Measurement of X-Rays from magnetrons 1

[BrianG]

Hi folks,

Does anyone have experience of the correct way to accurately measure X-ray emissions from magnetrons?

In one of our small radars we have a need to measure if any x-rays are generated, and if so, is the level "hazardous" to service personnel. The magnetron is running at an anode voltage of about 7kV and p.r.f. of 800Hz, so presumably any X-rays will be soft (7keV max). We have tried using hired-in instruments (all G-M tube based) with variable success: some read nothing at all, others suddenly read once a distance threshold is crossed, but with no expected distance / strength field attenuation. These meters may be responding to stray r.f., as a conventional field strength meter gives a power reading of about 0.2mW/cm2

Thanks in advance

 

[BrianR]

I wouldn't think you'd get much from only 7kV, but you could try a radiographers dosimeter badge which is good for low level measurements and not RF sensitive.

 

[IRstuff]

Or get some regular or x-ray film. If it fogs, then you'll know.

TTFN

 

[BrianG]

Thanks guys for the suggestion about using film. We know that some magnetrons do give off a small x-ray dose but unfortunately we need to quantify this as a specific rate in mSv /hour.

From what I remember about film badges, etc. is that they only measure an accumulated dose over a relatively long period via diferent energy level filters. The difficulty is that I'm not sure our x-rays are hard enough to get through some of the filters, then there's the problem of reverse-calibrating any exposed film back to a dose rate.

 

[biff44]

Have you heard of "google"? There are about one thousand different devices out there, such as:

http://www.seintl.com/products/inst...6&kc=abQ5tXFmYuoNS8CKhbFoQNAC5hEmiDMlMevJNtpG

 

[BrianG]

biff44: yes we did google for just about everything: x-rays, health physics regulations and different instruments and that is exactly the problem - there is so much information out there an there are thousands of instruments all with different thresholds and ranges. I was hoping to have found someone who has already measured this sort of x-ray emmision successfully to compare notes.

Since I first posted here, other forums have told us that you can't use g-m tube based detectors with magnetrons because of the pulsed nature of the drive voltage. It seems that an ion chamber type instrument may be the best sort for this application, so we will try that.

Thanks to all who replied.

 

[biff44]

I find that in these sensor companies, especially the smaller ones, that it is pretty easy to get ahold of a knowledgeable applications engineer that will steer you straight.

 

[lighterup]

You hopefully know the beam energy and the target material. The way I would approach this problem is to Google some theoretical articles on how to calculate emissions from your target given your beam energy, and calculate the worst possible case your system could emit. If it is below minimum safety limits, I think you could make a pretty good case to whatever agency is buying your system that you don't need to do an expensive measurement. The television folks went through this with CRT's and the bremsstrahlung from beam-phosphor interaction back in the 70's as I recall. You might look at some cyber-archeology on that subject, as the energy level is comparable to your system.
If there is any possibility you could exceed the limit, perhaps you should re-negotiate your contract to cover the cost of testing at a lab equipped to do so. Such labs could probably be suggested by vendors of the sensors you have had contact with.

 

[waveboy]

Hi, I used to be a diagnostic radiographer. You're right to suspect that 7kV may not get through the filters of a radiographers dose badge...because in radiography its not common to work below 50kV.

If your magnetron is emitting x-rays in to the open, your problem will be one of which part of the magnetron they are coming from. -So x-ray cassettes as used in hospital diagnostic imaging departments sound like a good idea here...you could stand them round the magnetron and start it up....then simply put them through a film processor in a hospital diagnostic imaging department. X-ray film is cheap..a 35cm*43cm sheet of film used to cost about £1.10 in 1995...the chemicals used in processing it will be a few pence. (in fact you can wet process in a big bowl as long as you have a dark room preferably with a safelight)..many new hospitals have dry-processors and there is no "wet chemical".....i think this would be an excellent and cheap and quick way to start as it would indicate where to put your dosemeters (when you know which type you want)....

give me a shout if you want details of how to get in touch with a hospital etc....i also used to get inspections from radiation guys so know who to point you to for advice if you want it.

 

[BrianG]

Thanks for the idea Waveboy; we do actually know where the leakage is occuring (around the h.v. lead to the cathode)but the problem is to quantify the typical leakage into a dose rate and whether this is suitably mitigated when the outer casing is in place. Is there a way to estimate this reliably from film exposure density?

 

[waveboy]

I doubt that film exposure density would be a good method. I could be wrong but doubt it would be accurate/reliable enough. My hospital xray dept were always given dosemeters to wear (some kind of lithium compound in them?)-At the end of a month we would send them off to be read. They were read by heating them up and measuring the light emitted from them (which was proportional to the amount of radiation they'd received). -I just remember sending a load of dosemeters to be read, and the dosemeter that read the highest, was often from people who had not been at work, but had taken a flight and accidentally took the dosemeter with them (perhaps it was the airport xray machine?) -so you need to think about how you will return used dosemeters to the lab. Also, just to check the lab is operating OK, order more doesmeters then you need, and avoid exposing some of them, and perhaps expose some of them to direct beam xrays (eg as in hospital), just to see that they are read as you would expect.

 

[waveboy]

Hi - i forgot to say that if a medical xray film cassette was left near the magnetron for a few minutes say, -and then when processed gave a film showing exposure density, -then it would be recommended to stay well away from the magnetron.

 

[BrianG]

Just to put this in perspective - this particular radar has been around for 40 years and is known to produce small amounts of x-rays from the back of the magnetron when the cover is removed for servicing, but without any apparent problems to servicing personnel in all that time.

Here in the UK with the ever-increasing "health and safety" culture we now have to do a risk assessment - OK - but then we have to quantify this risk with a specific dose rate measurement. Getting a meaningful reading, that's the real problem!

 

[waveboy]

Hi,

You could try emailing someone at

http://www.hpa.org.uk/radiation

for advice. Or if you do military work you could try the "Defence Radiological Protection Service" -Though I think (after re-arrangement) that this is now under

http://www.dstl.gov.uk

(or else some division of Qinetiq).
7kV might be relatively low -however, we were often taught that the "lower kV xrays" were potentially more harmful than those from higher kVs, since they would give rise to more (harmful) absorption by human tissue.
Best Regards.