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Author Topic: How does radiation blacken photographic film?  (Read 779 times)

Offline chris

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How does radiation blacken photographic film?
« on: 13/04/2016 21:39:22 »
In Becquerel's experiments pieces of uranium salts caused photographic plates to blacken in the same way that Roentgen's X-rays did. Why does this happen?


 

Offline alancalverd

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Re: How does radiation blacken photographic film?
« Reply #1 on: 13/04/2016 22:38:25 »
This is horrendously simplified - I have a rack of textbooks on the subject, and it's still something of a black art!

Silver halides dissociate when stimulated by photons, into elemental silver and halogen. In a photographic emulsion the halogen atoms can diffuse out of the halide crystal and react with the suspending gelatin before they can recombine with the silver atoms, leaving particles of silver as defects in the crystals. The development process removes the unexposed silver halide, which is slightly soluble in some reducing agents, leaving the insoluble metallic silver in the emulsion.   

Ionising radiations (x, gamma, alpha, beta...) deliver more energy than visible photons to the emulsion. It generally requires at least two visble photons to produce a permanent change in a halide crystal (you need to deliver enough energy, in a short enough time, to drive the halogen away from the silver atom) but a single gamma photon will produce several secondary photons and electrons when it interacts with the emulsion, and each of these can impart energy to the halogen atom, so pretty well every ionising radiation photon or particle will produce a permanent change.   

Uranium releases alpha, beta and gamma radiation as it decays, and all of these have a similar effect to x-rays in transferring energy to silver halide crystals.
 

Offline evan_au

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Re: How does radiation blacken photographic film?
« Reply #2 on: 13/04/2016 22:43:40 »
It is the same mechanism as light sensitivity in silver halides.

The necessary energy to trigger the chemical reaction can come from visible light, X-Rays, gamma rays, cosmic rays or even alpha/beta particles.

Quote from: Wikipedia
When absorbed by an AgX crystal, photons cause electrons to be promoted to a conduction band (de-localized electron orbital with higher energy than a valence band) which can be attracted by a sensitivity speck, which is a shallow electron trap, which may be a crystalline defect or a cluster of silver sulfide, gold, other trace elements (dopant), or combination thereof, and then combined with an interstitial silver ion to form silver metal speck.

Developing converts this speck of silver into a whole grain of silver, which absorbs light and appears black. 

See: https://en.wikipedia.org/wiki/Silver_halide#Light_sensitivity
https://en.wikipedia.org/wiki/Film_badge_dosimeter

Alan beat me by a few seconds...
 

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Re: How does radiation blacken photographic film?
« Reply #2 on: 13/04/2016 22:43:40 »

 

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