pulse totalizer/ rate meter

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Q
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pulse totalizer/ rate meter

Post by Q » Thu Jan 05, 2006 11:43 pm

in making my scintillator i have come across a bit of a dark area in my knowlege. how does one use an analog panel meter to indicate counts/min?
here's what i am trying to do:
i have a victoreen cdv-715 (the ionization chamber based survey meter) that i bought mainly for parts (it had the origonal "dry cell" still in it and was badly damaged)
i would like to use the casing and front panel meter (these parts are still good) for my scintillator. the meter is a 50ua type with an internal resistance of 1800 ohms.

now i assume all i would need to do is take the output from the pmt and put it through some to be determined resistance with possibly a capacitor in there somewhere and feed it directly into the meter.
to roughly calibrate it all i plan to use a pulse generator with a pulse length and voltage similar to the out put of the pmt. if a known number of pulses per time period are fed to the meter it i should be able to adjust the input resistance until the meter matches. repeat the process across the range of the meter's movement.

now, does this make sense? are there easier ways to do this? am i just completely off on this one?

Q

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Richard Hull
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Re: pulse totalizer/ rate meter

Post by Richard Hull » Fri Jan 06, 2006 2:57 pm

Q you need an integrator circuit which is indeed a resistor and a capacitor. Passively, it can be tricky to impliment.

Scintillator pulses are rather short, but in general, more frequent. Wtihout a scope, you will have to just lay a circuit out on a table and place different capacitors in the circuit until a given source lets the needle come off the peg. Again, without active amplifier buffering, this can be tough to impliment I would start with a 10k resistor in series with a 10ufd capacitor to ground and take the meter output across the capacitor. The final values that work could be far from these. be prepared also to play with the resistor value. possible resistor values 10K, 22K,47k, 100k possible capacitor values .1ufd, .5ufd, 1ufd, 10ufd, 100 ufd. It would be nice to take the output of a speaker or headphone output from the scintillator to use as the signal into this integrator.

Normally, an emitter follower transistor buffer feeds such integrators to create a low source impedance. Ranges are created by a rotary switch changing the time constant resistor of the integrator.

Short detector pulses are are often expanded to tens of microseconds with a monostable multivibrator for analog meter counting.

Other schemes use a fixed integrator TC and vary the pulse width out of the multivibrator to create different ranges.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Q
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Re: pulse totalizer/ rate meter

Post by Q » Sat Jan 07, 2006 1:12 am

thanks richard!
yeah, i'm not even going to try this without a good o'scope. i dont have a good one, but the local university does....

a monostable circuit, eh? actually i was thinking about that but i figured it would limit the number of pulses that made it through to the meter.

also, i would like to build a preamp into the scintillator for use with external devices. possibly one of the cremat units that carl reviewed not long ago. where would this be put into the circuit?

Q

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Richard Hull
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Re: pulse totalizer/ rate meter

Post by Richard Hull » Sun Jan 08, 2006 6:31 pm

All Scintillators must have a preamp. If you are going the passive integrator route mentioned earlier, make sure that the premp or final buffer output has a low impedance < or = 50 ohms.

A monostable is OK if it is retriggerable by design and type and the pulse width is on the order of one or two to five microseconds. You might put a pot in its timing circuit to let you select the best pulse width for you resultant circuit.

Remember, all of the foregoing is to either drive an analog meter or an audio port. For spectometry or critical data apps extreme pulse stretching is a no-no, normally. Sometimes pulse conditioning and shaping is allowed in critical apps and even demanded. What you asked for required really modifying the output pulses to drive slow responding real world stuff.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Q
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Joined: Mon Jan 12, 2004 10:45 pm
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Re: pulse totalizer/ rate meter

Post by Q » Sun Jan 08, 2006 8:43 pm

thanks again richard,
i enjoy projects like this. i learn a lot along the way. i could just buy a scintillator, but then i learn very little. :)

anyway, i have a simple preamp similar to your reverse engineered 3 transistor circuit to feed the monostable and integrator. for this i plan to take the output from the tube anode.
however, i have seen some text to indicate that for more than simple pulse counting the output from the pmt should be taken from the dynode string. origonally i had planed to use two preamps, one for the integrator and another for future use with spectometry, and simply switch between the two. but in light of the dynode referance i might not need to do that.
this is why i ask where to put the preamp.

Q

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