which is capable of record the information in spite of where the initial interaction occurred, this happens because the scintillation detectors that we have discussed don't have the spatial resolution, if scintillation counters had spatial resolution it could use in majority of imaging system, this could achieve by making a large detector plane from an array of individual detector pixels. In this method individual detector pixel don't have any spatial resolution, but the detector plane as a whole has spatial resolution. Even though this is the simplest way of achieving the task of having spatial resolution in scintillation detectors there are other ways of producing a single detector which has spatial resolution itself, for instance The Anger camera, Position-sensitive photomultipliers (PS-PMT).(b00k)
Shielding & Collimation
So far we have draw over attention to various types of detection techniques which could use in gamma-ray astronomy and meantime able to have an understanding what sort of techniques that the Integral had use to accomplished the task to have a better detection system, furthermore we can draw attention to apply the various detection techniques so far discussed to construct a feasible detector system that could compare with the Integral detector system. (not book)
Before that it is important to discuss reduction of background counting rates, this task could be fulfill by use of different combination of detectors and shields which help to build a good detector system.
When talking about shielding methods of gamma-ray detectors there are two methods they are namely passive and active shielding, in the passive shielding technique charge particles and non- source gamma-rays obstructed by a strongly gamma-ray absorbing materials such as lead or tungsten, to prevent them reaching the detector, there is a problem of using this method any absorber that can use will tend to transmit photons at characteristic energy levels ,therefore florescence photons which comes from the shield should be stop or reduce to a minuscule number, to full fill this task method called graded shielding could use, in this method apart from using one absorber material few materials are used, these high-Z, intermediate-Z, low-Z materials are used to prevent fluorescent photons cascade from the shield to detector, however there is another major issue with this technique when compared to the active shielding which we are going to discuss next, the problem is when fully absorbing incoming radiation without generating any secondary radiation at high energies, passive shielding becomes heavy, however passive shielding technique is worth to consider at low energies rather than in high energies.
Active shielding is easy to understand after having a idea how the passive shielding works, as we have discussed in the previous paragraph a passive shield must fully absorb the photon energy but there was a problem at higher energies interaction in the shielding material is a scatter, and number of scatters were required for fully absorption, basis of active shielding is to let the scatter event happen which means fully absorption is not need in the active shielding technique, which makes active shielding is much more efficient specially at higher energies.
Purpose of collimation is to restrict the angles from which photons can reach the detector which means restrict the angle of sky that a gamma-ray detector views. Therefore collimation essential in number for ways in gamma-ray detection system, most importantly it reduce the background counting rate in the detector and also produce a crude form of "imaging" which help the scientist to have a understanding of the source position.
When using passive collimation technique each part of the detector could only see a restricted part of sky any photons comes outside the angle of acceptance of the collimator are stopped this is similar to passive shielding actually main purpose of the collimator is to fully absorb the photons without scattering or secondary radiation, collimators are usually made by rolling sheets of metal to form a honeycomb construction to minimise fluorescence photons, to stop fluorescence photons cascade in to the detector, this could be achieved by using led or tungsten collimator assembly with a thin layer of lower Z-material, this is a similar technique that we have use for graded shield for passive shielding.
Collimators are also could use in imaging system such as coded masks in order to restrict the aperture, but it could seems to be pointless because there is no reason to restrict the field of view since that is part of the function of the mask, however it is essential to have collimator to reduce the background flux. When considering collimation in imaging system veto collimation is an option which means extending the veto all the way to the mask and it feels like the best solution for restrict the field of view because the only photons which can reach the detector much come through the mask, even though it appears to be the best solution it is not, there are several major problem with this technique, the first problem is veto material itself can become radioactive and the design of the veto can be very heavy when it is design to absorb all the photons. Passive collimation could be consider Instead of using veto collimation, in this technique collimator is used to restrict the detectors FOV to match the mask system this is a better way of to reduce the aperture background without causing serious problems to the imaging performance, however in practice collimators only work well up to 200 keV , so there is no point having collimators above these energy levels. Therefore having the detector not collimated above aforesaid energy level will not be an issue. (book)