Download Archives for cosmogenic radionuclides

‘methane & climate change’
or
‘atom counting reveals secrets of Earth’ past climate’
Dr Andrew Smith
Australian Nuclear Science and Technology Organisation
Climate change: reflecting back, looking forward
10 days of science
National Science Week, 15th -23rd August 2009
Cosmogenic radionuclides
Next is an AIRES simulation of what happens when a proton with
1Tev (=1012 electron volts energy) hits the atmosphere about 20km
above the ground. The shower is in a 20km x 5km x 5km box
superimposed on a scale map of Chicago's lakefront. Different kinds
of particles are coloured differently: electrons and positrons are green,
muons are red, and gamma rays are cyan.
Cosmic rays (discovered 1912)
• Cosmic rays are energetic particles from outer space that impinge
on Earth's atmosphere.
• ‘Ray’ a misnomer: cosmic particles arrive individually, not as a ray
or beam of particles.
• ~ 90% are protons, ~ 10% are helium nuclei (alpha particles), < 1%
heavier elements and electrons.
• Energies > 1020 eV, far higher than < 1013 eV man-made particle
accelerators can produce.
• Cosmic rays incessantly bombard Earth, smashing atoms and
molecules high in the atmosphere, producing cascades of
secondary particles that reach the surface.
The origin of cosmic rays
• energetic processes on the Sun
• Supernova
• unknown events in the farthest reaches of the visible universe.
Supernova RXJ1713.7-3946
[Suzaku X-ray observatory]
• This supernova remnant
is the gaseous remnant
of a massive star that
exploded about 1,600
years ago
• The contour lines show
where gamma-ray
intensity is highest
Credit: JAXA/ Takaaki Tanaka/HESS
Production rate modulation
• Cosmic ray flux
• Terrestrial magnetic field
• Heliosphere magnetic field
Cosmogenic radionuclides
radioisotope half-life (years)
'cosmogenic'
10
1.51  106
Be
14
5,730
C
26
Al
7.3  105
36
Cl
3.01  105
129
15.9  106
'primordial'
235
7.04  108
U
I
236
2.342  107
U
'anthopogenic'
239
2.410  104
Pu
240
Pu
242
Pu
(as 'spike')
6.56  103
3.75  105
radionuclide nomenclature
atomic mass, A
ionised charge state
(6 protons + 6 neutrons in nucleus)
(Z - number of electrons)
12
6
atomic number, Z
(number of protons in nucleus)
4+
C
element symbol
How do we measure cosmo-isotopes?
►By Accelerator Mass Spectrometry or ‘AMS’.
Example: ‘radiocarbon’ or 14C:
• Stable carbon isotopes: 12C (98.90%) and 13C (1.10%).
• Only 7.5kg of cosmogenic 14C produced globally in the entire
atmosphere per year: in equilibrium.
• Natural abundance: 14C/12C ~ 1.2 × 10-12: one in a trillion!
• 14C oxidised to 14CO2: radiocarbon dioxide
• Photosynthesis: living organisms in equilibrium with atmosphere.
• Radiocarbon dating: the clock starts on death: limit 10-16 (50ka).
• Carbon from sample chemically prepared as graphite.
Radiocarbon dating
• black square is carbon, mostly
12C (99%) and 13C (1%).
• yellow dots are 14C atoms,
initially 104 atoms.
• 14C atoms are radioactive and
disintegrate with a half-life of
5,730 years.
• When? It cannot be predicted
for a given atom.
• Dating old samples is difficult:
few 14C atoms remain.
• Modern natural carbon
contains ~ 50 million per mg.
credit: M. Blaauw 2007, chrono.qub.ac.uk/blaauw
The technique of accelerator mass spectrometry
ANSTO’s STAR accelerator: 2MV
Accelerator Mass Spectrometry at ANTARES
Australian
National
Tandem for
Applied
RESearch.
10MV
advantages of tandem AMS over mass spectrometry:
negative ions: elimination of 14N isobar
charge exchange: destruction of 12CH2 & 13CH in terminal
ionisation detector: E,M,Z  atom counting
features:
ultra-small samples ~ 0.1 mg
rapid measurement ~ 20 min
sensitivity: 1 in 1015
accuracy ~ 0.5%, background ~ 50 ka
Isotopes and climate science
Stable and radioactive isotopes:
• Isotopes of atoms provide valuable information about past climates.
and factors which have forced climate change.
• Natural radioisotopes provide additional, and often unique, data.
Archives for cosmogenic radionuclides:
• tree rings, rocks, coral, speliothems, sediments, ice cores…
Applications include:
• Past atmospheric composition.
• Timing of past climate change.
• Atmospheric circulation & transport.
climate signals recorded in polar ice sheets
Snowfall traps traces of
atmospheric gas and
impurities...
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When the firn depth
reaches 50-100 m, the
pockets of air close off
into bubbles and flow
with the ice
Law Dome
W20k
97/98 Law Dome firn air sampling
Law Dome DSS0506 thermal drilling
Gaseous components of the atmosphere.
The atmosphere is composed mostly of nitrogen, oxygen
and argon. Together, the remaining trace gases account
for only about 0.1% of the air.
nitrogen [78.1%]
oxygen [20.9%]
argon [0.9%]
carbon dioxide [330 ppm]
neon [18 ppm]
helium [5 ppm]
methane [2 ppm]
krypton [1 ppm]
nitrogen oxide [0.5 ppm]
hydrogen [0.5 ppm]
xenon [87 ppb]
Global warming in the anthropocene
From ~1700 AD to 2005 AD, CO2 has
risen 36% from 280 ppm to 379 ppm,
CH4 has risen 153% from 700 ppb to
1,774 [IPCC4]
quantity of air & ice needed for 14C AMS
[assuming 300 ppm CO2, 2 ppm CH4 and 50 ppb CO and 100 mL of air per kg ice]
CO2
CH4
CO
kilograms of ice for 100
micrograms of carbon
6.2
930
37,000
litres of firn air for 100
micrograms of carbon
0.6
93
3700
helping to determine the anthropogenic & natural sources of
the important greenhouse gas - methane
A new highly interactive exhibition exploring the complex
world of nuclear science, medicine and nuclear power.
On display in the Museum’s Experimentations gallery where
different areas of science are explained, Nuclear matters
aims to provide a greater public understanding of what
nuclear science is and how it plays a big part in our
everyday lives.