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

2. Scope of ANSEL Experiment 1
Ubiquitous presence of radiation on Earth, e.g., g-ray photons
Concepts of absorption coefficient and cross section
Introduction to g-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
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

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

4. Probability and Cross Section
Mass absorption coefficient m
Target x
Beam
Transmitted
Beam area A
Nucleus area s
Thin target approximation
Absorption upon intersection of nuclear cross section area s
j beam current density (#part.time x area) A area illuminated by beam L= /mol Loschmidt# NT # target nuclei in beam MT target molar weight rT target density x target thickness
[s]=1barn = 10-24cm2
W. Udo Schröder, 2010
elementary absorption cross section per nucleus
ANSEL Expt 1: Gamma Spectroscopy

5. Differential Cross Section/Probability
Projectile current j0
Ejectile numbers
W. Udo Schröder, 2010
Detector DWdet
Ejectile numbers measured
Projectile current j0
ANSEL Expt 1: Gamma Spectroscopy

6. Spherical Coordinates
Volume Element dV=dA·dr dr q f dq df
rsinq
rsinq df
r dq x y z dA
W. Udo Schröder, 2010
Unit of s.a. = sr (steradian)
ANSEL Expt 1: Gamma Spectroscopy

7. Scope of ANSEL Experiment 1
Ubiquitous presence of radiation on Earth, e.g., g-ray photons
Concepts of absorption coefficient and cross section
Introduction to g-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
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

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

9. 1. Photo-Absorption Coefficient
Absorption coefficient  m (1/cm)
“Mass absorption” is measured per density r
 m/r (cm2/g)
“Cross section” is measured per atom
 s (cm2/atom) Pt
Absorption Coefficient m/r (cm2/g)
Absorption of light is quantal resonance phenomenon: Strongest when photon energy coincides with transition energy (at K,L,… “edges”)
ANSEL Expt 1: Gamma Spectroscopy
Wave Length l (Å)
Probabilities for independent processes are additive:
mPE = mPE(K)+mPE(L)+…
W. Udo Schröder, 2010

10. 2. Photon Scattering (Compton Effect)q f l l’
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

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

12. 3. Pair Creation by High-Energy g-rays
{e+, e-,e-} triplet and one doublet in H bubble chamber
Magnetic field provides momentum/charge analysis
Event A) g-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). A B
ANSEL Expt 1: Gamma Spectroscopy
Magnetic field
W. Udo Schröder, 2010

13. Dipping into the Fermi Sea: Pair Production
Dirac theory of electrons and holes:
World of normal particles has positive energies, E ≥ +mc2 > 0
Fermi Sea is normally filled with particles of negative energy, E ≤-mc2 < 0
Electromagnetic interactions can lift a particle from the Fermi Sea across the energy gap DE=2 mc2 into the normal world  particle-antiparticle pair
Holes in Fermi Sea: Antiparticles
Minimum energy needed for pair production (for electron/positron)
Energy
-mec2
+mec2
normally filled Fermi Sea
normally empty
e-hole
e-particle Eg
-[mec2+Ekin]
+[mec2+Ekin]
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

14. The Nucleus as Collision Partner
recoil nucleus g
Excess momentum requires presence of nucleus as additional charged body. Pb
ANSEL Expt 1: Gamma Spectroscopy
Increase with Eg because interaction sufficient at larger distance from nucleus
Eventual saturation because of screening of charge at larger distances
1barn = 10-24cm2
W. Udo Schröder, 2010

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

16. Efficiencies of g-Induced Processes
Different processes are dominant at different g energies:
Photo absorption at low Eg
Pair production at high Eg > 5 MeV
Compton scattering at intermediate Eg.
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 Eg
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

17. Scope of ANSEL Experiment 1
Ubiquitous presence of radiation on Earth, e.g., g-ray photons
Concepts of absorption coefficient and cross section
Introduction to g-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
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

18. 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 Eph and time constant.
Advantage of inorganic scintillator: high density, stopping power  good efficiency
Disadvantage: slow response – ms 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 Eg, not absorbed.
ScintillationDet
W. Udo Schröder, 2007

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

20. Scope of ANSEL Experiment 1
Ubiquitous presence of radiation on Earth, e.g., g-ray photons
Concepts of absorption coefficient and cross section
Introduction to g-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
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

21. Shapes of Low-Energy g Spectra
Photons/g-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 Eg 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-
measured energy
measured intensity
The incoming photon may only scatter off an atomic e- and then leave the detector  Compton-e- energy spectrum (CE, dark blue)
ANSEL Expt 1: Gamma Spectroscopy
An incoming g-ray may come from back-scattering off materials outside the detector  backscatter bump (BSc)
W. Udo Schröder, 2010

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

23. ANSEL Expt 1: Gamma Spectroscopy
Quiz
Try to identify the various features of the g 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 AZ source in a Pb housing.
The g spectrum is the result of a decay in cascade of the radio-active daughter isotope A(Z-1) with the photons g1 and g2 emitted (practically) together
Start looking for the full-energy peaks for g1, g2,…; 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.
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

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

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

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

27. Spectrum of g Rays from Nuclear Decay
ANSEL Expt 1: Gamma Spectroscopy
CE g2
W. Udo Schröder, 2010

28. Spectrum of g Rays from Nuclear Decay
SE g2
ANSEL Expt 1: Gamma Spectroscopy
CE g2
W. Udo Schröder, 2010

29. Spectrum of g Rays from Nuclear Decay
DE g2
SE g2
ANSEL Expt 1: Gamma Spectroscopy
CE g2
W. Udo Schröder, 2010

30. Spectrum of g Rays from Nuclear Decay
511 keV
DE g2
SE g2
ANSEL Expt 1: Gamma Spectroscopy
CE g2
W. Udo Schröder, 2010

31. Spectrum of g Rays from Nuclear Decay
BSc g2
511 keV
DE g2
SE g2
ANSEL Expt 1: Gamma Spectroscopy
CE g2
W. Udo Schröder, 2010

32. Spectrum of g Rays from Nuclear Decay
Pb X-rays g1
BSc g2
511 keV
DE g2
SE g2
ANSEL Expt 1: Gamma Spectroscopy
CE g2
W. Udo Schröder, 2010

33. Spectrum of g Rays from Nuclear Decay
Pb X-rays g1
BSc g2
511 keV
DE g2
SE g2
ANSEL Expt 1: Gamma Spectroscopy
CE g2
g1+g2
W. Udo Schröder, 2010

34. Scope of ANSEL Experiment 1
Ubiquitous presence of radiation on Earth, e.g., g-ray photons
Concepts of absorption coefficient and cross section
Introduction to g-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
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

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

36. Scope of ANSEL Experiment 1
Ubiquitous presence of radiation on Earth, e.g., g-ray photons
Concepts of absorption coefficient and cross section
Introduction to g-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
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

37. ANSEL Expt 1: Gamma Spectroscopy
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 22Na g 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 22Na g energy spectrum in histogram form.
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

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

39. Lab Measurements Expt. 1 (cont’d)
Check the appearance of the NaI spectrum for the 22Na g 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 22Na g energy spectrum, perform a coarse calibration of the ADC channel numbers in g-ray energy. In this task utilize the well measured channel # positions of the full-energy peak (1.275 MeV), of the associated Compton edge (ECE = ?? MeV) and of the MeV annihilation peak.
Perform similar, individual measurements for the 60Co and 54Mn sources.
Verify that the main g lines for these sources appear in the spectrum approximately at the expected locations.
Measure the g-ray energy spectrum for the unknown source.
Remove all sources from the vicinity of the NaI detector and perform a measurement of g-ray energy spectrum of the room background. To accumulate sufficient intensity, accumulate data for at least several hours (possibly overnight).
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

40. ANSEL Expt 1: Gamma Spectroscopy
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 (Eg or ECE). In the fits keep track of experimental errors. Use Gaussians for g 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 (22Na, 60Co, 54Mn) vs. the experimental channel numbers for these features.
Perform a least-squares fit for the calibration data Eg (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 g-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 g-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.
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010

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