PB Beamline Information

      This document contains information about some of the properties of the Wide-band Photon beam used by the FOCUS (E831) experiment.


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Description of Secondary Beam

As shown in this schematic, the photon beam is produced from primary protons in three stages: neutral beam production from 800 GeV protons; electron and positron beam production and transport; bremsstrahlung photon beam production.

800 GeV primary protons interact with a liquid deuterium target (in PB4) to produce an array of secondary products which includes high energy photons from pi-zero decays. The charged secondaries and uninteracted protons are swept out of the beam and absorbed in the PB4 dump. An aperture in the dump transmits all neutral secondaries produced within 1.0 mr horizontaly and 0.7 mr verticaly of the incident proton beam trajectory. Deuterium is chosen as the target material because it has a high ratio of radiation length to interaction length and hence will absorb a smaller fraction of the secondary photons.

The neutral beam produced at the dump is passed through a 60% radiation length lead convertor where most of the photons will interact to produce an electron-positron pair. The secondary beamline transports either or both of the electrons and positrons to a lead radiator in PB6 while the remaining neutral secondaries (consisting of neutrons, kaons, lamdas, and uninteracted photons) are absorbed in the neutral dump in PB5. The secondary transport selects the momenta of the electrons/positrons, transports them in a double dog-leg with zero net bend, and focusses them onto the experiment target. A dump in PB5 is used to remove hadronic neutrals.

The focussed electron/positron beam is passed through a 20% radiation length lead radiator (in PB6) where it produces bremsstrahlung photons aimed at the emperiment's target. The recoil electrons are swept out of this photon beam and are absorbed in an electron dump.

Layout of Beam Elements and Enclosures

The following schematic shows the relative locations of all the Research Division enclosures housing the PB beamline elements. (These are the enclosures controlled by thr RD Operations Department.) Also shown are the locations of the major bends in this part of the beamline. The lengths of the enclosures and the distances between them are drawn to scale. The total distance from the entrance to enclosure P01 to the E831 experiment target is 4,358 feet. The length of the secondary (the distance from the PB primary target to the experiment target) is 1,242 feet. There is also a further 2,942 feet of beam line within the accelerator Switchyard where the primary protons are extracted from the Tevatron and split into the major fixed-target beams.

The first element shown is P01D which bends downwards all three of the Proton Area beams (PE, PC, and PW) coming from the accelerator Switchyard. This is immediately followed by the 3-way Lambertson magnets, PE1E, which separate the three Proton Area beams horizontally. The PE beam is of course bent to the east. The PE1ED string further bends the PE beam down and to the east. The PE2E 2-way Lambertson magnets horizontally separate the PB beam from PE by bending it to the east whilst leaving the PE beam undeflected. The PB3E cryogenic magnets are the last major bends in the primary beamline and again direct the PB beam east towards the targets in PB4 and the PB7 experiment hall.

The following diagrams show the layout of the PB beamline magnets, collimators, beam stops, targets, dumps, and SWICs for each of the above enclosures. The element positions along the beam for each enclosure are drawn to a common scale.

Enclosure P01

Enclosure PE2

Enclosure PE3

Enclosure PB4

Enclosure PB5

Enclosures PB6 and PB7

Beam Monte Carlo Studies

The expected characteristics of the Wide Band beam have been simulated using TURTLE interfaced with an electron production Monte Carlo. TURTLE is a standard beamline transport program maintained by the RD Operations Department. The electron production model contains the following features and assumptions.

  • The pi-zero differential production cross-sections are assumed to be the same as the pi-minus cross-sections for which published measurements on deuterium targets are available.
  • Pi-zero production from secondary interactions is ignored.
  • The pi-zero's are immediately decayed to two photons. The decay is isotropic in the pi-zero rest frame.
  • Depending on where in the target the interaction occurred some fraction of the photons will convert to electron-positron pairs in the target and be lost.
  • The pair production process is modelled using standard cross-section and energy sharing formulae. The resulting electrons and positrons are given no transverse momentum with respect to the incident photon. (i.e coulomb scattering and nuclear recoil effects are ignored.)
  • The electrons and positrons are allowed to produce bremsstrahlung photons and lose energy as they pass through the remainder of the convertor.

    The following post-script files show various properties of the electron beam as predicted by the Monte-Carlo program described above. The secondary electron beam line is tuned in TURTLE for a central trajectory of 250 GeV and the focussing has been optimized for maximum yield and target acceptance.

  • Plot 1. The momentum distribution and divergence of those electrons reaching the PB6 radiator. Note that due to the rapidly energy distribution of the electrons produced at the target, the average electron momentum is somewhat less than the nominal 250 GeV. The shaded section of the plot represents those electrons which are targetted on a 1 inch square target rotated 45 degrees from vertical.
  • Plot 2. The horizontal and vertical profiles of the electron beam at the radiator and projected to the experiment target.
  • Plot 3. These plots show the momentum of beam spot as projected to the experiment target. X is the horizontal co-ordinate and Y is the vertical co-ordinate. The focussing is complicated by the large momentum spread of the beam; at low momentum the electrons undergo a point-point-point focus while at higher momenta the focussing is point-parallel-point. Electrons with intermediate, orextreme momenta are not properly focussed and set a lower limit on the size of the beam spot.
  • Plot 4. This plot indicates the horizontal projection of electron trajectories between the radiator and target. The projections for those electrons which either miss or hit a 1 inch rotated target are plotted separately. Also shown on the plot are the various defining apertures in the beam.
  • Plot 5. The same as plot 4, but using vertical projections

    Beam Sheets

    The following are copies (dated 7-Jun-96) of the official PB beam sheets. Official copies can be found on fnalu at ~beamlibs/bsheets/pb*.bsheet.

  • PB primary beamline : Switchyard to PB4 target.
  • PB electron secondary beamline : PB4 target to PB7.
  • PB positron secondary beamline : PB4 target to PB7.