Ebook: Radiation and Particle Detectors

From the 20th to 25th of July 2009 the International School of Physics entitled “Radiation and Particle Detectors” was held in Varenna, which involved the use of detectors for the research in fundamental physics, astro-particle physics, and applied physics. At the school ten teachers and thirty students were present.

In the context of fundamental physics the High Energy Physics (HEP) plays an important role. In general the HEP experiments make use of sophisticated and massive arrays of detectors to analyze the particles which are produced in high-energy scattering events. This aim can be achieved in a large variety of approaches. Some examples are the following:

– Measuring the position and length of ionization trails. Much of the detection depends upon ionization.

– Measuring time of flight permits velocity measurements.

– Measuring radius of curvature after bending the paths of charged particles with magnetic fields permits measurement of momentum.

– Detecting Cherenkov radiation gives some information about energy, mass.

– Measuring the coherent “transition radiation” for particles moving into a different medium.

– Measuring synchrotron radiation for the lighter charged particles when their paths are bent.

– Detecting neutrinos by steps in the decay schemes which are “not there”, i.e., using conservation of momentum, etc. to imply the presence of undetected neutrinos.

– Measuring the electromagnetic showers produced by electrons and photons by calorimetric methods.

– Measuring nuclear cascades produced by hadrons in massive steel detectors which use calorimetry to characterize the particles.

– Detecting muons by the fact that they penetrate all the calorimetric detectors.

All these types of detectors are used in the largest accelerator ever built: the Large Hadron Collider (LHC). LHC is a proton-proton (also ions) ring, 27 km long, 100 m underground, with 1232 superconducting dipoles 15 m long at 1.9 K producing a magnetic field of 8.33 T. The figures of merit, for proton-proton operations, are beam-energy 7 TeV (7 × TEVATRON), luminosity 10³⁴ cm⁻²s⁻¹ (> 100 × TEVATRON), bunch spacing 24.95 ns, particles/bunch 1.1·10¹¹, and stored emergy/beam 350 MJ. For ion-ion operations we will have energy/nucleon 2.76 TeV/u, and total initial luminosity of 10²⁷ cm²s⁻¹. The main four experiments are two general purpose experiments (ATLAS and CMS), B-physics and CP violation experiment (LHCB), and heavy ions experiment (ALICE).

The international community of physicists hopes that the LHC will help answer many of the most fundamental questions in physics: questions concerning the basic laws governing the interactions and forces among the elementary particles, the deep structure of space and time, especially regarding the intersection of quantum mechanics and cosmology, where current theories and knowledge are unclear or break down altogether. The enormous success of the Standard Model (SM), tested at per mil level with all particles discovered except the Higgs boson, will hopefully be able to build a Cosmology Standard Model.

The issues of LHC physics include, at least:

– Is the Higgs mechanism for generating elementary particles masses via electroweak symmetry breaking indeed realised in nature? It is anticipated that the collider will either demonstrate or rule out the existence of the elusive Higgs boson, completing (or refuting) the SM.

– Is supersymmetry, an extension of the SM and Poincaré symmetry, realised in nature, implying that all known particles have supersimmetric partners?

– Are there extra-dimensions, as predicted by various models inspired by string theory, and can we detect them?

– What is the nature of the Dark Matter which appears to account for 23% of the mass of the universe?

Other questions are:

– Are electromagnetism, the strong force, and the weak interaction just different manifestations of a single unified force, as predicted by various Grand Unification Theories (GUTs)?

– Why is gravity so many orders of magnitude weaker than the other three fundamental interctions (Hierarchy Problem)? For all proposed solutions: new particles should appear at TeV scale or below.

– Are there additional sources of quark flavours, beyond those already predicted within the Standard Model?

– Why are there apparent violations of the symmetry between matter and antimatter (CP violation)?

– What was the nature of the quark-gluon plasma in the early universe (ALICE experiment)?

Obviously, for the construction of a Standard Cosmology Model, the astro-particle experiments are crucial with direct or indirect dark matter measurements. In particular, the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) experiment, which went into space on a Russian satellite launched from the Baikonur cosmodrome in June 2006, uses a spectrometer—based on a permanent magnet coupled to a calorimeter—to determine the energy spectra of cosmic electrons, positrons, antiprotons and light nuclei. The experiment is a collaboration between several Italian institutes with additional participation from Germany, Russia and Sweden. PAMELA represents a state of the art of the investigation of the cosmic radiation, addressing the most compelling issues facing astrophysics and cosmology: the nature of the dark matter that pervades the universe, the apparent absence of cosmological antimatter, the origin and evolution of matter in the Galaxy. PAMELA, a powerful particle identifier using a permanent magnet spectrometer with a variety of specialized detectors, is an instrument of extraordinary scientific potential that is measuring with unprecedented precision and sensitivity the abundance and energy spectra of cosmic rays electrons, positrons, antiprotons and light nuclei over a very large range of energy from 50 MeV to hundreds GeV, depending on the species. These measurements, together with the complementary electromagnetic radiation observation that will be carried out by AGILE and GLAST space missions, will help to unravel the mysteries of the most energetic processes known in the universe. Recently published results from the PAMELA experiment have shown conclusive evidence of a cosmic-positron abundance in the 1.5–100 GeV range. This high-energy excess, which they identify with statistics that are better than previous observations, could arise from nearby pulsars or dark matter annihilation. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in antiprotons is highly constraining on any model with hadronic annihilation modes. The light boson naturally provides a mechanism by which large cross-sections can be achieved through the Sommerfeld enhancement, as was recently proposed. Depending on the mass of the WIMP, the rise may continue above 300 GeV, the extent of PAMELA's ability to discriminate electrons and positrons. The data presented include more than a thousand million triggers collected between July 2006 and February 2008. Fine tuning of the particle identification allowed the team to reject 99.9% of the protons, while selecting more than 95% of the electrons and positrons. The resulting spectrum of the positron abundance relative to the sum of electrons and positrons represents the highest statistics to date. Below 5 GeV, the obtained spectrum is significantly lower than previously measured. This discrepancy is believed to arise from modulation of the cosmic rays induced by the strength of the solar wind, which changes periodically through the solar cycle. At higher energies the new data unambiguously confirm the rising trend of the positron fraction, which was suggested by previous measurements. This appears highly incompatible with the usual scenario in which positrons are produced by cosmic-ray nuclei interacting with atoms in the interstellar medium. The additional source of positrons dominating at the higher energies could be the signature of dark matter decay or annihilation. In this case, PAMELA has already shown that dark matter would have a preference for leptonic final states. They suggest that the alternative origin of the positron excess at high energies is particle acceleration in the magnetosphere of nearby pulsars producing electromagnetic cascades. The members of the collaboration state that the PAMELA results presented here are insufficient to distinguish between the two possibilities. They seem, however, confident that various positron production scenarios will soon be testable. This will be possible once additional PAMELA results on electrons, protons and light nuclei are published in the near future, together with the extension of the positron spectrum up to 300 GeV thanks to ongoing data acquisition.

S. Bertolucci, U. Bottigli and P. Oliva

Proton therapy represents the most promising radiotherapy technique for external tumor treatments. At Laboratori Nazionali del Sud of the Istituto Nazionale di Fisica Nucleare (INFN-LNS), Catania, Italy, a proton therapy facility is active since March 2002 and 200 patients, mainly affected by choroidal and iris melanoma, have been successfully treated. Proton beams are characterized by higher dose gradients and linear energy transfer with respect to the conventional photon and electron beams, commonly used in medical centers for radiotherapy. In this paper, we report the experience gained in the characterization of different dosimetric systems, studied and/or developed during the last ten years in our proton therapy facility.

This paper has been written following the two lectures given by the author on the same subject at the CLXXV Course Radiation and Particle Detectors, of the International “E. Fermi” School in Varenna, but should not be considered in itself a comprehensive text on the subject. The basic principles of Ion Beam Analysis (used to deduce the composition of a target material) and of Accelerator Mass Spectrometry (used to deduce the concentration of rare isotopes in a sample) are recalled, and the solutions implemented for their application in the field of Cultural Heritage are described. In particular, the specific requirements for the detectors and for some beam control systems along the lines of the accelerator are discussed in some detail.

This paper is intended as an introduction to and overview of calorimetric particle detection in high-energy physics experiments. First, the physics that plays a role when high-energy particles are absorbed in dense matter is described, with emphasis on issues that are important for the properties of calorimeters. Next, all aspects of the calorimeter response function are discussed: Mean value, shape, width, and the factors that determine these characteristics. Then, we elaborate on some practical issues that are important for those working with calorimeters: calibration and simulation. Finally, a brief overview of modern developments in this rapidly evolving field is given.

This lecture gives an overview of the design and current status of the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC). After a short introduction of the physics goals and design of the detector, the modular building of the detector is discussed as well as its commissioning using cosmic rays and beam induced muons from the first circulating LHC beams in September 2008. While eagerly anticipating the upcoming restart of the LHC, the CMS collaboration has invested a serious effort in studying the performance of the CMS detector as a scientific instrument using cosmic-ray data from a sustained run with full detector in readout and magnet switched on: the Cosmic Run At Four Tesla (CRAFT), recorded during a month-long campaign in October and November 2008. This was a crucial step towards the commissioning of CMS and understanding of the detector performance, improving the level of preparedness of CMS for physics analysis. The precise mapping of the CMS magnetic field using cosmics rays is highlighted as an example of a successful improvement that resulted from the analysis of the CRAFT data.

Gravitational waves were predicted in 1916 by Einstein as a consequence of the theory of General Relativity: accelerated masses can produce ripples propagating at the speed of light, which perturb the space-time metric. Thanks to the extremely weak coupling with matter, gravitational waves can cross the universe undisturbed and, hence, are a probe of the regions where they are produced which is not accessible by the eventual electromagnetic counterpart. The gravitational waves sources of detectable amplitudes are expected to be compact astrophysical sources such as the coalescence of binaries formed by black holes and neutron stars, the collapses of stellar cores, or the rotation of non-axis-symmetric neutron stars. For more than 40 years the search for gravitational waves has been pursued with resonant detectors made of metallic bars. The development of gravitational wave detectors based on laser interferometers started in the early seventies. After more than two decades of development, the construction of the first interferometers with kilometer scale arms started in the nineties. The sensitivity of such detectors is fundamentally proportional to its length, and with its 3 kilometer long arms Virgo is the largest gravitational wave detector in Europe, and the third largest in the world. It is located at the European Gravitational Observatory (EGO), close to Pisa, and it is designed to detect gravitational waves emitted by astrophysical sources in the frequency range between 10 Hz and a few kHz. Among the other current ground-based gravitational wave detectors, Virgo is the one having the best sensitivity at low frequency, thanks to the particular seismic attenuators, from which the mirrors are suspended. Construction started in 1996 and ended in July 2003. After a very intense commissioning phase, the performances of the detector are now very close to the design ones, and the detector is entering the operation phase. In parallel, the design phase of the second generation of interferometers should be finalized this year with a construction planned to start in 2011. Also, the conceptual design is under study for a third generation. The corresponding European project is called the “Einstein Telescope”.

Many indications suggest that the observed High Energy Cosmic Rays could be produced in astrophysical sources, namely SuperNova Remants, Gamma Ray Bursters and Active Galactic Nuclei, where Fermi acceleration mechanism of charged particles takes place. In this scenario, accelerated protons undergo photo-meson interaction with the ambient photon field, or hadronic interactions within the source and/or with close molecular clouds, producing high-energy gammas and neutrinos. In the last decade, the operation of air and water Čerenkov gamma ray detectors led to the discovery of about hundred cosmic TeV gammaray sources. At least few tens of the identified TeV gamma sources in the Galaxy are expected to be also high-energy neutrinos sources. Many other extragalactic sources, not seen in TeV gamma rays due to gamma ray absorption through interaction with the Cosmic Microwave Background, may also be high-energy neutrino emitters. Neutrinos, light and uncharged, are very promising probes for high-energy astrophysics since they can reach the Earth from cosmic distances and from astrophysical environments obscure to high-energy gammas and nuclei. Theoretical estimates indicate that a detection area of the order of a few km² is required for the measurement of HE cosmic ν fluxes. The underwater/ice optical Čerenkov technique is considered the most promising experimental approach to build high-energy neutrino detectors in the TeV-PeV energy range. After the first generation of underwater/ice neutrino telescopes (Baikal, AMANDA and ANTARES), the quest for the construction of km² size detectors has already started. At the South Pole the construction of the IceCube neutrino telescope is in an advanced stage, while the ANTARES, NEMO and NESTOR collaborations together with several other European Institutions take part to KM3NeT aiming at the installation of a km³-scale neutrino telescope in the Mediterranean Sea. Also limits for UHE neutrino detection were strongly improved in the last years, especially with the recent results of ANITA and Pierre Auger Observatory. An intense R&D activities is also ongoing on thermo-acoustic techniques that could provide a viable solution for Ultra High Energy neutrino detection underwater.