Richmond Nuclear Regan Day2

7/21/2019 Richmond Nuclear Regan Day2

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Making Gold : Nuclear AlchemyProf. Paddy Regan

Department of Physics

University of Surrey

Guildford, GU2 7XH [email protected]



Atoms (‘indivisible’) ……

~10-10 m, electrons (and their

orbital structure) determine chemistry of the elements, e.g., NaCl

Nuclei…..~10-14m across, protons determine the

chemical element (Z); neutron number (N) determines

the mass, (A = N+Z). > 99.9 % of the mass of the atom

is in the nucleus.

Nucleons (protons and neutrons ~10-15m) have a

substructure, three quarks in each nucleon

(‘ups’

and ‘downs’)…but they don’t exists on their

own.



Z=43

Tc Z=61

Pm Z=84

Po

Elemental composition of the Solar Nebula



What makes a nucleus ‘stable’?

•

There is an ongoing interplay and competition between coulomb

repulsion and strong nuclear force interactions

•

The result is that only certain combinations of Z and N give rise

to stable configurations (about 300 in total).

•

Other non-stable types can ‘radioactively decay’ (about 7,000

predicted).



10

Radioactive species in the body

Isotope Average amount by weight Activity

U-Uranium 90μg 1.1Bq

Th-Thorium 30 μg 0.11Bq

40K 17mg 4.4 kBq

Ra 31pg 1.1Bq

14C 22ng 3.7kBq

3H-tritium 0.06pg 23Bq

Po-Polonium 0.2pg 37Bq

Some variation-

for example smokers have 4-5 times more Po.



Mendeleyev



Moseley’s Law….evidence for

Atomic numbers….

‘Characteristic’

X-rays…with a

chemical (Z) dependence



X-rays come from

atomic ‘vacancies’

i.e. holes in the

electron shells

around the atom.

Quantum mechanics

means that theelectron orbits are

fixed in energy….

X-rays come from an

electron ‘dropping’

from one energy level

to a lower one



X-rays come from


i.e. holes in the

electron shells

around the atom.

Quantum mechanics


fixed in energy….

X-rays come from an



to a lower one



X-rays come from


i.e. holes in the

electron shells

around the atom.

Quantum mechanics


fixed in energy….

X-rays come from an



to a lower one

X-ray

emitted



Spectral Maps of the Galaxy

Ref http://adc.gsfc.nasa.gov/mw/mmw_images.html



Full-sky Comptel

map of 1.8 MeV

gamma rays in

26Mg following 26Al -decay.

Radioactive 26Al around the Galaxy….

Diehl et al., Astron. Astrophys

97, 181 (1993)



Z=43

Tc Z=61

Pm Z=84

Po

Elemental composition of the Solar Nebula



How it all starts….Hydrogen (Z=1) to Helium (Z=2)

The Proton-Proton Chain



Nuclear Fusion creates energy up to A~56 (Z=26 = Iron)

If the star is hot enough, nuclear fusion will fuel the star

and create elements up to A~56



B ~1.5 MeV

per A



Once you have 56Fe what next?

•

Top of the binding energy per nucleoncurve reached at A~56…fusion above thiscosts energy…bad news for the star -

supernova

•

BUT elements from 27-92 exist in nature–

how are these made ?

•

Neutron Capture –

neutrons have nocharge –

no electrostatic repulsion.



Beta –radioactive decay, (consequence of E=mc2)

2 types: (i) Beta-

plus proton changes to a neutron (Z ->Z-1)

(ii) Beta –

minus neutron changes to a proton (Z -> Z+1)



SN1987a before and after !!



A=N+Z = fixed



Mass Parabolas and Radioactive Decays



34

Proton Drip Line

Neutron Drip Line

Super Heavies

Fewer than 300 nuclei

For a give fixed A (isobar), we have

different combinations of Z and N.

e.g., A=Z+N=137 can be from

Z=56, N=81 →137Ba81

; or

Z=55, N=82 → 137Cs82 (see later)…

Mass Parabolas and Radioactive Decays

A = constant

Example of a mass parabola



Example of a mass parabola

Mass energy

(mc2

)

A=N+Z=125

p →

n + +

+

125Xe : Z=54; N=71

125Cs : Z=55; N=70

125I : Z=53; N=72

125Ba : Z=56; N=69

125Te : Z=52; N=73

STABLE ISOBAR

FOR A=125

125In : Z=49

125Sn : Z=50;

125Sb : Z=51;

n →

p + -

+

238



Q210Pb) = 5.41 MeVE

= 5.30 MeV

E(206Pb) = 0.11 MeV

T1/2 = 138 days.

‘218Po

=Radium A’

‘218 At

=Radium B’

C

D

E

210Po

=Radium ‘F’ Radon

=‘Emanation’

‘Radium’

C’

C’’

The Natural Decay Chain for 238U



Figure Wiescher, Regan &

Aprahamian, Physics World

Feb. 2002, page 33-38

Slow-neutron capture process

allows formation of elements

from A~56 to A=209 (Bi)...

terminates at 209Bi...why?

Neutron capture…

no electrostatic barrierto nuclear fusion….

all you need are enough

Neutrons…



Nuclear reactions in

Red giant stars create

‘spare’

neutrons

Stellar neutron sources in themiddle of Red Giant stars

(e.g., Betelguese)

13C+4He→17O*→16O + n22 Ne+4He→26Mg*→25Mg + n



So, how do you make Gold ?

•

Gold has 79 protons (i.e. Z=79)•

Start with Z=78 protons (i.e. Platinum)

•

Specifically

196

Pt ( Pt = Z=78, N=196-78=118)

•

Reaction is 196Pt + neutron to make 197Pt

•

197Pt is radioactive and ‘beta-decays’

to make197Au (i.e., normal ‘stable’

gold).



S-process makes

209

Bi from

208

Pb+n→209

Pb (T1/2

=3.2hr)→209

Bi

Neutron capture on stable 209Bi → 210Bi (T1/2

=5 days) → 210Po.

210Po →

+ 206Pb (stable nucleus, as is 207Pb and 208Pb).

Summary



Summary

What’s made where and how.

–

Hydrogen to Helium (in the sun, p-p

chain CNO cycles).

–

Helium to Carbon, triple-alpha process, special fusion.

–

Carbon to iron: nuclear fusion reactions, if hot enough.

–

Above 56Fe,•

(a) up to Z=92, 238U, supernova, rapid neutron captures...also spitsout material for future neutron capture in 2nd

/ 3rd

generation star

•

(b) can get up to

209Bi (210Po end-point) by slow neutroncapture



Tomorrow’s Lectures

•

Nuclear Weapons and Security (Dr. Gilfoyle

–

U. Richmond)

•

The Polonium-210 Story (Dr. Regan –

U. Surrey UK)

Richmond Nuclear Regan Day2

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