Anybody have experience dealing with gamma radiation?

[ghall 0]

I am installing a camera (CCD) in a linear accelerator (linac) used for cancer treatment and have experienced the gamma ray saturation problem of the CCD. Has anyone else combated this issue? Does anyone know if the radiation is accessing the CCD chips from all angles (through the camera walls etc...) or just through the lens? I'm looking at some leaded acrylic options. Any thoughts or concerns?

 

-Ghall

[Cooperman 0]

Ghall,

I haven't thought much about 'nuclear' related applications in years, so from memory, here are a few considerations.

 

I don't know what type of camera you are using, or indeed where it is located, or what it's looking at. General rules are to minimise, radiation exposure, which can be as simple as using lead 'flashing' as a liner, to cover all external camera surfaces , excluding the front of the lens. Try using the smallest possible front optic (e.g. a conventional, but expensive pinhole lens might be worth considering at a push), and / or if possible, locate the camera as far away from the target / radiation source as is practical.

 

You could think about shielding the camera and using a mirror, so with the camera (completely shielded) at right angles to the radiation source, you need only worry about a minimum of stray particles, rather than an intense source. A number of cameras have a 'mirror' switch so you can reverse the image to appear correctly on screen.

 

You need to absolutely minimise the cameras exposure to radiation; even low level exposure can damage 'sensitive' circuit junctions on a CCD imager, and you can end up with a steadily increasing dead pixel count, or worse.

 

Final thought; the accelerator manufacturer would almost certainly have been asked for advice on this precise problem, so it may be worth getting in touch with them. Many medical companies offer bespoke solutions to common technical problems, but boy do they charge for it!

[Cooperman 0]

Final, final thought!

I forgot to mention that you can buy a very limited range of 'non browning' radiation resistant lenses. Cheap they're not.

If you need a link, I'll have to dig one out.

[ghall 0]

Thanks for the info Cooperman... A link to the "non-browning lens" site may prove helpful. It sounds, from your response, like the majority of the exposure to the ccd happens through the lens. Do you know how critical it is to sheild the sides and other surfaces of the camera? Is the non-browning lens enough protection.

[Cooperman 0]

Physics was never my strong subject, but Gamma radiation is I think the most penetrative, and so it's not so much the material that causes the problems (i.e. lens glass, metal or plastic camera case etc.), but the intensity of the particle beam or proximity of the camera to a strong stray particle stream.

 

I may be wrong but the 'Non Browning' lenses aren't necessarily going to prevent radiation passing through, so much as resist the effects which would be suffered by ordinary glass; in other words, bombard a normal lens with radiation and watch the glass go brown.

 

As regards the degree of shielding required, this depends on the level of radiation which the camera is intended to resist. The idea of mounting the camera at a right angle behind a suitably constructed shield, and then using a mirror to view at a right angle, should ensure that only a relatively small percentage of stray particles will eventually find the camera.

 

If you have to work with affordable but 'disposable' cameras and low tech protection, so be it. It may well work out much cheaper than using a very highly engineered radiation resistant housing, unless of course the budget is available.

 

One link for Non Browning lenses is:- http://www.lens-design.co.uk/miniature_security_camera_lens.htm

 

and I'm not sure if a company like Dennard would have any radiation resistant camera housings up their sleeves. Certainly worth a try.