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Gennaro Miele » 12.Cosmic Rays - II

Cosmic Rays II – Outlines

  • CR observation
  • Air and space based experiments
  • Underground experiments
  • Ground based experiments

Cosmic Ray observation

The figure shows that several quite different kinds of detectors are necessary to study cosmic rays over their whole energy range.

Direct observations are only possible up to few  hundreds of TeV for charged CR. For γ-rays, whose flux is lower by several orders of magnitudes, this limit occurs around 100 GeV.

At higher energies primary CR induce in the atmosphere Air Showers detectable at ground.

CR experiments

In the knee region:Tibet, HEGRA, CASA-MIA, EAS-TOP, Dice, KASCADE – KASCADE GRANDE, …

At higher energies:Volcano Ranch, Akeno, Yakutsk, Fly’s High, Sugar, AGASA, HiRes, Havera Park, Auger, …

The yellow round bounds the Ultra High Energy Cosmic Rays region.

The yellow round bounds the Ultra High Energy Cosmic Rays region.

Aperture and Acceptance

The number of events detected by a given experiment depends, on one side, on the flux of the incoming particles and, on the other side, on the probability of interaction of such particles. This probability has a component depending on the physics of the microscopic event and another one connected to the geometry of the experimental apparatus. It is contained in a function, called the aperture:

\frac{dN}{dt} = D \int dE \, \frac{d \Phi}{dS \, d\Omega \, dt \, dE}\, A(E)

The aperture of a cosmic ray experiment, A, represents the product of the solid angle times the area viewed from the incoming particles and is measured in km2 sr. The acceptance is obtained from the aperture taking into account the efficiency of detection and the duty cycle, D,  during the period of observation.

A_c(E) = D \int dt \, \varepsilon(E,t) \, A(E,t)

with ε the efficiency.

Aperture and Acceptance

In the interval around 1 GeV, about 10 particles per second cross a telescope of the kind in figure (the aperture is small). A small detector flown at the top of the atmosphere in a balloon or satellite is therefore sufficient for measuring charge, energy and direction of the particles.

Measurement of the charge is based on the ionization energy loss in emulsions.

\frac{dE}{dx} \propto \frac{Z^2}{\beta^2}

Energy and mass are measured with calorimeters (emulsion chambers: lead foil and emulsions), scintillators, transition radiation detectors, and Čerenkov radiation detectors.

Advanced Composition Explorer (ACE)

Advanced Composition Explorer (ACE)


JACEE (Japanese-American Collaborative Emulsion Experiment) is a series of balloon-borne lead-emulsion chambers designed to directly measure the primary composition and spectra of cosmic rays at energies in the region of 1 TeV – 1000 TeV. JACEE-10 (1990) and JACEE-11/12 (1993) in Antarctica have pushed the exposure time to more than one week per flight, JACEE-13 was completed in Antarctica, January 1995, JACEE-14 flew in December 1995 – January 1996.

Launch of JACEE-11

Launch of JACEE-11

JACEE-14 flight

JACEE-14 flight


ASCA, Advanced Satellite for Cosmology and Astrophysics,  (formerly named Astro-D) is Japan’s fourth cosmic X-ray astronomy mission, successfully launched on February 20, 1993 and observed till 2001.

First direct evidence that supenovae can accelerate CR

First direct evidence that supenovae can accelerate CR


The Energetic Gamma Ray Experiment Telescope (EGRET), onboard the Compton Gamma Ray Observatory (CGRO), provides a very high energy gamma-ray window.

Its energy range is from 30 MeV to 30 GeV. EGRET is 10 to 20 times larger and more sensitive than previous detectors operating at these high energies and has made detailed observations of high energy processes associated with diffuse gamma-ray emission, gamma-ray bursts, cosmic rays, pulsars, and active galaxies known as gamma-ray blazars.

Muon detection

Detection of muons important for discriminating between different primaries: more muons are produced in air showers induced by heavy nuclei, because they develop relatively high in the atmosphere, where density is lower, and it is easier for charged pions to decay to muons.

1 \, {\mbox{km w.e.}} = l_{1 \, km} \, \rho_{w} = 10^5 \, {\mbox{cm 1g cm}^{-3} = 10^5 \, {\mbox{g cm}}^{-2}

Only muons (and neutrinos) can penetrate to large depths underground, till depths of tens of km of water equivalent.

Measurements from GRAPES collaboration (from PDG 2007).

Measurements from GRAPES collaboration (from PDG 2007).

Underground Experiments

Muons of a few hundred GeV and above have penetration depth of the order of a km even in rock and can be detected underground. Moreover, upgoing neutrinos can interact with nucleons in rock, water or ice.

The EAS-TOP experiment at Gran Sasso (Campo Imperatore, 2000 m a.s.l.) has been in operation between 1989 and 2000, for CR physics in the energy range 1013-1016 eV. It operated in coincidence with the underground experiment  MACRO,  which  made  interesting  measures in various fields, among which measurements of the vertical muons coming from downgoing cosmic rays and of the upgoing muons coming from the interactions of neutrinos with the earth rock.

EAS-TOP muon-hadron detector

EAS-TOP muon-hadron detector

Neutrino Telescopes

Neutrino astronomy offers the possibility of observing sources which correspond to the central engines of the most energetic astrophysical phenomena. The drawback, of course, is that the weak interactions of neutrinos imply that a very massive detector with extremely good background rejection is required to observe a measurable flux.

Since the Earth acts as a shield against all particles except neutrinos, a neutrino telescope uses the detection of upward-going muons as a signature of muon neutrino interactions in the matter below the detector. The muon detection medium may be a natural body of water or ice through which the muon emits Čerenkov light. Its detection allows the determination of the muon trajectory.


After the completion, in the same location, of the successful experiment Amanda-II, the extension to a km3, IceCube, has been installed at the South Pole during Austral summers over approximately six years. The IceCube In-Ice detector consists of  about 5160 optical modules deployed on 86 vertical strings buried 1450 to 2450 meters under the surface of the ice, and an IceTop surface air-shower detector array of about 324 optical modules.

From IceCube gallery

From IceCube gallery

South Pole station Right: IceCube

South Pole station Right: IceCube

Molière radius

The Molière radius is a characteristic constant of a material which describes its electromagnetic interaction properties. It is related to the radiation length, X0, by

R_M = 0.0265 \, X_0 \, (Z + 1.2)

where Z is the atomic number. RM is the reference scale in the lateral distribution of a shower. In fact, the average energy loss in a plastic scintillator of electrons, photons, and muons (in unit of the energy loss of vertically penetrating muons, VEM), is something like (Yakutsk and AGASA)

S(r) = N_e \, C_e \, R^{- \alpha} \, (1+R)^{-(\eta - \alpha)} \, f(r)

where R = r/RM .

Ground Experiments

For primaries with energy > 1015 eV, enough particles reach ground to be detected. A primary of 1019 eV will have about 1010 particles in the resulting cascade, and Coulomb scattering of the shower particles (mainly electrons) and the transverse momentum in the hadronic interactions will spread them over a wide area (over 10 km2 for 1019 eV primary), since the Molière radius in air at sea level is about 79 m.

A ground based experiment is made by a number of particle detectors in a regular array (about 1 km spaced for 1019 eV).

The particle detector used at the surface are scintillators and water- Čerenkov detectors. Radiation detectors are Čerenkov telescopes and fluorescence detectorskm spaced for 1019 eV).

Density sampling and fast timing

Density sampling: the shower particle density is measured only at some locations at ground, that is in the array of detectors. The energy is proportional to the total number of particles. Quantitatively, one identifies the shower core and measures the signal at 600 m from it, S(600), which is found to be quite insensitive to the primary composition and the interaction model used to simulate air showers.

Fast timing: the arrival time of signals in different detectors allow reconstruction of the direction to better than 3°


The most diffused are the plastic scintillators (inorganic salt or organic plastic). The plastic material, excited by the charged particle, emits light photons which are detected by a photomultiplier (PMT) and transformed in an amplified electric signal.

Water Cherenkov

When a charged particle cross the water faster than light, it produces a cone of Čerenkov light around its direction. This light can be reflected by the wall’s tank and conveyed to the PMT in the upper part of the detector.

The lateral density distribution of the water Čerenkov signal, ρ(r), in units of vertical equivalent muons (VEM) per m2 is given by

\rho(r) = k \, r^{-(\eta + r/4000)}

where r is the distance from the shower core in meters, k a normalization parameter and η a function of the zenith angle and shower energy.

PAO water Čerenkov

PAO water Čerenkov

Fluorescence detectors

The excitation of 2+ band of air nitrogen by the charged particles of an air shower of energy larger than ~ 3 1017 eV produces fluorescence light in the UV band (350-450 nm), which can be detected from very large distances. A cosmic primary of 1019 eV gives a fluorescence track of 10-100 km, depending on the mass of the primary and the inclination, which allows the reconstruction of the longitudinal profile and the depth of its maximum to within ±50 g/cm2.

This light, coming from a restricted field of view, is very weak, like a light bulb of a few watts, and requires to be detected dark nights (no moon or city lights) and clear time (no clouds or fog). This means that the duty cycle for this method of detection is less than 100%, usually 10%.

Fly’s Eye  fluorescence detectors

Fly’s Eye fluorescence detectors

PAO Los Leones fluorescence station

PAO Los Leones fluorescence station

Cherenkov detectors

The majority of air shower particles travel very close to the speed of light, in fact faster than the speed of light in the atmosphere. The resultant polarization of local atoms causes the emission of a faint, bluish light known as Čerenkov radiation. Depending on the energy of the initial primary, there may be thousands of electrons/positrons in the resulting cascade which are capable of emitting Čerenkov radiation. As a result, a large “pool” of Čerenkov light accompanies the particles in the air shower. This pool of light is pancake-like in appearance, about 200 meters in diameter but only a meter or so in thickness. Air Čerenkov detectors focuses this light to to a focal plane where it is detected by photomultipliers.


KASCADE (KArlsruhe Shower Core and Array DEtector) is a ground detector, located in Karlsruhe (Germany), made by scintillators and muon detectors. The energies explored are in the knee region.

KASCADE-Grande is an extension of KASCADE toward higher energies.





Ultra High Energy CR experiments

The first pioneeristic experiments used small configurations of ground arrays of Geiger-Müller or plastic scintillators: Pamir (1946), Agassiz (1956).


The Akeno Giant Air Shower Array was the first “giant” apparatus: 111 plastic scintillators on the ground, 1 km spaced, and 27 shielded muon detectors, spread over an area of ~100 km2. Also calorimeters (small) and water Čerenkov detectors.

AGASA water Čerenkov

AGASA water Čerenkov

AGASA: Observations

AGASA detected 6 events with energy above 1020 eV. The most energetic event was observed on December, 3rd 1993. AGASA closed in 2004.

AGASA at highest energy

Fly’s Eye

In 1981, the cosmic ray group of the University of Utah started the construction, in the Dugway desert, of the first experiment using the air fluorescence technique, following a successful trial at Volcano Ranch.

The two “eyes”, 3.3 km apart, were made up of 67 and 36 modules, respectively, with 12 photomultiplier tubes per module. The most energetic event was one with energy 3.2 1020 eV.


HiRes (High Resolution Fly’s Eye) is the successor to the Fly’s Eye experiment, which was operated at the same site over the period 1997-2006. It consisted of two detector stations (HiRes-I and HiRes-II) located 12.6 km apart. HiRes-I (HiRes-II) had 21 (42) modules pointing to 3°-17° (3°-31°) in elevation. 256 PMT were placed at the focal plane of each mirror. The data analysis was carried out in monocular mode, with best statistical power and wider energy range, or stereo mode, with best resolution but less statistic.

ICRC 2007      CR spectrum from HiRes – 13 events with E>10^20 eV

ICRC 2007 CR spectrum from HiRes - 13 events with E>10^20 eV

Pierre Auger Observatory

PAO is a hybrid detector, employing the two methods of ground array and fluorescence technique for the detection of EAS. Comparing results from the different types of detectors helps to reconcile the two sets of data and produce the most accurate results about the energy of primary cosmic rays. It started operation in 2004 and a northern site is planned in USA.

  • Located at Pampa Amarilla in Argentina
  • Size: 50×60 km2=3000 km2
  • 1600 water Čerenkov
  • 24 fluorescence telescopes in 4 stations
  • 17 nations involved
  • 300 physicists and 100 technicians
  • 38 Italian physicists from 8 Italian groups

Pierre Auger Observations

Arrival directions of UHECR at PAO

Arrival directions of UHECR at PAO

Telescope Array

The Telescope Array project is a collaboration between universities and institutes in Japan, Taiwan, China and the USA. The experiment is designed to observe cosmic-ray-induced air showers at extremely high energies using a combination of ground array and air-fluorescence techniques, like PAO. It is being deployed in the high desert in Millard County, Utah, USA. SD full operation started on the beginning of 2008.

Fluorescence site construction

Fluorescence site construction

Ground array deployment

Ground array deployment


EUSO is a space mission devoted to the investigations of cosmic rays and neutrinos at very high energy (> 1020 eV) by looking downward from space, under a 60° angle, at the fluorescence light produced when cosmic rays hit the air molecules. Phase-A study of EUSO under the ESA has successfully finished in July 2004. The phase-B study, however, has been postponed for a long time because of financial problems in ESA and Italy. Then, Japanese and U.S. teams re-defined EUSO as a mission attached to the Japanese Experiment Module/Exposure Facility of the International Space Station. They renamed it as JEM-EUSO and started the preparation targeting the launch after 2012 in the framework of second phase of JEM/EF utilization.


OWL (Orbiting Wide angle Light collectors) is a satellite project aiming at the observation of the air shower’s development from the space in mono and stereo mode. Three phases are planned: in the first one, two satellites will fly in formation with a separation of 10 to 20 km for about 3 months to search for upward-going showers from ντ’s propagating through the Earth. In the second one, they will separate to 600 km for ~ 2.5 years to measure the high energy end of the CR spectrum and in the third one the altitude will be reduced to 600 km and the separation to 500 km to study the CR flux near 1019 eV.

Claimed energy resolution is 14% at 1020 eV, angular resolution less than 1°, stereo aperture = 4 106 km2 sr, duty cycle ~ 10%.

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