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Radiation in the Natural World W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 1.

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Presentation on theme: "Radiation in the Natural World W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 1."— Presentation transcript:

1 Radiation in the Natural World W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 1

2 Scope of ANSEL Experiment 1 Ubiquitous presence of radiation on Earth, e.g., -ray photons Concepts of absorption coefficient and cross section Introduction to -interactions with matter Photo electric effect Compton scattering Pair production Operational principles of inorganic scintillation detectors Examples of energy spectra with NaI(Tl) detectors Experimental setup with a 3”x3” NaI(Tl) detector Lab measurements in Expt. 1 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 2

3 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 3

4 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 4 Probability and Cross Section Absorption upon intersection of nuclear cross section area  j beam current density ( #part.time x area ) A area illuminated by beam L= /mol Loschmidt# N T # target nuclei in beam M T target molar weight  T target density x target thickness []=1barn = cm 2 Target x Beam Transmitted Mass absorption coefficient  Thin target approximation elementary absorption cross section per nucleus Beam area A Nucleus area 

5 Differential Cross Section/Probability W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 5 Projectile current j 0 Ejectile numbers Projectile current j 0 Detector  det Ejectile numbers measured

6 Spherical Coordinates W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 6   dd dd rsin  rsin  d  r d  x y z dr dA Volume Element dV=dA·dr Unit of s.a. = sr (steradian)

7 Scope of ANSEL Experiment 1 Ubiquitous presence of radiation on Earth, e.g., -ray photons Concepts of absorption coefficient and cross section Introduction to -interactions with matter Photo electric effect Compton scattering Pair production Operational principles of inorganic scintillation detectors Examples of energy spectra with NaI(Tl) detectors Experimental setup with a 3”x3” NaI(Tl) detector Lab measurements in Expt. 1 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 7

8 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 8  -Induced Processes in Matter  -rays (photons): from electromagnetic transitions between different nuclear energy states  detect indirectly (charged particles, e -, e + ) Detection of secondary particles from: 1.Photo-electric absorption 2.Compton scattering 3.Pair production 4.  -induced reactions 1. Photo-electric absorption (Photo-effect) ħħ photon is completely absorbed by e -, which is kicked out of atom ħħ Electronic vacancies are filled by low-energy “Auger” transitions of electrons from higher orbits

9 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 9 1. Photo-Absorption Coefficient Absorption coefficient   (1/cm) “Mass absorption” is measured per density    (cm 2 /g)  “Cross section” is measured per atom    (cm 2 /atom) Absorption Coefficient  (cm 2 /g) Pt Wave Length (Å) Absorption of light is quantal resonance phenomenon: Strongest when photon energy coincides with transition energy (at K,L,… “edges”) Probabilities for independent processes are additive:  PE =  PE (K)+  PE (L)+…

10 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy Photon Scattering (Compton Effect)   ’

11 Compton Electron Spectrum Actually, not photons but recoil-electrons are detected Recoil-e - spectrum true finite resolution Compton Edge

12 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy Pair Creation by High-Energy  -rays {e +, e -,e - } triplet and one doublet in H bubble chamber Magnetic field provides momentum/charge analysis Event A)  -ray (photon) hits atomic electron and produces {e -,e + } pair Event B) one photon converts into a {e-,e+} pair In each case, the photon leaves no trace in the bubble chamber, before a first interaction with a charged particle (electron or nucleus). Magnetic field e-e- e-e- e-e- e+e+ e+e+  -rays A B

13 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 13 Dipping into the Fermi Sea: Pair Production Dirac theory of electrons and holes: World of normal particles has positive energies, E ≥ +mc 2 > 0 Fermi Sea is normally filled with particles of negative energy, E ≤-mc 2 < 0 Electromagnetic interactions can lift a particle from the Fermi Sea across the energy gap  E=2 mc 2 into the normal world  particle-antiparticle pair Holes in Fermi Sea: Antiparticles Minimum energy needed for pair production (for electron/positron) Energy 0 -m e c 2 +m e c 2 normally filled Fermi Sea normally empty e - hole e - particle EE -[m e c 2 +E kin ] +[m e c 2 +E kin ]

14 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 14 The Nucleus as Collision Partner Increase with E  because interaction sufficient at larger distance from nucleus Eventual saturation because of screening of charge at larger distances Excess momentum requires presence of nucleus as additional charged body. e+e+ e-e- recoil nucleus  Pb 1barn = cm 2

15 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 15  -Induced Nuclear Reactions Real photons or “virtual” elm field quanta of high energies can induce reactions in a nucleus: ( ,  ’ ), ( , n), ( , p), ( ,  ), ( , f) Nucleus can emit directly a high- energy secondary particle or, usually sequentially, several low-energy particles or  -rays. Can heat nucleus with (one)  -ray to boiling point, nucleus thermalizes, then “evaporates” particles and  - rays. n  nucleus   secondary radiation p incoming  -induced nuclear reactions are most important for high energies, E   (5 - 8)MeV

16 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 16 Efficiencies of  -Induced Processes Different processes are dominant at different  energies: Photo absorption at low E  Pair production at high E  > 5 MeV Compton scattering at intermediate E . Z dependence important: Ge(Z=32) has higher efficiency for all processes than Si(Z=14). Take high-Z for large photo-absorption coefficient Response of detector depends on detector material detector shape E 

17 Scope of ANSEL Experiment 1 Ubiquitous presence of radiation on Earth, e.g., -ray photons Concepts of absorption coefficient and cross section Introduction to -interactions with matter Photo electric effect Compton scattering Pair production Operational principles of inorganic scintillation detectors Examples of energy spectra with NaI(Tl) detectors Experimental setup with a 3”x3” NaI(Tl) detector Lab measurements in Expt. 1 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 17

18 ScintillationDet W. Udo Schröder, 2007 Scintillation Mechanism: Inorganic Scintillators Primary ionization and excitations of solid-state crystal lattice:  free (CB) e - or excitons (e -,h + ) sequential de-excitation with different E ph and time constant. Advantage of inorganic scintillator: high density, stopping power  good efficiency Disadvantage: slow response – s decay time, “after glow”, some are hygroscopic Electronic excitation: VB  CB (or below) Trapping of e- in activator states (Tl) doping material, in gs of activator band e transition emits lower E, not absorbed. 18

19 ScintillationDet W. Udo Schröder, 2007 Environment of  Scintillation Measurement 19

20 Scope of ANSEL Experiment 1 Ubiquitous presence of radiation on Earth, e.g., -ray photons Concepts of absorption coefficient and cross section Introduction to -interactions with matter Photo electric effect Compton scattering Pair production Operational principles of inorganic scintillation detectors Examples of energy spectra with NaI(Tl) detectors Experimental setup with a 3”x3” NaI(Tl) detector Lab measurements in Expt. 1 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 20

21 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 21 Shapes of Low-Energy  Spectra Photons/  -rays are measured only via their interactions with charged particles, mainly with the electrons of the detector material. The energies of these e - are measured by a detector. The energy E  of an incoming photon can be completely converted into charged particles which are all absorbed by the detector,  measured energy spectrum shows only the full-energy peak (FE, red) Example: photo effect with absorption of struck e - The incoming photon may only scatter off an atomic e - and then leave the detector  Compton-e - energy spectrum (CE, dark blue) An incoming  -ray may come from back-scattering off materials outside the detector  backscatter bump (BSc) measured energy measured intensity

22 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 22 measured energy (MeV)measured intensity Shapes of High-Energy  Spectra High-E  can lead to e + /e - pair production, e - : stopped in the detector e + : annihilates with another e - producing 2  -rays, each with E  = 511 keV. One of the 511 keV can escape detector  single escape peak (SE) at FE-511 keV Both of them can escape detector  double escape peak (DE) at FE MeV The energy spectra of high-energy g-rays have all of the features of low-energy  -ray spectra e + /e - annihilation in detector or its vicinity produces 511keV  -rays FE

23 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 23 Quiz Try to identify the various features of the  spectrum shown next (well, it is really the spectrum of electrons hit or created by the incoming or secondary photons), as measured with a highly efficient detector and a radio-active A Z source in a Pb housing. The  spectrum is the result of a decay in cascade of the radio-active daughter isotope A (Z-1) with the photons  1 and  2 emitted (practically) together Start looking for the full-energy peaks for  1,  2,…; then identify Compton edges, single- and double- escape peaks, followed by other spectral features to be expected. The individual answers are given in sequence on the following slides.

24 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 24 Spectrum of  Rays from Nuclear Decay

25 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 25 Spectrum of  Rays from Nuclear Decay 11

26 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 26 Spectrum of  Rays from Nuclear Decay 22 11

27 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 27 Spectrum of  Rays from Nuclear Decay 22 11 CE  2

28 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 28 Spectrum of  Rays from Nuclear Decay 22 11 SE  2 CE  2

29 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 29 Spectrum of  Rays from Nuclear Decay 22 11 SE  2 DE  2 CE  2

30 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 30 Spectrum of  Rays from Nuclear Decay 22 11 SE  2 DE  keV CE  2

31 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 31 Spectrum of  Rays from Nuclear Decay 22 11 BSc SE  2 DE  keV CE  2

32 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 32 Spectrum of  Rays from Nuclear Decay 22 11 BSc SE  2 DE  keV CE  2 Pb X-rays

33 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 33 Spectrum of  Rays from Nuclear Decay 1+21+2 22 11 BSc SE  2 DE  keV CE  2 Pb X-rays

34 Scope of ANSEL Experiment 1 Ubiquitous presence of radiation on Earth, e.g., -ray photons Concepts of absorption coefficient and cross section Introduction to -interactions with matter Photo electric effect Compton scattering Pair production Operational principles of inorganic scintillation detectors Examples of energy spectra with NaI(Tl) detectors Experimental setup with a 3”x3” NaI(Tl) detector Lab measurements in Expt. 1 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 34

35 W. Udo Schröder, 2004 Principles Meas 35 Slow Fast Produce logical signal  Principle of Fast-Slow Signal Processing Discrim CFTD PreAmp Amp Gate Gener ator Data Acquisition System Energy Gate 0 0 t t CFTD Output CFTD Internal t CFTD Input Principle of a Constant-Fraction Timing Discriminator:  t independent of E here f = 0.5 Source Produce analog signal  Binary data to computer NaI Det.

36 Scope of ANSEL Experiment 1 Ubiquitous presence of radiation on Earth, e.g., -ray photons Concepts of absorption coefficient and cross section Introduction to -interactions with matter Photo electric effect Compton scattering Pair production Operational principles of inorganic scintillation detectors Examples of energy spectra with NaI(Tl) detectors Experimental setup with a 3”x3” NaI(Tl) detector Lab measurements in Expt. 1 W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 36

37 Lab Measurements Expt. 1 With the help of the TA set up detector, electronics and data acquisition: Power up the NaI detector (+1750 V) and the electronics NIM and CAMAC bins. Place a 22 Na  source close (5 cm) to the face of the NaI. On the scope, follow the analog pulse along the slow circuit. –Check the effects of the settings of gain and time constant controls at the TC 248 main amplifier. On the scope, inspect the output of the fast (lower) part of the TC248 and feed it to a discriminator used to derive a digital signal for strobing the ADC in the CAMAC crate. Trigger the scope Ch 1 with the discriminator output signal, view on Ch 2 the analog signal and ascertain a proper (low) setting of the discriminator threshold. Feed analog signal to the ADC (Ch 7) Feed the digital signal to the CC-USB CAMAC controller Input 1 and use the signal appearing at Gate 1 output to strobe the ADC. Check on the scope the proper relative timing of analog and strobe signals. Start the EZDAQ data acquisition according to the EZDAQ setup checklist. Accumulate, display and save a 22 Na  energy spectrum in histogram form. W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 37

38 Source Info W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 38 Counts/keV Set of typical Calibration -ray sources Semi-log plot of a calibrated 22 Na -ray spectrum taken with a 3”x3’ NaI(Tl) detector

39 Lab Measurements Expt. 1 (cont’d) Check the appearance of the NaI spectrum for the 22 Na  source and place the dominant structure in the middle of the spectrum by adjusting fine and coarse gain of the main amplifier. After the above choice of gain (and previous integration) parameters, do not change the amplifier settings for any of the additional measurements. Take a final measurement for the Na source (5 min). Then remove this source and place it far away from the detector (in the cabinet). Based on the 22 Na  energy spectrum, perform a coarse calibration of the ADC channel numbers in -ray energy. In this task utilize the well measured channel # positions of the full-energy peak (1.275 MeV), of the associated Compton edge (E CE = ?? MeV) and of the MeV annihilation peak. Perform similar, individual measurements for the 60 Co and 54 Mn sources. Verify that the main  lines for these sources appear in the spectrum approximately at the expected locations. Measure the -ray energy spectrum for the unknown source. Remove all sources from the vicinity of the NaI detector and perform a measurement of -ray energy spectrum of the room background. To accumulate sufficient intensity, accumulate data for at least several hours (possibly overnight). W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 39

40 Data Analysis Expt. 1 Identify in the measured spectra for the three known sources the prominent spectral features and correlate their channel positions (ch#) with the known energies (E  or E CE ). In the fits keep track of experimental errors. Use Gaussians for  lines and half-Gaussians for Compton edges. Generate a calibration table and a plot of energies of the positively identified prominent spectral features from the three known sources ( 22 Na, 60 Co, 54 Mn) vs. the experimental channel numbers for these features. Perform a least-squares fit for the calibration data E  (ch#) and include the best-fit line in the calibration table and plot. Generate plots of all measured energy spectra as Counts/keV vs. Energy/MeV. Identify the -ray energies of prominent features in the spectrum for the unknown source. Based on the provided search table, suggest the identity of the unknown source (or source mix). Identify the -ray energies of prominent features in the spectrum for the room background. Based on the provided search table, suggest the identities of the various components. W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 40

41 Sample Spectrum W. Udo Schröder, 2010 ANSEL Expt 1: Gamma Spectroscopy 41 Counts/channel


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