B Which one is more efficient in generating high energy Gamma rays?

[Rev. Cheeseman]

TL;DR Summary

Is laser more efficient than large accelerators such as HERMES III?

According to https://apps.dtic.mil/dtic/tr/fulltext/u2/a351472.pdf the big pulsed power accelerator, HERMES III, generate electron beam with peak energy at 22 MeV and average electron energy at 16 MeV and the resulting photon energy which is bremsstrahlung radiation is approximately 2 MeV (actually 1.9 MeV) while in this link http://eprints.gla.ac.uk/80139/ this ultra compact laser driven plasma wakefield accelerator can produce more than 109 photons per pulse with a mean energy of 10 MeV. Another link https://www.nature.com/articles/s42005-018-0095-3 said laser-solid interaction generate γ-ray that reaches 0.74 PW with the brilliance of 2 × 1024 s−1 mm−2 mrad−2 (0.1%BW)−1 at 58 MeV. So, generating gamma rays using lasers is more efficient?

 

[gleem]

Summary:: Is laser more efficient than large accelerators such as HERMES III?

resulting photon energy which is bremsstrahlung radiation is approximately 2 MeV (actually 1.9 MeV)

That is the average energy of the x-rays, the maximum energy is 22MeV.

Efficiency would generally be measured as the ratio of the total energy of the useful flux of radiation to the energy used to produce this flux. Bremsstrahlung production itself is an inefficient process and the same for any method for accelerating electrons. The difference is in the energy losses due to the different methods of accelerating the electrons.

I am not very familiar with the Hermes or laser systems but let me make a few remarks. As it is presented the advantage of the laser based bremsstrahlung generator is that it is much smaller than other methods to accelerate the electrons. Linacs have losses because they use electric power to drive high frequency amplifiers to produce the electric fields for the waveguide that accelerate the electrons. Lasers must excite the lasing medium to produce the high intensity light beams that produce the electric fields. Not all this energy goes into producing the accelerating field. Their energy losses are different. Although both are pulsed systems the linac has(currently?) a higher repetition rate compared to the laser. The Los Alamos Hermes is a different beast producing a hyperintense electron pulse to produce the bremsstrahlung. You really cannot say it is a beam since its rep rate is only 7 pulses per day but each pulse produces a heck of a lot of x-rays.

So will the electric bill be less for a plasma wake field accelerator so the same thing as a linac? I do not know.

 

[Rev. Cheeseman]

That is the average energy of the x-rays, the maximum energy is 22MeV.

Efficiency would generally be measured as the ratio of the total energy of the useful flux of radiation to the energy used to produce this flux. Bremsstrahlung production itself is an inefficient process and the same for any method for accelerating electrons. The difference is in the energy losses due to the different methods of accelerating the electrons.

I am not very familiar with the Hermes or laser systems but let me make a few remarks. As it is presented the advantage of the laser based bremsstrahlung generator is that it is much smaller than other methods to accelerate the electrons. Linacs have losses because they use electric power to drive high frequency amplifiers to produce the electric fields for the waveguide that accelerate the electrons. Lasers must excite the lasing medium to produce the high intensity light beams that produce the electric fields. Not all this energy goes into producing the accelerating field. Their energy losses are different. Although both are pulsed systems the linac has(currently?) a higher repetition rate compared to the laser. The Los Alamos Hermes is a different beast producing a hyperintense electron pulse to produce the bremsstrahlung. You really cannot say it is a beam since its rep rate is only 7 pulses per day but each pulse produces a heck of a lot of x-rays.

So will the electric bill be less for a plasma wake field accelerator so the same thing as a linac? I do not know.

So if I'm not mistaken, if the laser system have a peak energy of say 1 GeV, the resulting bremsstrahlung will be 1 GeV at peak energy too. Is that correct?

If you see the isodose contours of HERMES III which can be seen in the first link in my question at page 40 especially the figure 6d, it is more like a field rather than a beam which measured 8 meters wide and 20 meters long according to the figure. The reason of why the radiation field is very large is because HERMES III is used to simulate nuclear gamma ray effects on very large test objects such as military tanks at very large open outdoor areas. I wonder what determined the size of the radiation field, is it the size of the accelerator or just the voltage and current of the diode regardless of the size of the accelerator?

 

[gleem]

So if I'm not mistaken, if the laser system have a peak energy of say 1 GeV, the resulting bremsstrahlung will be 1 GeV at peak energy too. Is that correct?

Yes but the yield will be extremely low compared too other energies.

I wonder what determined the size of the radiation field, is it the size of the accelerator or just the voltage and current of the diode regardless of the size of the accelerator?

The width of the radiation field depends on the diameter and divergence of the electron beam the distance from the conversion target.

The bremsstrahlung emitted from the target comes off at all angles and has an angular distribution that is strongly forward peaked so the radiation field is non-uniform transversely across the radiation field but radiation is being produced at all angles relative to the direction of the electron beam. As far as I can determine from the HERMES accelerator the target is very large compared to the systems that I am familiar with. The beam I expect is nearly as wide as the target. As the radiation leaves the target it diverges and expands laterally as you move away from the target. In linacs that are used to produce bremsstrahlung, the beam is mm's in diameter. To produce an isodose distribution that is constant across the field a flattening filter shaped like a cone is used to reduce the strong forward intensity of the field.

As far as I can determine HERMES does not use a flattening filter probably to maintain the high radiation field intensity that might otherwise be severely reduced by such a filter. Linac system with which I am familiar produces uniform fields up to 40cm wide at 1 m from the conversion target which can expand to 4 m wide at 10m from the target.

The size of the accelerator is determined by the size of the electron beam that must be produced to obtain the desired radiation field and the method of acceleration. Laser systems produce small electron beam diameters. Their advantage as I noted above is in their ability to produce high energy electrons in a small space.

You noted in your OP that a wake field accelerator produced 109 photons per pulse. This yield is extremely small compared to other methods of producing bremsstrahlung. Currently, the best wake field accelerators that have high yields per pulse are not useful for those applications that require high radiation fluxes since their repetition rates are too small. So replacing HERMES with a wake field accelerator is not yet feasible.

 

[Rev. Cheeseman]

Yes but the yield will be extremely low compared too other energies.

The width of the radiation field depends on the diameter and divergence of the electron beam the distance from the conversion target.

The bremsstrahlung emitted from the target comes off at all angles and has an angular distribution that is strongly forward peaked so the radiation field is non-uniform transversely across the radiation field but radiation is being produced at all angles relative to the direction of the electron beam. As far as I can determine from the HERMES accelerator the target is very large compared to the systems that I am familiar with. The beam I expect is nearly as wide as the target. As the radiation leaves the target it diverges and expands laterally as you move away from the target. In linacs that are used to produce bremsstrahlung, the beam is mm's in diameter. To produce an isodose distribution that is constant across the field a flattening filter shaped like a cone is used to reduce the strong forward intensity of the field.

As far as I can determine HERMES does not use a flattening filter probably to maintain the high radiation field intensity that might otherwise be severely reduced by such a filter. Linac system with which I am familiar produces uniform fields up to 40cm wide at 1 m from the conversion target which can expand to 4 m wide at 10m from the target.

The size of the accelerator is determined by the size of the electron beam that must be produced to obtain the desired radiation field and the method of acceleration. Laser systems produce small electron beam diameters. Their advantage as I noted above is in their ability to produce high energy electrons in a small space.

You noted in your OP that a wake field accelerator produced 109 photons per pulse. This yield is extremely small compared to other methods of producing bremsstrahlung. Currently, the best wake field accelerators that have high yields per pulse are not useful for those applications that require high radiation fluxes since their repetition rates are too small. So replacing HERMES with a wake field accelerator is not yet feasible.

Thanks for the lengthy response. I just found some articles about accelerators that generate electron beams over small areas, and the size of the vacuum window, anode, cathode, etc. are small especially according to this https://ieeexplore.ieee.org/document/1003878?reload=true&arnumber=1003878. While in this link Handbook of Radiotherapy Physics: Theory and Practice small focal spot is essential so the resulting ray can be projected into small areas. In addition, you also add that the target of HERMES III (I believe you're talking about the converter at the end of the diode) are larger than the targets of other accelerators that you are familiar with so I assumed that's the reason why the bremsstrahlung radiation fields from HERMES III are large. Even if you see the isodose contours of HERMES III for indoor cell irradiation in page 39 and 40 in the first link in my OP, the bremsstrahlung fields are still large. According to this https://www.osti.gov/servlets/purl/1390738 it can be used to irradiate test objects ranging in size from transistors to military tanks. I believe if it is operated indoor the size of the test object will be small and if it is operated outdoor the test object will be very large.

So in layman's terms, in order to irradiate very large areas, the size of the accelerator (including the size of the converter target, cathode/anode, etc.) must be large. I believe if in the future there are laser systems that can generate bremsstrahlung radiation fields with the same energy level as HERMES III over very large areas, the size of the accelerator or just some parts of it will be large in size.