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High Energy Observational Astrophysics

VIEWS: 3 PAGES: 18

									                       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              Collimator
 Coded mask collimator

 Spark chamber
 Compton telescope
                                        Tracking
 Wire chamber telescope
 Grazing incidence telescope
                        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 two-
level 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 mc 2
                    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.

								
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