Proper measurement of sub 100KeV photons

[davidtrimmell]

Hi, since there are some here who have Fusors that are in excess of 30KV, how are you folks measuring the x-rays produced? Since I have done some experiments with my x-ray setup, I have found that these lower energy photons are quite a different beast from the much higher energy gamma's I grew up with. From some research on the web I have found that the instrument of choice for radiation surveys around x-ray producing equipment are meters with Sodium Iodide detectors with a low Z window. Or one can also use a long end GM detector with an end window, but they tend to read high by a factor of 2-4. I personally have found to be my most reliable device to be the good 'ol SRD. We used an electroscope not to dissimilar to a SRD to calibrate our calibration sources. They work great at measuring the energy being deposited in air, but with these lower energy photons they may not give the 1:1 ratio we use for Roentgen to REM with Gamma's. Really the TLD is the only device I know of that would give a real tissue equivalent dose, but they are not really within reach of the average amateur. What do the experts here suggest for a correction factor for a SRD reading x-rays? Certainly the x-ray tube produces a IDLH situation, whereas the Fusor is somewhat more benign, but still could give a pretty high dose due to proximity and exposure duration. Thoughts on this anyone?

Regards,

David Trimmell

[Richard Hull]

The best recorder for gamma in the truest sense from 20kev up to 100kev is a cutie-pie ionization chamber with the mylar chamber shield cover, OFF. The cutie-pie measures x-rays the way the roentgen was defined in the first place....(how much energy is depositied in a volume of air.)

For X-rays under 20kev only a 2" pancake mica windowed GM tube will do well. I like the stock 200mr dosimeters for area measurement, but they give no running system warning like the cutie pie or GM counters do for stuff under 100mr/hr.

Overall, the good ole "cutie-pie" ionization chamber is best; for if it swamps due to a lethal blast, the needle will immediately pin or slam over to max on all ranges. A G-M tube swamping just goes quiet.....Not good.

Richard Hull

[pawinemaker]

While this method won't work for the higher end of the x-ray spectrum, there is some work being done using avalanche photodiodes to measure soft x-rays. The one advantage to this is that the output is proportional, so you can actually plot out an x-ray spectra with an appropriate MCA, although I am not sure that this will be all that useful for fusor work. The upper limit might also pose a problem for the useful range.

I have come into the possession of a large array avalanche photodiode, so I have been looking into this. It may not be useful, but it is worth some investigation.

An interesting paper on the noise in such devices is
http://fisica.usach.cl/~egramsch/LabOpt ... IeeeED.pdf

Some work was done at UNH for using these devices in satellites to measure soft x-ray spectra. A list of papers can be found at:
http://www.catsat.sr.unh.edu/papers

There are also a zipped set of word documents under the Theses link,
http://ssl11.sr.unh.edu/papers/forrest.zip

[davidtrimmell]

Yes! You know Richard I completly forgot about the "Cutie Pie". In the nuke plants they were considered old antiquated technology (even though they were not that much different from the RO-2 and RO-2a). Only when I work for the DOE did I get use them, I actually used one to measure the dose rate off of that old Madam Curie Radium source they had there in Grand Junction!

Thanks, I forgot about that one.

David Trimmell

[Jon Rosenstiel]

I'm using a "Cutie-pie" also.

Jon Rosenstiel

[Carl Willis]

Here's another satisfied cutie-pie user (RO-3).

The instrument's exposure response with the shield open is fairly flat from about 15 keV through 2.5 MeV. Provided the low-energy "grenz" rays of < 15 keV are an insignificant component of the field outside a device, the cutie pie can be considered reliable. Certainly that's the case in a fusor made from stainless steel.

The cutie pie ion chambers do have their little quirks. First, they are open to air. If the air pressure drops, the current for a particular exposure rate drops proportionally and the meter will under-report the exposure. So for example, a radioactive rock that shows 60 mR / hr in Moab, UT might read 75 mR / hr in Columbus, OH. The correct reading depends on where you had the 'pie calibrated (and the weather, to a minor extent). Second: water vapor will really screw up a cutie pie and cause it to act erratic, so the little parcel of indicating desiccant in the unit has to be replaced or reheated every couple months such that it stays blue in color. Third: the exposure indication on the cutie pie should not be confused with absorbed dose at the lower energies found in x-ray beams, below ~150 keV. It's OK to say that for most gamma rays, 1 R (Roentgen) / hr on the chamber means you'd get about 1 rad / hr. But for the low-energy stuff, bone is a much better absorber on a mass basis than air, and you might be getting 5 rad / hr to your bone in a field that reads 1 R / hr on the cutie pie.

So in conclusion the cutie pies are good for measuring low-energy gamma or x-rays (and energetic beta particles too). Users have to keep the desiccant dry and remember that at low energies the cutie pie reading isn't even close to accurate in giving the absorbed dose to parts of their own bodies.

-Carl

[davidtrimmell]

Yes, one of the first things you do when you get a open air ion chamber like this is change the desiccant!.

Carl, you have hit exactly on what my question is: how do you determine the "dose" equivalents of these lower energy photons? Just try and take a radiograph of soft tissue with a Co-60 source! If the energy is not being absorbed, then we have no shadows, obviously. Neutron dosimetry is the real bugger!

Regards,

David Trimmell

[Richard Hull]

It really is a matter of what is low energy. At fusor voltages (~20-30kv), it is more of a burning of the skin kind of radiation. Almost all of the X-rays are found at or below 1/2 v applied. There are few X-rays at the peak voltage energies.

Once the X-rays start penetrating the fusor shell, at very high voltages, ALL of the x-rays are just about at the max energy as the lower energy x-rays are filtered out.

When x-rays start pouring out of the shell, you really have to have proper lead shielding in place or use that point as the max voltage at which you can operate your fusor. To my knowledge, only Jon need worry about this and he has it covered.

Richard Hull