anp np slides l13 v2

7/23/2019 ANP NP Slides L13 v2

1/23

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


2/23

Alpha decay: changes N and Z


3/23

Homer at NukePlant


4/23

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)


5/23

Actually, only 4 possibledecay chains.. as a decay

changes A by 4.i.e. writing A = 4n + mChains for m = 0, 1, 2, 3


6/23

(R

1/")

(EQ)

Massive range in alpha lifetimes

Exponential depon Z/!Q


7/23

Picture alpha emission as:

Schematic of alpha emission


8/23

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


9/23


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


10/23

(R

1/")

(EQ)


Exponential dep= -1.7 x Z/!Q


11/23

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)


12/23

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


13/23

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.


14/23

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)


15/23

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


16/23

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


17/23

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.


18/23

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.


19/23

Characteristics of various types ofradiation

Typical energies ~ MeV

(=Q)


20/23

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


21/23

Positron Emission Tomography

crystals + photodetectors

annihilationphoton trajectories

object

Annihilationevent

coincidenceevents

Schematic of PETe+e- -> $$


22/23

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


23/23

Accelerators developed in labs used inhospitals

Around 9000 of the

17000 acceleratorsoperating in the Worldtoday are used formedicine.

Example: Hadron Therapy

Courtesy of IBA

anp np slides l13 v2

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