Xray detection 1

[xray]

I have a question. I have a successful photodetector circuit with 8-bit resolution from an on mcu adc. works well from a dark room to room well lit. The detection comes from a simple 1n4148 glass encased diode a couple of op amps to give a dc offset for low readings and approximatley 100 times gain. QUESTION: What would have to occur to make this an Xray detector. The radiation is in the medical diagnostic range 50kVp - 120kVp. The circuit will do nothing in the radiation beam and my thoughts would be it should register full on the same circuit as the photodetector; higher frequency beam higher energy and therefore a higher reading on the same circuit.

Any suggestion greatly appreciated; XRAY

 

[IRstuff]

X-rays are too energetic to be detected by conventional silicon diodes.

You could potentially wrap the diode with some sort of X-ray scintillating material. Do a web search on x-ray scintillator.

TTFN

 

[rmazzullo]

I believe a photodiode has unity gain if a scintillation material is covering the photodiode, and everything (photodiode and scintillation material) is enclosed in an opaque package (light tight). If you use an avalanche photodiode, the gain is much higher, but so is the price for such a device. The scintillation materials I am referring to are Gadolinium Oxysulfide (5 - 180 KeV) and Cadmium Tungstate (150 - 420 KeV), as well as silver activated Zinc Sulfide, but I have no immediate details on that (I think it's used for alpha rad)...

Hope this helps a bit..

 

[xray]

I am aware of a minimum energy required to create on electron whole pair within a Si junction: 1.1eV and the corresponding wavelength is 1.1Um. This is not my calculation it has already been done and I am just privileged to the knowledge. I am merely trying to take the acquired knowledge and use it in a slightly different way, photodetecotr to a xraydetector. If a photon, xray, has some 10^4 eV of energy the electron hole pairs created would surely be > 1. The photon must be able to create hole electron pairs when passing through the simple Si diode. Any thoughts on this?

 

[peebee]

That's IRstuff's whole point: there's too big an energy mismatch. There's 5 magnitudes of order of difference between the photon energy and the Si electron quantum energy jump. If you did get any Si atoms to jump states when hit with that photon, the electron would just be blasted away, and you would end up with something that looked a whole lot more like an ionized Si atom than an electron-hole pair. For the most part, though, the energy levels (and associated wavelengths) are so far off that few if any atoms would change state. That's the whole problem you're trying to solve.

Not that I have any practical answers for you.

 

[xray]

I would like to get the picture. Where are these electron blasted away to? Thanks.

 

[IRstuff]

The energy gap in Si does not imply that anything with a higher energy is captured; in fact, Si response to anything shorter than about 260 nm is essentially nonexistent.

Point of fact is also that Si is essentially transparent in x-ray regime, as fluoroscopes are often used to inspect die bonds, which require the intervening material to be reasonably transparent in the x-ray regime.

Most standard Si x-ray detectors use a fluorescing material to convert the signal to a visible signal.

TTFN

 

[peebee]

Exactly. Transparency means none of the energy is absorbed by the material, it's all passed right through, and no electrons have been displaced. The atom spacing and electron energy levels are of a very different scale than the wavelength. Problem is, thats common with lots of materials when it comes to xrays.

So far as "blasted away": far enough away that it's probably not gonna be recaptured by the atom. Same as any ion -- it's just short an electron. Kind of like a missle that reaches escape velocity, it's just gone for good.

You're talking a difference of 5 magnitudes. That's huge. Imagine the difference between a 1hp motor and a 100,000hp motor. No comparison. Trying to detect x-rays with a silicone diode is like using a bathroom scale to weigh an elephant, or using an AM radio to detect visible light.

 

[xray]

well that's put to bed. Thanks for your help.

 

[peebee]

Maybe Silicon PIN diodes will work for you. It looks like the high-impedance intrinsic (I) layer keeps the electrons from ionizing. See:

http://www.eng.auburn.edu/~troppel/courses/elec663spring2000/OralPresentations/Pres_Li_Ying.ppt

or

http://www.microsemi.com/brochures/pindiodes/appendix f.pdf

The electron drift detector seems to be a similar concept but in a very different configuration:

http://www.ketek.net/layer2/products/1comm_x_ray1.htm

This is an interesting site on x-ray detection in general:

http://www.canberra.com/literature/basic_principles/gamma.htm

X-ray fluorescence of visible light (XRF) might work for you, but I couldn't find any goon on-line information on what would be a suitable material.

 

[xrftom]

I think your silicon diode should make a fine x-ray detector. A few questions.
Is the diode in a light tight package. If not the visible photon flux will mask the x-ray output.
Is the window in front of the detector low Z so that the x-rays can penetrate it?
what is your source of x-rays. An x-ray tube will provide a good flux and should be detected. a smoke detector source is not intense enough to produce much signal.

 

[warpymoople]

I have designed an IC for an x-ray machine. I was
working for a company called Black Forest Engineering.
The project was for a company called Cardiac Mariners.
I'm sure you can find information on both of these
companies. To get to the point ... this system used
a compound of Cadmium Telluride. This compound was very
good at producing a lot of detectable current from just
one high-energy x-ray photon.

 

[wizardofx]

The X in my handle (wizard of X) stands for x-rays.
Pin diodes are used for detecting x-rays all the time.
You can get more than one electron per x-ray.
In fact I have a pin diode detector in my office
that can measure the energy of the x-rays by
measuring how many electrons are produced by
a single x-ray.


However, it takes a lot of low-noise expertese
to detect one x-ray at a time. As you move to
medical type hard x-rays the absorption of the
silicon drops, so you detect fewer of the x-rays
flying through the silicon. But at the kind of fluxes
that medical x-rays use, you should be able to
detect a signal.

A scintillator on top of the diode can get you much more signal
because they produce light, and because they
are thicker and have higher absorption.
Almost any phosphor will detect x-rays, as
will some fluorescent plastics. The common
scintillator used for this purpose is thallium
doped sodium iodide.

Best regards
mark

 

[xray]

I have since acquired some pin diodes and am looking to implement them in the circuitry I have put together. Thanks for the response.