How would you like to own a 335-page book on the physics and engineering of electron photoinjectors? For free!
That is what you will get if you click on the link. If you are ever interested in electron accelerators, especially at the "birthing" end where the electrons are generated and given the initial acceleration, this is the review book to get. It explores not only the engineering aspect of the photoinjectors, but also the physics of photocathodes, and what makes a good photocathode for accelerator applications.
Highly recommended.
Zz.
Showing posts with label photocathode. Show all posts
Showing posts with label photocathode. Show all posts
Tuesday, April 08, 2014
Tuesday, March 18, 2014
Single-Photon Detectors
This topic came up a few times during the past month in online discussions and with a few people that I've met. Most of these were in context with the photon detectors used in the EPR-type experiments, but a few came up due to the photon detectors used in detecting Cerenkov light from neutrino experiments.
A lot of people are confused with, and misinterpret, the meaning of "single-photon detectors". Most of them who are not familiar with it think that such detectors can detect every single photons that the detector comes in contact with, i.e. if there's a photon hitting a detector, it will detect it.
This is false. A single-photon detector is sensitive down to detecting single photons. So this is a sensitivity issue. However, it doesn't mean that it has a 100% efficiency. It doesn't detect every single photons that it encounters.
A photodetector such as a photomultiplier tube used in many photon detector is often made up of a photocathode (it converts the incoming photon into a photoelectron), an electron amplifier (something that multiply that single photoelectron into many electrons), and a signal generator/converter that converts the many electrons into an electrical signal. This is what we eventually detect in our electrical signal.
The problem here is that the photocathode does not have a 100% quantum efficiency. In fact, most photocathodes used in photodetector tubes have quantum efficiency less than 50%. What this means that if 100 photons hit the photocathode, less than 50 of them will be successful in generating a photoelectron each. The rest of the photons that hit the photocathode will generate no photoelectron and are lost.
So while the detector is sensitive down to the single-photon level, it is not 100% efficient. Single-photon detectors refer to the sensitivity, not the efficiency, of the detectors.
Zz.
A lot of people are confused with, and misinterpret, the meaning of "single-photon detectors". Most of them who are not familiar with it think that such detectors can detect every single photons that the detector comes in contact with, i.e. if there's a photon hitting a detector, it will detect it.
This is false. A single-photon detector is sensitive down to detecting single photons. So this is a sensitivity issue. However, it doesn't mean that it has a 100% efficiency. It doesn't detect every single photons that it encounters.
A photodetector such as a photomultiplier tube used in many photon detector is often made up of a photocathode (it converts the incoming photon into a photoelectron), an electron amplifier (something that multiply that single photoelectron into many electrons), and a signal generator/converter that converts the many electrons into an electrical signal. This is what we eventually detect in our electrical signal.
The problem here is that the photocathode does not have a 100% quantum efficiency. In fact, most photocathodes used in photodetector tubes have quantum efficiency less than 50%. What this means that if 100 photons hit the photocathode, less than 50 of them will be successful in generating a photoelectron each. The rest of the photons that hit the photocathode will generate no photoelectron and are lost.
So while the detector is sensitive down to the single-photon level, it is not 100% efficient. Single-photon detectors refer to the sensitivity, not the efficiency, of the detectors.
Zz.
Monday, October 24, 2011
Random Physics Picture
Inside the window on the 6-way cross is a molybdenum plug. The left, flat surface of the plug is the newly-deposited cesium telluride photocathode. This plug will be transferred, under ultrahigh vacuum, to an accelerator photoinjector, where it will become the source of electrons for a research linear accelerator. The "spring" that you can see imbedded around the plug is to enhance electrical contact of the plug to the wall of the photoinjector.
Zz.
Monday, May 21, 2007
Cathodes for Photoinjectors
A while ago, I did a table listing various "family" or types of photocathodes that are used or might be used in an accelerator photoinjector. It was one of the first thing that I did after I changed field and went into accelerator physics. The table made its debut at our Theory Institute workshop on high brightness photoinjector, and it has appeared in the workshop's White paper, but also in several other presentations. So I consider it as the very first impact that I had on a field that I was still trying to learn.
The table is quite brief and obviously omits a lot of other material. I'm working on expanding it to include as many photocathode material that I can find that are either suitable, or being considered as viable candidates for this application. Just because something can photoemit, it doesn't mean it can automatically be used in a photoinjector setting, especially an RF photoinjector. The material must be robust because it will be under high, alternating gradient (often up to 90 MV/m peak field), and it must be able to withstand back bombardment of electrons that are sloshing in the photoinejctor cavity, and must be able to perform for months without significant degradation. All of these are strict requirements of a photocathode for an accelerator photoinejector.
So here is the table that I made years ago. Hopefully, an updated version will be out soon.
Zz.
The table is quite brief and obviously omits a lot of other material. I'm working on expanding it to include as many photocathode material that I can find that are either suitable, or being considered as viable candidates for this application. Just because something can photoemit, it doesn't mean it can automatically be used in a photoinjector setting, especially an RF photoinjector. The material must be robust because it will be under high, alternating gradient (often up to 90 MV/m peak field), and it must be able to withstand back bombardment of electrons that are sloshing in the photoinejctor cavity, and must be able to perform for months without significant degradation. All of these are strict requirements of a photocathode for an accelerator photoinejector.
So here is the table that I made years ago. Hopefully, an updated version will be out soon.
Zz.
Tuesday, April 03, 2007
Modern Theory and Applications of Photocathodes
I wrote a while back on the legacy of Bill Spicer, a name that you may not have heard, but whose influence probably have affected your life in some way. One of such ways is the scenario where there are just some papers, no matter how old they are, that continue to be used, cited, and scrutinized. These papers continue to be highly relevant even today.
One such paper is this one, published in 1993, and written by Spicer and Herrera-Gomez. It deals with the physics of photocathodes. For those of us in the accelerator physics field, this paper continues to be cited and studied when we deal with photocathodes for particle accelerators. Other than Sommer's Photoemissive Material text, this paper is a tour de force in almost everyone one needs to know about basic photoemission (not angle-resolved or resonant etc.) processes that are relevant in a photoinjector.
As I've said before, this is roughly what Spicer started with when he delved into photoemission physics. It is fitting that, to this day, his work still has relevance in that field of study.
Zz.
One such paper is this one, published in 1993, and written by Spicer and Herrera-Gomez. It deals with the physics of photocathodes. For those of us in the accelerator physics field, this paper continues to be cited and studied when we deal with photocathodes for particle accelerators. Other than Sommer's Photoemissive Material text, this paper is a tour de force in almost everyone one needs to know about basic photoemission (not angle-resolved or resonant etc.) processes that are relevant in a photoinjector.
As I've said before, this is roughly what Spicer started with when he delved into photoemission physics. It is fitting that, to this day, his work still has relevance in that field of study.
Zz.
Friday, March 30, 2007
It's Shiny and Glittery, and Covered With Aluminum Foil!
One of the most common comment that I get when I show our deposition system to visitors is "What's with all the aluminum foil?" In fact, as you can see from the picture, you can't even see the vacuum chamber since it is almost completely covered by it.
Anyone who has done any work in trying to get to ultra-high vacuum (UHV) can tell you that one of the things we have to do after we vent and close our system is to do a "bake-out" during pumping. While all the vacuum pumps are running, we want to reach the vacuum system to above 100 C to drive out moisture and gases that may have been absorbed (or adsorbed) into the walls of the vacuum system. So the outer part of the vacuum chamber is often wrapped with heating tapes. To reduce heat loss and increase the heating efficiency, we also cover the vacuum chamber with aluminum foil. Thus, the presence of aluminum foil in many systems that go down to UHV level.
For the system in the picture, we usually do a bake-out for about 2 to 3 days, and after cooling, we can consistently get down to as low as 2.5 x 10^-10 Torr using a combination of ion and turbo pumps. Since we do have to vent the system pretty often, it is a lot easier to just leave the heating tapes and the aluminum foil "wrappings" on the chamber. So we haven't actually seen the whole chamber underneath all that aluminum foil for about a year (I'm guessing it's still there!).
Zz.
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