Radiation Signals and Signatures in a Detector (Gamma spectroscopy) Sangkyu Lee

Radiation Signals and Signatures in a Detector (Gamma spectroscopy) Sangkyu Lee

Photon interactions Photoelectric effect Compton scatter Pair production μ= τ + σ + κ μ = Total cross section τ = Photoelectric cross section σ = Compton scatter cross section κ = Pair production cross section Determination of beta attenuation coefficients by means of timing method E.E. Ermis, Ege University, Faculty of Science, Physics Department, 35100 Bornova, Izmir, Turkey

Photoelectric effect The photon is captured by an orbital electron The photon disappears all it s energy is imparted to the electron The electron is ejected from orbit http://commons.wikimedia.org/wiki/file:photoelectric_effect_schematic-de.svg

Photoelectric cross section a: constant N: atomic density m, n: 3 to 5 depending on Eγ The probability that a photon will pass through while traveling distance x is e τx ". The probability that a photon will undergo photo capture while traveling distance x is 1 e τx.

Photoelectric effect Photo peak E γ

Compton scatter The photon scatters off an orbital electron The photon imparts some (but not all) of its energy to the electron The electron is ejected from orbit http://hyperphysics.phyastr.gsu.edu/hbase/quantum/imgqua/compton.gif

Compton scatter physics Conservation of energy E γ + m e c 2 = E γ + E e p γ E γ E γ θ Relativistic p γ m e c 2 φ p e = 1 c E e 2 (me c 2 ) 2 p e E e Conservation of momentum x-axis : p γ = p γ cosθ + p e cosφ y-axis : 0 = p γ sinθ p e sinφ

Compton scatter physics Energy of the scattered photon Recoil energy of the electron

Compton continuum http://ns.ph.liv.ac.uk/~ajb/radiometrics/glossary/compton_continuum.html http://en.wikipedia.org/wiki/file:klein-nishina_distribution.png

Compton edge The maximum electron recoil energy is called the Compton edge E e = E γ E γ (minimum, θ =180) (E γ = E γ 1+ 1 cosθ E γ /m e c 2 = E γ 1+2E γ /m e c 2) Example(Cs-137=661.7 kev) E γ E e = E γ 1 + 2E = 661.7 kev γ m e c 2 = 477.37 kev 1 + 661.7 kev 2 661.7 kev 511keV

Backscatter Some photons will undergo Compton scatter before entering the detector. (The interaction happens at the outside of detector) For photons undergoing backscatter peak (θ=180): E E γ γ = 1 + 2E γ /m e c 2 Example(Cs-137=661.7 kev) E γ = E γ 1 + 2E γ /m e c 2 = 661.7 kev 2 661.7 kev 1 + 511keV = 184.3 kev

Photoelectric + Compton scatter Photo peak Backscatter peak Compton continuum Compton edge E γ = E γ 1 + 2E γ /m e c 2 E e = E γ E γ 1 + 2E γ m e c 2 E γ

Pair production The photon splits into an electron/positron pair Only happens in the presence of a nucleus E γ > 1022 kev http://commons.wikimedia.org/wiki/file:pair_production.png The positron will eventually annihilate with an electron to produce two 511 kev photons

Escape peaks by pair production e e + e Single escape peak E se = E γ 511keV Double escape peak e + E de = E γ 1022keV e e + Photo peak E γ = E γ

Annihilation peak (511keV) β+ γ: 511keV β- γ: 511keV Radiation Detection and Measurement - Glenn F. Knoll (Wiley) Annihilation peak (511keV) 1. Positron by pair production in surrounding materials 2. Positron by pair production in detectors 3. Beta + decay source

Characteristic X-ray http://www.amptek.com/xrf.html Radiation Detection and Measurement - Glenn F. Knoll (Wiley) X-ray peak 1. Surrounding materials 2. Detector materials

Pulse pileup Pulse pile-up happens when pulses arrives closer in time than the pulse resolution time for the system. http://www.radiationsolutions.ca/fileadmin/pdf/pulse_pile-up.pdf Example Cs-137: 661.7 kev + 661.7 kev = 1323.4 kev Co-60: 1173 kev + 1332 kev = 2505 kev

Photoelectric + Compton scatter + Pair production + X-ray + Pulse pileup Photo peak Annihilation peak X-ray peak Double escape peak Single escape peak Backscatter peak Compton continuum Compton edge X-ray escape peak Sum peak E E γ E E γ 0.511MeV γ = e = E γ 1 + 2E 1 + 2E γ /m e c 2 γ m e c 2 E γ = E γ 1.022MeV E γ = E γ 0.511MeV E γ 2E γ

HPGe and NaI detector http://www.amptek.com/grad.html

MCNP6 Simulation of HPGe and Count probability Count probability Count probability NaI detectors 10-1 NaI_Co-60 10-1 HPGe_Co-60 10-2 10-2 Count probability 10-3 10-4 10-3 10-4 10-5 10-5 0.0 0.5 1.0 1.5 2.0 MeV 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 MeV 2.5 3.0 3.5 Gaussian Broadening Function 10-1 NaI_Co-60(GEB) HPGe_Co-60(GEB) 10-2 10-2 10-3 10-4 10-5 10-3 10-4 10-6 10-5 10-7 10-6 0.0 0.5 1.0 1.5 2.0 MeV 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 MeV 2.5 3.0 3.5

Conclusion Even one photo peak can generate several peak signatures. (Photo peak, SE, DE, Back scatter, Compton edge, etc..) It is possible to predict(calculate) all signal and signature energies coming from a radiation source in a detector. Spectra might be differ by characteristics of detector. (Material difference Cross section, Energy resolution, X-ray)

Questions?

References [1] Glenn F. Knoll, Radiation Detection and Measurement, Forth Edition (2010) [2] Nicholas Tsoulfanidis, Measurement and Detection of Radiation, Third Edition (2010)