6 Calorimetry
253
6.3.6 Calibration and Monitoring of Calorimeter Response
Modern calorimetry operates frequently at the 1% accuracy level and requires
therefore appropriate calibration methods. An extraordinary effort went into the
development and deployment of adequate calibration techniques for the LHC
calorimeters. In general, the following tasks have to be performed:
– establishing the absolute scale of response of a calorimeter, averaged over an
entire data set
– assessing the uniformity and linearity of response
– monitoring the response as a function of time, locally and globally, in order to
correct for time dependent effects, rate effects, aging.
A few examples are discussed below to illustrate each of these tasks.
Energy Scale
(i) Low energy domain: one large-scale example is the Superkamiokande experiment, dedicated to low-energy neutrino interactions. After a careful calibration
of the gain of each of the phototubes, and an assessment of the water transparency (absorption length greater than 100 m), the absolute energy calibration
was made using two radiation sources for cross-checks:
– the beam of an electron Linac operated in-situ above the liquid volume was
sent through an evacuated beam pipe into several places of the detector
volume recording the corresponding light signals. The Linac was operated at
energies between 5 and 20 MeV. The absolute energy scale of the beam was
known to better than 1%;
– 16 N radioactive nuclei were produced in situ from 16 O nuclei of the water
volume using a neutron generator. The decay products to 16 O ∗ (beta emission
with an endpoint energy of 4.3 MeV in coincidence with a 6.13 MeV photon)
were then recorded during a few lifetimes of 16 N (7.13 s). The two methods
agreed to better than 0.6% rms.
(ii) Medium energy domain: one example is the Babar experiment at SLAC, which
used a CsI crystal electromagnetic calorimeter and employed three calibration
sources to cover the full energy range:
– at low energy, the 6.13 MeV photons of 16 N decays were used (see
Superkamiokande above). At this energy, the resolution of the calorimeter
was found to be 5 ± 0.8%.
– at high energy (~10 GeV) the Bhabha scattering was used. With a luminosity
of 3·10 33 cm −2 s −1 this reaction provided about 200 events per crystal in a
12 h run.
– finally the peak position of known neutral resonances decaying in two
photons were used for further checks. Figure 6.39 shows the recorded γγ
invariant mass spectrum. The π 0 peak was observed at the nominal mass of
135.1 MeV with a width of 6.9 MeV.
253
6.3.6 Calibration and Monitoring of Calorimeter Response
Modern calorimetry operates frequently at the 1% accuracy level and requires
therefore appropriate calibration methods. An extraordinary effort went into the
development and deployment of adequate calibration techniques for the LHC
calorimeters. In general, the following tasks have to be performed:
– establishing the absolute scale of response of a calorimeter, averaged over an
entire data set
– assessing the uniformity and linearity of response
– monitoring the response as a function of time, locally and globally, in order to
correct for time dependent effects, rate effects, aging.
A few examples are discussed below to illustrate each of these tasks.
Energy Scale
(i) Low energy domain: one large-scale example is the Superkamiokande experiment, dedicated to low-energy neutrino interactions. After a careful calibration
of the gain of each of the phototubes, and an assessment of the water transparency (absorption length greater than 100 m), the absolute energy calibration
was made using two radiation sources for cross-checks:
– the beam of an electron Linac operated in-situ above the liquid volume was
sent through an evacuated beam pipe into several places of the detector
volume recording the corresponding light signals. The Linac was operated at
energies between 5 and 20 MeV. The absolute energy scale of the beam was
known to better than 1%;
– 16 N radioactive nuclei were produced in situ from 16 O nuclei of the water
volume using a neutron generator. The decay products to 16 O ∗ (beta emission
with an endpoint energy of 4.3 MeV in coincidence with a 6.13 MeV photon)
were then recorded during a few lifetimes of 16 N (7.13 s). The two methods
agreed to better than 0.6% rms.
(ii) Medium energy domain: one example is the Babar experiment at SLAC, which
used a CsI crystal electromagnetic calorimeter and employed three calibration
sources to cover the full energy range:
– at low energy, the 6.13 MeV photons of 16 N decays were used (see
Superkamiokande above). At this energy, the resolution of the calorimeter
was found to be 5 ± 0.8%.
– at high energy (~10 GeV) the Bhabha scattering was used. With a luminosity
of 3·10 33 cm −2 s −1 this reaction provided about 200 events per crystal in a
12 h run.
– finally the peak position of known neutral resonances decaying in two
photons were used for further checks. Figure 6.39 shows the recorded γγ
invariant mass spectrum. The π 0 peak was observed at the nominal mass of
135.1 MeV with a width of 6.9 MeV.