Transcript
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Lecture 13Previously
Seen beta and gamma decay
Today
Look at alpha decaySee alpha decay chainsSee large range of alpha decay lifetimes
Due to quantum tunneling
See some applications of radiation
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Alpha decay: changes N and Z
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Homer at NukePlant
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Alpha decay chain
Includes !-decay- preferentially shed ns
(recall, for large A: N > Zbut for small A: N = Z.So as move to lower A,need to reduce N/Z ratio)
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Actually, only 4 possibledecay chains.. as a decay
changes A by 4.i.e. writing A = 4n + mChains for m = 0, 1, 2, 3
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(R
1/")
(EQ)
Massive range in alpha lifetimes
Exponential depon Z/!Q
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Picture alpha emission as:
Schematic of alpha emission
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What potential does alpha feel?
Schematic of alpha emission
+
=> Vmax
2(Z 2)e2
40rn
2Ze2
40r0A1/3
= 2.4 Z
A1/3 MeV"10s of MeV
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Massive range in alpha lifetimes
e2kd
e2
k(r) dr =e2G
G=
2mQ
h
Ze2
40Q=e2
2m
40h
ZQ
= 2.0 ZQ
MeV1/2
R=1
=ae
2G
logR= log a
(2log e)G= log a
1.7
Z
Q
d
V
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(R
1/")
(EQ)
Massive range in alpha lifetimes
Exponential dep= -1.7 x Z/!Q
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Practicalities
Problem sheetsPS3 last Friday, soln this wednesdayPS4 this Friday no assessed question
Past Exams
ALL PS contain past exam Qs, or equiv material,e.g. PS1 Qs: 1, 3, 4, 5
PS2 Qs: 1, 2, 3
PS3 Qs: 1, 2, 3, 4
Your Exam: 40% A, 60% NPP (~40%PP, 20% NP)
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Energy loss by charged particlesParticle with charge Z
i,
velocity
!=v/c
gas of electrons
Energy lost by charged particle per unit length traversedgiven by dE/dx:
dEdx ! ""Z
A
1
me
" Zi2"
1
"2
of medium of incident
particle
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Energy loss by charged particlesEnergy lost by charged particle per unit length traversed
given by dE/dx: dEdx ! ""
Z
A
1
me
" Zi
2"
1
"2
of medium of incident
particle
Recall: E ~ !mv2= !mc2!2
dE/dx #1/!2 #m/Ei.e. for a given E, dE/dx larger for larger m,
e.g. m#/me~7500.
Highly relativistic electrons lose most energy viaBremsstralung. This radiation (photons) is emittedwhen charged particles are accelerated or decelerated
e.g. by the nuclear electric field.
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Absorption of $rays in matter
Incomingphoton Outgoing
electron
Photoelectric Effect
Incoming photon outgoing photon
scatteredelectron
%
1
%
2
Compton Scattering
Pair Production
Energetic photon (E$> 1.02 MeV) convertd to e+e- pair
In intense electric field of nucleus.
Prob. of pair production Z2(E - 1.02)
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Energy loss by electrons & photons
Fractio
nalEnergyLos
s/UnitLength
Electron Energy (MeV)
Ionization
Bremsstrahlung
0.5
1.0
10 100Photon Energy (MeV)
Photoelectric
Pairproduction
Compton
K absorption edge
Abso
rptionCoefficien
t/(g/cm2)
0.1
1
10
0.1 10
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Interactions of neutrons with matter
n n
p
n-p elastic scattering
n
N
n
&
n
Inelastic scattering
N
A
N
A-3
NA+1
*
&
Neutron capture
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Radiation Units: Activity &Absorbed dose
Activity
Measures strength of the ionisingradiation.
Unit, the becquerel(Bq):1 Bq = 1 disintegration/second
Absorbed dose
Measures total energy absorbed per unitmass.
Unit, the gray(Gy): 1 Gy = 1 J/kg.
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Radiation Units: Equivalent dose
Measures biological effect of absorbed dose Obtained by multiplying the latter by thequality factor, Q, which describes how muchbiological damage the specific type of
radiation causes. Q = 1 for !& $rays, but
Q = 20 for #-particles.
Unit sievert(Sv), 1 Sv = = Q x dose[Gy]. Amount of damage also dep on dose rateand part of body exposed.
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Characteristics of various types ofradiation
Typical energies ~ MeV
(=Q)
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Typical Radiation Dose Rates fromCommon Sources
Source mSv/year
Cosmic rays 0.26
Natural backgrounds (U, Th, Ra) 1.65Within body (
40K,
14C) 0.30
Global fallout
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Positron Emission Tomography
crystals + photodetectors
annihilationphoton trajectories
object
Annihilationevent
coincidenceevents
Schematic of PETe+e- -> $$
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Pathway of an Innovation1928: Diracs description of electrons consistent withEinsteins special relativity and quantum mechanicsPredicted the existence of anti-particles (e.g positronbasis of PET) and explained spin (basis of MRI)1932: Operation of first cyclotron , the anti-electron(positron) discovered
Radionuclides (e.g. fluorine18 (half-life ~110min) usedin PET scanning are produced by cyclotrons in hospitals.PET cameras today use APDs (and Si PMs) and heavyscintillating crystals and starting to be combinedwithMRI scanner.
The scientific basis for all medical imaging(functional & physiological) are steeped in
nuclear/particle physics
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Accelerators developed in labs used inhospitals
Around 9000 of the
17000 acceleratorsoperating in the Worldtoday are used formedicine.
Example: Hadron Therapy
Courtesy of IBA
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