On the energy resolution of uranium and other hadron calorimeters

Abstract

The various components that contribute to the signal from a hadron calorimeter, and the factors that affect the energy resolution with which hadrons can be detected, are examined in detail. The role of the electromagnetic to hadronic signal ratio e h is crucial in this respect. Its value is determined by the Z of the absorber material, the thickness of the passive and active layers, the signal integration time of the detector, and the properties of the readout material, in particular the free proton content and the saturation or recombination properties for few-MeV proton detection. Readout media that contain free protons offer the possibility to tune the e h ratio to the desired value (1.0) through the sampling fraction. Signal equalization ( e h = 1.0) does not seem to be a property unique to 238U, but can also be achieved for lead and even iron calorimeters. The calculations show, on the other hand, that e h values are larger than 1.0 for any calorimeter using liquid-argon readout. The intrinsic energy resolution for hadron detection is largely dominated by fluctuations in the binding energy losses that occur in the nuclear reactions. Efficient neutron detection can considerably reduce these effects provided that energy loss through recoil protons dominates. Calorimeters using 238U, Pb or Fe absorbers, and plastic scintillator, liquid-argon, silicon or TMP readout were investigated. Experimental results on e h values and energy resolution, which are often considered confusing, are nicely reproduced and explained.