[ RadSafe ] So, in terms of health physics, what goes on with a television?
Richard L. Hess
lists at richardhess.com
Fri Dec 18 14:39:50 CST 2015
It was always my understanding that the cathode ray tube (CRT) was the
source of potentially "interesting" radiation from the health physics
perspective.
The second anode was close to the screen and was connected via a
connector on the side of the CRT to the high voltage (HV) module. The HV
rectifier was usually shielded. I recall something like 25,000 volts as
the acceleration potential. Sparks would jump 3/8 of an inch or so. TV
service people used to have large HV probes that had a gigohm resistor
in series which was then shunted by the 10 or 11 megohm input impedance
of a then-typical vacuum-tube voltmeter (VTVM), providing a 100:1
reduction of the signal for ease of measurement.
The traditional oscilloscope (brought to a high art by Tektronix) used
the same principal, but I don't know the early history of the
co-development of the oscilloscope and the TV. Early in the literature
there were discussions of oscillographs rather than oscilloscopes. The
difference between the oscilloscope and the typical consumer TV is that
the oscilloscope uses electrostatic deflection inside the CRT while the
television uses a magnetic deflection yoke placed around the neck of the
CRT just where it widens out.
The safety issues, as I understand them from the perspective of a
curious technology oriented person involved in the television
broadcasting field professionally from 1974-2004 (and now doing audio
tape restoration professionally and supporting a bunch of computer
equipment to enable that business), were well addressed by the use of
leaded glass in the CRT envelope in televisions.
Wikipedia has two paragraphs in an extensive article that pertain to
this discussion. Some of it is out of date. The link to the CFR is
broken...but it's a starting point.
> Ionizing radiation [from https://en.wikipedia.org/wiki/Cathode_ray_tube]
>
> CRTs can emit a small amount of X-ray radiation as a result of the
> electron beam's bombardment of the shadow mask/aperture grille and
> phosphors. The amount of radiation escaping the front of the monitor
> is widely considered not to be harmful. The Food and Drug
> Administration regulations in 21 C.F.R. 1020.10 are used to strictly
> limit, for instance, television receivers to 0.5 milliroentgens per
> hour (mR/h) (0.13 µC/(kg·h) or 36 pA/kg) at a distance of 5 cm (2 in)
> from any external surface; since 2007, most CRTs have emissions that
> fall well below this limit.[46]
> Toxicity
>
> Older color and monochrome CRTs may contain toxic substances, such as
> cadmium, in the phosphors.[47][48][49] The rear glass tube of modern
> CRTs may be made from leaded glass, which represent an environmental
> hazard if disposed of improperly.[50] By the time personal computers
> were produced, glass in the front panel (the viewable portion of the
> CRT) used barium rather than lead,[citation needed] though the rear of
> the CRT was still produced from leaded glass. Monochrome CRTs
> typically do not contain enough leaded glass to fail EPA TCLP tests.
> While the TCLP process grinds the glass into fine particles in order
> to expose them to weak acids to test for leachate, intact CRT glass
> does not leache (The lead is vitrified, contained inside the glass
> itself, similar to leaded glass crystalware).
>
> In October 2001, the United States Environmental Protection Agency
> created rules stating that CRTs must be brought to special recycling
> facilities. In November 2002, the EPA began fining companies that
> disposed of CRTs through landfills or incineration. Regulatory
> agencies, local and statewide, monitor the disposal of CRTs and other
> computer equipment.[51]
>
> In Europe, disposal of CRT televisions and monitors is covered by the
> WEEE Directive.[52]
I guess this is my once-a-year post to let you know I'm still alive and
not worrying too much about radiation.
Cheers,
Richard
On 12/18/2015 2:25 PM, Bob May wrote:
> I think the principal x-ray source from the old TV sets was the high voltage rectifier vacuum tube and not the CRT itself. I think theses high voltage rectifier tubes had metal (Al?) sleeves over them...
> Bob
>
> ------------------------------
>
> Message: 2
> Date: Thu, 17 Dec 2015 21:54:01 -0500
> From: Joseph Preisig <jrpnj01 at gmail.com>
> To: "The International Radiation Protection (Health Physics) Mailing
> List" <radsafe at health.phys.iit.edu>
> Subject: Re: [ RadSafe ] Speaking of a de minimis regulatory level
> ....
> Message-ID:
> <CAJ9CPDgjWu0esXzB+ikeFrjjgwF4aeWfgrK6OdTOnaRFMkqT9A at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
>
> Radsafe/Ted:
> 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?
>> _______________________________________________
>> You are currently subscribed to the RadSafe mailing list
>>
>> Before posting a message to RadSafe be sure to have read and understood
>> the RadSafe rules. These can be found at:
>> http://health.phys.iit.edu/radsaferules.html
>>
>> For information on how to subscribe or unsubscribe and other settings
>> visit: http://health.phys.iit.edu
>>
>
> ------------------------------
>
> Message: 3
> Date: Thu, 17 Dec 2015 19:22:24 -0800
> From: Ted de Castro <tdc at xrayted.com>
> To: "The International Radiation Protection (Health Physics) Mailing
> List" <radsafe at health.phys.iit.edu>
> Subject: Re: [ RadSafe ] Speaking of a de minimis regulatory level
> ....
> Message-ID: <56737BF0.8020200 at xrayted.com>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> 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:
>> Radsafe/Ted:
>> 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?
>>> _______________________________________________
>>> You are currently subscribed to the RadSafe mailing list
>>>
>>> Before posting a message to RadSafe be sure to have read and understood
>>> the RadSafe rules. These can be found at:
>>> http://health.phys.iit.edu/radsaferules.html
>>>
>>> For information on how to subscribe or unsubscribe and other settings
>>> visit: http://health.phys.iit.edu
>>>
>> _______________________________________________
>> You are currently subscribed to the RadSafe mailing list
>>
>> Before posting a message to RadSafe be sure to have read and understood the RadSafe rules. These can be found at: http://health.phys.iit.edu/radsaferules.html
>>
>> For information on how to subscribe or unsubscribe and other settings visit: http://health.phys.iit.edu
>
>
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--
Richard L. Hess email: richard at richardhess.com
Aurora, Ontario, Canada http://www.richardhess.com/
http://www.richardhess.com/tape/contact.htm
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