674 Nature Vol. 292 20 August 1981

antigens, complement allotyping and Theoretically the reasons for expecting of HLA and immunoglobulin allotyping additional enzyme markers would clarify HLA and immunoglobulin-gene linked data together with other genetic markers the point. associations with immune response in and environmental factors should allow Although in the face of the evidence general and autoimmune disease in autoimmune diseases to be predicted presented one tends to think of tissue particular are overwhelming: HLA-DR exactly. However, there is still a typing at birth (to predict the occurrence of antigens (or antigens in the same considerable amount of analysis of both autoimmune disease) or perhaps even chromosome area) are necessary for H LA-region genes and lgG-region genes to before one goes to the computer dating antigen handling and presentation by a be done in order to achieve this goal in the service, there are practical scientific lymphoid cell subset; markers in this region general population. reasons for being cautious. For example, (by analogy with the mouse) are important In Japanese families, the occurrence of Uno et a/. selected only 15 of the 30 for interaction ofT cells during a response; Graves' disease can be exactly predicted on families studied for inclusion without immunoglobulin genes are also involved in the basis of HLA and immunoglobulin saying how or why this selection was made. T-cell recognition and control; HLA-A,-B allotypes, but it is too early to start wearing Second, lgG allotype frequencies are very and -C antigens are important at the "Are you my H LA type?" badges outside different in Caucasoid and Japanese effector arm of the cellular response; and Japan. Nevertheless, elimination of populations. Third, Graves' disease is very finally, C2, C4, Bf and immunoglobulin autoimmune diseases by genetic strongly associated with B8 / DR3 in are involved at the effector arm of the counselling is now much closer to being Caucasoids but with DR5 and DRS in humoral response. Thus, HLA and realized: if two independent susceptibility Japanese populations. A study of Japanese immunoglobulin genes are active genes are necessary for disease onset, then families living in America might show throughout the immune response from it is possible to ensure that the genes will whether these differences are due to a differ­ recognition to control and it is slight not occur together in the next generation.! ' ent environmental factor interacting with differences in an individual allele's ability I Brewerton. I..>.A . f!l a/. l .uncer i, 904 ( 1973). different IgG or HLA haplotypes in the to function, or more exactly a summation 2. S ~ hlo <; _qein. l .. lcra,aki, P.l. & Hllll'\IOne, R . Ne w two populations or to putative of slight differences, which we are t 'nRI. J. Mer!. 288. 704 (19737 . .l . Batchdo r J . K. ~ ra t. / . ann~t i, 1107 ( 1980). susceptibility genes going with different measuring as disease susceptibility. 4. Whiningham. S. era!_ Chn. exp. lmmun. 43, XO ( 19XI ). allotypes in the two populations. Theoretically therefore we expect that use Iii _ Mt:

Positronium and positronium ions from T. C. Griffith

POSITRONS and positronium, the bound located at the exit side of the carbon film slow positrons of well defined energies state of an electron and a positron, have in will reflect the transmitted positrons back achieved at Bell Laboratories and at recent years been used in a rapidly to the carbon whilst accelerating the weak Brandeis University by Karl Canter and co­ increasing range of experiments in atomic beam of Ps- into a field-free drift region workers3. Technology has now reached physics, solid-state and surface physics and beyond the grid . A lithium-drifted such a level that within a few years low­ positronium chemistry. Positronium has germanium detector is used to measure the energy positron beams of intensities of 109 also played an important part in energy spectrum of the resulting positrons per second, or even several fundamental experiments designed to annihilation photons and a y-ray peak is orders of magnitude higher, can be check the validity of certain aspects of the found whose energy depends on the envisaged. Positron emitters created in situ theory of quantum electrodynamics accelerating potential applied to the grid. It in a nuclear reactor followed by carefully (QED). It is therefore pertinent that the was demonstrated that this peak can be designed moderators and beam transport recent detection of a new entity, the attributed to a Doppler-shifted systems may be used for this purpose. If positronium negative ion, Ps- (e·e + e·), annihilation y resulting from the 2y decay allowed to strike clean metal surfaces under initially predicted to exist as a bound state ofPs-. ultra-high-vacuum conditions these by Wheeler' in 1946, should be greeted as As well as being of interest in its own positrons can be converted, with almost an important historical landmark which right, Ps-is also important because it might 100 per cent efficiency, into low-energy could greatly extend the activities of be used to produce beams of positronium positronium atoms. We might therefore positron physicists. The careful experiment of well defined energies. This could be expect intense and highly localized sources performed by Allen Mills 2 at the Bell achieved by accelerating the Ps- to a given of positronium atoms which can be used to Laboratories, New Jersey clearly energy and then photoionizing it to give study the excited states of positronium demonstrates that Ps- can be produced in positronium which could, perhaps, be used using laser beams in experiments, similar to the laboratory under controlled in scattering experiments. Another those performed for atomic hydrogen, that conditions. suggested application is to use the will provide further high-precision tests of Ps- is the analogue of H- ions produced annihilation of Ps- in flight to provide a QED. by slow proton bombardment of thin foils tunable source of y rays of precisely known Even more exotic possibilities exist. and the method used by Mills incorporates relative energy shift. The intrinsic Quantum chromodynamics predict that the same ideas. In his experiment, a nearly properties of the Ps- such as its lifetime (- orthopositronium can decay into a new monoenergetic beam of low-energy 2 nanoseconds), its 2y angular correlation, elementary particle, the ax ion, and a y ray . positrons, guided by a longitudinal its (ly/2y) branching ratio, its magnetic The axion must have a mass of less than I magnetic field, strikes a thin film of carbon moment and measurement of its MeV for this to be possible and if this were in ultra-high-vacuum conditions. T!le photoionization cross-section are obvious the case, then an intense positronium energy of the positrons is adjusted such candidates for extensive studies. source from a low-energy positron beam that the beam is partially transmitted and The glamour of deteding the Ps- ions could well find application in a search for application of a suitable potential to a grid should not be allowed to overshadow the theaxion. . .. deeper significance of this experiment. The major triumph underlying this work is the dramatic technical advances associated 1 \\lln·k1 . 1. -\ . Ann . .\'.}' Acad. .\('(. 4M,219(1446). T. C. Griffith is in the Department of Physics 1 Milk A .P . Jr Phv,. RC\'. Leff. 46.717 ;JI.JXI) and A stronomy at University College London. with the production of intense beams of -~- Canter.~ . !- . k ~,·\ - k\\ to he publi-;ht·d in t'ltn. J Ph ~·\ .

0028·0836/ XI 1.1406 74-0 I $0 I .00 ~1981 Macmillan Journals Ltd