Note: Geiger (or Geiger-Muller [G-M]) detectors are described in the companion to this experiment description, "IONIZATION-BASED DETECTORS". Please refer to that document for a discussion of the appropriate background material.
To determine the proper high voltage (HV) setting for a G-M detector.
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The number of electrons reaching the central anode wire, and hence the pulse height produced, depends upon the HV applied across the sensitive chamber. A G-M tube should be operated in the region of least dependence on the HV, ideally where the detector response does not depend upon the HV. Thisplateau can be found experimentally.
Note the wiring and equipment set-up required for G-M counting. Be sure that the HV control is set to minimum, then turn on the counting set-up. If the scaler has a test mode, verify operation. Select continuous count mode, and place a source in the counting tray.
Slowly increase the high voltage until pulses are observed, record the threshold, then increase the HV in successive steps, recording the counts per minute (or other reasonable time increment). Plot each point (counts/min vs. HV) before proceeding to the next.
Only go to 250 volts above threshold, and reduce the voltage at once if the count rate increases drastically.
Identify the most nearly horizontal portion of the curve and set the operating HV at this plateau for the remainder of the experiment.
To determine the insensitive ("dead") time for a G-M detector.
Dead time is measured by the "method of paired sources", based upon superposition, employing two geometrically similar half-sources which fit into a standard counting planchet.
The method relies upon keeping the geometry of the following counts constant:
Remember that any one count has two components, the count due to emanations from the source(s), and another due to background radiation (b), so that the gross count is g = s+b.
To proceed: put one half-source in place, with the blank half-disc, and count for 5 minutes or more. Normalize the count to cpm. This observed count rate is r1, and the corresponding true count rate is R1. Note that R1 is the sum of two components: the true source count rate S1 plus the true background count rate RB.
Now, without moving the first source, remove the blank half-disc, add the second half-source to the planchet, and count for the same time, yielding a count rate r12. The corresponding R12 = S12 + RB.
Remove the first source, substituting the blank half-disc, and count the second in the same manner, obtaining r2. For this measurement, the corresponding R2 = S2 + RB.
Finally, remove the second half-source and count the background, yielding rB. This last count should be taken over a longer time to produce better statistical certainty.
We know that the true counts must satisfy the relationship:
S1 + S2 = S12
thus:
R1 + R2 = R12 + RB.
However, since each of these terms are different, the actual counts will differ due to the effects of insensitive time. Writing the relation in terms of the observed count rates produces an expression which may be solved for the insensitive time:
Solution to this equation produces a quadratic expression for tau.
Calculation of the 68.3% confidence level (1) requires a complex manipulation of the confidence levels for each measurement and is not required.
To determine the range of G-M detector efficiencies for both Betas and Gammas.
The counting efficiency for a detector is defined as the ratio of cpm recorded by the detector to the particles (or photons) emitted per minute from the source. It should be clear that the efficiency will depend upon many variables:
This exercise will focus establishing a range of efficiencies for typical laboratory situations.
Determine the counting efficiency for the beta and for the gamma selected. Plot (on one graph) the beta and the gamma efficiencies (%) vs. energy of the radiation (MeV), showing one straight line for the gamma and another for the beta results. Include the band of one standard deviation in reporting your data.
Be very careful in your assessment of the source: especially branching ratios and multiple disintegration paths.
