[ RadSafe ] Speaking of a de minimis regulatory level ....
Ted de Castro
tdc at xrayted.com
Thu Dec 17 21:22:24 CST 2015
Let me make it easier for you ......
Old CRT TV's with tube regulated HV circuits used a shunt regulated high
voltage supply (ie. shunt excess current through a series resistor to
regulate the voltage at the end of the resistor). With an HV of as much
as 45 kV on larger CRT models - the darker the screen the more the
current shunted through the regulator tube to maintain the HV. So -
what you have is a current limited diode vacuum tube with a 45 kV across
it and a current flowing - ie. an unoptimized x-ray tube ( make the
anode W and angle it and you have a pretty decent x-ray tube) --- SO
the x-ray emissions were out the back and the darker the picture the
higher the emission.
The actual HEW limit was 1/2 mR/hr at 2 inches from any accessible
surface with all (user) adjustments made so as to maximize x-ray
production - ie. HV at MAX and picture fully dark. There was never any
issue with x-rays out through the face of the picture tube.
The problem was initially solved by hooding the anode of the shunt
regulator tube and leading the glass of the tube envelope. Then quickly
came solid state HV power supplies and the problem completely went away.
NONE THE LESS - its still out there declaring 1/2 mR/hr as an acceptable
surface emission limit for a common household appliance - I'd call that
acceptance/declaration of a de minimis level.
On 12/17/2015 6:54 PM, Joseph Preisig wrote:
> Hmmmm. Don't know much about television dose limits. Older
> televisions work something like as follows. An electron
> beam starts out at the back of a TV, via an electron gun, (febetron???).
> The beam is shot towards the TV screen and sweeps horizontally across the
> TV screen (rather quickly). In some time step (also quickly) there is some
> vertical stepping so the whole TV screen is scanned. A phosphor on the TV
> screen is activated at each TV pixel location, and you see the TV picture.
> Newer TV's have LED or Liquid Crystal or other displays. Of course, the TV
> is receiving the signal via an antenna, cable system or whatever.
> I would believe, without specific knowledge, that the oscilloscope was
> a precursor to the TV. The television was developed by groups led by
> Farnsworth (read about him on the internet) or Vladimir Zworykin (RCA
> Laboratories USA). My Dad also worked at RCA Labs (Princeton, New Jersey)
> and he held a few patents and one patent on a portable color TV. He worked
> on vacuum tube technology and eventually solid state. When RCA ended, the
> TV division was absorbed by Thomson Electronics, the Solid State Division
> was absorbed by General Electric and there were other pieces. I think
> there are still some TV's manufactured with the RCA name.
> So, in terms of health physics, what goes on with a television? The
> electron beam is accelerated (at some current) via some voltage towards the
> TV screen, which is a fairly thick piece of glass. The television tube is
> under vacuum. The electron beam isn't directed at one position at the TV
> screen, but sweeps vertically and horizontally across the screen. I don't
> know the range of the electron beam in the glass, and I expect this number
> varies by television model/brand. I would hope not much electron beam is
> getting out of the TV glass, but if it does, it is being distributed all
> across the room where people may be watching. I suspect there are TV
> regulations/safety regulations on the internet, or in paper or book form.
> Not everything is on the internet.
> Have a good weekend. Anyone stick their fingers into xray units
> lately, or has anyone crawled into holes in radiation shielding and/or
> defeated interlocks and gotten some large doses??? The demand at
> Universities and/or research institutes must really be large these days
> with various people doing xray diffraction and xray flourescence, with
> people analyzing DNA samples etc. The NSLS II (the National Synchrotron
> Light Source 2) is starting to run nowadays at Brookhaven Lab. Brookhaven
> now has a pretty fair assortment of physics toys with the exception of not
> having a Large Hadron collider. The NSLS2 is there, along with all the
> accelerators associated with the Relativistic Heavy Ion Collider (200 GeV x
> 200 GeV protons and various Heavy Ions such as gold) --- a Cockroft-Walton
> accelerator, a linac, a booster accelerator, the Alternating Gradient
> Synchrotron, and RHIC itself. There is also a NASA beamline for
> radio-biology etc. studies and BLIP for accelerator production of
> radioisotopes. Many of the Health Physics technicians at Brookhaven have
> Masters degrees.
> Joe Preisig
> On Thu, Dec 17, 2015 at 8:38 PM, Ted de Castro <tdc at xrayted.com> wrote:
>> Recently while doing some consulting for an analytical x-ray installation
>> I explained the use of the HEW "Color TV limit" as a "bullet proof" leakage
>> criteria without needing to go any lower.
>> Thinking about that I realized that the HEW in declaring an acceptable
>> emission limit for an entertainment device in the home and often attended
>> by young children had in effect made a regulatory declaration of de minimis
>> acceptable radiation levels.
>> Never mind that few if any sets actually emitted this much, that the ones
>> that did only emitted that much out the back, that it applies only to now
>> obsolete technology and that it was stipulated under conditions that
>> rendered the image unviewable - never the less - its out there, in federal
>> law and effects to living rooms across the country!
>> I wonder what dose consequences were assumed in arriving at that standard?
>> Comments on this notion?
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