Introduction
 A more general title for this course might be
“Radiation Detector Physics”
 Goals are to understand the physics,
detection, and applications of ionizing
radiation
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The emphasis for this course is on radiation
detection and applications to radiological physics
However there is much overlap with experimental
astro-, particle and nuclear physics
And examples will be drawn from all of these
fields
1
Introduction
While particle and medical radiation
physics may seem unrelated, there is
much commonality
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Interactions of radiation with matter is the
same
Detection principals of radiation are the
same
Some detectors are also the same, though
possibly in different guises
Advances in medical physics have often
followed quickly from advances in
particle physics
2
Introduction
 Roentgen discovered x-rays in
1895 (Nobel Prize in 1901)
 A few weeks later he was
photographing his wife’s hand
 Less than a year later x-rays were
becoming routine in diagnostic
radiography in US, Europe, and
Japan
 Today the applications are
ubiquitous (CAT, angiography,
fluoroscopy, …)
3
Introduction
 Ernest Lawrence
invented the cyclotron
accelerator in 1930
(Nobel Prize in 1939)
 Five years later, John
Lawrence began studies
on cancer treatment
using radioisotopes and
neutrons (produced with
the cyclotron)
 Their mother saved from
cancer using massive xray dose
4
Introduction
Importance and relevance
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Radiation is often the only observable available
in processes that occur on very short, very
small, or very large scales
Radiation detection is used in many diverse
areas in science and engineering
Often a detailed understanding of radiation
detectors is needed to fully interpret and
understand experimental results
5
Introduction
 Applications of particle detectors in science

Particle physics
 ATLAS and CMS experiments at the CERN LHC
 Neutrino physics experiments throughout the world

Nuclear physics
 ALICE experiment at the CERN LHC
 Understanding the structure of the nucleon at JLAB

Astronomy/astrophysics
 CCD’s on Hubble, Keck, LSST, … , amateur telescopes
 HESS and GLAST gamma ray telescopes
 Antimatter measurements with PAMELA and AMS

Condensed matter/material science/
chemistry/biology
 Variety of experiments using synchrotron light sources
throughout the world
6
Introduction
 Applications of radiation/radiation detectors in
industry
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Medical diagnosis, treatment, and sterilization
Nuclear power (both fission and fusion)
Semiconductor fabrication (lithography, doping)
Food preservation through irradiation
Density measurements (soil, oil, concrete)
Gauging (thickness) measurements in manufacturing (steel,
paper) and monitoring (corrosion in bridges and engines)
Flow measurements (oil, gas)
Insect control (fruit fly)
Development of new crop varieties through genetic
modification
Curing (radiation curing of radial tires)
Heat shrink tubing (electrical insulation, cable bundling)
 Huge number of applications with hundreds of
billions of $ and millions of jobs
7
Introduction
8
Introduction
Cargo scanning using linear accelerators
9
Radiation
 Directly ionizing radiation (energy is delivered
directly to matter)
 Charged particles
 Electrons, protons, muons, alphas, charged
pions and kaons, …
 Indirectly ionizing radiation (first transfer their
energy to charged particles in matter)
 Photons
 Neutrons
 Biological systems are particularly sensitive to
damage by ionizing radiation
10
Electromagnetic Spectrum
Our interest will be primarily be in the
region from 100 eV to 10 MeV
11
Electromagnetic Spectrum
 Note the fuzzy overlap between hard x-rays
and gamma rays
 Sometimes the distinction is made by their
source

X-rays
 Produced in atomic transitions (characteristic x-rays) or
in electron deacceleration (bremsstrahlung)

Gamma rays
 Produced in nuclear transitions or electron-positron
annihilation
 The physics is the same; they are both just
photons
12
Nuclear Terminology
 Nuclear species == nuclide
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A nucleons (mass number),
Z protons (atomic number)
N neutrons (neutron number)
A = Z+N
 Nuclides with the same Z == isotopes
 Nuclides with the same N == isotones
 Nuclides with the same A == isobars
 Identical nuclides with different energy
states == isomers

Metastable excited state (T1/2>10-9s)
13
Table of Nuclides
 Plot of Z vs N for all nuclides
 Detailed information for ~ 3000 nuclides
14
Table of Nuclides
Here are some links to the Table of
Nuclides which contain basic information
about most known nuclides
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http://www.nndc.bnl.gov/nudat2
http://atom.kaeri.re.kr/ton/
http://ie.lbl.gov/education/isotopes.htm
http://t2.lanl.gov/data/map.html
http://yoyo.cc.monash.edu.au/~simcam/ton/
15
Table of Nuclides
 ~3000 nuclides but only ~10% are stable
 No stable nuclei for Z > 83 (bismuth)
 Unstable nuclei on earth

Naturally found if τ > 5x109 years (or
decay products of these long-lived
nuclides)
 238U,
232Th, 235U
(Actinium) series
Laboratory produced
 Most stable nuclei have N=Z
 True for small N and Z
 For heavier nuclei, N>Z

16
Valley of Stability
17
Valley of Stability
 Table also contains information
on decays of unstable nuclides
 Alpha decay

238
234
4
U

Th

Beta
(minus
or
plus) decay
92
90
2 He

137

Cs

Ba

e
 v(IT)
e
Isomeric transitions
56
4
U 234
90Th 2 He
238
92
137
55
Tc Tc fission

(SF)
99 m
99
43
43
 Spontaneous
256
100
112
Fm140
Xe

54
46 Pd  4n
18
Valley of Stability
19
Binding Energy
 The binding energy B is the amount of energy
it takes to remove all Z protons and N
neutrons from the nucleus

B(Z,N) = {ZMH + NMn - M(Z,N)}
 M(Z,N) is the mass of the neutral atom
 MH is the mass of the hydrogen atom
 One can also define proton, neutron, and
alpha separation energies
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Sp = B(Z,N) - B(Z-1,N)
Sn = B(Z,N) - B(Z,N-1)
Sα = B(Z,N) - B(Z-2,N-2) - B(4He)
 Similar to atomic ionization energies
20
Binding Energy
 Separation energies can also be calculated as
Sn  M  AZ1 X   M n   M ZA X 
S p  M ZA11 X   M 1H   M ZA X 
Note these are
atomic masses
S  M ZA22 X   M 4 He  M ZA X 

Q, the energy released, is just the negative of the
separation energy S
 Q>0 => energy released as kinetic energy
 Q<0 => kinetic energy converted to nuclear mass or
binding energy
 Sometimes the tables of nuclides give the
mass excess (defect)

Δ = {M (in u) – A} x 931.5 MeV
21
Example
Is 238U stable wrt to α decay?
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Sα = B(238U) - B(234Th) - B(4He)
Sα = 1801694 – 1777668 – 28295 (keV)
Sα = -4.27 MeV => Unstable and will decay
22
Radioactivity
Radioactive decay law
dN  Ndt
N t   N 0e t where N t  is thenumber at timet
1
t / 
N t   N 0e
where  is themean lifetime

Nomenclature
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λ in 1/s = decay rate
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λ in MeV = decay width (h-bar λ)
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τ in sec = lifetime
You’ll also see Γ = λ
23
Radioactivity
 t1/2 = time for ½ the nuclei to decay
N0
t / 
N t  
 N 0e
2
1
t
ln  
2

ln 2
t1 / 2   ln 2 

24
Radioactivity
 It’s easier to measure the number of nuclei that have
decayed rather than the number that haven’t decayed
(N(t))
 The activity is the rate at which decays occur
dN t 
 t
At   
 N t   A0e
dt
A0  N 0

Measuring the activity of a sample must be done in
a time interval Δt << t1/2
 Consider t1/2=1s, measurements of A at 1 minute and 1
hour give the same number of counts
25
Radioactivity
Activity units

bequerel (Bq)
 1 Bq = 1 disintegration / s
 Common unit is MBq

curie (C)
 1 C = 3.7 x 1010 disintegrations / s
 Originally defined as the activity of 1 g of
radium
 Common unit is mC or μC
26
Radioactivity
Often a nucleus or particle can decay
into different states and/or through
different interactions

The branching fraction or ratio tells you
what fraction of time a nucleus or particle
decays into that channel
A decaying particle has a decay width Γ
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Γ = ∑Γi where Γi are called the partial
widths
The branching fraction or ratio for channel
or state i is simply Γi/Γ
27
Radioactivity
 Sometimes we have the situation where
1
2
1 2  3
226
Ra Rn Po
222
218
 The daughter is both being created and
removed
28
Radioactivity
 We have (assuming N1(0)=N0 and N2(0)=0)
dN1  1 N1dt
dN2  1 N1dt  2 N 2dt
then
N 2 t   N 0
2
1

e

1t
e
2t

1
A2 t   2 N 2 t   A0
2
2

e

and maximumactivityat
ln2 / 1 
tmax 
2  1
1t
 e 2t 
1
29
Radioactivity
 Case 1 (parent half-life > daughter half-life)

This is called transient equilibrium
1  2
N1 t   N 0e 1t
N 2 t   N 0
1
2  1
e
1t
 e 2t 
becomes
2 N 2
2


1  e 2 1 t 
1 N1 2  1
A2
2

A1 2  1
30
Radioactivity
 Transient equilibrium
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A2/A1=2/(2-1)
Example is 99Mo decay
(67h) to 99mTc decay
(6h)
Daughter nuclei
effectively decay with
the decay constant of
the parent
31
Radioactivity
 Case 2 (parent half-life >> daughter half-life)
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This is called secular equilibrium
Example is 226Ra decay
1  2
N 2 t   N 0
1
2  1
e
 1t
 e 2t 
becomes
1
N 2 t   N 0 1  e  t 
2
2 N 2 t   N 01
2
A2  A1
32
Radioactivity
Secular equilibrium
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A1=A2
Daughter nuclei are decaying at the same
rate they are formed
33
Radioactivity
 Case 3 (parent half-life < daughter half-life)

What happens?
34
Units
 Sometimes I will slide into natural units used in
particle physics
  c 1
 Then at the end of the calculation or whatever
we’ll insert h-bar’s and c’s to make the answer
dimensionally correct
 And while it might not come up so often
e
2
1


4 0c 137
35
Electromagnetic Spectrum
What part of the EM spectrum has a
physiological effect on the human body?
36
Radioactivity
 Case 3 (parent half-life < daughter half-life)

What happens?
 Parent decays quickly away, daughter activity rises to a
maximum and then decays with its characteristic decay
constant
37
Electromagnetic Spectrum
What part of the EM spectrum has a
physiological effect on the human body?
38
Electromagnetic Spectrum
 Photon energy is given by
E  h 
hc

1.240 106
E eV  
 m 
h

 1.051034 Js  6.5810 22 MeVs
2
-19
1 eV  1.60210 J
c  197 MeVfm  200 MeVfm
39
Constants and Conversions
34
  1.05  10
22
Js  6.58  10
MeVs
1eV  1.6  1019 J
c  3  10 m / s
8
15
1F (fermi) 1  10 m
c  197.3MeVF
e2
1


4 0c 137
1b (barn)  1028 m 2
40