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Geiger counter design and prospects of scintillation spectroscopy

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Carbon_Rod

Mon Apr 08 2013, 09:02AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Just curious if anyone knows whether its possible to differentiate the emission energy level of Geiger tube spikes.

For example, does the spark gap avalanche event settling-time differ with ionizing sources?

Cheers,
Rod

Steve Conner

Mon Apr 08 2013, 09:21AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Short answer: No. Geiger tubes operate in the avalanche region, every discharge is a complete discharge of the tube. Different particles may touch it off in subtly different ways, but the information is lost in the resulting chaos. Essentially you have a flattened ski resort and you're trying to figure out exactly which snowflake started the avalanche.

Long answer: Your Geiger tube may show a "proportional region" when it's operated below its rated voltage.

Carbon_Rod

Mon Apr 08 2013, 10:07AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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Are they sampling a ramp generator (~80v to 400v) to garner â€œtriggerâ€ thresholds?

Would the method generalize to an alpha "spark detector"?

The Neon indicator tube in series with the Geiger tube tends to vary in intensity under background events, and it is operating at a well regulated 400v. Hypothetically, if it were completely discharged every event than when a Geiger tube enters into the saturation â€œglowâ€ condition it would remain discharged. Which wouldn't make sense.... confused I am...

Cheers,

Uspring

Mon Apr 08 2013, 03:46PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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CR wrote:

Are they sampling a ramp generator (~80v to 400v) to garner â€œtriggerâ€ thresholds?

No you would stay with a voltage just below the point of a continuous discharge. If you want to do spectroscopy you need to pick up the pulse height. Alphas typically produce a few thousand ion/electron pairs, which are multiplied somewhat in the proportional regime. You need low noise amps to do that.

Proud Mary

Mon Apr 08 2013, 10:13PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
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Have a read of this essential GM tube theory manual from Centronics:

Steve is right to say that some GM tubes can be operated in proportional mode. A few of the common Cold War era Russian GM tubes give proportional mode parameters in their data sheets. This is generally a band about 20V wide before the start of the GM plateau region.

Proportional mode requires a low noise linear pre-amp which can accurately amplify the pulses while maintaining their relative pulse amplitude. Discriminators can then sort these out into the number of bins appropriate to the practical resolution of the instrument.

Carbon_Rod

Tue Apr 09 2013, 06:36AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
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I was looking into the various tubes listed on this site:

These look like interesting options, but do not seem to disclose proportional operation:
LND7317,LND712,SBT11A

@Steve
Alternately, would it be possible to avoid proportional methods by using a precision Aluminum coated mica filter wheel to selectively retard beta emission. Thus, forming a profile by automatically stepping through 0.001mm to 1.000mm shielding on the tube aperture over several hours, and removing the background profile after each exposure interval. I am curious if anyone has attempted building a similar apparatus.

Thanks for the link PM, I will look over the info.
Rod

Uspring

Tue Apr 09 2013, 08:52AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Carbon Rod wrote:

Alternately, would it be possible to avoid proportional methods by using a precision Aluminum coated mica filter wheel to selectively retard beta emission.

Alphas have a much shorter range than betas. You can't shield against betas and not block alphas.
In a thin, transmissive type detector, an alpha will leave a higher signal than a beta due to its fatter ionisation track. If you're sensitive to pulse height, you might be able to distinguish between them.
Alternatively you could place a somewhat shielded (alpha insensitive) detector behind a thin alpha detector and use anticoincidence. A signal in both detectors will signal a beta and a signal only in the first one an alpha.

Proud Mary

Tue Apr 09 2013, 10:49AM

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

The use of filtration using filters of known properties - such as a sheet of thin aluminium foil - has been a basic analytic procedure since the invention of the GM tube in 1928. At the risk of over-simplification, if all the radiation from a sample is blocked by aluminium foil, then you have an alpha only emitter. If none of it is stopped measurably by 1 mm Pb, then it is gamma rays, and so on.

A great deal more can be done with GM tubes than simply detecting the presence or absence of radiation. Have a look at this wonderful old classic from the Golden Age of gaseous detector technology:

The use of the Geiger-MÃ¼ller counter X-ray spectrometer in an X-ray laboratory, with special reference to automatic recording

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