X-ray measurement

[AlexanderHun]

Hello!

I would like to know how radiation is measured when the radiation is not a natural source but produced at high voltage? Thinking about X-ray tube, I noticed the measurements were in many cases fake because the high voltage also excites the meter itself. Be it the Geiger Muller tube or the semiconductor diode counter.
I have two self-built radiation meters and I have a factory military. Each is capable of recovering from high voltage, has a smb-20 tube, a sbt-10a tube, and the military is a semiconductor diode.

[Mads Barnkob]

https://en.wikipedia.org/wiki/Geiger%E2%80%93M%C3%BCller_tube#Photon_energy_compensation

If a G–M tube is to be used for gamma or X-ray dosimetry measurements the energy of incident radiation, which affects the ionising effect, must be taken into account. However pulses from a G–M tube do not carry any energy information, and attribute equal dose to each count event. Consequently the count rate response of a “bare” GM-tube to photons at different energy levels is non-linear with the effect of over-reading at low energies. The variation in dose response can be a factor between 5 to 15, according to individual tube construction; the very small tubes having the highest values.

To correct this a technique known as “Energy Compensation” is applied, which consists of adding a shield of absorbing material around the tube. This filter preferentially absorbs the low energy photons and the dose response is “flattened“. The aim is that sensitivity/energy characteristic of the tube should be matched by the absorption/energy characteristic of the filter. This cannot be exactly achieved, but the result is a more uniform response over the stated range of detection energies for the tube.[5]

Lead and tin are commonly used materials, and a simple filter effective above 150 keV can be made using a continuous collar along the length of the tube. However, at lower energy levels this attenuation can become too great, so air gaps are left in the collar to allow low energy radiation to have a greater effect. In practice, compensation filter design is an empirical compromise to produce an acceptably uniform response, and a number of different materials and geometries are used to obtain the required correction.[4]

[AlexanderHun]

Thanks, but does this apply to semiconductor diode sensor counters? I noticed with a large electromagnetic field generated by the high voltage transformer, it produces erroneous measurements. For example, 30kV is a television hv transformer, 7-8mA connected to an x-ray tube, the military counter is 200-300 R / h, which is a lot, converting to 2-3Sv / h is impossible.

[klugesmith]

The fundamental method is to use an ionization chamber  Directly measure the amount of ionization in ordinary air, by collecting the charge without any gas multiplication.  Look up definition of roentgen (röntgen) unit.

I have measured X-rays at home, and at my dentist's office, using quartz fiber dosimeters with 200 mR full scale.
Like the one explained below:
https://www.personal-dosimeter.com/what-is-quartz-fiber-dosimeter-self-indicating-pocket-dosimeters-definition/

IIRC, ordinary dental radiograph setting gave about 10 mR in less than one second.
That's more than 36 R/hr at the distance of one's teeth.  There would be a lot more, if not for 2mm-thick aluminum filter usually built into the x-ray generator.  That blocks soft x-rays, which contribute to tissue dose but not to the picture.

[johnf]

7-8 mA @30kv is one very big dose and being relatively soft your flesh will absorb it all
it is about 3-4 ^10 xray photons per second ie 30,000,000,000 xray photons per sec

[AlexanderHun]

My X-ray tube is 60kV, at 30kV X-rays start to penetrate the glass wall.  I don't think 30kV was such a huge dose.  Rather, I know about the high voltage that disturbed the meter.  It is a military instrument, it has a warning sign, and it is forbidden to use near a gasoline engine.  Probably because of the spark plug spark ....

[AlexanderHun]

I measured 200-300 R/h, an impossible, huge dose.  This instrument is designed for post-nuclear exploration, with this small tube at this power should not unleash the scale.

[klugesmith]

At what distance from tube do you measure 200 or 300 R/h?  It doesn't take much electric power, in a Coolidge tube (intentional x-ray radiator), to match the output of a very hazardous radioactive source.  Well, maybe not if the radioactive source is strong enough for significant thermal heating from decay activity. 

I just downloaded and ran radprocalculator for the first time in years.  (online calculator version of x-ray computations is no longer available).  Entered 50 kV, 7 mA, a few cm distance, and the answer was more than 2000 R/h.  I don't completely trust that value, but don't think it is obviously ridiculous.  Whole body exposure at that rate, for an hour, would probably be fatal.  On Internet you can find pictures of amateurs with burned hands, from careless play with x-ray generators.

[edit] Found a forum where somebody solved an x-ray dose question 2 ways. https://www.researchgate.net/post/How_to_calculate_X_ray_dose_using_mAs_and_kV_without_using_any_detector
His answers are 1.8 and 3.2 R/minute, 1 meter away from a 60 kV 10 mA source with 3mm aluminum filtration.
Do you still think your 200 or 300 R/hr measurements are impossibly high?

 

[AlexanderHun]

 You should also know about the military counter that during my experiments, if a spark gap occurred at 30kV, the counter went off, there was no x-ray tube, only sparks.
The problem is using a translation program, I can't speak English, I can't express myself as I should.

[klugesmith]

Your language translation program seems to be serving well, so far.
To keep the message simple: 

If military radiation counter gives a scary answer, like one would expect in nuclear accident or war situations, it might be correct.
 
Little x-ray tubes can generate scary intensity of radiation, until anode gets too hot and tube is damaged.  Not scary strong in a whole room, or building, or city.

Radiation safety can be improved by distance,  short exposure time, and shielding.  And proper measurement, like you asked about. (GM tubes are not good for that)

[AlexanderHun]

Thanks, it's weird to have a dental X-ray head that operates at 220 volts.  The tube in it, 45kV 5mA, there maximum what I measured 80R / h.  It works with the factory transformer, while the other is a  television flyback transformer.




That's the X-ray tube I'm talking about

[AlexanderHun]

This is my military counter, red scale mR/h, black scale R/h, and below the safe residence time, there is a switch that controls the scale with a stepper motor, but I do not know.

[klugesmith]

Thanks for posting the pictures.

The dental tubehead says it has 1.5 mm thick aluminum filter in the beam.  I bet you would get a much higher radiation intensity (in R/h) without that part.  Mostly soft x-rays that deposit their energy in the first mm or cm of body parts, as John warned.

Here's a question for German speakers.  In Alex's datasheet, for example the heat limit section at bottom, the nominal Hochspannung is 60 kVs.  In English it would say 60 kVp, where p stands for peak (as opposed to the RMS voltage).  What's the s for?

[AlexanderHun]

I would add that I also put an aluminum plate in front of my tube when I measured it.

[Uspring]

The s in kVs could refer to "Spitze", which is the german word for peak.

As a student I was once criticised for using in a peak voltage measurement the unit Vp. There is no such unit as Vp. The p refers to the way of measurement or the quantity determined, not the unit. So the correct means of speaking is: The peak voltage is so and so V. Attaching an s to the V as in german can be confusing, since it might also mean second. Luckily no one uses ml or mw units for length and width quantities.

[klugesmith]

Thanks, Uspring, I bet you nailed it.  Spitze = peak.
Reminds me of the Mountbatten connection in British royal family.  Old Louis was born Battenberg, but in early 20th century that sounded too German.

We are in agreement that Vp is not a unit of measurement.  Nor are VAC and VDC. :-)
Vs is even worse, because s is the proper SI symbol for second, but it's still easy to forgive. 

This is an opportunity to praise the European (?) style of giving voltage or Spannung quantities U names instead of V names.  Capital V should be reserved for the unit of measurement, lest beginners confuse the two.
Example: for "voltage = current X resistance" we say v = I R (formerly E = I R) or U = I R, not V = A Ω .  For computing the charge on a capacitor, we say Q = v C, not C = V F.
E = m v^2 /2, not J = kg (m/s)^2 /2. 

The formulas with units of measurement are dimensionally correct, but some are numerically wrong (like the kinetic energy example).  All invite misinterpretation.

[MRMILSTAR]

When I was attending college we were taught to use the term "EMF" (electromotive force) rather than the term "voltage" in order to avoid the confusion with volt as the unit of measurement. I always liked to use EMF instead of voltage but it never seemed to catch on. It always struck me as strange that we seldom say "amperage", we say "current". But for EMF we almost always say "voltage".