This gets long.
> There must be some reason X-Ray techs wear lead aprons in hospitals.
> Perhaps because they protect vital parts from X-Rays? If the X-Rays from
> airport scanners are strong enough to pass easily through these lead bags,
> there's a good chance that they are also hazardous to unprotected people.
The assertion was that lead does not stop 100% of X-Rays, but only
reduces exposure; and that thus machine operators could turn up the
intensity of the beam to "punch through" lead shielding.
Given that lead _does_ stop _some_ of the radiation and thus cut
down the dose, it makes sense for medical X-Ray techs to wear the
lead aprons (and, for that matter, to cover nearby, non-imaged
body parts of patients. This does not contradict the statement
that a lead film bag won't stop them from irradiating your film
in checked baggage.
AFAIK the operators have no control over the beam intensity for the
hand-baggage scanners. These are presumably set to lower levels.
After all, if they can't see into a bag at the checkpoint, they can
just ask you to open it. Presumably the levels are such that the
radiation doesn't penetrate the shielding of the machine itself
enough to be a problem.
X-ray imaging technology is kind of interesting actually. I don't
know _much_ about it -- mostly stuff gleaned from being one of those
patients who asks an annoying number of questions, and several
years out of date -- but enough to have a glimpse into the medical
imaging mindset.
I do know that over the years, the radiation dose used for medical
imaging has decreased significantly. They came out with more
sensitive film so they could use less radiation. Then they came
up with more tightly controlled machines. Then they came out with
even more sensitive film. Then they came out with funky image
intensifier thingies (a plate or grid re-emitter placed between
the patient and the film, if I understood the explanation from the
tech) so that even places using older machines could turn down the
intensity. Since then, I'm betting they've upped the film
sensitivity even more. (Note that dose also varies with what
they're particularly trying to see.)
Now obviously they've decided that the tiny amount of damage done
by a few sets of X-rays done over a patient's lifetime add up to
a whole lot less risk than ignorance of what's going on inside, but
being a mostly cautious lot, doctors don't like any _extra_ exposure.
So when I had my chest X-rayed, they gave me an apron to cover my
groin. As for the techs wearing lead, well usually they go behind
a wall containing thicker lead than you can put in a protective
garment, but if they need to be right there when the image is made,
they wear lead. Well a medical X-ray machine is not a tidy,
enclosed box, shut around the body part being examined. It's
this emitter and film-carrier assembly that the patient gets in
between. Any radiation dispersed by less-than-theoretically-perfect
focus gets to bounce around the room. And radiation _scattered_
by the imaging process (say if a photon hits an atom in the
patient's body _just_right_ so that it's reflected instead of
absorbed) gets to bounce around the room. Do that day after day
after day after day, and you're getting more _cumulative_ exposure
than the patients. Lead aprons stop enough of that to bring things
back into the "acceptable risk" levels.
Let's compare this to checked-baggage imaging equipment: like the
hand-baggage scanner, items are moving through on a conveyor belt.
I've never examined a machine, but some things stand to reason: you
can put as much shielding as you like around that conveyor and
keep photons from escaping in the Y and Z axes. If there might
be enough reflection/scatter to throw things out along X -- along
the direction of the conveyor belt, it would be possible to either
put in a zig-zag light-trap (X-ray-trap) or to have lead doors
come down in front of and behind each bag before turning on the
beam. The safe power limits are a whole lot higher. They can
crank up the gain a long way without exposing the techs (and you
can bet the techs have to wear dosimeters just in case, if I know
anything about OSHA). Your limits are the designed power capacity
of the beam-generating equipment and the designed level of shielding
on the machine. If they put 5 cm lead walls around the imaging
chamber, they can crank up the beam intensity enough to push some
X-ray photons through an 8 mm thick lead bag. Maybe not _many_
photons, but some.
Off into "hope I remember my high-school physics"
land... Consider that there are two approaches to
"turning up the power", and I have no idea which
they use: you can turn up the energy of each photon
by increasing the frequency (decreasing the
wavelength) of the X-rays you're throwing, which
ought to make them a bit harder to stop/absorb; or
you can just "throw more light" -- throw more
photons of the same energy at the target. How does
that help? Well, matter has a lot of empty space in
it. If a photon smacks into a nucleus, it'll be
absorbed or reflected. If it plows into an
electron, it'll be absorbed but have a pronounced
effect on that electron -- it can "excite" the
electron by moving it to a higher-energy state, in
which case the electron will eventually decay to its
original state and _re_emit_ the photon (possibly at
a different frequency, though the only way I can
think of off the top of my head for that to happen
would be if the photon decayed in stages, emitting
two lower energy photons whose total energy added up
to that of the original one) or the electron can be
knocked clear off the atom (thus _ionizing_ it).
The effects I know of that make X-rays biologically
dangerous are a) heating and b) ionization. By
ionizing an atom in a molecule, you may split that
molecule. This can cause trivial damage, create
"free radicals" (ionized molecules that really,
really want to react with other molecules), or if
you're really unlucky, cause a break in the middle
of a DNA molecule and turn on an oncogene
(cancer-causing gene).
Okay, why should "throwing more light" help to
penetrate lead shielding? Lead stops X-rays as well
as it does mostly because it's so dense -- each
photon doesn't have much chance of getting through a
small amount of lead without running into a lead
atom somewhere along the way. But a few lucky
photons will make it through. The thicker the lead,
the longer the photon has to stay lucky to avoid
being absorbed. But if only .05% of the photons who
hit one side of the lead sheet get through, there's
still the fact that .05% of a hellofalota photons is
more than .05% of a tiny number of photons.
Note that ordinary _dirt_ can be used as radiation
shielding -- it just takes more of it. ISTR (and
could be mistaken) that something like a foot of
concrete provides the shielding of an inch of lead.
Obviously even lighter things absorb some X-rays;
otherwise soft tissue and bone wouldn't show up at
all in X-ray images as they'd be completely
transparent.
A hand-baggage scanner should see through an LP
record album (anybody remember those?), but it won't
see through a 100 cm thick stack of them (as I
discovered on my way to college[*]). Similarly, a
centimeter of lead shielding stops more X-rays than
a thin lead bag.
-- Glenn
[*] Okay, this wouldn't be so funny now, but here's the
situation: I had this roughly square LP carrying case with a
handle on top, and I'd stuffed a bunch of LPs in it. Having
some extra room, I tossed a notebook and some other stuff in
behind the LPs. Get to the airport and go through the X-ray
machine, and wait for my bags to come out on the conveyor belt.
Wait and wait. Finally realize nothing's moving and look at the
X-ray operator. She's got this look of dread on her face, and
she's a little pale. I lean over to see the screen, and there's
my bag. It's a square box with a handle, and neatly inscribed
in that is a circle, with a line coming off the top at a
tangent. It looked like one of two things: the flowchart
symbol for mag-tape or a *cartoon* bomb.
"Excuse me, ma'am. Is there a problem?"
She turned slowly to look at me. "Is that your bag, sir?"
"Yes, it is."
"What is that?"
"Records. Albums. Y'know, LPs? It's a carrying case for them
and I guess the stack was too thick for the X-rays to go
through.
She relaxed noticeably, then pointed at the "fuse" and said,
"What's this then?"
"I'm not sure. I don't remember what else I put in there."
She went all stiff and frightened again.
"But we can open it and take a look, if you'd like," I
suggested.
After convincing her that it was _safe_ to open the case, she
advanced the conveyor and I opened the case. On top of the
notebooks behind the records, I'd perched a tube of toothpaste.
Back then, toothpaste was sold in metal tubes.
All completely harmless and entirely unintentional on my part,
but it resulted in fear-induced total brain freeze in the X-ray
operator.
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