Space Instrumentation

Definition

How do we measure these particles?

h

p+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 ~ ve).

• Calculate momentum acquired by electron, e-.

• Impulse acquired by the electron = (electrostatic force) (time of collision)

o

b

me

zeV

€

Δp= F∫ dt = F⊥∫ dt = ze2

b2

⎛

⎝

⎜ ⎜ ⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟ ⎟ ⎟

bV

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

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- /cm3, b = impact parameter. €

dEdx ⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟ions

= 4πz2e4nmv2

⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟lnbmax

bmin

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

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 >> vorbital of

electrons in absorber.• For v << c, dE/dx varies as 1/v2.

• Energy transfer maximum when charged particles have low energy and spends more time in the vicinity of electron in the matter.

• z2 dependence means particles with high z have larger energy loss

(dE/dx for He++ > p+).

€

dEdx

=−4πNZz2e4

mov2 ln2mov2

I−ln1−v2

c2

⎛

⎝

⎜ ⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟ ⎟−v2

c2

⎡

⎣

⎢ ⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ⎥

Energy loss of meson in Cu

Energy loss of Ions through air

Range of ions

€

R= dEdx ⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟Eo

0∫

−1

dE

Si

€

dEdx

=−2πNZe4

mov2 ln mov2E2I 2(1−β 2)

−ln2(2 1−β 2 −1+β 2

⎡

⎣

⎢ ⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ⎥

+(1−β 2)+18

(1− 1−β 2 )2)

€

dEdx

=−NEZ(Z+1)e4

137mo2c4

4ln 2Em

oc2

−43

⎡

⎣

⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥

Range of Electrons

€

R= dEdx ⎛

⎝

⎜ ⎜

⎞

⎠

⎟ ⎟Eo

0∫

−1

dE

backscatter straggle

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)

€

dEdx

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟=−NEZ(Z+1)e4

137mo2c4

4ln 2Em

oc2

− 43

⎛

⎝

⎜ ⎜ ⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟ ⎟ ⎟

• Presence of E and Z2 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.

€

dEdx

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟= dE

dx

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟c

+ dEdx

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟r

€

dEdx

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟T

=

dEdx

⎛ ⎝ ⎜

⎞ ⎠ ⎟r

dEdx

⎛ ⎝ ⎜

⎞ ⎠ ⎟

c

≈ EZ700 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

€

E=hν −Eb

€

σ=k× Z n

(hν )3.5

Photon Interaction-2

€

hν '= hν1+ hν

moc2 (1−cosθ)

Photon Interaction-3

€

E−+E

+=hν −m

oc2

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.

€

I = Ioe−(μ ph+μc+μ pp)x

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

€

dEdx

=−2πNZe4

mov2 ln mov2E2I 2(1−β 2)

−ln2(2 1−β 2 −1+β 2

⎡

⎣

⎢ ⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ⎥

+(1−β 2)+18

(1− 1−β 2 )2)

€

dEdx

=−4πNZz2e4

mov2 ln2mov2

I−ln1−v2

c2

⎛

⎝

⎜ ⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟ ⎟−v2

c2

⎡

⎣

⎢ ⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ⎥

Empirical Formula for Energy loss

• Feather’s rule (electron)

R = 0.542E – 0.133 for E >0.8 MeV

in Al, but OK for other substance. R in gm/cm2, E in MeV.

For example, R~2 MeV/gm/cm2; 1 cm plastic scintillator will stop 2 MeV particles .

• Wilson’s formula (R. R. Wilson, 1951)

R = ln 2[1+E/(Ec ln2)]

Ec= 700/(Z+1.2) MeV defined as that energy at which the ionizatio loss = radiation energy loss.

Design a photon Instrument

€

I = Ioe−(μ ph+μc+μ pp)x

• 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