Transcript Part F
Main detector types Multi Pixel Photon Counter (MPPC) and Charge Coupled Devices (CCDs) How does it work? 1. Photon hits a pixel producing electron hole pairs. 2. This charge induces an avalanche in depletion region because the diode is so close to its reverse breakdown voltage and the pixel fires. 3. Since the diodes are not connected the number of fired diodes gives a measure of the total number of photons. Detector saturated with light Main detector types http://www.youtube.com/watch?v=VP__-EKrkbk&feature=related Multi Pixel Photon Counter (MPPC) and Charge Coupled Devices (CCDs) Charge-Coupled device (CCD) has been incorporated into the MPPC thereby providing track information. The figure below shows a CCD used in digital photography. A CCD is a device used for the movement of electrical charge, usually from within the device to an area where the charge can be analysed, Main detector types Multi Pixel Photon Counter (MPPC) and Charge Coupled Devices (CCDs) Imaging systems What will we be looking at? The detectors met so far measure energy and flux. But direction the photons come from is also useful. We cant use telescopes with lenses and mirrors to image X-rays and gamma rays as they penetrate material but … Honeycomb collimator Modulation collimator Coded mask collimator Spark chamber Compton telescope Wire chamber telescope Grazing incidence telescope Collimator Tracking Imaging systems Spark chamber Metal plates in a sealed box filled with a gas such as helium or neon. As gamma or particle travels through the detector, it ionises the gas between the plates. A trigger system consisting of two PMTs coupled to scintillators above and below box is used to apply high voltage to plates after the particle goes through the chamber. Breakdown occurs along the ionisation trail making it is possible to determine the path of the particle in the detector. Imaging systems Spark chamber Imaging systems Spark chamber http://www.youtube.com/watch?v=cAIKp0cu7UM Imaging systems Compton telescope Compton telescopes are twolevel instruments. Typically sensitive to photons between 300 eV and 30 MeV. Top level = photon Compton scatters in liquid scintillator. Bottom level = Scattered photon travels down and is absorbed by crystal scintillator. PMTs triggered on both levels. Imaging systems Compton telescope Line between two interaction points does not point back to direction of incoming photon because photon changed direction when it scattered. It is possible, however, to determine the angle of incidence the photon made with respect to this line. Imaging systems Compton telescope MISTAKE IN NOTES 1 Ec mc 2 cos E a ( E a Ec ) Ec mc2 cos 1 Ea ( Ea Ec ) Imaging systems Grazing incidence telescope Uses fact that x-rays and gamma rays at such short wavelengths behave like ordinary light rays if they strike surfaces at a shallow enough angle. Known as Wolter telescopes. Only work if angle of reflection is very low (typically 10 arc-minutes to 2 degrees). Imaging systems Grazing incidence telescope Chandra Wolter telescope Imaging systems Grazing incidence telescope ROSAT (1990-1999) the ROentgen SATellite, was an X-ray observatory designed to make an all-sky survey in soft x-rays (0.1 keV-2 keV). Its sensitivity to X-rays was over 1000 times greater than Uhuru. The X-ray mirror assembly was a grazing incidence four-fold nested Wolter I telescope with an 84 cm diameter aperture. Imaging systems Rotational modulation collimator Consists of two sets of very dense grids which are rotated. The rate of change in the brightness is dependent upon the angle from the Z axis and the orientation of the source relative to the telescope with respect to the rotating grid position. Complex computer reconstruction of the image provides the source location. Imaging systems Coded mask aperture collimator Big metal sheet with random holes in it that casts a shadow on the detector plates. Since scientists know what the aperture mask looks like, they can use the shadow cast by a source on the detector to pinpoint the location of the source. The detector is a thick semiconductor array. Imaging systems A honeycomb collimator Restricts the incident angle of the incoming photons. Composed of thousands of precisely aligned holes cast in a lead block. Conveys only those photons travelling directly along the long axis of each hole. Photons emitted in other directions are absorbed. Without a collimator in front of the imaging device, the image would be indistinct. Imaging systems Wire chambers Some are now capable of discerning the path of ionisation through the gas. Rectangular boxes containing grids of orthogonal wires (Multi-wire Proportional Chambers). Ionisation track is drifted . within an electric field toward grids. Upon arrival it creates a signal on both sets of wires and triangulation provides x and y coordinates. z coordinate is determined by measuring the drift time from the ionization event to the wires. Path of ionisation can be reconstructed. Imaging systems Wire chambers The ionisation trail has been determined using x ,y ,z, coordinates.