detectors for Xrays Xray detectors every type of

detectors for X-rays

X-ray detectors every type of electromagnetic radiation can be detected via its interaction with matter type, shape and efficiency of the detector determines the measurement strategy no detector simultaniously fulfills all demands perfecly: - time resolution - point resolution - energy resolution

X-ray detectors characteristics: - quantum efficiency (QE): - capability of absorption of incident photons ratio of detected photon to incident photons (0. 5 < QE < 0. 9) depends on photon energy and photon flux density - dynamic range - range between maximum (end of linearity) and minium (intrinsic noise) observable signal > 105 - linearity of counting rate - linearity of the QE as a function photon flux density

X-ray detectors characteristics: - sensitivity - minimale Anzahl an Photonen je Zeiteinheit, die detektiert werden können (als Stromfluss), bezogen auf das Rauschen des Detektors - spectral sensitivity - wie gut kann der Detektor Photonen unterschiedlicher Energie detektieren, z. B. wie ändert sich der dynamische Bereich wenn man Photonen anderer Energie nutzt - temporal stability - life cycle of the detector/ stability across the experiment chemical or physical degradation resistance to radiation damage precise measurement of the intensity during an experiment

X-ray detectors in general: - energy range: 5 … 25 ke. V - excitation of electrons in the active area of the detector (absorption, Compton) - secondary processes result from excitation: - gas ionisation and creation of electron ion pairs - creation of electron hole pairs in semiconductors - emission of optical/UV photons through fluorescence - creation of images through change of the valence state of chemical elements in photographic films or plates - all secondary effects (except for exposure of films) must be converted into electrical signal to be measured (amplification, storage)

X-ray detectors typical X-ray detectors (classification) dimensions process of photon detection gas ionisation semiconductor fluorescence 0 D proportional counter solid state detector scintillation counter 1 D linear position sensitive detector photo diode array 2 D multi wire detector CCD image plate, phosphorous screens chemical X-ray films

X-ray detectors 0 D detectors (point detectors): - photon counter - photons, incident on a active area are counted - convert photons in an electrical signal proportional to their number (photon flux) no resolution of location of the photon ‚impact‘ correlation with position (e. g. crystal coordinate system) through mechanical movement of the detector

X-ray detectors proportional counter - closed metal tube = cathode - thin metal wire in cylinder axis = anode (isolated agaiinst cathode) - tube is filled with working gas (e. g. air, Ar, Xe, additions of CH 4, CO 2) - detection of g-radiation, X-radiation, - and b+, b--radiation - tube lid must withstand pressure difference between environment and gas filling (up to several bar)

X-ray detectors proprtional counter

X-ray detectors proportional counter – working principle - DC between anode and cathode ionising radiation enters the chamber/tube gas molecules become ionised: electron-ion pairs charge separation: electrons travel towards the anode (+), ions towards the cathode (-) - magnitude of the tube voltage defines the working regime of the detector: - ionisation chamber - proportional counter - Geiger-Müller tube

X-ray detectors proportional counter – work principle - recombination - low voltage - e- recombine with ions during their travel to the anode - no information on intensity can be extracted - ionisation chamber - voltage ~ 100 V - all free e- reach the anode - propoertionality between energy of the incident beam and measurement signal - proportional counter - Geiger-Müller counter

X-ray detectors proportional counter – work principle - recombination - ionisation chamber - propoertional counter - - higher tube voltage (100 … 1000 V) e- are accellerated towards the anode further ionisation due to collisions with gas atoms charge avalanche develops localised to anode-near regions, hence the current pulse is independent of the locus of 1 st ionisation and proportional to the energy/intensity of the incident radation single ionisation is amlified by the avalanche requires constructur with one (ore more) thin anode wires - Geiger-Müller-Zählrohr

X-ray detectors proportional counter – work principle - recombination - ionisation chamber - proportional counter - Geiger-Müller counter - highest tube voltage every incident particle causes an instantaneous ionisation of the complete gas tube high sensitivity very long dead time no energy sensitivity impulse counter

X-ray detectors scintillation counter

X-ray detectors scintillation counter scntillator: - material, which gets excited by the passage by high energy photons and tranforms to the ground state by emission of photons in the visible range - doping of anorganic scintillators creates free electrons or electron-hole pairs (active centres) excited states travel in the material until they reach an active centre, excite the active centre, which decomposes through emission of light e. g. : Zn. S, Na. I(Tl), Pb. WO 4, … - intensity of light contains information on the energy of the incidetn photons - intensity is determined by the number of oscillations per time

X-ray detectors szintillation counter - scintillation crystal (covered) in the device head - generates light flashes when being hit by ionising radiation - optical photons hit a subsequent photocathode - external photoelectric effect: photon creates free electrons in the photoactive material (work function) - electrons are multiplied in the photomultiplier - flowing current can be measured

X-ray detectors scintillation counter - photomultiplier: - evacuated glass tube (10 -6 Pa) - created, free electrons are accellerated in an electric field, and hit a series of dynodes - a dynode is an electrode which accepts and emits electrons - accellerated electron creates multiple secondary electrons at the dynode surface, which become accelerated towards the next dynode by the potential between them - materials: Mg. O, Be. O - dynodes multiply the electron: multiplication factor ~ 106

X-ray detectors comparing proportional counter and scintillator (1) … scintillation counter (Na. I-crystal), (2) … proportional counter, filled with Xe, (3) … proportional counter filled with N 2 He. CH 4. all detectors obey the Poisson-statistics: 18

X-ray detectors solid state detectors (semiconductors) Ø better energy resolution than proportional or scintillation counters Ø operation range: 2 ke. V … 30 ke. V (6. 17 Å – 0. 41 Å) Ø energy resolution for Cu K (1. 542 Å): E ≤ 300 e. V Ø need to be cooled during operation to suppress thermal noise 19

X-ray detectors solid state detector - diode, switched in reverse direction (DC) - incident, ionising radiation creates electron-hole pairs (free charge cariers) - photon: moves an electron from valence to conduction band - eletron: has a high kinetic energy and thus creates more electrons - separation of electrons and holes due to high external voltage ( ~ 1 k. V) - charge carriers travel to the respective electrodes: current flows - photons deliver their total energy in one point - at very high energies, the Compton effect may occur - material: typically Li-drifted Si, Ge

X-ray detectors solid state detectors 21

X-ray detectors 1 D-detectors: - count the number of incidet photons per time contain sensitive elements to detect the photons which hit them cover a small part of the reciprocal space need to be moved like 0 D-detectors but allow for much higher setp sizes (detector opening)

X-ray detectors linear, position-sensitive detectors (LPSD) - are typically a series of 0 D-detectors (solid-state detectors or proportional counters) - thus, have a lateral resolution - gas filled LPSD: - current readout at both ends of the anode wire has low electrical conductivity to slow down current/electron flow measure time difference between the two signal at the two ends of the wire meet the parafocussing condition of the Bragg-Brentano geometry only in their centre (bad angular resolution) curved LPSDs have a constant distance between sample and detector maximum angular range 2 q < 120° - semiconductor LPSD: - chain of photodiodes = pixel on an Si-Chip typically 512, 1024, 2048 Px, 25 µm wide, 2. 4 mm high application started at synchrotron sources, meanwhile standard for (new) lab devices

X-ray detectors 1 D-detectors – proportional counter X-ray photon impulse ionisation of the working gas (Ar + Methan) electrical impulse is amplified, its position determined, separated in energy and counted 24

X-ray detectors 2 D-detectors: - photon counter photons per time unit are counted as a function of the position inside the detector in 2 dimensions scan a large(r) portion of the reciprocal space behaviour is similar to that of films, but they produce digital data 3 types: - multiwire proportional counter - phosphor screen and TV camera - CCD-detektors

X-ray detectors gas filled multiwire proportional counter (wire chamber detector) - 2 D-extension of the 0 D-proportional counters 3 parallel planar electrodes, and 2 (crossed) anodes between gas filling: Xe + CO 2 (needs to be refreshed periodically) resolution defined by ‚pixel size‘, created by the anode wires (> 0. 3 mm) charge buidup (wire thickness) limits resolution currently only usable in the proportional counter regime (low voltage) high dead time limits counting rate (~ 106 cps)

X-ray detectors 2 D detectors – proportional counter 27

X-ray detectors phosphor screens - X-rays are converted into visible light by the X-ray screen is observed with a camera e. g. Zn. S (250 … 500 ph. /X-ray photon) signal must be amplified, usually by a photocathode and a second phosphor screen for good statistics, multiple images are desirable

X-ray detectors CCD-detektors - 2 D-semiconductor array, usually metal oxide systems - pixel size approx. 1. 5 … 20 µm (1 Pixel is equivalent to one 0 D-solid state detector) - the larger the pixel, the high the sensitivity, the lower the resolution - Readout: - charge is not dissipated (0 D), but collected in a potential well (~ capacitor) size of charge proportional to the intensity of the incident signal collected charge is shifted pixel-wise upon readout, until they reach the amplifier (measurable voltage) - readout signal is serial, while image writing is parallel - electronic reconstruction - adv. : high dynamic range, works up to high photon energies (20 ke. V/200 ke. V), high lateral resolution - disadv. : cooling required (thermal noise), large arrays expensive (typically 30 x 30 mm 2)

X-ray detectors 2 D detectors – CCD with phosphor screen steps: 1. transformation of the Xradiation in visible radiation (phosphorous fluorescence plate) 2. compression of the image and transfer of the optical photons to a CCD chip (fibre glass wire) 3. CCD chip converts the visible radiation into electrical charge, which can be read electronically (and thereby also deleted) 30

X-ray detectors films - ealiest method to measure diffracted X-ray intensities - based on the decomposition reaction: Ag. Br Ag + Br (= illumination) - advantages: - covers a large area of the reciprocal space very good lateral resolution flexible, easily fits to different diffraction geometries constant sensitivity across the complete active area dynamic range: > 105, quantum efficiency ~ 1 - disadvantages: - low sensitivity high background noise slow readout process (photometer) single use

X-ray detectors 2 D detectors – films v photographic plates with emulsion layers for better latteral resolution v photographic plates with two emulsion layers (front and backside) for higher efficiency v Polaroid films for fast developing 32

X-ray detectors image plates - permanently phosphorising screen saves an image - storage: - active material: Ba. FBr: Eu 2+ - intensities are stored through conversion of Eu 2+ to Eu 3+ with the electrons being trapped in Br-vacancies (metastable condition) - half-life of the metastable state: < 10 h - latent image is read via laser stimulation (photostimulated luminescence) - back transformation of the active material to the ground state (Eu 3+ Eu 2+): under emission of luminescence radiation - luminescence is recognised by a scintillator and a photomultiplier - image plate can be used multiple times - optics of the read-out device determines lateral resolution and image quality - originally separate readout (as for films) - meanwhile, image plate post processing in detectors with included read-out system

X-ray detectors 2 D detectors imaging plate track of the read-out laser 34

X-ray detectors 2 D Ddetectors – imaging plate energy of the X-ray photons is stored in a phosphorous layer and read out by a laser beam 35

X-ray detectors 2 D detectors – imaging plate process steps: 1. erasure of the IP by illumination with visible light 2. exposure to X-rays 3. read-out of the stored information intensity of the fluorescence in the visible spectrum is proportional to the intensity of the X-radiation 36