Transcript Slide 1

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Rad. Int. with Matter: Gammas
Interaction of Radiation with Matter
Gamma Rays
W. Udo Schröder, 2008
g-Induced Processes
Rad. Int. with Matter: Gammas
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g-rays (photons) come from electromagnetic transitions between
different energy states of a system  important structural information
Detection of secondary
particles from:
1. Photo-electric absorption
2. Compton scattering
3. Pair production
4. g-induced reactions
1. Photo-electric absorption
(Photo-effect)
Ekin    En ; En  binding energy
En
Z  

 Rhc 
n
2
2
Moseley ' s Law
Rhc  13.6 eV Rydberg constant
screening constants
 K  3,  L  5, different subshells
W. Udo Schröder, 2008
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
Absorption Coefficient m/r (cm2/g)
Rad. Int. with Matter: Gammas
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1. Photo-Absorption Coefficient
Absorption coefficient
 m (1/cm)
“Mass absorption” is
measured per density r
Pt
 m/r (cm2/g)
“Cross section” is
measured per atom
  (cm2/atom)
Wave Length l (Å)
Probabilities for independent
processes are additive:
mPE = mPE(K)+mPE(L)+…
W. Udo Schröder, 2008
Absorption of light is
quantal resonance
phenomenon: Strongest
when photon energy
coincides with transition
energy (at K,L,… “edges”)
 PE ( Eg , Z )  Z 5  Eg7 4 low Eg
 PE ( Eg , Z )  Z 5  Eg1 2 high Eg
2. Photon Scattering (Compton Effect)
Relativistic E 2  ( pc) 2  (m0c 2 ) 2 photons : m0  mg  0
 Eg   g  pg c

4
l
f
Momentum balance :
pe  pg  pg 
pe2 c 2  Eg2  Eg2  2 Eg Eg   cos 
Energy balance :
Eg  me c  Eg  
Rad. Int. with Matter: Gammas
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l   l  lC  1  cos  
" Compton wave length lC "
2
lC 
 2.426 pm
me c
W. Udo Schröder, 2008
pe c   pg  pg  c
2
Eg  
 pec 
Eg
2
  mec

2 2
1   Eg mec 2  1  cos  
me c 2  0.511MeV
2
Compton Electron Spectrum
Scattered  photon energy
Actually, not photons but
recoil-electrons are detected
Eg
Eg  
Compton Energy Sp ectrum
0.644 0.7
Cross Section (b)
true
0.4
ddE( E)
0.3
0.2
Compton Edge
spectrum
0.5
Nexp( E)
Scattered recoil  electron energy :
Recoil-e-
0.6
Ekin  Eg  Eg  
0
0
0.2
0.4
0.6
E
Energy ( MeV)
0.8
1   Eg me c 2  1  cos 
Eg
Eg  
1  2 Eg me c 2
Maximum electron energy (Compton Edge) :
0
0
Eg  Eg mec 2  1  cos 
Minimum photon energy :   1800
finite
resolution
0.1
1   Eg mec 2  1  cos 
1
1
Ekin  ECE  Eg

2 Eg me c 2

1  2  Eg mec 2 
3. Pair Creation by High-Energy g-rays
g-rays
{e+, e-,e-} triplet and one doublet in
H bubble chamber
A
e-
Magnetic field provides
momentum/charge analysis
Event A) g-ray (photon) hits atomic
electron and produces {e-,e+} pair
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e+
Rad. Int. with Matter: Gammas
B
Event B) one photon converts into a
{e-,e+} pair
ee+
e
Magnetic field
W. Udo Schröder, 2008
In each case, the photon leaves no
trace in the bubble chamber, before
a first interaction with a charged
particle (electron or nucleus).
Dipping into the Fermi Sea: Pair Production
Dirac theory of electrons and holes:
World of normal particles has positive
energies, E ≥ +mc2 > 0
Energy
normally empty
Fermi Sea is normally filled with
particles of negative energy, E ≤-mc2 < 0
e-particle
7
+[mec2+Ekin
+mec2
]
Rad. Int. with Matter: Gammas
0
Eg
-mec2
-[mec2+Ekin]
W. Udo Schröder, 2008
e-hole
normally filled
Fermi Sea
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)
Eg  EThreshold  2mec2  1.022MeV
The Nucleus as Collision Partner
recoil
nucleus
e-
g
Eg  EThreshold  2me c 2


Actually converted : Eg  2me c 2  Ekin
 Ekin
 ....
Rad. Int. with Matter: Gammas
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e+
Pb
Excess momentum requires presence of
nucleus as additional charged body.
2
 P( Z , Eg )
d PP
2 1  e
Z


2 
dEkin
137  me c  Eg  2me c 2
2
5.81028 cm2
Eg  2 me c 2
P slowly varying
1barn = 10-24cm2
W. Udo Schröder, 2008
Increase with Eg because interaction
sufficient at larger distance from nucleus
Eventual saturation because of screening
of charge at larger distances
4. g-Induced Nuclear Reactions
Real photons or “virtual” elm field
quanta of high energies can induce
reactions in a nucleus:
secondary
radiation
n
9
g
p
incoming
Rad. Int. with Matter: Gammas
nucleus
Nucleus can emit directly a higha energy secondary particle or, usually
sequentially, several low-energy
g
particles or g-rays.
g-induced nuclear reactions
are most important for high
energies, Eg  (5 - 8)MeV
W. Udo Schröder, 2008
(g, g’ ), (g, n), (g, p), (g, a), (g, f)
Can heat nucleus with (one) g-ray to
boiling point, nucleus thermalizes,
then “evaporates” particles and grays.
Efficiencies of g-Induced Processes
Different processes are dominant at
different g energies:
Photo absorption at low Eg
Rad. Int. with Matter: Gammas
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Pair production at high Eg
W. Udo Schröder, 2008
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
Escape Geometries for Real Detectors
High-energy g-ray leading to e+/epair production, with e- stopped in
the detector.
g
g e+ is also stopped in the detector
511 and annihilates with another ekeV producing 2 g-rays of Eg = 511 keV
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Ge
crystal
Rad. Int. with Matter: Gammas
e-
g
511
keV
W. Udo Schröder, 2008
e+
each. If both g-rays are absorbed
 full energy Eg is absorbed by
detector  event is in FE peak.
If one g-ray escapes detector 
event is in SE peak at FE-511 keV
If both of them escape  event is
in detector  DE peak at FE-1.022
MeV.
Relative probabilities depend on
detector size!
Shapes of Low-Energy g Spectra
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 intensity
Rad. Int. with Matter: Gammas
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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.
measured energy
The incoming photon may only
scatter off an atomic e- and then
leave the detector  Compton-eenergy spectrum (CE, dark blue)
An incoming g-ray may come from back-scattering off materials
outside the detector  backscatter bump (BSc)
W. Udo Schröder, 2008
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+/epair production,
FE
e-: stopped in the detector
measured intensity
Rad. Int. with Matter: Gammas
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e+: annihilates with another
e- producing 2 g-rays, each
with Eg = 511 keV.
One of them can escape
detector  single escape
peak (SE) at FE-511 keV
Both of them can escape
detector  double escape
peak (DE) at FE-1.022 MeV
measured energy (MeV)
e+/e- annihilation in detector or its vicinity produces 511keV g-rays
W. Udo Schröder, 2008
Rad. Int. with Matter: Gammas
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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 doubleescape peaks, followed by other spectral features to
be expected.
• The individual answers are given in sequence on the
following slides.
W. Udo Schröder, 2008
Rad. Int. with Matter: Gammas
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Spectrum of g Rays from Nuclear Decay
W. Udo Schröder, 2008
Spectrum of g Rays from Nuclear Decay
Rad. Int. with Matter: Gammas
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g1
W. Udo Schröder, 2008
Spectrum of g Rays from Nuclear Decay
Rad. Int. with Matter: Gammas
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g1
W. Udo Schröder, 2008
g2
Spectrum of g Rays from Nuclear Decay
Rad. Int. with Matter: Gammas
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g1
W. Udo Schröder, 2008
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
Rad. Int. with Matter: Gammas
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g1
W. Udo Schröder, 2008
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
Rad. Int. with Matter: Gammas
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g1
W. Udo Schröder, 2008
g2
DE
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
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g1
g2
Rad. Int. with Matter: Gammas
511
keV
W. Udo Schröder, 2008
DE
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
BSc
Rad. Int. with Matter: Gammas
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g1
W. Udo Schröder, 2008
g2
511
keV
DE
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
Pb X-rays
BSc
Rad. Int. with Matter: Gammas
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g1
W. Udo Schröder, 2008
g2
511
keV
DE
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
Pb X-rays
BSc
Rad. Int. with Matter: Gammas
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g1
W. Udo Schröder, 2008
g2
511
keV
DE
g2
SE
g2
CE
g2
g1+g
2
Photo
Multiplier
Reducing Background with Anti-Compton “Shields”
g
BGO
Scintillator
Rad. Int. with Matter: Gammas
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g
W. Udo Schröder, 2008
e-
e+
Ge
crystal
High-energy g-rays
produce e+/e- pairs in
the primary Ge detector.
All e+ and e- are stopped
in the Ge detector.
e+ finds an e- and
annihilates with it,
producing 2 back-to-back
511-keV photons.
Escaping 511-keV photons
are detected by
surrounding annular
scintillation detector.
Escape events are
“tagged” and can be
rejected.
Compton Suppression Technique
Rad. Int. with Matter: Gammas
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The figure
shows a
comparison
between the
“raw” 60Co gray spectrum
(in pink) and
one (blue)
where Compton
contributions
have been
removed.
The disturbing Compton background has been reduced by app. a factor
8 by eliminating all events, where a photon has been detected by the
BGO scintillation shield counter in coincidence with a g-ray in the
corresponding inner Ge detector.
W. Udo Schröder, 2008
Rad. Int. with Matter: Gammas
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g-Ray/Photon Detectors
There is a variety of detectors for nuclear radiation,
including g-rays. A special presentation is dedicated
to the main detector principles.
The next image shows a section of one of the currently
modern g detector arrays, the “Gamma-Sphere.”
The sphere surrounds the reaction chamber on all sides
and leaves only small holes for the beam and target
mechanisms.
Each element of the array consists of two different
detector types, a high-resolution Ge-solid-state
detector encapsulated in a low-resolution BGO
scintillation counter detecting Compton-scattered
photons escaping from the Ge detectors
W. Udo Schröder, 2008
Modern g Detectors: “Gammasphere”
Rad. Int. with Matter: Gammas
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Compton
Suppression
W. Udo Schröder, 2008
Rad. Int. with Matter: Gammas
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e
n
d
W. Udo Schröder, 2008