Crystal LabFerrara · UniFe & INFN

Research › Accelerators and colliders

Oriented calorimeters

Faster electromagnetic showers in PbWO₄

Aligned with the beam, a scintillating crystal contains an electromagnetic shower in a fraction of its usual depth.

The physics.

Longitudinal profile of an electromagnetic shower in a misaligned and in an aligned crystalEnergy deposited per unit depth against depth, for the same high-energy electron, positron or photon. In the misaligned crystal the curve rises slowly and peaks deep inside. In the crystal aligned with the beam it peaks much earlier and falls to zero within a shorter depth. The two curves enclose the same area, the same energy. depth in the crystal energy deposited per unit depth aligned crystal misaligned crystal e⁺/e⁻ or γ of the same energy · illustrative shapes
Energy deposited per unit depth by the same high-energy e⁺/e⁻ or γ in a misaligned and in an aligned crystal. Illustrative shapes (Longo’s Γ-function profile, equal areas): in the aligned crystal the shower peaks earlier and ends within a shorter depth.

Electro­magnetic calo­rim­e­ters measure the energy of elec­trons, po­si­trons and photons by absorb­ing their whole shower: the par­ti­cle ra­di­ates, the photons convert into pairs, and the cascade multi­plies until its energy is spent in ionisa­tion and, in a scin­til­la­tor, in light. The depth needed scales with the radia­tion length X0 of the ma­te­ri­al, and ho­mo­ge­neous calo­rim­e­ters of high-Z scintillat­ing crys­tals such as lead tung­state (PbWO4, or PWO) give the best energy resolu­tion.

In a crystal aligned with the beam the picture changes. Along an axis the atoms form strings whose fields add up co­her­ent­ly. For elec­trons, po­si­trons and photons above a few GeV the Lorentz boost raises this field, in the par­ti­cle’s frame, beyond the Schwinger crit­i­cal field of 1.32 × 1016 V/cm. In this strong-field regime of quantum electro­dynamics brems­strah­lung and pair produc­tion are greatly en­hanced: the shower starts earlier and the ef­fec­tive radia­tion length is shorter.

The effect needs align­ment within a few mil­li­ra­di­ans, an ac­cep­tance that depends little on energy, while the shorten­ing of the shower stays pro­nounced at the highest en­er­gies. Hadrons do not feel the orienta­tion, so the dif­fer­ence between electro­magnetic and had­ron­ic showers grows, which helps to iden­ti­fy par­ti­cles.

OREO, the ORi­Ent­ed calO­rim­e­ter, an INFN project led from Ferrara with Legnaro and Milano Bicocca, builds the first calo­rim­e­ter of this kind: 3 × 3 ma­tri­ces of ultra-fast PWO crys­tals read by silicon pho­to­mul­ti­pli­ers, an ori­ent­ed layer in front of a second layer. The axis of every crystal is mea­sured by X-ray diffrac­tion, and the crys­tals are as­sem­bled with inter­ferometric feed­back so that the whole matrix points along the same axis. Uses range from compact calo­rim­e­ters for fixed-target experi­ments and beam dumps for light dark matter to lighter γ-ray tele­scopes in space.

What we have shown.

Align­ing the axes of PbWO₄ crys­tals with the beam makes the electro­magnetic shower develop faster. We built a matrix whose atomic planes are co-aligned within 100 µrad, combin­ing X-ray diffrac­tion, in­ter­fer­om­e­try and preci­sion me­chan­ics. On beam, ori­ent­ed crys­tals gave up to three times more energy deposit, with the effect still visible about one degree off axis: compact calo­rim­e­ters for fixed-target experi­ments and γ-ray sat­el­lites. CORAL con­tin­ues the line from 2026.

× 5

Shorter radiation length in a lead tungstate crystal aligned with its [001] axes, from the radiation of 120 GeV electrons measured at the CERN SPS and compared with simulations.

Physical Review Letters 121 (2018)

up to 3×

More energy deposited in oriented lead tungstate by 120 GeV electrons and by photons of 5–100 GeV than in the same crystal out of alignment.

European Physical Journal C 85 (2025)

100 µrad

Lead tungstate crystals bonded with their atomic planes aligned within 100 µrad, using X-ray diffraction to measure the miscut of each one.

Nucl. Instrum. Methods A 1069 (2024)

< 1 mrad

Alignment between all the crystals of the first 3 × 3 × 2 OREO matrix, assembled in Ferrara with Legnaro and tested with beams at the CERN PS and SPS.

Read first.

  1. Strong Reduction of the Effective Radiation Length in an Axially Oriented Scintillator CrystalBandiera L., Tikhomirov V.V., Romagnoni M. … Mazzolari A., Sytov A. et al. · Physical Review Letters 121 (2018)
  2. Strong enhancement of electromagnetic shower development in oriented scintillating crystals and implications for particle detectorsSoldani M., Monti-Guarnieri P., Selmi A. … Guidi V., Malagutti L., Mazzolari A. et al. · European Physical Journal C 85 (2025)
  3. A highly-compact and ultra-fast homogeneous electromagnetic calorimeter based on oriented lead tungstate crystalsBandiera L., Baryshevsky V.G., Canale N. … Guidi V., Malagutti L., Mazzolari A. et al. · Frontiers in Physics 11 (2023)
  4. High-precision alignment techniques for realizing an ultracompact electromagnetic calorimeters using oriented high-Z scintillator crystalsMalagutti L., Selmi A., Bandiera L. … Guidi V., Mazzolari A., Paternò G. et al. · Nucl. Instrum. Methods A 1069 (2024)
  5. Particle identification capability of a homogeneous calorimeter composed of oriented crystalsMonti-Guarnieri P., Bandiera L., Canale N. … Guidi V., Malagutti L., Mazzolari A. et al. · Journal of Instrumentation 19 (2024)
  6. Satellite-borne γ-ray astrophysics from coherent interactions in oriented crystalsMonti-Guarnieri P., Paternò G., Sytov A. … Fedeli P., Guidi V., Mazzolari A. et al. · Astronomy & Astrophysics 710, A255 (2026)
  7. Mechanical design and photodetector system of the first calorimeter prototypes based on oriented scintillating crystalsFedeli P., Selmi A., Bandiera L. … Cescato F., Guidi V., Malagutti L. et al. · Journal of Instrumentation 21 (2026)

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