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HERMES III (High Energy Radiaton Megavolt Electron Source) at the Sandia at Sandia National Laboratories is the world's most powerful gamma ray generator. HERMES-III produces a highly energetic beam that tests the capability of electronics to survive a burst of radiation that approximates the output of a nuclear weapon. It used a field emission diode, instead of a transistor.

I wonder why most accelerators used vacuum diodes instead of transistors. Transistors are much smaller. What are the reasons?

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    \$\begingroup\$ use a vacuum diode for what exactly? Such an accelerator doesn't only contain a single component... \$\endgroup\$ – Marcus Müller Sep 7 '20 at 8:05
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    \$\begingroup\$ ...used vacuum diodes instead of transistors In some cases a solid state diode can be used instead of a non-solid state diode (like a vacuum diode). A transistor is an entirely different device so why mention the transistor? Are smaller devices always better? What if the larger device is much more rugged (doesn't break so easily)? \$\endgroup\$ – Bimpelrekkie Sep 7 '20 at 8:17
  • \$\begingroup\$ @MarcusMüller according to an article about the HERMES III, "Diode designs are needed that efficiently convert electrical energy to gamma..-ray energy and that distribute that gamma-ray energy uniformly over a large area". Are you saying that it is possible to create an accelerator without tubes or solid-state devices? \$\endgroup\$ – SnoopyKid Sep 7 '20 at 9:25
  • \$\begingroup\$ @Bimpelrekkie gotcha. So, in theory, both vacuum electronic devices and solid-state devices can actually be used in accelerators. Right? If we are using transistors to generate, say, gamma rays the circuit will be much more different than when using vacuum electronic devices. Isn't it? \$\endgroup\$ – SnoopyKid Sep 7 '20 at 9:27
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    \$\begingroup\$ You REALLY need to educate yourself a lot more, what are gamma rays? How are they generated? (not: with a gamma-radiation generating device. But: by accelerating ... and letting these hit ...., gamma radiation is generated). Why is the acceleration needed? How it the acceleration done? You just seem to want to "collect facts" which is pointless and teaches nothing. Try to understand how it works instead. \$\endgroup\$ – Bimpelrekkie Sep 7 '20 at 10:25
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I started this answer on the second question you asked on this subject, and moved it here when the other question was closed. I've tried to make it address both questions.


The machines you are looking at use the bremsstrahlung effect to generate short wavelength electromagnetic waves.

The X-rays and gamma rays are generated inside the field emission diode you mention.

The field emission diode in the particle accelerator is basically an enormous vacuum tube diode, with the plate replaced with a very hard and dense metal.

It generates electromagnetic waves via the bremsstrahlung effect. Electrons are accelerated to stupidly high velocities and slammed into the hard plate at the end of the tube.

Bremsstrahlung is generated by braking (slowing down) a charged particle.

The word "Bremsstrahlung" is German. It is composed from two words:

  1. "Bremsen" - to brake.
  2. "Strahlung" - radiation.

Literally "radiation from braking."

The velocity of the charged particle (electrons) dictates the amount of energy available, which also dictates the wavelength of the generated electromagnetic waves.

In a semiconductor, electrons move at the drift velocity of the semiconductor material. This is very slow, on the order of meters per second or less.

The low speed makes it very unlikely for any high frequency (short wavelength) waves to be generated.

In vacuum tubes, electrons can be accelerated to very high speeds.

enter image description here

The very high velocities of the electrons in a vacuum tube makes it very likely that short wavelength electromagnetic waves (X-rays) will be produced if the electrons are braked by a suitable material (tungsten is used because it is dense enough to brake the electrons efficiently and hard enough not to be eroded by the heat caused by the electron collisions.)

Note that the electron velocities of that plot are in tens of thousands of kilometers per second. They are reaching a significant portion of the speed of light.

In summary, you use vacuum tube diodes because they can accelerate electrons to high velocities and slam them into a metal plate.

Semiconductors can't be used because they don't accelerate electrons to the velocity needed, and don't have any place to "slam" the electrons.


Bremsstrahlung generates a wide spectrum of wavelengths. The peak in the spectrum depends on the electron velocity.

At low speeds, the peak is at longer wavelengths - but it is statistically possible that a high energy photon (X-ray) could be released. The incidence of such photons would be too low to make a useful (or even detectable) X-ray source.

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  • \$\begingroup\$ Thanks for the response. If you're familiar with particle accelerators especially those that generated bremsstrahlung, there are such things as small area and large area accelerators. I believe they are talking about the size of bremsstrahlung radiation, as HERMES III is described as gamma ray simulator over very large areas to irradiate military tanks. What determined the size of the radiation? Is it the size of the source (e.g. diode)? \$\endgroup\$ – SnoopyKid Oct 22 '20 at 11:26
  • \$\begingroup\$ The number of electrons. The velocity controls the wavelength, and the number of electrons controls the number of photons and therefore the "intensity" of the output. More current= higher intensity, more voltage= shorter wavelength. \$\endgroup\$ – JRE Oct 22 '20 at 11:47
  • \$\begingroup\$ I'm actually talking about the length and width of the resulting radiation. HERMES III is a gamma ray simulator that was used to irradiate very large area and the size of the test objects are about the size of a transistor to the size of military tanks. In order to generate large area bremsstrahlung radiation so it can irradiate very large areas, is the size of the source must be huge too? \$\endgroup\$ – SnoopyKid Oct 22 '20 at 12:09
  • \$\begingroup\$ Look down the page for the answer from StainlessSteelRat. There's a drawing of the business end of Hermes III. The plate is over 60 centimeters in diameter. The plate has to be large enough for enough electrons to strike it at the required current. It also has to be large because the bombardment causes erosion. And, electrons try to avoid each other - they spread out so you need a larger area to "catch" all the electrons from a high current source. \$\endgroup\$ – JRE Oct 22 '20 at 12:30
  • \$\begingroup\$ Read that paper that StainlessSteelRat linked to. \$\endgroup\$ – JRE Oct 22 '20 at 12:31
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The correlation of MV (or MeV) in HERMES III (High Energy Radiaton Megavolt Electron Source) to diodes or transistors is not appropriate.

The use of diode in the HERMES III Diode implies an anode and cathode, but it does not imply scale. Helps to understand what is happening, but has led to your confusion.

From History of HERMES III Diode to Z-Pinch Breakthrough and Beyond by Thomas Sanford.

enter image description here

Estimates indicated that HERMES III (Fig. 9) needed to produce a ~20-MeV electron beam at the target on the order of ~700-kA in ~20 ns to produce the desired radiation dose rate of ~5 Trad/s over a useful area of 500 cm2. These electrical and radiation parameters became the HERMES III design goals.

That's 20,000,000 V at 700,000 A for 20ns.

An artists rendition of the Hermes III (Referenced Fig 9). Notice the two people in front of the model.

Hermes III diode

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    \$\begingroup\$ Thanks, so the diode isn't the average vacuum tube that we used everyday then. \$\endgroup\$ – SnoopyKid Sep 7 '20 at 15:25
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    \$\begingroup\$ More like Dr. Evil (experimental physist) meets unlimited budget to deliver theoretical physics. \$\endgroup\$ – StainlessSteelRat Sep 7 '20 at 15:29
  • \$\begingroup\$ Lol, thanks once again for your response and for clearing that up! \$\endgroup\$ – SnoopyKid Sep 7 '20 at 15:30
  • \$\begingroup\$ My initial comment was to have vacuum tubes are used in radio stations for high power output, but that would of not been useful. Read the paper and my previous comment will make sense. \$\endgroup\$ – StainlessSteelRat Sep 7 '20 at 15:36
  • \$\begingroup\$ Sorry, do you mean even vacuum tubes that are used in radio stations for high power output are also different from the diode that is used in HERMES III? I'll download the paper in a minute. \$\endgroup\$ – SnoopyKid Sep 10 '20 at 8:31
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"Diode designs are needed that efficiently convert electrical energy to gamma..-ray energy and that distribute that gamma-ray energy uniformly over a large area"

For gamma ray production, semiconductors are useless. How is that even supposed to work?

Just because you can replace e.g. the design of a vacuum tube amplifier with a transistor amplifier you can't replace every tube with everything else. This isn't an amplifier application of a vacuum tube.

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  • \$\begingroup\$ Thanks Muller. So, what are the non-vacuum electronics options to generate very high energy gamma rays? \$\endgroup\$ – SnoopyKid Sep 7 '20 at 9:49
  • \$\begingroup\$ there's none. That's the point. Having a semiconductor for that isn't useful. \$\endgroup\$ – Marcus Müller Sep 7 '20 at 9:49
  • \$\begingroup\$ How about these articles then pnas.org/content/115/40/9911, arstechnica.com/science/2017/11/…, hedp.osu.edu/research/experiment/neutrongamma, advances.sciencemag.org/content/6/22/eaaz7240 and spie.org/news/…? The gamma rays are generated by lasers. \$\endgroup\$ – SnoopyKid Sep 7 '20 at 10:02
  • \$\begingroup\$ Oh I forget about what reasons semiconductors are not useful for generating gamma rays, can you tell the reasons? \$\endgroup\$ – SnoopyKid Sep 7 '20 at 10:06
  • \$\begingroup\$ @MohamedObeidallah The gamma rays are generated by lasers Not really, read the article carefully. The laser is used to accelerate electrons. What you state is incorrect. What is correct: The gamma rays are generated with the help of lasers. \$\endgroup\$ – Bimpelrekkie Sep 7 '20 at 10:20

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