The sphere must be almost completely covered by photomultipliers to capture the faint flashes of light.
As the name indicates, a photomultiplier ‘’multiplies’’ weak electrical signals generated by faint light signals making them measurable. It is composed of a vacuum tube containing different types of electrodes. A photon that hits the photomultiplier causes the release of an electrone, which is then accelerated by an electric field and triggers a cascading multiplication process that generates hundreds of millions of electrons. These produce then a measurable electric signal.
Photomultipliers are commonly used in particle physics experiments, but they are also widely used in astronomy, in medical imaging, and even in film production to convert films into digital videos. A photomultiplier basically captures a glow of light - even a single photon – and converts it into an electric signal of proportional intensity. The operating principle of these detectors is based on two physical phenomena: the photoelectric effect and the secondary emission. The first consists of an emission of electrons from the surface of any material when this surface is hit by light. Historically it was a very important phenomenon because the explanation put forward by Albert Einstein - which earned him the Nobel Prize - helped to clarify the nature of light and develop Quantum Mechanics, the theory that describes the behavior and interactions between matter and radiation. In particular, from the photoelectric effect, it is clear that light is made of energy packages (quanta), i.e. the particles we call photons. The second effect, the secondary emission, is a phenomenon that is observed when an electron hits an electrode causing the emission of other electrons. The electrode in which an electron multiplication occurs is called a dynode.
A photomultiplier consists of a vacuum glass tube that contains three different types of electrodes: a photocathode, several dynodes (usually a dozen) and an anode (Fig.1). The photocathode is placed at the entrance of the tube and is made of a material that, when hit by a photon, emits one or more electrons due to the photoelectric effect. These electrons - called primaries - are directed towards a series of dynodes placed in succession with the purpose of multiplying the electrons. The first dynode has a more positive potential than the photocathode, therefore the primary electron accelerates towards it. Here the secondary emission takes place: the dynode hit by electrons emits other electrons. These electrons, called secondary electrons, are accelerated towards the next dynode, which is more positive than the first. This process is repeated in all the dynodes: the secondary emission multiplies the number of electrons and the electric field accelerates them. The cascade of electrons produced is collected by the anode (the electrode with the most positive potential placed at the end of the tube) generating a detectable electric pulse. A succession of dynodes creates an exponential multiplication of the number of electrons: starting from a single primary electron produced by the impinging light it is possible to obtain hundreds of millions of secondary electrons. A photomultiplier, as its name suggests, is a signal amplifier.
Fig. 1 Scheme of a photomultiplier. In red the initial photon, in blue the primary electron and the secondary electrons. The empty rectangles at the bottom represent resistors placed in the electrical circuit coupled to the dynodes to increase their electric potential in sequence. (Credits: Wikimedia Commons)
Fig.2 Side and front view of a photomultiplier of the model used in the Borexino experiment with LNGS. (Credits: Steve E. Hardy, PhD Thesis)
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