You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the brahms experiment at rhic consists of two magnetic spectrometer arms that will survey particle production in au - au collisions at 100 gev / c per nucleon .
the goal of the experiment is to study the system formed by the two colliding ions in two regions of rapidity .
a schematic description of the brahms setup is shown in figure [ fig : brahms_layout ] .
the midrapidity arm will probe the particles emerging near @xmath1 , corresponding to spectrometer angles between @xmath2 and @xmath3 with respect to the beam line . the forward arm will study particles of higher rapidities , @xmath4 , corresponding to spectrometer angles ranging from @xmath5 to @xmath2 .
the particle identification is accomplished in each spectrometer with a combination of time of flight arrays and erenkov counters .
of particular interest for this presentation is the particle identification system ( pid ) of the forward arm spectrometer , which must identify charged particles with momenta ranging from 1 to 25 gev / c .
particle identification at high momentum is done with the second time of flight wall h2 and the ring imaging erenkov detector rich .
assuming the expected time resolution of @xmath6 psec , h2 will be able to separate pions from kaons up to 5.8 gev / c . and kaons from protons up to 8.5 gev / c .
the function of the rich detector is to extend the particle identification up to 25 gev / c .
some considerations of its design are given in the following subsection .
figure [ fig : rich1 ] shows three aspects of the proposed detector , indicating various components and dimensions .
the lateral dimensions of the radiator volume are somewhat larger than the aperture of d4 @xmath7 .
the radiator has l = 1.5 meters of , which at @xmath8 c and 1.25 atm .
pressure has an index of refraction n = 1.00185 .
the spherical mirror has a focal length of 1.5 m and is rotated by an angle @xmath9 , thus shifting the ring image ( by @xmath10 ) to a focal plane outside of the volume illuminated by the direct particle flux .
the photon - imaging array consists of four hamamatsu r-4549 - 01 detectors , placed as indicated , defining an image plane some @xmath11 .    the expected performance of this detector , in terms of particle resolution and efficiency ,
have been outlined previously , @xcite,@xcite , and are supported by the results of the prototype tests presented in the following section and also in ref .
@xcite .
simulations have been performed using event generators appropiate for au - au collisions , in order to determine the multiplicities expected for the various detectors of the forward arm .
table [ table : pid_richrates ] shows the pertinent data for the rich detector , indicating typically one or at most two charged particles per event .
.average charged particle hits in the rich detector [ cols="^,^,^ " , ]
the prototype rich counter is shown schematically in figure 3 of ref .
the prototype is similar in general design to that shown in figure [ fig : rich1 ] , but with the optical system rotated by @xmath3 about the beam axis , such that the reflection angle is now in the horizontal plane .
the detector housing is constructed as a rectangular box , with 2 cm thick aluminum walls , having inner dimensions of 127@xmath1264@xmath1246 @xmath13 ( @xmath14 ) .
the construction is such that , using gasket and o - ring seals , the pressure of the radiator gas may be safely varied from 0 to 1.5 atm absolute . the particle beam enters and exits through mylar windows 0.25 mm thick and 15 cm in diameter , at opposite ends of the long dimension .
a 15-cm diameter spherical mirror , of focal length @xmath15 = 91.4 cm , is situated at a radiator distance of @xmath16 = 114 cm , rotated by @xmath17 = 8@xmath18 .
a single 100-channel pmt is centered at @xmath19 at the 91 cm distant focal plane .
the photon detector was a single hamamatsu r-4549 - 01 , one element of the four proposed for brahms .
this pmt is a 20-stage device having a @xmath20 photocathode , with the segmentation defined by a @xmath21 array of @xmath22 anode elements . at the full operating voltage of 2500 volts
, it provides a current gain of @xmath23 producing single - photon pulses large enough to be fed directly to an adc or timing electronics .
this specific module employs a special focussing electrode ( between cathode and first dynode ) which results in a rectangular flat - topped response function @xmath24 , with a very sharp fall off at the edges .
while the photocathode itself is quite uniform , the gain falls off a bit ( 50% ) near the edges of the array , and even more at the four corners .
however the signal remains clean , and so the gain can be compensated in software .
four identical drift chambers , placed in pairs upstream and dowstream of the prototype counter , provided tracking for determining the particle trajectory .
thus , the expected position of the center of each rich ring on the phototube is determined to a resolution of approximately 500 @xmath25 m in both the @xmath26 and @xmath27 directions .
in previous tests @xcite , time limitations did not permit a complete filling of the radiator volume with pure .
our filling system was based on the difference ( factor of six ) in the molecular weights of the fluorocarbon and argon , with the heavy gas displacing the lighter one as it is brought into the bottom of the detector . in reality
we found that even though the flow of into the detector was slow , some mixing occurs and 100% concentrations can not be reached in a single cycle .
the measurements reported in our previous publication were done with @xmath28 and @xmath29 argon .
this time we simply evacuated the radiator volume completely and then filled with pure .
measurements were made for several particle momenta , over the range @xmath30 , using the ( negative ) secondary beam from the bnl ags accelerator .
( the momentum calibration for the beam - line was established in a separate measurement , employing time - of - flight techniques , to an accuracy of better than 0.5% ) . for each setting ,
the tracking information from four drift chambers was used to project the expected ring centers onto the pmt matrix . given this information , a ring - fitting algorithm was employed to find and fit the ring radius on an event by event basis .
the results are shown in fig .
[ fig : momentum_scan ] , which plots observed ring - radius versus beam momentum for particles thus identified as electrons , muons , and pions .
the solid curves are calculated for the indicated particles , with the focal length and index of refraction as free parameters .
both results can be seen to agree well with the known value,@xmath31 , and the calculated index of refraction ( at @xmath8 c , 1 atm @xmath32 nm ) n = 1.001379 ( this index is calculated using measurements in the liquid phase reported in @xcite ) .
we have investigated the possibility that for larger rings ( r@xmath334.5  cm ) edge - effects may be of importance , resulting in extracted radii somewhat too small .
for this purpose , the spherical mirror was further rotated by an additional 1@xmath34 to @xmath17=9@xmath34 .
the projected ring center was therefore moved by 3.4  cm , such that one arc of the ring image was well inside the pmt s photocathode area .
the results for this comparison are shown in fig .
[ fig : momentum_scan ] for the electron data at 3  gev / c .
as can be seen , the effect produces , at most , a minimal displacement in the direction that might have been expected .
as remarked previously @xcite , the direct determination of the number of photons detected for a given event is precluded by the exponential shape of the pmt single - photon response function .
instead , we have measured ( over a large number of events ) the statistical variation in the total charge collected per event . the resultant distribution ( @xmath35gaussian )
is then fitted to determine the measured mean ( @xmath25 ) and standard deviation ( @xmath36 ) .
the number of detected photons ( n ) may then be estimated as @xmath37 , where @xmath38 is deduced by unfolding the exponential detector response function .
we thus estimated ( for 12 gev / c pions ) that @xmath39 , corresponding to @xmath40 .
the predicted behavior of the rich detector proposed for brahms has been investigated and confirmed by these additional measurements with the prototype detector , which were carried out under carefully controlled conditions .
the imaging capabilities ( see fig .  3 ) and figure of merit @xmath41 are found to be in good agreement with expectations based on the design and performance of the individual components .
this work was supported by u.s .
department of energy contract number de - ac02 - 76ch00016 , in part through the r.&d .
funds of the rhic project , and we thank t.  ludlam for his encouragement in this enterprise .
a ring - imaging erenkov counter , to be read out by four 100-channel pmt s , is a key element of the brahms experiment . 
 we report here the most recent results obtained with a prototype tested at the bnl ags using several radiator gases , including the heavy fluorocarbon . 
 ring radii were measured for different particles @xmath0 for momenta ranging from 2 to 12 gev / c employing pure as radiator .    ,    ,    ,    ,