CD-V dosimeters for X-ray measurement

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
Post Reply
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

CD-V dosimeters for X-ray measurement

Post by Rich Feldman »

Long ago, while messing around with Coolidge tubes and NST's, I reported that x-rays from 1/2-NST voltage (7.5 kV RMS) were apparently stopped by the aluminum tube of a pen-style fiber dosimeter. Full NST voltage (15 kV RMS) brought out x-rays that were easy to detect.

Fast forward to a few weeks ago, a week before Covid lockdown. A local dentist was moving to a new office, and didn't have to move his oldest x-ray machine because I took it off his hands.
rad5.jpg
.
The control box reflects my interests and image. It includes a switch-selectable autotransformer, timer, and long curly cord with remote-control button. Output had been wired to an ordinary wall outlet into which the X-ray head was plugged.

For expedient demolition in the field, wires were cut and plugs were removed. Not restored until last night, and tested by turning on a desk lamp for selected short intervals and selected brightness. X-ray head didn't join the party until this morning, when I was set up to measure the expected bursts of ionizing radiation.
rad4.jpg
It worked on the first try, and the next 10, all with me about 20 feet away, before all four 200 mR pen dosimeters were near full scale.
rad6.jpg
.
Note well: if timer and dosimeters are accurate, the point-blank intensity was around 300 to 500 R/hr.
A seriously scary level if seen on a regular Civil Defense meter. Soon I will figure out how many whole curies of gamma-emitting isotope would need to be switched on and off to get that intensity at that distance. And will put a lead bib on the gadget even though I'm 20 feet away and not planning to reproduce.

[edit]These systems have a mechanical hazard which could be lethal if overlooked. The manipulator pantograph, like many adjustable drafting tables and garage door openers, includes strong springs instead of counterweights. Unless restrained, a linkage can clobber you unexpectedly when a load is removed, or a brake or clamp is released.
Springs can beat high voltage capacitors, when it comes to storing energy for years with no loss.
rad7.jpg
All models are wrong; some models are useful. -- George Box
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: CD-V dosimeters for X-ray measurement

Post by Richard Hull »

Yes, when you are designed for x-ray production, (x-ray tubes), you do it very well and with a vengeance.

Fusors are terrible x-ray machines and even at 40kv max out at the window to a good, calibrated ion chamber register about 60mr/hr or 1 mr/minute if you put your chest in contact with the view port window.
The upshot is don't monkey with real X-ray tubes or systems. Unless, as above, you use distance and the inverse square law while playing with them.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: CD-V dosimeters for X-ray measurement

Post by Rich Feldman »

Got some numbers to emphasize the point:
intentional radiators of x-rays can easily, but briefly, match the output of scary amounts of radioisotopes.

Started with Rad Pro Calculator's dose-rate-to-activity calculator. Selected Am-241, whose alpha decays have 35% chance (?) of coming with a 59.6 keV gamma ray. http://www.radprocalculator.com/Gamma.aspx
To get 360 R/h (100 mR/s) at distance of 25 cm, it says you need 1345 curies.


As a sanity check, I took that activity value and ran it through a spreadsheet for RTG calculations,
extended using formulas from http://hps.org/publicinformation/ate/fa ... osure.html.
1345 Ci (50 TBq) is 392 grams of Am-241, producing 44 watts of heat from 5.486 MeV alphas.
Enough for 1.7 billion smoke detectors.
If 35% of the activity produces 0.06 MeV gamma rays, the gamma power is 167 mW.
At distance of 25 cm, the gamma fluence per cm^2 is 2.2e9 photons/s = 1.3e8 MeV/s.
Factoring in the mass attenuation coefficient for air, ion pairs per eV ratio, and ion charge,
we get 268 R/h. Close enough to make me confident that no decimal points were misplaced.
All models are wrong; some models are useful. -- George Box
User avatar
Rich Feldman
Posts: 1470
Joined: Mon Dec 21, 2009 6:59 pm
Real name: Rich Feldman
Location: Santa Clara County, CA, USA

Re: CD-V dosimeters for X-ray measurement

Post by Rich Feldman »

Dosimeter motion can be captured in real time with a video camera.
https://youtu.be/D4B9uxNR-IM (about 11 seconds)
rad8.jpg
.
The image darkens momentarily, as I cross the backlighting path on my way to sheltered place with x-ray control button.
We could probably measure the tube filament heating time by comparing audio and video.

The setup is touchy, with many degrees of freedom, but came together without much fabrication.
rad9.jpg
All models are wrong; some models are useful. -- George Box
Post Reply

Return to “Neutrons, Radiation, and Detection (& FAQs)”