If it is too thick, more photons will be absorbed, but fewer electrons will be emitted from the back surface however, if it is too thin, too many photons will pass through without being absorbed. The cathode is key, of course, and its design and thickness involve tradeoffs. In contrast, the PMT is a relatively low-noise, low-dark current device.Ī: Quantum efficiency is the percentage of impinging photons that are converted to electrons at the cathode a typical value is about 30%. Q: Why not just take the small current developed at the cathode, and amplify it using an electronic amplifier?Ī: Again, it’s a matter of noise the photon-derived current is so small that it would be “swamped” by the noise of any electronic amplifier and thus be lost. As in most applications with low signal levels, the SNR of the first stage limits performance, and it’s critical to have a high-gain, low-noise first stage. A high-end PMT has the lowest dark current of any photodetectors, with its dark current is due to thermal emissions of electrons from the photocathode, leakage current between dynodes, and even stray high-energy radiationĪlways keep in many that in many high-end designs from DC to RF (GHz and higher), internal noise is one of the limiting factors in the achievable performance. Dark current establishes a noise floor for the device, and the lower the dark current is, the better. Q: Is noise the figure of merit for a PMT (or other photodetector)?Ī: Noise is one, of course, as is gain, But the other figure of merit is “dark current,” which is the amount of current produced when there are no incident photons. It will actually degrade due to the amplifier’s own added noise.įig 1:The gain a of PMT is largely related to the number of dynodes, among other factors. While the apparent gain can be further increased by following it with a conventional low-noise electronic amplifier, the intrinsic noise will be “gained up “as well, and so SNR will not improve. In contrast, the gain of the basic APD is fairly low, the order of 10 2. The PMT has the highest internal gain with the lowest internal noise – analogous to SNR – of the available options.Ī: It varies, but gain is largely a function of the number of dynodes (Figure 1) and is on the order of 10 5 to 10 6 and higher. However, each of these has issues, primarily related to internal (intrinsic) noise and sensitivity, that makes each less suitable for some applications. These alternatives include the silicon photomultiplier (SiPM), the avalanche photodiode (APD, and the single-photon avalanche photodiode (SPAD). Q: Are there solid-state functional equivalents to the PMT?Ī: Of course there are, and they work quite well in many applications. ![]() ![]() This part looks into other PMT characteristics. Part 1 of this article explained the basics of the photomultiplier tube. My English is poor.We live in a world of solid-state active devices that have made most vacuum tubes obsolete, but a few types remain as they are still indispensable. If you copy only the box of circuit, the Child sheet will be empty). I didn't find a way how to get it in to library to use it in other projects, but you can always copy it (Parent and Child sheet. You can continue with building circuit in the parent sheet. You have to connect them by the wire as any other device. There you need to use INPUT and OUTPUT from the Terminals Mode (the icon right under the Subcircuit Mode icon in the first picture) and name them exactly the same as ports in the previous file. Then click on the box and choose "Goto Child Sheet". ![]() Give name to all ports by double clicking or using context menu and properties. You choose "INPUT", click on your box, where you want ports to be. In the list you now have highlighted "DEFAULT". Then click on the screen, drag and create a box you want your device to look like.
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