Photon Interactions When a photon beam enters matter, it undergoes an interaction at random and is removed from the beam I I 0e x I 0e x / is thelinear attenuat ion coefficent1/cm is theattenuat ion length or mean free pat hcm is themass attenuat ion coefficient cm2 / g 1 2 is themass attenuat ion length g / cm / 1 Photon Interactions 2 Photon Interactions Notes is the average distance a photon travels before interacting is also the distance where the intensity drops by a factor of 1/e = 37% For medical applications, HVL is frequently used Half Value Layer Thickness needed to reduce the intensity by ½ I 0.693 1 ln ln x HVL 2 I0 Gives an indirect measure of the photon energies of a beam (under the conditions of a narrowbeam geometry) In shielding calculations, you will see TVL used a 3lot Beam Hardening 4 Photon Interactions 5 Photon Interactions What is a cross section? What is the relation of to the cross section s for the physical process? s has units cm and has units1 / cm Ns where N is thedensity of atoms N Av s is thelinear attenuation coefficient 2 A cm2 as mentioned, in is morecommon g 6 Cross Section Consider scattering from a hard sphere What would you expect the cross section to be? α θ α b α R 7 Cross Section The units of cross section are barns 1 barn (b) = 10-28m2 = 10-24cm2 The units are area. One can think of the cross section as the effective target area for collisions. We sometimes take σ=πr2 8 Cross Section One can find the scattering rate by R I 0 NTs N / s N / s / cm cm N Av NT A 3 g / cm cm 2 / cm g 2 2 9 Cross Section For students working at collider accelerators R Ls L is t heluminosit yin / cm2 / s N s Ldt 2 Ldt is t he int egrat ed luminosit y in / cm In 2010 t he LHC delivered 35 pb-1 of int egrat edluminosit y In 2011t he LHC expect st o deliver 1 fb-1 A t ypicalcross sect ionat 7 T eV might be s tt 160pb 10 Photon Interactions In increasing order of energy the relevant photon interaction processes are Photoelectric effect Rayleigh scattering Compton scattering Photonuclear absorption Pair production 11 Photon Interactions Relative importance of the photoelectric effect, Compton scattering, and pair production versus energy and atomic number Z 12 Photoelectric Effect An approximate expression for the photoelectric effect cross section is 7/2 me c s pe 4 2 0 Z hv 8re2 25 2 0 6.65 10 cm 3 2 4 5 What’s important is that the photoelectric effect is important For high Z materials At low energies (say < 0.1 MeV) 13 Photoelectric Effect More detailed calculations show n Z s pe ~ m hv n variesfrom 4 - 5 m variesfrom 3.5 - 1 we' ll just taken 5 and m 3.5 14 Photon Interactions Typical photon cross sections 15 Photoelectric Effect The energy of the (photo)electron is Ee E Eb Binding energies for some of the heavier elements are shown on the next page Recall from the Bohr model, the binding energies go as me Z 2e4 1 13.6Z 2 En eV 2 2 2 2 n 40 2 n E1 13.6eV for H 16 Photoelectric Effect 17 Photoelectric Effect The energy spectrum looks like This is because at these photon/electron energies the electron is almost always absorbed in a short distance As are any x-rays emitted from the ionized atom 18 Photoelectric Effect and X-rays PE proportionality to Z5 makes diagnostic x- ray imaging possible Photon attenuation in Air – negligible Bone – significant (Ca) Soft tissue (muscle e.g.) – similar to water Fat – less than water Lungs – weak (density) Organs (soft tissue) can be differentiated by the use of barium (abdomen) and iodine (urography, angiography) 19 Photoelectric Effect and X-rays Typical diagnostic x-ray spectrum 1 anode, 2 window, 3 additional filters 20 Photon Interactions Sometimes easy to loose sight of real thickness of material involved Photon Interactions X-ray contrast depends on differing attenuation lengths Photoelectric Effect Related to kerma (Kinetic Energy Released in Mass Absorption) and absorbed dose is the fraction of energy transferred to the photoelectron T hv Eb hv hv As we learned in a previous lecture, removal of an inner atomic electron is followed by x-ray fluorescence and/or the ejection of Auger electrons The latter will contribute to kerma and absorbed dose 23 Photoelectric Effect Thus a better approximation of the energy transferred to the photoelectron is T hv PKYK hvK YK is theprobability of an x - ray when K shell is vacant PK is thefractionof PE interactions occurringin the K shell hvK is themean energy carried away by theK x - rays We can then define e.g. s tr pe hv PKYK hvK s pe hv trpe hv PKYK hvK pe hv 24 Photoelectric Effect Fluorescence yield Y for K shell 25 Cross Section dΩ=dA/r2=sinθdθdφ 26 Cross Section 27 Cross Section If a particle arrives with an impact parameter between b and b+db, it will emerge with a scattering angle between θ and θ+dθ If a particle arrives within an area of dσ, it will emerge into a solid angle dΩ 28 Cross Section From the figure on slide 7 we see b R sin and 2 then sin sin( / 2 / 2) cos( / 2) and b R cos( / 2) or 2 cos1 (b / R ) This is the relation between b and θ for hard sphere scattering 29 Cross Section We have ds ds d d where ds bdbd and d sin dd And the proportionality constant dσ/dΩ is called the differential cross section 30 Cross Section Then we have ds b db d sin d And for the hard sphere example db R sin d 2 2 2 ds Rb sin 2 R cos 2 sin 2 R 2 d 2 sin 2 sin 4 31 Cross Section Finally ds R s ds d d R 2 d 4 2 This is just as we expect The cross section formalism developed here is the same for any type of scattering (Coulomb, nuclear, …) Except in QM, the scattering is not deterministic 32 Cross Section We have ds ds d d where ds bdbd and d sin dd And the proportionality constant dσ/dΩ is called the differential cross section The total cross section σ is just ds s ds d d 33