Beamline and Stopping Target

In our proposed staged approach to this measurement we will first need a low intensity continuous muon beam, then an intermediate intensity beam, and finally a high intensity chopped beam. The properties of the existing PSI beam lines are well described in documents available on the WWW. For each beam line the flux, focus, purity and other properties are given. It is clear that several of the beam lines can provide surface muon and/or low energy muon fluxes at low and intermediate intensities, e.g. $\pi$E1. It is also clear that some beam lines can provide muon fluxes comfortably in excess of 10⁷ Hz. For example, $\pi$E3 provides in its achromatic mode a surface muon flux of 6  x  10⁷ Hz per mA of primary protons. With such an initial flux, even at a 10% duty factor, the primary requirement of 12 muons collected in 1 $\mu$s can be realized. Beam line $\pi$E3 was chosen by Kammel et al. [3] for their chopped-beam feasibility studies. There would be no difficulty in using, for example, $\pi$M3 for our low and intermediate intensity work, and the chopped-beam facility for our high intensity. The above mentioned beam lines also have appropriate focus and purity.

The chopped beam is central to the lifetime measurement. Three parameters serve to describe the beam: the rise and fall times, the repetition rate, and the extinction factor. The beam developed at LAMPF, described by Ciskowski et al.[17], had rise and fall times of 100 ns, a maximum repetition rate of 100 kHz, and an extinction factor of 0.3%. In their work they would collect muons for 1 to 5 $\mu$s and count for 10 to 20 $\mu$s. These times are well matched to the lifetime measurement. The extinction factor could, perhaps, have been better if the chopper were located further upstream, allowing magnet elements to deflect scattered muons. The extinction factor is not critical because it is monitored continuously with the highly efficient beam scintillator. It is clear then that an appropriate chopped beam is technically feasible. This conclusion is also made in the LOI of Kammel et al. [3].

The surface muon beam has a range of approximately 150 mg/cm². This limited range requires some attention to materials in the beam path. The muon beam can exit through the standard 150 $\mu$m mylar window. Upstream of this window in the beam line vacuum, we would suspend a 200 $\mu$m thick scintillator, which would be viewed directly by two photomultiplier tubes. A surface muon would deposit sufficient energy in the scintillator to provide approximately a 50 photoelectron signal in each photomultiplier tube. This signal should provide for greater than 99% efficient detection of beam particles. A highly efficient beam counter is needed to detect errant muon injection during the measuring period. This counter will be monitored using the same waveform digitizers which we will use for the detector timing tiles as described later.

The µLan Detector will be placed directly downstream of the vacuum window. The beam then must be transported 28 cm to the stopping target. The beam must be transported through helium to minimize multiple scattering. The detector allows the insertion of up to a 5.7 cm diameter cylinder. Helium at slightly above one atmosphere pressure can be efficiently contained in a 25 $\mu$m aluminized nylon bag. At least two stopping targets will be employed. We will use a non-depolarizing target such as Al for studies of the asymmetry of the detector. For the main experiment, a sulfur target of 1 mm thickness will be suspended in the helium bag. The stopping target would be approximately 5 cm in diameter. Multiple scattering in the beam scintillator and vacuum window together would produce a beam spot of 1.1 cm rms. This multiple scattering increases only slightly the typical beam focus of 20 x 30 mm². The sulfur target is known [18] to depolarize the muon distribution to a level below approximately 3%. We will assume that the muon population has a residual polarization of 3% in the systematic studies which are outlined in a later section.