Flash x-rays and phosphors / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Flash x-rays and phosphors

previous 1 2 3 4 next last

Move Thread

LAN_403

Proud Mary

Sun Mar 23 2008, 01:39PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Direct X-ray exposure of film is not very efficient so dedicated X-ray films have a much thicker coating of sensitive silver salts to stop more of the rays.

In addition, it is common to use an 'intensifying screen' between the X-ray source and the film. This screen is coated with very fine particles of X-ray phosphors, so it lights up when the X-rays hit it, and so exposes the film much faster and more efficiently than the X-rays alone would be able to do. Intensifying screens are always used in conventional medical X-radiography, to minimize the received dose to the patient and to the radiographer.

{After the first very expensive barium platinocyanide/ Ba[Pt(CN)4].4H2O screens used by RÃ¶ntgen, the main X-ray phosphor to be used from the old times was calcium tungstate/CaWO4 - {the mineral Scheelite, which is found in Norway} This was followed by cadmium tungstate/CdWO4 which being heavier can stop more rays. These simple X-rays phosphors have been replaced in modern X-ray equipment with more efficient, but more complex phosphors, but are still very good if you can get them.}

To expose a film with your flash system, I think you will have to use multiple exposures. Leslie Wright {"Plazmatron" of this forum} knows a lot more about this than I do, and will give better advice.

Did you know that you can detect X-rays quite well with a common PIN photodiode?

c4r0

Sun Mar 23 2008, 01:53PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Uzzors wrote ...
I have no experience whatsoever with photography, what kind of paper/film should I get and where? Won't I need some chemicals to expose the paper as well?

Every b&w paper will be good. Just for the beginning it can be also expired paper and you can buy it very cheap on internet auctions. You will also need chemicals: developer and fixer, a red light lamp (i use a few red LEDs

) and dishes for the chemicals. Here you can read about processing the paper:

It sounds difficult to do but in fact it's quite easy :)

uzzors2k

Sun Mar 23 2008, 06:23PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Thanks for the link c4r0, it cleared things up. I'll try taking a photograph first with my ghetto phosphor screens, and if that works at all, I'll invest in a real intensifier. Assuming I can find the chemicals and paper first of course.

c4r0

Sun Mar 23 2008, 08:08PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

First of all, wgen you get the chemicals and paper, try to develope a few ordinary photos instead od radiograms. To expose the paper you can place sth on it in darkroom and turn on the light for a few seconds. When you learn how it works then you can try doing radiograms and if it doesn't wor you can be sure that it's not development fault

For such tests you dont neet to waste whole sheet of photo paper ofcourse, you can cut a piece of it using scisors

In darkroom ofcourse.

uzzors2k

Sun Mar 23 2008, 08:11PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Since it's sort of on topic: How do you take ordinary photos, do I need a pinhole camera or something ancient?

c4r0

Sun Mar 23 2008, 08:17PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

I wrote that you can place something (a door key or something like that) on the photo paper and exposure it with flashlight for example - it will be a shadow type photo (in fact just like x-ray photos). You need to do that just to see how developing works. If you want you can make a pinhole camera too ofcourse :D

c4r0

Tue Mar 25 2008, 06:23PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Today I translated this part of my website to english:

You can read there something about how I made my paper radiographs :) And if somebody find some glaring mistakes in my english (probably a lot :P ) please let me know how to correct it :)

Proud Mary

Tue Mar 25 2008, 07:08PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Good work, Karol! A very interesting and informative page! Well done!

uzzors2k

Tue Mar 25 2008, 08:18PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Alright! Thanks. Making those photos isn't too hard then. I've found a store which stocks this stuff, so I'll buy some soon and try it out.

plazmatron

Wed Mar 26 2008, 10:44AM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Hi Uzzors,
although ZnS, is listed as an X-ray phosphor, it has appalling quantum efficiency. Although t is great for detecting heavy charged particles like Alpha. As someone else mentioned earlier, this is mostly to do with the Z number of the phosphor, which is why for a great number of years CaWO4, was used in intensifying screens. In fact the efficiency of CaWO4 is such that it became the fluorescence standard for x-ray screens, against which all other phosphors are measured.

CaWO4 screens and cassettes show up on fleabay all the time, and are well worth the investment.
Newer screens are available too, using modern rare earths, but I would avoid them if you are not using x-ray film. This is because modern rare earths, whilst more efficient, tend to emit in the green-orange portion of the spectrum, where common black/white film, is several times more sensitive to blue light (such as that emitted by CaWO4)

As for exposure time, with a marx firing twice per second, figure about 1 minute exposure using CaWO4 screens, and ordinary variable contrast B/W paper at 8 inches distance from the tube, and work from there, for your particular setup.

As you have already realised the x-ray pulse from flash setups is VERY penetrating, and it would be advisable to design some sort of shield for the tube, or at least be FAR away from it when it is running

Leslie

previous 1 2 3 4 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob