phys586-lec01-radioactivity - Experimental Elementary Particle
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Transcript phys586-lec01-radioactivity - Experimental Elementary Particle
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
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
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Introduction
While particle and medical radiation
physics may seem unrelated, there is
much commonality
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, …)
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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
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Introduction
Importance and relevance
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
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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
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
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Introduction
8
Introduction
Cargo scanning using linear accelerators
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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
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Electromagnetic Spectrum
Our interest will be primarily be in the
region from 100 eV to 10 MeV
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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
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Nuclear Terminology
Nuclear species == nuclide
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)
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Table of Nuclides
Plot of Z vs N for all nuclides
Detailed information for ~ 3000 nuclides
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Table of Nuclides
Here are some links to the Table of
Nuclides which contain basic information
about most known nuclides
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/
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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
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Valley of Stability
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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
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Tc Tc fission
(SF)
99 m
99
43
43
Spontaneous
256
100
112
Fm140
Xe
54
46 Pd 4n
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Valley of Stability
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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
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
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Binding Energy
Separation energies can also be calculated as
Sn M AZ1 X M n M ZA X
S p M ZA11 X M 1H M ZA X
Note these are
atomic masses
S M ZA22 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
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Example
Is 238U stable wrt to α decay?
Sα = B(238U) - B(234Th) - B(4He)
Sα = 1801694 – 1777668 – 28295 (keV)
Sα = -4.27 MeV => Unstable and will decay
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Radioactivity
Radioactive decay law
dN Ndt
N t N 0e t where N t is thenumber at timet
1
t /
N t N 0e
where is themean lifetime
Nomenclature
λ in 1/s = decay rate
λ in MeV = decay width (h-bar λ)
τ in sec = lifetime
You’ll also see Γ = λ
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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
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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
At
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
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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
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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
Γ = ∑Γi where Γi are called the partial
widths
The branching fraction or ratio for channel
or state i is simply Γi/Γ
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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
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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
ln2 / 1
tmax
2 1
1t
e 2t
1
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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
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Radioactivity
Transient equilibrium
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
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Radioactivity
Case 2 (parent half-life >> daughter half-life)
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 01
2
A2 A1
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Radioactivity
Secular equilibrium
A1=A2
Daughter nuclei are decaying at the same
rate they are formed
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Radioactivity
Case 3 (parent half-life < daughter half-life)
What happens?
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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 0c 137
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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
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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
1.240 106
E eV
m
h
1.051034 Js 6.5810 22 MeVs
2
-19
1 eV 1.60210 J
c 197 MeVfm 200 MeVfm
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Constants and Conversions
34
1.05 10
22
Js 6.58 10
MeVs
1eV 1.6 1019 J
c 3 10 m / s
8
15
1F (fermi) 1 10 m
c 197.3MeVF
e2
1
4 0c 137
1b (barn) 1028 m 2
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