The goal of the experiment is to measure the strong interaction ground state shift and width in pionic hydrogen with a high resolution crystal spectrometer. The experiment will concentrate on the improvement of the precision of the width measurement by one order of magnitude. The knowledge of the value for the shift will be improved also by almost an order of magnitude because of better statistics and by extensive studies of sources of systematic errors like the pressure dependence of the measured shift.
It will be demonstrated below that the combination of the cyclotron trap II and spherically bent crystals with an extended CCD detector provides a sufficient resolution with acceptable rates and measuring times.
The main difficulty in the experiment is to quantify the influence
of Doppler broadening on the line shape caused by Coulomb deexcitation. One
way to solve this problem would be to perform the measurement with transitions
from high levels 
where no sizable Coulomb acceleration can occur.
Unfortunately the yield of these transtions is too low for a high statistics
study of the line broadening. Also using low pressures will not eliminate the
Doppler broadening completely as recent experiments [68] show the Coulomb
acceleration still exists in the mbar region. The high statistics experiment
is thus restricted to pressures 
and to the transitions from 
for pionic hydrogen and 
for muonic hydrogen because of intensity
reasons.
The present experiment will be performed with the
and the
transitions in pionic hydrogen. The corresponding
transitions in muonic hydrogen will be measured simultaneously with a special
arrangement of two 
crystals working with different reflection
planes. This saves beam time and, more importantly guarantees that no systematic
error occurs because of change of experimental parameters. The effect of the
Doppler broadening as determined from the muonic measurement will deliver the
input parameters for the fitting of the pionic lines as discussed in chapter
2.2.4. The soundness of this procedure is then checked by comparing the results
for the width extracted from both the
and the
transitions measured at different pressures, typically between 
and
. The strong interaction width must be independent of the
transition line and the pressure if all corrections have been treated properly
. This will also allow us to check that other minor effects like the
X-ray broadening due to transitions during the Stark collisions do not affect
the accuracy of the measurement.
The measurement of higher transitions in pionic hydrogen, like the
transition and transitions at the series limit, are presently
not considered as they require different Bragg crystals, but they could be the
subject of a following proposal.
As we heavily rely on the two crystal method for a simultaneous measurement of pionic and muonic atoms, we propose to do extensive preparatory experiments with electronic X-rays before setting up the experiment at the pion beam. These experiments are planned to take place in the NA hall at PSI. They will use an ECR source and fluorescence X-rays as mentioned in chapter 4.1. The corresponding experimental set-up will be described in chapter 6.1.