Adjusting the Discriminator Setting

What does the discriminator level do?

• When a 14C+ ion enters the detector, it excites a cascade of electrons in the detector gas that are collected as an electrical pulse on the anode.

• The discriminator rejects any incoming pulses that fall below a certain level.

  • For example: if a discriminator level of 2.5 V is set, the detector will not register any pulses under 2.5 V high as a 14C count.

When to adjust the discriminator level?

• If you recently flushed and replaced the detector gas

• If you recently installed a new detector window

• As an additional check if the 14C/12C ratios are too high/low

Why adjust the discriminator level?

• Changing the detector gas can change the height of the pulses induced in the detector by incoming 14C+

• After installing a new detector window, the glues that bind the window can outgas over time

  • The impurities generated by this outgassing can reduce the signal pulse height over time.

  • If the average pulse height of 14C+ ions falls below 2 V, it is recommended to flush and replace the detector gas followed by readjusting the discriminator level

How to set the discriminator level with an oscilloscope

1. Put in a tuning standard.

2. Turn pulsing off in ACS

3. Read the output of Signal Out 1 from the detector electronics.

4. Measure the average pulse height of the incoming 14C+ ions.

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 level 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

   1. The 14C/12C ratio should come up to a plateau which should be near the reference value.

   2. As the discriminator is further lowered, eventually the ratio will sharply increase as electrical noise is eventually counted as 14C counts as well.

5. The average pulse height will be roughly the discriminator setting the 14C/12C ratio of the standard is half it’s reference value.

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The discriminator is part of the detector electronics.

Example with an OxII:

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.
   
   

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   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:
   
   

5. 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.

6. 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?

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