Sciencemadness Discussion Board

Why does strontium block x-rays?

CrimpJiggler - 6-1-2012 at 16:42

The glass screens of old cathode ray tube TVs contain about 8% strontium oxide which is added to prevent the TV from emitting x-rays. How does strontium oxide block x-rays? Does it absorb them?

Ozone - 6-1-2012 at 17:08

I thought the glass was leaded or "bariated" to block whatever X-rays you might have. In any case, blocking an x-ray involves a scattering event which can lead to a lower energy photon moving in some random direction (perhaps not out-the-front of the CRT). The more scattering events you have, the lower the energy of the photon (and the more likely it will be to deposit its energy into its surroundings, LET increases as energy decreases).

Other things can happen as electrons are removed via energy transferred from a photon (see photoelectric effect). If the electron was in a lower shell, the electrons will "fall in" from the outer shells to fill the gap. This gives a cascade of x-rays of much lower energy than the incident photon (Auger electrons may also result), the spectrum of which is unique to the target atom. This gives the basis for x-ray fluorescence (XRF) analysis. (see also k-alpha edge)

My guess is that the Sr was present as part of at least one of the phosphors (rather than for any shielding purpose).

Cheers,

O3

[Edited on 7-1-2012 by Ozone]

watson.fawkes - 6-1-2012 at 18:48

A good introduction to CRT glass: http://spie.org/samples/TT54.pdf. It doesn't explain the physics of the x-ray absorption of barium-strontium glass, but it does go into the glass compositions used.

Ozone - 6-1-2012 at 19:14

Cool!

Ah, I see. It's used to increase electron density in non-leaded glass formulations (I was familiar with the use of bariated fronts; they still used leaded glass for the rear-part of the vacuum envelope). In any case, the words "block" or, worse, "absorb" are (while commonly used) mechanistically misleading. Sr is almost certainly used in the phosphor compositions, as well.

Interestingly, is the note that the glass composition is non-browning--this means that the x-rays are not knocking electrons out of the glass matrix. Are the x-rays in a CRT close to the k-edge for Sr* or Ba? My books are at work, and I'll not get back to them until Monday :(.

Hmm,

O3

*If I get time, perhaps tomorrow, I'll calculate that for accelerating voltages from 35-50 kV.



[Edited on 7-1-2012 by Ozone]

watson.fawkes - 6-1-2012 at 20:30

Quote: Originally posted by Ozone  
Interestingly, is the note that the glass composition is non-browning--this means that the x-rays are not knocking electrons out of the glass matrix.
Which is essentially saying that lead glasses brown by the Pb losing electrons, going from Pb(2+) to Pb(4+) oxidation state. Group II alkali earths don't have a stable second oxidation state to enter.

Ozone - 7-1-2012 at 07:32

Plain glass browns very nicely in x or y-ray flux. As the electrons are knocked out (of silica tetrahedra? and "frozen"), the optical transmission decreases (through the creation of so-called "color centers"). The glass can be annealed, returning the electrons to place (whatever that means in an amorphous material--I'd expect this effect to be more pronounced in a crystalline material with a fixed lattice) and with that, optical clarity returns.

The fact that the Pb/Sr/Ba dopant prevents/inhibits this is the interesting part. Perhaps the intimate addition of electron density donates electrons to full the holes created by the ionization event?

"Color center" makes sense in a organic material where a chemical change (e.g. conjugation, etc.) has taken place. In glass I suppose this refers to event-specific points where browning-induced-by-ionization has occured-- these probably have some radius about the point-of-impact which is proportional to the energy of the photon and its LET in the medium.?

Hmm,

O3

[Edited on 7-1-2012 by Ozone]

neptunium - 7-1-2012 at 14:53

we used to lower flask of glass when i was a student in nuclear physics into a very strong y ray source...after a little whille the glass was completely black ...heating it would restore it transparency...
this was a cheap glass though witgout any Strontium in it

watson.fawkes - 8-1-2012 at 08:33

Quote: Originally posted by Ozone  
Plain glass browns very nicely in x or y-ray flux. As the electrons are knocked out (of silica tetrahedra? and "frozen"), the optical transmission decreases (through the creation of so-called "color centers"). The glass can be annealed, returning the electrons to place (whatever that means in an amorphous material--I'd expect this effect to be more pronounced in a crystalline material with a fixed lattice) and with that, optical clarity returns.
What's happening here is that you're getting internal ionization. The electron leaves the vicinity of one atom or bond and hops over to another. Radiation-induced free electrons are "hot", which just means high velocity, so they can hop over Coulomb barriers. When they cool off (collisions etc.) they're stuck where they land, if there's an orbital where they can stably reside. The result is a frozen dipole from that ionization event.

Now a sea of frozen dipoles is exactly what increases the opacity of the glass. If the frozen dipoles where arranged in an orderly fashion, as in some crystals, you'd get macroscopic effects like birefringence. In an amorphous material, you get a lot of scattering. The effect of this is to increase the effective optical path length of each individual photon. Since the underlying material has some initial absorption, the net effect is even more absorption.

What heating the glass does is increase electron mobility. A frozen dipole is in a higher energy state, but facing a kinetic barrier to decaying. The heat lowers the effect of that kinetic barrier by raising the thermal floor of the electron population.

My best guess, therefore, is that the reason that the Ba/Sr glass remains clear is that its oxide structure has no readily available LUMO for a knocked-out electron to land in and reside there. This could be either because the next energy level is high or its decay time is short, or both; I don't know. It could also be the availability of many intermediate states, which existence would entail short decay times of higher-energy MO's.

So an x-ray expends some of its energy ionizing an electron in a HOMO, but the electron migrates back there quickly. There would be photon emission when the electron returns, but it won't be at the same energy. A single x-ray induces multiple ionizations, so the net effect is to take a single event of ionizing radiation and convert into multiple events of non-ionizing radiation.

AndersHoveland - 20-1-2012 at 22:23

Quote: Originally posted by Ozone  
Plain glass browns very nicely in x-ray flux. As the electrons are knocked out, the optical transmission decreases. The glass can be annealed, returning the electrons to place, and with that, optical clarity returns.


Not only does optical clarity return, but if a crystal of sodium chloride is irradiated enough, a flash of vissible light will be observable in a darkened room while the salt is being heated over a hot plate.
http://www.teachersource.com/Product.aspx?DeptID=971
http://faraday.physics.uiowa.edu/modern/7D10.13.htm
Quote:

Ordinary table salt, NaCl, when exposed to very high ionizing radiation, turns brown. When this salt is heated on a hot-plate ( 190 °C), a flash of visible light can be observed and the salt returns to its white color (only safe visible light is given off). This principle is used in many dosimeters that measure radiation exposure. When placed in our reactor, gamma radiation from a cobalt-60 source promotes electrons in the table salt crystals to higher energy levels. In this excited state, electrons in the salt can absorb certain wavelengths of visible light, resulting in the observed brown color. Heat enables the excited electrons to fall back to the ground state with the emission of a flash of visible light and the return of its white color.
Quote: Originally posted by CrimpJiggler  

Why does strontium block x-rays?


To answer your question,
The little gnomes, who live inside the material, catch them all ! :P




[Edited on 21-1-2012 by AndersHoveland]

neptunium - 21-1-2012 at 07:49

oh you naughty little gnomes....

[Edited on 21-1-2012 by neptunium]

strontium

Diablo - 19-2-2012 at 19:16

After some reading on lead, I found somewhere that lead is used in the glass of crt's but not in the front because it causes discoloration.

magnus454 - 13-3-2012 at 20:45

I believe it has to do with electron configuration in the outer electron shells. Some lastics are very good at blocking beta/gamma radiation, if I remember right Polyethylene is a good blocker of radiation.