paul-Nobel prize lecture 1989 (1248333), страница 4
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Accordingly,one can lower the magnetic wall on the inside by omitting the two inwardcurrent leads. The resulting superposition of the magnetic and the centrifugal potential still provides a potential well with its minimum at the beamorbit. But there is no barrier for the inflected neutrons.It is obvious, that the toroidal trap in principle works analogous to thestorage rings for high energy charged particles.
In many respects the sameproblems of instabilities of the particle orbits by resonance phenomenaexist, causing the loss of the particles. But also new problems arise like, e.g.undesired spin flips or the influence of the gravitational force. In accelerator physics one has learned to overcome such problems by shaping themagnetic field by employing the proper multipole components. This technique is also appropriate in case of the neutron storage ring. The use of themagnetic force µ· gradB instead of the Lorentz force being proportional toB just requires multipole terms of one order higher.
Quadrupoles forPhysics 1989focusing have to be replaced by sextupoles and e.g. octupoles for stabilization of the orbits by decapoles.In the seventies we have designed and constructed such a magneticstorage ring with a diameter of the orbits of 1 m. The achieved usable fieldof 3.5 T permits the confinement of neutrons in the velocity range of 5 - 20m/s corresponding to a kinetic energy up to 2 * 10-6 eV. The neutrons areinjected tangentially into the ring by a neutron guide with totally reflectingwalls. The inflector can be moved mechanically into the storage volume andshortly afterwards be withdrawn.The experimental set up is shown in Fig.
15. A detailed description of theW. Paul619storage ring, its theory and performance is given in [27]. In 1978 in a firstexperiment we have tested the instrument at the Grenoble high flux reactor. We could observe neutrons stored up to 20 min after injection bymoving a neutron counter through the confined beam after a preset time.As by the detection process the neutrons are lost, one has to refill the ringstarting a new measurement.
But due to the relatively low flux of neutronsin the acceptable velocity range, their number was too low to make relevantmeasurements with it.In a recent experiment [28] at a new neutron beam with a flux improvedby a factor 40 we could observe neutrons up to 90 min, i.e.
roughly 6 timesthe decay time of the neutron due to radioactive decay. Fig. 16 shows themeasured profile of the neutron beam circulating inside the magnetic gap.Measuring carefully the number of stored neutrons as a function of time wecould determine the lifetime to τ= 877 ± 10 s (Fig. 17).The analysis of our measurements lets us conclude that the intrinsicstorage time of the ring for neutrons is at least about one day. It shows thatwe had understood the relevant problems in its design.Figure 16.
Beam profile of the stored neutrons inside the magnet gap 400 sec after injection.620Physics 1989Figure 17. Logarithmic decrease of the number of stored neutrons with time.The storage ring as a balanceThis very reproducible performance permitted another interesting experiment. As I explained the neutrons are elastically bound to the symmetryplane of the magnetic field.
Due to their low magnetic moment the restoring force is of the order of the gravitational force. Hence it follows that theweight of the neutron stretches the magnetic spring the particle is hangingon; the equilibrium center of the oscillating neutrons is shifted downwards.The shift z0. is given by the balance mg = µgradB. One needs a gradient %= 173 Gauss/cm for compensating the weight. As the gradient in the ringincreases with z and is proportional to the magnetic current Zone calculatesthe shift z0 toIt amounts in our case to z0 = 1.2 mm at the highest magnet current Z = 200A and 4.8 mm at 50 A accordingly.By moving a thin neutron counter through the storage volume we couldmeasure the profile of the circulating neutron beam and its position in themagnet.
Driving alternating the counter downwards and upwards in manymeasuring runs we determined z0 as a function of the magnet current.The result is shown in Fig. 18. The measured data taken with differentexperimental parameters are following the predicted line. A detailed analysis gives for the gravitational mass of the neutron the valueIt agrees within 4 % with the well known inertial mass.Thus the magnetic storage ring represents a balance with a sensitivity of10 -25 g.
It is only achieved because the much higher electric forces play norole at all.w. Paul621I am convinced that the magnetic bottles developed in our laboratory asdescribed will be useful and fruitful instruments for many other experiments in the future as the Ion Trap has already proved.Figure 18. Downward shift of the equilibrium center of the neutron orbits due to the weight ofthe neutron as function of the magnetic current.622Physics 1989REFERENCES[1][2][3][4][5]H. Friedburg and W. Paul, Naturwissenschaft 38, 159 (1951).H. G.
Bennewitz and W. Paul, Z. f. Physik 139,489 (1954).H. G. Bennewitz and W. Paul, Z. f. Physik, 141, 6 (1955).C. H. Townes, Proc. Nat. Acad. of Science, 80, 7679 (1983).a) W. Paul and H. Steinwedel, Z. f. Naturforschung 8a, 448 (1953); b) GermanPatent Nr. 944 900; USA Patent 2939958.[6] W. Paul and M. Raether, Z. f. Physik, 140, 262 (1955).[7] W. Paul, H. P.
Reinhardt, and U. v. Zahn, Z. f. Physik 152, 143 (1958).[8] F. v. Busch and W. Paul, Z. f. Physik, 164, 581 (1961).[9] U. v. Zahn, Z. f. Physik, 168, 129 (1962).[10] P. H. Dawson: Quadrupole Muss Spectrometry and its Application, Elsevier, Amsterdam 1976.[11] W. Paul, 0.
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Klempt and G. Werth, Zeitschrift f. Physik 222, 201 (1969).[24] R. S. van Dyck, P. B. Schwinberg, H. G. Dehmelt, Phys. Lett. 38, 310 (1977).[25] B. Martin, Thesis Bonn University 1975.[26] A. L. Migdal, J. Prodan, W. D. Phillips, Th. H. Bergmann, and H.J.
Metcalf,Phys. Rev. I.ett. 54, 2596 (1985).[27] K. J. Kügler, W. Paul, and U. Trinks, Nucl. Instrument. Methods A 228, 240(1985).[28] W. Paul, F. Anton, L. Paul, S. Paul, and W. Mampe, Z. f. Physik C 45, 25(1989)..
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