Transcript Slide 1

ANSEL Expt 1: Gamma Spectroscopy
1
Radiation in the Natural World
W. Udo Schröder, 2010
Scope of ANSEL Experiment 1
• Ubiquitous presence of radiation on Earth, e.g., g-ray photons
ANSEL Expt 1: Gamma Spectroscopy
2
• 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
W. Udo Schröder, 2010
W. Udo Schröder, 2010
ANSEL Expt 1: Gamma Spectroscopy
3
Probability and Cross Section


4

N  N0  e    x
N0  j  A
Beam
area A
Nucleus
area s
x
Target
Absorption upon intersection of nuclear
cross section area s
W. Udo Schröder, 2010
Mass absorption coefficient 
Transmitted
Beam
j beam current density (#part.time x area)
A area illuminated by beam
L= 6.022 1023/mol Loschmidt#
NT # target nuclei in beam
MT target molar weight
rT target density
x target thickness
[s]=1barn = 10-24cm2
ANSEL Expt 1: Gamma Spectroscopy
Pabsorption 
 # nuclei  
x  

 
in
beam

  per nucleus 
 L
 s 

 
rT    A  x     

 MT
  A





N abs  N 0  N  N 0  1  e    x

Thin target approximation

N abs


 L rT Ax 

s
 MT 
current density j
N0
 N0     x  
A
 NT js

N abs
s 
Nnucl  j
elementary absorption
cross section per
nucleus
Differential Cross Section/Probability
Reaction A  a  B  b
Reaction cross section s : s A( a ,b )B
5
A( a ,b )B
d
Projectile
current j0


Ejectile
numbers
dNb  ,  
W. Udo Schröder, 2010
dNb  ,    j0  NT 
dNb  ,  
NT
Projectile
current j0
Flux of particles b
d
Detector
Ddet

ANSEL Expt 1: Gamma Spectroscopy
ds  ,  
d
 d
 ds
 j0  NT 
ds  ,  
d
Ejectile numbers measured
 dNb  ,  
ds  , 
DNb  ,   
  Ddet 
d
 d  
Spherical Coordinates
Volume Element dV=dA·dr
z
r
Spherical Coordinates
x  r sin   cos 
y  r sin   sin 
z  r cos 
6
Volume Element
d 3 r  dx  dy  dz



 r 2  dr  d  sin   d 

Solid angle element
W. Udo Schröder, 2010
d   d  sin   d 
dA
r2
Integral :
Total Solid Angle :
dA 4 r 2
   d  
 2  4
2
r
sphere r
ANSEL Expt 1: Gamma Spectroscopy

2
0
0
   d    d sin 
 d  4
Unit of s.a. = sr (steradian)
Scope of ANSEL Experiment 1
• Ubiquitous presence of radiation on Earth, e.g., g-ray photons
ANSEL Expt 1: Gamma Spectroscopy
7
• 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
W. Udo Schröder, 2010
g-Induced Processes in Matter
ANSEL Expt 1: Gamma Spectroscopy
8
g-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. g-induced reactions
Ekin    En ; En  binding energy
En
Z s 
 Rhc 
n
2
2
Moseley ' s Law
Rhc  13.6 eV Rydberg constant
screening constants
s K  3, s L  5, different subshells
W. Udo Schröder, 2010
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
Absorption Coefficient /r (cm2/g)
ANSEL Expt 1: Gamma Spectroscopy
9
1. Photo-Absorption Coefficient
Absorption coefficient
  (1/cm)
“Mass absorption” is
measured per density r
Pt
 /r (cm2/g)
“Cross section” is
measured per atom
 s (cm2/atom)
Wave Length l (Å)
Probabilities for independent
processes are additive:
PE = PE(K)+PE(L)+…
W. Udo Schröder, 2010
Absorption of light is
quantal resonance
phenomenon: Strongest
when photon energy
coincides with transition
energy (at K,L,… “edges”)
s PE ( Eg , Z )  Z 5  Eg7 4 low Eg
s 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
10
l
Momentum balance :


pe  pg  pg 
pe2 c 2  Eg2  Eg2  2 Eg Eg   cos 
Energy balance :
Eg  me c  Eg  
ANSEL Expt 1: Gamma
Spectroscopy
2
l   l  lC  1  cos  
" Compton wave length lC "
2
lC 
 2.426 pm
me c
W. Udo Schröder, 2010
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
Actually, not photons but
recoil-electrons are detected
true
0.4
dsdE( E)
0.3
0.2
Ekin  Eg  Eg  
Compton Edge
Cross Section (b)
0.5
0
0.2
0.4
0.6
E
Energy ( MeV)
0.8
Eg  Eg mec 2  1  cos  
1   Eg me c 2  1  cos  
(" Backscatter ")
Eg  
Eg
1  2 Eg me c 2
Maximum electron energy (Compton Edge) :
0
0
1   Eg mec 2  1  cos  
Minimum photon energy :   1800
finite
resolution
0.1
Eg
Scattered recoil  electron energy :
Recoil-espectrum
0.6
0
Eg  
Compton Energy Sp ectrum
0.644 0.7
Nexp( E)
Scattered  photon energy
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
12
e+
ANSEL Expt 1: Gamma Spectroscopy
B
Event B) one photon converts into a
{e-,e+} pair
ee+
e
Magnetic field
W. Udo Schröder, 2010
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
13
+[mec2+Ekin
+mec2
]
0
Eg
-mec2
ANSEL Expt 1: Gamma
Spectroscopy
-[mec2+Ekin]
W. Udo Schröder, 2010
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
14
g
Eg  EThreshold  2me c 2
e-


Actually converted : Eg  2me c 2  Ekin
 Ekin
 ....
e+
Excess momentum requires presence of
nucleus as additional charged body.
Pb
5.81028 cm2
ANSEL Expt 1: Gamma
Spectroscopy
2
 P( Z , Eg )
ds PP
2 1  e
Z



dEkin
137  me c 2  Eg  2me c 2
2
Eg  2 me c 2
P slowly varying
1barn = 10-24cm2
W. Udo Schröder, 2010
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
15
g
p
incoming
ANSEL Expt 1: Gamma
Spectroscopy
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, 2010
(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
ANSEL Expt 1: Gamma
Spectroscopy
16
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
W. Udo Schröder, 2010
Scope of ANSEL Experiment 1
• Ubiquitous presence of radiation on Earth, e.g., g-ray photons
ANSEL Expt 1: Gamma Spectroscopy
17
• 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
W. Udo Schröder, 2010
Scintillation Mechanism: Inorganic Scintillators
Primary ionization and excitations
of solid-state crystal lattice:
ScintillationDet
18
 free (CB) e- or excitons (e-,h+)
sequential de-excitation
with different Eph and time constant.
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.
W. Udo Schröder, 2007
Advantage of inorganic scintillator:
high density, stopping power
 good efficiency
Disadvantage: slow response – s
decay time, “after glow”,
some are hygroscopic
ScintillationDet
19
Environment of g Scintillation Measurement
W. Udo Schröder, 2007
Scope of ANSEL Experiment 1
• Ubiquitous presence of radiation on Earth, e.g., g-ray photons
ANSEL Expt 1: Gamma Spectroscopy
20
• 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
W. Udo Schröder, 2010
Shapes of Low-Energy g Spectra
measured intensity
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-
ANSEL Expt 1: Gamma
Spectroscopy
21
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, 2010
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
FE
High-Eg can lead to e+/e- pair
production,
e-: stopped in the detector
measured intensity
22
e+: annihilates with another eproducing 2 g-rays, each with
Eg = 511 keV.
ANSEL Expt 1: Gamma
Spectroscopy
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-1.022 MeV
measured energy (MeV)
e+/e- annihilation in detector or its vicinity produces 511keV g-rays
W. Udo Schröder, 2010
ANSEL Expt 1: Gamma
Spectroscopy
23
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, 2010
ANSEL Expt 1: Gamma Spectroscopy
24
Spectrum of g Rays from Nuclear Decay
W. Udo Schröder, 2010
Spectrum of g Rays from Nuclear Decay
ANSEL Expt 1: Gamma Spectroscopy
25
g1
W. Udo Schröder, 2010
Spectrum of g Rays from Nuclear Decay
ANSEL Expt 1: Gamma Spectroscopy
26
g1
W. Udo Schröder, 2010
g2
Spectrum of g Rays from Nuclear Decay
ANSEL Expt 1: Gamma Spectroscopy
27
g1
W. Udo Schröder, 2010
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
ANSEL Expt 1: Gamma Spectroscopy
28
g1
W. Udo Schröder, 2010
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
ANSEL Expt 1: Gamma Spectroscopy
29
g1
W. Udo Schröder, 2010
g2
DE
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
ANSEL Expt 1: Gamma Spectroscopy
30
g1
W. Udo Schröder, 2010
g2
511
keV
DE
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
BSc
ANSEL Expt 1: Gamma Spectroscopy
31
g1
W. Udo Schröder, 2010
g2
511
keV
DE
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
Pb X-rays
BSc
ANSEL Expt 1: Gamma Spectroscopy
32
g1
W. Udo Schröder, 2010
g2
511
keV
DE
g2
SE
g2
CE
g2
Spectrum of g Rays from Nuclear Decay
Pb X-rays
BSc
ANSEL Expt 1: Gamma Spectroscopy
33
g1
W. Udo Schröder, 2010
g2
511
keV
DE
g2
SE
g2
CE
g2
g1+g2
Scope of ANSEL Experiment 1
• Ubiquitous presence of radiation on Earth, e.g., g-ray photons
ANSEL Expt 1: Gamma Spectroscopy
34
• 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
W. Udo Schröder, 2010
Principle of Fast-Slow Signal Processing
Produce analog signal 
Source
NaI Det.
Slow
PreAmp
Amp
35
Fast
Produce logical signal 
Principles Meas
CFTD
Input
W. Udo Schröder, 2004
Data
Acquisition
System
Gate
t
CFTD
Internal 0
CFTD
Output
Discrim
CFTD
Gate
Gener
ator
Energy
t
Principle of a
Constant-Fraction
Timing Discriminator:
t independent of E
0
t
here f = 0.5
Binary
data to
computer
Scope of ANSEL Experiment 1
• Ubiquitous presence of radiation on Earth, e.g., g-ray photons
ANSEL Expt 1: Gamma Spectroscopy
36
• 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
W. Udo Schröder, 2010
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
37
•
•
•
ANSEL Expt 1: Gamma Spectroscopy
•
•
•
•
W. Udo Schröder, 2010
22Na
g energy spectrum in histogram form.
Source Info
Counts/keV
ANSEL Expt 1: Gamma Spectroscopy
38
Set of typical
Calibration g-ray
sources
W. Udo Schröder, 2010
Semi-log plot of a
calibrated 22Na g-ray
spectrum taken with a
3”x3’ NaI(Tl) detector
Lab Measurements Expt. 1 (cont’d)
•
39
•
•
ANSEL Expt 1: Gamma Spectroscopy
•
•
•
•
•
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 0.511 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).
W. Udo Schröder, 2010
Data Analysis Expt. 1
•
40
•
ANSEL Expt 1: Gamma Spectroscopy
•
•
•
•
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.
W. Udo Schröder, 2010
ANSEL Expt 1: Gamma Spectroscopy
W. Udo Schröder, 2010
Counts/channel
41
Sample Spectrum