Author Topic: Dental X-ray picture dosage measurement  (Read 3501 times)

Offline Mads Barnkob

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Dental X-ray picture dosage measurement
« on: May 04, 2017, 02:56:45 PM »
I brought my uRADMonitor KIT1 dosimeter to the dentist at my latest appointment and we had a little chat about medical x-ray and when he first started his business.

He had to make sure all walls in the two dental rooms had an extra thick concrete wall, lead padded doors and I am actually not sure about ceiling. Another thing is windows, different position switches are added to the x-ray head and dentist chair so that it can NOT fire if the head is at an angle where x-rays could be emitted through a window into the public streets.

We pointed the x-ray head directly at the dosimeter, 1-2 cm (same distance as from the cheek), just like if a picture was taken of a persons teeth.

The unit was set up for 160 ms ontime at 65 kV, 7.5 mA. This resulted in a alarm from the dosimeter, showing 462 CPM and 2.92 uSv/h when we got back in the room, so it properly fell a little from shot to entering again, some 3-4 seconds.

The total dose was not yet calculated here, but half a minute later it showed 0.038 uSv, the unit had been turned on for about 1+ minute at that time.
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Offline klugesmith

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Re: Dental X-ray picture dosage measurement
« Reply #1 on: April 21, 2020, 07:40:50 PM »
Adding to this old thread because its Subject fits exactly.  And there's a gap to fill between Mads's data and mine.

Got dental X-ray total dose measurements on two occasions, using old civil-defense pen shaped dosimeters.  Quartz fiber electroscopes.  Got a few that were made for training, with full scale of 200 milliroentgens (close to 2 mGy or 2 mSv).

First was a few years ago, during a visit with tooth-puller across the hall from cavity-filler.
Brought a recently charged dosimeter, he gave it a normal zap, and the device indicated a change of about 10 milliroentgens (close to 0.1 mSv).

More recently, a week before the Covid lockdown, a dentist who was moving to new offices gave me his oldest X-ray generator (the one not registered with California for safety and tax).  That deserves a topic of its own in this forum.   Since fixing wires that had been cut at demolition time, I have not tried to make radiographs or even light up a fluorescent sheet.  Just zapped the same old dosimeter and a couple of its siblings, using remote control from another room.


For this thread, it looks like typical dose would have been well over 50 mR (0.5 mSv).  Is that 'cause it was set for traditional silver emulsion film instead of a digital sensor?  Or did my tooth puller set his machine low as a show of safety?
Note to AlexanderHun: my readings all correspond to exposure rates higher than 300 R/h.

How can we explain a three orders of magnitude difference between these numbers and the 0.038 uSv reported by Mads?

We might shed some light on the matter by measuring actual electrical current or charge in a home-made ionization chamber with known air mass.  Another case for new Topic in this forum.

p.s. Mads, how did you get your pictures to appear as clickable thumbnails instead of inline images?  Have I overlooked a button in the Attachments dialog?
« Last Edit: April 21, 2020, 07:58:37 PM by klugesmith »

Offline Mads Barnkob

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Re: Dental X-ray picture dosage measurement
« Reply #2 on: April 21, 2020, 08:22:42 PM »
How can we explain a three orders of magnitude difference between these numbers and the 0.038 uSv reported by Mads?

We might shed some light on the matter by measuring actual electrical current or charge in a home-made ionization chamber with known air mass.  Another case for new Topic in this forum.

p.s. Mads, how did you get your pictures to appear as clickable thumbnails instead of inline images?  Have I overlooked a button in the Attachments dialog?

Maybe the uRadMonitor KIT1 is not fast enough to count all the events? Maybe its something with the tube used in it?

If you just add attachments and do not put them inline, they will appear underneath the post :)
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Offline klugesmith

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Re: Dental X-ray picture dosage measurement
« Reply #3 on: April 21, 2020, 09:55:43 PM »
Good point, I had forgotten about saturation in event detector-counters.  The dental x-ray generator, at minimum distance, delivers billions of photons per second per cm^2.  Can be reduced by increasing distance, but by the time 1/r^2 is enough to make the rate countable, the air will be absorbing and scattering much of the x-ray flux.

We might use the metal equivalent of "partial shade cloth" as a spectrally neutral density filter.

Even without saturation, don't GM tubes have trouble with spectral sensitivity variation in the 10-60 keV range, if you want to measure sieverts?  I think there are some metal filters designed to flatten the response.

Offline klugesmith

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Re: Dental X-ray picture dosage measurement
« Reply #4 on: April 28, 2020, 09:46:26 PM »
Still posting to this thread because it's about the dosimeter, not about my x-ray machine.

Took the opportunity to get a video of dosimeter moving during x-ray shot. If you listen carefully you can hear bird sounds.
/>(about 11 seconds)

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.

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Re: Dental X-ray picture dosage measurement
« Reply #4 on: April 28, 2020, 09:46:26 PM »

 


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