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Studies of a lead-scintillator calorimeter with fast response Roy, Jean

Abstract

We describe in detail a fast photon calorimeter for the endcap region of the BNL (Brookhaven National Laboratory, U.S.A.) experiment 787 detector. The experiment, conducted by a collaboration of physicists from BNL, Princeton University (U.S.A.) and TRIUMF (Canada), studies the rare kaon decay K⁺ → π⁺vv and aims at measuring its branching ratio with a sensitivity of 2 x 10⁻¹⁰, almost three orders of magnitude better than the current best measurement. A short description of the physics involved is given as well as a description of the overall detector. The endcap calorimeter is formed of two almost identical systems, one at each end of the cylindrical detector. Several requirements must be met : compact design, high efficiency of photon detection, low energy threshold and fast counting ability. This is achieved with a segmented lead-plastic scintillator sampling calorimeter with wavelength shifter read out system. Each endcap is formed of 24 individual modules tightly assembled to minimize non-active areas. The scintillator-wavelength shifter combination used, NE-104 and BBOT, was found to give very fast pulses of approximately 10 ns FWHM. The light output obtained is about 10 photoelectrons per MeV deposited in the scintillator. A detailed description of the modules and their working principle is given. Since the endcaps will operate in a high rate environment, radiation damage to NE-104 scintillator was studied. It was found that the light output diminishes by a factor of two for a dose of about 18 x 10⁶ rad. However,with an expected dose in the endcaps for the whole duration of the experiment of about 1 x 10⁶ rad, we find that radiation damage is not a cause of concern. Two endcap modules were tested in a particle beam at TRIUMF to determine the linearity of the energy response and the resolution as a function of energy. The linearity was found to be excellent over the energy interval 30 to 140 MeV. The energy resolution can be well represented by R = (6.2 ±0.1)%/ √E where E is the energy in GeV. This is comparable with the results obtained for a similar calorimeter, used in the ARGUS detector at DESY, which uses a wavelength shifter (BBQ) with a much slower decay time. The energy calibration of the endcaps was done using monoenergetic muons from the decay K⁺→ μ⁺v[sub μ]. Doing the calibration in the experimental conditions insures that the systematics will be the same as for the physics data. The expected energy in the endcaps for this decay was determined by a Monte Carlo calculation using a programme called UMC, specifically designed for E787. A verification of the calibration could be done by measuring the energy of the two photons coming from the decay of the monoenergetic π⁰’s of the K⁺→ π⁺ π⁰ decay. The value obtained is consistent with what is expected. The latter decay is also used to calibrate the endcaps' TDC's. It is achieved by selecting events with well identified decay products, including one photon in the endcaps. Offsets which make the time of the endcaps events the same as the time of the positive pion are calculated using this data set. Both the energy and time calibration are described in details. Online random vetoing by the endcap, i.e. events rejected by the endcap energy veto because of random background hits, is calculated to be about 1 % , using a clean sample of K⁺→ μ⁺v[sub μ]. This rate is also found to be dependent on the kaon beam intensity. Finally, we found that the calorimeters described here performed very satisfactorily for an extended data collection period. Nothing at the moment indicates that they could be a limitation to the photon detection efficiency required to attain the ultimate goals of the E787 experiment.

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