DASP, the double arm spectrometer detector at one of the intersection regions on the DORIS - storage rings at DESY, photographed when the two arms were pulled back. The central section, which is receiving attention, and the counters, which are visible on the right and on the left, are part of the inner detector elements. DASP was the source of the F meson discovery.

(Photo DESY)

due to this loss. Firstly, beam scrapers record. On 20 June the polarized ion non-charm quarks. Mesons called D have been mounted above and below source gave a current of 960nA; the made of charm quark and non-strange the beam plane so as to direct any external beam current at 500 MeV was quarks (cG or cd), as well as baryons beam lost vertically to localized spots 120nA with over 70% polarization. carrying charm, have already been on the wall. Secondly, the space The improvement is attributed to found. Mesons consisting of a charm between the tank walls and the cleaning and adjustment of the quark and a strange quark (cs) called F yoke has been filled with matching section between source and mesons were still at large. graphite blocks surrounded by acceleration tube. The source is of the A team from Aachen / DESY / Ham­ boriated gypsum sheets; this has given Lamb shift type with a Sona zero field- burg / Max-Planck-lnstitute, Munich / a factor five improvement in residual crossing region. The design aim was Tokyo reported evidence for the adiation at the centre of the tank and 60 nA. elusive F meson at the Budapest ten at the wall. Thirdly, 5cm thick lead Conference. The discovery was made shields are remotely installed inside DESY with data collected over several the tank wall during shutdowns; these months in the double arm give a further factor two in the middle The evidence spectrometer DASP on the electron- and five at the wall. As a result, after six for the F meson positron storage rings DORIS. months' running at 3 and 10uA, the The way the F meson was found activity is still only 5 mrem/hr over The discovery of the J/psi family of amongst the massive background of most of the 1 7 m diameter tank and 1 0 particles added a fourth type of quark, other events from electron-positron mrem/hr at the wall. called the charm quark c, to the annihilation resembles feats of The first regular operation at familiar three quarks u, d, and s. Since archeology. The strategy was as fol­ 100uA, for say one shift / week, is the c quark should be able to combine lows : if the F is not much heavier than scheduled for the beginning of 1978. with any of the three others, an im­ the D, it will decay only weakly and will To achieve this the permanent beam mediate consequence was the ex­ show up mainly in states containing a dump, thermal neutron source, and istence of particles made of charm and strange quark and its antiquark (ss). beam line extension to it must be com­ pleted and commissioned. To make this possible the high intensity beam line and the Meson Experimental Hall will be closed in September for three months. In addition, the front ends of two additional lines (one for and one for pions) will be installed on the high intensity line. There are three longer term plans. Next year it is planned to introduce third harmonic r.f. into the cavity resonators to 'flat top' the r.f. wave. This will improve regular operation and make it possible to extract separate turns with an energy spread of 0.1 MeV rather than the present 1.5 MeV. Secondly, the possibility of extracting beams at lower energies (70-180 MeV) is being studied and the feasibility of extracting a 75 MeV beam for isotope production has recently been demonstrated experimentally. Thirdly, it is planned to raise the cur­ rent to 300 uA, extracting at 450 MeV to avoid electric stripping losses. The 114uA is not the only new

235 Thus, the final states will contain a pair the FF* and F*F* hypotheses. Four of in the full-sized ISABELLE system.) of kaons or, for example, an eta meson the events gave F and F* masses which A vacuum of 6 x 10~12 torr was which has a sizable ss component. were the same to within 40 MeV with achieved after the initial bake-out us­ A likely F decay mode is therefore no other events in the vicinity. ing a combination of titanium sublima­ F+-^n+r|- The mass values for F and F* are tion pumps and vac-ion pumps and an There was a good chance that F 2.03 ± 0.06 GeV and 2.14 ± 0.06 aluminium vacuum chamber. (The mesons have production features like GeV. Hence F and F* are about one ISABELLE design now calls for a D's, that is that near threshold the F pion mass heavier than their charm stainless steel vacuum jacket.) The meson appears mainly associated with non-strange counterparts D and D*. vacuum was much better than the its first excited state F*. The F* was ex­ ISABELLE design requirement of 3 x pected to be only slightly heavier than 10~11 torr. The main magnet power the F and it therefore would decay into BROOKHAVEN supply, as well as the supplies required the F by emitting a photon. If the F* is for the various tuning windings in each almost at rest the energy of the photon ISABELLE half-cell of the , were controlled by a is of the order of the (F* - F) mass dif­ tests PDP 1 1/10 computer system. ference and therefore small. The recipe Of primary concern was a test of to look for F mesons was therefore to A superconducting half-cell of the methods for 'quench' protection. The search for an eta meson associated ISABELLE magnet lattice has been scheme employed in the half-cell uses with a low energy photon. powered to full field. The assembly, the switching capabilities of silicon Having decided how to look, the erected in a mock-up tunnel section, diodes to protect the other magnets next question was where to look. The consists of two prototype dipoles plus when one of them quenches (fails to most promising place appeared to be a quadrupole connected in series, both remain superconducting). One or more the region just above the charm electrically and cryogenically, in a con­ such diodes are connected across each threshold at energies 4 to 4.5 GeV figuration simulating an element of the magnet. If a magnet quenches, the in­ where the total cross-section exhibits proposed storage rings. duced voltage exceeds the diode several spikes, for example at 4.4GeV. Refrigeration, vacuum, and controls forward bias voltage (which normally In scanning the energy region from systems were all in action. The prevents any leakage current from 4 to 5 GeV, DASP found about sixty refrigeration is based on forced con­ flowing) and the main magnet series events at 4.4GeV which had an eta vection of high pressure (15 atm) current (3300 A at the full excitation of together with a low energy photon in a supercritical helium gas. Prior to en­ 4T) passes through that diode, bypass­ nonmagnetic part of the detector. No tering the string of magnets, the ing the magnet. such signal was observed at other refrigerant is further cooled when it The first quench occurred at a dipole energies. The eta-photon events had passes through a subcooler heat ex­ bore field of 3.9 T (corresponding to 48 the taste of the F meson and a frantic changer containing a bath of liquid T/m in the quadrupole). With subse­ search was made for other events helium at subatmospheric pressure. quent 'training', the dipole field which, besides eta and a low energetic The bath is achieved by pumping on reached approximately 4.3T on the photon, had a pion detected in the the helium vapour with an ejector noz­ sixth and last quench initiated. This is magnetic spectrometer. Nineteen zle of novel design. (The nozzle will be close to the expected limit at the events were found, six of which fitted replaced by a turbocompressor system presently attainable temperature

236