low energy accelerators – compact ams systems josé maría lópez gutiérrez universidad de...

Low energy accelerators – Compact AMS systemsJosé María López GutiérrezUniversidad de SevillaCentro Nacional de Aceleradores

OPAC School. London, July 10th 2014

Overview

• A bit of history

• First applications of (today) low-energy accelerators

• What to do with the “old” accelerators?

• Accelerator Mass Spectrometry

▫ Decay Counting or counting atoms (AMS)

▫ Key physics points in AMS

▫ Accelerators

▫ Stripping

▫ Detectors

▫ Problems at low energies

▫ Where are the limits?

▫ Challenges


Energies in the atomic and subatomic world

J. Holmes, USPAS, January 2009


A bit of history

• 1906: Rutherford bombards mica sheet with natural alphas and develops the

theory of atomic scattering. Natural alpha particles of 1911 Rutherford

publishes theory of atomic structure.

• 1919: Rutherford induces a nuclear reaction with natural alphas.

▫ ... Rutherford believes he needs a source of many MeV to continue research

on the nucleus. This is far beyond the electrostatic machines then existing,

but ...

• 1928: Cockcroft & Walton start designing an 800 kV generator

encouraged by Rutherford.

• 1932: Generator reaches 700 kV and Cockcroft & Walton split lithium atom

with only 400 keV protons. They received the Nobel Prize in 1951.

• 1932: Van de Graaf invents a 1.5 MV accelerator for nuclear physics

research.

• Some years later, Van de Graaf type accelerators increase their potential to

more than 10 MV and also Tandem accelerators are invented.


First applications of (today) low-energy accelerators

• Nuclear physics:

▫ Nuclear reactions

▫ Nuclear energy levels

▫ Excited levels

lifetimes

▫ Decay schemes


• The energies that could be reached by the accelerators used before the 1950’s were too low for the proposed nuclear physics experiments.

• New applications had to be found in order to give use to them:

• Ion Beam Analysis techniques: PIXE, PIGE, RBS…

• AMS• Nuclear Physics• …

What to do with the “old” accelerators?


Accelerator mass spectrometryA technique going for every time smaller accelerators


Discovery of AMS in 1977 AMS-PioneersRochesterA.E. LitherlandK.H. PurserH.E. GoveR.P. BeukensR.P. CloverW.E. SondheimR.B. LiebertC.L. Bennet

The Rochester MP Tandem accelerator (12 MV)

McMasterD.E. Nelson, R.G. Korteling, W.R. Stott.


•How many atoms we need for a good measurement?

–N: Number of atoms–A: Activity–: Decay constant

•Reasonable assumptions:–Measurement time: 106 s (12

days)–Minimum count rate: 0.01 cps–Detection efficiency: 100 %

Decay Counting

Willard F. Libby

Nobel Prize in Chemistry 1960


•With AMS the number of atoms is counted!!

N: Number of atoms

tot: Overall efficiency

T: Transmission

•Typical values:

Negative ion yield ion: 0.5-30%

Instrument transmission T: 10-50%

Detection efficiency det : 100 %

Total efficiency few %independent of half-life

Counting atoms (AMS)

At least 4 orders of magnitude better!!!


Traditional AMS systemTandem AcceleratorE = (1+q)

eV

Detection systems

(E, dE/dx, v…)

Ion source

E,q0

Magnetic deflector(ME/q2)

M

Magnetic Analyzer(ME/q2)

EM/q2

Electrostatic deflector

(E/q)

E/qM/q


Traditional AMS system

The use of high energies makes it possible to use nuclear properties (like

stopping power) to reduce interferences at the detector

Under certain conditions, molecules are broken in the

accelerator stripper


Interferences MS Interferences

Radioisotop

e

T1/2

(years)

Isotopic

abundance in

environmental

samples

Analyze

d ionIsobars Molecules

E/q and M/q

ambiguities

10Be 1.51·106 10Be/9Be=10-11-10-5 10Be+ 10B+ 9Be1H+ 20Ne2+

14C 5730 14C/12C=10-14-10-11 14C+ 14N+

12C1H2+,

13C1H+

28Si2+

32Si 172 32Si/28Si=10-15-10-12 32Si+ 32S+ 31P1H+ 64Ni2+

36Cl 3·105 36Cl/35Cl=10-15-10-8 36Cl+ 36S+ 35Cl1H+ 72Ge2+

41Ca 1.03·105 41Ca/40Ca=10-14-10-11 41Ca+ 41K+ 40Ca1H+ 82Se2+

129I 1.57·107 129I/127I=10-12-10-7 129I+ 129Xe+

127I1H2+,

128Te1H+------

239Pu 24110 106 atoms 239Pu+ ------ 238U1H+ ------

240Pu 6564 y 106 atoms 240Pu+ ------ 238U1H2+ ------


Key physics points in AMS

• Sputtering ion source

• Sripping process

▫ Coulomb explosion at high AMS energies

▫ Interactions with residual stripping gas ambiguities

on E/q and M/q

• Beam analysis and transmission

▫ Focusing

• Detection system

▫ Isobar discrimination

▫ Similar masses and energies discrimination


Sputtering ion source• High efficiency, good

stability, low

dispersion, low memory

effects.

• Typical extraction energy:

tens of keV

• Charge state: -1

• Non-stable negative

ions:

▫ 14N-

▫ 129Xe-

▫ …

Lens

Ion beam

Cs reservoir

HeaterIonizer

Sample

Acceleration10 kV


Tandem accelerators

+ + + + + +

+ + + + + +

Iones negativos

Tubo de aceleración

Canal de stripping

Terminal Correa

Cadena de resistencias

Oscilador

Canal de stripping

Electrodo RF

Módulos rectificadores

Tubo de aceleración Tubo de aceleración

Entrada de gas

a b

+ + + + + +

+ + + + + +

Iones negativos

Tubo de aceleración

Canal de stripping

Terminal Correa

Cadena de resistencias

Oscilador

Canal de stripping

Electrodo RF

Módulos rectificadores

Tubo de aceleración Tubo de aceleración

Entrada de gas

a b

Van de Graaf

Cockcroft-Walton

Higher stabilityLower terminal voltages (up to 6 MV)


Tandem accelerators

Leibniz AMS 3 MV facility, Kiel, GER

VERA AMS 3 MV facility, Vienna, Austria


Stripping

• Electron-loss • Break-up of

molecules• Energy straggling• Angular straggling


Minimum gas pressure needed for stable distribution

Higher charge states result from stripping at higher energies

Stripping

Golden rule of molecular destruction: high efficiency for charge state 3

No surviving molecules

TV 2.5 MV

Bonani et al. (1990)


Detection system• Best option Gas Ionization Chamber

▫ Able to give information on total energy and energy loss.• Bethe-Bloch formula:

• For heavy ions qef instead of Zp:

2 4 2

2

24 lnp e p

te p

Z e m vdEZ

dx m v I

0.68801 1.034exp( / )ef p p pq Z v v Z

E (36Cl) E (36S)

Eres (36Cl) Eres (36S)


Traditional 3-6 MV AMS systemsLeibniz AMS 3 MV facility, Kiel,

GER

≈ 10

-15

m

HZDR 6 MV Tandetron AMS facility, Rossendorf, GER

VERA AMS 3 MV facility, Vienna, Austria

20 -

25 m


What if we go to smaller energies???

• Advantages:

▫ Smaller facilities

▫ Lower cost

▫ Less (or no) specialized personnel needed

• Conditions:

▫ High transmission at the stripper

▫ Good sensitivity

▫ High reproducibility


Several problems arise…

• Charge states 3 after stripping very low probability

• Lower charge states after stripping: “Surviving”

molecules??330 kV

[Jacob et al., 2000]


• Lower energies

▫ Higher angular straggling

Low beam transmission

(stripping channel

acceptance)

▫ Higher energy dispersion

in the beam Difficult ion

beam transmission and

worst separation at the

detector

Several problems arise…Energy dependence of

angular straggling

Transmitted beamintensities

2 µg /cm2

stripper gas (Ar)

VT (MV)dstripper

(µg/cm2)

E0

(MeV)

Ef

(MeV)

ΔE

(keV)ΔE/ Ef (%) q

ΔE/( Ef+qVT)

(%)

14C 3 0.2 3 2.998 0.44 0.015 3 0.004

14C 0.6 2 0.6 0.595 2.26 0.38 1 0.2


Several problems arise…

• Possible separation at the detector?

▫ Relevant nuclear stopping

▫ Energy losses and dispersion at the detector window

▫ Influence of electronic noise, etc.


Stripping Process

• Electron-loss • Electron capture • Break-up of

molecules• Energy straggling• Angular straggling

Charge state distribution

14C-

13CH-

12CH2-

1

108

109

14Cq

13Cq

12Cq13CHq

12CH2q

Hq

q=1-, 0, 1+, 2+, 3+,..

σ: dissociationcross section

Injected negativemass 14 ions

Destruction of molecular ions in q=1+


Charge state yield of 14C ions in Ar gas

0 1+ 2+ 3+ 4+

Traditional AMS2.5 - 9 MV

Multiple ion gas collisions Coulomb disintegration

Compact AMS

0.2 - 1 MV


Angular straggling• Different stripper channel design:

▫ Shorter▫ Wider▫ Higher pumping capacity

X- Xn+

Bomba Turbomolecular

Apertura de entrada del gasAr, N2, O2

Medidor de presiónCanal del stripper

Válvula

Sistema de recirculación del gas

X- Xn+

Bomba Turbomolecular

Apertura de entrada del gasAr, N2, O2

Medidor de presiónCanal del stripper

Válvula

Sistema de recirculación del gas


Energy straggling

• Design of achromatic optics

Deflector electrostático Deflector magnetostáticoDeflector electrostático Deflector magnetostáticoElectrostatic deflector Magnetic deflector


Use of specialized gas ionization chambers

5 cm

gas detector electrodes"innards"

CREMAT preamp modulesmounted directly on the anodes(Electronic noise (protons): 16 keV)

E-Eres anodes

Frisch-grid

Cathode

CF 100

Ions


Compact AMS Systems (1 MV- 500KV)

AMS facility, Seville, Spain

1 MV Tandetronaccelerator

≈ 4.5 m

KECK AMS facility, Irvine, USA

≈ 3

m≈ 6 m

≈ 5

m

Tandy AMS facility, Zurich, CH

≈ 6

m

≈ 3.5 m


Where are the limits?Cross sections of

molecule destruction in Ar

Molecular species

Energy dependence of angular straggling

Transmitted beamintensities

• Cross sections are comparable to molecular sizes

• Only weak energy dependence

• @ 230 keV cross sections are about 10 % lower

New concepts can be applied at stripping energies below 250 keV!!

Deal with ion beams of large

divergence

2 µg /cm2

stripper gas (Ar)


Inside view of vacuum insulated acceleration system

q=1+

acceleration section LE

acceleration

section HE

q=1-

Vacuum pumps

Stripper gas flow

1 m


200-250kV- AMS systems

5.4 m

6.5

m

SSAMS - High Voltage platform (open air)

2.5

m

3.0 m

BernMICADAS, Universtity of Bern

Compact lab-sized instrument–Designed for operator safety–No open high voltages–Easy to operate–Easy to tune–Fully automated


Moore’s Law of radiocarbon AMS

0.01

0.1

1

10

100

1975 1980 1985 1990 1995 2000 2005 2010 2015

14Ci one n

erg y

/MeV

Year / AD

MP-Tandem AMS

System Rochester

EN-Tandem AMS Systems:ETH, Oxford, Lower Hutt,

Utrecht, Erlangen,….

IONEX (Ken Purser) Arizona, Oxford, Gif-sur-Yvette,….

HVEE-Tandetron (Purser)AMS Systems:

Woods Hole, Groningen, Kiel,…

ETH-“Tandy”(Compact)-AMS Systems:

Zurich, Georgia, Poznan, Irvine…

NEC 500 kV Pelletron

ETH-“MICADAS” AMS Systems

Zurich, Davis, Mannheim, Debrecen, Seville,…. 200 kV PS (vacuum

insulated)

SSAMS Systems (NEC)Lund, ANU, SUERC,…

250 kV HV-deck

FN-Tandem AMS System McMaster University

?


Nitrogen stripper gas

Physical properties of molecule dissociation


He stripper gas

He areal density of ≈ 0.5μg / cm2 should be sufficient to get rid of molecules

Physical properties of molecule dissociation


Angular acceptance of

stripper:

max = 30 mrad

Ion Scattering Beam losses due to small angle scattering


Ion Scattering Beam losses due to small angle scattering

Angular acceptance of

stripper:

max = 30 mrad


ETH radiocarbon MS (μCADAS)


Stripping

• New stripping gasses as

He

• Optimization of vacuum

out of the stripping

channels Ion sources

• Reduction of memory effects

and cross contamination

• Selection of specific chemical

compounds Combination

with other techniques

Sample preparation

• Reduction of background

(isobars, neighbours,

molecules…)

• Small samples

• Liquid and gaseous samples

Development of new

detectors

Challenges (there’s a lot of work to do!)

• Reduction of electronic

noise through new designs

• Modified detection

techniques


Acknowledgements

Thank you very much to

Hans-Arno Synal (ETH-PSI, Switzerland)

Elena Chamizo (CNA)

for providing me of ideas, graphics and pictures


Thanks for your attention!

low energy accelerators – compact ams systems josé maría lópez gutiérrez universidad de...

Documents