Search

A search for the decay μ + → e + γ , performed at PSI and based on data from the initial three months of operation of the MEG experiment, yields an upper limit on the branching ratio of BR ( μ + → e + γ ) ⩽ 2.8 × 10 − 11 (90% C.L.). This corresponds to the measurement of positrons and photons from ∼ 10 14 stopped μ + -decays by means of a superconducting positron spectrometer and a 900 litre liquid xenon photon detector.

The intensity of scintillation light emission from liquid xenon at room temperature was measured. The scintillation light yield at 1 deg. was measured to be 0.64 +/- 0.02 (stat.) +/- 0.06 (sys.) of that at -100 deg. Using the reported light yield at -100 deg. (46 photons/keV), the measured light yield at 1 deg. corresponds to 29 photons/keV. This result shows that liquid xenon scintillator gives high light yield even at room temperature., Comment: 16pages,12figures

A liquid-gas double phase xenon detector for WIMPs with a 0.3l active volume has been constructed and tested. Proportional scintillation signals are observed after ionization electrons drifted 8 cm to a multiwire anode mounted in gas phase. Basic properties of the detector and electrons lifetime were measured. The efficiency of background rejection for grays is tested and discussed.

The feasibility of a large-volume liquid xenon Compton telescope based on full event imaging in a time projection chamber has been demonstrated with the development of the balloon-borne instrument LXeGRIT. With its 400 cm2 sensitive area and 7 cm drift gap, the liquid xenon detector images γ-rays in the energy range from 200 keV to 25 MeV. The precise 3-dimensional localization of γ-ray interactions within the sensitive volume provides excellent background reduction capabilities. Together with the large efficiency of a homogeneous detector volume, LXeGRIT addresses the primary instrumental limitations encountered in this energy band. Following engineering tests at balloon altitude in 1997, LXeGRIT has been upgraded with a new trigger and data acquisition system, integrated with the existing readout electronics. Enhanced data transfer capability and onboard data storage were also implemented. LXeGRIT was successfully flown from Ft. Sumner, NM on May 7, 1999. The instrument worked as expected at balloon altitude. The large amount of data gathered during the 9.5 h flight are currently being analyzed. We describe the instrument and present some results from calibration data and preliminary results from the flight.

Large two-dimensional position-sensitive silicon detectors (PSDs) with good position linearity, resolution and thickness uniformity have been developed. The PSDs have an effective area of 62 mm×62 mm and a thickness of 500 µm. The non-linearity and position resolution for the whole sensitive area are found to be less than 0.26 mm in rms and 0.44 mm in fwhm, respectively, when an energy of 157 MeV is deposited in the detector and a shaping time constant of 12 µs is used in the pulse processing. The non-uniformity of detector thickness is ≤0.1% in rms. The PSDs recently developed are good enough to separate iron isotopes in a telescope aboard the GEOTAIL spacecraft scheduled to be launched at July in 1992.

From the view-point of application to gradient magnets for a strong focusing proton synchrotron, the magnetic and mechanical properties of oriented low carbon steels as well as typical non-oriented low carbon steels are investigated in detail. One gradient magnet of non-oriented steel and two of oriented steel, were constructed and the distributions of field gradient for these magnets were measured at various excitations. As a result of the measurements, it is shown that in the gradient magnet made of oriented steel the use of the field up to 17 kG is possible. Furthermore, the computer calculations of the field gradient distribution for gradient magnets of oriented steel were made by using the magnetic property in the direction perpendicular to the median plane of the magnet gap. The results are in fairly good agreement with those of the field measurements.

The criticisons of Ascarelli1,2 regarding Shibamura et al5’s measurements of electron drift velocity in liquid Ar, Kr and Xe are answered. (AIP)

The scintillation of helium is explained by de-excitation of the excited molecule formed by a three body collision of a metastable atom with ground state atoms.

A stack of CR-39 sheets has been exposed to 96 MeV/n and 2.1 GeV/n Ar beams in order to study uniformity of response and the response curve. Variation in response of the area of about 50 cm × 50 cm for one sheet was within 1.5 %. Roughness of the surface after 27 hr etching and the charge resolution were improved by changing temperature-time curing cycles. Steeper response curve at high REL region was observed and it was confirmed that our CR-39 has higher sensitivity in the recording of nuclear tracks down to Z/β ≃ 6 for vertical incidence.