Help with Radiation

[trebor]

Kerbal looks great, thanks for bringing that up.
I think my rocket designs are a bit crap though.

[ka9q]

Yes, it was drilled into us by our physics teachers that if the units weren't right, the numbers were totally meaningless.

BTW, dimensionally all radiation exposures have units of energy per mass. The SI unit of radiation exposure is the Gray (Gy), equal to one joule per kilogram, released by energetic particles as they deposit their energy in a target. 1 Gy = 100 rad, because 1 rad = 100 ergs (energy) / gram.

Corresponding units of Sieverts (Sv) and Rontgen Equivalent Man (REM), with 1 Sv = 100 REM, have the same units of energy per unit mass but scaled with a dimensionless effectiveness factor that depends on the type of radiation. For photons (X-rays and gamma rays) 1 Sv = 1 Gy, but for particles like neutrons, protons and electrons the damage is greater so each Gray of absorbed dose results in more than one Sievert of damage.

Knowing this, and knowing the energies and numbers of your incoming particles and how much of their energy they deposit in each type of material (shielding and the people behind it) you can compute radiation exposures for yourself. It takes a lot of particles to accumulate even 1 Gy because the amount of energy carried by even a highly energetic particle (e.g, 100 MeV) is a tiny fraction of a joule, and that joule is divided by the entire mass of the body.

This is something I think the HBs don't get because they seem to think of each charged particle as something like a bullet, able to kill all by itself if it gets through your shielding. Bullets move vastly slower than most charged particles, but they are even vastly more massive so they carry more than enough energy in each one to kill you. Basically, it's energy that kills though the amount required to do so varies a lot depending on how it's delivered.

[JayUtah]

Jan 31, 2012 14:48:43 GMT -4 ka9q said:

BTW, dimensionally all radiation exposures have units of energy per mass.

It's more accurate to say that this is a measure of absorbed dose. But yes, energy deposited per unit mass. The difference being that exposure can be considered from the point of view of the source, whereupon it's measured in particles per unit time.

Shielding is measured in mass per unit area, not per unit volume. This is one of those things that makes sense when you work with it, but isn't immediately obvious if you are just fumbling through it on intuition.

Watching Jarrah stumble around brings to mind the saying, "That's not right. That's not even wrong." Sadly most conspiracy theorists want you to consider the problem from the perspective of their model. They don't ever stop to think that it's the model that's wrong.

[nomuse]

Jan 31, 2012 5:45:56 GMT -4 Daggerstab said:

Jan 31, 2012 1:39:40 GMT -4 Vincent McConnell said:

Well that was a great load of help! Thank you!
It's called "Kerbal Space Program" and it's a load of fun. It has taught me a lot about Orbital Mechanics.

Ah, another KSP fan.

An early version of the game is available for free download as a demo version. It looks like a sillier, more user-friendly version of Orbiter (Orbiter doesn't have green-skinned bug-eyed astronauts). I recommend at least trying the demo.
kerbalspaceprogram.com/

Here's a video demonstrating a flight to the Mun:
www.youtube.com/watch?v=bGd_BFu9e10

And yes, it works as a nice introduction/demonstration of orbital mechanics. I laughed the first time I thought "Now I'm going to reach apogee and execute a circularization burn" and it worked.

I picked up those rudiments...circularizing an orbit, etc., with the VIDEO game "Space War." They had one at the local pinball arcade and you could play for hours on a quarter as long as you were careful.

[Bob B.]

I've found the following, which might be helpful:



The outer radiation belt consists of electrons with energies lying mostly within the 0.1-10 MeV range; the majority being below 1 MeV. From the above figure we can see that a 10 MeV electron will penetrate less than 1000 mils of aluminum (1 mil = 1/1000 inch), and a 1 MeV electron will penetrate less than 100 mils. The scale is logarithmic so you cannot scale linearly between the values. I estimate the penetration depths to be about 800 mils and 80 mils for the 10 MeV and 1 MeV electrons.

(Edit: Fixed typos in the last sentence above... I got some decimals points in the wrong places.)

The inner belt consists mostly of protons in the 10-50 MeV range, though some may exceed 100 MeV. I estimate the penetration depth for a 50 MeV proton is about 400 mils, and for a 100 MeV proton about 1400 mils. At the lower end, a 10 MeV proton has a penetration of about 25 mils.

The Apollo CM had a shielding rating of about 7 to 8 g/cm². Given that aluminum has a density of 2.7 g/cm³, this rating equates to an aluminum thickness of about 1021-1167 mils (25.9-29.6 mm).

Note that the inclination of the translunar trajectories to the geomagnetic equator caused the spacecrafts to pass well above or below the zones with the highest flux and highest energy particles (see illustrations here). Also note that the zone with the highest energy protons (100 MeV or higher) was traversed in just about 10 minutes. By about 15 minutes after TLI, it appears there was only a very small chance of anything penetrating the spacecraft.

[Bob B.]

Jan 30, 2012 20:50:30 GMT -4 Vincent McConnell said:

Jarrah says the outer belts had an average of 10-100MeV. I told him we could work on the principle that they were 50MeV.
Now, here is what he told me. To find the "aerial density?", we must multiply 50 by .545cm^2.
We have 27.25cm^2.

Now he said we have to divide that by the density of the material itself. Aluminum has a density of 2.7cm^3.
We get about 10.09cm^3 worth of shielding.

OK, I see what Jarrah is doing. Surprisingly, I actually think he’s right this time even though the units are all messed up.

Aerial density, or more commonly areal density, is the mass per unit area for a sheet of armor. I don’t know where Jarrah found his number, but his claim appears to be that the areal density needed to stop a particle of 1 MeV is 0.545 g/cm², and that this number scales linearly is proportionate to particle energy. Figure 2 in my last post agrees with this. For example, I estimated 800 mils of penetration in aluminum for a 10 MeV electron and 80 mils for a 1 MeV electron. As you can see, this is a linear proportional relationship. Furthermore, if I convert this to areal density I get 0.549 g/cm²-MeV. This is good confirmation of Jarrah’s number. (See post #24 for clarifications and corrections to this paragraph.)

The next step Jarrah takes is to calculate the areal density required to stop a 50 MeV electron,

50 MeV x 0.545 g/cm²-MeV = 27.25 g/cm²

Next he says that if this shielding is aluminum, the thickness required is,

27.25 g/cm² / 2.7 g/cm³ = 10.09 cm

Now that I’ve corrected the units, he’s actually correct, at least mathematically.

The problem is that I don’t see any indication that there are electrons as energetic as 50 MeV, and if they do exist, they are rare. From what I’ve read, 10 MeV seems like a more reasonable maximum, with the majority of electrons <1 MeV. For a 10 MeV electron, the areal density of the required shielding reduces to 5.45 g/cm². As previously stated, the CM was rated to 7-8 g/cm².

Jarrah makes the same rookie mistake that many other hoax believers make, that is, he seems to think that the thin pressure hull was all that lay between the VARB and the astronauts. The hull was actually made up of a couple sheets of aluminum sandwiching aluminum honeycomb, another couple sheets of stainless steel sandwiching stainless steel honeycomb, fibrous insulation, and a phenolic resin heat shield. (Note that the heat shield wasn’t just across the base – it was just thickest there. The sidewalls were also covered with heat shielding.) There were also electrical panels and other equipment mounted on the interior walls. All this added up to a considerable amount of protection. There is no requirement that the shielding must consist of a thick aluminum sheet as Jarrah implies.

Although the electrons are of generally low energy, there are higher energy protons in the inner belt. It is my understanding that mission planning was more concerned about the protons. The electrons in the outer belt were of low enough energy that I don’t think they were of much concern.

The problem with Jarrah’s numbers is that they aren’t applicable to protons. The 0.545 g/cm²-MeV figure is apparently for electrons only. According to the previously posted Figure 2, the areal density per MeV for protons is not linear and is much lower than for electrons. Earlier I estimated penetration depths for three different energies of protons. Converting the penetration depths to areal densities, I get 0.017, 0.055 and 0.096 g/cm²-MeV for 10, 50 and 100 MeV protons respectively.

What I can conclude from all this is that the only thing to get worried about is the very highest energy protons, i.e. about 100 MeV and above. Fortunately these protons make up only a very small portion of the particles in the inner belt. Furthermore, the spacecrafts managed to skirt the edge of this zone and was at risk to these particles for only about ten minutes. Exposure to these particles surely contributed to the astronauts’ absorbed dose, but the low flux and brief duration kept the dose low.

[trebor]

Jan 30, 2012 20:50:30 GMT -4 Vincent McConnell said:

Jarrah says the outer belts had an average of 10-100MeV. I told him we could work on the principle that they were 50MeV.

Where is he getting that figure for the electron energy range from?
I had a look and have found nothing which shows there is any at all with that high an energy in the outer VAB.

[theteacher]

Jan 31, 2012 19:11:10 GMT -4 Bob B. said:

The Apollo CM had a shielding rating of about 7 to 8 g/cm². Given that aluminum has a density of 2.7 g/cm³, this rating equates to an aluminum thickness of about 1021-1167 mils (259-296 mm).

Wouldn't that be 25.9-29.6 mm?

[Bob B.]

Feb 1, 2012 8:35:47 GMT -4 theteacher said:

Jan 31, 2012 19:11:10 GMT -4 Bob B. said:

The Apollo CM had a shielding rating of about 7 to 8 g/cm². Given that aluminum has a density of 2.7 g/cm³, this rating equates to an aluminum thickness of about 1021-1167 mils (259-296 mm).

Wouldn't that be 25.9-29.6 mm?

Oops. You are correct; I'll fix that.

I also want to correct/clarify something in my last post. I implied that the areal density versus electron energy was a linear function. If you look at Figure 2 you can see that this is clearly not the case. It just happens to be approximately linear over the range of energies I was looking at. The other key point I was trying to make, but did a poor job of expressing, is that the density is roughly proportional to the energy in that same 1-10 MeV range. This proportionality doesn’t necessarily hold true when we move to other energy ranges.

I also want to be fair to Jarrah about something. I inferred he’s claiming the areal density is 0.545 g/cm²-MeV across all electron energies, but since Vincent provides just one sample calculation for just one energy, I can’t make that inference. I don’t know Jarrah’s source, so I don’t know how he came up with that number. He may very well have different numbers for different energies.

[Bob B.]

Can I suggest that we break off the discussion about the lost ascent stages into a separate thread? It sounds like it could be an interesting discussion and it deserves its own appropriately titled thread. Since the original topic of this thread is still alive, the introduction of a second off-topic discussion is making the thread confusing.

[LunarOrbit]

Feb 1, 2012 9:48:41 GMT -4 Bob B. said:

Can I suggest that we break off the discussion about the lost ascent stages into a separate thread? It sounds like it could be an interesting discussion and it deserves its own appropriately titled thread. Since the original topic of this thread is still alive, the introduction of a second off-topic discussion is making the thread confusing.

Sure, I can do that. I've moved the related posts to this thread:

The inclinations of the Apollo lunar orbits

[Bob B.]

Thanks, LO.

[Bob B.]

Something else worth mentioning... If a 100+ MeV proton does penetrate the shielding, it is no longer a 100+ MeV proton by the time it gets through. It loses much of its energy penetrating the shielding. Therefore, the amount of biological damage it can do if it does happen to hit an astronaut is greatly reduced.



edit spelling

[trebor]

Feb 2, 2012 10:45:49 GMT -4 Bob B. said:

Something else worth mentioning... If a 100+ MeV proton does penetrate the shielding, it is no longer a 100+ MeV proton by the time it gets through. It looses much of its energy penetrating the shielding. Therefore, the amount of biological damage it can do if it does happen to hit an astronaut is greatly reduced.

Protons? I thought the particular discussion was focussing on electrons?

[Bob B.]

Feb 2, 2012 11:04:27 GMT -4 trebor said:

Feb 2, 2012 10:45:49 GMT -4 Bob B. said:

Something else worth mentioning... If a 100+ MeV proton does penetrate the shielding, it is no longer a 100+ MeV proton by the time it gets through. It loses much of its energy penetrating the shielding. Therefore, the amount of biological damage it can do if it does happen to hit an astronaut is greatly reduced.

Protons? I thought the particular discussion was focussing on electrons?

It started out that way, but I think we've pretty much dismissed electrons as a problem. At least none of the thread participants seem to be contending that they're a problem. In my posts #19 and #20 I expanded the discussion to include protons because it seemed silly to me not to include them in a discussion about the potential threat of the VARB to manned spaceflight. Their inclusion is not directly pertinent to the opening post, but protons are pertinent to the topic in general.