paul-Nobel prize lecture 1989 (1248333), страница 3
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This enables a mass selective detection of the stored ions. If oneconnects the cap electrodes with an active r.f. circuit with the eigenfrequency Ω, in the case of resonance Ω = o,, the amplitude of the oscillationsincreases linearly with time. The ions hit the cap or leave the field through abore hole and can easily be detected by an electron multiplier device. Bymodulating the ion frequency determining voltage V in a sawtooth modeone brings the ions of the various masses one after the other into resonance,scanning the mass spectrum. Fig. 11 shows the first spectrum of this kindachieved by Rettinghaus [19].The same effect with a faster increase of the amplitude is achieved if oneinserts a small band of instability into the stability diagram .
It can begenerated by superimposing on the driving voltage V cos ω t a small additionalrf voltage, e.g. with frequency ω/2, or by adding a higher multipole termto the potential configuration [5b,20].In summary the ion trap works as ion source and mass spectrometer atthe same time. It became the most sensitive mass analyzer available as only afew ions are necessary for detection. Its theory and performance is reviewedin detail by R.E.
March [21].613W. PaulFigure 10. a) Pseudo crystal of 7 magnesium ions. Particle distance 23 µm. b) The same trappedparticles at “higher temperature”. The crystal has melted [18].The Penning TrapIf one applies to the quadrupole trap only a d.c. voltage in such a polaritythat the ions perform stable oscillations in the z-direction with the frequency CO: = , Ix, ; the ions are unstable in the x-y-plane, since the field isdirected outwards.
Applying a magnetic field in the axial direction, thez-motion remains unchanged but the ions perform a cyclotron motion ωin the x-y-plane. It is generated by the Lorentz force F directed towardsthe center. This force is partially compensated by the radial electric forceF, = & r. As long as the magnetic force is much larger than the electric one,stability exists in the r-y-plane as well. No r.f.
field is needed. The resultingrotation frequency calculates toLFigure Il. First mass spectrum achieved with the ion trap. Gas: air at 2 . 10 -9 torr [19]It is slightly smaller than the undisturbed cyclotron frequency eB/m. Thedifference is due to the magnetron frequencywhich is independent of the particle mass.The Penning trap [22], as this device is called, is of advantage if magneticproperties of particles have to be measured, as for example Zeeman transitions in spectroscopic experiments, or cyclotron frequencies for a veryprecise comparison of masses as are performed e.g.
by G. Werth. The mostspectacular application the trap has found in the experiments of G. Gräff[23] and H. Dehmelt for measuring the anomalous magnetic moment of theelectron. It was brought by Dehmelt [24] to an admirable precision byobserving only a single electron stored for many months.Traps for neutral particlesIn the last examination I had to pass as a young man I was asked if it wouldbe possible to confine neutrons in a bottle in order to prove if they areradioactive.
This question, at that time only to be answered with “no”,pursued me for many years until I could have had replied: Yes, by means ofa magnetic bottle. It took 30 years until by the development of superconducting magnets its realization became feasible.Using the example of such a bottle I would like to demonstrate theprinciple of confining neutral particles.
Again the basis is our early work onfocusing neutral atoms and molecules having a dipole moment by means ofmultipole fields making use of their Zeeman or Stark effect to first andsecond order [1,2,3]. Both effects can be used for trapping. Until now onlymagnetic traps were realized for atoms and neutrons. Particularly, B.
Martin, U. Trinks, and K. J. Kügler contributed to their development with greatenthusiasm.W. Paul615The principle of magnetic bottlesThe potential energy U of a particle with a permanent magnetic moment µin a magnetic field is given by U = -µB. If the field is inhomogenous itcorresponds to a force F = grad(µB). In the case of the neutron with its spinh/2 only two spin directions relative to the field are permitted. Therefore,its magnetic moment can be oriented only parallel or antiparallel to B.
Inthe parallel position the particles are drawn into the field and in theopposite orientation they are repelled. This permits their confinement to avolume with magnetic walls.The appropriate field configuration to bind the particles harmonically isin this case a magnetic sextupole field. As I have pointed out such a field Bincreases with r2, B = 3 * P and the gradient $ with r respectively.In such a field neutrons with orientation p tt B satisfy the confiningcondition as their potential energy U = + µB ~ r2 and the restoring forceµgradB = --Cr is always oriented towards the center. They oscillate in thefield with the frequency ω2 = q. P articles with ,u t 1 B are defocused andleave the field. This is valid only as long as the spin orientation is conserved.Of course, in the sextupole the direction of the magnetic field changes withthe azimuth but as long as the particle motion is not too fast the spin followsthe field direction adiabatically conserving the magnetic quantum state.This behaviour permits the use of a magnetic field constant in time incontrast to the charged particle in an ion trap.An ideal linear sextupole in the x-z-plane is generated by six hyperbolicallyshaped magnetic poles of alternating polarity extended in y-direction, asshown in Figs.
12a, b. It might be approximated by six straight current leadsFigure 12. a) Ideal sextupole field. Dashed: magnetic field lines, dotted: lines of equal magneticpotential, B = const. b) Linear sextupole made of 6 straight current leads with alternatingcurrent direction.616Figure 17. Sextupole spherewith alternating current directions arranged in a hexagon. Such a configuration works as a lense for particles moving along the y-axis.There are two possibilities to achieve a closed storage volume: a sextupolesphere and a sextupole torus.
We have realized and studied both.The spherically symmetric field is generated by three ring currents in anarrangement shown in Fig. 13. The field B increases in all directions with r 2and has its maximum value B0 at the radius r0 of the sphere. Using superconducting current leads we achieved B0 = 3T in a sphere with a radius of 5 cm.But due to the low magnetic moment of the neutron µ = 6 * 10-8 eV/T thepotential depth µB o is only 1.8 * 10-7 eV and hence the highest velocity ofstorable neutrons is only v max = 6m/s .
Due to their stronger moment for Naatoms these values are 2.2 * 10-4 eV and 37 m/s, respectively.The main problem with such a closed configuration is the filling process,especially the cooling inside. However, in 1975 in a test experiment wesucceeded in observing a storage time of 3 s for sodium atoms evaporatedinside the bottle with its Helium cooled walls [25]. But the breakthrough inconfining atoms was achieved by W. D.
Phillip and H.J. Metcalf using themodern technique of Laser cooling [26].The problem of storing neutrons becomes easier if one uses a linearsextupole field bent to a closed torus with a radius R as is shown in Fig. 14.The magnetic field in the torus volume is unchanged B = $Q * ? and hasno component in azimuthal direction. The neutrons move in a circular orbitwith radius Rs if the centrifugal force is compensated by the magnetic forcew. Paul617Figure 14. Sextupole torus. Rs orbit of circulating neutrons.In such a ring the permitted neutron energy is limited byIt is increased by a factor ($ + 1) compared to the case of the sextupolesphere. As the neutrons have not only an azimuthal velocity but alsocomponents in r and z directions they are oscillating around the circularorbit.But this toroidal configuration has not only the advantage of acceptinghigher neutron velocities, it also permits an easy injection of the neutrons inthe ring from the inside. The neutrons are not only moving in the magneticpotential well but they also experience the centrifugal barrier.
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