Nuclear astrophysics:
synthesis of the chemical
elements
Zsolt Podolyák
University of Surrey, UK
Earth:
~1890 Kelvin: ~20-40 Myears
radioactivity
1905 Rutherford => billions of years
age: 4.55 billion years
(radioactive dating)
Universe:
26Al
all-sky map:
T1/2=0.74 My
Eγ =1.8 MeV
 continuous
nucleosynthesis
picture by COMTEL
What are things made of?
Air: 78% nitrogen, 21% oxygen
Pebbles:
47% silicon,
53% oxygen
(by weight)
+ traces
Fe, Mn
What is the world made of?
Logarithmic scale
Abundances in the solar system
Hydrogen: 74%
Helium: 24%
Other: (oxygen, carbon etc) 2%
Elements and isotopes
electrons
neutrons (N)
protons (Z)
Helium
3He
4He
chemical and physical properties: element (Z)
nuclear properties (and synthesis of elements): nuclei (Z and N)
Magic numbers: 2, 10, 18, 36, 54, 86,118
Chart of nuclei
Super Heavies
Fewer than 300 nuclei
Proton Drip Line
Neutron Drip Line
Magic numbers: 2, 8, 20, 28, 50, 82, 126
How do we know what the world is made of?
Emission and Absorption (fingerprint) Lines
Absorption lines
(observation)
Star light
Galactic Chemical Evolution
From big bang till today
Expansion of the universe
7p & 1n
74% H & 24%He
98% of known matter
Where does the rest
come from?
http://www.davidreneke.com
The life cycle of stars
Stars: hydrogen burning (the most important)
1H=proton
Net effect: 4 1H -> 4He +2 e+ +2 ν e +energy
Why does it take billions of years?
1H+1H
-> 2H + e+ +νe
Two steps:
(i) p -> n + e+ + νe
E=-1.8 MeV it needs energy!
it cannot happen (classical phys.)
sometimes (quantum physics)
Bottle neck
(ii) p+ n-> 2H
In laboratory never observed!
How are elements heavier than helium made?
The Hoyle state
EKIN
excited state
‘right’ energy (7.65 MeV)
‘right’ quantum numbers 0+
energy
m(8Be)c2+m(4He)c2
groundstate
m(12C)c2
• No Hoyle state, no elements beyond helium
• The energy of Hoyle state determines the
amount of heavier elements
Onion structure of (heavy) stars
H burning: ~109 y
He burning: ~106 y
C burning:~103 y
Ne, O burning: ~1 y
Si burning:~ 10-2 y
Creation of elements up to iron is understood
In burning (fusion) processes
in stars
Based on the US
National Academy of
Science Report
[Committee for the Physics
of the Universe (CPU)]
Question 3
How were the elements from
iron to uranium made ?
The Abundances of the
elements for A = 70 - 210
double peaks at neutron number
46/50, 76/82, 116/126
N=50, 82, 126 are magic!
(equivalent to noble gases)
T1/2(neutron)=614 s
=> it has to happen in stars
They are due to production
by the two separate
processes
Slow neutron capture process: s process
low neutron flux;
beta-decay time < neutron capture time
abundances peaks at A=84, 138, 208
Z+1
Neutron capture
Beta decay
Z
Rapid neutron capture process: r process
high neutron flux
beta-decay time > neutron capture time
abundances peaks at A=80, 130, 195
The slow neutron capture process
(well understood)
80Br,
proton number
(n,g)
(b-)
(b+)
t1/2=17 min, 92 % (b-), 8 % (b+)
Sr
Rb
p-only
Kr
Br
Se
As
Ge
Ga
Zn
Cu
Ni
Co
Fe
85Kr,
79Se,
Zr
Y
t1/2=11 a
t1/2=65 ka
r-only
t1/2=12 h, 40 % (b-), 60 % (b+)
63Ni, t =100 a
neutron
number
1/2
64Cu,
The rapid neutron capture process (r-process)
movie from http://compact-merger.astro.su.se/movies.html#rproc
The mystery of the r-process
Where does it happen (neutron density, temperature)?
supernovae explosions?
neutron star mergers?
What are the properties of the nuclei involved?
the majority of them never studied
Neutron-star mergers
movie from http://compact-merger.astro.su.se/movies.html#nsbh
Nuclear physics input
The majority of nuclei involved in
the r-process cannot be studied
Excite a nucleus:
p
n
First particles are emitted..
p
Then gamma rays (bursts of energy)...
Good description of what is known
FAIR (Facility for Anti-proton and Ion Research)
FUTURE
>1000M Euro
Solar system abundances
From solar and meteoritic abundances
J.J. Cowan and F.-K. Thielemann, Physics Today, Oct. 2004, p.47
Universality of the r-process
abundances
(there are other stars with the same abundances)
Frebel, Norris
The origin of the elements
Big Bang
H,He Li-B C
massive stars
(hydrostatic
buring)
~Fe
SN
type II
SN
type Ia
Sr
Pb
Unknown – SN type II/NS merger ?
(r-process)
massive
stars
(C,O burning
(He
weak
s-process)
Low mass AGB stars
burning, main s-process)
SN type Ia or II (p-process)
13C,17O,..
92,94 Mo,96,98
Novae (“rp process”)
Unknown
X-ray bursts? (rp process)
Ru
Note: yellow-red all related to massive stars (>8-12 solar masses)
Th,U
Summary
Elements created in nuclear reactions
Fusion processes in heavy stars create elements up to iron
proton+proton takes billions of years
all nuclei heavier than helium created via the Hoyle state
Elements beyond iron created in neutron rich environment
We still do not know where
(NS mergers? Supernovae?)
and how heavy elements
were made (r-process)