E864 is a mass spectrometer with high rate capability, large acceptance and
excellent resolution. The detector measures magnetic rigidity, charge, time
of flight, and energy (via calorimetry). High redundancy in the measurements
is a key feature. The layout of the detector elements is shown in
Fig. 
.
The magnet M1 is a standard AGS 18D72, and M2 will be borrowed from SLAC.
The symbols H1, H2, and H3 refer to scintillation counter hodoscopes. The counters are vertical slats and are read out at each end by phototubes so as to provide accurate time measurement via the mean time of the two photomultiplier tubes.
The symbols S1, S2, and S3 refer to straw tube proportional chamber
stations. Each station consists of six planes: two vertical planes, two planes
tilted at 
, and and two planes tilted at 
. Each
coordinate (x,y,u) needs two planes in order to assure full efficiency.
Station S1 is located inside the vacuum chamber.
Considerable study has gone into the design of the collimation and vacuum
chamber. Background tracks from the vacuum chamber walls, magnet pole tips,
etc. were studied using a detailed GEANT simulation. A satisfactory design
was worked out, and is indicated in Fig. .
The dimensions of the major detector elements are given in Table .
We plan to use the first two straw tube arrays as simple proportional counters without drift time information. The third straw tube will be used in the drift mode. The diameter of the tubes in the first two arrays is chosen to give acceptable occupancy, and is small enough to give the necessary resolution without drift time information. The third array is sufficiently far downstream so that a larger tube diameter (0.8 cm) can be tolerated and, with drift time information, excellent position resolution can be obtained. The straw tube detector will be the responsibility of the Penn State group, who have had experience with similar systems in Fermilab E760 and E706.
Table: Dimensions of the E864 Spectrometer Elements
The thickness of the hodoscope counters is still under investigation. The
dimensions indicated in Table are known to be adequate, but we are
investigating the possibility that the second and third hodoscope could be
made somewhat thinner. Our design specification for the RMS mean-time
resolution of the hodoscopes is 0.2 ns. We demonstrate in the chapter on
experimental methods that this resolution can be achieved. The Yale (WNSL)
group is taking major responsibility for this system.
The calorimeter, a lead/scintillating fiber ``spaghetti" design, offers many
advantages for the E864 application. This is the fastest known calorimeter,
and has an expected RMS time resolution of 0.5 ns. The design is inherently
hermetic, unlike calorimeters using wavelength shifters, and can be made completely
compensating. Because of the compensation and the high degree of sampling,
the spaghetti design offers the best known hadronic energy resolution. A
stochastic term between 
and 
is achievable, depending on how we trade off cost and resolution. We have
kept in close touch with the SPACAL collaboration at CERN, and our
requirements have been met or exceeded by prototypes they have already built
and tested [12].
It would be wasteful and expensive, however, to simply copy the SPACAL design. The SPACAL calorimeter is 2 m long so that it can measure particle energies of several hundred GeV. The E864 calorimeter may detect particles with energies as high as tens of GeV, but the depth of these showers will be dictated by the energy per baryon of the particle, which will be limited to the beam's 12 GeV/nucleon. The size of the constant term in the resolution is of much greater importance for the higher CERN energy application than for E864. Finally, we are crucially interested in the timing accuracy of the calorimeter, while the CERN program is not. For these reasons we will carry out a modest R&D program to optimize the spaghetti design for E864. This is further discussed in the section on experimental methods.
The few disadvantages of the spaghetti design, such as difficulty in longitudinal segmentation and possible sensitivity to radiation damage, are not important considerations for the E864 application. Finally, the Yale (YAUG) group is responsible for construction of the spaghetti calorimeter.
The groups building the remainder of the E864 apparatus are: The University of Mass, MIT, and the University of New Mexico are responsible for the trigger counter system. BNL is responsible for the DA system and late energy trigger. The BNL AGS department is taking responsibility for the beam line, counting house, AC power, etc.