Photomultiplier Tubes (PMTs) for High Energy Physics

With regard to the goals and objectives of high-energy physics, one of the priority techniques for particle detection with fast detectors can be the use of Cherenkov detectors based on quartz or glass emitters in direct optical contact with photomultiplier tubes (PMTs). Such PMTs are produced by VTC Baspik, namely Topaz-M and Sapphire-2AM.

PMTs most adapted for the detection of signals with high temporal resolution are PMTs with linear-focused, grid, metal-channel and microchannel dynode systems, but only PMTs with grid type dynode systems and microchannel plates can operate correctly in magnetic fields above 100 mT without the need for additional large-scale magnetic shields.

Topaz PMT has time resolution of ~ 1ns, which in combination with the selection of optimal scintillator will allow to use it for the detection of radiation in different kinds of experiments at accelerators of elementary particles. In addition to the intrinsic time resolution of the PMTs the resulting timing response of the detector is also greatly influenced by the scintillator parameters, including its geometry, decay time and spectrum. The best to date scintillators for timing applications have a decay time of 0.8 ns – 1.4 ns (the fast component of BaF2 crystal and the plastic scintillator EJ-228, respectively), but their maximum characteristic dimensions usually do not exceed 10 cm, so in many cases it is more appropriate to use emitters of Cherenkov radiation which is the radiation generated when a charged particle moves in a medium at a velocity exceeding the speed of light propagation in that medium. The fundamental limit for the duration of a Cherenkov flare is only in the range of few femtoseconds, while the practical limit for the time resolution of the vast majority of Cherenkov detectors is determined by the emitter geometry and size, and the photodetector intrinsic time resolution based on photon statistics limited by the intensity of the Cherenkov flare.

When it is necessary to detect particles in a large range of angles and/or several particles simultaneously, such detectors as RICH (Ring Imaging Cherenkov) are used. If the trajectory of the charged particle is not parallel to the counter Z axis, but has angular coordinates ϴр, φр, with respect to it, then the ring in which the Cherenkov light is collected in the focal plane of the spherical mirror will be displaced: its center will have polar coordinates r0 = R/2×tgϴр, φ0р

Thus, by determining the center position and the radius of the Cherenkov ring it is possible not only to measure the velocity of the particle, but also to find the angular coordinates of its trajectory, which is implemented in RICH counters. Instead of a ring chart, sensors capable of effectively detecting individual photons and measuring their coordinates, such as photomultipliers with a small photocathode diameter, are placed in the focal plane.

Later, IHEP (Institute of High Energy Physics) designed and manufactured a 10-meter RICH counter with an array of 2848 PMTs for experiments at the Fermi National Accelerator Laboratory in the United States. The counter was filled with neon at a pressure of 1 atmosphere. It enabled separation of pions and kaons up to energies of 185 GeV and pions and protons up to 320 GeV.

The only method for γ – quanta and one of the main ones for electrons to determine the energy in the region of more than several GeV is the method of total absorption of electromagnetic showers created by them in the detector material. Having traveled about 2 cm, the γ-quantum will be converted into an electron and a positron, which in turn, will lose about half the energy for the bremsstrahlung radiation at the same distance of about 2 cm. Bremsstrahlung photon having traveled another 2 cm, will generate new pairs, etc.  The electrons, positrons, and γ-quanta formed in the matter will constitute an electromagnetic shower. Naturally, shower electrons and positrons emit Cherenkov light in the glass. The total track length of all electrons and positrons in the shower and therefore the number of Cherenkov photons is proportional to the energy of the primary particle. Thus, by measuring the intensity of a Cherenkov flare in a thick block of glass using a photomultiplier tube, it is possible to determine the energy of the particle that generated the shower.

An important factor when working in the field of high-energy physics is the radiation resistance to hard radiation. MCP-PMTs have a high radiation resistance as compared to semiconductor photomultipliers and are capable of withstanding high doses of irradiation without a significant decrease in performance.