Accelerator Mass spectrometry, current status, new developments

and opportunities with FRIB

Philippe Collon, University of Notre Dame

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

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).

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

Typical AMS setup

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???????

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

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-

Gas filled magnet setup

Scattering chamber

Beam

Enge Spli t Pole spectrograph

PPAC + ionisation chamber

Nitrogen (10 torr)

39K

39Ar

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

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:

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

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

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

ATLAS layout

Split-Pole Enge Spectrograph

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:

39K count rate in a “clean system”: ~5x108

cps

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

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

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

Blocking Shield

Gas-Filled Magnet†

10 Torr N2

TargetChamber

Gas Filled Magnet (GFM) Spectrograph

146Nd146Sm

80Kr

Rapid beam switching at ATLAS

Switching between 146Sm22+,

147Sm22+ and152Sm23+

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.