Reflecting Gamma Rays w/ Electron Gas for Spacecraft

[Ryan_m_b]

In a discussion recently I came across the idea of an antimatter photon rocket, to quote http://en.wikipedia.org/wiki/Nuclear_photonic_rocket#Power_sources"

A design proposed in the 1950s by Eugen Sänger used positron-electron annihilation to produce gamma rays. Sänger was unable to solve the problem of how to reflect, and collimate the gamma rays created by positron-electron annihilation; however, by shielding the reactions (or other annihilations) and absorbing their energy, a similar blackbody propulsion system could be created. An antimatter-matter powered photon rocket would (disregarding the shielding) obtain the maximum c specific impulse; for this reason, an antimatter-matter annihilation powered photon rocket could potentially be used for interstellar spaceflight.

Looking through various sites via google I've had a few answers as to how gamma rays may be reflected, the general consensus is that it cannot practically be done (multiple layers of shielding may do so but would absorb a lot of energy) however I have come across sites that have mentioned "pure electron gas" but I can't seem to find the original source.

Does anyone know what this is and if it is realistic that it could be practically created for spacecraft propulsion? In compliance with https://www.physicsforums.com/showpost.php?p=3404598&postcount=4" I'll clarify that my knowledge of physics should best be treated as a layman with some extra knowledge. Maths I haven't studied since school and my knowledge of particle physics is simplistic (electrons orbit nuclei etc).

 

[sophiecentaur]

Actually, they make X ray (which overlaps Gamma rays on the spectrum) telescopes, using curved surfaces of (heavy) metals which reflect at a very oblique angle. So you can. in fact, get reflection and, indeed, focussing. I was pretty amazed when I first heard of that.
Not much use for the idea of propulsion, though.

 

[Ryan_m_b]

Yes I've heard that gamma rays can be reflected at extreme angles, interesting stuff! But as you point out not very useful for propulsion.

 

[mathman]

Gamma ray reflection is incoherent. The essential point is that gamma rays lose energy when undergoing any scattering, losing most at 180 deg. X-ray scattering is mostly coherent, where there is no energy loss.

 

[wolfson109]

Technically, for propulsion purposes you wouldn't need to be able to reflect the photons given off. Even if you just absorbed them you would gain a propulsive force from the radiation pressure, but the force would only be half of the maximum available from pure reflection. I don't know if that would be enough of a propulsive force for it to have an advantage over other potential forms of propulsion though.

 

[M Quack]

@mathmqn: You are talking about Compton scattering.

You can also get perfectly elastic total reflection of x-rays and gamma rays. But the surfaces have to be very very flat and the angle of incidence has to be very small.

If the mirror has a small curvature it can be used to focus the x-rays/gamma rays.

This was successfully employed at Rosat which deorbited last year. Laboratory x-rays of 20 keV and more are routinely focused with such mirrors. The higher the energy, the smaller the angle has to be, so for several 100s of keV from pair annihilation is would be exteremely difficult.

http://en.wikipedia.org/wiki/ROSAT

For propulsion this is useless, of course. You want near back-reflection for that.

 

[sophiecentaur]

Reflection involves twice the momentum change as absorption so twice the propulsive force.

 

[sophiecentaur]

The X rays hit the reflector at a ver oblique angle so they do not penetrate the surface. they consist of a portion of a paraboloid that is way up on the side and they are arranged as a 'toast rack( with several reflectors side by side. A delightful bit of invention

 

[mathman]

@mathmqn: You are talking about Compton scattering.

You can also get perfectly elastic total reflection of x-rays and gamma rays. But the surfaces have to be very very flat and the angle of incidence has to be very small.

If the mirror has a small curvature it can be used to focus the x-rays/gamma rays.

This was successfully employed at Rosat which deorbited last year. Laboratory x-rays of 20 keV and more are routinely focused with such mirrors. The higher the energy, the smaller the angle has to be, so for several 100s of keV from pair annihilation is would be extremely difficult.

http://en.wikipedia.org/wiki/ROSAT

For propulsion this is useless, of course. You want near back-reflection for that.

I don't think one should use the terms x-rays and gamma rays interchangeably. Gamma rays don't coherently scatter, x-rays do.

 

[M Quack]

Both x-rays and gamma rays are electromagnetic waves. Radiation from electronic processes are usually called x-rays, from nuclear processes are usually called gamma rays. The energy ranges overlap. Moessbauer gammas from 57Fe for example come from the nucleus, but have an energy of only 14.4 keV - a lot softer than the x-rays you receive at a medical examination.

The distinction is purely artificial.

 

[mfb]

Another issue with absorption and blackbody radiation is the involved heat, which limits the thrust. ~3000K (quite hot) gives a pressure of just ~1/20 Pa.