Physics News Update, Number 466

Number 466, January 12, 2000 by Phillip F. Schewe and Ben Stein

NEUTRONS HAVE BEEN CAPTURED AND STORED IN A MAGNETIC TRAP, a development which should lead to a better estimation of the neutron's lifetime and in turn a better understanding of the weak nuclear force. Neutral atoms have been confined in magnetic traps before (even uncharged atoms can have a magnetic moment which can be influenced by a strong magnetic field), but neutrons are more difficult to deal with in the same way since their intrinsic magnetic moment is so much weaker.

Now a collaboration of scientists from Harvard, NIST, Los Alamos National Laboratory, and the Hahn-Meitner Institute (Berlin) has succeeded in trapping neutrons in a magnetic bottle, thereby restricting neutron movement in all three dimensions (a decade ago, neutrons were magnetically trapped in a storage ring, but this confined neutron motion in only two dimensions). To bring about 3D trapping, a beam of already cold (11 K) neutrons from a reactor was directed into a trapping vessel surrounded by magnetic coils and filled with liquid helium at a temperature of less than 250 mK. The helium acts as a coolant, slowing the neutrons, and as a scintillator for recording the subsequent decay of neutrons into a proton, positron, and anti-neutrino.

The neutron lifetime measured in this experiment was 750 seconds, with an uncertainty of +300 and -200 seconds. The researchers hope to push their method to an accuracy of a part in 10⁵, which would exceed the accuracy of the currently accepted best value for the neutron lifetime, 886.7 (+/- 1.9) seconds. (P.R. Huffman et al., Nature, 6 January 2000.)

TWO-ELECTRON PRISON BREAK. New experiments studying the cooperation among electrons undergoing ionization show that electrons do not act alone when intense light liberates two of them at once from helium and other rare-gas atoms. When an intense light pulse removes more than one electron from an atom, it's simplest to assume that electrons respond to the light independently of their brethren and leave one by one. However, this "independent electron model" fails by many orders of magnitude in predicting double-ionization rates of atoms.

Using the COLTRIMS "momentum microscope" for atoms and molecules (Update 373), two multi-institutional experiments in Germany at the University of Marburg and the Max Born Institute in Berlin have measured the complete 3D momentum values for singly and multiply ionized helium (Reinhard Doerner, University of Frankfurt, 011-49-69-798-24218, doerner@hsb.uni-frankfurt.de) and neon (Robert Moshammer, University of Freiburg, 011-49-761-203-5741, moshammer@physik.uni-freiburg.de). If the electrons had acted independently, and left one by one in two successive steps, then the momentum data for double ionization would look like single ionization occurring twice. But the data show otherwise, leaving only the possibility of coordinated behavior.

Going further, the authors of the neon observations suggest that their data support a cooperative-behavior scenario known as "rescattering": the laser pulse's oscillating electric field first removes one electron, then pushes electron and ion back together, and finally the electron knocks out one of its comrades. These experiments can begin to test the extensive theoretical models of strongly interacting electrons in intense light fields. (Weber et al. and Moshammer et al., Physical Review Letters, 17 January 2000; images at Physics News Graphics; papers available from Select Articles.)

SCANNING GATE MICROSCOPY Scanned probe microscopes not only provide images of surface atoms, they also allow one to move atoms and to study the spectroscopy (the quantum energy levels) of those surface atoms (or molecules or metallic clusters). Concerning the latter, physicists at the Delft University of Technology (in the Netherlands) can better assay the energy levels of target particles at a surface by positioning a second probe right next to the main probe in a standard scanning tunneling microscope (STM) setup, giving it a tong-like appearance (see figure at Physics news Graphics).

The second probe acts much like a gate in a transistor: by shifting energy levels of the target particle it allows or disallows the passage of the tunneling current. In the reported experiment, the so-called Coulomb blockade (the difficulty of yet another electron to join many other electrons already on a tiny electrode) for single-electron tunneling in a 20 nm gold cluster was controlled using the gate electrode. (Gurevich et al., Applied Physics Letters, 17 January 2000; Select Article.)