I'm working a counting circuit for radiation detection. The input signal comes in as a 5V TTL signal, however the pulse width is an issue. According to the detection equipment datasheet, it should be a 500ns pulse, but it is very old and I'm guessing it is coming out less. I built a 4026 based counter which does work (tested clock input to positive 5V rail, it does indeed count). When A known radiation source is used, however, the counter doesn't get a single count (there are short bursts on my scope at the 5V line, however it isn't good enough to allow me to see the pulse length since it's an old non-storage scope).

I'm thinking that a monostable multivibrator (perhaps 555 based) would work between the radiation detection equipment and the counter. Any suggestions on this schematic? It seems that most multivibrators are designed to have high input with low pulses, whereas I have low input with high pulses. How should I go about building a multivibrator to lengthen this pulse?

Also any other ideas aside from a multivibrator?

 

What is your means of detection? The last stuff I worked with generated pulses with heights determined but the energy of the photons, so you could use a window discriminator to sort energies. Pulse widths ran something under 500 ns, though.

 

The BF3 neutron detector is running through a windowed single channel analyzer, it was just the output pulse of that window analyzer going into the counter that was giving me problems. I managed to increase the pulse driver inside the analyzer to be 8V and now it's counting fine.