The BRAHMS ring imaging Cherenkov detector

0.36 MB·English

by  R. Debbe

ˇ
The BRAHMS Ring Imaging Cerenkov
Detector
7
0
R. Debbe a C. E. Jørgensenb J. Olness a Z. Yinc
0
2
n aBrookhaven National Laboratory, Upton, New York 11973 USA
a bCERN, PH-EP, CH-1211 Geneva 23, Switzerland
J

4 cInstitute of Particle Physics, Huazhong (Central China) Normal University,
2
430079 Wuhan, China

]
t
e
d
- Abstract
s
n
i A Ring Imaging Cˇerenkov detector built for the BRAHMS experiment at the
.
s
Brookhaven RHIC is described. This detector has a high index of refraction gas ra-
c
i diator. Cˇerenkov light is focused on a photo-multiplier based photon detector with
s
y a large spherical mirror. The combination of momentum and ring radius measure-
h
ment provides particle identification from 2.5 GeV/c up to 35 GeV/c for pions and
p
[ kaons and well above 40 GeV/c for protons during runs that had the radiator index
of refraction set at n 1 = 1700 10−6.
1
− ×
v
1
8
1 Introduction Particle identification at momenta
2
1 lower than 7 GeV/c is done via
0 the time-of-flight technique and the
7
0 more challenging task of identifying
/ TheBRAHMS(BroadRAngeHadron high momentum particles (up and
s
c Magnetic Spectrometers) Collabo- above 30 GeV/c) is tackled with a
i
s ration is one of the four heavy ion photo-multiplierbasedRingImaging
y
h experiments running currently at Cˇerenkov detector (RICH) described
p the Brookhaven RHIC. This experi- in the present paper. This detector
:
v ment was designed to study particle is the last element of a two section,
i
X production from colliding systems as movable spectrometer called For-
r varied as p+p, d+Au and Au+Au in ward Spectrometer (FS). This spec-
a
a wide rapidity range that extends trometer was designed to measure
from 0 to 4 units of rapidity ( max- high momentum charged particles at
imum beam rapidity: 5.4 ) These angles with respect to the colliding
measurements are primarily based beams that range from 30 to 2.3◦.
on the extraction of inclusive trans- Tracking and momentum measure-
verse momentum distributions of ment is done with four dipole mag-
fully identified particles with moder- nets, two TPCs in the first section of
ate reach in transverse momentum.
Preprint submitted to Elsevier January 31, 2002
the spectrometer where occupancy 2 Design and construction
can be high, and three drift chamber
modules in the back section where
lower occupancy and higher resolu- Cˇerenkov light is a particular form
tion provide precise measurement of of energy loss present whenever a
momentum. charged particle moves through the
bulk of a medium and its velocity is
higher than the velocity of light in
The BRAHMS RICH shown in Fig.
the medium c/n; this emitted light
1 has a nominal radiator length of
appearsasa conical“retardedwake”
150cm, and a 55cm 70cm spher-
× of light that follows the particle.
ical mirror with a radius R = 3m.
The mirror was manufactured as a
TheenergylossassociatedtoCˇerenkov
slump formed glass substrate later
light emission is evaluated at dis-
polished to best commercial mirror
tances much greater than the typical
surface quality. The polished surface
atomic lengths of the material and
was coated with Al and a protective
it appears as radiated photons of
layer of SiO. The mirror is rotated
◦ frequency ω whenever the velocity
by8 tokeepthephotondetectorout
of the particle β attains values such
of the spectrometer acceptance. The
that the dielectric constant of the
photondetectorisaphoto-multiplier
material ǫ(ω) and the particle veloc-
(PMT) based system described in
ity satisfy the relation β2ǫ(ω) > 1
section 3. This array of PMTs is
see REf. [2]. The energy radiated per
placed on the focal plane of the ro-
unit length is known as the Frank-
tated mirror. More details about the
Tamm relation:
BRAHMS experimental setup can
be found in [1].
dE (ze)2 1
= ω(1 )dω
dx c2 − β2ǫ(ω)
ǫ(ω)Z>β12
The integrand between brackets is
equal to sin2(θ) where θ is the angle
of the emitted photon with respect
to the particle velocity. From these
relations it is possible to extract an
estimate of the amount of light gen-
erated in a particular detector. The
fact that Cˇerenkov light emission
Fig. 1. Photograph of the RICH in-
does not depend on a microscopic
stalled at the end of the BRAHMS FS
spectrometer. The interaction region is description of the medium implies
located some 20 m to theright of the that the emission is coherent, and
∼
picture. it is another quantum effect visible
in macroscopic systems. The index
2
of refraction of the radiator medium Q is the quantum efficiency of
PMT
written as: n = ǫ(ω) can be mea- the photomultiplier photo cathode.
sured for the freqquencies of interest. The PMTs used in the BRAHMS
A set of such measurements can be RICH have borosilicate windows
found in [3] and [4]. that set their quantum efficiency
Q lower wave length cutoff at
PMT
Theconicalwave ofCˇerenkov light is 250 nm and bialkali photocathodes
transformed into a plane wave by re- withamaximuminefficiencyof20%.
flection on a spherical mirror. If the The calculated integral of Q for
PMT
angles of incidence to the mirror are these PMTs is equal to 0.33 eV. A
small, all photons with the same an- fraction of the light, estimated to be
gle of incidence are reflected onto a equal to 9.7%, is lost when the pho-
point on the focal plane, (located at tons traverse the 2.5 cm thick quartz
R/2where R is theradius ofthemir- window. The transmission of that
ror) and as Cˇerenkov light is emitted window τ is thus set to 0.90. Fi-
quartz
uniformly in the azimuthal angle φ nally the photon detector has dead
(defined with respect to the particle areas between PMTs that amount
velocity), the focus of Cˇerenkov light to an fiducial term f equal to 0.7.
det
reflected by an spherical mirror is a The expected figure of merit of the
ring centered around a point related BRAHMS RICH is then equal to:
to the angle of incidence of the par- N = 69 cm−1.
0
ticle with respect to the axis of the
mirror. Within the approximation of
small angles, an spherical mirror fo- Thefocusedlightdoesn’t formaper-
cusingCˇerenkovlightonaphotonde- fect ring, chromatic and geometrical
tector placed on its focal plane mea- aberrations will distort it. The posi-
sures the charged particles angle of tionresolutionofthephotondetector
incidence and their velocity. will also contribute to the spread of
the photons away from a perfect cir-
Thenumberofphoton-electronsorig- cle. The angular or radial resolution
inatingfromapathoflengthLinthe of a Ring Imaging Cˇerenkov detector
radiator volume is written as: is critical at high values of momen-
tum where the bands of radius ver-
sus momentum for different particles
N = N Lsin2(θ) begin to merge. The width of these
detected 0
bandssetstheparticlediscrimination
where N (in units of (cm)−1 ) con- power. As mentioned above, the res-
0
tains all the detector parameters: olution of the detector will be writ-
ten as the sum in quadrature of the
chromatic aberrations and the single
N = 370ε τ f Q (ω)dω
0 m quatz det PMT
hit radius resolution contribution:
Z
where ε is the reflectivity of the
m
spherical mirror set as indepen- ∆r 2 ∆r 2
= +
dent of frequency and equal to 90%. r r
(cid:18) (cid:19) (cid:18) (cid:19)chromatic
3
∆r 2 ∆r 2 ∆nintheserelationsissetbythedy-
+
r r namic range of the photon detector
(cid:18) (cid:19)geometric (cid:18) (cid:19)detector
and can be evaluated from measured
values. In the case of the BRAHMS
The contribution from chromatic
RICHthephotondetectorisanarray
aberrations(∆r/r) ispresent
chromatic ofmultianodephoto-multiplierswith
because the index of refraction in
aFWHMquantumefficiencyequalto
a dielectric depends on the fre-
2.7 eV (from 250nm to 517nm). The
quency of the electromagnetic wave
radiator is a mixture of two heavy
as n = ǫ(ω). Photons of different
fluorocarbon gases C F and C F
4 10 5 12
energy fqocus at different radii. For from 3M [5].
a particular detector sensitive to a
finite band of wave lengths, the con- The index of refraction of the gas
tribution of this distortion to the mixture was calculated using mea-
resolution of the detector can be surements performedinliquidphase,
simplified at high momentum values [3] and the Lorentz-Lorenz equation.
whereβ 1andthemaximumangle Assuming an average index of refrac-
∼
of emission can be written as: [4] tion within the dynamic range of the
PMTs equal to n 1 = 1900 10−6,
− ×
the chromatic aberration contribu-
tiontotheresolutioninthemeasure-
1 (n+1)(n 1)
θ = 1 = − ment of a single photon is equal to:
max
s − n2 s n2
∆r
= 0.0466
r
thechromaticaberrationcanthenbe (cid:18) (cid:19)chrom
written as:
The four pixels of each R7600photo-
multiplier have square shapes with
1.1 cm on each side. For each of the
∂θ ∂θ ∂n ∂θ
∆θ = ∆λ = ∆λ = ∆n measured photons, the error intro-
∂λ ∂n∂λ ∂n
duced in the ring radius measure-
ment by the assumption that the
and
photon intersects the detector plane
at the center of the pixel is listed as:
∆θ 1∂θ
= ∆n
θ θ∂n ≈
∆r s
=
1 ∂θmax ∆n (cid:18) r (cid:19)detector r√12
∆n =
θ ∂n n(n2 1)
max
−
with s = 1.1 cm and maximum ring
radius of 8.7 cm the contribution
Theradiusofasinglefocusedphoton
to the radius resolution is equal to
has a fractional spread produced by
0.036.
chromatic aberration:
∆r ∆θ ∆n To estimate the overall effect of the
r ≈ θ ≈ n(n2 1) geometricalaberrationswehavesim-
(cid:18) (cid:19)chrom −
4
ulated the geometry of the detector
and ray traced all Cˇerenkov pho-
tons produced by charged pions with
fixed momentum set to 19 GeV/c.
Two cases are considered, in the first
one, a set of pions move all along
the axis of symmetry of the detector.
The deviation from a perfect ring is
shown in the a and b panels of Fig 2.
Panel a shows the distribution of the
distance between the photon inter-
section with the detector plane and
a nominal center of the ring. The az-
imuthal dependence of the deviation
from a perfect ring (for pions at 19 Fig. 2. Geometrical aberrations from
GeV/c and n 1 = 1960 ppm) is ray tracing in the actual geometry of
−
shown in panel b. These deviations thedetector.Panelsaandbcorrespond
to the first set of pion moving all along
are all the result of the 9 degree ro-
the axis of the detector. Panels c and
tation of the spherical mirror and
d were obtained with measured tracks
the different positions of the emitted
ˇ that have different angles of incidence
Cerenkov photons along the track
as well as different entrance point loca-
of the charged pion. The photons
tions, this set shows the strongest ge-
with the smallest deviations are the
ometrical aberrations. For both sets of
onesemittedclose tothemirror.The
pions,theleftpanelshows thedistribu-
overall effect of mirror rotation and
tion of radii, and the right ones display
photon emission position is small
therelativedeviationfromaperfectcir-
and transforms the ringsinto ellipses cle as a function of the azimuth angle.
with the big axis along the x axis of
the photon detector. volume. We can thus quote a max-
imum value for geometrical aberra-
A second set of charged pions with tions as (∆r/r) = 0.025 even
geometric
the same fixed momentum was used though the majority of the rings are
to extract an overall effect from ge- containedinthebrightbandofpanel
ometrical aberrations. This time the d that corresponds to 0.7% Once a
tracks belong to detected particles particle is identified it is posible to
and give us the most realistic distri- correct the geometrical aberrations
butions in slopes and intersections inaniterativeway.Thepresentanal-
with the radiator volume. The result ysis does not include this correction.
ofraytracingtheCherenkovphotons
fromthissecondsetofpionsisshown A prototype of this detector was
in panels c and d of Fig. 2. The an- tested in one of the experimental
gles of incidence into the RICH are halls at the BNL AGS. These stud-
small, the most distorted rings are ies aimed at developing a photo-
produced by charged particles that multiplier based photon detector
fly close to the edges of the radiator in collaboration with Hamamatsu
5
Corp. From an earlier PMT version 4 biasing bases and sockets. Each
with 256 pixels that provided the matrix has two high voltage con-
first rings, the development work nections such that a single channel
continued to reduce the amount of of a HV power supply can bias ten
charge shared between neighboring PMTs. The photo-multipliers placed
pixels. The second PMT version had in these matrices were selected to
100 1cm2 pixelarrangedina10X10 have similar gains and pulse height
×
matrix. This device included an ad- resolutions. The signal from each
ditionalfocusingstagebetweencath- anode is routed out of each matrix
ode and first dynode reducing the through RG17 cables. The complete
chargespreadto10%atthecenter of photondetectorconsistsoffourPMT
thenext pixel [6],but hadpoorpulse matrices, and 70% of the total area
height resolution, and was deemed is covered by active pixels.
too difficult to manufacture. Finally
we tested the R5900 four pixel PMT TheoutputofthesePMTsisfastand
mounted in a compact metallic can. hassufficient pulse height resolution,
In order to achieve close packing af (in average the single photo-electron
these tubes Hamamtsu produced the peak appears as a shoulder of the
R7600 03 M4F that was selected to distribution) but their gain is insuf-
be used in this detector. Results ob- ficient to send the signal through
tained with the prototype can be some 100 meters of cable (RG58) to
found in Refs. [6] and [7]. be integrated in an ADC with 50 fC
coulomb resolution. This shortcom-
ing of the PMT was compensated by
the addition of one stage of amplifi-
3 The photon detector
cationmountedrightonthematrices
of PMT bases. Fast charge ampli-
Thephotondetectorisanarrayof80 fiers were AC coupled to the anode
four-pixel photomultipliers R7600- output through impedance match-
03-M4F [8]. Each PMT has a single ing 24Ω resistors to ground and a
photocathode plane evaporated on a 0.1µf capacitor. Fig. 3 shows one of
borosilicate window. Eleven dynodes the bias array with the five amplifier
amplify the electron extracted from cards connected to it.
the cathode and the total charge is
collected in four independent anode The response of the R7600 PMTs
plates. A focusing stage is placed was studied with a well collimated
between cathode and dynodes to LEDpositionedinfrontofthephoto-
reduced the charge shared between cathode with a two axis stepping
anodes. The complete system is en- motor system set to move in steps
cased in a flange-less metallic can of 1 mm. The amount of light pro-
held at the cathode voltage. duced with green LED is constant
during the duration of the scan of
Close packing of these PMTs was one photo-multiplier and can thus
achieved by Hamamatsu engineers be used to study the position depen-
with the design of a matrix of 5 x dence of the response of a particular
6
pixel. As the LED is 1 mm into the C F was introduced till the pres-
5 12
neighboring pixel it has 20% of the sure reached 392 Torr (or 41.3% of
average pulse height in the center. the final mixture pressure of 1.25 at-
Two mm further away that fraction mospheres). After that,C F is sent
4 10
drops to 6% and is negligible at into the radiator volume till the final
∼
the center of the next pixel. mixture pressure is reached. Once
the filling is done, a small sample of
The 320 signal cables are connected the gas was used to measure the in-
to a LeCroy 1885F Fastbus ADC af- dex of refraction by counting fringes
ter suitable delay to allow for a trig- in a Young interferometer as one of
gerdefinitionforeachRHICcollision. the split beams of laser light goes
Thechargeoneachchannelwasinte- through a volume that is filling with
grated during a gate that lasted 120 the gas sample, and the other passes
ns. through an equal length of vacuum.
Fig.4displaysaportionofthefringes
detected with a PIN diode to convey
theirgooddefinitionthatmakes pos-
sible to make measurement of the in-
dex of refraction with a resolution of
one part in a million.
0.5
olts]0.45
V
put [ 0.4
ut
Fig. 3. Photograph of 5 amplifier cards ode o0.35
mounted on one of the 5X4 PMT base di
o-
matrices. hot 0.3
P
0.25
400 450 500 550 600
Time [sec.]
4 Filling procedure.
Fig. 4. Interference fringes detected
with a PIN diode while a sample of the
radiator gas is brought into one of the
Work performed on the prototype
cavities of the interferometer while the
detector proved that the only filling
other is kept at vacuum.
procedure that guaranties the high-
est concentration of the gas mixture The highest index of refraction
in the radiator volume is the one achievedwiththismixtureat1.25at-
that starts by evacuating the vol- mospheres was n 1 = 2029 10−6.
− ×
ume. The drawback of this method Later, the focus of the collaboration
is the need of a vessel designed to shifted to studies at higher p values
T
withstand a full atmosphere pres- and the gas mixture and operating
sure differential. Once the radiator pressure were changed to reduce the
volume was evacuated, a boil off of index of refraction to lower values
7
(n 1 = 1600 10−6).
− ×
10
5 Data analysis
5
]
m
c 0
[
Tracks reconstructed with the FS y
spectrometer tracking system are -5
used to find the nominal center of
-10
rings. In case there are several tracks
in the event, the loop over tracks is
ordered such that it starts with the -10 -5 0 5 10
x [cm]
one with the highest momentum.
Oncearingcenter isdefined,thedis-
Fig. 5. Two rings of Cˇerenkov light fo-
tancefrompixel center toring center
cused on the photon detector. The big
is calculated for all pixels that have
ring (red pixels online) has been pro-
charge above pedestal. The radius of
ducedby a 20 GeV/c pion.Thesmaller
a ring candidate is set as the average
ring (blue pixels online) was produced
of those distances. The set of pixels
by a 17 GeV/c proton.
is accepted as a ring if their number
exceeds a minimum set by default at low momentum, kaons are well
to be equal to 4 and is at least 40% separated from pions (at the three
of an estimated number of photo- standard deviation level) up to 25
∼
electrons with a figure of merit set GeV/c. The index of refraction of
low as N = 55. the radiator gas throughout the runs
0
was equal to n 1 = 1690 10−6
− ×
Pixels that were included in the his- and the spectrometer was placed
togram are marked and will not be at four degrees with respect to the
used in the search for another ring in beam line. The rings with ”satu-
events where there is more than one rated” radii extracted from runs
track in the RICH. A more detailed where the index of refraction was
description of this algorithm can be set to n 1 = 1560 10−6 have an
− ×
found in [9]. average of 38 9 photo-electrons.
±
The measured figure of merit of this
detector is thus N = 81 16cm−1.
0
±
6 Performance
ParticleidentificationwiththeRICH
detector can be done with two inde-
Fig. 6 is a composite of five field pendent and consistent methods, the
settings of the FS spectrometer, no first one is based on the difference
effort was made to normalize the between measured ring radii and the
yields, the main purpose of this fig- expected radius of a ring produced
ure is to show the remarkable mo- byacandidateparticle.Ifsuchdiffer-
mentumrangeofthisdetector;itcan encefallswithinasetnumberofstan-
identify electrons, muons and pions dard deviations the measured parti-
8
σm2 is the standard deviation of the
mass square distribution.
Fig.6.TheradiusoftheCˇerenkovrings
produced by negative particles at 4 de-
Fig. 7. Mass-squared as a function of
grees with respect to the beams in p+p
momentum. The dashed curves show
collisions at √s = 200 GeV/c, as a
function of their momentum multiplied the ±2σm2 cut used by the mass based
particle identification method. The red
by the charge. Different magnetic field
dashed curve shows the threshold for
settings of the FS spectrometer are in-
Cˇerenkov light emission as function of
cluded in this figure. No effort is made
momentum.
tonormalizethedifferentdatasamples.
Monte Carlo simulations of the
cle identity is set to be the one of the
BRAHMS spectrometer, together
candidate particle. This method re-
with information extracted from
quires the correct value of the index
data, show the high efficiency of
of refraction of the radiator gas ex-
this detector; Fig. 8 displays the
tracted previously fromthe data and
efficiency as function of the ratio
storedinaruninformationdatabase.
γ/γ where γ is the value of
thrsh thrsh
Thismethodincludestoolsdescribed
theγ factorattheparticlethreshold.
insection 6.2 tohandle high momen-
The efficiency values shown in this
tum particles whenever the pion and
figure were obtained with protons.
kaon band start to overlap. The sec-
The simulations show a 4% ineffi-
ond particle identification method is ∼
ciency due to interactions with ma-
based on the calculated mass using
terial at the entrance of the RICH.
themomentumoftheparticle,thera-
dius of the Cˇerenkov ring and the in-
dex of refraction of the radiator.The 6.1 Relative radius resolution
resolution of this calculated mass is
momentum dependent and has con-
tributions from the momentum reso- The particle discrimination power of
lutionaswellastheradiusresolution. this detector is set by the relative
Fig. 7 shows the distribution of mass radius resolution at each momen-
squared as a function of momentum. tumvalue. Fig.9 shows thestandard
The particle identification is done in deviation obtained from gaussian
thisparticularcasewitha 2σm2 cut fits to the distributions of the ratio
±
indicated with dashed curves, where (r r )/r , where
measured calc measured
−
9
6.2 Particle identification at high
1.2
momentum
1
y
c0.8
n
e Fig. 6 shows that kaons can be sepa-
ci
e0.6
eff rated from pions with simple cuts of
h
c0.4 the radius of the rings for momenta
Ri
smaller than 25GeV/c, and that
0.2 ∼
protons are well separated beyond
0
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 the momentum range of the figure.
g /g
thrsh In order to extend the separation
of kaons and pions to higher val-
Fig. 8. Efficiency near threshold calcu-
ues of momentum it is necessary to
latedusingprotons.Thehorizontalaxis
parametrize the uncorrected abun-
is a normalized value of the γ of the
dance of kaons and pions in small
particle
momentum bins in order to allocate
probabilities or weights to events
r is the expected radius for rings
calc where the kaon band has started
produced by pions, kaons or protons.
merging with the pion band. That
parametrization was obtained at
The horizontal axis displays the ve-
each full field spectrometer setting
locityoftheparticlesshownastheirγ
by fitting projections of narrow mo-
factor.Pionsareshownwithredfilled
mentum bands (500 MeV/c) onto
circles and above γ 40 have a con-
∼ the radius axis. The functional form
stant relative radius resolution with
usedforthese fitswasthesumoftwo
a value as low as 1.2%, kaons shown
gaussians with equal widths, the free
withbluefilledtrianglesandtheanti-
parameters of the fit were the nor-
protons shown with filled star sym-
malizations, centroids and the com-
bols have a worsening resolutions as
mon width. Fig. 10 shows the results
their momentum diminishes.
of those fits.
Oncetheabundanceofkaonswithre-
0.08
spect to pions is know for a particu-
0.07
lar full field spectrometer it is possi-
0.06
ble to assign probabilities to events
0.05
thatlieintheoverlapofthekaonand
r0.04
r/
0.03 pion bands. Fig. 11 shows the sepa-
D
0.02 ration between kaons (blue hatched
0.01 histogram centered around 8.1 cm)
0 and the more abundant pions shown
-0.01 as the red histogram centered at 8.4
0 100 200
g
cm) in a wide momentum bin (from
30 to 44.6 GeV/c). Protons are also
Fig. 9. A fit to the width of the pion
present in this figure but their dis-
band.
tribution is not gaussian because at
10