Sizes. W. Udo Schröder, 2007 Nuclear Decay 2 Decay Types There are many unstable nuclei - in nature Nuclear Science began with Henri Becquerel’s discovery

Sizes

W. Udo Schröder, 2007

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Decay Types

There are many unstable nuclei - in nature Nuclear Science began with Henri Becquerel’s discovery (1896) of uranium radioactivity

and man-made:

+30 30 30 J oliot &Al( ,n) P Si ,1934

Th source

Curi

, E

e

6 MeV

Types of decay:

1 1

1 1 1

A A 4Z N Z 2 N 2

A AZ N Z 1 N 1 e

A AZ N Z 1 N 1 e

A AZ N Z 1 N 1 e

A A A x yAZ N Z N Z Z y N x

decay : X Y

decay : X Y e

decay : X Y e

e capture : X Y ( e )

Fission : X F F xn yp

Various rare heavy particle(cluster ) decays

“weak” decays


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Domains of Nuclear Decay Modes

N=126

Isotones

Z=82 Isotopes

A

Neutron Dripline

Bn = 0

SHE Z=118 discovered (?)

Proton Dripline Bp = 0

Ef = 0

Segré Chart

stable nuclide

A = 132 Isobars

Stable nuclides


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Example: ThB (212Pb) Decay Scheme

212Pb ground state decays spontaneously by e- emission (- decay)

212Bi ground state has branched ( and ) decays

208Pb ground state is stable


Pri

nci

ple

s M

eas

5

Measuring “Decay Curves”: Fast-Slow Signal Processing

Radiation

Source

Slow

Fast

PreAmp

Amp

Produce timing signal

electron. Clock(TAC)

Data Acquisition System

Energy

E-Tag

Produce analog signal Binary

data to computer

EnergyDiscriminator

TimeTrigger

Start

Stop

External Time reference signal t0

Detector

Measured: Energy and time of arrival t=t-t0 (relative to an external time-zero t0) for radiation (e.g., -rays), energy discriminator to identify events (A) in a certain energy interval E by setting an identifier “tag.”

Calibrate t axis channel # time units (s, y,..)Watch that t-channel .

0 100 200 300 4000.01

0.1

1

i

t

Act

ivit

y

A(

t)/

A(t

0)

t (Channel #)

t

A( t ) exp


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Example: Determination of the 198Au g.s. Lifetime

E. Norman et al., http://ie.lbl.gov/radioactivedecays/page2

411.8 keV

411.8 keV

Spectrum of delayed 198Au -rays

Spectrum of delayed 198Au -rays decay of 198Hg exc. state is prompt:

11 measurementsEach spectrum ran for 12 hours real time#11 taken 5 days after #1

# 1

# 11

Kinetics of Nuclear Decay: Logarithmic Decay Law

2.303

:

# /

( )

( 2.1828..)

( ) ( 0)

( 10)

( ) 1( 0)

( 2)

( 0

0

( )

1

)

t

t

e l

First order process

Activity of decays unit time

dA N N t N

dt

exponential law base e

N t N t

exponential law base

N t N t

exponential law b

ife time

ase

N t N t 0.

/ 2

1

1

6932

0.

:

6931

t

Deca

ha

y w

lf

idth

life t

0 100 200 300 4001 10 2

0.1

1

i

t

0 100

200

300

400

0

0.2

0.4

0.6

0.8

1

Disintegration of Radioactive Sample

i

ti

Sam

ple

Act

ivit

y A

(t)/

A(0

)

time t

time t


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Radio-Activity/Units

1

10

Number of nuclei in sample : N N(t ) changes in time

Differential probability for a nucleus to decay (dimension time )

Activity A(t ) : N(t )

Units of activity : 1Bq(Becquerel ) 1disintegration / s

Old : 1Ci (Curie) 3.7 10 di sintegr

226

3

ations / s ( 1g Ra)

radiation energy abs.Absorbed dose : D

mass of material m

Unit [D] 1Gy (Gray ) 100 rad 1J / kg

Old : 1R(Roentgen) 8.8 10 J / kg in air (material dependent )

Bio log ically equivalent dose H weight D

unit [H] 1Sv (

2

Sievert ) 1 J / kg

Old unit 1rem 10 Sv

19

18

1eV 1.602 10 J

1 J 6.242 10 eV


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Average Radiation Exposure

From Lilley, Nuclear Physics, Principles and Applications, J. Wiley & Sons, Ltd., 2001

(238U, 232Th, 40K)

(219Rn, 220Rn, 222Rn)

Tobacco absorbs Rn 210Pb, 210Po“hot spots” in lungs cancer risk

Note unit: Sv


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Biological EffectsAnn. ICRP 26, 1994

Heavy charged particles:High ionization density, localized: maximum near end of range (Bragg peak)

Electrons (+Bremsstrahlung):Long range, diffuse multiple scattering, low ionization density, delocalized absorption

Neutrons:Indirect ionization, capture H(n, ) for En<100 eV, keV-neutrons scatter elastically (np), 2-MeV neutrons have = 6 cm to thermalization and high ionization density. Photons:Like electrons, low ionization density, Compton scattering + photoelectric absorption, delocalized absorption

Indirect chemical effects:Free radicals (neutral atom or molecule with an unpaired e-)

2 2

2 2

2 2

2

2

RH OH

H O rad H O e

H O e H O

H O H OH

H O H OH

R organic

RH H R H

R O RO peroxy radical

2 2

Oxygen Effect (Chain Re ac

RO RH R

tion)

O H R


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Biologically Relevant Dose Weighting Factors

Type Energy Range

Weighting wR

e± all 1

neutrons < 10 keV 5

10-100 keV 10

100 keV – 2 MeV

20

2 - 20 MeV 10

>20 MeV 5

protons < 20 MeV 5

, FF, clusters

20

tissue radiation tissue,radiation

T R T ,R

T R T ,RR

T TT

Biol. equivalent dose (Damage due to energy deposition)

H w D

Brief : H w D

Combination of different rads,H w D

Combination of different tiss. Equivalent dose E w H

Tissue Weight wT

Gonads 0.20

Red bone marrow 0.12

Colon 0.12

Lungs 0.12

Stomach 0.12

Bladder 0.05

Breast 0.05

Liver 0.05

Oesophagus 0.05

Thyroid 0.05

Skin 0.01

Bon surface 0.01

All remaining tissue 0.05

Ann. ICRP 26, 1994


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Effect on Complex Molecules

Enzyme deoxyribonuclease (DNAse)

Splits DNA

Rad exposure decreases activity exponentially with dose

•High DNAse concentrations: expect more direct hits, direct damage of molecule

•Low DNAse concentration: expect fewer direct hits, less direct damage.Observation due to more free OH. and H. radicals from H2O dissociation.

DNAse in Water

Recovery: DNA molecules can repair themselves after moderate damage by X rays and minimum ionizing radiation.

Approximately total recovery after small doses, no accumulation of effects

More damage and less recovery after irradiation with neutrons

X-rays, Assay at t0

t0+5hrs

Neutrons Assay at t0

t0+5hrs

DNA Survival Rate

After Lilley, Nuclear Physics, Principles and Applications, J. Wiley & Sons, Ltd., 2001


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Risk Factors

Tissue Effect Probability/Sv

Breast Cancer 2.0 x 10-3

Red bone marrow Leukemia 5.0 x 10-3

Lung Cancer 8.5 x 10-3

Thyroid Cancer 8.0 x 10-4

Bone surface Cancer 5.0 x 10-4

Other tissue Cancer 3.4 x 10-2

Whole body Cancer effects 5.0 x 10-2

After Lilley, Nuclear Physics, Principles and Applications, J. Wiley & Sons, Ltd., 2001


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0 100 200 300 4001102

0.1

1

i

t

Act

ivit

y A

(t)/

A(t

0)


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Intrinsic Nuclear Spin

Nuclei can be deformed can rotate quantum mech. collective spin and magnetic effects (moving charges)

Intrinsic spin? Nucleons have spin-1/2Demonstrate via interaction with external B

odd-A : I= half-integer multiple of ħeven-A: I= integer multiple of ħeven-Z & even-N: I = 0

Quantum mechanical spin:

2 2, ,

, ,

1 3ˆ ( 1) 0, ,1, ,....2 2

ˆ ( 1)

2 1

ˆ

I m I mI I

z I m I I m II I

I I I I

I I I I

I different magnetic substates

I m I m I

“good” quantum numbers: I, mI

2B Ip

eW B g I B

m

Interactions via magnetic moment

mIħ

z

x

yI

quanti

zati

on

axis

I

I 2Np

em

Nuclear Magneton

expt fact

Documents

Sizes. W. Udo Schröder, 2007 Nuclear Decay 2 Decay Types There are many unstable nuclei - in nature Nuclear Science began with Henri Becquerel’s discovery