by George Dowell » Mon Sep 12, 2011 4:16 pm
This is just a stub/ I'll drop the whole article here somewhere in a fresh thread when it is better formatted for this forum.
MOVING TARGETS.
We have a number of parameters to play with in the Home Rad Lab equipment.
Ludlum makes it easy for end users because they know their market well. All their Model 3's are
nothing more or less a set of parameters. a particular set of tools. By this I mean, the Model 3 is ideal for use with a GM Pancake and GM thin walled metal beta gamma probe.
There are no input sensitivity adjustments, because they are not needed. To have them COULD compromise the proper operation of the instrument in the field ( Double Pulsing etc).
Model 3's DO have adjustable HV, but no way to meter it. The meter, probes, scales, HV, even the cable are set up as a unit by a bench tech (me- as a hobby) and that's that. Nothing to adjust so nothing to screw up.
Models 3's have been made for decades. Early Model 3's are very different in HOW they get the operating requirements to work, but ALL Model 3's operate to the same set of requirements ( Battery Check, number of ranges- 4, adjustable HV 400-1500, no meter readout of HV, Fast/Slow response switch, audio on-off switch.fixed input sensitivity).
Add an alarm feature, it becomes a Model 3A.
Change one thing, make it 3 ranges instead of 4 ranges, it is now a Model 2!!
There will always be a Model 3 because there will always be a requirement for a meter that does what it does, nothing more, nothing less.
Depending on the vintage, the input sensitivity (that is the lowest signal in milliVolts to which the meter responds) is 30 or 40 mV, negative going.
Ideal for GM and even SOME scintillator applications > by no means ALL scintillator applications, far from it).
Much more on this key parameter shortly.
Jump next to a Ludlum Model 12. It is not an improvement or advancement on a Model 3, it is simply a different bag of tools. Like the Model 3, the 12 has been made for decades with all the technical electronic advancements available at the particular vintage, but they all do the same things.
With the added features on the '12 you can monitor the HV on the meter, and most importantly set up the meter's input sensitivity (LLD, Lower Level Discriminator, Threshold)> 1 mV to about 100 mV range here and the HV range is expanded to 400-2500V.
Next up are the "analyzers" which have all the features of a Model 12 but add an additional control called WINDOW ( ULD, Upper Level Discriminator). Adjustable from 0-60 mV above LLD.
With the WINDOW control you can set another parameter, where the meter will respond ONLY to a signal that is above LLD and below ULD. This makes it by definition a Single Channel Analyzer. Very handy if needed, totally devastating if ser wrong.
So one up the line it goes until the Model 2221 which has every control, but each and every control MUST BE SET to make it do what you want it to do with a particular probe.
THERE IS A REAL NEED TO UNDERSTAND THE FUNCTION OF LLD WHY IT IS SET A CERTAIN WAY FOR CERTAIN PROBES>PERIOD> THIS IS DONE WITH A PULSER> MORE ON THIS ANOTHER TIME.
The probes.
Different class probes work vastly differently and a meter used with that class probe must be set up a certain way.
GM probes can give huge pulses/. The HV must be set somewhere within the operating range of the probe, they are not too critical.
Scintillator probes can give smaller pulses, but the height of those pulses means something. The height of the pulse is related to the HV setting, and for some scintillator materials, the energy that creates the pulse in the first place. By diddling with the HV setting, LLD and ULD, a window can be made just about at any pulse height to respond to a given energy. Usually we do this in a precise manner so that a certain gamma energy is equal to a certain pulse height ( and sometimes read all at once as in MCA).
Proportional probes give VERY small pulses and the height of those pulses can mean something.
One last thing while I pause for discussion. Sometimes we use a scintillator with MORE THAN ONE type of
detector in it, reading say Gammas and Betas, or Alphas and Betas at the same time but independently. This is accomplished by pulse height detection.
The picture shows a typical Geologists Gamma Ray Spectrometer with remote probe, servo loops for temperature feedback and "downhole" HV generation and pulse shaping. With these we look at 5 things at the same time. 1- TC or Total Counts of all energies, #2 U window - we monitor the Bi-214 line at 1764 keV for Uranium , #3 Th channel We monitor the Tl-208 @ 2620 keV for TH 232 , and the K-40 line (1460 for K-40). The last channel is called a stability channel and is an artificial channel introduced by either an isotope or a small LED lamp. The precise position of this channel is adjusted by HV electronics to keep the whole system calibrated. There are a number of schemes, the newest uses onboard K-40 for continuous calibration, the fraction added being removed by software.
Geo
- Attachments
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- Geologists Gamma Spec