Bulletin Number Four 1985

system. The quality o f precious stones, such as jade, has been studied by means o f neutron activation. (2) A computerized thermoluminescence (TL) spectrometer has been fabricated for archaeological dating (and hence authentication) o f ancient pottery. Absolute dating is now in process, using improved sample preparation and TL dosimeters. Cosmic-ray Physics A cosmic-ray observation station installed w ith a Nishina type ion-chamber has been in operation since 1969 (in collaboration w ith The Institute o f Physical and Chemical Research, Tokyo). In this observatory, measurements o f the diurnal variation o f cosmic-ray intensity in Hong Kong has been going on since 1973. The data o f measurements have been supplied regularly to the World-Data Centre for Cosmic Rays for distribution to the worldwide research laboratories. In the meantime, studies have been made on the rigidity responses o f ion-chambers and long-term changes in cosmic ray diurnal variations, using the data accumulated from the observations. Using a plastic-scintillator telescope we have also measured the spectra and zenith angle depend­ ence o f slow muons and atmospheric effects on slow muons. A study o f multi-particle production in dense material o f sea-level cosmic rays has been in progress for a few years and the result shows an interesting comparison w ith the theory. Theoretical Physics - K . Young One major area o f theoretical work is in electro dynamics. Professor Y.W. Chan has for a number o f years studied the interaction o f charges w ith strong electromagnetic fields using classical concepts. Im portant examples include the interaction w ith electro magnetic waves (generalization o f the Compton effect), w ith standing waves (the Kapitza-Dirac effect) and w ith spatially periodic magnetic fields in wigglers and undulators. The last o f these is extremely import ant for understanding the basic mechanism in various devices proposed as free electron lasers, which are now being developed in many research centres around the world as a possible source o f coherent x-rays and gamma rays. When free charges appear not singly, but in large numbers, collective effects become important in the so-called plasma state, which is important both astrophysically and in controlled fusion. The recent works o f Dr. H.M. Lai in this area include the study o f radiation emitted in such plasmas in the presence o f a background magnetic field, the effect o f radiation reaction on relativistic plasmas, and particle acceler­ ation by electromagnetic wave pulses. Dr. Lai has also studied in great detail the parallel lateral displace ment o f a light beam upon total internal reflection (Goos-H ä nchen's effect). The interaction o f electromagnetic field w ith dielectrics (e.g. something as simple as water) is even more common but historically controversial. Dr. H. M. Lai and K. Young have achieved an understanding o f this problem from both amicroscopic and athermo dynamic point o f view, and provided quantitative accounts for a number o f experimental situations, including the photoacoustic experiments carried out by colleagues in the Department. Current work centres on unexpected features o f thermodynamics in the presence o f long-range forces (including viola tions o f the zeroth law o f thermodynamics), w ith dielectrics providing a test ground for the theory. While physics may be said to be the search for order in nature, the idea o f disorder surprisingly provides another thread which runs through several aspects o f theoretical research. Dr. K.L. Liu is interested in the behaviour o f disordered magnetic systems, through which one can attempt to under stand how disorder affects material properties. Dr. Liu has addressed this question by calculating a number o f relevant physical quantities to very high order accuracy. Another kind o f disorder appears in dynamical systems (e.g. turbulent fluids) which become appar ently unpredictable and chaotic. Dr. K.L. Liu and K. Young have recently established certain regularities on one o f the more common routes to chaos. The interaction among elementary particles is a subject o f interest to many physicists. The recent work o f K. Young is directed towards the case o f extremely high energies, where the situation is correspondingly simple and in many ways analogous to optical diffraction. Current work exploits the concept o f maximum disorder to understand hadron- nucleus scattering. A number o f other projects illustrate the close collaboration w ith experimental colleagues. Many amorphous materials soften rather suddenly at the so-called glass transition. Theoretical work supported by parallel experiments has proved conclusively that the transition is basically a kinetic phenomenon. The surprisingly negative axial thermal expansivity o f polymer crystals measured in experiments both here and to low temperatures in other laboratories has been understood via an ab initio quantum mechanical calculation. The quasi-one-dimensional nature o f polymer crystals has also enabled an understanding o f its thermal conductivity through a fairly rigorous formalism. This work has also prompted another look into the theory o f thermal conductivity, which has been neglected for several decades. RECENT DEVELOPMENTS 29