Time-of-flight distribution of polarized very slow muons emitted from solid argon layer for a flight path of 1 metre at Swiss Paul Scherrer Institute. The very slow muons are accelerated in a two-stage system with the aluminium substrate held at a potential of 8kV. The peak at317ns corresponds to an initial muon kinetic energy of 10 eV; 1 eV muons appear at 332 ns, and 50 eV muons at 295 ns.

Under the heading 'nuclear chromodynamics' it was vigorously argued (S. Brodsky) that nuclei can change the QCD environment to produce interesting phenomena such as colour filtering to measure colour transparency and test the hypothesis of an intrinsic heavy flavour content in the proton. Altogether, it was made quite clear that QCD is well established and tested. Yet, "the problem of deeply inelastic scattering ... is [only] half solved" (to quote F. Wilczek at the Aachen 1992 QCD workshop) and open problems such as small-x and non-perturbative QCD will continue to attract much attention.

By Gunnar Ingelman (DESY)

PSI Very slow polarized muons

At the 'pion factory' of the Swiss Paul that widely used to produce very slow tor inserted into a secondary surface Scherrer Institute, a collaboration of positrons (although the underlying muon beam can used as the PSI, Heidelberg and Zurich (ETH) physics may be different) - energetic source of a tunable tertiary beam of headed by Elvezio Morenzoni has positively charged particles slowed polarized muons with energies recently produced intense beams of down in certain materials can be re- between 10 eV and a few tens of positive muons which have kinetic emitted with kinetic energies of just a keV. energies as low as 10 eV and with few eV. This moderation technique PSI's new high-intensity beamline, complete polarization (spin orienta­ was first applied to muons at the which after completion of the current tion). Canadian TRIUMF Laboratory in accelerator upgrade will deliver The new results were achieved at a Vancouver in the late 1980s. several hundred million surface surface muon channel, transporting At PSI, it has now been shown that muons per second, can give a source positive muons from the decay of by using a solid argon or neon layer, flux of about 104 very slow polarized positive pions stopped at the surface the probability of converting a surface muons per second. of a pion production target. Surface muon (kinetic energy 4 MeV) to a Such tertiary muon beams have a muons with 4 MeV kinetic energy muon of a few eV can be made as vast potential of applications in were transported by a conventional large' as 104 and, most important, condensed-matter and particle secondary beam channel and par­ with the muon retaining its initial physics. The muon spin rotation tially stopped in a moderator consist­ polarization. technique with secondary surface- ing of a layer of solidified noble gas This is an essential step towards muon beams is well established, and deposited on a cold metallic the use of muons in a number of new at PSI a large community of users substrate. The method is similar to applications. A solidified-gas modera­ applies it to investigations in solids,

16 CERN Courier, January/February 1994 Exploded view of the INDRA detector at the French GANIL Laboratory. liquids and gases. Because of the large penetration depth of the surface muons, the technique is presently limited to the study of bulk characteristics. How­ ever with the new tertiary muon beams, the penetration depth in matter can be varied between a few and a few thousand angstroms, extending the muon spin rotation technique (for the study of magnet­ ism, defects, and diffusion) to sur­ faces and thin films. Investigations on the muonium atom, a preferred object for studies of fundamental interactions and conser­ vation laws, will also benefit.

GANIL INDRA and the hot measurements of their number, size Building ionization chambers with a (charge, mass), spatial distributions low energy threshold and almost nuclei and energy. Physicists interested in complete solid angle coverage was a studying the multifragmentation real challenge and is certainly the The formation and decay of hot process at GANIL decided in 1989 to most original INDRA feature. The nuclear systems formed in heavy ion build INDRA. structures of the detector compo­ collisions at intermediate energies is This detector, which has to operate nents used two different techniques - a major research axis at GANIL in a vacuum, covers about 90% of fibre-glass moulding or glued (Grand Accelerateur National d'lons the space around the target. It can be multilayer printed circuit boards. The Lourds, Caen, France). The ultimate described schematically as an anodes and cathodes of aluminized goal is to form highly excited nuclei at ensemble of 336 cells arranged in 17 mylar foils were glued on the 1 mm the "limit of stability". rings centred on the beam axis. The thick walls of the structures and have In the past few years theoretical number and the size of cells on a ring to support a gas pressure of 70 approaches predicted new decay depend upon its polar angle and mbars. modes for these nuclei, especially have been chosen in order to have The second challenge was the huge the possibility of a "sudden an approximately uniform counting dynamic range required for the multifragmentation" of the nucleus, rate. Charge identification (up to Z = electronics : 4000 to 1 for energy accompanied or not by a phase 50) is achieved by energy measure­ measurements in the silicon detec­ transition. The -excitation of very ments in two or three detection layers tors (from 1 MeV to 4 GeV) with a hot nuclear systems is characterized of increasing thickness: a low pres­ 100 keV energy resolution. The by the emission of a large number of sure gas-ionization chamber, 300 5m electronics modules were designed what are called in the trade "light thick silicon detectors and cesium and built by the INDRA collaboration charged particles" (in fact hydrogen iodide scintillators thick enough to laboratories, using mainly the new and helium nuclei) and, to a less stop all particles. Hydrogen and VXI Bus - an extension of the VME extent, heavier fragments. helium isotopes are identified using Bus for analog processing. This Experimental studies require an pulse shape discrimination tech­ considerably reduced the number of event-by-event detection of all niques on cesium iodide detector modules by regrouping many func­ particles and fragments, with precise signals. tions in the same module (all func-

CERN Courier, January/February 1994 17