Converting a contraband detector into a spectrometer

[Jerry B]

Friend gave me an old contraband detector like what is used to detect drugs hidden in cars and materials. Its pretty simple, a Ba-133 source provides gammas which bounce off the contraband and are detected by a scintillator and detector, older ones used a small PMT, newer ones use silicon detectors. The one I have uses a PMT. Denser objects reflect the gammas more and the electronics detect that.

So I took it apart. The source is in a tungsten carbide wheel which rotates to expose the source when a button is pushed which also activates the electronics. The detector sits to the side and itself is in a tungsten carbide housing to isolate it from anything not in front of it. Im not sure what it uses for a detector, I cant find any info on it, I know they do use NaI detectors in other products so it might be that. It's sealed with a stainless panel.

The internals are separated into two sections. Bottom board is voltage regulation, HV, and preamp and pulse shaping. There is a little board that surround the PMT that handles the dynode power. Instead of the usual resistor string it uses a C-W multiplier and taps off each stage for the different dynode voltages.

The top board has the display, uC, and the rest of the supporting circuitry. They are connected by a 4 pin header, +5v, GND, pulse, and unknown. The pulses are just TTL pulses so I had to do some digging and found a cleaned up proportional signal I was able to tap off of, it's still higher voltage so I turned it down with a "volume" pot to drive the input of the little soundblaster USB dongle I picked up. Seems I am getting something out, think I have the signal a bit high still.

IMG_2787 (2) by Jerry Biehler, on Flickr

IMG_2788 by Jerry Biehler, on Flickr

[Sesselmann]

Jerry,

The configuration you describe sounds like some kind of x-ray fluorescence spectrometer. Does it have a model number or make?

Steven

[Jerry B]

This is about all the information you will find, they were kind of restricted. [Broken link removed - Steven]
The electronics themselves dont care about pulse height, just pulse frequency. Basically geiger mode. It actually has a mode where it can be used as a geiger counter but the display does read anything useful. Just it beeps more for higher radiation.

[Jerry B]

Oh. Now I see what you are thinking. Reading the fluorescent off what is reflected. No. It’s quite dumb. It just measures density, it can’t tell you what’s hidden. Though I wonder if it might be able to be converted to do something like that. Subtract the Ba-133 spectra out and see what remains.