3.2.2 The p E1 beam line
The p E1 beam line consists of nine quadrupole and two dipole
magnets used to select the momentum of the particles transported to the
experimental area. In addition, there exist vertical and horizontal beam
collimators as well as two carbon degraders, all of which can be controlled
remotely. For the summer 1996 beam time, there were three additional
quadrupoles and a large dipole in the p E1 area. It was quickly
determined that the dipole magnet spread the beam spatially and extended the
focal point of the last quadrupole enough to significantly spread the beam
momentum. The decrease in beam quality during data runs due to the effects of
the field of the dipole magnet in 1996 caused the omission of this magnet
during data runs in 1997.
The facility is capable of producing up to 2.4 × 108 pions per
second and momenta of up to 200 MeV/c. For the 1996 and 1997 beam times, a pion
momentum of 116 MeV/c was used. A large pion momentum means fewer pions will
decay in-flight due to kinematics and relativity. The beam line collimators
were used to reduce the pion stop rate such that the detector saw good events
at a rate slightly higher than the data acquisition rate (~100 Hz). This
minimized the number of accidental coincidences without compromising our rate
of collection.
The beam line magnets at PSI are under user control. A computer program called
TRANSPORT, written at PSI, allows users to determine optimal magnet
settings for a given momentum tune and particle type. This program requires
some experience to use properly. Pibeta beam times typically start out using
beam tunes from past experiments then incorporate the use of another computer
program called OPTIMA. This program uses a configuration file to change
the setting for each magnet in the beam line one at a time. OPTIMUM
checks the rate of a user-supplied signal to determine the optimal magnet
settings for maximum beam rate. The user-supplied signal can be as simple as a
single scintillator/phototube detector placed in the beam, or a more
complicated logic made from fast electronics. Pibeta beam times usually used a
2 × 2 mm2 "pill" counter positioned at the target center to
determine a beam tune focused at that point.