Inside some Radcal Accu-kV sensors

The kV sensors in one of my Radcal 9000 kits failed calibration, and unfortunately Radcal no longer has spare detector modules available to rebuild the sensors anymore, so I had them just recalibrate the ion chambers and send everything back.

I use the Radcal kits primarily for making fluoroscopy exposure measurements and the Accu-kV meter and kV sensors don’t really get much use these days. I have other meters that I used for x-ray tube voltage and exposure measurements, so losing the Accu-kV sensors isn’t a big deal.

Since I’m taking them out of service anyway, I thought I’d crack the sensors open to see what’s in them. I only had to undo a few screws to get the cover off

Under the cover is a stepped copper filter that attenuates the x-ray beam by different amounts. The ratio of attenuation through the different filter thicknesses is used to calculate the x-ray tube voltage. The filters are attached to a block of lead that blocks x-rays from getting to the rest of the sensor. A little bit of wiggling and gentle prying let me lift the block out to look at the insides.

The sensor module itself fits snugly into the lead block and is held in place by a brass bar screwed into the lead. The circuit boards contain a couple of AD822 op amps and supporting components that take the signal from the sensor module and send it to the 4082 meter.

kV sensor module
kV sensor module

The kV sensor module itself appears unremarkable. There’s a white plastic 4 x 6 x 40 mm bar glued to the black carrier board. I have a vague memory of the 40×5 kV sensors being photodiode type detectors, so the white plastic would probably be some kind of scintillator material, and there would be some photodiodes underneath. Not positive about that though, so I’ll have to do a bit of digging to find out.

The 40×5-MO mammography kV sensor is similarly constructed, and aside from having to undo a few more screws, came apart pretty easily.

The sensor module in the mammography sensor fits into a brass block, and the stepped filters are much thinner (possibly aluminum?). The sensor module itself is virtually identical to its 40×5-W counterpart.

When I get some spare time, I’ll get some x-ray images of the sensor modules to see what’s in them. Then I’ll put them back together and they’ll become part of my museum collection.

Update: Here’s an x-ray image of the detector modules. The row of pin headers is in the middle, and the square blocks are the individual detectors.

X-ray of the 40x5-W and 40x5-MO detector modules
X-ray of the 40×5-W and 40×5-MO detector modules