cosmic rays i long - istituto nazionale di fisica nucleare

Astroparticle Course 1

Cosmic Rays ICosmic Rays I

Cosmic rays continually bombard the Earth.

In fact, about 100 000 cosmic rays pass through a person every hour!





Where do they come from?

How are they accelerated to

such high energies?



The discovery of The discovery of cosmic rayscosmic rays

Cosmic ray and particle Cosmic ray and particle physicsphysics

CR deflections inCR deflections inmagnetic fieldmagnetic field

CR from the SunCR from the Sun Shower theoryShower theory


Some essential bibliographySome essential bibliography

• Cosmic rays: A dramatic and authoritative account by

Bruno Rossi

• Cosmic Rays and Particle Physics, Thomas K. Gaisser

• Origin and propagation of Extremely High EnergyCosmic Rays, P. Bhattacharjee & G. Sigl, Phys. Rept.

327 (2000) 109.

• Observation and implications of the ultrahigh-energy

cosmic rays, M. Nagano & A.A. Watson, Rev. Mod. Phys. 72 (2000) 689.


Just beforeJust before……

When scientists first started studying radiation in the early 1900s, they found 3 different types of rays:

• α rays: turned out to be Helium nuclei

• β rays: turned out to be electrons and positrons

• γ rays: turned out to be e.m. radiation

Of the known radiation, the one emitted by radioactive substances had the highest energies (MeV).

Cosmic ray physics had to involve much greater energies, till 1020 eV!


The discoveryThe discovery

“At six o’clock on the morning of August 7, 1912, a balloon ascended from a field near the town of Aussig, in Austria…”

from Cosmic rays, Bruno Rossi

Victor F. Hess took with him three electroscopes up to an altitude of about 16000 feet (without oxygen!).

“The results of my observations are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above.” Physikalische Zeitschrift, November 1912

Hess won the Nobel prize in 1936 for his discovery of cosmic rays. Millikan gave the name cosmic rays to the new radiation.


Atmospheric depthAtmospheric depth

When comparing radiation absorbers of different substances, it becomes necessary to consider the density as well as the thickness of the absorber. Thus, it is customary to define an ab-sorber not by its geometrical thickness, but by the mass of a

column of unit cross sectional area. This quantity – the mass per unit area – is usually measured in grams per square centimeters (g/cm2). For an absorber of constant density, the mass per unit area is just the product of its thickness and its density: so, it’s like a length which takes into account the density. The mass per unit area of the atmosphere above a given level is known as atmospheric depth.


New particlesNew particles

Colombo, searching for a new route to India, discovered America. In the same way physicists, searching for a solution to the cosmic ray puzzle, discovered a zoo of new particles, opening an entirely new field of research: at the beginning, cosmic ray physics and elementary particle physics were strictly connected.

The instrument which made possible these discovers is the cloud (or expansion) chamber, invented by Wilson in 1911.

e+

e-

Photon conversions γ→e+ e−

Photo of α-particles emitted by radioactive

source


Cloud chamberCloud chamberThe cloud (or expansion) chamber was invented by

Wilson in 1911. The expansion of the gas in the chamber

causes condensation around the ions present, producing a

visible track along the trajectory of a charged particle.

However, to be detected, the particle must traverse the

chamber at some time during the so-called expansion

phase: so the chamber, in its early version, was sensitive

for a period of about 0.01 second at each expansion.A major technical achievement was the counter-

controlled chamber, which was triggered by Geiger-

Müller counters when they were hitted by a CR particle

(Blackett & Occhialini, 1932).

For a given velocity, the density of ions per unit length

increases with increasing charge of the initial particle.

For a given charge, it decreases with increasing

velocity. The ion trail of smallest possible density is

one left by a singly charged particle moving at nearly

the velocity of light (minimum-ionizing particle).


An elementary zoo: the positronAn elementary zoo: the positron

Anderson, 1932Anderson, 1932

The positron in the figure is identified as the particle that enters the cloud chamber from below and curves sharply to the left after traversing the lead plate.

At first Anderson thought the positive particles were protons. But the ionizing power estimated by the observation should have been greater for a particle of mass larger than the electron one.


An elementary zoo: the positronAn elementary zoo: the positron

Anderson, 1932Anderson, 1932

The positron in the figure is identified as the particle that enters the cloud chamber from below and curves sharply to the left after traversing the lead plate.

At first Anderson thought the positive particles were protons. But the ionizing power estimated by the observation should have been greater for a particle of mass larger than the electron one.

Ion density in multiples of the density of a

minimum-ionizing particle

Magnetic rigidity


An elementary zoo: the An elementary zoo: the muonmuon

Anderson & Anderson & NeddermayerNeddermayer, 1937, 1937Physicists observed that cosmic rays contained a soft and hard component; the particles of the latter could penetrate as much as 1 m of lead. They could not be e+-e-, since their estimated energy should have been absurd, and their energy losses did not agree with the Bethe-Heitler theory. Moreover, the penetrating particles often occurred in groups, as they were secondary products of the interaction of primary cosmic rays.




Electron energy losses




Photograph by Blackettand Occhialini




The circle is the result of the measurement

relative to the track in figure.


MuonMuon decaydecay

In measuring the numbers of CR at various altitudes in the atmosphere, physicists found a very puzzling result: contrary to the earlier findings of Millikan, it looked as if air absorbed CR more effectively than solid or liquid matter. Moreover, the low density air at very high altitudes appeared to be a better absorber then the denser layer in the lower atmosphere.

The German physicist H. Kuhlenkampff proposed a solution based on the fact that the newly discovered cosmic ray meson were unstable, with a decay time of the

order of µs. In a 10 cm layer of water, equivalent to a 16000 cm layer of the high atmosphere air, none of the mesons will have the time to decay.


MuonMuon decaydecay

In measuring the numbers of CR at various altitudes in the atmosphere, physicists found a very puzzling result: contrary to the earlier findings of Millikan, it looked as if air absorbed CR more effectively than solid or liquid matter. Moreover, the low density air at very high altitudes appeared to be a better absorber then the denser layer in the lower atmosphere.

The German physicist H. Kuhlenkampff proposed a solution based on the fact that the newly discovered cosmic ray meson were unstable, with a decay time of the

order of µs. In a 10 cm layer of water, equivalent to a 16000 cm layer of the high atmosphere air, none of the mesons will have the time to decay.

The µ meson enters the cloud chamber from above, loses most of its energy in traversing an aluminum

plate, then decays giving an electron track (minimum

ionizing track)


Nuclear emulsionsNuclear emulsionsThe cloud chamber has inherent limitations: because of the low density of the

gas, very few of the particles entering it collide with nuclei or stop inside the

chamber. In the middle 1940s, physicists succeeded in perfecting the nuclear

emulsion technique (Powell&Occhialini). Ionizing particle “sensitize” the grains of

silver bromide that they encounter along their path. An appropriate “developer”

solution will then reduce the sensitized grains to silver, in such a way that, under

a microscope, the trajectories of ionizing particles appear as rows of dark grains.

The density of the silver grains

along the track is proportional

to the density of ion pairs that

the particle would produce in a

gas, and decreases with

increasing velocity. If the

particle stops in the emulsion, it

is possible to measure its

range, which depends on its

energy and mass.Grain density in

multiple of the one for a minimum ionizing particle

Residual range in the emulsion


An elementary zoo: the An elementary zoo: the pionpion

Lattes, Lattes, OcchialiniOcchialini, & Powell, 1947, & Powell, 1947

But, the results of an experiment made by Conversi, Pancini and Piccioni, using a magnetic lens, and different materials (lead, carbon, magnesium), showed that in light elements negative mesons could escape nuclear capture. Only later, Lattes, Occhialini and Powell

identified the π meson in emulsions.

In 1935, H. Yukawa had postulated the existence of a subatomic particle associated with the nuclear forces, like the photon was associated to the

e.m. ones. Physicists thought that the µ meson was such a particle. Next, Tomonaga and Araki pointed out that positive and negative µmesons should behave differently after coming at rest in matter.


An elementary zoo: the An elementary zoo: the kaonkaon

Rochester & Butler, 1947Rochester & Butler, 1947 Just a few months after the

discovery of the π meson, Rochester and Butler published two cloud-chamber photographs. Neither the neutral particle invoked to explain the first event, nor the charged particle in the second could possibly be identified as any known particle.

Two years later, Powell’s group found in nuclear emulsion a particle, with mass intermediate between that

of a π meson and a proton, which appeared to decay in three particles,

one of which was a π meson.


An elementary zoo: more and moreAn elementary zoo: more and more……

For a while there was a great deal of confusion about the number and properties of the particles required to explain all the experimental data. Then a classification was made in mesons, baryons, and leptons.

π+

p

Discovery of Ω-


Magnetic rigidityMagnetic rigidity

ZeBvR

mv=

2

Ze

pBR =

The product BR is called magnetic

rigidity. From the definition of eV it

follows that:

Z

eVEcBR

)(=

Z

eVEcmRgaussB

300

)()()( =

and inserting unity of measure:

A moving charged particle in a magnetic field experiences a deflecting force. The

radius, R, of the circle described in a uniform field (Larmor radius) is obtained

from the condition that the centrifugal force and the Lorentz force must balance.

relativistically correct


Magnetic field deflections: latitude effectMagnetic field deflections: latitude effectIn 1930 the notions about the possible effects of

the earth’s magnetic field upon cosmic rays

were still rather nebulous. Consider a particle

that circles the earth at the geomagnetic

equator: it has to move from east to west if it is

positive and on the contrary if it is negative. The

product BR, known as magnetic rigidity of the

particle, has to be

cmgausscmgaussBR 88 1021038.632.0 ⋅=⋅=

which correspond to an energy of

about 60 GeV. This means that

charged particles with energies of

this order or less must be strongly

deflected by the earth’s magnetic

field at the geomagnetic equator, and

CR should somehow be channeled

toward the poles (latitude effect).


Magnetic field deflections: EMagnetic field deflections: E--W effectW effect

He found that there existed a special class of

trajectories, called bounded ones, with the

property of remaining forever in the vicinity of

the earth. For each point on the earth, there

exists a Störmer cone with the axis pointing to

the East (West), which contains the bounded (so

forbidden) directions for positive (negative) CR.

Then, the Norwegian physicist Carl Störmer

computed the trajectories of particles with

different magnetic rigidities approaching the

earth, and distinguished them in allowed (a)

and forbidden (b) ones.

Störmer cone for positive particles

Störmer cone for negative particles


Van Allen radiation beltVan Allen radiation belt

On November 3, 1957, USSR launched

Sputnik II, and USA satellites Explorer I

and III followed on February 1 and March

26, 1958. At every revolution, Explorer Iand III swung from several hundred km to several thousands km. Above 2000 km,

the counters, installed aboard by the CR group under J. Van Allen, apparently

stopped working and started again at lower altitudes. The only explanation was

that they become “jammed” when were exposed to a radiation of excessive

strength.

It is generally understood that the inner and outer

Van Allen belts result from different processes. The

inner belt, consisting mainly of energetic protons, is

the product of the decay of albedo neutrons which

are themselves the result of cosmic ray collisions in

the upper atmosphere. The outer belt consists

mainly of electrons that are injected from the

geomagnetic tail following geomagnetic storms.


Low energy CR from the SunLow energy CR from the Sun

When systematic measurements were

undertaken at altitudes and latitudes where

primary CR particles of lower energy could

also be observed, it became apparent that

the low-energy portion of the cosmic

radiation had to do primarily with events in

the sun. SOHO images of the flare that occurred on the 15 July 2002

The CR particles from these events, recorded at

earth, have energies of the order of tens of GeV,

since the effect is usually much smaller near the

geomagnetic equator than at high latitudes. The

same conclusion is indicated by the fact that neutron

detectors record a much greater increase than µdetectors, since µ leptons are produced abundantly only by protons with greater energies.


The solar cycleThe solar cycle

The general pattern of solar activity follows

an 11-year cycle. When cosmic ray

observations began to accumulate, it was

found that the flux of cosmic rays also

changes systematically during this cycle.

In the plot it is reported the intensity of CR

measured at a geomagnetic latitude of 88°N by

H.V. Neher of CalTech in 1954 and 1958. At the

highest altitude, the intensity doubles. The

interpretation of these data is that the plasma

emitted by the Sun carries materially away with

it the magnetic field, which acts as a partial

screen against CR particles entering the solar

system from the outside.

1991

1995


A changing perspectiveA changing perspective

During several years physicist belief on cosmic rays changed continuously. At the beginning, Millikan though that they were mainly photons and resulted from the synthesis of heavy elements like nitrogen, oxygen or silicon. But the coincidences observed by Bothe and Kohlhörster in Geiger-Müller counters were difficult to explain by double Compton effects. The observations seemed to indicate that the primary cosmic radiation consisted of charged particles.




Experimental set-up by Bruno Rossi

Then, other experiments showed that high-energy cosmic rays occasionally produced secondary ionizing particles in the matter of the shields. It soon became clear that this was not inusual, but a characteristic of these particles, which arrived to earth in showers.






Photograph by Blackettand Occhialini






Photograph by the MIT cosmic ray group


The discovery of extensive air showersThe discovery of extensive air showers

Extensive air showers were discovered in the

1930's by the French physicist Pierre Victor Auger.

In addition to his contributions to the field of

cosmic rays, Pierre Auger was most well known

for his discovery in the 1920's of a spontaneous

process by which an atom with a vacancy in the K-

shell achieves a more stable state by the emission

of an electron instead of an X-ray photon,

commonly known as the Auger Effect.

After physicists began to experiment with coincidences, it became a common

practice to test the operation of the equipment by placing the counters out of

line, usually on a horizontal plane. Several experimenters noticed that the

number of coincidences recorded was too large to be accounted for entirely by

chance. In 1938 Pierre Auger and collaborators undertook a systematic study

that established beyond any doubt the occurrence of air showers and provided

preliminary information about their properties.


Shower developmentShower development

A high-energy primary CR particle

(e.g. a proton) collides with a nucleus

(O, N, Ar) in the atmosphere

producing other particles, mainly

pions and kaons. These particles

have energies high enough to

produce more particles (mainly

hadrons). This is called air shower

(or hadronic cascade). At very high

energies this is an Extensive Air

Shower (EAS).

Neutral pions quickly decay into two photons, which start electromagnetic

cascade. Photons produce e+e--pairs, which generate photons in their turn via

bremsstrahlung radiation. Eventually, π, K and other unstable particles decay into muons and neutrinos (or electrons and neutrinos), whereas low energy

electrons lose energy via ionization without generating more photons.


Branching modelsBranching modelsAs a result, at first the particles increase in

number while their energy decreases. Eventually,

as the original energy is shared among more and

more particles, individual particles have so little

energy that they no longer produce new particles

(they arrive to the so called critical energy, Ec),

but lose energy by ionization: the shower particle

number stops increasing and gradually goes to

zero.

Longitudinal shower

distribution

After n branchings the

number of particles is

λ/2)( XXN =

Simple branching model of an air shower (Heitler, 1944)

λ=collision length

The energy per particle is )(/)( 0 XNEXE =

The number of particles

at maximum is cEEXN /)( 0max =

Then, Xmax is given by2ln

)/ln( 0max

cEEX λ=


Particles and energyParticles and energyThe growth and decline of the number of charged particles of a shower can be defined using various mathematical models. One of these is the Gaisser-Hillas profile (1977):

−

−

−=

−

λ

λ XX

XX

XXNXN

XX

max

0max

0max exp)(

0max

αsin

vertXX =

Slant depthSlant depth

parametersfreeXand 0λ

να EXNdXE += ∫∞

0

0 )(The primary energy is given by the track length integral plus the energy carried away by neutrinos:

α = energy loss per unit length per particle

First interaction

point


Shower characteristicsShower characteristicsProton induced showers have larger fluctuations than iron or photon induced

ones, and the average depth of the shower maximum is intermediate between

them. The first thing is due to the fact that a heavy primary like an iron nucleus

is viewed as a collection of independent nucleons (superposition model) and

the result of collision is similar to an average on its constituents. On the other

Eprimary=3 1020 eV

side, a photon primary produces an

e.m. shower, where the fluctuations

are reduced with respect to a hadronic

shower.

)]/(ln[ 0max cAEEX λ∝proton FI: 70 g/cm2

Fe FI: 15 g/cm2

at PeV energies

The second feature depends on the

fact that the interactions probabilities

of the nucleons in the superposition

model add, leading to a faster

development of the shower and a

somehow different formula for Xmax:


Elongation rateElongation rateThe average of Xmax is related to the primary energy. For the simple Heitler branching model, for example:

The elongation rate is different for different primaries and can be used for obtaining information on the composition of cosmic rays.

XEd

dXER 3.2

log

max ==

aEXaEXaEXEE

X c +=+=+≡= log3.2log10lnln2ln

)/ln(0

0max λ

The elongation rate is the increa-se of Xmax per decade of energy


Shower distributionsShower distributionsThe evolution of a shower is of

statistical nature, since the exact point

where a given photon materializes or a

given electron radiates, or how the

energy is shared between the two

particles produced in a single event, is a

matter of chance. One may, however,

inquire into the average behavior of

showers.

Longitudinal shower distribution

Lateral shower distribution

Three component: e.m., muonic, and hadronic


Fluorescence and Fluorescence and ČČerenkoverenkov lightlight

Moreover, as Blackett first realized in 1948, charged particles that travel faster than light in the atmosphere emit detectable Čerenkovradiation on a narrow cone around the direction of the particle. The opening angle is a function of the density of the air and, thus, of the height of emission.

A possible source of radiation, practically isotropic, from an air shower is the excitation of air nitrogen by the charged particles, mainly electrons (more correctly, it is scintillation light). First used by the experiment Fly’s Eye.


Neutrinos as universe messengersNeutrinos as universe messengers

High energy neutrino astronomy is one of the most promising research line in astroparticle physics. Similarly to photons and unlike charged cosmic rays, they keep directional information which can be used to perform astronomy. Differently from gamma rays, they are emitted only in hadronic processes and travel unimpeded to the Earth.

Vertical neutrino induced showers cannot be distinguished from ordinary CR showers. But in very inclined showers it is possible to identify different features for the different primaries.

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