Ring Imaging Cherenkov (RICH) Detector
Overview
The RICH is part of SBS. In the SIDIS experiment E12-09-018, it will be located just behind the GEMbased tracker of SBS, where it will be used for charged hadron identification (pions, kaons and
protons). The SBS will be located at a central angle of 14 degrees to the beam left at a distance of 2.5
meters from the target to the magnet yoke. The RICH will detect and identify charged particles at
momenta above 2 GeV with polar scattering angles in the range of 10 ≤ θ ≤ 22 degrees, with an
azimuthal angular acceptance of approximately ±60 degrees. The SBS RICH consists of one of two
identical halves of the RICH detector from the HERMES experiment. After the end of the HERMES
experiment, one half of the RICH was preserved and sent to Charlottesville, VA, where it was stored
under the custody of the UVA group (Cates) from 2009-2014. In early September of 2014, the RICH
was sent to the University of Connecticut, where the group of Andrew Puckett is now responsible
for testing and preparing the detector for use in SBS. The RICH is based on the dual-radiator design
built and operated successfully by the HERMES collaboration from 1998-2008. Silica aerogel tiles
with a refractive index of 1.03 and C4F10 gas1 with an index of 1.00137 were used to separate pions,
kaons and protons with momenta from 2-15 GeV/c using the combination of the effective threshold
and the reconstructed emission angles for each particle type in each radiator. The Cherenkov
radiation emitted by fast-moving charged particles is collected by a spherical mirror array onto a
matrix of ¾”-diameter photomultiplier tubes.
Function
The RICH is used for charged hadron identification in the single-hadron SIDIS reaction 3He(e,e’h)X,
where h = π±, K±.
Performance Requirements
The performance specifications of the SBS RICH are driven by the PID performance requirements of
the SIDIS experiment. The most stringent performance goal is that the pion contamination of
identified kaons, which is the product of the pion  kaon misidentification probability and the flux
ratio of pions/kaons, be less than roughly 10%, in order to keep the systematic uncertainty
associated with the pion background subtraction from the kaon observables manageable. Averaged
over all momenta from 2 GeV up to the maximum momentum of about 10 GeV in the SIDIS
1
While C4F10 was the gas used in the HERMES experiment, C4F8O will probably be used in SBS, as it has very similar
optical properties and is safer, more widely available, and less costly.
experiment, the expected π+/K+ flux ratio is about 6, while the expected π-/K- ratio is about 20. In
the worst-case scenario of negative kaons, the π- misidentification probability must therefore be
less than 0.5%, implying ~200:1 kaon rejection for the combined aerogel/gas radiators. This
specification on the PID performance leads to the following criteria for the detector performance:
1. Timing: TDC readout of the RICH PMTs with sufficient resolution that the effective PMT
occupancy in the offline analysis is preferably less than 1% and in the worst case no more
than 5%. Given the expected background rates of the SIDIS experiment, this implies an
effective offline timing window of not more than 10 ns (and a corresponding time
resolution of 2-3 ns or less).
2. Resolution: The resolution of the Cherenkov emission angle θC is limited to about 8 mrad by
the pixel size of the photon detector array, the uncertainty of the location of the emission
vertex along the particle trajectory, the chromatic dispersion in aerogel, and several other
minor effects. The design of the HERMES RICH defines the minimum acceptable resolution
as that which provides 4.6σ separation between pion and kaon emission angles in the gas
radiator at a maximum momentum of 15 GeV/c and between kaon and proton emission
angles in the aerogel at the kaon gas threshold momentum of 9.4 GeV/c, for typical average
numbers of fired PMTs on a ring. The maximum momentum of SIDIS hadrons in SBS is
about 10 GeV. The main requirement regarding resolution is that the refractive index of the
425 aerogel tiles to be used in SBS be uniform to within Δn/n ≤ 0.001.
3. Efficiency/gain uniformity/noise level: The PMTs will be supplied with positive anode HV
in groups of 32 PMTs powered from a single HV channel. This places some restrictions on
the level of gain variation among PMTs in the same HV group, as well as the noise level of
the PMTs. Since most of the signals (and those from aerogel in particular) are detections of
single photons, the threshold for readout of a PMT signal will be quite low, perhaps 0.1
photo-electron or about 0.5 mV. The PMT noise level/dark counting rate must be low
enough at the applied HV to maintain the effective occupancy below 1% as described
above.
Calibration
The major in-beam calibration tasks for the RICH include:
1. Calibration of the Cherenkov emission angle (θC) reconstruction.
a. Software alignment of the spherical mirror array using high-energy (β ≈ 1) electron
and/or pion tracks
b. Determination of the Cherenkov yield and effective average index of refraction for
both radiators.
2. Absolute calibration of PID efficiency using exclusive electro- and/or photo-production
channels and/or time-of-flight.
Both of these tasks require that the RICH be installed in the SBS with the GEM-based tracker and
HCAL-based trigger. PID efficiency calibration using exclusive coincidence electroproduction
channels such as p(e,e’pi+)n and p(e,e’pi+pi-)p also requires the use of BigBite.
Physical Characteristics
Detector entry and exit windows: 1 mm-thick Aluminum, area 187.7 × 46.4 cm2 (entry) and 257.0 ×
59.0 cm2 (exit).
Aerogel wall: 425 tiles. Tile dimensions 11.4 × 11.4 × 1.13 cm3. Stacked in 5 columns, 17 rows, 5
tiles deep. Thin Tedlar spacers between tiles absorb photons crossing track boundaries. Aerogel is
stored in containment vessel with dry N2. 3.2 mm-thick UVT-Lucite exit window seals aerogel from
RICH gas volume and absorbs UV photons, yield of which from aerogel is dominated by Rayleigh
scattering.
Spherical mirror: Constructed from array of eight smaller mirror segments. Common radius of
curvature of 2.2 m and common sphere center.
PMTs: 1,934 XP1911UV PMTs. 0.75” diameter (15 mm minimum active photocathode diameter).
Close-packed hexagonal arrangement in 73 rows of alternating 26 and 27 columns. Custom HV
divider attached to PMT; signal cable soldered to base.
Gas system: C4F8O supply at slight overpressure with respect to atmosphere. RICH box has existing
gas inlet and outlet connections. Need Hall A infrastructure for gas supply and circulation.
Electronics
HV: 32 PMTs powered by a single HV channel. Need 61 channels of HV power supplies and cables.
Patch panels, connectors and cabling for HV distribution to PMTs exist. Typical operating HV
~1,350 V.
Signal cables: 1,934 existing signal cables consist of LEMO 50 Ohm coaxial ending in custom
connector to existing PCOS4 electronics. Existing cable length ~2 meters. Need patch panel from
existing custom connectors to NINO front-end card and cables from NINO front-end card to Fastbus
TDCs.
Front-end electronics: Plan to use NINO front-end, available from CDET, not in use during SIDIS,
1,934 channels.
Readout electronics: Intend to use Fastbus 1877S TDCs to read out RICH PMTs in ~50 ns effective
timing window.