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X-Ray Chicken Wing Inspector

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LAN_403

EEYORE

Fri Jan 08 2010, 11:51PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Here are some pictures...
Matt

uzzors2k

Sun Jan 10 2010, 05:17PM

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

Nice!

1) I wouldn't worry about the filament drawing 2,5A at 3,3V, but I wouldn't dare take it any higher. I suggest you adjust the filament temperature so it provides just a few milliamperes of anode current, this should reduce reduce strain on the tube. There's no point in running full intensity unless you need to.

2) How is your setup grounded? I would ground the filament transformer core, 0V on the filament transformer and the HV supply negative. If that's done there shouldn't be any major potential differences at the cathode end. Can you provide some details on your HV supply?

EEYORE

Sun Jan 10 2010, 07:53PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello,
So do I need to only concern myself with current through the filament then? I dont have a spec sheet to go off of, just what the seller says. I am still gaining an understanding of x-ray tubes and feel things are still fuzzy for me. Is it okay to under power the filament if I am looking to keep anode current low? I am going to order a 1mA analog current meter to use for measuring current. I dont plan to go over 1mA of anode current anyways.

As for grounding, I am grounding everything to the house ground (earth). I have the filament, HV return, flyback core all connected to earth ground. I will also ground the filament xfmr core. I think the problem is now from HV+ spray. It seems impossible to stop this spray! I attempted to measure the output voltage by using two metal spheres and got about 1in arc distance. I dont understand how the voltage is enough to spray like mad with the tube running. I am wondering if the voltage is climbing somehow...The flyback driver is a bit strange and seems to change its output depending on the load. (Its an electronic halogen light PSU).

The larger x-ray tube doesnt seem to get too hot when running in air, but the HV will arc to the lead shield I made. The area around the filament gets warm, but I figure this is due to the filament? The anode stays cool. Should this be okay to run in air? I may need to put it in oil....I still havent tested the small tube aside from its filament. (Works).

I was messing around with RadPro and it seems that I will need 4mm of lead for shielding with 50kV and 0.5mA for the x-ray tube. Does this sound okay? Right now I have about 2mm of lead wrapped around the middle of the tube with a hole drilled for the output (Be window in the anode). I have a tube made of lead going around this output window as well. My CDV700 doesnt report any radiation through the lead, but sure does from the ends of the tube! This will be tricky to shield considering the HV connections...

uzzors2k

Sun Jan 10 2010, 09:32PM

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

I would take the seller's filament quotes as maximum values to get the rated 15mA. For my tube, and any other tube with adjustable anode current, you simply adjust the filament temperature by increasing voltage until you reach the desired anode current. Tube's are not my area of expertise, so you should check with Stella on this, but I think the filament must be run in a certain temperature range or else the lifetime is decreased. It seems counterintuitive to me, but apparently having the filament too cool is just as bad as being too hot! So maybe you should aim for 2 or 3mA of anode current instead.

Flyback xfrmrs can have incredible open circuit voltages, which will vary widely with load. What I did for my supply was make a HV resistor divider using 100M resistors. This loads down the CW enough that the open circuit voltage doesn't drop much when the x-ray tube is on. With an analog meter I was able to measure the HV, my digital multimeter would just go nuts. This is the only reliable way to measure the anode voltage, measuring spark gaps and such is too inaccurate, and doesn't give you the whole picture.

The cathode area will heat up from the filament, but I would expect the anode to become the warmest. After all, nearly the entire HV * anode current product end up as heat there, which is possibly several hundred watts. If it stays cool you're either not running much power or not long enough.

I wouldn't bother with oil unless you need longer run times so heating becomes an issue, or if arc-overs occur.

4mm of lead provides a good attenuation level. The thing is, most of the x-rays will exit through the window and bounce back at you through x-ray scatter (Think of how stuff can be illuminated by indirect light sources), so you should also shield the object being viewed if possible. A small lead-lined box would do wonders. Is radiation leaked at the anode end too? I would have thought the anode to shield most, if not all of it. Shielding the cathode is no problem as it's at ground level already.

EEYORE

Wed Jan 13 2010, 03:56AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello,
I am just as surprised about radiation leakage around the anode! But sure enough, my gieger counter pegs itself when the probe is brought near the anode...This makes for tricky shielding. I have no idea how to stop the HV from arcing to the shielding. Perhpas it is much more than the 25kV I thought...I had originally assumed I could approximate the voltage by measuring the max arc lenght between two spheres. I guess I need to order some high voltage, high resistance resistors for a voltage divider huh?

I had another question. Do I need to worry about the glass expanding due to heating? I was going to slide a tight fitting PVC pipe piece covered in lead sheeting over the tube for shielding. Also, I managed to scratch the glass a little when trying to wrap lead directly over the glass. (Maybe some sand got in there). The scratch is small. Should I be okay? I dont know how thick the glass is...I guess I will use some felt or something else to keep it from scratching.

Matt

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