Calorimeters in HEP Add hermiticity CC event - calibration Paul Thompson Add hermiticity CC event - calibration DIRC Charged particles through matter Electrons and positrons Electromagnetic/hadronic showers Calorimeter design Absorber Incident particle Detector

What is a Calorimeter? ….. A device for measuring energy! Charged AND NEUTRAL particles incident on thick dense material Deposit energy which leads to eg ionisation and thus measurable signal In total contrast to gaseous tracking detectors, we want particles to deposit all of their energy in the calorimeter, so we can detect and measure it. The measured signal is then (hopefully) linearly proportional to the particle energy Different techniques for electromagentic and hadronic calorimetry How do EM and HAD particles interact with matter?

Charged Particles Through Matter Lose energy through ionization Bethe-Bloch Tmax is the max. kinetic energy that can be transferred to e in a single collision for A=1 g/mol

Interactions of Electrons and Positrons in Matter Bremsstrahlung radiation when accelerated by nuclei. Strongly dependent on particle mass Dominant energy loss for low mass particles (electrons) at high energy Define `Radiation length’ X0 = mean free path of electron before it radiates = distance over which electron energy reduced to fraction 1/e Apart from particle mass, X0 depends on material density and ~A/Z2 If thickness expressed in X0, radiation loss independent of material

Some Radiation Length Data X0 W Pb Sn G10 Fe Cu Al glass Z k/A … Z2 expresses charge of scattering sources, … 1/A ~ expresses the number of sources per unit volume Rate of energy loss ~ 1/X0 X0

The Critical Energy `Critical energy’ when Bremsstrahlung and Ionisation losses equal. …. Will be important later dE/dx 10 MeV 1/E dE/dX electrons-positrons Bremsstrahlung Ionization f(Z) 1 Critical Energy Ionisation energy loss ~ Z/(A.b2) (Bethe Bloch) So Bremsstrahlung has stronger Z dependence … … very roughly, critical energy Ec~600/Z (MeV) Projectile energy

More on Critical Energy Ec Roughly Ec~1/Z …. independent of material density Z

Energy Loss for Photons Photons interact with matter in 3 main ways: 1) Pair production (high energy) 2) Compton scattering (low energy) 3) Photoelectric absorption (even lower energy) At high energy, electron Bremsstrahlung plus photon pair production can lead to a ``shower’’ Mean free path for ge+e- ~ X0 ~1/Z2

Photon scattering cross section spe= Atomic photo-effect (electron ejection, photon absorption) scoherent = Coherent scattering (Rayleigh scattering-atom neither ionised nor excited) s incoherent = Incoherent scattering (Compton scattering off an electron) kn = Pair production, nuclear field ke = Pair production, electron field snuc = Photonuclear absorption (nuclear absorption, usually followed by emission of a neutron or other particle) Photon total cross sections as a function of energy in carbon and lead, showing the contributions of different processes Pair production dominates at high energy

Electromagnetic Shower Development Shower maximum occurs when shower particles on average have an energy equal to the critical energy

Modelling Electromagnetic Showers Bremsstrahlung and pair production ... Shower deposits constant energy for each radiation length Each step the number of particles in the shower doubles The energy per particle reduces by a factor of two The emission angles are small so narrow shower develops Width mainly comes from multiple scattering When the energy falls below the critical energy Ionisation and collisions dominate the energy loss Showers stops over a relatively short distance Longitudinal Transverse

Hadronic Showers Much more complicated than electromagnetic showers! Hadrons can interact strongly as well as ionising the sampling material … nuclear reactions become an important energy loss mechanism. Define nuclear interaction length l via E(x) = E(0) e-x/l Roughly, l ~ 35 A1/3 g cm-2, quite long even for dense materials, so hadronic showers are longer and broader than electromag showers Other difficulties compared with electromagnetic showers: `Invisible’ energy caused mainly by neutrons interacting by exciting or breaking up nuclei. Generates transverse momentum Large component from p0 -> gg …. Hadronic showers have an electromagnetic core!

Development of Hadronic Showers Nuclear interactions lead to large energy loss, but have long l (l ~ 50 cm for protons in lead)

Breakdown of a Hadronic Shower 20 GeV hadronic showers in copper …. Charged particles Full shower Large transverse component, especially in neutrals

Basics of Calorimeter Design Calorimeters require: A dense ABSORBER material in which to build shower A SAMPLER as a means of collecting deposited energy / getting a signal out! “Homogeneous” Calorimeters: Absorber and Sampler are the same thing Usually built from crystals Good resolution, but expensive and hard to grow big crystals! Often used for electromagnetic calorimetry 2) “Sampling Calorimeters: Alternating layers of absorber and sampler Used for all hadronic and many electromagnetic calorimeters

An Example Homogeneous Calorimeter Individual lead glass blocks - OPAL EM Calorimeter Also used for electromagnetic calorimeter at CMS (lead tungstenate crystals)

Homogeneous Calorimeters Use special optical properties of crystals or glasses to generate detectable signal from electrons and photons in shower: Scintillation light or Cerenkov radiation. e.g. Crystal Calorimeters Electrons and positrons in electromagnetic shower emit Cerenkov radiation at optical wavelengths. Radiation collected in photomultiplier tubes Total light amplitude collected proportional to total energy deposited by incident e / g Cerenkov radiation Analogous to a supersonic shockwave. Charged particle with v > c/h emits coherent radiation at an angle cos q = 1/hb to incident particle …. photon rings

Sampling Calorimeters Can fill large and awkward volumes at manageable cost Only a fraction of deposited energy is sampled

Some Example Sampling Calorimeter Designs Scintillator readout with wavelength shifter bars (or wavelength shifting fibers) Scintillator readout Noble liquid readout Readout with gaseous detectors (MWPCs or streamer tubes)

e.g. Hadronic Calorimeter at ALEPH Iron return yoke of solenoid used as absorber material Active material is limited Streamer tubes

e.g. ATLAS Calorimetry EM calorimeter with lead absorbers and liquid argon active material Hadronic calorimeter with alternating tiles of stainless steel active scintillator (polystyrene)

Calorimeter Resolution

Example: CMS At low E, s(E)/E~1/E, dominated by noise, pile-up etc At high E, s(E)/E~1/E1/2, dominated by `intrinsic’ statistical sampling error NB! Calorimeter resolution improves with increasing energy Tracking resolution deteriorates with increasing energy … the two types of detector are often complementary!

Summary Basic ideas of how particles interact with matter introduced Differences between EM and HAD showers lead to different designs and materials of calorimeters Met some of the most common designs in HEP Concepts such as components of calorimeter resolution useful to know and may arise in your viva!

Compensation For EM showers most energy seen as ionisation For hadronic shower 30% of energy is not seen due to nuclear excitations/break-up Ways to compensate for `unseen’ energy so that response to EM and hadronic showers is the same Use software to reweight the hadronic response. Reweighting algorithm based on test beam and simulation. `Artificial’ compensation needs fine segmentation of EM/hadronic parts Use Uranium 238 fissile isotope releases neutrons and fission products increase the ionisation. But expensive and dangerous to health. Usually worse EM resolution but much better hadronic.