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Maurizio Paolillo » 6.Instruments for X-ray and γ-ray Astrophysics – Part I


Contents

  1. X and γ-ray mission.
  2. γ-ray interaction processes.
  3. X and γ-ray detectors:
    • Gas-filled detectors, proportional and Geiger counters.
    • Solid state detectors, CCD,
    • scintillators,
    • Photomultipliers.

γ-ray line sources

Observed astrophysical gamma-ray sources.

Observed astrophysical gamma-ray sources.


History of High Energy astrophysics

High Energy astrophysics started late in the history of modern astronomy, compared to other wavelength ranges, since it required the development of space flight through rockets and satellites.

Main events in the history of High Energy Astrophysics.

Main events in the history of High Energy Astrophysics.


High Energy missions

Schematic view of gamma ray missions before year 2000.

Schematic view of gamma ray missions before year 2000.


γ-ray interaction processes

Gamma-ray interaction processes (Davisson, 1966).

Gamma-ray interaction processes (Davisson, 1966).


Spectral signature of high-energy photons

Spectral signature of high-energy processes in a gamma-ray detector.

Spectral signature of high-energy processes in a gamma-ray detector.


Ideal and real detectors

Left: Spectral signature of monocromatic photons revealed by an ideal very large gamma-ray detector. Right: in real detectors the signal is the combination of all the different processes, and of the escaped energy.

Left: Spectral signature of monocromatic photons revealed by an ideal very large gamma-ray detector. Right: in real detectors the signal is the combination of all the different processes, and of the escaped energy.


Gas filled detectors

Gas filled detectors work through photoionization of filling gas. Ionization potential is 10~30 eV (compared to 1~5 eV of solid state devices).

Depending on the applied voltage they work in different regimes (linear, saturated) and take different names (proportional counters, Geiger counters).

Scheme of a gas filled detector.

Scheme of a gas filled detector.

The operating regimes of gas filled detectors.

The operating regimes of gas filled detectors.


Solid state detectors

Properties of Semiconductor detectors for gamma-rays.

Properties of Semiconductor detectors for gamma-rays.


Solid state detectors: CCDs

They work as seen for optical astronomy (cf. lecture of Prof. Brescia) but with some differences:

  • CCD work in single photon detection mode, through fast reading of pixel array (e.g. every 3.2s in Chandra).
  • Cosmic rays are removed based on the ‘grade’ of each event, i.e. the configuration of activated pixels.
  • Electrons deposited in pixels are proportional to the energy of the incident photon, allowing to measure the energy even though with low (R~10-50) resolution.

CCD in X-ray band thus give 4-dimensional arrays containing X,Y (or Ra,Dec), energy and time.

CCD: Pattern selection

Grade selections allows to optimize the Signal to Noise ratio of an observations retaining only “significant” events: direct removal of cosmic rays and off-axis photons (from the XMM proposers manual).

Grade selections allows to optimize the Signal to Noise ratio of an observations retaining only “significant” events: direct removal of cosmic rays and off-axis photons (from the XMM proposers manual).


CCD: Effective area

CCD rely on photoelectric absorption of incident X-ray photons. Thus the probability of detecting a photon is approximately given by:

e-τ(det)•(1-e-τ(mirr))

The resulting efficiency, expressed in terms of effective area, shows thee edges tipical of photoelectric absorption.

In designing a telescope you often want to place absorption edges of the detector within your energy range to increase the detection efficiency, while avoiding those of the mirrors. This is done through a proper choice of the building materials.

Left: effective area of EPIC and RGS on XMM. Right: Combined effective area of all telescopes (from the XMM proposers manual).

Left: effective area of EPIC and RGS on XMM. Right: Combined effective area of all telescopes (from the XMM proposers manual).


Scintillators and Photomultipliers

Left: Scintillator and solid state detector interacting with a 1 MeV photon. Right: Scheme of a photomultiplier.

Left: Scintillator and solid state detector interacting with a 1 MeV photon. Right: Scheme of a photomultiplier.


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