[time-nuts] How dangerous if a Rb lamp broken?

Robert Atkinson robert8rpi at yahoo.co.uk
Thu Jul 11 02:32:23 EDT 2013


Hi,
Not quite,  The raduim is still very active. The glow stops because the ZnS breaks down. The light emissson relies on the crystaline structure of the ZnS plus a small amount of doping, typically silver. The alpha particles break down the crytaline structrure causing the glow to weaken. Most will still show a very faint glow if you let your eyes adapt in complete darkness for about 15 minutes.
One reason why stopped hands can darken the srystal of a watch more than the numerals is distance. Inverse square law applies, thae hands are about half the distance so four tine the radiation. Alphs leaves are even lower as even a inch or two of air will stop them.
 
Robert G8RPI.


________________________________
From: Bill Hawkins <bill at iaxs.net>
To: 'Discussion of precise time and frequency measurement' <time-nuts at febo.com>; 'Tom Miller' <tmiller at skylinenet.net> 
Sent: Thursday, 11 July 2013, 0:11
Subject: Re: [time-nuts] How dangerous if a Rb lamp broken?


Since this thread doesn't appear to have a half-life, perhaps this needs
some explanation.

The zinc sulfide fluoresces when an atom or more of radium decays. The
fluorescence will still occur in the presence of ionizing radiation. The
radium, OTOH, is nearly dead. Probability says that some number of atoms
will leave their radioactive state and become inert (Lead? I haven't got
time to look it up so I'll share my ignorance with you. Someone will
correct me and the thread will never die.) The rate at which a mass of
radium becomes inert is expressed as its half-life, which is the TIME
that it takes for half of the remaining radium to become inert. The
decay is exponential, as are so many natural things. You'll need a
sensitive Geiger counter to see if there's any life left in the radium.
Or you could expose the watch to a photomultiplier in total darkness to
see if it scintillates.

In other words, it doesn't burn out.

Bill Hawkins


-----Original Message-----
From: Chuck Harris
Sent: Wednesday, July 10, 2013 5:29 PM

The dial is painted, the hands are actually metal frame, and the
luminous paint is a wax that is put on the hand kind of like a soap
bubble.

The luminous material in the paint is very dead, as the hands and digits
no longer glow at all.  I guess there is a limit to how long ZnS:Cu can
take the exposure and still glow.

-Chuck Harris


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