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\uD83D\uDCD8 How to set the discriminator level without an oscilloscope

1. Put in a tuning standard and, for best results, let the 12C current stabilize.

2. Set a single run sufficiently long to perform the study (e.g. Cycle time 10s, 250 Cycles)

3. Select a starting discriminator high enough that most to all of the 14C counts are filtered out

4. Reduce the discriminator by 0.1 V, dwelling on each setting for 5 cycles before moving on to the next

5. Compute the average 14C/12C ratio over each set of 5 cycles and plot them against the discriminator setting.

6. The average pulse height will be roughly where the 14C/12C ratio of the standard is cut in half.

7. Set the discriminator to 2/3 this average pulse height

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Example case:

1. An OxII standard was inserted.

2. A Single run with 250 ten second long cycles was run and the currents were allowed to settle.

3. Starting at Cycle 100, the discriminator was set to 2.2 V

4. Every 5 cycles, the discriminator was lowered by 0.1 V until electronic noise began to saturate the count-rate (0.7 - 0.5 V). See the following plot for the view in ACS of this test.
   

   

   
   

5. The above data was copied into Microsoft Excel. The average 14C/12C ratio was computed for each segment of 5 cycles and plotted against the corresponding discriminator value as seen below:
   

   

6. Examining the graph, the 14C/12C ratio is cut roughly in half between 2.0 to 2.1 V on the discriminator. This is where the average pulse height is.

7. Two thirds of 2.1 V yields a setting of 1.4 V as a final discriminator setting.
   

Is my average pulse height too low?

What causes the pulse height to change?

Particularly in newer systems or following a new detector window installation, impurities in the detector gas due to outgassing can degrade the average pulse height.

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