Questions about radiation

[startanewww]

1) Why can hitting a heavy metal target with high-speed electrons produce X-rays?

 

2) Alpha particles have a larger mass and lower speed. Thus, they are more likely to knock electrons out of air molecules. Why? Is it about momentum? I thought particles with high speed can knock electrons out of air molecules easily.

 

3) Can we completely block gamma radiation? My teacher said that we can't. A piece of lead of 25mm can only reduce the strength by 2 times. If you add one more lead, then the strength will be reduced by 4 times. This goes on and on and then eventually the strength is still not zero. Is he incorrect? I think we can come up the number of leads that are used to block gamma radiation. (Am I right?)

 

4) I found on the Internet that "lead is good for shielding against X-rays, gamma rays because lead has lots of electrons the rays can interact with". Then what about those elements that have high atomic numbers? Are they really good at shielding?

 

Thank you so much for your help.

[swansont]

1) Why can hitting a heavy metal target with high-speed electrons produce X-rays?

 

2) Alpha particles have a larger mass and lower speed. Thus, they are more likely to knock electrons out of air molecules. Why? Is it about momentum? I thought particles with high speed can knock electrons out of air molecules easily.

 

3) Can we completely block gamma radiation? My teacher said that we can't. A piece of lead of 25mm can only reduce the strength by 2 times. If you add one more lead, then the strength will be reduced by 4 times. This goes on and on and then eventually the strength is still not zero. Is he incorrect? I think we can come up the number of leads that are used to block gamma radiation. (Am I right?)

 

4) I found on the Internet that "lead is good for shielding against X-rays, gamma rays because lead has lots of electrons the rays can interact with". Then what about those elements that have high atomic numbers? Are they really good at shielding?

 

Thank you so much for your help.

 

1. There are two sources. One is the acceleration of the electrons; any acceleration of a charge produces EM radiation, in proportion to the acceleration. A tiny electron hitting a massive atoms gives you a large acceleration. This is Bremsstrahlung, German for "braking radiation". The second way is that the electron can ionize an inner electron from the atom. Recombination can give off x-rays

 

2. Usually this is discussed in the context of being more likely in a certain volume. Yes, it's about momentum and also the time spent interacting. Slow means more time, and thus an interaction is more likely. The large mass difference means the alpha continues on its path, almost undeflected. An electron will zig-zag. An alpha can deposit a large fraction of its initial energy in a very small volume.

 

3. Your teacher is right. The attenuation is an exponential decay with distance. It never goes to zero — there's always some amount of gamma rays that can make it through.

 

4. High atomic numbers are generally good for that reason, and Pb has Z = 82. Anything heavier than lead is radioactive (though Bismuth is only marginally so). Having a shield that is radioactive is kind of self-defeating.

[Enthalpy]

2) The slower alphas have more time to eject an electron from the molecule. They also act by two protons.

 

3) Yes and in that, photons differ radically from electrons and ions, which have a finite (though somewhat variable) path in matter. It's a serious annoyance, because when the gamma flux vastly exceeds the acceptable, the thickness increases correspondingly.

 

To protect electronics, cameras or worse, humans in a space vessel, one has to stop a few extremely energetic particles, which may prove impossible within possible mass, say for a 10 months travel to Mars. Worse: stopping the protons creates gammas which cannot be stopped easily.

 

Or think at radioisotope batteries. Everyone designer of such generator would love to use waste from uranium reactors, especially ⁹⁰Sr. But its child, ⁹⁰Y, emits a gamma in one disintegration over many, so shielding is difficult and costs thick lead, prohibitively heavy on a spacecraft, especially because these ugly poisons shall please be shielded on an individual pellet basis, not as a whole generator that would break apart in an accident. Result: the only source is ²³⁸Pu, a pure alpha emitter, which must be produced on demand, and is scarce and expensive.

 

4) There are more fundamental reasons favouring heavy elements against X and gammas. Against X, the absorption by a deep electron, typically 1s, removes like 90keV at once and with a high probability. Against gammas over 1022keV, the strong electric field near a heavy nucleus favours the creation of electron-positon pairs, which is an efficient absorption process.

 

So much that a shield equally efficient is lighter if using a heavier element - which the density of electrons wouldn't explain.

 

Beyond lead, you have thorium and natural or depleted uranium, which are very little radioactive and are used (infrequently) as shields.

 

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Complete absorption curves of photons, electrons, protons, alphas... for every element and a few compounds are available at Nist's website, really nice of them.

Edited October 23, 2013 by Enthalpy

[startanewww]

 

1. There are two sources. One is the acceleration of the electrons; any acceleration of a charge produces EM radiation, in proportion to the acceleration. A tiny electron hitting a massive atoms gives you a large acceleration. This is Bremsstrahlung, German for "braking radiation". The second way is that the electron can ionize an inner electron from the atom. Recombination can give off x-rays

 

2. Usually this is discussed in the context of being more likely in a certain volume. Yes, it's about momentum and also the time spent interacting. Slow means more time, and thus an interaction is more likely. The large mass difference means the alpha continues on its path, almost undeflected. An electron will zig-zag. An alpha can deposit a large fraction of its initial energy in a very small volume.

 

3. Your teacher is right. The attenuation is an exponential decay with distance. It never goes to zero — there's always some amount of gamma rays that can make it through.

 

4. High atomic numbers are generally good for that reason, and Pb has Z = 82. Anything heavier than lead is radioactive (though Bismuth is only marginally so). Having a shield that is radioactive is kind of self-defeating.

 

Thank you so much for the explanations!!

 

Could you also tell me why a tiny electron hitting a massive atoms gives a large acceleration?

 

What does "larger mass and lower speed" tell us about momentum?

2) The slower alphas have more time to eject an electron from the molecule. They also act by two protons.

 

3) Yes and in that, photons differ radically from electrons and ions, which have a finite (though somewhat variable) path in matter. It's a serious annoyance, because when the gamma flux vastly exceeds the acceptable, the thickness increases correspondingly.

 

To protect electronics, cameras or worse, humans in a space vessel, one has to stop a few extremely energetic particles, which may prove impossible within possible mass, say for a 10 months travel to Mars. Worse: stopping the protons creates gammas which cannot be stopped easily.

 

Or think at radioisotope batteries. Everyone designer of such generator would love to use waste from uranium reactors, especially ⁹⁰Sr. But its child, ⁹⁰Y, emits a gamma in one disintegration over many, so shielding is difficult and costs thick lead, prohibitively heavy on a spacecraft, especially because these ugly poisons shall please be shielded on an individual pellet basis, not as a whole generator that would break apart in an accident. Result: the only source is ²³⁸Pu, a pure alpha emitter, which must be produced on demand, and is scarce and expensive.

 

4) There are more fundamental reasons favouring heavy elements against X and gammas. Against X, the absorption by a deep electron, typically 1s, removes like 90keV at once and with a high probability. Against gammas over 1022keV, the strong electric field near a heavy nucleus favours the creation of electron-positon pairs, which is an efficient absorption process.

 

So much that a shield equally efficient is lighter if using a heavier element - which the density of electrons wouldn't explain.

 

Beyond lead, you have thorium and natural or depleted uranium, which are very little radioactive and are used (infrequently) as shields.

 

----------

 

Complete absorption curves of photons, electrons, protons, alphas... for every element and a few compounds are available at Nist's website, really nice of them.

 

Thank you so much for the extra information!!!!!

 

Photons have a finite path in matter, and so gamma rays can be completely blocked?

 

Why does stopping the protons create gamma rays?

 

What does an individual pellet basis mean?

 

Why does a strong electric field near a heavy nucleus favour the creation of electron-positron pairs?

 

I'm sorry I have so many questions...

Edited October 24, 2013 by startanewww

[swansont]

Could you also tell me why a tiny electron hitting a massive atoms gives a large acceleration?

Small mass hitting a large mass can scatter through a large angle and even backwards, and the small mass means this happens quickly — changing direction quickly means a large acceleration.

[startanewww]

Small mass hitting a large mass can scatter through a large angle and even backwards, and the small mass means this happens quickly — changing direction quickly means a large acceleration.

Thanks a lot! I get it now.

[Enthalpy]

Heavy nuclei have also more protons that produce a stronger electric field to deflect the incoming electron. The strong electron deflection radiates stronger X and gamma rays.

 

If the incoming electron is almost as fast as light (energy >> 511keV), then an aligned observer sees the electron begin to deviate, and later the electron end to deviate - but the beginning is observed after a longer propagation delay than the end, and the delay difference almost compensates the duration of the event, so that the deflection event is extremely short for the observer. This makes the emitted X or gamma shorter and of higher energy, stronger and more concentrated in one direction. It's synchrotron radiation. If using magnetic fields, reversed over millimetric period, it's called a free electron laser (which also needs many electrons to synchronize).

 

Protons and charged particles have a finite path in matter - a short path at MeV energies. The number of surviving photons decreases exponentially, so some survive to any distance; this number may become acceptable for health, or negligible, like 0.01 photon over the experiment duration.

 

Protons stopping in matter create gammas only because cosmic rays have huge energies. Deflection by surviving nuclei wouldn't explain it as for electrons. The nature of the collisions differs from electrons and involves the destruction of nuclei, the creation of new particles - including at energies inaccessible to the LHC. Among those brutal events, gammas are created as something banal, during collisions, annihilation of particles...

 

----------

 

A radioisotopic generator contains enough radioisotope material to run very hot - only its cooling keeps it solid. In a rocket accident (11km/s for instance) the generator and its cooling would be lost. To keep the radioisotope contained, which means solid and not too hot, it consists of individual pellets, which are small enough (1cm) to cool naturally in air when they're dispersed. Though, dispersed pellets need an individual radiation shield, which aso resists the possible re-entry in Earth's atmosphere (it happened with Apollo 13), the impact on the ground, the corrosion in the Ocean...

 

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

 

The alphas emitted by ²³⁸Pu are blocked by Pu itself and by few µm shielding, enabling this individual shielding of pellets. Gammas emitted by ⁹⁰Sr, or rather ⁹⁰Y and by bremsstrahlung of the emitted electrons, demand a thick shield, too thick for individual pellets. It's a collective shield for the whole generator, hence unsafe in an accident. Some projected Moon probes plan to use radioisotopic generators, and I dislike this idea, since batteries exist and Sunlight is plentiful on the Moon.

 

----------

 

Electron-positon: as I imagine to understand it, any photon >1022keV could create them, but it takes an electric field to separate them, so they don't recombine immediately. An additional reason is that photons have some energy-versus-momentum relation, electrons have a different one (slightly different if ultra-relativistic), and simultaneous conservation of energy and momentum demands an additional mass that takes recoil - here the nucleus.

[swansont]

Protons stopping in matter create gammas only because cosmic rays have huge energies. Deflection by surviving nuclei wouldn't explain it as for electrons. The nature of the collisions differs from electrons and involves the destruction of nuclei, the creation of new particles - including at energies inaccessible to the LHC. Among those brutal events, gammas are created as something banal, during collisions, annihilation of particles...

Protons stopping in matter can interact with a nucleus, but they don't have to. They can lose energy by ionization the same as alphas do, and the photons they produce will be less than their initial kinetic energy. Not all protons are cosmic rays.

[startanewww]

Heavy nuclei have also more protons that produce a stronger electric field to deflect the incoming electron. The strong electron deflection radiates stronger X and gamma rays.

 

If the incoming electron is almost as fast as light (energy >> 511keV), then an aligned observer sees the electron begin to deviate, and later the electron end to deviate - but the beginning is observed after a longer propagation delay than the end, and the delay difference almost compensates the duration of the event, so that the deflection event is extremely short for the observer. This makes the emitted X or gamma shorter and of higher energy, stronger and more concentrated in one direction. It's synchrotron radiation. If using magnetic fields, reversed over millimetric period, it's called a free electron laser (which also needs many electrons to synchronize).

 

Protons and charged particles have a finite path in matter - a short path at MeV energies. The number of surviving photons decreases exponentially, so some survive to any distance; this number may become acceptable for health, or negligible, like 0.01 photon over the experiment duration.

 

Protons stopping in matter create gammas only because cosmic rays have huge energies. Deflection by surviving nuclei wouldn't explain it as for electrons. The nature of the collisions differs from electrons and involves the destruction of nuclei, the creation of new particles - including at energies inaccessible to the LHC. Among those brutal events, gammas are created as something banal, during collisions, annihilation of particles...

 

----------

 

A radioisotopic generator contains enough radioisotope material to run very hot - only its cooling keeps it solid. In a rocket accident (11km/s for instance) the generator and its cooling would be lost. To keep the radioisotope contained, which means solid and not too hot, it consists of individual pellets, which are small enough (1cm) to cool naturally in air when they're dispersed. Though, dispersed pellets need an individual radiation shield, which aso resists the possible re-entry in Earth's atmosphere (it happened with Apollo 13), the impact on the ground, the corrosion in the Ocean...

 

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

 

The alphas emitted by ²³⁸Pu are blocked by Pu itself and by few µm shielding, enabling this individual shielding of pellets. Gammas emitted by ⁹⁰Sr, or rather ⁹⁰Y and by bremsstrahlung of the emitted electrons, demand a thick shield, too thick for individual pellets. It's a collective shield for the whole generator, hence unsafe in an accident. Some projected Moon probes plan to use radioisotopic generators, and I dislike this idea, since batteries exist and Sunlight is plentiful on the Moon.

 

----------

 

Electron-positon: as I imagine to understand it, any photon >1022keV could create them, but it takes an electric field to separate them, so they don't recombine immediately. An additional reason is that photons have some energy-versus-momentum relation, electrons have a different one (slightly different if ultra-relativistic), and simultaneous conservation of energy and momentum demands an additional mass that takes recoil - here the nucleus.

 

Thanks a lot for your reply!!!!

[Delbert]

 

3. Your teacher is right. The attenuation is an exponential decay with distance. It never goes to zero — there's always some amount of gamma rays that can make it through.

Not sure that's right. Not right because gamma rays are corpuscular (as we all know). Once the intensity is down to a single gamma particle it therefore can't be made less - it can only reduce to nothing.

[swansont]

Not sure that's right. Not right because gamma rays are corpuscular (as we all know). Once the intensity is down to a single gamma particle it therefore can't be made less - it can only reduce to nothing.

 

But that gamma has a chance of penetrating a large distance. Since we're typically talking about large numbers, the point where the statistics fail won't be a realistic scenario. The big picture is that alphas and betas have a finite penetration distance, while gammas (and neutrons) do not.

[Enthalpy]

Protons stopping in matter can interact with a nucleus, but they don't have to. They can lose energy by ionization the same as alphas do, and the photons they produce will be less than their initial kinetic energy. Not all protons are cosmic rays.

Sure! But through ionization, they produce only X-rays, not gammas.

[swansont]

Sure! But through ionization, they produce only X-rays, not gammas.

 

I don't know what your point is. You equated protons with cosmic rays, and that's not the case. Not all energetic protons are comic rays.

[Delbert]

 

But that gamma has a chance of penetrating a large distance. Since we're typically talking about large numbers, the point where the statistics fail won't be a realistic scenario. The big picture is that alphas and betas have a finite penetration distance, while gammas (and neutrons) do not.

Not quite sure what you're saying. I was answering the apparent statement that gamma ray intensity reduces over distance but never gets to or touches zero. My answer to that was that being corpuscular, granular or whatever characteristic you care to use for something formed of particles (as we understand particles!), there is a point whereby it gets to zero. That point follows when the intensity has reduced to a single particle, and once that decays there's nothing. In other words it will and therefore does reduce to zero or nothing.

[swansont]

Not quite sure what you're saying. I was answering the apparent statement that gamma ray intensity reduces over distance but never gets to or touches zero. My answer to that was that being corpuscular, granular or whatever characteristic you care to use for something formed of particles (as we understand particles!), there is a point whereby it gets to zero. That point follows when the intensity has reduced to a single particle, and once that decays there's nothing. In other words it will and therefore does reduce to zero or nothing.

 

I was saying this was a semantic point that has no applicability to the discussion. It is both true and completely irrelevant.

[Enthalpy]

 

I don't know what your point is.

By definition, rays produced by ionization are X-rays at most, not gammas.

[swansont]

By definition, rays produced by ionization are X-rays at most, not gammas.

 

Yes. And entirely unrelated to my objection.

[Delbert]

 

I was saying this was a semantic point that has no applicability to the discussion. It is both true and completely irrelevant.

I'm well known for irrelevant comments. I'm thinking of making irrelevant my middle name.