From: SFPhysics@aol.com
Date: Thu Mar 31 2005 - 02:25:06 PST
From: SFPhysics@aol.com Message-ID: <1d9.39b0b435.2f7d2a02@aol.com> Date: Thu, 31 Mar 2005 05:25:06 EST Subject: Cathode Ray Tubes...
In a message dated 3/30/05 11:15:45 PM Pacific Standard Time,
pinhole@exploratorium.edu writes:
<< It seems to me like what you're describing is a cathode ray tube?
I remember reading once about someone who put up a Geiger counter up
next to an old classroom cathode ray tube, like the kind you see in
chem/physics textbooks. The radiation reading was WAY high on the
Geiger counter. I'm not sure how you'd do this experiment in a way
that students could both see and be shielded from fast e- (which I
think is the same as beta radiation).
- Geoff
>>
Dear Geoff - I would bet your Mom told you not to sit too close to the
television. ;-)
The real danger is from implosion. Cathode Ray Tubes have a high vacuum and
will dangerously implode if struck or dropped. Students handling them must
have face shields in case of accident. It is always a good idea to have the CRT
demo in the center of the lab table where it cannot be knocked over or
accidentally struck.
However, there is virtually no danger with a proper classroom CRT
demonstrator. I used open demonstration Cathode Ray Tubes for years. The CRT
demonstrators of years past were Oscilloscope tupes and are the same type of tube used
in televisions and in most computer monitors (not flat screen plasma or liquid
crystal types). These tubes do give off "soft" X-rays which will generally
not be either penetrating or dangerous for short periods of exposure.
Television tubes use higher voltages than CRT demonstrators, especially in color tubes,
and use internal shields with lead in the glass to keep radiation levels very
low. The old clear glass 5BP1 tubes will let you make an electron beam that
can be moved by either a magnetic or electrostatic field. Since they do this
with around 9 kV accelerating voltage there is not much radiation produced.
Add to that the beam current is normally in the microAmpere region so there is
not much energy involved. Since the target is a phosphor coating on the
inside of the glass envelope the X-rays are very weak and the majority of the
energy is sent out as visible photons.
HOWEVER, if the Physics lab has an old X-ray tube lying around, don't use it
for demonstrations. These have a tungsten target that generates "hard" X-rays
from any current through the tube. These are highly sought after by vacuum
tube collectors who treat them with great respect because of the implosion
hazard. The same can be said for the early Maltese Cross Cathode Ray Tubes and
radiometer vane CRTs.
As for detecting radiation, the GM (Geiger-Müller) tube will not normally
detect low level X-rays. Holding a Geiger counter tube next to a large TV or an
average CRT demonstrator will show little activity indistinguishable from
background radiation. To detect low level X-rays a type of scintillation counter
is used. It has a crystal that emits visible photons when struck by higher
energy X-ray and gamma ray photons. The flashes of light are detected and
amplified for an indication by a meter that sums the radiation activity. In the
1950s we had both types handy in case of nuclear war.
Kindest regards to the list,
Al Sefl
Who was around when Crookes perfected the CRT...
And can remember the X-ray machines in shoe stores that let you see your foot
bones in your shoes...
Which explains his size 22 feet... with suction cups on each toe... and
green scales...
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