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RE: Smoke Alarms & Photon Beams Don't Mix




We ran into similar problems in my former life operating a fast burst
reactor.  Remember that this type of smoke detector uses an ion chamber
concept - the ionization of smokey air is different from clear air.
Therefore, anything that ionizes the air in the chamber will set it off.
(They also work real well when somebody is jack-hammering the concrete floor
underneath them - believe me, I've been there too :-)

You might consider photoelectric smoke detectors as an alternative, but the
best solution for us was conversion to heat detectors rather than smoke
detectors.

Doug Minnema, Ph.D, CHP
Defense Programs, DOE
<Douglas.Minnema@ns.doe.gov>

what few thoughts i have are truly my own

> -----Original Message-----
> From:	Chris Alston [SMTP:alstonc@odrge.odr.georgetown.edu]
> Sent:	Monday, February 14, 2000 5:48 PM
> To:	Multiple recipients of list
> Subject:	Smoke Alarms & Photon Beams Don't Mix
> 
> >Date:         Fri, 11 Feb 2000 15:52:19 -0800
> >Sender: Medical Physics Mailing List <medphys@LISTS.WAYNE.EDU>
> >From: "David J. Misisco, M. S." <david.misisco@CHOMP.ORG>
> >Organization: Community Hospital of the Monterey Peninsula
> >
> >Here's one for the text books.  We just opened a new center here, new
> >vaults, new linacs, etc.  While conducting the radiation survey on the
> >second linac, I directed the gantry at 45 degrees in order to make
> >measurements outside the room in a public area.  Within seconds of
> >energizing the 18 MV photons the fire alarm system went off.  Since this
> >was a new facility, we were accustomed to random fire alarm system tests
> >and thus ignored the alarms - until the fire department and about 6 guys
> >with fire extinguishers came running into the control room.
> >
> >It seems that the smoke alarm was mounted in the plane of rotation of
> >the gantry, and happened in this case to be in the beam during this
> >measurement.  The charged particles set in motion by the photons in the
> >smoke alarm must have disrupted the alpha particle stream, thus
> >simulating smoke.
> >
> >Well, the fire department was relieved that they didn't have to fight a
> >fire, but our engineers are a bit embarrassed.  One to remember the next
> >time you have the opportunity to design a new site.
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