Calorimeter (particle physics)

Sandwich calorimeter, consisting of scintillator plates and absorber material, exhibited at DESY .

Calorimeter of the UA2 detector of the former Super Proton Synchrotron , exhibited at CERN .

In particle physics , a calorimeter is an instrument for measuring the total energy of a single particle . In particle physics experiments, the calorimeter is an essential component of the particle detector .

In the calorimeter, incoming, fast particles are completely stopped and the energy released is determined. When high-energy particles hit a calorimeter, secondary particles are created which themselves generate further particles until the available energy is exhausted. This cascade of secondary particle generation is called a particle shower . In addition to energy, (approximate) location information is often determined in a calorimeter.

Traditionally, calorimeters are differentiated according to the type of predominant interaction .

Electromagnetic calorimeters

An electromagnetic calorimeter is used to determine the energy of particles that essentially interact via the electromagnetic force . These are electrons and positrons as well as gamma particles (high-energy photons ), and to a lesser extent muons .

Electromagnetic shower

The mode of operation of a so-called sandwich calorimeter, in which layers of absorber and readout material are arranged alternately, can be explained particularly well. A sequence of bremsstrahlung and pair-forming processes develops in the absorber (each proportional to the atomic number Z ²). An electron hitting the absorber emits a photon, the photon forms an electron-positron pair, which in turn emits photons, etc. The process continues until all electrons have reached the critical energy E _k and then essentially through Ionization release the energy. Part of this ionization energy is measured with the readout material ( scintillator ) sandwiched between them .

Let E _{0 be} the energy of the primary particle, so the number of shower particles results in:

${\displaystyle N_{\mathrm {max} }={\frac {E_{0}}{E_{\mathrm {k} }}}}$

In the simplest model one assumes that the number of particles doubles after a radiation length χ ₀ . Then at the end of the shower one has particles with the energy E _k after n radiation _lengths . The number n of radiation lengths is thus: ${\displaystyle 2^{n}}$

${\displaystyle 2^{n}={\frac {E_{0}}{E_{\mathrm {k} }}}}$

${\displaystyle n={\frac {\ln \left({\frac {E_{0}}{E_{\mathrm {k} }}}\right)}{\ln(2)}}}$

The shower depth only increases logarithmically with the primary energy : ${\displaystyle E_{0}}$

${\displaystyle t_{\mathrm {max} }\propto \ln {\frac {E_{0}}{E_{\mathrm {k} }}}}$

The unit of length is the radiation length χ ₀ . As the number of shower particles N is proportional to the energy of the error of N but is, we have: ${\displaystyle {\sqrt {N}}}$

${\displaystyle N\propto E}$

${\displaystyle \sigma _{E}\propto {\sqrt {E}}}$

${\displaystyle {\frac {\sigma _{E}}{E}}\propto {\frac {1}{\sqrt {E}}}}$

The relative error therefore becomes smaller with increasing energy. In the case of magnetic measurements of the pulse, on the other hand, it increases with the energy (because the curvature becomes smaller and smaller). Therefore, at energies above about 10 to 20 GeV, only calorimetric measurements are possible even with charged particles .

Types of electromagnetic calorimeters

• Lead glass calorimeter (e.g. in the CMS detector at the LHC at CERN ).
• Liquid inert gas calorimeter (z. B. in ATLAS detector at the LHC at CERN ).
• Semiconductor calorimeters (e.g. in the GERmanium Detector Array (GERDA) in the Heidelberg-Moscow experiment in the Laboratori Nazionali del Gran Sasso ).
• Sandwich calorimeter, consisting of alternating layers of scintillators and absorber materials (e.g. in the ZEUS calorimeter at HERA at DESY ).

Hadronic calorimeters

In a hadronic calorimeter, particles can be detected that are predominantly subject to the strong interaction . Since hadronic particles penetrate scintillating material almost unhindered, a structure like the electromagnetic calorimeter is not possible. For this reason, hadronic calorimeters are often designed as “sampling” calorimeters in layers, with sensitive detection layers alternating with insensitive layers that only serve to dissipate energy. The interaction processes of the hadronic particles take place in these absorbers, which then generate electrons, photons, nuclear fragments and hadrons; these are in turn detected in the subsequent sensitive layer by generating scintillation light.

Research and Development

With the constantly increasing energies and intensities in particle physics , the demands on quality , complexity and radiation resistance of the detectors , the essential component of which is the calorimeter, also increase. The development and construction of this special detector component has become an independent branch of science.

Web links

• Calorimeter - Fundamentals of Particle Physics , accessed May 26, 2013.

Individual evidence

1. ↑ C. Grupen, Particle Detectors, Spectrum Academ. Verlag, 1993, ISBN 978-3411165711 .
2. ^ WR Leo, Techniques for Nuclear and Particle Physics Experiments , Springer-Verlag, 1987, ISBN 3-540-17386-2 .
3. ↑ ^{a b} B. Povh , K. Rith, C. Scholz, F. Zetsche, Particles and Kernels , Springer-Verlag, 1997, ISBN 3-540-59438-8 .
4. ↑ A. Meyer-Larsen, Construction, setup and calibration of a blrei-tungstate calorimeter close to the beam tube for use in the ZEUS experiment , dissertation from the University of Hamburg , 1999, internal report from the University of Hamburg (PDF; 3.24 MB), accessed on June 12, 2013.
5. ↑ D. Schroff, study on the electronic calibration of liquid argon calorimeters and on the discovery of invisibly decaying Higgs bosons in the ATLAS experiment , dissertation of the Albert Ludwigs University of Freiburg , 2004, internal report of the University of Freiburg (PDF; 3.57 MB), accessed June 12, 2013.
6. ^ G. Lutz, Semiconductor Radiation Detectors , Springer, 1999 ISBN 978-3540716785 .
7. ↑ ^{a b} R. Wunstorf: Systematic investigations on the radiation resistance of silicon detectors for use in high-energy physics experiments , dissertation, Department of Physics at the University of Hamburg , 1992, internal report: DESY FH1k-92-01 (PDF; 93.1 MB) , accessed June 9, 2013.
8. ↑ ^{a b} A. Dannemann: Investigations into the radiation resistance of polymer materials for use in experiments in high-energy physics , dissertation, Department of Physics at the University of Hamburg , 1996, internal report: DESY F35D-96-06 (PDF; 5.8 MB), accessed on May 25, 2013.
9. ^ I. Bohnet , U. Fricke, B. Surrow, K. Wick, Investigation of non-uniform radiation damage observed in the ZEUS Beam Pipe Calorimeter at HERA , Nuclear Physics B (Proc. Suppl.) 78 (1999) 713-718 .
10. ↑ G. Lindström, Radiation Damage in Silicon Detectors , Nucl. Instr. Meth. A 512, pp. 30-43, 2003.
11. ^ Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Elsevier , accessed May 26, 2013.

Remarks

1. ↑ What these so-called hadrons have in common is that they are made up of individual quarks . Examples are protons , neutrons , pions or kaons .
2. ↑ E.g. the Large Hadron Collider (LHC) is filled with around 2800 particle packets in full operation, which orbit the currently largest particle accelerator over a period of hours at a frequency of 11 kHz. This means a collision at the interaction points (the particle detectors or calorimeter) every 25 nanoseconds, corresponding to a luminosity of 10 ³⁴  cm ⁻² s ⁻¹ . The associated radiation dose for the technical components is enormous.