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Space Instrumentation. George K. Parks Space Sciences Laboratory UC Berkeley, Berkeley, CA Introduction (Basic Principles) Detectors Light Emitting Material ( Scintillators ) Electron Multipliers Semiconductor Detectors Cluster and Wind Experiments. Definition.
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Space Instrumentation George K. Parks Space Sciences Laboratory UC Berkeley, Berkeley, CA Introduction (Basic Principles) Detectors Light Emitting Material (Scintillators) Electron Multipliers Semiconductor Detectors Cluster and Wind Experiments
Definition • Particles in Space - Electrons e- and positrons e+ - Protons p+ - Ions X+n - Elementary particles p+, m+, etc.. - Neutrals - Neutrinos n - Electromagnetic radiation, IR to g-rays (Photons) • Particles in our Universe have energies from < 1 eV to > 1021eV.
How do we measure these particles? Source hn Device Signal p+ e- • A Device must convert incident flux and energy of particles into electrical signalsthat contains the original information. • Call this a Detector which may include several components.
Short History • Radiation measurement tied to early development of modern physics. • In the beginning of the twentieth century, physicists studying disintegration of unstable nuclear substances wanted to quantify what was observed. • To determine how many disintegrations occurred per unit time, fluorescent screens were used to count the particles hitting them. • Since then, numerous recording devices have been developed (Gaas counters, Scintillators + PMTs, semi-conductor detectors, windowless electron multipliers “channeltrons”, MCPs, etc..). • Two major discoveries were important: Photoelectric effect and production of Secondary electrons.
Early Detectors • In practical application, “amplifiers” are required to boost the power so the output signal can be measured. • This amplifier must NOT change the input signal while boosting the signal to measurable levels. • A simple amplifier applies to signals that vary only in time (Electrical output of a microphone). • Consider a picture. Signal now has both space andtime variations. • A way to amplify such signal is to convert the image to purely time pattern. • Accomplish by scanning the image in systematic way (old TV cameras). • Scanning allows reconstruction of 2D images from 1D time varying signal.
Early Imaging Device (Image Intensifier) photoelectron Fluorescence screen photocathode Image …………......... Signal processing Light 0V 1000V Lens
Early Image Intensifiers • Image focused on the cathode. Each area of the photocathode emits electrons in numbers determined by the brightness of the image in that area. • Electrons are accelerated to anode. • Flux of electrons that reach anode corresponds to brightness variations of the image on the photocathode. • Photocathode and anode placed close together to preserve spatial resolution. • An electron striking anode excites and ionizes atoms in the material. • A fraction of the energy is released as photons of visible light as electrons return from excited state to ground energy levels. • The emitted photons constitute the image that is visible on the fluorescent anode.
Early Image Intensifier (cont’d) • A photon can only eject at most onephotoelectron. • Efficiency of photocathode ~10% (1 in 10 photons ejects electrons). • Efficiency of anode ~30% (30% of energy from accelerated electrons converted to light). • In spite of these inefficiencies, there is a net gain (amplification) of the luminosity of the image, because the increase in the energy of each emitted electron can be several thousand times. • BUT image signal cannot be amplified to any level by increasing Voltage. • Two-electrode image tube cannot be operated beyond certain Max applied Voltage because charge begins to flow spontaneously across the small gap and overwhelms the system. • Effect could be reduced by larger gap, but spatial resolution deteriorates. • The fact that each photon can tranasferat most one electron to anode is a fundamental limitation!
Major Discovery • Electrons striking a material can produce secondary electrons. • Multiplies number of electrons reaching anode. • Example: PMTs(photomultiplier tubes).
Dynodes •Dynodes progressively at higher potentials. • Incident electron hits first dynode, knocks out several electrons, accelerated to second dynode, etc.. • Achieve large multiplication arriving at anode. • High amplification but NO spatial resolution. • Dynode geometry does not guarantee straight line path and image is lost!
Continuous Electron Multiplier (CEM) • A "continuous-dynode" structure is possible if the material of the electrodes has a high resistance. Functions of secondary-emission and voltage-division are merged. • A glass coated inside with a thin film of semi-conducting material, with negative high voltage at input end, and positive voltage at the output end. • Electrons emitted at any point are accelerated down the funnel before impacting the surface. • Output end a separate electrode (anode) collects the multiplied electrons. Channel electron multiplier (CEM) commercially called Channeltron.
Detector (Modern) • Technology advances: Assemble millions of CEMsin a geometric array. • Each CEM in the array is ~5-15 microns in diameter that can brighten a small well-defined portion of an image. • The entire array of CEMs operating simultaneously and in parallel, functions as an image intensifier, making faint images brighter without destroying the spatial information of the input signal. • Such array of microscopic CEMs is a MicroChannel Plate (MCP). • MCP combines the gain of electron multiplier and the spatial resolution of an image intensifier.
Modern Image Intensifier • With MCP, each channel is an electron multiplier. • A channel is a pixel and high resolution images can be obtained.
Principles of Detectors • How do detectors work? • Detection of particles and photons relies on the physics of how particles and photons interact with matter. • Consider acharged particle penetrating a material surface. The particle collideswith electrons that surround the nucleus of the matter. • Many collisions occur: small amount of energy is lost with each collision. • Collisions produce many secondary electrons that are collected as an electrical signal. • A photon impinging on a surface ejects a photoelectron (photoelectric effect). • The electron interacts with the material in a “similar” way as it migrates through matter losing energy producing many secondary electrons. (Evans, Atomic Nucleus, 1955; Knoll, Radiation detection and measurement, 2000)
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) Electron me o b V ze Ion
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.
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 theabsorber. • For v << c, only first term in bracket significant. • Equation valid for different types of charged particles if v >> vorbitalof 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+).
Energy loss of Ions through air • Specific energy loss for different particles (along particle track). • Non-relativistic, dE/dx~1/energy. • At energies of several hundred MeV, v ~ c, and dE/dx~constant, with a broad minimum value. • The specific loss is about 2 MeV/gm-cm2. • Such relativistic particles are minimum ionizing particles.
Range of ions • Range = total distance traveled by the particle until its kinetic energy = 0. • Range of a particle is defined as • Linear in log-log plot suggests empirical formula R = aEb b similar for various particles Si
Theory of Electron Interaction • Electrons more complicated because it can lose energy by (1) ionizationand (2) radiation. • Bremstrahlung (breaking) X-radiation is produced when electrons undergo accelerationnear nucleus. Detector Design: • X-rays are easily generated when energetic electrons strike high Z material. • High Z materials should be avoided for space applications.
Energy Loss of Electrons (Bethe formula) • Ionization loss where b = v/c. All other symbols same. • Radiation loss:
Range of Electrons • Range of electrons • Concept of R less definite because electrons scatter in material and the path length much greater that the distance travelled in the initial velocity vector. • Also, for same energy, energy loss of electrons << than heavy charged particles. Electrons path length much longer. 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) • 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. Useful Formula
Photon interaction with Matter • Possible processes by which X-and g-ray interact with matter (Fano, 1953): Photons can Interact with 1. atomic electrons (a) Complete absorption 2. nucleus (b) Elastic scattering (coherent) 3. E-field surrounding (c) Inelastic scattering (incoherent) nuclei or electrons 4. meson field surrounding nucleons 12 ways of combining Columns 1 and 2 indicating12 different ways by which photons can be absorbed or scattered. hn ~0.01 to 10 MeV: dominant processes are (1a) photoelectric effect, (1c)Compton effect, and (3a) pair production.
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 • X- and g-ray interaction with matter depends on the incident energy of hn. • Photoelectric process(hn ≤ 100 keV) - hn absorbed by matter. Electron is ejected from bound shell of matter. - The interaction is with the whole atom, not just with electrons. - For g-rays, interaction with bound electrons in the K-shell. - Electron propagate through matter, produces secondary electrons. The photoelectron energy is where Eb is the binding energy. Process enhanced for materials of high Z. Approximate equation: where n = 4-5
Photon Interaction-2 Compton Process (hn ~50 keV – MeV) • Incident photon hn is scattered by electrons in the matter. - Incident photon hn loses energy to electrons, becomes hn’, scattered photon. - The difference is picked up by the electron, Ee = hn – hn’. • Compton shift of the photon energy is where the incident photon is scattered by the angle qand moc2 is the rest mass of the electron (0. 51 MeV). • Photon eventually interacts photoelectrically and the photoelectron interaction with matter produces secondary electrons.
Photon Interaction-3 • Pair Production Process (hn > 1.02 MeV): - Photon produces electron-positron pair (e-, e+) - Cross section low until g-ray is several MeV. - All excess energy of initial hn shared by e- and e+ - e+ subsequently annihilates itself in matter • Energy conservation requires • Electrons and positrons travel in the matter and lose all their KE to the absorbing medium, producing many secondary electrons.
Design a photon Instrument • Designing an X- and g-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.
Protons in Silicon dE/dx
CASINO -" monteCArloSImulationof electroNtrajectory in sOlids". • A very useful simple tool that simulates electron propagation within solids http://www.gel.usherbrooke.ca/casino/index.html • A Monte Carlo simulation of electron trajectory in solid specially designed for low beam interaction in a bulk and thin foil. • Single scattering program specifically designed for low energy beam interaction. • Used to generate many of the recorded signals (X-rays and backscattered electrons) in a scanning electron microscope. • Efficiently used for accelerated voltage in field emission scanning electron microscope(0.1 to 30 KeV)
Energy Loss of Electrons (Bethe formula) • Ionization loss where b = v/c. All other symbols same. • Ionization loss for ions (Reminder):
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 • 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.
