Johann Tischhauser

The European Hybrid Spectrometer - a facility to study multihadron events produced in high energy interactions

Abstract The European Hybrid Spectrometer is described in its preliminary version for the NA16 charm experiment. The performance of the small hydrogen bubble chamber LEBC and the detectors of the spectrometer is discussed. In particular the combination of the bubble chamber information with the spectrometer data is described in detail. The track reconstruction efficiency is 90%. The precision with which vertices seen in the bubble chamber are reconstructed is around 10 μm and the two track resolution is 40 μm. Therefore very complex event configurations, in particular charm particle decays, can be reconstructed correctly.

A rapid cycling hydrogen bubble chamber with high spatial resolution for visualizing charmed particle decays

Abstract For the purpose of directly detecting the decays of charmed particles, a rapid cycling hydrogen chamber with high spatial resolution has been constructed. In several runs it was exposed to 360 GeV proton and negative pion beams and has taken a total of 1.63 million pictures during more than 20 million expansion cycles. The resolved bubble diameters were around 40 μm with 80.5 counted bubbles/cm at 33 s −1 chamber and 17 s −1 camera cycling rates.

The CERN cryogenic test facility for the ATLAS barrel toroid magnets

The superconducting magnet system of the ATLAS detector will consist of a central solenoid, two end-cap toroidal magnets (ECT) and the barrel toroid magnet (BT) made of eight coils symmetrically placed around the central axis of the detector. The magnets will be tested individually in a 5000 m/sup 2/ experimental area prior to their final installation at an underground cavern of the LHC Collider. For the BT magnets, a dedicated cryogenic test facility has been designed which is currently under the construction and commissioning phase. A liquid nitrogen pre-cooling unit and a 1200 W@4.5K refrigerator will allow flexible operating conditions via a rather complex distribution and transfer line system. Flow of two-phase helium for cooling the coils is provided by centrifugal pumps immersed in a saturated liquid helium bath. The integration of the pumps in an existing cryostat required the adoption of novel mechanical solutions. Tests conducted permitted the validation of the technical design of the cryostat and its instrumentation. The characteristics of one pump were measured and pressure rise of 300 mbar at nominal flow of 80 g/s confirmed the specifications.

Long-term operational experience and upgrade of the cryogenic system for the superconducting cavities in the CERN SPS accelerator

Abstract Cryogenics at liquid helium temperature has been requested in the SPS for testing and operating superconducting accelerating RF cavities: in 1988 and 1989 a single LEP prototype cavity made of bulk niobium, in 1990 two cavities made of niobium sputtered copper and finally, in 1991, a single niobium sputtered cavity. To cope with the various cooling needs, we used the same refrigerator (120 watt at 4.5 K) and the associated cryosystem has been optimized to reduce the thermal load. The original transfer line has been replaced by a flexible and thermally shielded one made of six coaxial bellows. The achieved thermal performance for the various combination of superconducting cavities, the long-term behaviour and operational experience of the cryosystem are given. While the first cryosystem is maintained in operation, a new one (using a refrigerator of 400 watt at 4.5 K) is now being tested to increase, in 1992, the SPS accelerating field by means of two additional superconducting cavities. A description and the preliminary results of the new cryogenic facility are presented.

Design Criteria of the Cryogenic System for the CERN LHC Injection Lines

Protons to the CERN LHC will be supplied from the SPS accelerator via two new injection lines. The lines include three bending arcs for horizontal beam deflection and are characterized by a strong inclination (7.2 % maximum slope) to overcome the difference in height (40 m) between the two machines. They will be equipped with superconducting dipoles operated at a field of 5.4 T and 4.4 K. The distribution to the magnets of liquid helium in strongly inclined channels and over a distance of several hundred meters represents a considerable cryogenic challenge. A preliminary design of the cryogenic system is under way and the cooling schemes suitable to the unusual geometry are reviewed. The basic requirements of the refrigerating system and a tentative layout of the cryoplants are presented.

A Rapid Cycling Hydrogen Bubble Chamber with High Spatial Resolution to Visualize Charm Decays

In the Autumn of 1979, mass values and decay modes for D-mesons were already established from leptonic production with e+, e--colliding beams. Emulsion exposures in neutrino beams indicated lifetimes between 10-12 s and 10-13 s, and neutrino beam dump experiments yielded indirect evidence for hadronic charm production with estimated cross sections from 10 µb to 50 µb; In striking contrast to leptonic charm production, hadronic interactions should produce associated charmed pairs and visual observation with a bubble chamber seemed to be the only clean method of separating these double decays from the strange particle background. In this talk we report on the contruction and operation of such a bubble chamber, named LEBC, which stands for LEXAN bubble chamber.