One type of tracking material is a spark chamber, which is a gas-filled region criss-crossed with wires. Another type of tracking material is silicon strip detectors, which consists of two planes of silicon. In one plane the strips are oriented in the "x"-direction, while the other plane has strips in the "y"-direction. The position of a particle passing through these two silicon planes can be determined more precisely than in a spark chamber.
By reconstructing the tracks of the charged pair as it passes through the vertical series of trackers, the gamma-ray direction, and therefore its origin on the sky, are calculated. In addition, through the analysis of the scattering of the pair (which is an energy-dependent phenomenon) or through the absorption of the pair by a scintillator detector or a calorimeter after they exit the spark chamber, the total energy of the initial gamma-ray is determined.
This animation shows how the Large Area Telescope on the Fermi Gamma-ray Telescope works. A gamma ray (purple) interacts with the detector, creating an electron-positron pair which cascade down the tower. Using the paths that the electron and positron take through the telescope, the direction of the original gamma-ray can be determined (shown in purple). (Credit: NASA's Goddard Space Flight Center Conceptual Image Lab)
Photo of one of the HESS telescopes. The HESS array detects Cerenkov light from high energy gamma rays entering the Earth's atmosphere. (Credit: HESS Collaboration)
While a typical gamma-ray detector must be flown with a balloon or on a satellite above the Earth's atmosphere to avoid absorption of the gamma-ray photon, the air Cerenkov telescope makes the atmosphere part of the detector. When gamma rays encounter Earth's atmosphere, they create an "air shower." This process involves the original photon undergoing a pair production interaction high up in the atmosphere, creating an electron and positron. These particles then interact, through bremsstrahlung and Compton scattering, and give up some of their energy to create energetic photons. These in turn create more electrons, resulting in a cascade of electrons and photons that travel down through the atmosphere until the particles run out of energy.
These are extremely energetic particles, which means that they are traveling very close to the speed of light. In fact, these particles are traveling faster than the speed of light "in the medium of the atmosphere." Remember that nothing can travel faster than the speed of light in a vacuum, but that the speed of light is reduced when traveling through most materials (like glass, water and air). The resultingpolarization of local atoms as the charged particles travel through the atmosphere results in the emission of a faint, bluish light known as "Cerenkov radiation", named for Pavel Cerenkov, the Russian physicist who made comprehensive studies of this phenomenon.
Depending on the energy of the initial cosmic gamma ray, there may be thousands of electrons/positrons in the resulting cascade that are capable of emitting Cerenkov radiation. As a result, a large "pool" of Cerenkov light accompanies the particles in the air shower. Air Cerenkov detectors, as the name implies, rely on the detection of this pool of light to detect the arrival of a cosmic gamma ray.
Illustration of the process of detecting a gamma ray using Earth's atmosphere. (Credit: Diagram by NASA's Imagine the Universe; telescope image from the HESS Collaboration)
Air Cerenkov detectors begin with one or many large optical reflectors, and are usually placed at mountain sites where standard optical observatories might be located. The mirrors used can be of lesser quality than those used in optical telescopes, since they are reflecting the light of this large local pool rather than directly imaging an astronomical source. The Cerenkov light reflected from this mirror is then detected in the focal plane by one or many photomultipliers that convert the optical signal into an electronic signal to record the gamma-ray event. The light in this pool is very faint and can only be detected cleanly on dark, moonless nights. Even so, it helps that the total pool passes through the detector in only a few nanoseconds. This allows further separation of the faint signal from the ambient light from the rest of the night sky.
Once the light has been detected in a phototube, fast electronics are used to record the signal. Many modern detectors use an array of 100 or more small phototubes in the focal plane rather than a single phototube. In this way, a crude image of the Cerenkov light pool is recorded. This is very important because these detectors, in addition to detecting cosmic gamma-ray photons, detect a large cosmic ray background. Cosmic ray protons and nuclei interact in the atmosphere in much the same way, creating their own Cerenkov light pools. These showers induced by cosmic rays come uniformly from all parts of the sky and mask the desired photonic signal. Less than 1% of the events detected are due to photons. The rest are cosmic rays.
Updated: October 2013
https://imagine.gsfc.nasa.gov/science/toolbox/gamma_detectors2.html
gamma ray产生原理
放射性原子核在发生α衰变、β衰变后产生的新核往往处于高能量级，要向低能级跃迁，辐射出γ光子。原子核衰变和核反应均可产生γ射线。其为波长短于0.2埃的电磁波[3] 。γ射线的波长比X射线要短，所以γ射线具有比X射线还要强的穿透能力。
伽马射线是频率高于1.5 千亿亿 赫兹的电磁波光子。伽马射线不具有电荷及静质量，故具有较α粒子及β粒子弱之电离能力。伽马射线具有极强之穿透能力及带有高能量。伽马射线可被高原子数之原子核阻停，例如铅或乏铀。
gamma ray测量方法
γ光子不带电，故不能用磁偏转法测出其能量，通常利用γ光子造成的上述次级效应间接求出，例如通过测量光电子或正负电子对的能量推算出来。此外还可用γ谱仪（利用晶体对γ射线的衍射）直接测量γ光子的能量。
由荧光晶体、光电倍增管和电子仪器组成的闪烁计数器是探测γ射线强度的常用仪器。
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