PHY6040 Particle Detectors Dr C N Booth

Hadronic Calorimetry

Neutrons, protons, pions etc. interact with nuclei through the strong interaction.  In passing through matter, a hadron can therefore build up a shower through multiple interactions, in a similar way to that discussed for electrons in electromagnetic calorimetry.  The shower can be parametrised by a nuclear interaction length, similar to the radiation length for electromagnetic showers.  However, unlike the case of bremsstrahlung by high energy electrons, it should be noted that: Fluctuations in the amount of energy deposited are largely due to the variable fraction of the shower which is converted into an electromagnetic shower, by the production of fast neutral pions and their subsequent rapid decay into energetic photons.

Practical Hadronic Calorimetry

The nuclear interaction length of "active" detector material is so large (e.g. 68 cm for scintillator) that calorimeters have to be sampling devices.  A typical calorimeter therefore contains alternate layers of "absorber" (e.g. iron or copper) and detector (e.g. plastic scintillators, proportional counters or liquid ionisation chambers).

The typical energy resolution σ/E is about 50%/E  (with E measured in GeV), limited largely by fluctuations in the size of the electromagnetic part of the shower.  The resolution can sometimes be improved by one of two methods:

Most practical calorimeters are combined electromagnetic and hadronic detectors.  Because the nuclear interaction length is so much bigger than the radiation length, most hadrons pass through the electromagnetic front compartment and interact in the hadronic part behind.  The illustration below shows a module of a combined calorimeter with a lead/scintillator front electromagnetic section followed by an iron/scintillator rear hadronic section.  BBQ sheets are used to read out the scintillator with photomultipliers behind the module.  In this way, modules can be stacked next to each other with minimal dead space between.

Other Calorimeter Designs

Other Considerations

For a calorimeter to provide accurate energy measurements, more is needed than a good intrinsic energy resolution.  It should be highly uniform and stable, and there should be a means of detecting any changes and correcting for them.  These features are provided by a calibration system. The most significant limitation to the long term stability of a calorimeter depends on the type of readout.

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