22
P. A. Caraveo
Cocconi [2]. Unfortunately, detecting and studying the spectrum and the composition of the cosmic radiation could not provide clues on its sources since cosmic
magnetic fields, known to exist both at the interstellar and intergalactic scale, deviate
the particles’ paths. To study the origin of the very-high-energy particles, Morrison
in a seminal paper [3] proposed to focus on gamma-rays, the highest energy photons
of the electromagnetic spectrum which are certainly produced when high-energy
cosmic rays interact with interstellar matter and travel unaffected by interstellar and
intergalactic magnetic fields. Moreover, the same processes responsible for the acceleration of cosmic rays could also yield high-energy gamma-rays which could be used
to pinpoint their enigmatic sources. Since high-energy photons are absorbed by the
atmosphere, Morrison proposed to use balloon-borne detectors, the state of the art
technology at the time, prior to the advent of space instruments. At the same time,
Cocconi, already familiar with particle induced air showers proposed to exploit the
same technique to measure TeV gamma-rays from the Crab Nebula. Indeed, while
Cocconi proposed to use an array of particle detectors to measure very high-energy
photons able to produce secondary particles energetic enough to reach the ground,
others were investigating methods to see the vast majority of air showers that stop in
the atmosphere.
In fact, when entering our atmosphere, particles as well as gamma-ray photons
interact with its nuclei and produce a shower of highly energetic secondary particles
moving with a speed greater than that of light in air (even if their velocity is lower than
the speed of light in vacuo). In 1934, the Russian physicist Pavel Cherenkov, working
at a particle accelerator, noted that this phenomenon produced bluish luminescence,
conceptually similar to the sonic boom in air. It is a very short-lived emission (few
billionths of a second) and a very faint one (less than 1/10,000 of the average night sky
background, but, at nanosec time-scale, the flash exceeds the night sky background).
Since high-energy photons and charged particle interact with the atmosphere in a
similar way, one must find ways to discriminate showers induced by photons from
showers induced by charged particles. Luckily, showers originating from hadrons
can be discriminated from electromagnetic ones using their shape since gammaray induced cascades are much narrower. The dominant multiplication processes
in electromagnetic showers is electron/positron bremsstrahlung, producing gamma
rays and electron–positron pairs from gamma-ray conversion. Hadronic showers are
wider owing to more complex cascading process involving also pions and muons
which render the cascade less symmetric. Moreover, the flashes of Cherenkov light
provide a distinctive signature that makes it possible to indirectly reconstruct the
energy and arrival direction of the incoming very high-energy photons.
The very first Cherenkov instrument was built in 1953 by Galbraith and Jelley.
It was a search-light mirror viewed by a photomultiplier installed in a garbage can
which provided protection against stray light [4]. Such pioneering set-up (and those
which followed with bigger mirrors and more photomultipliers) detected fast flashes
of radiation, showing that the technique was a promising one, but failed to achieve
significant results. Indeed, the coming of age of the Cherenkov technique required
decades of hardware and software development, as described in [5].
P. A. Caraveo
Cocconi [2]. Unfortunately, detecting and studying the spectrum and the composition of the cosmic radiation could not provide clues on its sources since cosmic
magnetic fields, known to exist both at the interstellar and intergalactic scale, deviate
the particles’ paths. To study the origin of the very-high-energy particles, Morrison
in a seminal paper [3] proposed to focus on gamma-rays, the highest energy photons
of the electromagnetic spectrum which are certainly produced when high-energy
cosmic rays interact with interstellar matter and travel unaffected by interstellar and
intergalactic magnetic fields. Moreover, the same processes responsible for the acceleration of cosmic rays could also yield high-energy gamma-rays which could be used
to pinpoint their enigmatic sources. Since high-energy photons are absorbed by the
atmosphere, Morrison proposed to use balloon-borne detectors, the state of the art
technology at the time, prior to the advent of space instruments. At the same time,
Cocconi, already familiar with particle induced air showers proposed to exploit the
same technique to measure TeV gamma-rays from the Crab Nebula. Indeed, while
Cocconi proposed to use an array of particle detectors to measure very high-energy
photons able to produce secondary particles energetic enough to reach the ground,
others were investigating methods to see the vast majority of air showers that stop in
the atmosphere.
In fact, when entering our atmosphere, particles as well as gamma-ray photons
interact with its nuclei and produce a shower of highly energetic secondary particles
moving with a speed greater than that of light in air (even if their velocity is lower than
the speed of light in vacuo). In 1934, the Russian physicist Pavel Cherenkov, working
at a particle accelerator, noted that this phenomenon produced bluish luminescence,
conceptually similar to the sonic boom in air. It is a very short-lived emission (few
billionths of a second) and a very faint one (less than 1/10,000 of the average night sky
background, but, at nanosec time-scale, the flash exceeds the night sky background).
Since high-energy photons and charged particle interact with the atmosphere in a
similar way, one must find ways to discriminate showers induced by photons from
showers induced by charged particles. Luckily, showers originating from hadrons
can be discriminated from electromagnetic ones using their shape since gammaray induced cascades are much narrower. The dominant multiplication processes
in electromagnetic showers is electron/positron bremsstrahlung, producing gamma
rays and electron–positron pairs from gamma-ray conversion. Hadronic showers are
wider owing to more complex cascading process involving also pions and muons
which render the cascade less symmetric. Moreover, the flashes of Cherenkov light
provide a distinctive signature that makes it possible to indirectly reconstruct the
energy and arrival direction of the incoming very high-energy photons.
The very first Cherenkov instrument was built in 1953 by Galbraith and Jelley.
It was a search-light mirror viewed by a photomultiplier installed in a garbage can
which provided protection against stray light [4]. Such pioneering set-up (and those
which followed with bigger mirrors and more photomultipliers) detected fast flashes
of radiation, showing that the technique was a promising one, but failed to achieve
significant results. Indeed, the coming of age of the Cherenkov technique required
decades of hardware and software development, as described in [5].