Kern- und Teilchenphysik SS2012Kern- und Teilchenphysik SS2012 Johannes Blümer Vorlesung-Website....

KIT – Universität des Landes Baden-Württemberg undnationales Forschungszentrum in der Helmholtz-Gemeinschaft

KIT-Centrum Elementarteilchen- und Astroteilchenphysik KCETA

www.kit.eduKIT – Universität des Landes Baden-Württemberg undnationales Forschungszentrum in der Helmholtz-Gemeinschaft

KIT-Centrum Elementarteilchen- und Astroteilchenphysik KCETA

www.kit.edu

Kern- und Teilchenphysik SS2012

Johannes BlümerVorlesung-Website

KT2012 Johannes Blümer IKP in KCETA

Übergangsstrahlung, Fluoreszenz, Radioemission

2

Übergangsstrahlung

Fluoreszenzlicht (in N2)

Radioemission (von Schauern)


Photonen: Photoeffekt, Comptonstreuung, Paarbildung

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Paarbildung und -Vernichtung in Blasenkammer

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KT2012 Johannes Blümer IKP in KCETA5

Photon energy

100

10

10–4

10–5

10–6

1

0.1

0.01

0.001

10 eV 100 eV 1 keV 10 keV 100 keV 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV

Abs

orpt

ion

len

gth

h (

g/c

m2)

Si

C

Fe Pb

H

Sn


294 27. Passage of particles through matter

Photon energy

100

10

10–4

10–5

10–6

1

0.1

0.01

0.001

10 eV 100 eV 1 keV 10 keV 100 keV 1 MeV 10 MeV 100 MeV 1 GeV 10 GeV 100 GeV

Abs

orpt

ion

len

gth

λ (

g/c

m2)

Si

C

Fe Pb

H

Sn

Fig. 27.16: The photon mass attenuation length (or mean free path) λ = 1/(µ/ρ) for various elemental absorbers as a functionof photon energy. The mass attenuation coefficient is µ/ρ, where ρ is the density. The intensity I remaining after traversal ofthickness t (in mass/unit area) is given by I = I0 exp(−t/λ). The accuracy is a few percent. For a chemical compound ormixture, 1/λeff ≈

∑elements wZ/λZ , where wZ is the proportion by weight of the element with atomic number Z. The processes

responsible for attenuation are given in Fig. 27.10. Since coherent processes are included, not all these processes result in energydeposition. The data for 30 eV < E < 1 keV are obtained from http://www-cxro.lbl.gov/optical constants (courtesy ofEric M. Gullikson, LBNL). The data for 1 keV < E < 100 GeV are from http://physics.nist.gov/PhysRefData, throughthe courtesy of John H. Hubbell (NIST).

Photon energy (MeV)1 2 5 10 20 50 100 200 500 1000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

C

Pb

NaI

Fe

Ar

HH2O

P

Figure 27.17: Probability P that a photon interactionwill result in conversion to an e+e− pair. Except for afew-percent contribution from photonuclear absorptionaround 10 or 20 MeV, essentially all other interactions inthis energy range result in Compton scattering off an atomicelectron. For a photon attenuation length λ (Fig. 27.16),the probability that a given photon will produce an electronpair (without first Compton scattering) in thickness t ofabsorber is P [1 − exp(−t/λ)].

27.4.6. Photonuclear and electronuclear interactions at stillhigher energies : At very high photon and electron energies,where the bremsstrahlung and pair production cross-sectionsare heavily suppressed by the LPM effect, photonuclear andelectronuclear interactions predominate over electromagneticinteractions. At photon energies above about 1020 eV, forexample, photons usually interact hadronically. The exact cross-over energy depends on the model used for the photonuclearinteractions. At still higher energies (>∼ 1023 eV), photonuclearinteractions can become coherent, with the photon interactionspread over multiple nuclei. Essentially, the photon coherentlyconverts to a ρ0, in a process that is somewhat similar to kaonregeneration [54].

27.5. Electromagnetic cascades

When a high-energy electron or photon is incident on athick absorber, it initiates an electromagnetic cascade as pairproduction and bremsstrahlung generate more electrons andphotons with lower energy. The longitudinal development isgoverned by the high-energy part of the cascade, and thereforescales as the radiation length in the material. Electron energieseventually fall below the critical energy, and then dissipate theirenergy by ionization and excitation rather than by the generationof more shower particles. In describing shower behavior, it istherefore convenient to introduce the scale variables

t = x/X0 , y = E/Ec , (27.32)

so that distance is measured in units of radiation length andenergy in units of critical energy.

Longitudinal profiles from an EGS4 [55] simulation of a 30GeV electron-induced cascade in iron are shown in Fig. 27.18.The number of particles crossing a plane (very close to Rossi’s

IKP in KCETAKT2012 Johannes Blümer

Detektorbeispiele

Pionierarbeiten und BeispielePositron; Anderson 1933Blasenkammer, CERNGasdetektorenCherenkov-DetektorSzintillatorHalbleiterdetektorenKalorimeter

Moderne GrossdetektorenCMS, Pierre Auger, AMS

Rutherford-StreuungExperimentAbleitung der Rutherford-StreuformelGröße von Atomkernen

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Entdeckung des Positrons: Anderson 1933

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Blasenkammer

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Blasenkammer

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Gasdetektoren

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Gasdetektoren

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Cherenkov-Detektor

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Ionization excitation of base plastic

Forster energy transfer

a

a

base plastic

primary fluor(~1% wt /wt )

secondary fluor(~0.05% wt /wt )

photodetector

emit UV, ~340 nm

absorb blue photon

absorb UV photon

emit blue, ~400 nm1 m

10<4m

10<8m


Szintillator; Photomultiplier

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Szintillator

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Halbleiterdetektoren

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Energieauflösung

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Kalorimeter

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Compact Muon Solenoid CMS am LHC

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Pierre Auger-Observatorium

solar panel

GPS+data

electronics

1 of 3 PMTs

battery 12 m3 pure water in Tyvek liner

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Alpha Magnetic Spectrometer AMS an der ISS

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