Date: Fri, 11 Oct 2002 08:45:34 -0400

Author: Sam Sampere

Subject: Re: Cathode ray tubes for demos

Post:

Good job. Aren't you glad you checked?

Now that we have these numbers, we must interpret them. Are doses this high ok
for an instructor using the device for a minute or two? How about a demo person
who sets this up and checks it in advance? He/she might get this dose for 2
minutes or more. Where can we find guidelines with this info?


Thanks,

Sam

"Jason St. John" wrote:

> Sammy, Cliff, and anyone else who was thinking the same thing,
>
> You guys are right, of course; I jumped to the conclusion that these
> things are totally harmless based on very little information. They've
> been around so long and seem so innocuous that I pretty much assumed they
> generated negligible ionizing radiation/ill health effects.
>
> We have several, with different geometries and probably, and Cliff
> points out, any of a wide range of internal pressures. So, like Sammy,
> I'll be checking them out tomorrow.
>
> Much obliged for your collective alertness and safety awareness,
> -jmsj
> ____________________________________________________
> Jason St. John 617.353.2634 stjohn@bu.edu
> Boston University Physics Lecture Demonstrations
>
> On 2002-10-10.14:14 owner-tap-l@listproc.appstate.edu sent:
>
> Jason,
>
> I measured one tube at 12 R/hr (that's R not mR) 6 inches away.
>
> They can emit a lot depending on how evacuated they are, the current and
> voltage and the material in the tube.
>
> Cliff
>
> ----- Original Message -----
> From: "Jason St. John"
> To: "tap-l"
> Sent: Thursday, October 10, 2002 2:02 PM
> Subject: Re: Cathode ray tubes for demos
>
> > I don't think that these old tubes accelerate (decelerate) enough
> > electrons to generate appreciable X-ray radiation. Not enough current.
> >
> > -jmsj
> > ____________________________________________________
> > Jason St. John 617.353.2634 stjohn@bu.edu
> > Boston University Physics Lecture Demonstrations
> >
> > On 2002-10-10.14:49 owner-tap-l@listproc.appstate.edu sent:
> >
> > Would that include the old demo at this website?
> >
> > http://www.physics.brown.edu/Studies/Demo/modern/demo/7b3510.htm
> >
> >
> > ----- Original Message -----
> > From: Cliff Bettis
> > Sent: Thursday, October 10, 2002 1:42 PM
> > To: tap-l@listproc.appstate.edu
> > Subject: Re: Cathode ray tubes for demos
> >
> > The voltage you apply determines the energy, the current determines the
> > intensity. You can limit the emission by keeping the voltage and current
> as
> > low as possible. The problem is that, traditionally, these tubes were run
> > off of induction coils and the only regulation was a spark gap in parallel
> > with the circuit.
> >
> > As far as detecting the radiation, a Geiger counter or film badge will do.
> A
> > friend of mine turned this issue into a class project using the tube to
> make
> > some x-ray photographs!
> >
> > Usually the audience is far enough away from the device that there is not
> > much risk for them, however, the lecturer or set-up person has to be up
> > close. There is also the matter of the law and in our state we would
> violate
> > the law if we operated an unlicensed x-ray source. So we generally use
> > cathode ray tubes designed to run at low voltages (a few kV) with heated
> > cathodes.
> >
> > I don't mean to alarm anyone, just to point out something you should be
> > aware of if you operate these tubes. I, personally, wear a film badge if I
> > set up one.
> >
> > Incidentally, gas filled tubes (plasma globes and the like) don't emit
> > x-rays because the electrons in them never get going fast enough before
> they
> > collide with a gas atom and have to start accelerating over again.
> >
> > Cliff
> >
> > ----- Original Message -----
> > From: "Jim Kernohan"
> > To:
> > Sent: Thursday, October 10, 2002 11:24 AM
> > Subject: Re: Cathode ray tubes for demos
> >
> >
> > > tap-l@listproc.appstate.edu writes:
> > > >
> > > >Be careful when you use these old tubes as they can emit x-rays.
> > >
> > >
> > >
> > > UNder waht conditions will they emit x-rays? Should I shorten the length
> > > of time? lessen the voltage?
> > >
> > > How do I easily detect if I'm emiting x-rays?
> > >
> > > thanks!
> > >
> > > JimGet more from the Web. FREE MSN Explorer download :
> http://explorer.msn.com

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