Tricky Tubes: Newsletter Challenge (July 2017)

Xrays are emitted when electrons decelerate when hitting the screen of the CRT, so the answer is the front of the CRT.

You can get x-rays from plastic tape also!

http://www.nature.com/news/2008/012345/full/news.2008.1185.html

Sticky tape emits light - and X-rays - as it unpeels.

..when I think about the countless rolls wasted trying to escape the supply closet..

The electron gun in a CRT (cathode ray tube) accellerates electrons using voltages ranging from 7000 to over 20,000 volts. The electromagnetic 'yoke' magnets around the CRT neck help direct the electrons to the screen. The screen is composed of phosphors that glow when the electron beam hit them; this layer of phosphors is coated with a thin layer of aluminum which is grounded.

So the electrons are shot from the gun and strike the screen. It is this deceleration of the electrons, called Bremsstrahlung, or braking radiation, which requires that the electrons give up their energy, which comes out as x-rays.

The CRT makers were well aware of this x-radiation and minimized its intensity by using a thick, heavily leaded glass for the CRT screen. The lead in the glass attenuates the x-rays. The thickness of the glass was such that even if the yoke were to fail, the x-ray dosage from the thin beam of electrons striking the screen would be within safe limits under ordinary conditions.

You also get x-rays of of the high voltage rectifier, if it used a vacuum tube rectifier. On some color sets they used a high voltage triode as a shunt regulator on the DC to the CRT anode and that was another source. Typically these tubes were located inside a metal box to reduce the emissions.

this is not the only source of x-rays in some types of color televisions; the high voltage rectifier as a tube and the high voltage regulator tube were also sources,most times the rectifier was enclosed but not always the case with the regulator , in fact some types of tubes were recalled with reward rebates as known to cause harmful radiation especially dangerous to children setting on floor directly in front of these sets I worked on some of these sets along with my 40 year career with electronics. comment from Roland Kelts . cet(by nea).

So that is why I now have a receding hair line at a youthful 63! Dern CRT's, and dern Monahans TV station coming through all snowy with the only cartoons on.

Don't forget the Ion Trap! The cathode electron gun was not aimed directly down the tube neck but at an angle (mostly upwards a few degrees from what I recall). A permanent magnet on a ferrous yoke clamped to the neck just downstream of the gun(s) deflected the electrons toward the screen but left the X-Rays to go straight on.

Perhaps that's why the flowers on top of my parents' TV never lasted.

Seems everybody digresses.

I believe that the answer is not at the front of the CRT. The sides would be most likely to show more x-radiation, since we sit in front of the leaded front glass since viewing from the sides is pretty dull! But, being in that repair business, I did my share of viewing from the back

The amount or strength of radiation is affected by the square of the distance from its source to the viewer(s).

To measure radiation dose, I will point the x-ray detector on the viewer closest to the TV set.. It should show the effective dosage of radiation being received..

Someone of an experimental bent with an old TV set and some photographic film could probably provide some actual data.

Well that is a common approach used in most hospitals. Staff are required to wear a strip of film clipped in their badges. Which are regularly analyzed , quantifying how much x-ray radiations the wearer been routinely exposed to ...

Unless the xrays scatter back (unlikely) you won't get much of a reading.

The concern is the distance between the radiation source and how much the viewer is supposedly being exposed to...

In my view radiation can be cumulative over a certain period of time, while watching or by just sitting in front or around the TV). It is the distance from the source that matters. Regardless whether the radiation is scattered, direct, reflected, etc.

The x-rays produced by the electron beam are in a diffuse pattern and also get refracted by the "target" materials. Since the front of the tube (and a small amount onto the sides) has the heavy leaded glass coating to absorb the x-rays heading towards the viewer, you will measure a higher amount of them from the sides of the tube and a smaller amount from the rear.

--JMM

Turn off the horizontal and vertical deflection circuits to concentrate the beam at the center of the screen and measure the X-ray strength here. This should give you the maximum possible reading.

See my comment about Ion Trap earlier. Most X Rays exit through the neck (usually upwards in the ones I have seen) and do not head direct for the screen in order to prevent 'ion burn'. Most likely place to have X Rays exit the TV is just above the screen at the front if my memory of the gun angle is correct.

Here's a drawing showing a typical offset offset gun.

https://antiqueradio.org/art/Hoffman7M112RaulandCRTPatentDrawing.jpg

I agree that the x-rays would be directed up from the neck in a well designed set, and if they weren't they should have been. I meant that the way I described should give the maximum dose emitted by the screen front. ideally this will be a reading of zero. Some of the later sets had, from memory, circuitry to minimize X-ray emissions. If I weren't chasing so many rabbits at once I could pull a circuit out and reverse engineer it to find out how it worked. The next time I get to my files and have a spare minute I might even do that. Its been a loooong time since I did any repairs on predigital tvs and someone snuck those in while I wasn't looking.

I propose another experiment: Leave the tube inside its box (the TV)!

Mount a geiger tube on a goniometer set-up, and rotate from the low front to around low to the back, making sure to pass over the TV set. Log readings, and note the goniometer position (another use for high resolution angular displacement sensor), and log that too. Now align the geiger tube at the maximum zenith position of x-ray intensity, and scan the azimuthal readings for that zenith position.

The point of maximum radiation (my guess, not the experimental result, this is hypothesis) is about 80 degree from front horizontal as zenith, and 90 degrees azimuth, where 90 degree refers to straight up, zero would be horizontal at the right side from front.

I used to read CRT spec sheets from the manufacturers in my job helping to design CRT displays for cockpits.

The electrons that 'boil' off the cathode don't generate xrays until they experience the high voltage acceleration and then deceleration at the screen. The manufacturers' warnings were all about xrays at the crt screen; I never saw any warning about xrays anywhere else.

The only other warning was a caution about touching the anode cap. You could get a nasty shock if you touched the anode cap even weeks after the last time the CRT was turned off.

OK, I certainly agree about charge storage at the anode cap.

I must also disagree on one point:

If the voltage is applied to the electrons at the distal end of the tube, and they are fired off axis to the perpendicular of the screen, then whatever lies in the pathway of the electrons (that are already accelerated to that kinetic energy), could potentially produce Bremsstrahlung (x-rays), but in truth as you stated, they should be bent by the magnetic field applied, and all strike the screen, thus producing phosphor excitation. If there were no magnetic field to bend the electron path, I suppose a lot of x-rays might be seen off axis.

This has the earmarks of a trick question.

Regardless, you point the detector at the television set.

The question is where do you LOCATE the detector.

Initially the x-rays were projected from the screen and the detector was placed where the user would be located. Once the heavy leaded glass screen covers were put into use, the emissions were primarily to the sides, top and bottom, but because the user rarely sits behind the TV, the emissions testing still placed the detector in front of the TV.

You could survey all around the TV, but you still point the detector at the TV.

Isn't that why the air behind the TV used to smell phunny?

No, James, that smell is the missing pet hamster,"Sparky".

LOL, I guess wired hamster just does smell the same as grilled.

Wired hamster? What's his pharmaceutical of choice?

He bit the big red wire insulation and liked it too much. At first we called him Lew, but after that we changed his name to "Benny". I don't know where you came up with Sparky, that was my pet road lizard.

It may sound like a tricky question but its not!.... The question in my perspective is about the proper detector positioning, to measure the amount of x-ray radiation the TV viewer(s) received while watching TV...

We all agree that x-rays are being radiated and regardless which TV section is giving off the maximum emission, the distance between the TV set to the viewer is what the question is all about?

Methinks the only logical place for the detector is still by the viewer.. Whichever part of his/her body is positioned closer to the TV set receives the maximum..

During Radcon training in the navy, it was not unusual to have a worker wearing several dosimeters at different locations on their body because different parts of the body tolerate exposure differently. A thyroid or chest are more sensitive to radiation than were hands or feet. It would make sense to position an array of detectors in the location of the viewer or do a locational scan with a single detector to find the safest and most dangerous locations to answer such burning questions as, "should I lie on my back with my thyroid 24" below the television and watch the programs in a mirror?"

There was a brief period where dosimeters were electronic and chirped every few microrems. Dosages went up and we found out that the radiation workers were wandering around to see where they could get the dosimeters to chirp the fastest, like a contest.

VSAR wrote:

It may sound like a tricky question but its not!.... The question in my perspective is about the proper detector positioning, to measure the amount of x-ray radiation the TV viewer(s) received while watching TV...

We all agree that x-rays are being radiated and regardless which TV section is giving off the maximum emission, the distance between the TV set to the viewer is what the question is all about?

Methinks the only logical place for the detector is still by the viewer.. Whichever part of his/her body is positioned closer to the TV set receives the maximum..

Two different cases. If you are trying to find the viewer's received dose then of course you must measure at their position. However, if you are trying to find the direction of maximum radiation by the TV, then from my memory of working on TVs decades ago, the strongest X-radiation would have been just above the front of the tube, due to the ion trap previously mentioned.

NeilA.....

It is very possible that the strongest radiation maybe coming just above the front of the TV....

At issue in my view is the radiation dose....which I'm interpreting as the amount of radiation the viewer receives while watching?

Regardless where the strongest emissions are coming from, the dosage is still the amount measured as received by the viewer... An amount that is affected by the spacing or distance between the xmtr and the rcvr plus whatever physical interference that maybe in between..

Is that why my bird died every time the canary cage rested on top of the Phillips?

Let's see if I can disremember my Radiation Safety Officer training from the early 80's:

The way a Geiger-Mueller tube works, you hold the tube parallel to the surface of the screen. An X ray passing through the Geiger Mueller tube causes a spike in the charge and releases an electron (making the click we often associate with the old Geiger counters), and is best detected if the tube is transverse to the flux of photons meaning parallel to the Screen, and probably in front of the center of the discharge.

As I recall, the folks used to "fuss" at me about sitting directly in front of the TV, now I have to wear glasses (since 8th grade). Darn it!

If I had sat off to the sides or back, then I would be a mutant. Oh snap! I am a mutant - in the manner of Bull Shannon on Night Court.

I think the problem really came to light with the "newer" larger screen color CRT's. To achieve brighter images the acceleration voltage was increased. In the color CRT's there is a steel shadow mask that blocks the three beams when they are not aligned with the three color phosphorus dots. I believe that it is when the electron strike the steel that they produce the most and higher energy x rays. The steel mask provides some shielding for the rays coming out the front. Most of the x rays come out the sides and back of the tube.