On Sat, 5 Apr 2008 17:09:39 +1100, "Phil Allison"
wrote:

"Jeff Liebermann"

Most of the x-rays emitted by CRT's come out the BACK of the tube, not
the front. The shadow mask blocks most of them.

X-rays are generated at the point where maximum electron deceleration
occurs - ie as the moment they

1. Hit the colour phosphors.

2. Are intercepted and absorbed by the shadow mask while on the way
there.
d
Yep. See the scribbling of the Coolidge Tube at:
http://en.wikipedia.org/wiki/X-ray_tube#Coolidge_tube
Note the x-rays are produced when they hit the metal anode targe (A)
and are *REFLECTED* to wherever they need to be going. The same thing
happens in a CRT. The accelerated electrons from the filament hit the
shadow mask and produce x-rays which are reflected back towards the
filament.

Also see comments on x-rays (ionizing radiation) at:
http://en.wikipedia.org/wiki/Cathode_ray_tube#Ionizing_radiation

In case 1, x-rays travelling towards the viewer are absorbed only by the
face glass.

The metal CRT shadow mask is fairly impervious to x-rays. What
electrons go through the holes in the shadow mask to light up the
phosphor dots, do not produce x-rays.

In case 2, x-rays are generated on the reverse side of the shadow mask
and then travel both forward and backwards. Those travelling forwards
are absorbed by the face glass.

Most travel backwards (reflected) which is why the funnel of the CRT
has much more lead in it than the screen.

The face glass is many times thicker than the rest of a CRT's glass - so it
contains most of the of lead.

See appendix B at:
http://www.eiae.org/chemicals/files/EIA_CRT_5-01.pdf
For a 19" CRT, the amount of lead is:
neck 0.027 lbs
funnel 2.1 lbs
panel 0.5 lbs
frit 0.079 lbs

The shadow mask itself is made from very thin alloys sheet ( " invar " or
nickel-iron) so will not absorb x-rays to any great extent.

The shadow mask is made from Invar for mechanical stability. The
alignment of the electron beam to the phosphor dots (or lines) is
critical to maintain proper convergence. The tube gets warm and
having the aperature screen drift would be a bad idea.

When I was young (and stupid), I took some sealed Polaroid ASA 3000
speed "film" sheets, attached some coins to the surface, and plastered
them all over a late 1960's vintage color TV, where I was working.
After running the TV all day, I developed the pictures, and found a
noticeable lightening around the coins. (Polaroid "film" is positive
exposure, not negative). The shadow wasn't very distinct. The "film"
on the front screen was barely exposed, while the "film" near the Hi-V
cable was more noticeable. The "film" had to be attached to the CRT
to get any kind of exposure. Those on the cabinet showed no shadows.
About the only change that this prompted in my lifestyle was to not
leave my loaded film camera on top of the TV set.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558