RHIC PERFORMANCE. Page: 4 of 6
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-9- BRAHIS ? STAR - PHOENIX -n-- PHOBOS
Figure 3. Evolution of the collision rate at the four RHIC detectors during a typical store.
installed to change transition energy quickly during acceleration to effectively jump across
Fig. 2 shows a typical acceleration cycle which includes filling the blue ring with
56 bunches in groups of 4 bunches, filling the yellow ring in the same way and then
simultaneous acceleration of both beams to storage energy. Residual loss of beam that
was not captured by the rf system occurs at the beginning of acceleration and from
transition crossing about one minute later. Beam losses due to the two beams interacting
is also noticeable at injection energy.
During the last month all four experiments were in data taking mode. Typical stores
lasted about 5 hours. Fig. 3 shows the evolution of the collision rate in the four exper-
iments during a typical store. After optimizing longitudinal and transverse steering the
initial observed collision rate agreed well with the expected rate as calculated from the
beam parameters listed in Table 2 which is 314 Hz for BRAHMS and PHENIX. Note
that since STAR and PHOBOS are located at the interaction regions with * = 8 m their
collision rates are reduced by a factor of 8/3 from the rates of the other two detectors.
The drop of luminosity during the store is due to both beam loss and transverse emit-
tance growth most likely caused by the non-linearities of the beam-beam interaction and
Coulomb (intra-beam) scattering within a bunch. The latter is particularly important for
these fully stripped, highly charged gold beams.
The collision rate was measured using identical Zero Degree Calorimeters (ZDC) at
all four interaction regions. The ZDC counters detect at least one neutron on each side
from mutual Coulomb and nuclear dissociation with a total cross section of 10.7 barns
at 65 GeV beam energy[l]. The peak luminosity for the initial rate shown in Fig. 3
is then 3.3 x 1025 cm2 s- for BRAHMS and the average luminosity for the store is
1.7 x 1025 cm-2 s-. Independent determination of the luminosity using Vernier scans
gives a cross section for the ZDC counts of 9.1 1.8 barns which is consistent with the
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Roser, T. RHIC PERFORMANCE., article, January 14, 2001; Upton, New York. (https://digital.library.unt.edu/ark:/67531/metadc717060/m1/4/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.