Mass Spectrometry

Brief introduction(part1)

I. Sivacek flerovlab.jinr.ru

2012 Student Practice in JINR Fields of Research 1.oct.2012

Mass spectrometer• Ion source – analyzing magnet – detector (basic setup)

– Isotope identification– Molecule identification– Chromatography– Microscopes – AMS– RIBs and many more…

Charged particles and ions

• Neutral atoms (p = e)• Charged atoms = ions (p ≠ e)

• Neutral particles (f.e. neutron)• Charged particles (f.e. proton, electron…)

• Charged = with electric charge

How to create ion ?• If energy is given to neutral atom

– Excitation, de-excitation (a)– Ionization (b)

• Thermal ionization• Chemical ionization• ECR (uniform magnetic field; plasma; 2,45 GHz microwave

power)

E=hν

Incident radiation

-

(a)

(b)(Or other kind of energy)

Ions with kinetic energy

• Interactions of charged particles and ions: – ionization – excitation– elastic scattering– bremmstrahlung

Ion ionization• Bethe formula for ion energy loss

• α ~ 1 MeV/cm(1 atm = 101 kPa = 760 Torr)

• Bragg curve – stopping power

=> Vacuum is needed !

Electron density of target Ion charge Ion velocity

Electric and magnetic field

• Electric field:(accelerates)

• Magnetic field E = |U2 – U1|

Lorentz Law

(1)

Mass separation in magnetic field

• Mass-over-charge ratior

Generally at E-M field

mag. field: E=0; |Fm| = qvB.sinα

radius

=1

=> r = (m/q)(v/B) (2)

Cyclotron

• Lorentz force in magnetic field keeps orbits

• Electrical field between duants accelerates ions

• Cyclotron frequency (particle velocity does not depend on time of flight thru magnetic field)*

Duants (electrodes with HF oscillating electric field)

Magnetic field

Beamline

* (2) => r = const. v

Detection of particles

• Generally, all types of detector can be used– Spectroscopic detectors

• Semiconductor• Scintillators• Gas detectors• Time-of-flight (and many more)

– Counters (quantitative analysis)• Gas detectors• Pick-ups• Faraday cups (and many many more…)

Warning: Since high ionization

ability of charged ions, proper

choice of detector strongly

depends on application and

characteristics of detected beam !!

Mass resolution

• Ability of device to resolve between two masses m and m+Δm

• Defined as (by FWHM)

cislo stripu

inte

nzi

ta [n

A]

Stripstrip number

Dispersion on ion source

Resolution on ion source-magnet-detector array

Radius of ion in magnetic field

Dispersion after magnetic sector D [mm/%p]

D = Δx/ΔBρ = Δx/ΔpTranslation on focal plane Ion impulse

NOW YOU ARE FREE FOR A WHILEEnd of part 1