Accelerator Mass spectrometry, current status, new ...mantica/radio-frib/collon-acsboston.pdf ·...
Transcript of Accelerator Mass spectrometry, current status, new ...mantica/radio-frib/collon-acsboston.pdf ·...
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Accelerator Mass spectrometry, current status, new developments
and opportunities with FRIB
Philippe Collon, University of Notre Dame
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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
b) count the number of atoms themselves
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
In a Mass Spectrometer a sample material is converted to an ion beam that is then magnetically (and electrostatically) analyzed
MS separates ions by their mass only
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Goal of AMS
However in many cases high background (molecular, isotopic, isobaric, …) orders of magnitude higher makes it impossible to separate the ions of interest.
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- Charge state- ……….
An unambiguous (A, Z) identification solves this problem
(A, Z)
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 µA = 1 x 1013 ions).
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MS vs. AMS
E
E∆
Mass spectrometry
Accelerator mass spectrometry
Tandem accelerator
Stripper
Negative ion sourceElectrostatic analyser
Detector setup
Cyclotron
Posit ive ion source
Low-energyanalysingmagnet
Low-energyanalysingmagnet
High-energyanalysing
magnet
Low-energyanalysingmagnet
ion source
ion detector
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Typical AMS setup
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From carbon dating the Ice Man:
To nuclear Astrophysics:
The measurement of the cross-section of the suspected main production channel of 44Ti: 40Ca(α, γ)44Ti
The detection of the decay of 44Ti by Compton gamma-ray obs.
A clear indicator for ongoing 44Ti nucleosynthesis
14C age = 5300 years
But FRIB???????
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Pushing the detection limit
• Gas-filled-magnet techniques– Notre Dame, Munich, ANL,…
• Full striping at high energy– NSCL,….
• Rapid beam-energy and acceleration changes (especially for large systems)– ANL
Challenges: Overall stability, reproducibility, transmission and beam purity
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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
Bρ ∝ mv / q-
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Gas filled magnet setup
Scattering chamber
Beam
Enge Spli t Pole spectrograph
PPAC + ionisation chamber
Nitrogen (10 torr)
39K
39Ar
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The importance of 60Fe as a direct observable of stellar nucleosynthesis
Supernovae
Cassiopea A, x ray images of a supernovae remnant
Credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.
Broadband x-rays from iron (from Chandra)
ESA’s INTEGRAL satellite
n pn p
ννe-
e-
60Fe 60Co* 60Ni* 60Ni
β deacy
β deacy
τ = 1.5 My
τ = 5.3y
γ 59 keV (2%)
γ 1.173 MeV (99.9%)
γ 1.332 MeV (99.9%)
0+
5+
2+
0+
4+
2+
60Fe60Co
60Ni
SATELLITE
Produced in stars
In order to link observations to our models we have to know this decay half-life precisely. The present accepted value of 1.5 My has been questioned by a new measurement of 2.5 My (Munich AMS group)
In addition to x-ray observation we can use isotopic specific information by measuring the gamma decay lines
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Notre Dame involvement in the measurement of the production of 60Fe
Production of a 60Fe sample using the superconducting cyclotron of the National Superconducting cyclotron Laboratory The sample produced will be used at Notre Dame for
the half-life determination using Accelerator Mass Spectrometry (AMS) and gamma activity measurement.
Predicted distribution of 60Fe radioactivity along the galactic plane. The GRASP (Gamma-Ray Astronomy with Spectroscopy and Positioning) project that we are part of, will aim at mapping sources of 60Fe in the galactic plane
Improve our knowledge of 60Fe galactic distribution:
Improve our knowledge of the 60Fe half-life:
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Negative Sputter Ion Source
90 degree Injector Magnet
Stripper Foil
90 degree Analyzing Magnet
F.C
F.C
F.C
F.C
Wien Filter
11MV FN Tandem
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Position (Channel number)
58Ni
58FeBeam tuned to 80MeV 58Ni11+
Fe/Ni Mixed cathode (200:1) injected
MANTIS @ 2.9 Torr N2 , B = 0.484T
Ene
rgy
Los
s (C
hann
el n
umbe
r)
Separation of 60Fe vs 60NI using MANTIS
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Detecting 39Ar at and below natural levels
• 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% [Loosli1983]
• Subsurface production can be significant in rocks with high uranium content 39K(n,p)39Ar
t1/2 = 269 years39Ar/Ar = 8.1 x 10-16
Applications to oceanography (global ocean circulation) and WIMP dark Matter detectors
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ATLAS layout
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Split-Pole Enge Spectrograph
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Later detector set-up
Pπ = 113.6 MeVBooster = 348.8 MeVATLAS = 464 MeV
Cath: - 430 VAnode: + 575 VGrid: + 300Div: +240V / -365V
N2 = 12.1 TorrPPAC = 3 torr (Isob)IC = 21 torr (Isob)
Beam:
Detect:
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39K count rate in a “clean system”: ~5x108
cps
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AMS and applications to FRIB?????
• Many AMS measurements rely on small systems
• AMS requires long run times• Most AMS measurements rely on samples
that have gone through chemistry (i.e long half-lives)
• Beam purity (even using HP materials) is an issue
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AMS developments important for FRIB
• In AMS you have to know what you are looking for. It is not a “one size fits all” technique
• AMS relies on a number of factors and has developed:– Good isobaric and isotopic separation– Stability of the entire system– Reproducibility of tunes and settings– High overall transmission– Beam purity analysis
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146Sm146Nd
146Sm/147Sm~ 10-12
146Sm t1/2 measurement using AMS
146Sm-146Nd separation
PII
ECR-II
BOOSTER LINAC
ATLAS LINAC
FN TANDEM INJECTOR
ECR-I
GFM Spectrograph
146Sm22+/146Nd22+
840 MeV
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Blocking Shield
Gas-Filled Magnet†
10 Torr N2
TargetChamber
Gas Filled Magnet (GFM) Spectrograph
146Nd146Sm
80Kr
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Rapid beam switching at ATLAS
Switching between 146Sm22+,
147Sm22+ and152Sm23+
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What can AMS bring to FRIB
• Many of the problems have similarities• AMS has reached reliable measurements into
the mid 10-17 levels (isobaric ratios)• AMS allows the separation from background
levels many orders of magnitude higher• AMS relies on stable reproducible beams.