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Ground State of Protonium

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Ground State of Protonium Spectrum of coincident pairs of X-rays from proton-antiproton atoms. Selecting a transi­ tion to the first atomic excited state (L line) enables the first ground state transition (K- alpha line) to be unravelled from the other K lines. beams, and can help the experi­ ments wanting decelerated parti­ cles. The electron gun from the ICE ring was converted for LEAR. The cooling had to be achieved along a shorter interaction length, a 1.5 m straight where the electron beam overlaps with the orbiting particles, compared to 3 m in ICE, and the problem of firing an intense elec­ tron beam into the very high vacu­ um of LEAR (2.5 A into 10"11 torr) had to be confronted. Special diag­ nostics had to be developed, in­ cluding scattering of laser light on the electron beam, observing the microwave radiation from the spi­ ralling of the electrons in the mag­ netic field, and observation of the X-rays caused by stray electrons. In the October tests cooling was observed from the very first injec­ tion of a proton beam into LEAR using Schottky scans and the de­ tection of neutral hydrogen. This latter technique can only be applied while working on proton beams - neutral hydrogen atoms emerge periments. The cooling of a the system used for these experi­ from the straight section unde- bunched beam was demonstrated ments into a reliable system for viated by the magnetic fields of the and very short bunches were regular operation of LEAR, how­ ring or of the electron cooling sys­ achieved. The electron energy was ever the tests have shown convinc­ tem. This is a useful tuning aid, moved off the optimum value and ingly that electron cooling adds since hydrogen production in­ the proton energy was seen to fol­ another effective weapon for tam­ creases as the two beam velocities low. Too much cooling induced ing beams of antiprotons and other approach (the ideal condition for beam instabilities. The equilibrium ions. cooling). Production rates of a few between the stochastically intro­ thousand hydrogen atoms per se­ duced heating of the proton beam cond were observed and the meth­ for beam extraction and the elec­ Ground state od still worked with low proton in­ tron cooling gave a measure of the of protonium tensities, about 106, when Schott­ cooling power. ky scans were no longer sensitive. Further tests with antiproton 'Exotic atoms' - where everyday Major achievements included beams are eagerly awaited, when orbital electrons are replaced by slimming the proton beam from a the cooling rates can be compared other negatively charged particles few centimetres across to a few with those of protons. Experiments (muons, pions, kaons, antiprotons) millimetres in seconds, while the at Novosibirsk indicate that over a provide physicists with another energy spread was pared from sev­ range of parameters electron cool­ window on the strong nuclear for­ eral parts per thousand to better ing works more than twice as fast ce to supplement what is learned than one part in ten thousand. The for negatively charged beams (like from scattering experiments. short time available for the tests antiprotons). These synthetic atoms have long saw a number of beam physics ex­ Much work remains to convert been a speciality of CERN research, 12 CERN Courier, January/February 1988 and the availability of intense beams of antiprotons at the LEAR Low Energy Antiproton Ring gave a boost to the study of atoms with orbital antiprotons. Like everyday electrons, antipro­ tons are held in their atomic orbits by the electromagnetic attraction of the positively charged nucleus. Ho­ wever because antiprotons are al­ most 2000 times heavier than elec­ trons, they pass very close to the nucleus, experiencing at close quar­ ters the effect of the nuclear force, LEP progress Not a mock-up this time but the real thing. A section of the 27 kilometre LEP electron-positron collider at CERN, where installation work is pushing ahead quickly. (Photo CERN X312.11.87) Magnets being readied to go down into the EP tunnel. At the rear are the 'concrete' di- pole elements, with quadrupole and sextu- pole assemblies in the foreground. (Photo CERN X340.11.87) measured through the resulting changes in atomic behaviour. Because their inner orbits are so small, antiprotons are frequently swallowed by the nucleus before they can reach the ground state or­ bit, making the corresponding X- ray spectra (K lines) difficult to see. This was overcome in different ways by three experiments at LEAR (see December I986 issue, page 12). The simplest such atom is 'pro- tonium', or antiprotonic hydrogen, CERN Courier, January/February 1988 Crystal Materials for your applications. HILCERi Hilger Analytical Limited Westwood Margate Kent CT9 4JL I Analytical i Tel: 0843225131/224261 Telex: 96252 High quality Cryo-Components AT EC A • VACUUM HEAT TREATMENT • VACUUM BRAZING • CRYOGENIC TREATMENT • HELIUM CONTROL • LASER YAG - CFA0 - 5 AXES - SOLDERING Valves, Flow orifice gauges, Nozzles, Ejec­ - CUTTING tors, Cryostats according your require­ - DRILLING ments with full quality assurance - MARKING - SUPERFICIAL LOCALISED TREATMENT • CAO • STUDYING AND REALISATION OF MECHANICAL PARTS Alfred Battig AG CH-8400 Winterthur/Switzerland Phone: Telex: Telefax: Verlhaguet 82000 MONTAUBAN (FR) 052/25 27 69 896 371 valv ch 052/25 02 24 Tel.: (63) 63 78 97 Telex: BSC - ATECA 306 254 F 14 CERN Courier, January/February 1988 with a lone nuclear proton and an Later the beam was transported PETRA beam test orbital antiproton, giving deep in­ through 11 HERA modules in sights into the nuclear interaction 130 metres of tunnel, giving a final of superconducting between particle and antiparticle. particle intensity of 8 x 108. cavities However the individual K lines were One interesting test involved difficult to separate or were obscu­ picking out single bunches from red by large backgrounds. To over­ the beam stored in PETRA with Superconducting 500 MHz cavities come this, the ASTERIX experi­ the help of very fast ejection kick­ have been developed at DESY for ment looked for two or three coin­ ers, without disturbing the rest of increasing* the electron beam ener­ cident X-rays due to successive the bunches. The kickers operate gy in the HERA storage ring. The atomic transitions as the orbital an­ faster than the bunch separation basic 4-cell design follows closely tiprotons tumbled from one orbit to of 96 nanoseconds, allowing the the corresponding CERN layout the next, landing up in the ground injection process to be studied in (see page 7). At DESY new higher state. depth. Video images of the spots order mode couplers have been Selecting a particular line (an L produced by single bunches on developed to supply enough damp­ transition to the first excited state) thin fluorescent screens are stored ing of higher cavity modes at the in coincidence with a K line selects electronically and analyzed to HERA current of 60 mA. In this the first K X-ray line (K-alpha) from optimize injection. design the dominant higher order the complete pattern of K lines. PETRA was operated at 7 GeV mode resonances are much more This requires a detector with good (in future it will be 14 GeV) for strongly damped than in a normal angular coverage, low threshold electrons. Throughout the test, a conducting copper cavity. A tight- and good efficiency, as only one K- 4-cell superconducting cavity oper­ fitting helium vessel is welded to alpha X-ray is emitted per hundred ated in the PETRA ring. the cavity, and the additional use L lines. Unravelling the K-alpha of aluminium fillers reduces the from its neighbours in this way ma­ amount of liquid helium required kes for cleaner nuclear physics in­ to 100 litres per 4-cell unit, an formation. important safety aspect. The com­ plete assembly, including couplers and helium vessel, is fabricated, PESY chemically cleaned and processed by industry. Two such assemblies Accelerators tested will be mounted under dust-free conditions in 4.2 m cryostats As mentioned briefly in our Decem­ supplied by industry. ber 1987 issue, the new chain of In the first prototype cryostat electron accelerators at the German with its two 4-cell assemblies a DESY Laboratory in Hamburg un­ cold leak in one cavity had to be derwent a 14-day trial. The elec­ fixed. Subsequently the cavity tron linac, the new DESY II syn­ quenched at 2.5 MV/m at one of chrotron and the PETRA ring (pre­ the repaired electron beam welds. viously an electron-positron collider The other cavity assembly was and now used as an intermediate limited at 200 kW forward power accelerator) injected particles into by sparking at the high power input the first completed section of the window. At this level the acceler­ electron ring for the HERA electron- ating gradient was 5.1 MV/m. Field proton collider. emission at this gradient lowered On 5 November, single bunches the quality factor (Q-value) to 1.3 were injected from PETRA into an x 109 (compared to 2 x 109 at electron module of HERA at time Helium transfer line under test at DESY for lower fields). the superconducting magnets of the HERA intervals of 8 seconds using two electron-positron collider now under The main purpose of the test septum- and three kicker-magnets. construction. was to explore the behaviour of CERN Courier, January/February 1988 15 .
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