Fisica Nucleare e Subnucleare II · PDF filePer ÒVelaÓ, la sorgente gamma pi...

Fisica Nucleare e Subnucleare II, 2007-2008, A. Capone Lezioni 8-9 1
Fisica Nucleare e Subnucleare II
apparati rivelatori di particelle elementari
rivelazione di gamma nello spazio
rivelazione di raggi cosmici primari con
energia < 100 GeV
misura della composizione dei raggi cosmici
primari

Considerazioni preliminari
Latmosfera terrestre circa 28 r.l. e 11 lunghezze di
interazione. assorbe i gamma HE
Il flusso dei raggi gamma dalle sorgenti astrofisiche basso e
diminuisce sensibilmente con lenergia
Es. Per Vela, la sorgente gamma pi forte, !(E>100MeV)=1.3x10-5
fotoni cm-2 s-1 e lindice spettrale -1.89. Area riv. " 1000 cm2
N(E>100MeV) " 1 #/minuto; N(E>2GeV)" 1 # in 2 ore
Il flusso dei raggi cosmici carichi molto maggiore di quello
gamma :
11-1-276.26TeVsrscm109
!!!"#
TeVE
dE
dN

Energy range Name Technique
10-30 MeV Medium Satellite
30Mev-30Gev High Energy (HE) Satellite
30 GeV - 30 TeV Very High Energy (VHE) Cerenkov Array (g.b.)
30 Tev - 30 PeV Ultra High Energy (UHE) Ground Based Array
30 Pev -> Extremely High Energy (EHE) Ground Based
Nomenclatura usata in astrofisica dei raggi gammaNomenclatura usata in astrofisica dei raggi gamma

CGRO (1991- 2000)CGRO (1991- 2000)
Compton Gamma Ray Observatory

Compton Gamma Ray Observatory
Since the number of gamma-ray photons from celestial
sources is very small compared to the number of optical
photons, large instruments are needed to detect a significant
number of gamma rays in a reasonable amount of time. The
combination of these instruments can detect photon energies
from 20 thousand electron volts (20 keV) to more than 30
billion electron volts (30 GeV).
Oriented
Scintillation
Spectrometer
Experiment50 KeV-10 MeV
Burst
And
Transient
Source
Experiment
These four instruments are
much larger and more sensitive
than any gamma-ray telescopes
previously flown in space. The
large size is necessary because
the number of gamma-ray
interactions that can be
recorded is directly related to
the mass of the detector.
1-30 MeV
20 MeV - 30 GeV
20 KeV - 1 MeV

CGRO - BATSE
At the heart of the BATSE detectors are NaI
crystals which produce a flash of visible light
when struck by gamma rays. The flashes are
recorded by light-sensitive detectors whose
output signal is digitized and analyzed to
determine the arrival time and energy of the
gamma ray which caused the flash. Each
BATSE detector unit consists of a large area
detector sensitive to faint transient events
along with a smaller detector optimized for
spectroscopic studies of bright events.
The Burst And Transient Source Experiment (BATSE) serves as the all-sky monitor for
the Compton Observatory, detecting and locating strong transient sources called gamma-
ray bursts as well as outbursts from other sources over the entire sky. There are eight
BATSE detectors, one facing outward from each corner of the satellite, which are
sensitive to gamma-ray energies from 20 keV to 1 MeV.

CGRO - OSSE
The Oriented Scintillation Spectrometer
Experiment (OSSE) consists of four NaI
scintillation detectors, sensitive to energies
from 50 keV to 10 MeV. Each of these
detectors can be individually pointed. This
allows observations of a gamma-ray source
to be alternated with observations of nearby
background regions. An accurate subtraction
of background contamination can then be
made.
The OSSE instrument has produced observations of the energy spectrum of
nuclear lines in solar flares, the radioactive decay of nuclei in supernova remnants,
and the signature of matter-antimatter (electron-positron) annihilation in the
Galactic center region.

COMPTEL's upper layer of detectors are filled with a liquid scintillator which scatters an
incoming gamma-ray photon according to the Compton Effect. This photon is then
absorbed by NaI crystals in the lower detectors. The instrument records the time, location,
and energy of the events in each layer of detectors which makes it possible to determine
the direction and energy of the original gamma-ray photon and reconstruct an image and
energy spectrum of the source.
The Imaging Compton Telescope
(COMPTEL) utilizes the Compton
Effect and two layers of gamma-ray
detectors to reconstruct an image of
a gamma-ray source in the energy
range 1 to 30 million electron volts
(MeV). Gamma rays from active
galaxies, radioactive supernova
remnants, and diffuse gamma rays
from giant molecular clouds can be
studied with this instrument.
Il "telescopio" Compton: COMPTEL

Fotoelettrico in Piombo
Compton

Rivelatori su satellite
- Rivelano il fotone primario
- Anticoincidenza
- Tracking detector
- Calorimetro
Area Efficace
convoluzione dellarea
geometrica con la efficienza di
rivelazioneCARATTERISTICHE DI EGRET
E! " E/E "# (FWHM) Aeff (cm2)(MeV) FWHM gradi
100 26% 5.5 930
500 20% 2 1570
1,000 19% 1.2 1300
10,000 26% 0.4 690
TE
NEA
)()(eff
!=
CGRO - EGRET

EGRET lo strumento di maggiore sensibilit e vola a
bordo del Compton Gamma Ray Observatory (CGRO)
dal 5 aprile 1991.
EGRET formato da una spark chamber multistrato con
convertitori al tantalio.
Lenergia viene misurata da un calorimetro a Ioduro di
Sodio di 8 lunghezze di radiazione
Un sistema di tempo di volo seleziona il verso di
propagazione delle particelle entranti nell'apparato
Il flusso minimo di fotoni di energia maggiore di 100 MeV,
rivelabile da EGRET circa 5x10-8 fotoni/cm2/s in una
presa dati di due settimane.
CGRO - EGRET

The Energetic Gamma Ray Experiment
Telescope (EGRET) provides the highest
energy gamma-ray window for the Compton
Observatory. Its energy range is from 20
million electron volts (20 MeV) to 30 billion
electron volts (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 blazars.
The EGRET instrument produces images at these energies (20 MeV- 30 GeV) using high-
voltage gas-filled spark chambers. High-energy gamma rays enter the chambers and produce
an electron-positron pair of particles which cause sparks. The path of the particles is
recorded allowing the determination of the direction of the original gamma ray. The particle
energies are recorded by a NaI crystal beneath the spark chambers providing a measure of
the original gamma-ray energy.
CGRO - EGRET

EGRET sky map E# >100 MeV
The EGRET all-sky map shows an image of the sky at gamma-ray energies above 100
MeV in Galactic coordinates. The diffuse emission, which appears brightest along the
Galactic plane, is primarily due to cosmic-ray interactions with the interstellar medium.
The Vela, Geminga and Crab pulsars are clearly visible as bright knots of emission in the
Galactic plane in the right portion of the image. The blazar 3C279 is seen as the brightest
knot of emission above the plane. This map was produced by combining EGRET
observations from the first year of Compton Observatory operations.

CRAB and GEMINGA
This EGRET image shows Geminga
(above and left of center) and the
Crab nebula (below and right of
center), two bright sources of
gamma rays in the direction of the
Galactic anticenter. Both Geminga
and the Crab contain pulsars which
pulse at gamma-ray energies.
Strikingly, Geminga mainly pulses in the X-ray and gamma-ray regimes with very
little pulsation at other wavelengths, while the Crab pulsar is observed to generate
optical and radio pulses as well. The faint source visible just below center is the
active galaxy designated PKS 0528+134.

Distribuzione spettrale dei Raggi Cosmici
Distribuzione in energia dei
raggi cosmici osservati da
diversi esperimenti - Simon
Swordy - Universit di
Chicago
Misure dirette della rad. cosmica primaria
nello spazio (satelliti)
nellalta atmosfera (palloni)
Misure indirette
sciami estesi
Cherenkov in aria
rivelatori a Terra
laboratori sotterranei
~ 1000 particelle/(sm2)
nuclei ionizzati:
90% protoni
9% particelle !
nuclei pi pesanti
quale e lorigine dei raggi cosmici ?
nel sistema solare ? una piccola quantit
associata a

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