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C. W. Fabjan and D. Fournier
surrounding gas. Both the primary and secondary photons are detected by a set of
248 VUV photomultipliers, with 78 mm diameter and 35% quantum efficiency at
175 nm, disposed in the liquid at the bottom of the vessel, and in the gas above
the multiplication region. The light distribution in the top and in the bottom circles
gives the position and lateral extension of the emitted signal. The time between
the primary and secondary signals gives the vertical coordinate. All construction
materials of the detector were selected for low radioactivity. The experiment is
operated in the LNGS laboratory near the Gran-Sasso tunnel, shielded from cosmic
background. It is furthermore enclosed in several layers of passive and active
shielding. The remaining background is dominated by electron recoils from residual
γ emitters, and nuclear recoils from residual neutron background. The former are
very much suppressed by a requirement on the ratio of ionization over primary
scintillation. The electron lifetime, which depends critically on the extreme liquid
purity, and affects the magnitude of the ionization, is measured with photon to
electron conversion signals generated in the liquid. A neutron generator is used
to calibrate the energy response to recoils. The PMTs and electronics chain are
calibrated with blue light pulses sent in fibers ending in the liquid volume. The dark
count rate of the PMTs during the first science run was about 10 to 20 Hz. A first
science run of about 30 days demonstrated that Xenon1T is the most sensitive device
for WIMP masses above 10 GeV presently running. A science run of two years is
planned. An enlarged version of the detector, Xenon-nT, with 8 tons fiducial volume
is under construction. Its sensitivity should allow to approach the “neutrino floor”
given by coherent scattering of solar neutrinos on nuclei.
6.7.3 The CMS Electromagnetic Crystal Calorimeter
The largest crystal calorimeter operated so far is the PbW0 4 calorimeter of the CMS
experiment at the CERN LHC [110], clearly aimed at the Higgs → γγ discovery.
The calorimeter consists of a cylindrical barrel part (inner radius ~ 1.3 m) and two
planar end-caps closing the cylinder at about 3 m from the proton-proton collision
point (see Fig. 6.44). Each of the 61,200 barrel crystals is a tapered bar covering
a φ × δη solid angle of 0.018 × 0.018, and has a depth of 23 cm (24.7 X 0 ). In
the end-caps, the calorimeter is preceded by a lead-Silicon strip preshower. Basic
properties of PbW0 4 have been given in Sect. 6.3.1.
The calorimeter is located inside the hadronic calorimeter, which in turn is inside
the 3.8 T superconducting solenoid. Barrel crystals are readout by APDs, while the
end-cap crystals (somewhat bigger) are readout by phototriodes chosen for their
better radiation resistance.
The front-end electronics processes signals corresponding to energy deposits of
up to ~1.5 TeV (3.0 TeV) in the barrel (end-caps). The equivalent noise per crystal
is ~30 MeV. This figure is likely to increase after high luminosity running, due to
increased leakage current in the APDs.
C. W. Fabjan and D. Fournier
surrounding gas. Both the primary and secondary photons are detected by a set of
248 VUV photomultipliers, with 78 mm diameter and 35% quantum efficiency at
175 nm, disposed in the liquid at the bottom of the vessel, and in the gas above
the multiplication region. The light distribution in the top and in the bottom circles
gives the position and lateral extension of the emitted signal. The time between
the primary and secondary signals gives the vertical coordinate. All construction
materials of the detector were selected for low radioactivity. The experiment is
operated in the LNGS laboratory near the Gran-Sasso tunnel, shielded from cosmic
background. It is furthermore enclosed in several layers of passive and active
shielding. The remaining background is dominated by electron recoils from residual
γ emitters, and nuclear recoils from residual neutron background. The former are
very much suppressed by a requirement on the ratio of ionization over primary
scintillation. The electron lifetime, which depends critically on the extreme liquid
purity, and affects the magnitude of the ionization, is measured with photon to
electron conversion signals generated in the liquid. A neutron generator is used
to calibrate the energy response to recoils. The PMTs and electronics chain are
calibrated with blue light pulses sent in fibers ending in the liquid volume. The dark
count rate of the PMTs during the first science run was about 10 to 20 Hz. A first
science run of about 30 days demonstrated that Xenon1T is the most sensitive device
for WIMP masses above 10 GeV presently running. A science run of two years is
planned. An enlarged version of the detector, Xenon-nT, with 8 tons fiducial volume
is under construction. Its sensitivity should allow to approach the “neutrino floor”
given by coherent scattering of solar neutrinos on nuclei.
6.7.3 The CMS Electromagnetic Crystal Calorimeter
The largest crystal calorimeter operated so far is the PbW0 4 calorimeter of the CMS
experiment at the CERN LHC [110], clearly aimed at the Higgs → γγ discovery.
The calorimeter consists of a cylindrical barrel part (inner radius ~ 1.3 m) and two
planar end-caps closing the cylinder at about 3 m from the proton-proton collision
point (see Fig. 6.44). Each of the 61,200 barrel crystals is a tapered bar covering
a φ × δη solid angle of 0.018 × 0.018, and has a depth of 23 cm (24.7 X 0 ). In
the end-caps, the calorimeter is preceded by a lead-Silicon strip preshower. Basic
properties of PbW0 4 have been given in Sect. 6.3.1.
The calorimeter is located inside the hadronic calorimeter, which in turn is inside
the 3.8 T superconducting solenoid. Barrel crystals are readout by APDs, while the
end-cap crystals (somewhat bigger) are readout by phototriodes chosen for their
better radiation resistance.
The front-end electronics processes signals corresponding to energy deposits of
up to ~1.5 TeV (3.0 TeV) in the barrel (end-caps). The equivalent noise per crystal
is ~30 MeV. This figure is likely to increase after high luminosity running, due to
increased leakage current in the APDs.