Space Instrumentation

Definition

How do we measure these particles? h p+p+ e-e- Device Signal Source

Short History

Early Imaging Device (Image Intensifier) photocathode photoelectron ………… Light 1000V 0V Fluorescence screen Lens Signal processing

Early Image Intensifiers

Early Image Intensifier (cont’d)

Major Discovery

Dynodes

Continuous Electron Multiplier (CEM)

Detector (Modern)

Modern Image Intensifier

Principles of Detectors

Coulomb Interaction (Classical) During “collision”, moves very little, so electric field can be calculated (Not valid if V ~ v e ). Calculate momentum acquired by electron, e -. Impulse acquired by the electron = (electrostatic force) (time of collision) o b meme ze V Ion Electron

Coulomb Interaction (Cont’d) As charged particles lose energy by electromagnetic interactions, electrons of the matter are raised to excited energy states. - If to continuum, electron ionized (otherwise electrons excited) The rate of energy loss per unit of path length by ions z = charge of the particle, n= number of e - /cm 3, b = impact parameter.

Energy loss of charged particles (Ions) Energy loss of heavy charged particle through matter is (H. Bethe) where v and ze are velocity and charge of the primary particle, I is average ionization potential of the absorber (detector), and N and Z are the number density and atomic number of the absorber. For v << c, only first term in bracket significant. Equation valid for different types of charged particles if v >> v orbital of electrons in absorber. For v << c, dE/dx varies as 1/v 2. Energy transfer maximum when charged particles have low energy and spends more time in the vicinity of electron in the matter. z 2 dependence means particles with high z have larger energy loss (dE/dx for He ++ > p + ).

Energy loss of  meson in Cu

Energy loss of Ions through air

Range of ions Si

Range of Electrons backscatterstraggle

Range of Electrons Range similar in different material

Electron Backscattering When an electron hits an atom it can undergo a very large angle deflection, (can often scatter out of the material). Larger Z has more backscattering.

Electron Energy Loss by Radiation (Bremsstrahlung) Radiation loss (Bethe) Presence of E and Z 2 in the numerator indicates radiation losses important for high energy electrons and for material of high atomic number Z. For monoenergy electron, bremsstrahlung X-ray spectrum is continuous and extends to as high as the electron energy. Shown is 5.3 MeV electron on Au-W target

Energy loss electrons (Cont’d) Total Loss Ratio where E is in MeV and Z is the atomic number of the absorber. For Silicon, for example. Z~14. Radiation loss ~Collision loss when E ~ 50 MeV. For Pb, Z=82, so E ~8.5 MeV. Useful Formula

Photon interaction with Matter

Photon interaction with matter Photoelectric effect: the photon kicks loose an electron. The energy of the electron is the incident photon energy minus the binding energy. Compton effect: the photon hits an electron and some of the energy is transferred but the photon keeps going. Pair production: the incident photon interaction in the matter creates electron positron pair. Each of these processes produces electrons (positrons) interacting with scintillators (matter) that emit photons (uv-visible) characteristic of the scintillator that the PMTs can “see.”

Photon Interaction-1

Photon Interaction-2

Photon Interaction-3

Absorption coefficient in Si

Design a photon Instrument Designing an X- and  -ray instrument requires taking into account all three interaction processes. For example, if the goal is to measure of X-ray energy spectra, one needs to reduce Compton effect. Compton scattering degrades energy spectra. Here, x must be thick enough to capture the photon with good efficiency but thin enough to minimize the Compton interaction.

Simulation Tools

Ion Simulation Software

CASINO Simulation

Protons in Silicon dE/dx

Alpha particles in Silicon

CASINO -" monteCArloSImulationof electroNtrajectory in sOlids".

CASINO Simulation result in Si

Electrons in Silicon

The End

Empirical Formula for Energy loss Feather’s rule (electron) R = 0.542E – for E >0.8 MeV in Al, but OK for other substance. R in gm/cm 2, E in MeV. For example, R~2 MeV/gm/cm 2 ; 1 cm plastic scintillator will stop 2 MeV particles. Wilson’s formula (R. R. Wilson, 1951) R = ln 2[1+E/(E c ln2)] E c = 700/(Z+1.2) MeV defined as that energy at which the ionizatio loss = radiation energy loss.

Design a photon Instrument Designing an X-ray instrument requires taking into account all three interaction processes. For example, if the goal is to measure of X-ray energy spectra, must reduce Compton effect. Compton scattering degrades energy spectra. Here, x must be thick enough to capture the photon with good efficiency but thin enough to minimize the Compton interaction.

TRIM/SRIM Ion Simulation