gamma ray detection

Silicon drift detectors are used to detect x-rays (see Amptek for example), but due to the low atomic number they have poor sensitivity for higher energy gamma rays.

High purity germanium detectors at liquid nitrogen temperatures have excellent energy resolution, but can easily cost more than $100k each. Mercury zinc telluride crystals are used for gamma detection, but need pixelated electrodes for the best performance as well as sophisticated electronics to deal with the high density of p-type traps and are also expensive.

If you only want to detect gammas and not measure their energy, you might want to look a plastic scintillators or perhaps thallium doped cesium iodide crystals as less expensive alternatives to semiconductors.

Bret Cannon

A google search yields some papers on gamma ray detection with photodiodes, but a quick look indicates they use something in front of the photodiode to create a second emission.

As you probably know, on a geiger counter you need to put a shield over the tube so it only measures gamma rays. Perhaps you can set up some sort of mechanical scheme which can block gamma rays in all but one direction, then mechanically scan the device.

I know experiments have been done with film and sources in shielded boxes, though I don't recall which "rays" this experiment is sensitive to. It may be a very low tech solution like exposing film to a source is the way to go.

If you want similar performance to a geiger tube, why not a geiger tube? It'd be difficult to match the active volume of a modest geiger tube with semiconductor detectors, and cost and convenience (a GeLi detector can die permanently if you let its cryogen tank run dry) favor the Geiger.

I understand that some kind of "drift" Germanium (flower power) device may do the job, but that "drift" diffusion apparently makes it rather HEXpensive.

: > Is there a semiconductor replacement for the traditional geiger : > avalanche tube with similar performance?

: If you want similar performance to a geiger tube, why not : a geiger tube? It'd be difficult to match the active volume

The 'multipixel' version of the Geiger counter is known as the Wire Chamber.

Regards, Mikko

A client of mine is making radiation detectors using conductive polymers (I designed him a front end for them). Large area, low cost, good for alphas, betas, gammas, and even neutrons. Email me if you want me to put you in touch with them.

Cheers

Phil Hobbs

One cute wire chamber version uses a single long wire that zigzags between the mylar sheets, making a big s-curve. One uses high-resistance wire, tungsten maybe. The current at the ends is modulated by the wire resistance from the hit site, so you can localize the hit location pretty well, than map that into an x-y array.

ftp://jjlarkin.lmi.net/Peach.JPG

Wire chambers can be built by mere mortals, people who don't own semi fabs.

I had to photograph that screen. How does one capture a full DOS screen running under XP?

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

DOSBox.

I've done XRF with ccds from E2V but that was

Hi John,

That is very cool, 400 pixels with a single wire! :) I guess the mylar sheets are charged like a HV capacitor, and the wire is not charged but has a constant current source put through it from the measurement circuit, and somehow the modulations on the current can tell you accurately what part of the wire had an electron/charged particle fly by it? I guess it is like a network analyzer or something hooked up to the wire. How does the sensitivity to gamma rays compare to a typical off the shelf geiger counter? Also do you use electrical insulation between adjacent columns and rows to prevent detection cross-talk? Sorry for all the questions but it is an interesting device. :)

I think one limitation though may be detecting nearly spaced or simultaneous events, since there is only one wire, if there are multiple electrons/charged particles flying by in different locations I'm not sure if that could be detected with one wire?

cheers, Jamie

One applies the HV to the wire and grounds the aluminized mylar sheets. There's some gas mixture that makes nice ion multiplier effects. The wire is almost invisible, just a few mils in diameter, so the electric field gradient close to the wire in insane. There's a huge ion multiplier gain, close to geiger mode but not quite.

As I recall, we terminated each end of the wire, amplified the pulses a bit, and triggered an ADC on the peak. Some simple math mapped the two pulse amplitudes into position along the wire, with some calibrations maybe. The product was a Safeway shopping cart sort of thing with a big, like 1m square, detector on the bottom. The idea was to sweep a floor looking for hot particles. Lots of facilities need to do this. This sort of thing is a low-rate detector that will get confused by multiple hits.

I also did a classic wire chamber thing for UCLA/CERN. That had a zillion parallel wires per plane, with an amplifier, discriminator, and time-digital converter per wire. Looking at the timing data, one can interpolate the hit position to a fine fraction of the wire pitch. Multiple planes gathered X-Y data and particle path curvature in the magnetic fields. These were Gev particles that made a lot of ions in a lot of chambers without slowing down much. These arrays generate mountains of data on thousands of channels, and the problem is to process huge rates of junk hits down to a set that's possible to archive and analyze. We used a bunch of FPGAs in a data flow peristaltic sort of thing. Messy.

John

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properly

Hi,

That X-Y wire chamber sounds very cool. I wonder if anyone ever tried hooking it up to a strong gamma source instead of a particle source? The experiment I was interested in was to see if a single gamma ray can be detected by multiple "chambers" at the same time. It sounds like the setup's you've worked with are capable of finding that out. Ie. one experiement could be to record the X-Y grid distances between simultaneous detections as they occur over time, and then average them to see if the average distance is less than the distance that would be expected if these simultaneous detections were from separate gamma rays. If the distance is less then I think it could be assumed that a single gamma ray was being absorbed twice.

cheers, Jamie

Printscreen and paste the buffer into your favourite graphics app.

Regards, Martin Brown

Hijaak?

The same as in DOS, or any MS OS since. Hold down the 'CTRL' key and tap the 'Print Screen' key. The screen will be copied to the clipboard. Open 'Paint' or another graphics program and paste it.

That seems to work fine for a DOS *window*, but a when running a full screen DOS session CTRL + ENTER from the DOS command line, it does not work. At least on my machine it doesn't, I get only whatever was on the screen in Windows.

^^^^^^^^^^^^

Correction ALT + ENTER.

That's the obvious thing to do. And it doesn't work.

John

All I can ever capture is a plain white screen with black borders at the top and bottom. This is a full-screen DOS graphics thing, from a DOS PowerBasic program, written in 1997 to run under DOS.

John