Photomultiplier Tubes, or PMTs,
are electron tube devices which convert light into a measurable electric
consist of a photocathade and a series of dynodes in an evacuated glass enclosure (see figure). When photons strike the photocathode, electrons are emitted via the photoelectric effect. The electrons are focused onto the first dynode by an electric field, and when they strike the dynodes, more electrons are emitted. The current of electrons multiplies as it moves down the successive dynodes until it reaches the anode.
Summary of BooNE's PMT Specs.
Made By Hamamatsu.
|Min. Effective Area (mm)||Spectral Response Range (mm)||Peak (mm)||Photocathode Material||Window Material|
|R5912||190 dia.||300 to 650||420|| Bialkali
SbRb - Cs
The electrons are emitted from the photoemissive cathode by the photoelectric effect. When energy, in this case light, strikes the cathode, the electrons in the atoms absorb the energy and leave the energy levels of the atoms. A potential difference between the photocathode and the first dynode causes the electrons to accelerate toward the dynode and an electric field focuses them onto the dynode. For the photocathode's used in BooNE, the maximum effeciency is apporximately 420 nm, or blue light. The photocathode material is a bialkali composed of antimony an alkali metals. This forms a semiconductor material, much more favorable than metals or other photoelectric substances because semiconductors have much greater quantum effeciencies for converting a photon to a usable electron.
Electron-Optical Input System:
The electron-optical input system is responsible for collecting and focusing the electrons onto the first stage of the multiplier section. It is therefore important for the system to be capable of collecting, effeciently, as many electrons as possible and making sure that the time it takes for an emitted electron to travel from the photocathode to the first dynode is as independent as possible of the point of the electron's emission.
The focusing electrode is responsible for focusing the emitted electrons onto the first stage of the multiplier section, the first dynode. Between the photocathode and the first dynode is what's called an accelerating-electrode. The accelerating-electrode looks much like a wall with a small gate in it (noticable in the top figure). Positioned behind the gate is the first dynode. The accelerating-electrode sets up a high potential between itself and the photocathode which accelerates the emitted electrons toward the "wall." It is the job of the focusing electrode, using an electric field, to focus the accelerating electrons through the "gate" and onto the first dynode.
The electron-multiplier system amplifies the primary photocurrent using a series of dynodes to produce a measurable current at the anode of the photomultiplier. When electrons strike the dynodes, energy is transmitted into the dynode material allowing secondary electrons to escape. This is very similar to the photoelectric effect except that the photon is replaced with an electron. This process snowballs, thus creating a current. The dynode materials are also made of semiconductors and insulators. However, unlike the photocathode, the dynodes must supply a constant electric field to keep the electrons accelerating through the multiplier. Thus the secondary emission material must be deposited on a conducting material. This can be done by forming an alloy of an alkali or alkaline earth metal with a more noble metal. There are three important characteristics of a good dynode material:
1) high secondary emission factor, i.e., a high average number of secondary electrons emitted per primary electron
2) stability of secondary emission effect under high currents
3) low thermonic emission, i.e., low noise.
The PMT used by BooNE is sturctured in what is called Box and Line form with 10 stages of dynodes.
The anode is connected in series to a load resistance, the anode voltage will fall as the anode current increases. Thus a change in the potential difference between the anode and the last dynode will occur. This allows for the entire chain of dynodes to be sustained at a constant voltage. The output signal at the anode, then, is a current or charge pulse whose total charge is proportional to the initial nimber of electrons emitted by the photocathode.
Specifications and Diagrams of BooNE's Photomultiplier Tubes R5912