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Grenadier's big thread of RÃ¶ntgen related shenanigans

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Adam Munich

Thu Oct 21 2010, 08:18PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Well before I do anything, I'll need a new radiation detector. I still have that 700V end window probe, but I have no way to use it. I'll need a 700V geiger circuit (audio only, no need for a meter). Any ideas?

Proud Mary

Thu Oct 21 2010, 09:34PM

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

Grenadier wrote ...

Well before I do anything, I'll need a new radiation detector. I still have that 700V end window probe, but I have no way to use it. I'll need a 700V geiger circuit (audio only, no need for a meter). Any ideas?

GM tubes are not a suitable means of measuring X-rays.

The graph below shows the energy response curve of a typical GM tube. You'll see that it charges uphill in a breathless rush until it gets to 60keV, and then slides down again, so beyond this peak the count rate goes down while the photon energy goes up! Moreover, there are two points, one on either side of the peak, where X-ray photons of widely different energies will give the same number of counts.

1287695254 543 FT0 Gm Tube Energy Response

Now as a rule of thumb, two thirds of the emitted photons will be in the lower third of the energy spectrum. For example, if you had 75kVp on your target, then two thirds of the output photons will have an energy of less than 25keV, much of which will be blocked by a GM tube outer wall, or lack the energy to knock electrons out of the cathode, to cause ionization, and the Townsend avalanche which causes a click.

You also have to consider dead time compensation, - compensation for the fact that the tube cannot detect particles in the recovery time following each Townsend avalanche, even though more radiation continues to arrive - always an issue with GM tubes.

A GM counter can usually - but not always - detect the presence of X-rays above a certain minimum photon energy - but it cannot measure them.

More suitable means include ionization chambers, scintillation counters with NaI(Tl) or CsI(Na) or BGO etc, and PIN photodiodes.

Adam Munich

Fri Oct 22 2010, 05:18AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I don't need to measure them, just detect their presence. My tube is mica windowed, so things could get in easily. An ionization counter is completely useless at the distances I'll be from the thing, and I sure as hell can't afford a scintillator. I'll just feel safer with that clicking noise. If it speeds up I'll know something's wrong.

Proud Mary

Fri Oct 22 2010, 04:36PM

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

If there is some reason why you feel you must use a GM tube, then I would suggest one with a bismuth cathode, such as the Victoreen 6306, which has about double the X-ray detection efficiency at 50keV compared with an ordinary Cr/Fe cathode tube liner.

plazmatron

Fri Oct 22 2010, 08:18PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Grenadier wrote ...

As for overheating + Reciprocity law failure, multiple short exposures solve the problem.

Multiple short exposures, will help with overheating, however, they will work against you as far as reciprocity failure is concerned.

Grenadier wrote ...

..........or behind a car/big tree (for portable use).

Using an x-ray machine in a public area, is likely to invite trouble. Mine is bolted to my worktop, and I wouldn't dream of running it in the presence of others. That way, any mistakes, are mine, and mine alone.

Grenadier wrote ...

I don't need to measure them, just detect their presence. My tube is mica windowed, so things could get in easily. An ionization counter is completely useless at the distances I'll be from the thing, and I sure as hell can't afford a scintillator. I'll just feel safer with that clicking noise. If it speeds up I'll know something's wrong

This has been mentioned a few times on this thread, by a few people. Geiger counters really are wholly unsuitable for this task, and in fact can be dangerously unsuitable.

Not only do they have an atrocious response curve, they can easily be driven to saturation resulting in........ no clicks!
So you could be sitting there getting your last ever tan, and never realize it.

I urge you to sort out some kind of ion chamber detector. I find self reading quartz fiber dosimeters to be relatively inexpensive, and reliable. Try and acquire low range types.

For example I have a 500mRem (5mSv) gamma/xray dosimeter, and a 2mSv Gamma/xray dosimeter (The 2mSv dosimeter has a linear response from 50keV to 3MeV), as well as a bunch of high range dosimeters for beam measurement.

You should have at least two detectors that agree with one another, preferably more. I wear three.

Les

Adam Munich

Fri Oct 22 2010, 08:34PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I was thinking of a dosimeter, but my problem is they're only sold on egay and I can't buy things on there anymore. Though it would solve the problem to get one + a charger, I am stuck with a gieger counter. However, if anyone wants to trade me a low range dosimeter + charger for my geiger probe, be my guest.

As for you're public area concern, the only ones in any danger are a few squirrels, and possibly the occasional mouse.

Proud Mary

Sat Oct 23 2010, 12:15AM

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

To the best of my knowledge, the last dedicated X-ray GM tube , the Centronics ZP1600, has been out of production for some time, and at any rate is no longer in the current Centronics GM Tube Selection Guide.

ZP1600 was designed to detect X-rays from 6 - 20keV, and so was filled with argon (and the usual halogen quench gas) at about 60% of atmospheric pressure to increase the probability of collisions. These so-called 'high pressure' tubes require much higher working voltages than ordinary GM tubes - 1.6kV in the case of ZP1600.

LND still have a rough equivalent to ZP1600 in their list - LND 72219 - for which you should expect to pay $350 or so. The 77219 data sheet is here:

Note how long the Minimum Dead Time is: 100Î¼s

Adam Munich

Sat Oct 23 2010, 04:46AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I'm going to take your advice and go with the dosimeters then, but how will I acquire them?

Also, please help me pick a head. Which one do you think is good? Whomever's interested Please PM me for the link because lots of people read this forum and I don't want them to get all snatched up.

Proud Mary

Sat Oct 23 2010, 09:19PM

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

Here are the two types of personal dosimeter that I use:

There are deliberately no user controls, and no on/off switch, though the older red Gammatell II unit does have a pressel switch to light the red LED display. Batteries typically last about a year of continuous use at lower exposure rates. Obviously if it is bleeping all the time, battery life will be that much shortened.

The alarm level options are set by a jumper wire, as shown on the card in the yellow unit. Both types use ZP1300 series compensated GM tubes. The DC/DC 450V converter module is the black cast resin block next to the battery in both cases.

Both types are still in use in the UK, though the older Gammatell II with its LED display has been largely displaced by the newer LED type, and the snazzy all-singing all-dancing Siemens EPD Mk 2, which takes X-ray dosimetry down to the 15keV level, and is marketed under the Thermo brand.

All my dosimeters are made in the UK, but US made units will be very similar in form and function.

klugesmith

Sun Oct 24 2010, 03:25AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Grenadier wrote ...

I was thinking of a dosimeter, but my problem is they're only sold on egay and I can't buy things on there anymore. ...

As for you're public area concern, the only ones in any danger are a few squirrels, and possibly the occasional mouse.

If you wait for a low-range (200 mR) training dosimeter "CDV-138" to be offered on ebay, I would be glad to bid on your behalf. Even have it shipped directly to you. I have a few of my own, from ebay 1 year ago, but not quite ready to part with any of them. The chargers are ubiquitous -- same as the ones used with "standard" CDV dosimeters.

Here is a picture of a CDV-138 in action:

[edit] and here is one low-range quartz fiber dosimeter on ebay right now:

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