Physics 736. Experimental Methods in Nuclear-, Particle-, and Astrophysics. Lecture 3

Transcription

1 Physics 736 Experimental Methods in Nuclear-, Particle-, and Astrophysics Lecture 3 Karsten Heeger heeger@wisc.edu

2 Review of Last Lecture a colleague shows you this data... what type of reaction is this?

3 Interaction of Neutrons neutrons elastic scattering A(n,n)A inelastic scattering A(n,nʼ)A*, A(n,2nʼ)B radiative n capture n captures fissions (n,f) hadron show production at high E

4 Interaction of Neutrons Neutron Energies

5 Interaction of Neutrons Neutron Moderators neutron cross-section neutron energy distribution after several elastic scatterings 6 (t,4,.9 u c, o o Hro '-"--i_-_pn'f3!_ pnotons \- \ 6 p lcrl t t0- lo-r Energy [Mev] tol original monoenergetic neutron - average lethargy change is constant - greatest delta E from early collisions

6 Lecture Goals passage of radiation through matter [neutrons] photons charged particles

7 Interaction of Radiation with Matter photons photoelectric effect Compton scattering pair production nuclear photo dissociation (γ,n) charged particles inelastic collisions w/ atomic e- elastic scattering Cherenkov radiation nuclear reactions Brehmsstrahlung

8 Photons Gamma rays, x-rays, visible light, and radio waves are all forms of electromagnetic radiation. Difference is the frequency and hence the energy of the photons. Gamma rays are the most energetic. γ- rays x- rays

9 Interaction of Photons photons photoelectric effect Compton scattering pair production nuclear photo dissociation (γ,n)

10 Interaction of Photons photoelectric effect K-edge Encrgy [!lev] In the original photoelectric effect, the photon energy of the photon is of the same order as the energy binding an electron to a nucleus, a few ev

11 Interaction of Photons photoelectric effect Absorption coefficient of Al Absorption coefficient of lead higher Z materials favored for absorption by photoelectric effect cross-section depends on atomic number Z

12 Interaction of Photons Compton scattering

13 Interaction of Photons Compton scattering original figure from Compton A. H. Compton, Phys. Rev. 21, 483 (1923a); 22, 409 (1923b) 6' c rt Ct t I rc-z o o 6 o (J total Compton cross-section Energy [uc[

14 Interaction of Photons Compton edge When a gamma-ray scatters out of detector and escapes, only a fraction of its energy is registered. The highest energy that occurs from this process is the Compton edge. ) EnergY li"levj escape peak In a gamma or X-ray spectrum, the peak due to the photoelectric effect in the detector and escape, from the sensitive part of the detector, of the X-ray photon emitted as a result of the photoelectric effect.

15 Interaction of Photons Compton scattering Inverse Compton scattering - in Compton scattering the incoming photon scatters off an electron that is initially at rest. - electron gains energy and the scattered photon has a frequency less than that of the incoming photon. - inverse Compton scattering takes place when the electron is moving, and has sufficient kinetic energy compared to the photon. In this case net energy may be transferred from the electron to the photon. - inverse Compton effect is seen in astrophysics when a low energy photon (e.g. of the cosmic microwave background) bounces off a high energy (relativistic) electron. Such electrons are produced in supernovae and active galactic nuclei

16 Interaction of Photons Rayleigh Scattering scattering from particles up to about a tenth of the wavelength of the light.

17 Interaction of Photons Pair Production Pair Production in bubble chambers

18 Interaction of Photons Pair Production Cross-Section What does the energy dependence of the pair production cross-section look like?? o a o { c.9 o o U I l0 t00 Energy [Mev]

19 Interaction of Photons Example: Ge detectors

20 Interaction of Photons Electron Photon Showers

21 Interaction of Charged Particles charged particles inelastic collisions w/ atomic e- elastic scattering Cherenkov radiation energy loss of electrons and positrons collisions Brehmsstrahlung Coulomb scattering nuclear reactions energy loss distributions

22 Interaction of Charged Particles characteristic features energy loss deflection of particles from incident direction classes of particles e +,e - heavy particles: μ, π, p, α primary processes inelastic collisions elastic scattering

23 Stopping Power stopping power (MeV/(g/cm 2 ))

24 Stopping Power stopping power for muons in copper stopping power (MeV/(g/cm 2 )) stopping power less because ions attach electrons - nuclear inelastic collisions if you consider nuclei instead of muons - bremsstrahlung important for electrons and muons minimum ionizing particles

25 Stopping Power stopping power for muons in copper

26 Stopping Power Bethe-Bloch - # = 2 n N^ r! ^, " p + il^ (ry^) -' P with density and shell corrections G de dx typically de/dx depends only on β (given a particle and medium)

27 Bethe-Bloch t -wrth correctrons --.wilhout cortections >t0 D :, d tot to3 Energy [Mev] los

28 Stopping Power Bethe-Bloch At low β -de/dx 1/β 2 decreases rapidly as β increases. reaches a min at βγ 3 (a particle at the energy loss min is called mip). typically de/dx depends only on β (given a particle and medium)

29 Stopping Power Bethe-Bloch de/dx lor l03 Energy ['lev] ro5 low momentum region where -de/dx 1/β 2 and the relativistic rise depend on m so can be used for particle identification (PID)

30 Stopping Power Bethe-Bloch For a given particle (z) and target (I,N,Z,A), the energy loss depends only on the velocity of the particle! Most relativistic particles have energy loss rates close to the minimum (mip = minimun ionizing particles)~ 2 MeV/g/cm2

31 Stopping Power Bragg Curve particle is more ionizing towards the end of its path

32 Karsten Heeger, Univ. of Wisconsin NUSS, July 13, 2009