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Title: ACCELERATOR MASS SPECTROMETRY from dating the ice man and tracing oceans to the stars


1
ACCELERATOR MASS SPECTROMETRY from dating the
ice man and tracing oceans to the stars
  • Philippe Collon,
  • University of Notre Dame

2
What is Accelerator Mass spectrometry (AMS)
The determination of the concentration of a given
radionuclide in a sample can be done in 2
ways a) measure the radiation emitted during
the decay
In many cases where concentrations and/or small
or long t1/2 this becomes impractical
1mg carbon 6 x 107 at 14C ? 1
decay/hour
b) count the number of atoms themselves
In a Mass Spectrometer a sample material is
converted to an ion beam that is then
magnetically (and electrostatically) analysed
MS separates ions by their mass only
3
Goal of AMS
However in many cases a high background
(molecular, isobaric, ) makes it impossible to
separate the ions of interest.
An unambiguous (A, Z) identification would solve
this problem
The use of an accelerator in AMS makes it
possible to go to much higher energies (several
MeV vs. keV) and the measurement of a range of
properties that do not depend on ionic
charge. - Range - Stopping power - TOF
The high sensitivity of the method makes it
possible to measure down to several counts per
hour from a beam of the order of microamperes
(1.6 mA 1 x 1013 ions).
4
MS vs. AMS
5
Typical AMS setup
6
From carbon dating the Ice Man
14C age 5300 years
To nuclear Astrophysics
The detection of the decay of 44Ti by Compton
gamma-ray obs. A clear indicator for ongoing
44Ti nucleosynthesis
The measurement of the cross-section of the
suspected main production channel of 44Ti
40Ca(a, g)44Ti
7
Schnals valley
North (Austria)
South (Italy)
Aug. 1989, G. Patzelt
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The Iceman Oetzi discovered in an Alpine Glacier
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Some applications of AMS
AMS can be used in many different fields and
adapted to different isotopes.
The following slides will illustrate - the
application to environmental studies - AMS
technique developed for 2 different isotopes
AMS is however also applied to - Nuclear
physics (t1/2 measurements, cross-section,)
- Nuclear astrophysics - Archeology (14C,10B,
)
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Cosmogenic radionuclides as tracers
14
Atlantic conveyor belt circulation
15
Concept of the Conveyor belt
The application of 39Ar dating to groundwater is
limited by the fact that underground production
in granitic rock 39K(n,p)39Ar can be substantial.
16
Properties of 39Ar
t1/2 269 years 39Ar/Ar 8.1 x 10-16
  • Mainly produced through cosmic ray induced
    spallation on argon in the atmosphere 40Ar(n,
    2n)39Ar Q -9.87 MeV
  • Anthropogenic production is estimated to be below
    5 Loosli 1983
  • Subsurface production can be significant in rocks
    with high uranium content 39K(n,p)39Ar

17
Activity of 1 l water
  • 1 l ocean sea water contains 6500 39Ar atoms
    (In ocean water Ar solubility ? 0.4 cm3 STP/l)
  • Activity(t0) 5.3x10-7 Bq or 17 decays per year.

This tends to make statistics rather poor
18
How can 39Ar be counted?
  • Low Level Counting
  • Possible on large samples (1000 l), done by H.H.
    Loosli in Bern
  • Laser
  • The extremely low concentration makes this a
    very difficult isotope for laser techniques
  • AMS (with small vol. samples)
  • several difficulties (DM/M, low concentration, )

19
AMS for 39Ar
  • 5 Main difficulties
  • The 39Ar/Ar 8.1x10-16 ratio
  • Isobar separation between 39K and 39Ar (DM/M
    1.55x10-5)
  • A tandem (as used in traditional AMS lab) cannot
    be used for noble gasses
  • Source efficiency
  • Overall transmission

20
Principle of the gas filled magnet
In the gas filled magnetic region, the discreet
charge states coalesce around a trajectory
defined by the mean charge state of the ion in
the gas
-
Br ? mv / q
21
Gas filled magnet setup
22
ATLAS layout
23
Split-Pole Enge Spectrograph
24
Experimental setup I
  • Initial beam tuning
  • As it is not possible to tune on 39Ar8 it was
    decided to use as pilot beam 78Kr16 from the
    ECR source
  • Beam energy
  • 78Kr16 Energy 464 MeV resulting in a 39Ar8
    beam with 232 MeV
  • Total transmission 20 (without stripping)

25
Later detector set-up
Pp 113.6 MeV Booster 348.8 MeV ATLAS 464 MeV
Beam
Cath - 430 V Anode 575 V Grid 300 Div
240V / -365V
N2 12.1 Torr PPAC 3 torr (Isob) IC 21 torr
(Isob)
Detect
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Using a quartz liner in the plasma chamber
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How to sample ocean water?
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R/V Nathaniel B. Palmer
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Nathaniel B.Palmer cruise 0106
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Water sampling rosette
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Ocean water samples
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Results from May 2002 AMS run
3.76 x10-5 ? 8.1 x 10-16
45
University of Notre Dame
  • 8000 Undergraduate students
  • 2200 Graduate students

46
70 years of electrostatic accelerators at ND
Initial budget 900 Cost overrun 450 Max
voltage 2MV
47
Further accelerators
48
NSL Facilities Layout
FN Tandem Accelerator 11MV
Browne-Buechner Spectrograph
KN Single ended Accelerator 3-4MV
200kV Inplanter ISIS
JN Accelerator 1MV
49
AMS for nucl. astrophysics at Notre Dame
50
The passage of time....before and after
51
Experimental Layout and AMS Facility
52
Detection System
PPAC and Ionisation Chamber (IC) for position and
energy determination Both containing Isobutane
gas Thin Mylar windows, low energy loss TOF can
aid in particle identification
53
MANTIS- ND AMS system
54
Isobar separation in the GFM
58Fe
58Ni
dt 20 min Bspec 0.620 Tesla Entrance foil
Mylar
55
First AMS measurement
56
Short-lived radionuclides in meteorites
An important result concerning the formation of
the solar system is the discovery of several
short-lived nuclides (with half-lives varying
from 105 to 108 years) in meteorites (10Be,
26Al, 36Cl, 41Ca, 60Fe, 53Mn..)
There are 2 generally accepted possible models
for the production of short-lived radionuclides
at the formation of Calcium-Aluminium-rich
inclusions (CAIs).
They originated in the in-situ irradiation of
nebular dust by energetic particles (mostly, p,
a, 3He X-wind irradiation model
They either originated from the ejecta of a
nearby supernova
60Fe/Fe
Provides a model for the formation of both CAIs
and Chondrules in primitive solar nebula
K. Knie et al., Phys. Rev. Lett. 93(2004)171103
57
The Origin of Mankind
Accelerator mass spectroscopy with long-lived
radioactive isotopes 60Fe
Dr. Collon Dr. Wiescher
58
Simulation to Detection
Only 1 unit of Z separation
59
SNO Motivation
2090 m to surface
105 m to upper atmosphere
Control room
1011 m to Sun
Vectran support ropes
1020 m to Galactic centre
Urylon liner
12 m diameter acrylic vessel
Norite rock
Support structure for 9500 PMTs, concentrators
With thanks to Kara Keeter
1700 tonnes light water
5300 tonnes light water
1000 tonnes heavy water
60
Preparation and Samples
Sample 1 Starting material 5g Cathode
material 100 ?g
Yarn sample 2 Starting material 5.28g
Cathode material 100 ?g
Acrylic sample 1 Starting sample 6.46g
Cathode material unknown
With thanks to Jaret Hise
61
Sample Running Proof of Principle
All measurements at present are only relative to
reduced background
62
Running Measurements
Cts/s/?A(65Cu)
Sample Runs
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