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PD500 X-Ray Machine

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oxodoes

Fri May 22 2009, 11:30AM

Registered Member #2134 Joined: Fri May 22 2009, 11:01AM
Location: Germany
Posts: 4

Hello everybody

While searching for some further information on amateur xray systems I hit upon this forum and discovered that a lot of people whose websites I had visited before are represented here. This encouraged me to sign up in order to present my own not yet sucessful project.

My system is based on a PD500 shunt stabiliser triode, which has been sucessfully used for similar projects before.
At the moment I'm usually operating the tube at 35kV (I could go to 50-60kV with better cooling of the transformer) with a current of 0,35 mA (At a filament configuration of 5,7V and 0,3A.).

As I understand it the breakthrough voltage for this tube is about 25 kV, so in theorie I should generate xrays.

Yet neither my Geiger Counter nor a scintillation detector is picking up any radiation. I also placed a ZnS screen and a xray intensifying screen in front of the tube and haven't been able to see anything with the naked eye.

Is the HV sufficient and can you spot any potential flaws in my current setup??

Thanks for your advice
Martin

PS: And YES, I am using shielding...

uzzors2k

Fri May 22 2009, 07:12PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Welcome to the forum!

Jochen Kronjaeger has summed up making x-rays with rectifier tubes here.

I think the problem is that you are heating the filament.

HV rectifiers and such will usually generate too little radiation to light up an x-ray cassette sufficiently to be visible, unless it's really dark. You should try exposing photographic paper with the intensifying screen instead. At 350ÂµA meager exposure should occur after 30s.

Proud Mary

Sun May 24 2009, 05:12PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Welcome to our online community!

We have four members whose main interest is radiography, and now we have one more!

Can I ask you about your measuring instruments? When you say you have 35kV anode voltage, is this by direct measurement when the tube is running, or just what you think you should have by Cockroft-Walton multiplication?

What is the make and type of your GM tube?
What is the type of scintillator in your counter.

As Uzzors says, do not expect visible results from a ZnS screen.

You're in the right place and we will get it up and running for you!

oxodoes

Sun May 24 2009, 09:44PM

Registered Member #2134 Joined: Fri May 22 2009, 11:01AM
Location: Germany
Posts: 4

Thanks for your quick replies.

@ Uzzors: I read about the approach to not heat the filament and instead connect the tube in reverse, but I can not really see any advantage as one does not have direct control over the tube current any more. Can you explain this any further??

@ Harry: The voltage is measured directly with the help of an high voltage probe (which is supposed to be capable of 40kV bit hasn't even blown up at over 60keV). So I really am sure that my high voltage source is appropriate.

I today sovled my cooling problems with the transformer by replacing an outdated mosfet (high internal resistance) with a series of modern mosfets, so that I was able to push the voltage to over 60kV. And WHOLLA the Geiger counter started ticking away happely.

Some further researche then showed that the GM tube (a thin window alpha-beta-gamma detector) I'm using is supposed to be sensitive to gamma radiation above 70keV only. So I guess I really only had a detection problem.

Sadly I do not have any further information on the scintillator and it does not react at 60kV, so I guess it's pretty much useless.

I now ordered an image intensifier tube (amplification factor of 30 000), which may enable me to take radiographs without having to use old style photo-paper (plazmatron has already build a system like this before). So I may be getting the first pictures by the end of the week.

I may be a bit too optimistic but if this works I may place the object that I want to xray on a rotating table and do some 3D reconstruction like in a CT scan...

Proud Mary

Sun May 24 2009, 11:20PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The maximum energy that an x-ray can have is numerically equal to the anode voltage.

The largest number of x-ray photons are emitted with an energy of about 33% of the maximum photon energy (Kvp) i.e. if you have 45kV anode voltage, the majority of your photons will fall below 15kVp

Now, if we assume that your PD500 valve glass is fairly opaque to X-rays <15kVp, you'll see why not much happened until you wound up the anode voltage good and proper. There will have been emission, but perhaps too feeble to detect with your unsuitable apparatus. (make an ionisation chamber if you are serious)

In Kronjaeger's cold cathode arrangement, the idea is to keep the impedance of the tube as high as possible, (i.e. no heater) thus maximizing the voltage drop across it. His idea of reversing the cathode-anode
connections is to increase the impedance still further.

The efficiency of x-ray production is completely independent of the anode current. Irrespective of the current selected, the efficiency of x-ray production remains constant. The efficiency of x-ray production increases with increasing projectile-electron energry. At 60 kVp, just 0.5% of the electron kinetic energy is converted to x-rays, but at 120 MeV, it is 70%.

oxodoes

Mon May 25 2009, 06:54AM

Registered Member #2134 Joined: Fri May 22 2009, 11:01AM
Location: Germany
Posts: 4

First of all I have to make a slight correction to my previous post: My GM tube is not only sensitive to gammas above 70keV, but should react from 7keV onwards. (It was nearly midnight when I posted this so please excuse me.)

This means that I did really have no detection problem and is in perfect agreement with your argumentation Harry:
60 kV anode current -> xray intensity peak at about 20keV (just enough to escape the PD500)
I realise that the Geiger counter is still not the most suitable device and will begin building an ionisation chamber as soon as I can.

I understand that the efficiency is only depending on the voltage, yet as I have no way to increase the voltage to over 80keV, the only way to increase the xray intensity is to increase the current (momentarily at 0,5 mA) (intensity = efficiency * current).

Increasing the impedance as in Kronjaegers design does reduce the current, so that the overall voltage drop should actually stay the same (V=I*Z), while the intensity is reduced. I guess it has an advantage if ones high voltage source is not capable to deliver the necessary voltage and current at the same time.

PS: I will post some pictures of my setup later today.

Proud Mary

Mon May 25 2009, 12:04PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Isn't 500uA rather low?

Doesn't the Phillips PD500 Data Sheet (which should be exceeded for X-ray producton) indicate 1.6mA?

oxodoes

Mon May 25 2009, 01:01PM

Registered Member #2134 Joined: Fri May 22 2009, 11:01AM
Location: Germany
Posts: 4

Indead it does. But I thought 1.6 mA to be rather high, so I didn't ground the screen and some of the pins that say "internally connected", which does reduce the current significantly.

I now know that I am producing xrays and might ground the pins I mentioned above If I'm not able to get any image within the next couple of days.

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