To the best of my abilities, this page offers what little I've come to understand of space radiation, as that greatly derived from the above chart.
About Earth L2 (EL2): radiation dosage per year (based upon aluminum shielding)
@0.0 mm = 4+^5 rem
@2.2 mm = 4.14^3 rem
@22.22 mm = 2.85^2 rem
Away from EL2 (fully solar exposed), perhaps we should, for good measure, multiply upon the above by a factor of 10
That's 40+^5 rem/year at 0.0 g/cm2 (based upon but one solar max event).
Further calculating for all of the typically lesser flares per year, lets multiply by another factor of 5.
Now we're topping out our TBI at 20^6 rem/year at 0.0 g/cm2 (no shielding). That's .548^5 rem per day ( or 54.8^3 rem/day)
@22.22 mm (6 g/cm2) we've dropped this exposure to 14.25^3 rem/year = 39 rem/day
Even if we forego the 5X for accommodating all those multiple solar flare accumulations, that's still a whopping 7.8 rem/day while outside of EL2, where obviously for those Apollo missions which involved at least 10 such full exposure days external to EL2 amounts to 78 rem plus their 5+ hours worth of to/from Van Allen zone dosage, which is at another rate of least 15 fold more rem per day (4.875 rem/hr). In other radioactive words; even at the relatively thick 22.22 mm shielding (6 g/cm2 which is better than 10 times that of what much of the Apollo mission had to work with), the least of what the Van Allen zone represents is roughly 120 rem/day or 5 rem/hr.
Something Apollo, as based upon 6 g/cm2 shielding;
5 hours worth of Van Allen zone time = 25 rem
10 days of open space travel/exposure = 78 rem
EVA exposures of perhaps 5 hours @6 rem/hr = 30 rem
Misc. dosage in just getting to/from 1000 km = 2 rem
All toll we seem to have accumulated 135 rem and, that's certainly more than enough to cause externally visible and of not to mention measurably damaged internal biological issues, of measurable long term DNA damage and even of some irreversible issues (like film fogging).
and BTFW; I don't believe the Apollo command module was constructed and outfitted with even half as much shielding and, the lunar lander wasn't of 10% shielding as I've utilized above. EVAs were almost zilch worth of radiation shielding (at best .5 g/cm2). Doing the math on those lesser shields is yet another not so pleasant surprise, where such lesser shielding would surely enable another 10 fold radiation dosage; I believe that an accumulation of 1350 rem is almost assured death on the spot, as within a day or so.
It's also become rather noteworthy, of the 64 mrem/day being reported as a typical shuttle mission, of the cruising altitude of under 250 miles (that's roughly a third of the way before you're getting yourself sort of microwave cooked within just the initial Van Allen zone of death) and, besides that rather significant advantage of being magnetically shielded and as nowhere near that radiated death zone, remember that the shuttles are spending nearly half of their daily flight as otherwise fully shielded from the sun by Earth itself, not even an option for the greater portion of those Apollo mission exposures and still, as being compared to Apollo, those shuttle missions as such were taking in twice the daily Apollo dosage (actually 4X if a shuttle being fully solar exposed), which most certainly seems a little more than odd.
As you can see for yourself, from the official L2+MAP chart and, as due to all the different primary radiation elements and of secondary interactions, as the radiation shield thickness/density goes up, the radiation dosage goes down by a somewhat greater extent (nonlinear). In other words, a ten fold increase in thickness (taking 1 g/cm2 up to 10 g/cm2) clearly indicates that it offers a 27 fold reducton of overall radiation dosage. Presumably the next level of going to 100 g/cm2 will induce perhaps represent another 54 times reduction, getting us down to a fairly respectable dosage (excluding all but one solar maximum flare, while this is dreaming, as there's bound to be another maximum solar vent as well as for hundreds of lesser discharges that ott to require a 5X safety margin of error applied so as to obtaining the base exposures). Leaving the santuary of L2 is like stepping from the cool shade of a large tree into the blasing hot sunsine, thus to be calculating towards the safe side of being prepaired for the worst, as for open/free space travel, I still believe that one ott to multiply the L2 dosage as indicated by those charts by a safety factor of 10. In other words; 100 g/cm2 by itself might not even be sufficient.
As for the types of radiation held within the Van Allen zone of death, the reduction afforded by increasing shielding from 1 g/cm2 to 10 g/cm2 offers a 35 fold reduction. However, of the unshielded radiation dosage within the Van Allen zone is nearly 1000 times greater than being at EL2, that's still representing nearly 15 times greater TBI dosage as for being situated behind 10 g/cm2 worth of shielding.
Presumably a shield comprised of differing layers (instead of just aluminum), of mixed alloy (say of alternating alloy steel and aluminum), ott to improve things by further cutting those secondary radiation affects. Possibly a blend of heavily leaded UHMW is another form of density layering that's worth applying as for subduing some of those secondary radiation issues, not to mention the added energy absorbson and thermally self healing aspects in case of taking physical impacts.
For the Venus L2 (VL2) interpretation, simply multiply the above dosage estimates by a factor of 1.9
According to the 1000 km to 7000 km Van Allen zone chart, of one year's worth of typical exposures, if we elected upon the maximum listed shield of 10 g/cm2 and, if that represents 1.35^3 rem/year (per including one and only one solar max flare event per year), then the hourly dosage will become 0.154 rem/hr.
For some further reasons I don't even understand, as according to those same charts, where at 10 g/cm2 a solar minimum year somehow represents roughly another 1.25 to 1.6 factor increase in "Total rads(Si)" for the most shielded, thus the secondary radiation must be somewhat of greater concern.
Due to the size and mass of ISS/VL2 package becoming roughly 1000-t. If the necessary travel trajectory route and duration of Van Allen exposure were 20 hours worth, obviously we've accumulated a mere 3.08 rem. I believe we could and should reduce that to a factor of below 1 rem (possibly as low as 0.5 rem) if the additional shielding applied about the crew's sheltered cabin were configured as sufficiently multi-layered alloys accommodating 100 g/cm2.
I'm not trying to be intentionally morbid, just practical as all get out:
The idea of constructing personal survival pods, as sort of multi tasking coffins that are constructed of the same multi-layered 100 g/cm2 should prove most helpful, as whenever cabin radiation levels are sufficiently high, crew would simply lounge within their personal coffin, perhaps remaining familiar to this small space because, that's potentially what their irradiated body is eventually coming home in. Just in case Phantom Works is still running amuck with their laser cannon testing and/or, should another COLUMBIA event happen, as these coffins once again will likely save your sorry butt, as for residing inside one of these suckers and you're most likely to survive reentry, where at some opportune time the lid would be blown off and yourself ejected along with a parachute.
While onboard ISS, these personal pod/shield-coffins could be made relatively interactive between all the others, thus life could sort of go on in spite of the fact that the ISS interior remains as a radiated ghost town and otherwise looking very much like a mortuary or perhaps a morgue full of really big coffins.
What this all boils or radiates down to is that of my estimate, of there being roughly and of at least an open environment of 20^6 rem/year in free inner space (zero shielding), of which a 100 g/cm2 of mixed alloy layers will affordably reduce that to a ratio of 200^3:1, or that of 100 rem/year.
While the solar environment near Venus is 1.9 times greater, the protection afforded by the L2 position should more than compensate by a reduction factor of 10. Even though some of solar flare energies will circumvent the planet and strike whatever is situated at VL2, chances are that in the final outcome this will be a trade off, so that of what's to be expected at VL2 may for now be calculated as averaging at something less than 100 rem/year as shielded by 100 g/cm2, whereas the average to/from (fully exposed) travel may need to be calculated at something above 100 rem/year. Overall, for a two year mission, an average of 100 rem/year is sufficient knowledge, whereas the added protection in the manner of utilizing coffins as backup, this should not only cut those exposures to something well below 100 rem/year but entirely avoid many if not all of the "what if's" associated with the unknowns, by giving safe haven from the worst of the worst, short of ISS being pierced by something entirely unexpected.
Mission survival may ultimately depend upon the good health of the crew as well as their having banked amounts of their own bone marrow, thus able to being sort of revived at the last possible moment (somewhat exactly like taking TBI chemotherapy prior to bone marrow transplant). Perhaps some of this bone marrow could even be safely stored onboard, as with in liquid nitrogen or simply space refrigerated, as well as for being additionally shielded within an additional containment mass of 100 g/cm2), where between having the cash of frozen bone marrow and of the extra shielding might just save the day, not to mention your butt.
As I've mentioned previously; if the nuclear pumped Xenon or of some other capable and sustainable rocket engine were made available, ISS could be safely navigated to a sustainable position of 0.95 VL2, possibly even as close as 0.9 VL2, which ott to greatly reduce or allude solar flare energies (of course we lose out on PV panel energy, thus nuclear again comes in real handy), as well as for giving the communications platform an uninhibited view of essentially the entire nighttime season of Venus, as well as just about anywhere else they care to look.
Actually, the real concern may not come down to space radiation shielding nor of rocket technology but, as to whether or not Earth is still in one piece and/or worth returning to after two years, as at the current level(s) of button pushing (especially of those Islamic, Muslim and Palestinian buttons) is tenuous, where all we need is for warlord Bush taking on the notion that someone other than us has WMDs (even invisible ones) and, in a flash we could be over our heads in another demise that's not going away any time soon.