I've been reading with interest blog posts at spaceweather.com about a group of high-school kids using balloons and instruments on board commercial aircraft to measure (primary and secondary) cosmic radiation. They have a facebook page here.
It's very interesting, especially as direct measurement of some high energy particles is difficult, it's been long known that cosmic radiation levels increase in a curve with altitude, but peaking at an altitude coincidentally at just above where we like to fly long-range jet aircraft. I'm not a physicist, but understand that this is due to collision between some of these particles with atoms in the air forming a "shower" of particles, which sounds kind of benign, like a shower of raindrops, but because of the extremely high energy level / speed (close to light speed) of these primary cosmic "ray" particles, the secondary cosmic "ray" "shower" has only slightly reduced velocity / energy level and only a small deflection in trajectory. These secondary particles collide again, and it ends up as a cascade of collisions with eventual energy dissipation. There's a whole lot of other stuff going on, ionisation, chemical reactions etc. Much of the "shower" consists of neutrons (this is where cosmogenic radioisotopes like C14 are formed, ie nitrogen + 1 neutron give you C14 plus a proton). The altitude where the peak in ionising radiation is observed is sometimes called the Pfotzer Maximum, the radiation measured normally looking for charged particles and electromagnetic radiation, xrays, gamma radiation etc which can be easily detected, but not directly measuring fast neutron spectra. Above the Pfotzer maximum, the air is less dense, so fewer collisions, so radiation level falls. I guess the peak in fast neutron density will be lower (altitude) than the Pfotzer Maximum.
Back to the tinfoil hat side of things - here goes:
Methods for determining occupational radiation exposure risk are "derived", then typically monitored with maximum exposure limit set at a level where stochastic effect is deemed to be acceptable risk. (IIRC, US astronaut career total exposure limit carries a risk of about 15% increase in radiation induced cancer - something that would be totally unacceptable in any normal occupation, but might not be something you'd fret over if you were a keen astronaut). Note that the calculation of bioeffective dose (Sv etc) from actual exposure is determined by committee.
The data being produced by these kids - particularly on measurement of high energy fast neutrons - may be upsetting the applecart here. Exposure to fast neutrons may be observed (ie neutrons with energy level ~< 2 MeV in fission reactors) then a bioeffective safe exposure calculated. But the energy level spectrum of these cascades of neutrons "secondary cosmic rays" includes neutrons with much higher energy levels than produced by fission - spectrum is more like that from a hydrogen bomb (or probably far higher). Bioeffective dose equivalent at these energy levels isn't really known. I suspect that these kids are showing us something about which we need to find out much more - about bioeffective / assumed stochastic effect of fast neutron dose at the spectra which actually exists at (below) the Pfotzer Maximum zone - where we're "flying high".
To quote from their facebook page:
As a matter of fact, the radiation detected by our sensors during a 4 hour flight from Vegas to Boston was approximately equal to one dental X-ray, and we are measuring only a small fraction of the total radiation. Our sensors do not pick up neutrons or any ionizing radiation greater than 20 MeV. Therefore, our measurements represent a lower limit. So, if you flew all day on the plane we were on, you would be absorbing at least 6 dental X-rays per day, and probably several times more than that. Airline pilots should not feel re-assured by these numbers.
In their latest experiment they took a neutron bubble chamber on a commercial plane flight. Their measurement of total radiation exposure was 140 times greater than observed at sea level.
(IOW, the measurement and asumption that high altitude radiation exposure is no worse than a dental x-ray or two - in other words nearly nothing at all - may be flawed, as the typical response is that exposure in say mSv is at well and truly "safe" levels - "nothing to worry about")
That sounds a little alarmist, I'll throw in something worse:
Air crew have ~ double the risk of contracting malignant melanoma than the general population, and also have a significant increase in risk of breast cancer. The correlation has been known for more than a decade. Possible explanation for the correlation include lifestyle, disruption of circadian rhythm, and in the case of melanoma then exposure to high UV levels in aircraft cockpits. The widely held conclusion/belief is that the correlation may not be epidemiologically significant because of the poor way data was collected and analysed - fair enough, but there seems to be a smoking gun and IMO they really need to try again and try much harder.
I don't accept those explanations any more - I suspect they're wrong. Worse, if there is a link to exposure to secondary cosmic radiation / fast neutrons and it is this exposure casuing the correlation (these fast neutrons surely can and will damage DNA), then it's just going to get worse and perhaps far worse - as stochastic effects are by definition cumulative, crew are flying higher, for longer, and are now accumulating more career time flying at those high altitudes.
